Peavey Triumph 120 / 60 tone shaping and theory

[wavetuner]

I have a 120 and am interested in a few concepts in the amp. The final preamp tube half in the OD circuit has a 10K from B+ followed by a 100K plate resistor in series. The coupling capacitor proceeds to the Post control from this junction. This is a somewhat strange configuration for me. This is clearly a 10:1 voltage divider but why? The EFX impedance? My goal is to tame the shrill sound of the overdrive and get some harmonic bloom. Clearly the coupling caps will be replaced. There is a similar thread but it is rather long. This may not be a good candidate for what I want (classic blues and rock, not metal) but is sure has the power amp!

[Chuck H]

The 10k followed by a 100k plate load is a typical circuit. I'm assuming the 10k is a series resistor in the HV rail??? Is there a decoupling cap between the 10k and the 100k plate load? How about a schem?

Don't worry about the voltage division. What is the voltage? And why is this circuit suspect in your endeavor to reduce shrill tone? ALL PEAVEYS ARE BRIGHT! Peavey amps are always accused of being too glassy bright. There are mods on line here and there. But the best solution IMHE is different speakers. The right speaker can make all the difference in the world. FWIW I loved my PAG 60 with the original speaker (can't remember if it was a BW or a Scorpion though?). But it was even better through a quad of G12M70's. That speaker is known to have a high mid bump and a top end roll off. So there you go. Try a boxy speaker. I might suggest a couple of Emminence models as most Emminence speakers seem to have a rolled off top end compared to Celestion and Peavey speakers.

I had a friend with a Classic 50. He complained that the top end was too shrill. There are a ton of mods for this problem on the "Classic" amps. I replaced the two "Blue Marvel" 12's with Emminence Private Jacks and the unanimous concensus was that it sounded great. No need for further modification. The speaker is your amps voice.

[wavetuner]

The 10k followed by a 100k plate load is a typical circuit. I'm assuming the 10k is a series resistor in the HV rail??? Is there a decoupling cap between the 10k and the 100k plate load? How about a schem?

Don't worry about the voltage division. What is the voltage? And why is this circuit suspect in your endeavor to reduce shrill tone? ALL PEAVEYS ARE BRIGHT! Peavey amps are always accused of being too glassy bright. There are mods on line here and there. But the best solution IMHE is different speakers. The right speaker can make all the difference in the world. FWIW I loved my PAG 60 with the original speaker (can't remember if it was a BW or a Scorpion though?). But it was even better through a quad of G12M70's. That speaker is known to have a high mid bump and a top end roll off. So there you go. Try a boxy speaker. I might suggest a couple of Emminence models as most Emminence speakers seem to have a rolled off top end compared to Celestion and Peavey speakers.

I had a friend with a Classic 50. He complained that the top end was too shrill. There are a ton of mods for this problem on the "Classic" amps. I replaced the two "Blue Marvel" 12's with Emminence Private Jacks and the unanimous concensus was that it sounded great. No need for further modification. The speaker is your amps voice.

Thanks Chuck,
This is not a rail dropping resistor, it is a 10K in series with the 100k plate load. These are R22, R23 respectfully (triumph 60/120) the output of the stage is tied between these which is weird to me. Most of the gain is at the plate not ABOVE the 100k plate load resistor. The 12ax7 seems to be cathode biased as usual otherwise.

[pdf64]

See Peavey Diagrams, Schematics and Service Manuals for FREE for schematic.
The R22 & R23 plate circuit arrangement is known as a split load, and its purpose is probably to reduce the subsequent signal level, to about 1/10 of the level at the plate.
If you want to roll off more highs without following Chuck's sage advice, R97, R98 & C63 are acting to halve the highs above a certain frequency (-6dB shelving filter); if R98 was shorted out, it would change from being a shelving filter to a -6dB/octave roll off filter. Or alternatively increase the value of C64, eg double it to 4n7; or do both (short R98 and double C63).
Pete

[Chuck H]

Not wanting to rob thunder here, but the R97, R98, C63 circuit is fairly early in the gain cascade stages. I only mention it because one of my favorite things about my PAG60 was how the harmonics just lept out of it. Squashing the top end early in the cascade might reduce this nice feature. Of course this may be what the OP wants. It's a different sort of sound.

C54 & C55 are late stage bleeders like what was used on the SF Fender amps. They're also a very small value at 100pf. Probably for preventing ultrasonic oscillations. The OP could try changing these to a higher value, like 1000pf, to get the effect of this circuit into the range of human hearing. There are other bleeders too that could be changed. I think the BlueGuitar site has some mods for the Rockmaster preamp. Which I've read is a very similar circuit. There are also threads here about the Rockmaster that could be looked into.

And thank you Pete for posting the schem on the OP's behalf.

[Enzo]

Look again, that voltage divider doesn't drop the signal to 1/10 its size to pass along, it drops it to 9/10 its size. Or really whatever 100k/110k works out to be. 0.90909...;

[Chuck H]

I'm actually seeing it the way Pete is. There is 10k to 0 impedance (or an ohm or two) and 100k to the plate and the signal is taken from that point. Then through the cap tuned 100k/100k/.0022uf voltage divider for roughly 1/20th on frequencies that pass the cap and 1/10th for all others. The circuit is feeding the effects loop. So I can see why it needs to be padded so hard. Then the recovery stage, with a 220k plate has a gain better than 60x. Not a real good circuit. From what I've read a split load can increase power supply noise by the same factor it pads. Then the tiny signal is re amplified. I did it this way on one of my designs and don't have any problems. But that was before I knew WHY it was a bad idea. We'll see how it goes as the filters age!?!

[Enzo]

You're right, I was screwed up. The B+ is the stuck end, the output node is only 1/10 of the way towards the moving end.


Hey it was 3PM, not 3AM when I am usually reasonably sharp.