Cheap 'N' Loud Q&A

          Q & A page for questions ask on the AX84 Forum


The reason for this page is as follows:

I frequently post questions on the AX84 tube amplifier forum. Sometimes I would like to show a schematic or go into a more detailed question than would be appropriate for the forum. If I want to expand on a question I will reference this page and give a question or item number and a date.

The page title, Cheap ’N’ Loud is one of several pages documenting some guitar and amplifier builds.

When answering my questions please note.  I do not play any instrument, I wear a hearing aid, and I am too old and lazy to learn to draw load lines. Please not too many equations, I still add and subtract by counting on my fingers, I am easily confused by anything beyond ohms law.

 Any builds will be copies of something that already works or I will diddle the bias values until something does work. I will check for output swing and distortion by putting the inputs and outputs into the X &Y inputs of my ancient scope and looking at the Lissajous pattern. 

I am interested in guitar amps because I have a 50 year collection of tubes and I have discovered two dealers with lists of $1.00 tubes. I feel I must build and see my old tubes light up (except the metal ones) and make noise. I hope to build a tube bread boarding fixture some time this year (2016) and stop asking so many questions of you good people. I received Merlin’s pre-amp book for Christmas, so this should cut down on the questions.

All pictures and schematics on this page are in the thumbnail format, click on the small image to see a larger version.



Question 1  1-14-2016  I have built two cigar box guitars, I am using two piezo pickups and mixing or selecting them with a pair of 2 meg pots. This loads the pickups with one meg. I put every thing together before receiving Merlin’s pre-amp book and learning that piezos should look into 10 megs or more. If I plug these guitars into an amp with a 1 meg input impedance, the piezos will be looking into 500K. I will already have them loaded with 1 meg, so I would like to eliminate any additional loading .


Comments on the posts to question #1

Once again I would like to thank everyone for their comments and help.

Kursad K, Thank you for pointing out the dangers of not having an input capacitor if a grid to plate short should occur in the first stage tube. If this should occur, the voltage would only be present on the center conductor of the guitar cable and the tip of the plug. I always use an isolation or power transformer on my builds and a 3 prong power plug, connecting the green earth ground to the common star ground on the amplifier. Even with a tube failure the guitar cable shield and the plug sleeve would still be at ground (earth) potential. I just checked my Gibson, and the strings show continuity to the sleeve (shield) guitar cable connection. I will however be using a cap on the input, I will be discussing this and how I intend to configure the amplifier after these long winded replies.

rob rob, Thank you for validating my idea. I thought it would work, but some times others see things that I do not. As Paul Fawcett pointed out he left this resistor out of a build and did not notice it.

Paul Fawcett, The amplifier I am building will have even less current available at the plate than the situations you discussed. The B+ on the plate resistor will be around 160 to 180 volts and the plate resistor will be 180K to 220K. I will probably not need a bootstrap input circuit because of changes I will be making. I tend to shy away from bootstrap input circuits. I used one years ago to interface a very hi Z transducer to an opamp. The co-ax at the input looked like a resonant circuit and it would oscillate at VHF frequencies. The input device was solid state, I doubt that a tube with its stopper resistor and high Miller effect capacitance would have this problem.

PaulP, You are right, the 2 meg resistors are a bad idea. I did not realize this until I read Merlin’s book. I am making changes to get rid of them. I will not be putting a buffer amp in the guitar body only because I don’t like the idea of a battery hanging off of the guitar. Buffering the high Z piezos would probably reduce hum pickup. I will be connecting the strings attachment point to the sleeve shield of the output jack so that the player will be grounded through a brass slide and provide more shielding. The external buffer box for which you provided a link is a good idea. The 2N5457 looks like a good choice for this application. The schematic shows a MOS fet in this position which could be damaged when plugging in a cable, but the 2N5457 is a jfet which is a more robust part than a MOS device.

James Merchant, I did consider a bootstrapped circuit but I will not be needing it due to changes I will be making in the guitars and amp.


