6SJ7 - taming the beast?

[polaris26]

Hello all,

I am in the process of refining a little amp I made a while ago that uses a pentode input stage. The amp is all octal - current lineup is 6SJ7 in pentode mode, 6SL7 PI, pair of 6V6 cathode biased, no global negative feedback. The basic design is based on a Valco Supreme 510-1B circuit. I have added a simple 1-knob tone stack between the Pentode input stage and the PI.

The main problem I am having is a persistant hum. It is not coming from the PI and goes away when the volume knob is down. It is definitely coming from the 6SJ7. Unfortunately I can only find one 6SJ7 in my tube stash (I have more coming in the mail) so maybe it is a bad tube. I have read some people like the sound of the 6SJ7 compared to say an EF86 and from what I can hear it has a really nice squishy clean sound that really would be nice with the output stage, but the hum and buzz makes it not very usable at this point. I think it is a heater induced hum or perhaps the tube is picking up noise - I don't know the frequency b/c I haven't analyzed it yet but it sounds like more than just a low freq hum.

My chassis is very compact and made of aluminum, which may or may not be a problem. I am using the star grounding scheme a la Aiken, although my amp started out with a can cap I have since modified it as much as I can to have added C/R's with returns to the respective local grounds. The input jack is isolated and return runs to the local cathode return point for the input stage. I elevated the heaters by tying the mid point resistors to the cathodes of the 6V6's - so up about 17 Volts.

Things I have not yet tried:
1) Tying the 6SJ7 "shield" pin to the local ground point - is this advisable?
2) Subbing in a battery supply for the 6SJ7 heater to see if hum goes away - I don't have a handy battery or floating supply yet; I will see what I can scrounge up today.
3) Trying a different tube - I have only one and more on the way (eBay)

I noticed that many of the classic amps used grid leak biasing. One possible advantage I can see for this is that the cathode is directly grounded, so any hum leakage from the heater to the cathode would presumably be grounded out - is this one reason why grid leak was used?

Mine is cathode biased and I tried adding another cap in parallel - right now I have 122uF across 1200 ohm resistor and it might have helped some but not enough. Maybe I need an even larger cap (122uF has about 21 ohms reactance at 60 Hz) ?

Another thing - I tried biasing the 6SJ7 per the charts in the back of an old tube manual and the voltages seemed very low to me (like 22 volts on the screen, 65 volts on the plate) but maybe this is normal? The charts don't list voltages but in the section of the manual on the 'typical operation' for a 6SJ7 they list two options - plate 250/screen 100 and plate 100/screen 100. My B+ at that node is only about 270 volts to start with and with 220k plate resistor and 1meg screen resistor I can in no way see getting those voltages on the plate & screen. My cathode bias resistor is currently about 1200 ohms.

As always, any input most appreciated!

Thanks,
Dave

[EFK]

I've never been able to get those things to shut up unless running them on DC heaters. I realize others have, just not me. Also, they tend to easily pick up radiated hum from the chassis unless the sockets are mounted on little rubber shock washers so that neither the socket nor the socket retaining bolts contacts the chassis. Also (again), I find the metal can tubes noisier than the glass tubes. The glass tubes often will require a home-made tube shield (aluminum or steel) however for ultimate noise reduction. I've fiddled a lot with theses in old GA20 amps.

[terminal]

1) Tying the 6SJ7 "shield" pin to the local ground point - is this advisable?

Yes. same for EF86.

[polaris26]

I just wanted to follow up on the 6SJ7 thread - I ended up completely re-working the amp, with a new grounding scheme and a new circuit board to accommodate some other ideas I had (tone stack, added adjustable neg feedback lop, etc) The amp is now surprisingly quiet, even without DC heaters.

I am now focusing on the output section to make it more compressible at full volume and am trying to actually lower the B+ quite a bit as I realized the original circuit must have had a B+ in the 250 volt range owing to the losses in the field coil speaker that was used.
Dave