switchable parallel se?

[ALBATROS1234]

i was given a box of xfrmrs and ordered some new tubes. i am going to build my next guitar amp with 2 output tubes which i got a deal on 5 12AQ5 nos. i was wondering is it ever done where you make a switchable from se 2 tube parallel to se 1 tube. i am thinking you still have to provide voltage to the second tube so the circuit voltages remain balanced, perhaps put a spst switch to take the output signal away en route to the ot therefore dropping volume. also do you need a phase inverter for a se parallel and if so do you have to disable this or perhaps in switching send both outputs of the pi to the main tube. would i better of switching both output tubes from pentode to triode mode and are thee any amps in this config i can study schematics on.

does anyone ever switch from 2 tube parallel se to push pull. what would be advantages/disadvantages of these 2 configs.

what would be a better config for filament voltage feeds using a 12 volt xfmr to parallel to each tube(i got 12SL7 tubes coming for preamp) or use a 24volt xfrmr in series.

[shooter]

advantages/disadvantages

going from PSE to PP and back has little if ANY advantage, pick one, dial it to perfection and you're way ahead
my2c

[sluckey]

If I were gonna do this I'd just use a SPST switch between the two control grids. Each tube must have it's own grid return resistor.

No phase inverter needed.

I would prefer a 12V transformer for filaments. It would be capable of powering 12A?7 preamp tubes also.

[jjasilli]

The simple way is to put a vol control > one power tube.  That way you can turn that tube to -0- or in between.  Meanwhile both tubes remain ON, and with no change in p-p impedance.  No SW-ing & no popping.

One method:  use a pot for one of the grid leak resistors.


(This can also be done with PP, effectively making the circuit SE, as the complimentary signal wave is reduced to -0-.)

[terminalgs]

i was given a box of xfrmrs and ordered some new tubes. i am going to build my next guitar amp with 2 output tubes which i got a deal on 5 12AQ5 nos. i was wondering is it ever done where you make a switchable from se 2 tube parallel to se 1 tube. i am thinking you still have to provide voltage to the second tube so the circuit voltages remain balanced, perhaps put a spst switch to take the output signal away en route to the ot therefore dropping volume. also do you need a phase inverter for a se parallel and if so do you have to disable this or perhaps in switching send both outputs of the pi to the main tube. would i better of switching both output tubes from pentode to triode mode and are thee any amps in this config i can study schematics on.

You mean two 12AQ5's in parallel with a single SE OT, right?    to make an such amp that could be switchable in terms of going from 1 to 2 12AQ5's,  you'll have a lot of problems to sort out (mostly revolving around not destroying tubes when you switch)  maybe more than it's worth.  your bias method would need to account for it.  You couldn't pick a single cathode resistor that would serve both well.  You'd have to switch the cathode resistor and take the tube out of the loop,  then it changes your PT's load in the equation..  to me, more trouble than it's worth.      If you want a 4W SE amp, go with 1x12AQ5.  If you want a little more maybe 7-8W, parallel 12AQ5's.  If you were dead set on doing it,  maybe build it as a 2 12AQ5 SE amp and to switch:  power off and pull a tube :-) . but then you have to pick a bias resistor that runs hot in SE mode and cold in Parallel mode (and one would unfairly sound better than the other..., and that's how you'd leave it "switched" forever :-D )

what would be a better config for filament voltage feeds using a 12 volt xfmr to parallel to each tube(i got 12SL7 tubes coming for preamp) or use a 24volt xfrmr in series.

12V.   

[sluckey]

No need to mess with bias. Just simply kill the grid signal to one of the tubes. Both tubes will continue to idle just as before.

[DummyLoad]

No need to mess with bias. Just simply kill the grid signal to one of the tubes. Both tubes will continue to idle just as before.

exactly! nor the need for load matching/switching or switch popping suppression.  :-)


--pete

[ALBATROS1234]

thanks guys i just got 5 12AQ5s a few 12SL7s tubes coming and wondered as i often do. i figured i would have to do something that would allow both power tubes to still suck juice or else the circuit would become unbalanced and do weird things.i figured there wouldnt be much difference between power factor of push pull vs parallel se. i just kinda thought if i wanted to drop the power level as sometimes amps do it may be easier to do it on a se.i still have to look at switching from pentode to triode mode as i have seen some amps do.i suppose that is just what it sounds like open the circuit to the screen grid and suppressor grids then they are triodes with much less output.

 the reason i brought up the 24v filament in series is because i noticed on the old rca radio i have as well as other simple amp schematics, im talking mainly transformerless like an old harmony amp, i noticed that they ran the filaments in series and the total would add up to approx 120 like a couple 35volt tubes and 4 12volt which gives you 118volts i was thinking maybe its due to the voltage drop of each as long as the total amount equals applied voltage so since i have a handful of both 12volt and 24volt transformers i wondered if there was any advantage to either. i have enough iron to build several amps now ,even large and powerful ones a few of the transformers can supply 460 to 480 vac at a few hundred volt/amps . thanks again for replies and advice.

[PRR]

With AC heat, you want *low* AC heater voltage around your sensitive audio stages. That's why we had 2.5V and 4V tubes (before car-radio made 6.3V the popular heat).

In the 115V strings, the 1st Audio stage would be at the *bottom* (groundy end) of the string for least hum. Not that hum was a big deal with a 4" speaker in an open box.

Also a series string must have all tubes the *same current*. Mixing 0.15A, 0.3A, and 0.45V types gets messy.

No, keep your AC heater as low as is convenient.

For _DC_ heat, is different. DC rectifier losses are around a Volt, which looms large for 6V work but looks small in 25V work. So series looks good. You still have to match currents. This usually sets your Power tubes apart. DC heat has less benefit in a power stage, and Peavey did thousands of amps with 6V AC on the big jugs and 24V DC on the little ones.

But for what amounts to a funny-bottle Champ, all this is just over the top. It sounds like you could run 12V. These days a little 12V DC supply from China is cheap as chips. But a 12V transformer in the hand is cheaper.

[ALBATROS1234]

makes sense thank you sir