There was much discussion on this post about getting shocked and skin resistance, if you all can stand some long winded BS stories I will share some of my experiences with you. I do tend to carry on in my old age. The following items have nothing to due with tube amps so skip them if you are not interested. I will however discus what I will be doing to the cigar box guitars and amplifier at the end of this page.

When I was young an foolish and wanted to check for the presence of power and did not have a meter, I would lightly brush two fingers of the same hand across the terminals in question. Sometimes if the fingers were dry you could not feel 115v unless you wet your fingers. 230 volts you could feel wet or dry. I once talked to an old maintenance electrician who had thick calluses on his fingers, he said he would check for up to 440 volts this way. Any thing higher than 440 volts would really smart because it would draw an arc and burn his fingers.

I wandered over to where a newly minted engineer was having a tech check the output from a black box with a scope. There was no signal present so the engineer was about to declare the box defective until I pointed out that they we looking at the output of an open collector gate and that they needed a pull-up resistor. Under normal conditions the box under test was plugged into another box that had the required pull-up resistor. We would have had to have a procedure written and approved to add a resistor to the test. I told the engineer I could use my fingers to perform a quick test. I took hold of a test lead with 5 volts on it with one hand and with the other hand took hold of the box output and the scope lead. A nice digital signal appeared on the scope. The engineer wanted to know how many ohms I was, so showing off I said 10K. Later we used a VOM and checked me out. I squeezed the test leads just enough to read 10K. Still showing off, I ask the engineer if she would like to see 2K. I wet my fingers and squeezed the VOM test leads just enough to read 2K. Having fun with young engineers.

12 volts in salt water will get your attention! Many years ago some of my diving buddies and I wanted to do a night dive. Lacking dive lights we decided to improvise. We floated a 12v car battery in an inner tube and connected several 50 foot lengths of zip cord to the battery. At the other end of the zip cords we attached sealed beam car headlights. To turn the lights on and off we left one lead disconnected, and would twist two bare leads together to turn the light on. Upon entering the water we immediately noticed we were getting shocked. We had to touch the two bare wires together and then twist them together to keep from getting shocked. It was great fun for about 20 minutes until the lights crapped out. I did not think the current drain between the light terminals would be great enough to cause any problem so they were not insulated. After about 20 minutes electrolysis would eat through seals, letting water into the bulb and burning it out. We were all surprised at how you can get zapped with only 12 volts.

Back to guitars & amps, With all of the good input from everyone, I have decided which way to go. I will be removing the 2 meg pots from the guitars and replacing them with small toggle switches. I found a cigar box large enough at C. B. Getty to hold all of the components of a tube amp so I don’t have to go over to the dark side and build a solid state amp. To get the high impedance input I need I will use a small input cap and a 10 meg grid resistor with no cathode resistor. The plate resistor will probably raised to 200K or greater. Looking at Asden Pitmans book “Tube Amp Book” I think I have found an amp I want to copy, it is the Supro 1 X 10 on page 163. The power supply will be different, with only 160 to 180 volts on the output tube plate. The output tube will be a 6005. Should be good for about 2 watts. I had thought about using the Marshall one knob tone control, but the input tube may not

be able to drive it with the 200K plate resistor. I don’t know if I will have enough gain to take advantage of it, but I will add a master volume control. I have not drilled any holes so my option are still open.


Question 2

Your basic power supply design sounds OK. As Merlin noted the PSRR on this op-amp is very good, spec sheet says it is better than 80db so your Zener noise is probably not entering through the power pins but through some of the signal circuitry. Like the stopper resistors on a tube, all resistors connected to the amp should be connected as close to the input pins as possible. Long leads(several inches or more) leading from the summing junction to input and feedback resistors can act as an antenna and pick up noise. The open loop gain from the summing junction to the output can be as great as 100,000 or more. A small amount of capacitive or inductive coupling will go a long way. You may be operating your Zeners in a region where they are noisy. If the Zeners have insufficient current flowing through them they may be operating in the “knee” region and be very noisy. Google Zener knee current, there is much information available on biasing Zeners.

