Switch between fixed and cathode bias?

[SwingFit]

Hi. I think of installing a DPDT switch to be able to switch between cathode and fixed bias. It would also be nice to have a volume pot installed to adjust the volume between the two.
My thought is that I would use fixed bias for chord play and cathode bias for solo play.
Any thougts about this? is this a good idea? complicated?
A fixed bias amp is more dynamic with a clearer definition between simultanously ringing tones which is great for chord play. A cathode biased amp gives a more compressed, fat, singing sound which is great for single note play. A lot of sustain even with clean sounds.
My amp, a Fender Princeton Reverb II is originally a fixed bias amp and after some mods I am really pleased with the sound when playing chords and fill-ins. When I cathode bias the amp, it really sings and the most beutiful solo sound appears. However, chord play is not as good as before. Thats the reason I asked about it.
I understand that it may be much work to get it foot switchable but it would be nice to at least have a switch on the back of the amp so I can choose bias type when recording.
But, -if anyone can provide me a scheme for having it footswitchable I would be very greatful.
I tried cathode bias after reading Gerald Webers book "Tube amp talk for the guitarist and tech".
This text is from the book "The two grid return resistors, 220K, that connect each output tube grid to the negative voltage supply is disconnected from the voltage supply and grounded to the chassie. Disconnect the ground wire that connects each cathode to ground and run a wire connecting both cathodes. Now put a resistor from one of the cathodes to ground. The actual value of the resistor will vary depending on what kind of amp and what kind of tube. Connect an electrolytic capacitor in parallel with the cathode resistor, putting the negative to the ground."

There is no problem using a DPDT switch to do this but there will probably be loud pops as well as differences in volume.

Have anyone tried this?

[Enzo]

Sure, not very new, really. DPDT switch. One side switches the grid resistors between either bias supply or ground, and the other half switches the cathodes from either ground or a cathode resistor and bypass cap.

We did this to a Peavey Classic 30, and the notes are over at www.blueguitar.org. Yours would be similar other than the layout.

I don't recalling the pops as bad, and not a lot of volume shifting either, though on any particular circuit, things could react different.

My customer did not wind up switching on the fly, he tended to use it one way or the other as his mood dictated. I think it might be more subtle than warranting a footswitch. If I were to go with foot switch, I would mount a DPDT relay for the circuits, and control the relay with the footswitch and panel switch.

[stokes]

I did it to a Mesa MKII,but never switched on the fly,just didnt think it would be a good idea to do so.No real reasons for my concern,just kind of paranoid about the posssibility of the tube having no bias voltage for the instant the switching was taking place.The difference in that amp between the two was so subtle it didnt warrant switching on the fly,like for soloing,the switch between rythm and lead were more than adequate for that.In fact the cathode biased mode sounded terrible on the lead channel in that amp.I ended up removing it for just that reason,to me in cathode bias it became a one channel amp.

[Jammin'John]

Deluxe

I have a Deluxe that is set up with a switch.
I don't think I'd want a footswitch though.
I mostly end up using it cathode bias.


JJ

[HTH]

I've done this too - nothing new, I turn mine into standby when switching though I'll probably remove it next time I'm in there as it sounds MUCH better on the cathode bias setting.

[nooneknows]

We did this to a Peavey Classic 30, and the notes are over at www.blueguitar.org.

Hi,
I know I could seem a bit dumb but I'm not able to find the cathod bias mods instructions for the classic 30 on Steve's site, I've found bias mod for the C50 but nothing else.
Could you please help me? This amp have become my first gigging friend, since I stop using my Jcm 800 (too loud to make it work well in the small places I play in), and I'd like to improve further the sound trying to open it up a bit.

Thank you

[Enzo]

Blue Guitar
Site Map
Schematics
Peavey
Mod notes
third one on the list

[merkaba22]

any final values

A cathode biased amp gives a more compressed, fat, singing sound which is great for single note play. A lot of sustain even with clean sounds.... it really sings and the most beutiful solo sound appears.

I tried cathode bias after reading Gerald Webers book "Tube amp talk for the guitarist and tech".
This text is from the book "The two grid return resistors, 220K, that connect each output tube grid to the negative voltage supply is disconnected from the voltage supply and grounded to the chassie. Disconnect the ground wire that connects each cathode to ground and run a wire connecting both cathodes. Now put a resistor from one of the cathodes to ground. The actual value of the resistor will vary depending on what kind of amp and what kind of tube. Connect an electrolytic capacitor in parallel with the cathode resistor, putting the negative to the ground."

I just bought a PRII -- It came loaded with ScumBack H75 12" which I am very impressed with -- I plan to replace the (25 year old) power and output transformers and choke from Mercury Magnetics and am looking for ways to improve the gain channel (I love the clean now but apparently on a design like this there will be compromise that the MM stuff will help with),

I wonder if you have refined the mod you referred to above and can share any final values you determined; right now the amp has JJ 6V6's but I may opt for a MM transformer that allows for 6L6's as well?

[Bruce / Mission Amps]

I've done this mod at least three or more dozen times over the years and the interesting thing is that of the ones I am still aware of... all the owner players leave them in either fixed or cathode mode, never switching them around for any reason what so ever.
Moral:
I'd try both modes to see what you like and then make the chosen mode permanent. It's not much of a real feature once you decide what fits you.

[Enzo]

I'd agree, the switch usually stays flipped one way after installed. The switch is mainly there for the customer who is not sure about the change.

[Mandopicker]

I have made a SE 5f2 design amp with a Fixed/Cathode Bias Switch.

I recently intalled a MV in place of the 220K resitor to ground, off the coupling cap and all works very well. The amp is a 5F2 design.

Everything sounds great until I select the fixed bias mode, and turn the volume to 10 and the MV to 10...(Test the extremes essentially.)

Basically, my output tube seems to react like in a tremolo in that it starts beat or throbb quickly. Like its going off and on...so to speak.

Thinking it has to do with the grounding scheme or an oscillation, but not sure until I dig into it later tonight. The throbbing stops when I turn the MV down to about 7 ish.

This does not happen in the Cathode Bias Mode.

Theoretically speaking, what would make the output tube throb in that fashion. Seems really strange.

Thanks in advance for your ideas.

[Mandopicker]

Fixed it...lesson...never ground the CT in similar location as Main Filter ground.

I knew this from past experience, but just thought otherwise for the new experiment. (Bias type switch/MV etc.)

Amp sounds amazing!

[tubeswell]

Switchable Bias Modes

FWIW I realise that this thread is somewhat old now but here's my schematic version

[leadfootdriver]

FWIW I realise that this thread is somewhat old now but here's my schematic version

I was contemplating on how to handle the bias supply in CB mode.

So it's indeed alright to leave the bias supply open in said CB mode?

[MWJB]

"So it's indeed alright to leave the bias supply open in said CB mode?" Yes, it's fine.