Power amp request

[rzenc]

Hi gang!
A friend asked me to build him a power amp which will be used with a rocktron voodoo valve preamp + eventide eclipse + parametric e.q. He plugged it @ poweramp in on a 300W SVT-CL head and thanks god thought it was kinda too much  So, he asked me to build him along these lines, but 50W
I came up with the schem attached. I would like to hear your opinions.
Thanks in advance.
Best Regards.

Rzenc

[PRR]

Where did you get these values? 22K? 5K??? 33K?? 0.01u??

[rzenc]

Where did you get these values? 22K? 5K??? 33K?? 0.01u??

12AU7 is a C.F. to buffer 12AY7 gain stage from 6550 grids. In 6550 datasheet, maximum grid resistance is quoted as 50K in fixed bias mode. I thought it might load down 12AY7 plates too much. I planned a D.C. C.F. in order to try to avoid blocking distortion and buffer gain stage from P.A. Since 12AY7 plates @ 110V quiescent, and C.F. has 400V of supply, I also thought that letting cathode sit @ 110V would be a good idea...???....12AU7 is capable driving some current and 110V @ cathode could be achieved with 22K, assuming 5mA flowing thru each triode. Plate to cathode voltage would be: 290V. Max signal swing could be
V.S.= HT/(1+(ra/RK)) = 400/(1+(29K/22K)) = 172V
According to 6550 datasheet, it’s necessary ~30V to drive full power.
I’m not sure about it but I think C.F. Zout is Rk//RV, them:
Zout= 22K//(470+5K) ~4K3????
Do you think a 5K pot will load it down too much? If so, How much should I raise it’s value?
I have used 22K grid stoppers with 6550 builds and liked it a lot. However, since maximum G1 resistance in fixed bias mode is 50K, I thought a grid stopper of 22K should be taken into consideration and thus I choose 33K as bias resistor.
I would like the P.A. to be capable of resolving 60Hz very well, so I thought Capacitor value of 0.01uF would gimme what I want. I thought I could use this formula to find the necessary C value:
C=1/(2*pi*55K*60) which results in 4.8*10-8, 0.0047uF.

Feel free to correct me, I will greatly appreciate the lesson!
Thanks in Advance for you time and attention.
Best Regards,
Rzenc

[rzenc]

Hi,

I have a question regarding P.A. on ampeg SVT.... it seems to me that last C.F. is D.C. coupled to output power tube grids?? Voltage difference between C.F. grid (-77V) and cathode (-47V) is 30V?? Also, C.F. grid seems to be referenced to ground thru (150K+150K+bias adj.+22K), however it also seems that C.F. grid is bootstrapped with cathode, which is referenced to ground thru (47K+39K+bias adj.+22K) and the bias control seems to be more about setting bias condtions on C.F. grid instead of controlling power tubes bias???


I changed voltage supply nodes and I am feeding L.T.P.+C.F. from the same point, in this case node "C". I think this would not pose a problem but I'm not sure.. 12AY7 gain stages are 180° out of phase from each other and C.F. is a non-inverting stage..I would like to know what you guys think about it..
Last, I was thinking of implementing N.F.B. on the L.T.P. part of my plan, do you guys think it is an appropriate place to introduce NFB?? All insights are welcome!!
Thanks in advance
Best Regards

Rzenc

[PRR]

> seems that C.F. grid is bootstrapped with cathode, which is referenced to ground thru (47K+39K+bias adj.+22K)

Point D is a 90uFd cap to ground. No signal here. No bootstrapping.

> the bias control seems to be more about setting bias condtions on C.F. grid instead of controlling power tubes bias???

It is a cathode FOLLOWER. Whatever you do to the grid, the cathode tries to follow. Therefore fiddling the CF grid _will_ fiddle the power tube grids' DC bias voltage, just a few volts different.

I see the "-77V" on CF grid but doubt it is correct. Even with the lame meter specified in the NOTES.

[rzenc]

Thanks for the answer...

Regarding my schem... The presence control comes from a circuit drawn in merlin's book, pg.194Fig.8.19.a, I have been reading his book but feedback theory is not really clear to my brain  
Generally, the second triode grid is grounded, whilst mine is receiving an 180° inverted sgnal with respect to first triode grid from the cathodyne P.I. So, my question is if NFB is applied to the L.T.P. and is said to be negative, I believe it's referenced as inverted with respect to the incoming signal to the P.I. but now I have both grids receiving "identical signals" but opposite in phase. If I inject in the junction of R10 and R13, won't it be negative with respect to one signal and positive with respect to the other? Could it lead to oscillation - low or hi freq....????
Still about NFB: if I change the point where NFB is injected into the cathode of the first gain stage do I need/want a previous gain stage??

thanks in Advance
Best Regards

Rzenc

Edit: I also decoupled first gain stage from cathodyne P.I.