AC15 questions

[imo]

I am in the process of finishing up a Ac15 style amp. I built with Mercury mag power and output trans, as well as choke. All parts are standard, but i have a couple of questions regarding the build.
The first is that my supressor grid voltage is higher than the plate voltage on both tubes. Originally i had two different nodes powering this area. the first was right after the rectifier, and that went to OT center tap, second went to the suppressor grid after the choke and then 100R resistor. When i couldn't bring the voltage down, i tried moving both to the same power node(after the choke) they plates are around 360, and the supp grid is around 368..Seems wonky to me..Any ideas on where to go with this?
I am also trying to get the right bias on the powertubes. Again stock cathode value is 130R, but i am using the Mercury power tran which outputs more voltage so my voltages are up throughout the circuit. Right now i have a 150R in there(with the cap of course) and getting around 13 volts on the cathode. I have checked the plate current and with the 130, it was around 42-43ma per tube. I think i would like to bring it down a bit still. Any suggestions here? It sounds great, but even for this design it seems that the tubes are being pushed too hard. This may be the supressor grid wierdness, but either way it sounds so good i want to button it up and get it out on a gig!
As always, much gratitude for your generosity and knowledge
Best
Ian

[Mars Amp Repair]

HMM,
I believe you are actually referring to the 'Screen' grid, not the suppressor grid. The suppressor grid is usually internally connected to the cathode & serves to prevent what's called secondary emmissions which is electrons bouncing off the plate & returning to the tube. 2ndary emmisions cause 'shot' noise.

What power tubes are you using? I'll assume that your AC215 uses EL84 tubes. According to my RCA tube manual, you are really pushing the limits of this tube & possibly exceeding them. Even if you're using 6V6's, that seems to be pushing it a bit.

The screen voltages might come down if you put a series resistor of 470 or 1Kohms in there. I believe the value is not that critical as the screen doesn't draw nearly the current of the plate. You don't want it to be drawing any more than it is designed to. If it does you can actually see the screen grid getting bright red through the holes in the plate. My experiece is that you can destroy the tube pretty quick in that situation.

As far as bias goes, if all your other voltages are too high, then the bias will follow. If you make the cathode resistor too high to get the bias down, you will begin to effect the ability of the tube to produce the wattage you desire.

It sounds to me that you really need to get the proper power transformer or drop all the voltages (which means a lot of wasted energy to heat) to do this right.

I'm not a designer & I'm certain someone like Bruce-mission amps is much better qualifited to give advice here, but that would be my take...glen

[imo]

you are correct. I meant screen grid. I am using El84's, and i am using the power transformer meant specifically for the AC15. Shouldn't the screen grid be at a lower voltage than the plates? So the resistor change is really the only thing to do to bring the voltage down on the screen?

[Bruce / Mission Amps]

you are correct. I meant screen grid. I am using El84's, and i am using the power transformer meant specifically for the AC15. Shouldn't the screen grid be at a lower voltage than the plates? So the resistor change is really the only thing to do to bring the voltage down on the screen?

Here's what's happening:
The DC resistance of the OT is higher or the same as the choke.... when current flows through a DC resistance there is a voltage drop. Way more current is flowing through the OT then the choke so there is more voltage drop... so the plate voltage of the power tubes appears to be lower then the screen voltage of the power tubes.
A simple pair of 1w to 2w 470 ohm to 1000 ohm resistors in series with each screen to the B+ rail will help balance it back out.
Also, it is not a big deal if the screen voltage is a little bit higher then the plate voltage. This is one of those Net driven myths.
But it does make the tube more sensitive to bias voltage and the tubes will naturally draw more idle current as the screen voltage goes higher and higher.
That means setting or getting the relative bias voltage a little more negative to compensate.
IMHO, it is probably a good thing to keep the screen voltage under control. So, at least try to get it close to or less then the plate voltage.

[imo]

Bruce,
Thanks so much. Thats exactly the info that i was trying to clarify. I definitely don't want to be spoonfed answers, rather trying to really understand the "whys" of circuits i am working on. I love the sound of the AC15 circuit, and being able to get more of an intuitive feel for the flow of the circuit really helps to make changes that are in line with what needs to happen
Thanks again
Ian