"The Bursar" Push-Pull Amplifier

[wrenchone]

Attached is the preliminary schematic for an amp I'm naming after an old school colleague. The schematic came out of a discussion on this list regarding some old Baldwin iron I've been characterizing. Strange to relate, none of that iron will be used in this amp. Power and output iron will come from the guts of a Fisher 400 I picked up earlier this year from a local surplus store (just the iron). I'll start out using some small toroidal power transformers with dual primaries for the interstage autotransformers. I may award the amp with some better iron if the sound warrants it. Another possibility would be to use the primary windings of some cheap P-P transformers like the Edcor XPP10-8-8k.

The schematic looks a little busy because I've included the bias strings. The output stage uses a mixture of fixed and cathode bias. The amount of cathode bias is kept relatively small to allow me to squeeze out some extra watts, and will be used to trim the output stage balance. Those who insist on absolute simplicity could try omitting the source follower.

On the input side, one half of the 6BN11 dual pentode is shown, implying that the other half would be used in the other channel. There is a shield between the two pentodes in the tubes that I have (pretty much all GE manufacture, though of various brands), so I don't expect a problem with crosstalk between channels. If you feel queasy about that, I suspect a pair of 6EW6 or 6EJ7 would work as well, as they have similar transconductance for the same amount of plate current. I find that using the dual pentode makes the top panel layout more pleasing and a little less cluttered.

If you do use the 6BN11, remember to connect the suppressor grids to the cathode at the socket - this connection is not shown in the schematic, and the suppressor grids are brought out in the 6BN11 rather than being connected internally to the cathode.

[scott wurcer]

The pain, the pain. All those wasted afternoons watching Lost in Space reruns.

Actually this really happened. One semester I dropped all my courses and went to the bursar’s office and asked for all my tuition back. To my surprise they did it. Picture that happening today.

[wrenchone]

Upon reflection, this setup, with some changes in the output bias, may be just the ticket for kicking the living snot out of some of those stubby little 17-18W horizontal deflection tubes in screen driven mode. The P-P topology is just what is needed to take advantage of their high peak current capability. An all-Compactron amp would look pretty neat. Having said that, though, I'd probably want output iron with a little hotter turns ratio for that experiment, so I'll stick to the 6P3S-E for this go-round.

[wrenchone]

Scott - it's interesting to hear the real story - I always thought it was a play on the name, plus undergraduate whimsy... BTW, I can't even imagine paying the tuition these days, much less trying to get it back.

[kevinkr]

Originally Posted by scott wurcer

The pain, the pain. All those wasted afternoons watching Lost in Space reruns.

Actually this really happened. One semester I dropped all my courses and went to the bursar’s office and asked for all my tuition back. To my surprise they did it. Picture that happening today.

I had a friend in college who did pretty much the same thing with "Space 1999" - but unlike you he did not have the forsight to drop all of his classes and managed to pull a GPA of just 0.2 for the quarter. I still remember the look on his Dad's face as he helped him pack up after he flunked out..

My GPA was based on something somewhat resembling the cosine waveform with significant positive offset and some pretty big peaks AND valleys..

Tuition cost these days is pretty appalling..

[dhaen]

Will it be stable? How much feedback is there (I'm lazy and you'll know..)

[scott wurcer]

No zero's for me one "D" (pass no credit) from Millie Desselhaus herself with the promise that I would never be a physicist. A brilliant wonderful woman BTW, I didn't appriciate it at the time.

[wrenchone]

Dhaen -
I did some rough back-of-the-envelope calculations, and it looks like the amp has only 7-8 dB of feedback. The current value of the feedback cap (C3) is probably too small - I would start with around 1000- 1500 pF and adjust depending on the square wave response.

As for stability, I know from experience that the output transformers are the main limiters on the bandwidth of the P-P circuits I've built thus far. The Fisher transformers are pretty good, but not exceptional. A lot will depend on where the extra poles from the autotransformer are located. If the autotransformer has similar to or wider bandwidth than the output transformer, then there is a chance to roll off the gain gracefully before trouble starts. I'm going to see what I can accomplish with the cheap autotransformers first, as I already have everything in hand - there is always the option of better iron, and the Electra-Print autoformers are only $30 a pop.

[dhaen]

Yes, my mental estimator suggests that less than 10dB of feedback is likely to be safe with 2 inductive loaded stages.

My reason for asking is that I really like iron-coupling. With a stiff bias supply it allows the possibility of favourite class: A2...

[jerluwoo]

I have an old Delco car amplifier that uses vibrator power supply running a pair of 12V6 pushpull. It uses the center tapped input choke as the splitter and sounds just awesome. Had to replace the vibrator circuit using a 24V transformer into the stock transformer. It uses a 0v4 (not totally sure its not where im at atm) gas rectifier. I was amazed at how good this little thing sounded.