Strange buzz after socket replacement

SilkWilk

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Hi all - awhile back I had posted about a no sound issue with my 100 watt ptp super lead clone. Turns out I had a bad ground on the impedance selector. While fixing this I also replaced my V6 power tube socket as it had some carbon tracing from an old tube that went south on me and arced Badly.

I’ve got a matched quad of JjEL34s that are about a year old. Today I had the chassis out and was going to check bias as I’ve noticed v6 and v7 seem to be running hotter then v4 and v5. Not redplating but physically they are hotter to the touch you can even feel the increased heat just getting close to them.

Well I noticed when checking plate voltage that I’m getting a faint but very apparent electrical buzz right on v6 the socket I replaced inside of chassis it’s not an apparent sound in the speakers. This amp does also have a ppimv.

My connections look ok and amp works fine what could this be just interference better shielding, bad solder job? Should I even be worried?

Voltages on all tubes were as follows

Pin 3 - 515v +\- 1-2v vdc
Pin 4 - same as pin 3
Pin 5 - (49.5)vdc

I haven’t checked heater voltage and I ended up not being able to find my bias probe so not sure on plate current.

Any initial obvious thoughts
 

thetragichero

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so it only buzzes while on the bench? likely because it's guts up without any shielding
 

SilkWilk

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Do you think it’s odd that only one socket buzzes and it’s the one I had my hands on haha maybe I should just reflow the solder joints

After checking heaters and bias
 

william vogel

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The impedance selector isn’t grounded. It simply selects the secondary tap of the output transformer and the selection is wired to the speaker jacks. The speaker jacks have the common connection to the output transformer and this is then also grounded at the common speaker connection.
 

SilkWilk

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The impedance selector isn’t grounded. It simply selects the secondary tap of the output transformer and the selection is wired to the speaker jacks. The speaker jacks have the common connection to the output transformer and this is then also grounded at the common speaker connection.
You're right my bad - it was the secondary tap from the impedance selector to the output socket that had come loose
 

SilkWilk

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You're right my bad - it was the secondary tap from the impedance selector to the output socket that had come loose
I measured heater voltage it was as follows

Preamp tubes 6.8 Vac
Power tubes
V4 and V5 - 6.8 Vac
V6 and V7 - 6.9 Vac

Waiting on new bias probe to confirm plate current.

I swapped tubes around as well still only light buzz on V6. I tried moving wires around with chopstick to see if it was something with inter fence. Nothing changed.
 

SilkWilk

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I’m getting 125v at my outlet. Not sure about PT but I would presume it’s wired for a 120v tap.

Personally I think I should just rewire the socket and replace the screen resister I can’t remember if I did that last time or if I reused the old one because it ohms out fine.

I didn’t realize my wall voltage was that high. I’ve considered getting a variac now I guess I have good reason too
 

SilkWilk

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I realize all my talking is moot until I get a good reading on plate current. My bias might be way to high for all I know it’s been a long time since I checked and I’ll be honest and embarrased to say I threw these tubes in a year ago since they were matched and figured I’d run with it - to my ears amp sounded fine.

Also earlier I think I said my bias voltage on pin 5 was (49.5) but it is (45.9)
 

SilkWilk

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I got a new bias probe and hooked it up to V6 and I was not surprised bias was high around 39ma. This is an el34 at 515v plate voltage so it should be set more around 32-34ma for 70% dissipation.

Here’s the bigger concern I’m not getting any plate current on the other 3 power tubes. I tried them all individually and let the amp warm up each time and just flat zero.

What is this telling me?

Again I’m getting consistent 515v on pin 3 and 4 all power tubes and all of them have (45.9) on pin 5.
 
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SilkWilk

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Sorry to bump but I'm at a loss - I've tried multiple times - each time I check plate current I get nothing at v4 v5 and v7 but I'm getting a consistent plate current on v6. it was 39mA I lowered it to 32mA its still holding that when I go back and retest. I replaced v6 socket a couple months ago - I've never done anything to the other output tubes except replace tubes.
 

SilkWilk

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Sorry to bump but I'm at a loss - I've tried multiple times - each time I check plate current I get nothing at v4 v5 and v7 but I'm getting a consistent plate current on v6. it was 39mA I lowered it to 32mA its still holding that when I go back and retest. I replaced v6 socket a couple months ago - I've never done anything to the other output tubes except replace tubes.
I measured screen resistors as well they are all at 1k ohms resistance (1k 5 watt xicon)
 

Pete Farrington

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As only the recently replaced socket is conducting current, all we can assume is that those other sockets are bad :shrug:
If you’re able, I suggest to try retensioning their contacts (very carefully).
Failing that, see if replacing another achieves current flow.
 

SilkWilk

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As only the recently replaced socket is conducting current, all we can assume is that those other sockets are bad :shrug:
If you’re able, I suggest to try retensioning their contacts (very carefully).
Failing that, see if replacing another achieves current flow.
Yeah I suppose it’s worth a try.

I retested all my voltages just now -
Pin 3 - 509v
Pin 4 same
Pin 5 - -47v

Consistent I’m getting 509 at center tap as well.
 

SilkWilk

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Well now I'm starting to think maybe my no sound issue wasn't just a bad solder joint on the impedance selector but maybe it was that I shorted out all the sockets (somehow maybe a massive surge) and like you said I replaced the one since it had a carbon tracing on it (which was from a prior tube shortage) that allowed current to flow and I'm getting sound from that amp but maybe that v6 is running so hot because its carrying the entire load of the amp.

Is that even possible though I mean its a push pull if I wasnt getting current on another power tube then I should have a no sound issue right? I can't imagine its possible to have sound with only one tube conducting.
 

SilkWilk

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Yeah I read further elsewhere that technically it can run and produce sound on one tube. I’ve played the amp a handful of times since repairing it he v6 socket to my ears the sound wasn’t terrible but I was keeping the volume fairly low.
 


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