Half power Princeton Reverb

[Jrusto]

I am obviously showing my lack of depth in understanding amplifiers but I am trying hard to learn.  So my question is can a Princeton Reverb be run on only 1 of the 6V6 and get half power. 

My guess is probably not because output transformer is not designed for single ended configurations.

Thanks for helping educate me.
Rusty

[mresistor]

Guess you could leave the power tubes installed and turn off the signal to one. Don't know if that's going to be precisely half-power. 
I would not run the amp with only one tube installed.
Other folks will be along to expound on this.

[AmberB]

The push pull output setup is not designed to run with just one tube.  If the amp was biased as a true class A output, you might be able to get away with it, but I have no experience with that.  I think the power tubes would need to be cathode biased to get the power tubes to run in class A.  Of course, if you were to cathode bias the amp, you could use a switch to choose between different bias points that would change the output of the amp, using different cathode resistors.

[Dave]

I read a post from a guy one time who claimed that he had an original blackface Super Reverb and that one of the power tube's plate connection was missing from the factory. He stated that he never knew that the connection was not there and that he had played the amp like that for 40 or 50 years and had always loved the way it sounded.


On the other hand, I don't think it's best practice.


You could, however, wire it in "triode mode" which a lot of people mess around with and call it half power.


Dave

[d95err]

Mesa Boogie has amps where you can switch from push-pull to single-ended, per channel. They disconnect the output from one side of the phase inverter and switch the other side to cathode bias, as well as adjusting the anode voltage. For example, see the Mesa Boogie Express 5:25 or 5:50.

The Peavey Valveking’s Texture control gradually shifts between push-pull and single-ended, by adjusting the amount of signal out of one side of the phase inverter.

The power in single-ended mode will typically be less than half power. For a Princeton Reverb, I’d guess perhaps 3-4W.

[tubenit]

Kevin O'Connor's  SOMA 84 has a way of dialing it into a single ended amp sorta kinda.  I tried this idea and didn't care for it.


I think 6K6's or 6MB8 tubes are a "better" tone way to go for lower volume amps. However, you have to be sure that the voltages on the power tube plates aren't too high.


With respect, Tubenit

[mresistor]

I think 6K6's or 6MB8 tubes are a "better" tone way to go for lower volume amps. However, you have to be sure that the voltages on the power tube plates aren't too high.
With respect, Tubenit

I was thinking to use 6K6s too  but its tough to bring PR plate voltage down to say 325V - assuming the OP is using a PR PT.


I've found triode mode to not be very good at all.  Sounds kind of anemic to me. Have it installed in a Super Reverb and never use it.


But if the OP plans on using half power mode all the time then wouldn't it be better to build the amp with less power output to begin with?   

[tdvt]

Between this thread & the 6K6 thread, I think a new project is taking shape.


I have a Princeton, but I am finding even lower volume amps appealing at the moment. A Reverb Champ was the last scratch build. But a lower power P-P amp would be nice too.


By chance, I started building a cabinet last week for a BF Champ chassis I have had around for some time, but without any real game plan as what to put in it.

Then, earlier this AM sorting through stuff in a box, I tested an unknown PP OT that measured out somewhere around 8K:8Ω. I am thinking that is pretty close to what 6K6s would like.

Then I came across a 275-0-275 PT from a previous project that was dismantled, so it seems the planets are aligning...

[mresistor]

Look at Robinette's JCM800 Micro..   I have been thinking it would be killer with 6K6 output tubes. 
Tubenits TOS is a possibility too.

[Jrusto]

Thanks for the discussion

[HotBluePlates]

... So my question is can a ... be run ... and get half power. 

"Half Power" is barely quieter.  Apartment neighbors will call the cops just as fast for a cranked 50w amp as for a cranked 100w amp.

"One-tenth power" is also "-10dB" and seems "half-loud."  So for a ~15 watt Princeton, you're looking for 1 watt or thereabouts.

Guitar speakers are pretty loud for applied power.  95-100dB for 1 watt applied, which puts them at power-tool-loud.  One-tenth watt can still be "too loud" (85-90dB) at full-clip, but the quieter parts of playing could fall in a happy-range.  Notice how the digital amps with switchable power go down to the part-watt level.

[pdf64]

I am obviously showing my lack of depth in understanding amplifiers but I am trying hard to learn.  So my question is can a Princeton Reverb be run on only 1 of the 6V6 and get half power. 

My guess is probably not because output transformer is not designed for single ended configurations...

Well, yes there's the OT design being intended for 0 or minimal standing magnetisation (eg whilst idling).
But that's minor, mainly affecting the bass response, compared to the effect of losing up to 1/2 the push pull waveform.
Consider that as each push pull side handles only the upper or lower half of the waveform, and if the output valves had been biased to class B, then removing one valve would only allow the upper or lower half of the signal to pass. That means the signal would be bigger but half wave rectified.
As an amps power output is usually assessed 'clean', eg at the onset of clipping, the clean power of a half wave rectified signal would be pretty much 0.
Obviously our valve guitar amps aren't operated at or close to class B, the amp would be biased to idle at eg 20mA. So the anode current can swing 20mA down below its idle level, to 0. And as it's now running in class A but on only half the OT's primary winding (hence its previous class B loadline of about 2k), the peak current it can draw (at Vg1-k=0) will still be quite high, about 180mA.
So the unclipped power current swing will be +/-20mA, whereas overdriven it can swing +180mA to -20mA.

So that's a really low clean output, perhaps a fraction of a watt, but still a fairly high overdriven output, perhaps several watts.
Which is kinda the opposite of what we normally want in a guitar amp (eg 'clean to mean' being available from a twist of the instrument's volume control being a desirable characteristic).

[pdf64]

The push pull output setup is not designed to run with just one tube.  If the amp was biased as a true class A output, you might be able to get away with it, but I have no experience with that.  I think the power tubes would need to be cathode biased to get the power tubes to run in class A. 

No, class A operation can be achieved with cathode bias, fixed bias or a mix of both. It just has to idle near the middle of the loadline, centre bias, preferably without breaching the valve type dissipation limit.
It could be biased hot of centre, but that would reduce power output whilst increasing idle dissipation.
Biasing cold of centre would probably result in AB operation, and the (1/4 impedance) class B loadline would kick in.

Of course, if you were to cathode bias the amp, you could use a switch to choose between different bias points that would change the output of the amp, using different cathode resistors.

Yes that will tend to reduce power output, but there's several other mechanisms that affect the amp's tone / response that kick in with that method.