Bias on 5E7

[Buschman]

Hey ya'll, I have not been here in a while. I stopped building amps a started building acoustic guitars. I decided I just had to have a tweed Bandmaster. Trying to get my brain back into tube amps.
I am having trouble with bias. B+ is 419vdc with a gz34. I measure 52mv across a 1ohm resistor on pin 8 to ground. If I measure with the shunt method I read 74ma. If I measure using the OT resistance method I get a whopping .131. Bias voltage is -41. I am using 470 ohm grip stoppers and 1.5 screen resistors. What is my bias? I am using a fluke87 meter. I am using brown base 5881.

[martin manning]

I think you mean 1.5k grid stoppers and 470 ohm screen resistors?

The shunt and OT primary resistance methods should get very similar results, and using the 1Ω Rk should be a bit higher, maybe 5% or so. I suspect you are getting an oscillation when you put your probes on the OT primaries, so I would tend to believe the 52 mV = 52 mA (21.8W dissipation or 72% of 30W max for 6L6GC). You are not as likely to upset things measuring there.
Are you sure you have negative feedback, i.e. the OT primaries are not reversed?

[Stevem]

I would go by the current readings your getting across the 1 ohm resistors that you have on each pin 8, that is as long as they resistance test at 1 ohm or a tad more.

With 419 volts on the plate and a current draw of .052 amp your idling those outputs on the hot side at real close to 70%.
I would back that down to 60% or the equivalent of like .043 to .044 amp if your going to be cranking the amp well into clipping when playing, if not then stick with 70% providing you like how the amp will be clipping the output stage sooner and have less head room.

You may not like however that you will eat up output tubes 10% faster.

You might also want to audition how the amp responds to your ears and play style idling at 50% which would be .035 amp.

My experience is that once you have a A/B output stage with 6L6 type tubes idling at 60% your not really going to get the amp sounding any fatter at 70% , your just bringing in the clipping and saturation sooner.

Once again you be the judge.

[Buschman]

You know I have wondered about the OT primaries. The amp does not squeal but my presence knob does not seem to do anything. I may switch them and see if that changes. Yes I would like to cool the bias down some. I will have to make some circuit adjustments. Currently using a 56k in series with a 10k pot. I have some grey and blackplate rca that bias cooler. I could just run those.

[Buschman]

I swapped the primaries. No squeal and the presence might be working a bit but very subtle. I still get a 12v drop from ct to pin 3 which seems high. The E series Fenders have the choke in a bit of a strange place. Could that be affecting my readings?

[martin manning]

What DC resistance did you get for half of the primary? Does a 12V drop square with the 52 mA?

[Buschman]

Blue to red is 115.4 ohm brown to red is 105.2. I have not disconnected red from the amp but would not think that would matter. 12/115=.104 which is twice what the 1ohm is telling me.

[martin manning]

What would be nice is if you could monitor the cathode current with one meter while you measure the plate current with another. If the Ik measurement jumps up then you are definitely inducing an oscillation with the plate measurement.

[Buschman]

Is there a sure fire way to know if the primaries are in the correct place? The leads are still long if that matters. Thank You for your time and input Martin!

[sluckey]

Is there a sure fire way to know if the primaries are in the correct place?

Temporarily put a 22K or 15K across that 56K NFB resistor. That should give you enough feedback to know for sure.

[Buschman]

I paralleled with a 68k because I had one. No squeal. I think I will leave the primaries there.

[rooster]

This period in Fender amps was a bit odd. First off, understand that you can tie the screens directly to the plates of the 6L6 without harming the tube. This is a design feature of the tube, btw. Past this, you mentioned the choke placement seeming odd. Well, this is in an odd placement by most 'contemporary' guitar amplifier standards. This choke is trying to filter line noise before the P/P OT, and here the P/P circuit (by design) is doing this already, so this didn't make a lot of sense. I will assume it was something that must have come from Western Electric at the time but I dunno. And Fender eventually abandoned this idea because - although this layout created a unique sag/compression (along with the 5U4) that many guitar players liked - the long-tailed pair arrived (along with the need for more powerful guitar amps), and also a new circuit location for the choke. History.

The upshot here is that this Fender amp and the other 6L6 P/P models (5E5 & 5E5-A Pro, 5F4 Super, 5E6 & 5E6-A Bassman, 5E8 Twin) of this time period have their screens tied to the plates, albeit separated by the OT's P/P windings. This element, along with the 'older' PI will never have that 'clean' aggressive Fender BF amp sound no matter where you set the bias, so 35 mA is ballpark. Speakers are all important here, 3 alnico 10s was a good match and of course SC pickups, also note the dual 16uf caps after the choke in the Bassman and Twin, and easy mod. Have fun with it, Bushman, glad to see you back here.