EF86 noise- suppressor grid and internal shield

[Bloomfield]

I'm just finishing up a new build with an EF86 in the preamp. I've had it running for a while and I've got it very close to what I want. My only real complaint has been the noise level from the EF86. Part of that is microphonics from chassis vibration (it's a combo) so today I finally found some M3 nylock nuts, and I was just about to make some sort of suspended mount for the tube socket when I realised that I hadn't connected the internal shield or the suppressor grid.

I did a little reading and jumpered pins 2 and 7 and ran a wire from 7 to my ground bus. I also connected the suppressor grid (pin 8) to the cathode (3.) I switched on, hoping to hear less noise, but found it to be similar to before. Within a few seconds a loud low-frequency hum came in. Turning down the bass pot and volume had no effect. Looking around the back of the amp, I noticed the power tubes were red-plating, so I shut it down.

Any ideas? I suspect some sort of ground loop maybe, although I'd guess the frequency was under 60Hz. I should add there was nothing connected to the input at the time.

Thanks,

Andy

[g1]

First pull the EF86 and see if you still have the power tube issue. Could be an unrelated problem that just popped up.

[Bloomfield]

Thanks g1, that was quick. I pulled the EF86, and sure enough same problem, although maybe a bit quieter. I held a tuner to the speaker and it's a sharp B-flat, so it is 60 Hz after all.

[g1]

Still red-plating?
If it's 60Hz, could be the bias filter cap. (make sure + end to ground )

[Bloomfield]

I've been looking around a bit more. It's cathode-biased, so no bias supply filter. I did measure the cathode resistor, and got a very low reading. I took it out and checked it and it was right on 470R. I pulled all the tubes prior to this. I suspected the bypass cap might be shorted (admittedly a 25v part with 30.6v across it) so I took it out and measured the DC resistance, checked out open circuit same as a new part, so I put it back in.

Something was appearing in parallel with the cathode resistor. The only other suspect was the heater circuit, as I have the centre tap connected to the power tube cathodes to elevate the heater voltage. I disconnected the C.T., and with all the tubes out I read less than 1 ohm from C.T. to ground, same for each side of the heater wiring. I think the filament winding must have shorted to ground inside the transformer. I checked all the tubes for shorts and they seem fine.

[g1]

Disconnect the other heater wires and see if CT still measures low resistance to ground.

[Bloomfield]

You're right, I was just about to do that before I got hungry and wandered off for dinner. I disconnected the secondary from the heater wiring, and it looks like the transformer is fine. One side of the heater wiring is shorted to ground somewhere. I'll poke around some more and hopefully I'll find it.

[g1]

Is this a one off build, or based on a common schematic?

[Chuck H]

I suspected the bypass cap might be shorted (admittedly a 25v part with 30.6v across it) so I took it out and measured the DC resistance, checked out open circuit same as a new part, so I put it back in.

Maybe if you don't have a 50V part on hand. I wouldn't. And I wouldn't rule out failure in this at working voltage.

[Bloomfield]

Well, I got it fixed. First off, I intend on replacing the bypass cap with 50v or better; I hadn't anticipated needing so much bias voltage, and 25V caps were all I had around. I do have a 10uF @ 450 that I could use for now. Way overkill on the voltage rating, but losing a little low-end response wouldn't be a terrible thing. This amp is a one-off for now. It started off as that parallel reverb idea I was working on, but after reading a lot more on here and in Merlin's book it changed some. It now has a 12AU7 input stage going into an EF86 into a cathode follower driving the tone stack into one side of a long-tail pair. The other side of the inverter is fed by a pretty standard Fender-type reverb which comes after that same input stage. The power amp is 2x 6V6, cathode biased and diode-rectified. I might try the zener diode on the cathode resistor I saw Chuck talking about in another thread. A little bit more headroom might be nice to have, but it's not bad as is.

I figured out where the short must be when I was walking back to the shop. Sure enough, the bare jumper I used to connect the internal shield had cut into the heater wiring. I took the opportunity to take out the socket, and I found some small grommets around, so I drilled out the screw holes and mounted the socket on those grommets along with washers and nylon lock nuts. Between that and the now functioning suppressor grid and shield, the amp is WAY quieter now, with or without a guitar plugged in. It's way too late to crank it up here though, so that'll have to wait for another time. Thanks for the help, you guys are the best.

Andy

[Chuck H]

No reason you couldn't use two 25V caps in series. Not sure about the math for balancing voltage across the caps in that circuit, but I might use something like 10k and see how it goes rather than leave a single 25V part in place.

Glad you located the problem Sometimes I have to stew and poke off and on for days before locating problems. One of the reasons I don't get paid for my part in this avocation.

[pdf64]

With a class AB cathode bias amp, the voltage across the cathode resistor will rise, probably significantly, with signal level, may nearly double when the output is pushed to max square wave. As it's idling at 30Vdc, I suggest a cap rated >=60V.
Ensure a good degree of physical and thermal separation between the cathode resistor and its bypass cap.
A cathode resistor per tube (eg double the shared value), bypassed, enables non matching tubes to idle at closely similar cathode currents (ie they match themselves).

[Bloomfield]

Hi guys, thanks again for the help. I was away for a week, but when I got back I swapped the cathode bypass cap for that 10uF/450v that I had lying around. The amp sounds great and made it through a 45 minute set that night with no problems (other than not being allowed to turn it up loud enough.)

Have a great new year everyone.

Andy