JTM45 Clone Bright channel

[Khron]

If it cures the problem, why wouldn't it be a possible solution?

https://www.redcrab-software.com/en/Calculator/Electrics/RC-Parallel-Circuit
100k in parallel with 100pF acts like a 100k resistor at up to 2-3kHz, after which the impedance of the pair begins to drop (thus attenuating the gain of the higher frequencies).
By about 27.5kHz it's down to an impedance of 50k.

 

[Matador]

Many amp manufacturers limit gain in this way.

If I had to guess, the square wave response shows there is a lot of high frequency gain: there is significant overshoot on the square wave response, and adding a cap in parallel with an anode resistor is one of myriad ways to limit high frequency gain. I've had problems with V2 before, due to its lack of a grid stopper. A 10K here helps greatly.

I find it helpful to limit high frequency gain in each stage, so the sum of the response yields a roughly critically damped system (clean rising edge of a 1kHz square wave with minimum overshoot) once feedback is applied. A clean square wave means you have response up past (roughly) 10kHz, after which there's not much point in having gain in a guitar amp, and it only causes problems like supersonic oscillation, etc.

An alternate arrangement it to place a small (10pF or less) cap between the plate and grid, which is easy since those pins are next to each other in a typical 9-pin duel triode pinout. This reduces high frequency gain through local plate->grid feedback. You can also add a grid stopper, or a snubber on the grid, etc.

This is a great resource for tuning tube amplifiers (skewed towards HiFi, but still relevant):

https://www.angelfire.com/electronic/funwithtubes/T-E-Smith-NFB.html

 

[ilfungo]

Many amp manufacturers limit gain in this way.

If I had to guess, the square wave response shows there is a lot of high frequency gain: there is significant overshoot on the square wave response, and adding a cap in parallel with an anode resistor is one of myriad ways to limit high frequency gain. I've had problems with V2 before, due to its lack of a grid stopper. A 10K here helps greatly.

I find it helpful to limit high frequency gain in each stage, so the sum of the response yields a roughly critically damped system (clean rising edge of a 1kHz square wave with minimum overshoot) once feedback is applied. A clean square wave means you have response up past (roughly) 10kHz, after which there's not much point in having gain in a guitar amp, and it only causes problems like supersonic oscillation, etc.

An alternate arrangement it to place a small (10pF or less) cap between the plate and grid, which is easy since those pics are next to each other in a typical 9-pin duel triode pinout. This reduces high frequency gain through local plate->grid feedback. You can also add a grid stopper, or a snubber on the grid, etc.

This is a great resource for tuning tube amplifiers (skewed towards HiFi, but still relevant):

[URL

The cap between plate and grid is the best solution for me!!!

Seems more dynamic instead of a plate parallel cap,

It's only my impression?

Thanks to all for great help!!!

 

[CJ]

the first waveform on page one looks normal. it should change when you move the treble control. that circuit is very bright, but with 4 x 12 celestions a lot of that goes away. what is cathode bypass value? some use 0.68 or something like that, using 25/25 lytic will add gain and bass. inserting sq wave into git amp will usuially produce sawtooth by the time the signal gets through the circuit and output xfmr.

 

[ccaudle]

the first waveform on page one looks normal.

What causes that ripple on the falling edge? That asymmetry looks very strange to me, but I generally work on circuits that aren't intentionally band-limited, and are solid state, so I could be missing something that is common for guitar amplifiers.

 

[ilfungo]

the first waveform on page one looks normal. it should change when you move the treble control. that circuit is very bright, but with 4 x 12 celestions a lot of that goes away. what is cathode bypass value? some use 0.68 or something like that, using 25/25 lytic will add gain and bass. inserting sq wave into git amp will usuially produce sawtooth by the time the signal gets through the circuit and output xfmr.

Hi CJ
V1 bypass cap is 1uf, but I've used 3 position switch to parallel caps,
1uf -5uf-10uf .
With Les paul I prefer 1uf and bright input 1
With Stratocaster 5 or 10uf and bright input 2...
The cab is Marshall 1960 tv 4x12
Greenback

 

[ilfungo]

Little update

The amp sond great!!!

I've lowered a bit the preamp voltages and the response is more gradual from clean to full.

I've also tried 330pf vapacitor parallel to 68k input resistor as someone suggest in another forum and I like it.

Can someone explain it?

Thanks!!!