The knee region is a high impedance area just as the Zener begins to conduct and is noisy. To operate correctly, the current through a Zener should be sufficient to push it into the low impedance region where the I/V curve is almost vertical. Every Zener acts differently when operated at low currents in the knee area, even those from the same batch. Some are quiet, but I have seen some that exhibit a small amount of negative resistance and some show quick current jumps like a diac diode. The old rule of thump was to operate a 400Mw Zener at a minimum current of 5Ma. Your Zeners are larger than 400Mw so you may need to push more current through them to get them above the knee. Maybe this is the reason your larger Zeners are more noisy than the smaller ones, they may need more current to get them above the knee. There is no reason to cook your Zeners, just apply enough current to get them into a quiet zone.


Below is a graph taken from an old 1966 Motorola diode book.  It shows the region to avoid to keep Zeners quiet.  The image is in a thumbnail format, click on it to see a larger readable version.


It would be difficult to use 7815 regulators in your situation. The Zeners are shunt regulators, where the 7815s are series regulators. I think the Zeners are the way to go if you can keep them quiet enough for your needs. Some schematics are attached to this post in thumbnail format, click on them to see a readable version. Figure 1 shows one version of 7815s supplying 15 and 30 volts. The input voltage range shown is what I think you could get away with. If you follow the data sheet to the letter the input voltage would be 35 volts. The regulators require a 5 volt drop to work properly the max input is 35 volts. If the 7815 regulators were used they would require some sort of pre- regulation. Also note the 30 volt regulator would need a 15 volt Zener in the reference leg so you would be back to square one with the Zener noise showing up in the output of the regulator. The resistor from the regulator output to the junction of the Zener and reference pin is to push enough current through the Zener to move it out of the knee region. I have never tried the circuit shown in Figure 2 but I think it would work. You gave a very good description of your circuit in your first post, I have drawn a schematic of what I think you have built in Figure 3 just so that I know that we are on the same page. I stated in a previous post that I think the noise is entering your op-amp circuit at the 15 volt junction of the two Zeners. Check out the circuit arrangement in Figure 4. This would decouple the Zener noise from your circuit reference. I am unfamiliar with the op-amp circuit you are using and this would only work if no power were supplied at the 15 volt level, only signal returns. There is a high voltage series regulator that would probably work off of a b+ supply with a voltage divider. The TI TL783, it will take an input voltage up to 125 volts. I have never used one of these but will be building a variable filament supply with one of for a tube bread board soon.


Question 3 5-30-2016


Darren, I developed a circuit 40 years ago which you may be interested in. I will probably present way more information than you care to read, but bear with me I will eventually get to the point and discuss a filter which may work for you. A senior engineer

came to me an said he would like to develop a circuit which would lower the voice pitch of deep divers which were breathing a helium/oxygen mixture. Management said we could have a few days to work on it as long as we did not buy any parts. We had a good supply of spare parts and stuff to build items for other divisions that needed oddball devices. The first idea was to use some kind of heterodyne device, but this would lower all frequencies by the same amount. (lower 1Kc by 1Kc and you get zero) It would be like a miss tuned SSB signal. I talked everyone out of going down this path, besides I had no idea how to design or build it. I proposed an idea which would divide the voice frequency range into 4 bands with filters, hard limit the filter output with comparators, put the square wave output of the comparators through D flip flops dividing the original signal by two. The output of the Flip Flops were then to be sent through another 4 filters tuned to frequencies one half of the original input filter frequencies. Shortly after we started this project, management cut it off because we needed to get back to the real work of our division, flight test/flight simulation. I did get as far as building the filters and comparators. As long as we had come this far, we decided to put some sound through the bread board to see what it sounded like. The input filters were tuned to the normal voice band of 300 to 3000cps for a test of concept. We had no budget to buy a tank of helium to test with and no volunteers if we could not get a mix with oxygen. We hooked a mike amp to the filters input and mixed the comparator outputs with 4 resistors which were connected to an audio amp and speaker. To everyone’s astonishment, very understandable speech came out of the speaker. It sounded weird and distorted but you could even recognize the person speaking, and all this from mixed square waves. Darren does this sound like what you are looking for?

