Turning "off" One Tube in PP Output Stage

[ChrisM]

Did it sound bad Tom? I never played an amp with only one tube or one pair running.

Hi Chris,
Here's the link to the Vajra circuit that dartanian mentioned earlier. This amp also allows you to switch between fixed bias and cathode bias.
TUT5 has a couple amp circuits that have a pot to dial out the input to half of the output tube(s).
http://mhuss.com/18watt/schematics/Vajra18Scheme.jpg
Take care
dave

Hey Dave, thanks for the link.

However I am not understanding what we have here. We got a 750K bias feed/leak resistor in parallel with a 500K pot wired as a rheostat. Why? Seems like the pot is just altering the bias feed resistor's value.

I thought you would keep the bias feed resistors the same then just add a variable voltage divider pot after them. After thinking about it some more that would still change the value of the bias feed resistor as the two are in parallel.

In cathode bias mode the pot takes the grid of one half of the p-p pair to ground, "grounding out" the signal. In fixed bias mode it ties the grid to Vbias (by making the value of the bias feed resistor zero), which is the same as grounding it out. In fixed bias, the bias feed resistors make up most of the AC load seen by the PI stages.

My amp is fixed bias with with 220K bias feed resistors. I would like to add a switch for emulated class A. How would I do this, add a parallel resistor so one bias feed resistor becomes very small?

[martin manning]

My amp is fixed bias with with 220K bias feed resistors. I would like to add a switch for emulated class A. How would I do this, add a parallel resistor so one bias feed resistor becomes very small?

See the attached clip from the MESA/Boogie patent. Maybe read that MESA patent through carefully. Skimming through, it looks like they suggest switching in a cap to AC-short out one of the bias feed resistors if you are starting from fixed-bias p-p. I'm not sure what you'd do with the bias points of the output tubes. I thought maybe you could switch in a cathode resistor to reset them, but I don't know. In the circuit below the cathode resistors are just switched from parallel (for p-p) to discrete (for SE) to keep the unused side from being driven from its cathode in SE mode, and the quiescent point wouldn't move. Mr. Wizard?

[ChrisM]

Hmm coming back to this now that I have time...

So judging by the Mesa patent and the link Dave posted http://mhuss.com/18watt/schematics/Vajra18Scheme.jpg
I should be able to just add a switch to short the control grid to ground or Vbias. I'd do Vbias as the amp is fixed bias.

Yeah?

[Structo]

One day I forgot I had one of the two 6L6's pulled from my 50w D'lite amp, turned it on and started playing.
I couldn't figure out why it sounded the way it did until it dawned on my it only had one power tube installed.

I was actually surprised it even worked and sounded as good as it did.
But I didn't use it long like that because I was afraid of damaging it.

[ChrisM]

Hmm coming back to this now that I have time...

So judging by the Mesa patent and the link Dave posted http://mhuss.com/18watt/schematics/Vajra18Scheme.jpg
I should be able to just add a switch to short the control grid to ground or Vbias. I'd do Vbias as the amp is fixed bias.

Yeah?

Any thoughts on this?

[ChrisM]

Hmm
So I found this old link I had bookmarked and lost
http://www.ampbooks.com/home/amp-techno ... t-7173488/


In the article Richard Kuehnel says for fixed bias to emulate SE you place a large cap between the grid and ground. Now I did this a while back (before reading this article) and I got the tube red plating, glowed pretty nicely.

I forget if I had the 220mF cap on the grid or before the grid stopper, don't know if it would matter?

[Kagliostro]

Mesa Boogie Blue Angel

http://www.mesaboogie.com/manuals/Blue%20Angel.pdf

Do you mean this ?

Kagliostro