Build Log - John Mayer Amp Clone

[sluckey]

Actually, the mismatched values for R49 and R50 is an attempt to compensate for unequal gains of the LTP triodes. (same purpose as adding a trim pot in some designs)

Aiken says it this way, "When only one signal input is used (ignoring feedback inputs) R1 is usually made 10% - 20% lower than R2 to compensate the unbalanced gains of the two tube sections and make the two output amplitudes equal."

[norburybrook]

Actually, the mismatched values for R49 and R50 is an attempt to compensate for unequal gains of the LTP triodes. (same purpose as adding a trim pot in some designs)

Aiken says it this way, "When only one signal input is used (ignoring feedback inputs) R1 is usually made 10% - 20% lower than R2 to compensate the unbalanced gains of the two tube sections and make the two output amplitudes equal."

ah, thanks Steve I always thought it was there to intentionally add harmonic content by mismatching that's my Dumble experience coming to the fore.

If they're supposed to be equal how do you know which side is higher or lower on the two triodes? the tube can only fit one way so you could be making it worse no ? ……….just asking for a friend.....


M

[rootz]

There seems to be some debate about the values of the PI plate resistors. Or better said: where and how to set the trimmer. Some do it by ear to for harmonic content. Some match the output wave forms on a scope. This would give the least amount of 2nd harmonics from the output tubes (if they are really closely matched). Doing it by ear to get more harmonic content out of the output stage would suggest intentional mismatching and more 2nd harmonics.

IIRC the bottom triode always has less gain in a LTP with a normal tail resistor. That’s because the resistor is not a perfect current source and thus doesn’t mirror the current in the top triode to the bottom one perfectly like for example a transistor cascode (or other well designed current source) would. Take a look at a standard Marshall PI. Input triode (preamp in) uses 82k, the other triode 100k.

[sluckey]

It's not the tube that causes the difference in gain. It's the nature of the circuit. Even if you used exactly matched triodes the circuit gain will be different if you use equal value plate resistors. Here's Aiken's info on LTP...

https://www.aikenamps.com/index.php/the-long-tail-pair

...and here's his more in depth LTP info...

https://www.aikenamps.com/index.php/des ... e-approach

[Bombacaototal]

I am of the option that the ODS amps have the LTPI matched for minimum distortion, leaving only the overdrive channel as a source of distortion.

I also think that the SSS amps are intentionally unmatched for 2nd harmonic content and hence why all of them have equal plate resistors

[erwin_ve]

Also the pi tube is not an ideal linear amplification device.
When set at a certain input level for optimum output balance, the optimum balance will be gone by raising or lowering the input level. Some might call this bloom, when the optimum balance is gone after the initial input attack and the overtones appear created by the appearing imbalance.
Fascinating stuff really

[Roe]

I prefer setting the balance with minimal negative feedback and when the power amp starts to break up. its not so much about harmonics as about a punchy, powerful response. Also, its about minimizing hum and ghosting from an unmatched push-pull amp

[Johnny_Boy]

Almost Done! But more questions
OK, so I finally got it all wired up late last night. Even though I wanted to fire it up at 1 AM, I fought the urge and went to sleep, so I can triple check everything before I really fire it up today. Good thing. As I've found a couple wiring mistakes which I had to correct it today. I walked through the whole thing using the layout once, and then re-walk through it with the schematic. Marked every path with a highlighter as I validated, so I don't get confused.

• tracing.jpg
• Alldocuments.jpg

Updated the Schematic
Updated the earlier post with the latest schematic. mostly I've updated the heater wiring section, as I've realized that I don't have the center ground tap, and I was wiring up the 12AX7 with 6.3V instead of 12.6V.

• Heater.jpg

A couple more question on the schematic.

1. Contour pot wiring - per the schematic, it looks like I need to join two lugs together, and send out on the 3rd lug. But on the pictures and the layout diagram they are not joined together. What do I need to do here? Same scenario happens on the Bass pot, but on the bass pot it actually does join two lugs together?
   contourpot.jpg
2. Shorting Jacks on the Speaker output? - The updated Wunderbar 50W schematic that I've been using shows shorting jack on the speaker output. But I don't see that on the layout file or the photos of the unit. Should I switch out the regular mono one for the shorting kind? I am assuming this is for protection in case you forgot to plug in the speakers when you turn on the amp?
   Jacks.jpg

[Johnny_Boy]

Fired it up for the first time! What bias setting should I use?