The filters used were implemented with Hall networks. The Hall network is a notch filter when used in the negative feed back loop of an amplifier, ups the gain at the notch frequency. There is more going on than just this but I like a simple explanation that I can understand. Attached schematic, Figure F1 shows a Hall band pass filter using an OP-AMP. I use these filters every time I need one because they are easy to adjust. Only one pot is required to set the frequency and only one pot or resistor to set the Q (bandwidth).

The rules for using these filters are simple.

C1,C2,and,C3 are all the same value and chosen to cover the frequency required.

The resistance of R5 is 10 times greater than R3, The resistance of R4 is 10 times greater than R5. These resistor ratios give a tuning range of 3to1. Don’t try to fudge these resistor ratios or you will end up with an oscillator instead of a filter.

The Q or bandwidth is adjusted by changing the value of R2. Max Q is obtained when the value of R2 is ( R3+R4+R5 )*6. In other words add up the values of R3, R4, and R5 and multiply by 6. To get the wider band width you will need for your filters you will have to lower the value of R2 so that all bands will overlap.

For an astrometry experiment, I needed a filter that covered below 120 cps to over 180cps. The values for C1,C2, and C3 for this range were .022uf. Check out the following link for some good info on Hall networks.

I have included a schematic of what I think may work with a tube Hall filter. Figure F2 I have not tried it so I cannot verify that it will work. It is essentially a tube OP-AMP with out all of the DC stuff needed for analog computing. The AC stuff should be all you need. The filter requires a high input impedance, low output impedance, and gain. The pentode will not have the open loop gain of a silicon OP-AMP, and the cathode follower will not have as low impedance but what the hay give it a try.

I have some freebies you may be interested in. Check out this link,

go down to item 9 .You could use these items as filters for some experiments. If you are interested Email me at

you will have dibs on these items unless you let me know otherwise.

The following schematics are in a thumbnail format, click on them to see a larger readable image.


  Question 4  6-14-2016

Don’t try to put 220 volts into a transformer that was designed for 115 volts. The core will saturate during much of the AC cycle. A transformer with a saturated core looks like it is shorted, it will exhibit an input resistance little more than its DC resistance. A 115v transformer with 220v input will hum loudly, start to smoke within seconds, and if the winding does not open it will go up in flames.

What ever voltage you decide on for your tube filament supply (12.6v or 6.3v) stick with it for all of the transformers low voltage side.

Figure T1 Would be the simplest way to go. It almost duplicates the original schematic other than it would run on 220v. It would require that T1 be a 220v to (12.6 or 6.3v) and T2 be 115v to (12.6v or 6.3v).

Figure T2 Would use two 220v to (12.6 or 6.3v) for both T1 and T2. The output of T2 would be twice that of a 115v transformer, so the voltage doubler would not be required. I have not specified a value for the selected resistor, but a WAG would be around 39k to 47k. At this point I would like to ask a question of other forum members. Should there be a 1k or 2k resistor between the cathode of the 1N4007 and the 10uf capacitor to limit the charge current peaks and spread the charge time over a longer time? I have specified 350v filter capacitors because the voltage could rise to over 300volts until the tube warms up, or if it burns out or is removed from its socket.

