gofar99 wrote:
Hi, V(GK) runs in the 5-6 volt range. With the mod to add some positive voltage (between 5-8 volts) to the grids to raise the cathodes up by a like amount to get over the kink that happens when the LM317 gets driven too low the voltage on the grid to ground can range from 10-14 volts typically it is about 10. The drive however remains at + and - 5 volts (10 total) as that is all that is needed to drive the tubes full on or off. It is an easy chore for the 5751.
Hi Bruce, I should have been more specific, here I will break my thoughts down more clearly.
Here is what I calculate for gain for each stage:
"SRPP" ~48
output stage (EL84) ~22
output stage (6P15P) ~28
10k:8 OPT ~ -35.3
Your feedback resistors are 150 and 3k3 giving us a feedback factor of .043 150/(150+3300)=.043
With the EL84 open loop gain is roughly 30 and w/ feedback it's 30/1+(30*.043)=13
With the 6P15P open loop gain is roughly 38 and w/ feedback it's 38/1+(38*.043)=14.4
Let's look at the feedack factor again: EL84 30*.043=1.29 6P15P 38*.043=1.63
The 6P15P is higher.
Because you have not adjusted the feedback factor of .043, the larger open loop gain from the 6P15P yields a larger voltage at the cathode of the first gain stage, since the input of that stage is across the grid and cathode you deduct the now larger signal (compared to EL84) from the input. e.g. You need more input voltage to drive the
5751 for the same output of the that stage. The increase in negative feedback is probably why you get less distortion in the amp with 6P15P's.
The voltage from grid to cathode of the 6P15 should be about 1.5v less compared to the EL84, and since the 6P15P has more gain I was saying
they require slightly less drive voltage for full output.