Ideas how to convert Audio power amp to Guitar power amp?

[ModulusMK]

I want to use audio power amp as guitar power amp. Have had built one years ago but till now it collected dust only and want to give it a use. The sound I get with guitar preamp is fine, I modded just the input stage of the amp to eliminate the 'drive' that occurs. It went away almost 90% but the rest 10% are still here with small difference affecting the drive with volume.

Clarity is also here only the unwanted 'drive' I want to remove and it should be fine.

I'm using the preamp of my Roland Cube 60 Bass.
It is transistor amp with 2SK135 and 2SJ50 output stage.
First gain stage of the amp is with couple of BC 546, second is couple of BF 470 and then the high power ones for output.
I raised the input impedance and some cap mods and is much better, only the little drive that is still here needs to be removed...

Any ideas what to do more?

[Ronsonic]

I'm not entirely sure what you're saying, apparently the preamp is overdriving the input of the power amp.

Without a schematic to hand it's hard to guess. I'd just pad down the input and be done with it. J135 and K50 are extremely rare valuable these days, enjoy them.

[Steve Conner]

Yes, I don't really understand what you mean either.

Does the power amp module make its rated power without audible distortion?

If yes, then you're overdriving it, so you have to decrease the input signal and accept that it won't be as loud as you want. Or overdrive the preamp instead.

If no, then you need to fix the power amp. Test it with a signal generator and scope, look for parasitic oscillations and other kinds of instability, check the bias and so on.

Those MOSFETs need +/-50V supplies and should make about 100 watts into an 8 ohm load. Some designs claim 160 watts into a 4 ohm load, but I think that's pushing it a little. I used to have a Maplin Mosfet amp kit with these devices, and it worked great as a bass amp through a 4x10". It also sounded surprisingly good as a guitar amp, when I tried it with a JFET preamp to dirty the sound up a bit. The recommended idle current for that kit was 100mA.

The K135/J50 are not available any more, and they are somewhat sought after because they were used in a bunch of famous solid-state amps. But if you blow them there are plenty of compatible devices still in production from Exicon and Magnatec.

[ModulusMK]

Well yes, the preamp drives the amp and makes non-audible distortion. I don't think the volume of the preamp is the problem for the drive because no matter what volume, even 5% of max volume drives the amp.

Maybe it's the impedance, that's what I've been modding in the input stage of the amp and now I got more clear sound with much less non-audible distortion. And I think I reached the maximum with my mods (resistors and caps at input).

I tried with the guitar directly in the amp (passive) and the drive is even more present and sounds like crap haha. Added active electronics and sound got clear a little bit.
When connected through the preamp of my Roland Cube it is way much better with very little non-audible distortion that I want to remove. So I think it might be the impedance... I'm not sure what could it be :/ Never have had been adapting audio amp for guitar (the schematic was intended for audio amp, not guitar)
I hope you got me now.

Power supply is +/- 42V, as from the schematic it can go from 36V to 50V. It is set to work in A-B class at 60W (music). Idle current is about 70mA.

I don't have a scope to test it with. Bias on what I should check? On all stages it is around 40V except the output which is full from power supply.