Figure T3 Gets a little more complicated but would permit the use of 115v transformers on 220v. It does however require 3 transformers and some testing to get them phased correctly. The primaries are connected in series and the secondary’s in parallel. With out testing, there is a 50 50 chance of getting them phased correctly, get it wrong and plug it into 220 your transformers will go up in smoke. To test the phasing, a small light bulb is placed in series with the transformer primaries to limit the current flow. The low voltage secondary output wires of the transformers are labeled A through D on the schematic so the connections can described. To test phasing, power is applied through the current limiting bulb and wires A and C are temporally connected together. If the bulb lights up it means the secondary’s are out of phase. If there is little or no change in the bulbs light out put then the phasing is correct. If the bulb lights connect B to C and use A and D as the test points and repeat the test. Another way to check phasing is to put power to the primaries and measure the voltage at the test points. If there is 25 volts there with 12.6v transformers they are out of phase, the 6.3v transformers will show over 12 volts when out of phase. In either case swap the wires as described above and retest. A good test can also be performed with a 28v pilot light to look for voltage. When the correct phasing has been determined, the paralleled secondary’s of T3 and T4 should be connected to the low voltage winding of the third transformer to step the voltage up to 115 (transformer T2 on Figure T1).

Figure T1.

Figure T2.

Figure T3.


Question 5             6-27-2016


Figure 1C shows a partial schematic of a clipper/compressor which could be a starting point for a build. A provision for biasing the positive clipper diode is shown. With 250v B+ pot R3 could be adjusted to provide up to 1.8v peak to peak clipping. At first glance it looks as if the clipping would be unsymmetrical with only one diode biased, but because the signal to the diodes is cap coupled, an input signal will bang from diode to diode and even out some where between the two. Resistors R1 and R2 would set the output of the clipper at a mid point between the two diodes so that the first cycle of a note would be symmetrically clipped. These resistors could probably be eliminated, but with out them the grid of the following tube would pull the clipper output negative until the negative clipper diode started to conduct. The first guitar note struck would again bang between the two diodes and maybe take several cycles to settle out. This would drive the average grid bias of the second tube more positive and shift its operating point. This might cause a pop or thump, if so resistors R1 and R2 should cure this problem. This circuit has not been built or tested, I am just throwing it out as an idea. Comments please.

Figure 1C


Item 6   10-5-2016

 Below is a schematic for a question I ask on NFB on the forum.



Item 7  10-16-2016     

Below is a hand drawn rough approximation of the response of a two control tone control. This graph in no way resembles the response of a real circuit, but is just indicate what I mean by “ the cross over point “. What I am asking is, what should the frequency be for a guitar amp where the arrow is pointing on the drawing.



Item 8  12-26-2016

I found a schematic of a power amp using a 12AX7. They were popular for use as a modulator for mobile low power transmitters. Because of their low B+ draw, a transmitter could be powered from a car radio B+ supply.


Item 9  3-12-2017

I found this tone control circuit in a 1950 issue of Radio-Electronics magazine.


Item 10 3-21-2017

My test speaker is complete. It is a bit overkill for a piece of test equipment, but I wanted something that would work with any amp I might connect to it, and also something that a guitar player could use when I am through with it

The cab is from Seismic, and the speakers are an Eminence Texas Heat, and a Speaker Warehouse G12C. The Texas heat was chosen because it sounded good on sound clips. The resonant frequency of the Texas heat is 91 Hz. The G12C was chosen because it sounded good and has a resonant frequency of 134 Hz. Both speakers have almost the same spl. Both speakers are 16 Ohm models, so when connected in parallel they present a 8 Ohm load to an amp. The thought was to parallel speakers with different resonant frequencies, so that when one was resonant, its high impedance would be shunted by its non resonant partner and thus presenting a more constant impedance to the amp. The installed wiring that came with the Seismic cab had every thing wired in parallel, the two jacks on the back, and the two speakers. The wiring was changed so that each speaker went to its own individual jack, and a switch was added to connect the speakers in parallel so that either jack could be used to drive both.

Test Speaker.

Rear view two rear panels removed Texas Heat left G12C right.

Test Speaker.

Front view Texas heat right larger voice coil. Warehouse Speaker G12C left smaller voice coil.

Test Speaker.

Front view grille installed Gibson signal generator leaning against cab.


