Moderate signal passed even when Volume pot at zero in EF86

[chopstuck]

I'm revisiting my EF-86 Stank-Ray build. I'm scratching my head wondering where the signal is coming from when the 1 meg pot is at zero. It's bypassed with a 50pf disc cap but it sounds quite full range. While I'm here, are the input jacks wired to show a 34k load in the hi gain and 68k in the low gain ? I'm getting a little too much dirt..in a clean amp.

[Structo]

This is the schematic I have.


Most likely when you have the volume pot turned all the way down there is still some signal getting past and of course through the 50pf bright cap.

Nothing really to worry about as most amps leak a bit like that.

Does it change if you turn the tone control all the way down?

[chopstuck]

Yes the tone control works on the leaky signal. I'm also red plating a bit on half my power tubes. It tried to change the Kr from 50 to 135/pair and split them but it really just thins the tone and continues to cherry 2 out of four tubes. Related ? Hums a bit too...darn it. There is some good tone buried in there though.

Thanks for the reply,

Marc

[M Fowler]

The Stangray uses a higher secondary voltage than say the liverpool or Rocket and measured voltage of 293v at the rectifier. Are you using a choke?

Cathode is a 50R which is what I have but I have seen 68R used as well.

Newer Stangrays different slightly from the picture floating around.

Mark

[Structo]

Yes, if you are redplating you need to increase the common cathode bias resistor.

I don't know what the normal plate voltage is supposed to be on the EL84's but it's easy to figure where your tubes are biased at.
Just measure the voltage across the cathode resistor and divide that with the resistor's value.
Of course that will be for 4 tubes so divide by 4.

If you want to get real accurate you would also figure the screen current and subtract that from the plate current for the actual bias.

The hum could be from a couple different sources but if you feel your lead dress is up to par, I would suggest trying some different tubes.
If you happen to have an extra EF86 you can try that.
Also try a different 12ax7.

I have read that those tubes can be problematic, especially the new production ones.

[Bob-I]

I had an issue on a Fender type build where the sound was getting through with the volume on 0. I followed that little bitty signal all over with my scope and it's my belief, and I may be wrong, that the signal is passing through....

Rp on V1a through the PS rail to the plate resistor on V1b. Bazaar? that's what I thought but that's the only place I could see signal all the way.

That little bitty signal was also present at all PS nodes, but there wasn't enough at PI plates to be seen.

Bottom line is this wasn't causing any problems for my amp, I was just curious.

[DonMoose]

That little bitty signal was also present at all PS nodes, but there wasn't enough at PI plates to be seen.

That sounds like inadequate filtering in the divider string, or a poorly-chosen ground for those particular filters.

Hope this helps!

[jaysg]

Wires behave like antennae. Some transmit and others receive. Look for parallel wires in your layout. Sometimes you can simply route a plate wire in an S shape.

[LOUDthud]

The question I always have to ask when talking about hum is; does the hum die instantly when you put the amp in standby mode or does it fade out over a period of a second or more?

If it fades out, look at the heater wiring and any signal wires running near the heater wires. Consider applying a positive bias on the heater winding center tap. Another possibility is an imbalance in the output stage, swap the tubes around. Does it come through the volume control? EF86s are known to have hum problems.

If the hum dies instantly, it's usually a ground issue which can also be the cause of the volume control bypass issue. If all the filter caps are in one location and the grounds are all tied together, start there and move the grounds for the B+3 and B+4 caps to the ground side of the cathode resistors in their respective stages. Got input jacks grounded to the chassis and transformer bolts used to ground center taps? Move grounds for the EF86 stage to the input jacks.

[Bob-I]

That little bitty signal was also present at all PS nodes, but there wasn't enough at PI plates to be seen.

That sounds like inadequate filtering in the divider string, or a poorly-chosen ground for those particular filters.

Hope this helps!

I agree. The Fender AB763 has too many plates per PS node. I think on this node there were 4 plates connected so signal get through. The PI was on another node so less signal got through

[chopstuck]

Well, I rebuilt the old salvaged PS. New can caps and axials. I then replaced every ceramic I left in there (3). The biggest change came when I replaced my power tube sockets. Just before the fuse blew I could see a bright pinpoint of light from below one of the EL-84's. I replaced the fuse, pulled the tube and looked at the underside of the chassis while powering it up and - again bright light. The pin must have had a carbon track so I replaced the sockets. I then moved my input farther away from my PI tube. I now have much less squeal when the cut is turned off. Replaced the two prong cord with a grounded power connector and rerouted some suspicious wires. The amp is now pretty quiet -still a little fixed 60 hz, I suppose it's the bane of the ac- 30 mold. The amp sounds great now! Crystal clear and quite hifi. I'm still getting some red-plating I can see only in the dark but not as much as before. My voltages are in line with the published Stank layout.

I have just about thrown everything out of the donor except the chassis, transformers, pilot light and recto socket. What a giant waste of time this has been vs. a scratch build.

Is there another way to reduce bias current without changing the Rk and changing the tone ?