Any way to mute PP tube poweramp "after" LTP?

[dimashek]

I would like to switch the current setting Resistor in the LTP  (by channel switch) in an el84 amp
1k2 for "voxish" sound and 470 for overdrive channel. 

So just tried to "switch" a 1k parallel to 1k2 with an OPTO-Mos (Solid state relay) - it plops (as expected)
Not loud but unnoing. 
I would like to avoid using Vactrols because of "floating" on-resistance.

What would be the best way to mute the amp in this case?

Will it work to connect the el84 gates together (using second opto-mos)?
Any other suggestions?



Regards
dimashek

 

[shabtek]

Is there -v supply for bias? If so output valves could be cut-off.

 

[dimashek]

Forgot to mention - it's cathode biased, so no bias voltage.

add:
shorting Rcathode will make it biased more hotter, so not really muted

 

[Bo Deadly]

I would be careful about swapping out the cathode resistor while the amp is on. Changing the load like that could wreak havoc on the tubes. Even if the switch is the shorting variety it's still probably not a good idea.

You could have two resistors in series (470R + 730R) and then AC bypass the lower resistor (730R) by switching in a large cap. That's not going to change the bias current of course but it will change the AC impedance which may yield the effect you looking for. Not sure. And the RC nature of the circuit will almost certainly deaden that pop.

Then again, I'm not really a tubes guy.

 

[PRR]

"Muting" makes a several-step process. Mute. Change. Wait. Un-mute.

Slowing the transition may be good enough. We don't usually need a cathode cap, but in the extra long-tail form I do not think it would do any harm. The abrupt change becomes a "slow" change; 50mS as shown. Thus may thump but not "Crack!".

 

[dimashek]

You could have two resistors in series (470R + 730R) and then AC bypass the lower resistor (730R) by switching in a large cap. That's not going to change the bias current of course but it will change the AC impedance which may yield the effect you looking for. Not sure. And the RC nature of the circuit will almost certainly deaden that pop.

This is a guitar amp, so the LTP will clip! It's very important to have right bias current for "right" clipping.

"Muting" makes a several-step process. Mute. Change. Wait. Un-mute.

Slowing the transition may be good enough. We don't usually need a cathode cap, but in the extra long-tail form I do not think it would do any harm. The abrupt change becomes a "slow" change; 50mS as shown. Thus may thump but not "Crack!".

Not sure I get your schem right - is it a 47u choke and a light bulb?
The choke will shorten all the current this way, I can't see, how it could help...

using optocoupler will be slow enogh to have almost no cracks, but this beasts are not very stable (Ron)

 

[abbey road d enfer]

I would like to switch the current setting Resistor in the LTP  (by channel switch) in an el84 amp
1k2 for "voxish" sound and 470 for overdrive channel. 

Can you post a link to the schematics you refer to, because I suspect the effect of changing the common cathode resistor in the phase-splitter is not clear. In both cases, the tubes operate in their linear portion with almost identical gain. The plate voltage reduction does not significantly change the way it clips, even at 20dB overdrive, in which case the output tube distortion should be seriously dominant.

 

[dimashek]

Tried to find easy-to-read AC30 schematics  ;D
Attached are the LTP from 2 different amps.  And some statistics of other el84 amps.

VHT20  470 cathode, 22k tail, 4k7 FB
Friedman PT-20 : 470 cathode, 27k tail, no NFB
mesa mini recto: 470 cathode, 10k tail, 4k7 FB, switchable NFB

You can easily hear the effect of changing this R.

1k2 will give nice "shime" LTP overdrive, but will sound too harsh if driving too hot.
470 - almost no LTP clipping, much better sound with overdrive channels, but too clean, (steril) for VOX. 

I will also switch the tail R from 47k to 27-33k to have more el84-compression in OD channel,  in case I'll find the way to "mute" the poweramp

 

[abbey road d enfer]

You can easily hear the effect of changing this R.

1k2 will give nice "shime" LTP overdrive, but will sound too harsh if driving too hot.
470 - almost no LTP clipping, much better sound with overdrive channels, but too clean, (steril) for VOX. 

Honest, I'm too lazy to open my AC30 to check that, but I have serious doubts about so much difference when changing operating current from 0.7mA to 0.9.
I believe there are so many differences between those amps (particularly the OT and the speakers) that the effect of changing the op point is drowned. IMO the "overdrive sound" comes more from the output stage than anything else.

 

[dimashek]

It's 0.8mA vs about 1.2mA,  8) measured on my Amp. 
Huge difference for 12ax7,  Cold vs very hot biased.  = 2k4 vs 940 Ohm for a single triode.

I'm speaking about sound differences on this particular amp. They are easily audible.  At least for me
Tested the vox with 470 , Tested the overdrive with 1k2 - both not good

 

[walter]

Your idea to short the grids to the output tube should work to mute the output. That is how some amps standby works.

 

[PRR]

Light Bulb??

The (X) is your switch.

 

[dimashek]

Your idea to short the grids to the output tube should work to mute the output. That is how some amps standby works.

thanks! will try that.

Just tested it in Spice - it works OK with perfect matched power valves, but will leak some "Cracks" in the real world. 
It mutes only differential, common mode plops will be attenuated by CMRR of power valves only.  (not very high with real valves)

Light Bulb??

The (X) is your switch.

I got it after a while Yes, putting a smoothing cap to slow down the transient is a good idea. Will see how the big cap is changing the sound.