Item 11 3-23-2017

A new project. A speaker that goes with my 16mm movie projector has gone missing. Probably buried in a pile of unsorted junk. I ask a retired local contractor if he could build a cabinet for me. What he came up with looks too to good to store away with the old projector. I will use something else for the projector. The new cab looks like it is something straight out of the early 1930s. It is built out of ½” MDS and is really heavy, the end grain stained a darker color than the pressed sides so it looks as if it has some fancy edge trim. No Tolex for this cab. The price was right, I bartered some work for this cab. Mark Ball, the retired contractor is doing some volunteer work at a local museum, he needs some post holes drilled so I will do that for him. Several hours of my time and a ½ gallon of Diesel fuel seem to be a good trade for a nice cab. A post by Dave Morin on a type 47 power tube amp, the new cab, and the following web sites (link)  (link) have sparked an interest in building a combo that looks and hopefully sounds like something one might have come across in a roadside beer joint during the dust bowl days. I have chosen an Eminence 820H for this project and the speaker hole will be covered by some 1930s Philco replica grille cloth. Edcor power and output transformers are on order. I have 2 new type 45s and an 80, so this will be the power output and rectifier. An interstage transformer will be used to drive the 45s. The circuit will be a combination of the National Dobro 6107A and LA278. The National Dobros use 2A3 tubes but I have decided to use 45s for two reasons: 1, The 2A3s use a huge amount of filament current and it would cost much more for a transformer to light the 2A3s than the 45s. 2, If I used 2A3s in a guitar amp I would have a mob of cork sniffers with pitchforks and torches in my front yard ready to string me up. I am still undecided what tubes to use in the two preamp stages. I have been unable to locate a power transformer with two 2.5 volt filament windings, these were once widely available as replacements for early 1930s radios. A stand alone filament transformer will be used to light the 45s The Edcor transformer has a ct 6.3 volt winding so it can with some creative use of the ct and some .4 Ohm resistors to light 2.5 volt tubes. A 6.3 volt 77 and 76 would work directly off the 6.3 winding. The 76 did not appear on the market until 1934 so this build would have to be a 1934 clone. I had considered using some 1929 tubes, a 24A and a 27. I know I would have to stay out of the dreaded tetrode kink area with the 24A. I would like some comments on this tube. The only time I have seen this tube used as an audio amplifier is in the Loftin-White circuit. I briefly considered using some metal octals, but that would be cheating it would then be a 1936 clone.

New Cab.

Top view. Note the dark stained end grain looks like trim.

New Cab.

Front View. Cut out for 8" speaker.

New Cab.

Rear View.


Item 12   1-14-2018

 Once again I have written too much in answer to Denny Gacey's post to post on the forum, so it will appear here.

I will take another shot at this. I will repeat a lot of what I said before but it will be pertinent to what I perceive to be the explanation to your question. The reason different voltages are observed when a load is applied to the AC side of a bridge or to a capacitor input DC output of a bridge is due to the duty cycle during which a certain amount of energy is supplied to the load(tube filaments). Any power supply has a certain amount of output impedance, in this case the power supply being the filament winding on your power transformer. The more current you pull from it the lower the output voltage. The lost voltage will appear across the output impedance. The filament supply bridge or not is a simple series voltage divider. The output impedance of the filament winding is the upper leg, the filaments are the lower leg. When the filaments are powered on the AC side they draw current continuously a 100% duty cycle. Enough energy is transferred during this time to light the filaments. When the filaments are powered from a capacitor input power supply the same amount of energy must be transferred to the filaments in a much shorter time. The filter capacitor is charged during the voltage peaks of the sine wave. With a shorter time to transfer the same amount of energy the bridge/capacitor must draw more current during this time. This extra current draw will cause more voltage drop across the output impedance of the filament transformer leaving less voltage to charge the capacitor. It would be difficult to observe all of this occurring with a volt meter especially with a “true RMS reading meter” because 90% of the AC wave form would not be affected by the bridge and capacitor. A scope display is worth a thousand words, so if you have access to a scope I would suggest the following tests. Disconnect the power tube filaments and connect them to some jumper leads so that they can be connected at various points in your circuit. Connect the scope across the AC filament winding, connect the filaments across the AC winding. A reduction of voltage should be observed across the full AC cycle. Still observing the AC winding, connect the tube filaments across the bridge/capacitor. A flattening of the sine wave peaks should be observed with little effect on the remaining wave form. A third test can be performed which might be enlightening. Remove the capacitor, observe the AC with the scope, load the output of the bridge with the tube filaments. The wave form change should be similar to that observed when the tubes loaded the AC side. With a choke input a power supply exhibits better voltage regulation under load variations than a capacitor input because it has the effect of lengthening the duty cycle during current draw from a power source (I know, this is an overly simplified explanation).