Ugh, it did not work at first. No sound. No smokes. But it would make sound for a few seconds when I turn off the power (with that dying fading crackle sound). Then I realized that I have not set the bias yet, and the pot was pretty much turned all the way up. I just turned it all the way down and it is now working (except for the reverb, so I need to go trouble shoot what I messed up there).
(I've never seen an amp where I've set the bias setting way too high causing the amp to not work at all. I wonder if the pot has too much range?)

[Updated later - I had to reverse wire the Output Transformer's primary (blue and brown wires) from one set of 6L6 to the other, because the moment I turned it on it started to squeal very loudly. I guess positive feedback loop causing the tubes to oscillate? If you run into this, just reverse the wires and it will change the phase)

1. How do I measure bias on this amp and what should it read? 50ma?
2. Do I use 12AX7, 5771 or 12AT7 for V3? (Edited later - Use 5771 (lower gain version of 12AX7) or 12AT7, which has the Mu of 70 and 60 respectively, while 12AX7 has Mu of 100)

[alkuz1961]

you can use bias calculator
https://www.amplifiedparts.com/tech-art ... calculator

[pompeiisneaks]

If I'm reading the schematic right, you have 2 1 ohm resistors on the power tubes to ground. Measure mV from either side of that resistor and you'll get basically the mA of the current due to ohms law.

V=I*R (current times resistance which is 1) so volts equals amps.

Then as alkuz said follow the bias calculator and set them to be under the 70% diss for the tube.

[Johnny_Boy]

If I'm reading the schematic right, you have 2 1 ohm resistors on the power tubes to ground. Measure mV from either side of that resistor and you'll get basically the mA of the current due to ohms law.

Measuring it that way it gave me 100mv. If I measure it using the mA setting, i read around 90mA. assuming the resistor has +-10% error, they seems to tell me the same answer.
I should be reading around 47mA per the linked calculator! No wonder the amp did not work when the bias pot was cranked pretty high. Who knows what that setting was.

How should I fix this? change the 10K bias pot to something smaller like 5K?

(measuring the voltage across B3, B4, B5, I am only about 10V higher than the layout spec. Maybe my voltage coming off the blue/red transformer lead->1R->1N4007->bias pot is not correct? What should that read?)

EDIT - OK, I think I found out what the problem is. The schematic shows the bias tap as 50V, but my ClassicTone power transformer spec document shows that red/blue bias tap is 60V. so I am 10V too high which is probably why my 10K pot is turned all the way down and still can't get low enough.

[talbany]

Nothing wrong with having a bias pot sending your tubes into cutoff. As long as it's at the back end of the range on the pot.
As long as you have a fairly wide swing range (around 20/75) there and the pot is not real touchy or to sensitive, you should be fine. After I get the final set I put a drop of nail polish or thread lock on the pot's shaft (if it has one) just to lock it in place

Tony

[pompeiisneaks]

You'll need to adjust the resistors in the Bias supply to drop that extra 10V. My best guess would be that 1R resistor out of the bias tap maybe could go up higher to create a larger voltage drop before you hit the diode etc.

Someone else here probably is better at the math than I am, and I don't know what negative bias voltage you'd need for this amp anyway, I would basically be looking up the plate voltage, calculate the per tube mA needed for 70% diss at that voltage, and then do the math with an online calculator etc.

It might also work to just shunt more to ground by lowering the resistor to ground so more voltage heads to earth instead of through the bias circuit.

I'd consult smarter minds than mine to ensure you get the bias right You can just pull the output tubes and work until you get a better negative bias for what you should have in this amp... then put the tubes back in.

~Phil

[Bombacaototal]

You'll need to adjust the resistors in the Bias supply to drop that extra 10V. My best guess would be that 1R resistor out of the bias tap maybe could go up higher to create a larger voltage drop before you hit the diode etc.

Someone else here probably is better at the math than I am, and I don't know what negative bias voltage you'd need for this amp anyway, I would basically be looking up the plate voltage, calculate the per tube mA needed for 70% diss at that voltage, and then do the math with an online calculator etc.

It might also work to just shunt more to ground by lowering the resistor to ground so more voltage heads to earth instead of through the bias circuit.

I'd consult smarter minds than mine to ensure you get the bias right You can just pull the output tubes and work until you get a better negative bias for what you should have in this amp... then put the tubes back in.

~Phil

You probably need 1K instead of the 1R. It had a great range on my 100W (4x6L6) which uses the same PI