Hiwatt V2 overdriving

[bea]

I am aware on a similar discussion fairly recently (http://music-electronics-forum.com/t41041/) but i feel that it might be better to start a more specific discussion which hopefully might go a bit deeper.

It is about overdriving Hiwatts. To me the only resonable place in the amp for doing this is somewhere before the master volume and hence V2, especially in the larger models.

My reference is may Mywatt 200. Its preamp follows the "Mid 70s Four Input" layout: http://hiwatt.org/Schematics/DR_Pre4Input_v2.gif with a minor and negligible difference in the tone stack. BTW: there is also a 2nd "victim": my Epiphone Valve Junior, where i would like to be able to overdrive stage 2 and need leave the power stage clean.

Both V1 and V2 are modified: i added "large" capacitors to the cathodes of all three stages, in V2 mainly with the intention to tame the sharpness of the sound while playing clean. The normal channel is fullrange, and the bright channel cuts 1st order below 70 Hz.
The resistors of cathode and anode have not been touched; their values are still 1 kOhm / 100 kOhm.
V2 can be overdriven, and when that occurs, it sounds a bit muddy and also a bit harsch. The harshness is less pronounced when i use the bright channel. An indication for blocking distortion - despite the grid stopper of 470 kOhms?

No what can be done about that?


Having a look at an earlier version of that preamp: V2 is configured with 2.2 kOhm / 220 kOhm. Not overdriven this should lead to a cleaner sound than in my Mywat, shouldn't it? (the distortions of 2 stages with similarly cold bias should compensate to some degree, and with 220 k at the anode the ECC83 should produce less distortion than with 100 k anyway, correct?

What would happen if i overdrive that stage? A different tonal spectrum due to the colder biasing? Earlier yet maybe sharper onset of distortion due to the smaller anode voltage caused by the larger Ra?

What Do You think?

[uneumann]

I am aware on a similar discussion fairly recently (http://music-electronics-forum.com/t41041/) but i feel that it might be better to start a more specific discussion which hopefully might go a bit deeper.

It is about overdriving Hiwatts. To me the only resonable place in the amp for doing this is somewhere before the master volume and hence V2, especially in the larger models. ...

Both V1 and V2 are modified: i added "large" capacitors to the cathodes of all three stages, in V2 mainly with the intention to tame the sharpness of the sound while playing clean. The normal channel is fullrange, and the bright channel cuts 1st order below 70 Hz.
The resistors of cathode and anode have not been touched; their values are still 1 kOhm / 100 kOhm.
V2 can be overdriven, and when that occurs, it sounds a bit muddy and also a bit harsch. The harshness is less pronounced when i use the bright channel. An indication for blocking distortion - despite the grid stopper of 470 kOhms?

No what can be done about that?


Having a look at an earlier version of that preamp: V2 is configured with 2.2 kOhm / 220 kOhm. Not overdriven this should lead to a cleaner sound than in my Mywat, shouldn't it? (the distortions of 2 stages with similarly cold bias should compensate to some degree, and with 220 k at the anode the ECC83 should produce less distortion than with 100 k anyway, correct?

What would happen if i overdrive that stage? A different tonal spectrum due to the colder biasing? Earlier yet maybe sharper onset of distortion due to the smaller anode voltage caused by the larger Ra?

What Do You think?

So much can and has been said about such issues... it's hard to summarize all the issues into one post. Here are a few general comments.
Big bypass caps extends the bass range, which sounds full when playing clean, but bass response tends to muddy the tone when the stage is overdriven.
Unbypassed cathodes are an option - but the gain drops (depending on the cathode and load resistor values) and the overdrive tone changes, for good or bad - depending on your taste.

Grid stoppers help with blocking of bypassed cathode stages, but they create hard grid-current clipping (what I call dR clipping) during overdrive, which can be harsh.

There are too many variables in even a simple CF stage to come up with simple cause-effect rules about changing any one (or more) values. The impact of changing a cathode resistor depends on load resistor, bypass cap (or not), and grid stopper, the tube type, and probably other factors I'm not even considering this early in the morning.

If you don't like harsh overdrive, I suggest avoiding large grid stoppers (I'm sure that will get some responses ) and deal with blocking effects by attenuating input signal levels, and removing the bypass cap.

[bea]

Big bypass caps extends the bass range, which sounds full when playing clean, but bass response tends to muddy the tone when the stage is overdriven.

The Mywatt is used mainly as a bass amp.

Unbypassed cathodes are an option - but the gain drops (depending on the cathode and load resistor values) and the overdrive tone changes, for good or bad - depending on your taste.

Well, i do actually think of enlarging the load resistor, even to 400 kOhms or so. But this might have an effect on the transfer characteristics of the following tone stack. i tried in the small amp, and i just did a test of the tone stack in LTSPICE: with a source resistance of 40 kOhms the treble response is up to 3 dB more pronounced that with a source resistance of 70 kOhms. The latter is expected if the load resistance is large and/or there is no cathode capacitor.

Grid stoppers help with blocking of bypassed cathode stages, but they create hard grid-current clipping (what I call dR clipping) during overdrive, which can be harsh.

In the Hiwatts there are the large decoupling resistors of the passive mixer before V2 which also act as grid stoppers. So there is not too much room to reduce their size.

[mhuss]

The Mywatt is used mainly as a bass amp.

Depending on how much overdrive you're after, you could try adding a triode stage as done with the OL models. This makes the preamp a bit noisier, but you can can get a variety of overdriven sounds by doing this. The OLs were made by building 'flying' components right on the V2 socket (see Hiwatt Internals).

[bea]

I do not need much more than the distortion actually availably in the amp, just better sounding. Maybe resembling the sound of a mildly overdriven EL84 in SE at 250 V - that's mainly a bass amp.