Item 13  4-6-2018

If you use the 26 volt B+ supply, this circuit should come close to driving the tube to full output. You only need 7 volts out of each side of the phase inverter. I did a quick and nasty circuit analysis of the circuit and came up with about 9.5 volts drop across the 3.3K collector resistors. I have never used a circuit like this and I don’t know what percentage of the drop across the collector resistors this circuit will swing. I am sure that if you fudge some of the resistor values this swing could be increased. Much more current flows in this circuit than would be required to drive a tube grid. You could probably remove the 10K balance pot, I think it is there more to balance the DC output than the AC gain. The AC balance would be more affected by the actual values of the 3.3K collector resistors. You might also consider substituting a constant current source for the 1.5K emitter resistor. Good luck on this project. I have just purchased several 12L8GTs that will be running 160 volts on the plates & screens. Another interesting low power twin pentode.


                     New 4-16-201

I am a little pressed for time, but I will share what I have found out so far on the solid state PI. I got it fired up and took some quick readings with 40 year old test equipment that has never been calibrated. Next week I will have more information on this page.

All tests were performed with 26 volts powering the board.

The outputs looked fairly clean swinging up to 16 volts Peak to Peak.

The following readings were taken with the outputs swinging 10 volts peak to Peak.

Gain greater than 160

-3db bandwidth 30 Hz to 220KHz

To verify the gain I plugged my Gibson signal generator into the input. A twang on the strings pushed the circuit into overload. Needs a volume control at the input. If this circuit is used to drive the dual pentode, no other preamp is needed.

                       New 4-20-2018

There is some magic regarding the 2N5210 transistor, I have almost 100 in my junk box but only a few 2N3904s. There are some other characteristics of the 2N5210 which are advantages for use in a circuit like this. A Ft of 50 Mhz as compared to a 2N3907s Ft of 300 Mhz makes it less likely to oscillate in high gain circuits due to some sneak positive feedback loop. The very high beta ( Hfe/current gain) of the 2N5210 permit’s the use of almost any value of resistors in the base voltage divider. The transistors in the Q1,Q2 positions are a matched pair with a beta of 680. When I got out an envelope of 2N5210s one fell out, so it became the current source. When testing I noticed a slight oscillation on the driven transistor ( the 180 degree output ) just a bit of hair on the sine wave. It turns out it did not like driving a long test lead terminating in a 30pf load of the scope input. A 6.2K resistor between the output and the test lead suppressed the oscillation. I would suggest using  stopper resistors between the outputs of this board and the tubes grids. Just to experiment, the constant current source on the board was disabled and a resistor was substituted in its place as in the original circuit. With no other values changed, a 1.4K resistor gave the greatest voltage swing. With the resistor in the emitter line the board would only output 6volts peak to peak and with a lot of distortion, The peaks of the sine waves were rounded off and flat topping. The current source was re-enabled and the original test results were verified. The circuit was pushed to see how it reacted to overdrive. With an output of 16volts peak to peak the sine wave still looked clean and undistorted, pushed further the tops of the sine wave started to round off and pushed even further it rounded off even more until it turned into a square wave. The circuit was tuned by placing a volt meter between the collectors of Q1 and Q2 and with no signal input, adjusting the trimmer R2 to read the least voltage possible. With a signal input current source resistor R10 was selected to give the largest output voltage swing. Test frequency was 261 Hz ( middle C/C4 ).

