Anyone with UHF/VHF tube experience?

[HotBluePlates]

I'm beating my head against the wall trying to eliminate a buzz in my new 1/3w build.  Here's where I'm at:

 -  I built a PRR 1/3w using a 6AK5 preamp tube and a 6AU6 output tube.  This pentode channel works fine with no noise issues.
 -  I also built a triode channel, using two 6BC4's in a Fender blackface-style preamp.

The 6BC4 channel has had a buzz since the beginning, which sounds the same as buzz from single-coils or from a guitar lead plugged into the amp but not in a guitar.  Because this type has 4 grid pins (for the same single grid), I wondered if having used only 1 and leaving the other three alone could contribute.  However, strapping all those together didn't alleviate the noise.

Wondering if the criss-cross layout of the tone wiring contributed, I unhooked all of that and have the volume control moved near the 2nd 6BC4 triode.  But wiring only the 1MΩ Volume control wiper to the 2nd 6BC4 grid and that pot's ground lug to ground, I still have buzz.  No input to the Volume pot is wired up; the Treble pot and other tonestack circuitry has been disconnected.  What I hear is any time the Volume control is turned up much above 0, there is buzz.  With the Volume on 0, the channel is dead-quiet.

What I wonder is since this tube was designed for grounded-grid operation, is it likely that this tube type simply won't be quiet without being run in a grounded-grid configuration?  Or could it be only the length of the wiring between the Volume & the tube?  Straight-line distance from grid pin to Volume control is only ~2 inches, though the actual wire length for the grid & ground wires is ~4 inches due to dressing against the chassis.  The Volume pot ground is connected directly to the ground-end of the cathode resistor for this stage.

I know I'm exceeding the recommended grid resistance for this tube, but the buzz starts in the 1st quarter-turn of the volume control, when I should be well under 100kΩ grid-to-ground.  Ripple at the B+ node for this stage is ~4-6mV.

I'm running out of things to try, and wondering if I need to ditch this tube and/or only try to use it grounded-grid.

[silverfox]

If you're not already running DC heaters, perhaps you could run the 6bc4 filaments on DC and see if that's the source of the hum. I found other references to preamp's constructed with the same tube. Apparently it's a variant of the 12AX7.

silverfox.

[HotBluePlates]

I've have a.c. heaters.  However, the pentode channel of the 6AK5 to 6AU6 is dead-quiet, so the heaters seem unlikely to be the problem.  I did however, have the same thought, and went to the trouble of applying +25vdc to the artificial center-tap of the heaters, and no change.

If d.c. heat is/was absolutely required for these 6BC4's, then I'm just going to scrap using them; it's just not worth the hassle of deriving d.c. heat for me to use them (I've always used a.c. heaters and gotten quiet amps).

FWIW, the noise is a buzz and not hum.  Turning down treble did help reduce it slightly, but I've now gone the step further of unhooking the tone controls and first gain stage.

[Ritchie200]

I've got a bunch of 703A and 316A triode doorknob UHF transmitter tubes if that counts?   I had read an article about how a guy in Japan was using them to drive some big obnoxious tubes for some cork sniffer stereo.  Alone they produced a couple of watts so I got to thinking they would make a cool build - even bought custom sockets.  Not much info on how to use them for audio but the article had layout and BOM.  I lost the article and the website is gone....  They are now gathering dust. 

Anyway...  Probably a stupid suggestion but are you sure those 6BC4's are healthy?  Have you swapped them?  Do you have more than those two to sub in?  Almost sounds like a crappy tube?

Jim

[HotBluePlates]

Anyway...  Probably a stupid suggestion but are you sure those 6BC4's are healthy?  Have you swapped them?  Do you have more than those two to sub in?  Almost sounds like a crappy tube?

Not a stupid question!

I had gone through at least some of the 6BC4's and sorted out those with heater-to-cathode leakage earlier, as I was getting hum which went away with extra cathode bypass capacitance.  But since I did the d.c. reference for the heaters that has become a non-issue.

So I did try going back and swapping through at least 6 or so 6BC4's.  Where the noisiest pair started buzzing in the first 1/3rd of Volume rotation, the quietest starting buzzing in the last 1/4 or so.  So it definitely seems related to the tubes themselves.

But I also think the 1MΩ volume pot is a problem.  When I pull the input 6BC4 (leaving only the Volume running into the 2nd 6BC4, which then goes to the output tube), the buzz is gone at the very top/bottom of Volume rotation, but is maximum when the Volume is about on 7 (which would be something like half-resistance).  I think I need to switch to a 500kΩ or even 250kΩ Volume pot.  The 6BC4's have very low internal resistance, and my plate loads are only 15kΩ or so... So the smaller pot shouldn't be a load issue, and may help buzz (along with sorting through the 100 or so 6BC4's I've got).

[silverfox]

Well my guess is their may be a relationship between the heater and grid capacitance that is not significant when the grid is grounded. Unless shielded cable on the grid would get rid of the noise. Another guess is some sort of capacitive relationship between the tube and the controls. With the tube pulled is there any hum visible on a scope? I know the other channel works fine.

Another suggestion would be, from what I've read the 6bc4 is a 12AX7 compatible tube. Without seeing the schematic I don't know if the anode resistor are compatible with simply substituting an AX7 tube to see if the hum goes away. I'm thinking the hum has to do with the tube characteristics.

I don't know one way or the other but what is are the reasons for using this particular tube in the design?

silverfox.

[kagliostro]

Ciao HotBluePlates

If you can find one, give a try to those sockets that has a small ground pipe on the center, something like this



BTW have you tried a shielded socket ? (one effect is that it changes some capacity on the tube)

Little ferrite beads on wires ??



Franco


p.s.: OK, you don't want to go DC, but, just to knowledge, what happen if you go with DC heaters ?

[kagliostro]

I've read that there are Core Audio Tube Buffers that uses 6BC4 tubes

so seems that they are usable in audio

Franco

[shooter]

I have the head-cold-from-hell, sooo....
have you split the 2tube pre, giving each it's own B+ tap, maybe, spit the ground also., Bypass each half tube?   I spent 38yrs fixing RF fom 10gig, to low MHZ, was good at it, but spend zero time wondering why the real engineers split a foil trace, bent it funky, added a cap here or there, and used it to make things work

[Paul1453]

If you are not committed to using 6BC4s for your input.

I am having very good results using Joe Befumo's 6DJ8 totem pole input design.

It produces a very clean, clear and articulate sound with no hum issues,
that can still push the PI into nice tube distortion tones.

You may not have a big bag of 6DJ8s to use like I do.