12X6550 LF amplifier

[Colas LeGrippa]

Hello my firends, I am preparing my voyage back  to Cuba and «mientras yo pensaba que ustedes estarian ineresados de ver un aplificador muy raro».
Ok, I am sure you understood my spanish, and I wont repeat that you sure would be interested to see a 12 x 6550 low frequency amplifier designed for a vibrating table, which we doesn't see very often.. Un fortunately, the guy ( mi amigo ) who gave me the amplifiers, ( 2 amps, 24 X gold Lions !!! ) removed the HHHUUUGGGEEE trannies and of course the GOLD LIONS 6550. ( though he gave me one NOS brand new in a seales plasic bag with the teste results performed on it before packing it , and hand writed !!!!!! KNow what ? THis tube was worth approx 700$ before my error. ( opening the sealed bag).
Ok now I want you to take a look at the circuit: 1k 2w screen grid resistors on each tube, 1k 1/2 w grid stopper resistors on each tube as well. ( también ) and one 64K grid leak resistor for each set of 6 tubes.
The bias is a tube driven type ( I've never seen that before ). The construction is pretty well done with very solid industrial terminal strips. check the pics !!!

Colas

[Colas LeGrippa]

forgot to mention: 1 OHM resistors on each cathode, for bias measuring purpose.

[Colas LeGrippa]

more pics, see the bias circuit, in fact there are two bias circuits one is 12au7 driven and the other is 6an8a driven.......
could an experienced tech talk about that, por favor ? ( please )

[PRR]

Why do you say "bias circuits"?

I think it is the front-end for a Dynaco: 6AN8 pentode-triode. The eyelet connections are very Dynaco. That's fine for two power bottles (Dyna had one model with four); but at *twelve* power bottles it needs a buffer between the Dyna front-end and the power bottles' grids, so a 12AU7 was inserted.

[Colas LeGrippa]

...and 6an8a is a preamp tube ?

and could you tell me why there is only one grid leak resistor  for every package of 6 bottles , while there is  one grid leak and one screen grid resistor per 6550 ?
Thanx

ColasLeGrippa

[Colas LeGrippa]

error:...while there is one grid stopper per bottle ( not grid leak)

[Willabe]

and could you tell me why there is only one grid leak resistor for every package of 6 bottles, while there is one grid leak and one screen grid resistor per 6550 ?

It's not much protection to use a shared screen grid R. Each screen should have it's own R.

A grid stopper interacts with the tubes capacitance (Miller effect?) at the tubes grid pin to stop oscillation. Putting the stopper R as close to the socket grid pin with as little lead from the stopper R to the socket lug works best. So they need a separate R for each power tube.   


 

[Colas LeGrippa]

Thanx guys !