I was surprised at the differences between using a resistor as opposed current source in the emitter leads. I am sure someone has done this, but has anyone put a pentode in the cathode leads of a 12AX7 phase inverter?

JaapK , I have added a 75K volume control pot and a ¼” jack to the input so you can play with it straight out of the box. It will be in the mail tomorrow. Please excuse the bird shit soldering job on certain items. As an 81 year old cattle rancher, my dexterity and eye sight are not what they once were.

I forgot to mention in the previous text that the 2N5210 has a noise figure lower than the 2N3904. This is of little consequence in a high level circuit like this PI. With a 2N5210 and some metal film resistors, a low noise pre-amp can be constructed. I wonder if a high beta transistor like the 2N5210 would work in a Big Muff?

                           New 4-25-2018

JaapK, Thanks for the original post. Building and testing the PI circuit was an interesting project and I learned a lot. As to your original question would the “Nuts and Volts” circuit drive a 26A7? I think the answer would be no. I think the modified circuit with the current source/sink in the emitter lead would, It would probably drive a pair of 6V6s or 6005s also. I lied to you when I said I would mail the board on the 21st, the 21st was a Saturday and the post office was closed, the board is now in the mail.

Paul Fawcett, If I had some 2N3904s I would have used them, The parts used were what I had available in my junk box. I did not try degaining the circuit with emitter resistors because there was no room on the board to add more terminals.

Some more thoughts, This circuit overloads very smoothly making a the transition from clean to clipping to square wave with no glitches in-between. With a volume control at the input to control drive, and a dual pot on the output to act as a master volume control and driving two 6V6s a self powered distortion box could be created. One problem which might be encountered with this circuit is DC drift due to parts aging and requiring the re-adjustment of the balance pot at certain intervals. Any good designers out there who can design a differential op amp to sense a DC mismatch between the collectors and steer them back to null?



PI Schematic.

Top of circuit board.

Bottom of circuit board.


Item 14   4-28-2018

The output circuit is a cascode amplifier. Check out this Merlin page (link)  Now imagine that V1 is an NPN power transistor and V2 is the parallel 6AC7s with not 1/3 HT on the grid but +16 volts. The transistors are biased on by the 100 ohm pot TR1. The pot is tied across a combination of two forward biased 1N4003 diodes D15 and D16 . As wired, one side of the pot would see +.6 to +.7 volts the other +1.2 to +1.4 volts. By adjusting pot TR1, the idle current through the tubes can be set. I will pull a tube bias value out of thin air to make a talking point. Lets say the tubes need -20 volts grid to cathode to draw the desired idle current. In this case with +16 volts on the grids the cathodes would have to be setting at +36 volts. The base drive to the transistors would have to be set to sink the proper amount of current to achieve this cathode voltage. With the 3.9 ohm resistors in the emitters of the transistors, they would act as constant current sinks and would be fairly balanced even given different betas. I have never heard of the transistors used. I would think something with a 200 volt CE , 10 watt, and a 1 amp ratings would work. As Murphy would say ( I’m making this up ) If you have a 200 volt transistor in a circuit with 700 volts somewhere in it some time some way the 700 volts will find the transistor and blow it up. Think of a tube arc over. A horizontal output transistor from a CRT TV with a 1000 volt rating might be a good choice in this amp. To trouble shoot I would look at the +- 16 volt power supplies. Loosing either one would render all of the op-amps inoperable, also the transistor bias supply comes off of the +16 supply with its own regulator. I remembered a power circuit similar to the output stage of this amp only it used a FET in the lower leg. I was able to find it after a short search.(link)








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