amp that starts to hum and slowly gets louder and louder

[toner]

ive got a new build supro with a pair of 6973 tubes. amp works fine, but after a few minutes of being "on" a hum slowly starts creeping up and gets louder the longer its on. after about 15 minutes the hum is loud enough to tell something isn't right so i turn it off before something bad happens. i thought maybe it was the 250ohm cathode bias resistor on the power tubes since it was getting hot, and i replaced that with a 20w resistor and that clearly wasn't the problem. then i thought maybe one of the PI caps was leaky so i pulled the 6973 power tubes and left it on for 15 minutes. set my meter to 10vDC and couldn't get any reading on the 6973 side of the caps so i guess that rules out leaking caps? i haven't replaced the bias cap in parallel with the 250ohm 20w resistor, just figured it was new. i did take the amp to a gig and it blew a fuse. i checked it out and one of the power tubes bases cracked and it de-gassed which blew the fuse. anyone got any other ideas how i can troubleshoot this one?
thanks!

[M Fowler]

If you built this amp yourself then I suggest you refer to the schematic with the layout to make sure it is wired correctly. Also the voltages are noted on the schematic.

Willing help but the only way to possibly help out is for you to upload clear photos.

Mark

[Structo]

Be sure to watch the power tubes when you start up the amp.

It sounds like a tube is drawing too much current.

Being cathode biased, simplifies the bias circuit to troubleshoot but it could be several things but tubes should always be the first suspect.

But since it is a new build, most likely you made an error in the wiring so you should go over the amp with a fine toothed comb.

Check for leaky caps at the phase inverter. C19, C20

[toner]

thanks for offering to help guys! yeah, i checked those two caps for leakage (described in my first post) by pulling the power tubes and they appeared to be fine by that test i described. yes i built it, but ive had the amp for about a year and done some gigs with it with no problems.. i triple checked the wiring a while back so i don't think thats it. ill do it again anyways though. i put NOS rca's in so i really don't want it to be the tubes because ill cry if it is ive got some new EH 6973's on order so ill pop those in and see if that fixes it when they get here. i did measure the voltages in a couple spots and none of them seemed to be rising after the amp was on.. i did check the voltage readings with the schematic when i first put it together a year ago and they were all spot on.. ill go back and check that again. i really don't think its a wiring problem, just a failing part.. hopefully not the tube but maybe thats all it is... if you have any other ideas in the meantime of specific things i could check while i wait for the tubes let me know! thank you!!

[M Fowler]

It starts humming after the tubes warm up so each tube needs to be checked one at a time.

If no grounds have worked loose, it has only been a couple years since the amp was built so the filter caps should be good but you could parallel a new one across the existing caps to test.

[Structo]

Another test for caps is to see how much DC is leaking past the cap when the amp is powered on.

If you aren't comfortable working around high voltage then please don't try this.
But best practice is to clip your test leads on the test point when the amp is off, then power up to read the voltage.
Then power off and drain caps before removing the test leads.

[toner]

Another test for caps is to see how much DC is leaking past the cap when the amp is powered on.

If you aren't comfortable working around high voltage then please don't try this.
But best practice is to clip your test leads on the test point when the amp is off, then power up to read the voltage.
Then power off and drain caps before removing the test leads.

yes, thats what i did. pulled the power tubes and turned the amp on and i couldn't get any DC voltage on those two caps on one side. i wasn't sure if that was a definitive test for those caps being good under load though. thanks!

It starts humming after the tubes warm up so each tube needs to be checked one at a time.

If no grounds have worked loose, it has only been a couple years since the amp was built so the filter caps should be good but you could parallel a new one across the existing caps to test.

no, no grounds are loose. the mallory power supply caps are a year old but i could try that if i get desperate. i have a power tube tester that works great for finding bad octal tubes.. but it doesn't work on my 6973's. any procedure while in the amp to check them one at a time you recommend? just follow the DC voltages as the hum grows and see how stable it is?
thanks for your help!

[toner]

ok, i checked all the voltages per the schematic. the power tubes appear to be fine and the bias is stable at around 23v. all the voltages are correct except on V1 preamp tube. on pin 3 the schematic (attached) says 2.7v. but i get a reading of .8v. weird, i wonder why that is. i changed the 35uf cathode bypass cap and the reading was the same so its not that. anyone got any ideas to try? thanks for your help!!

[Structo]

Verify that it is indeed a 1K5 cathode resistor.
If that bypass cap is bad it could be taking too much to ground.

Since it is a cathode biased power amp, the bias should be stable.

I had an amp once that did something similar but it was fixed bias.

The bias pot was messed up and didn't keep a steady voltage.

Another thing I have gotten hum from is having two power tubes that draw very different amounts of current (extremely mis-matched).

[martin manning]

Did you try changing V1? the low voltage on its cathode may be an indication that it is going bad.

[es345]

I have read through the thread and looked to the schematic. I have 2 comments/hints:

- You have a common resistor for the bias at the power stage (R38). If one tube is starting to die (no more emission) the other one is going red because the resulting bias will be much too low based on the current of only one tube running through R38. You have total imbalance at the OT > hum. More safe is to have 2 separate resistors each e.g. 510 Ohm and 2 separate caps e.g. 47 uF.

- Her is one simple method I have used very often to localize the root cause of such an issue. I understand that you have new tubes now. Does the hum behave like before? Pull out V1 and V2. If the hum disappears, pull in V2. If the still there is no hum the problem is stage 1, otherwise stage 2

If you have done that already - just forget it

[tubeswell]

Do a tube swap in V1, then if that doesn't fix it, check your cathode resistor for that 0.8V triode - that voltage looks to be about 1/2 what it should be.

[toner]

thanks guys.. the cathode cap was replaced and no change so thats not it.i know that cathode resistor is the correct value. i haven't remeasured it though. i remember changing the preamp tubes around but didn't measure so ill do that again and remeasure and see what that does.
as far as the 6973 power tubes. if one was bad wouldn't the voltage on the bias resister/cap not be stable? they aren't red plating either so thats why i've moved on from that being the problem. the voltage being off on that one 1st preamp tube is where i think the problem is. i did pull v3 preamp tube which makes the amp sound a little better but the trem doesn't work. i would think that would make the voltage on pin 3 higher even, not lower.. i'll put it back in and measure and see what that does. thanks so much for your help.. ill report back!

also, when i pull the preamp tubes the amp goes totally silent.

[tubeswell]

i know that cathode resistor is the correct value. i haven't remeasured it though.

The cathode voltage is telling another story.
(Think about Ohms Law: E = I x R: .001A following through 1500R would be 1.5V, so your cathode resistor may actually measure 0.8V/.001A = ~800R - not that I think this would fix the hum problem)

So you did a pre-amp tube swap?

[Tillydog]

Re: V1 pin 3 voltage-

Does the build have the tremolo circuit as the schematic? If so, the voltage across R9 (pin 3 to gnd) will have a contribution from the current through the CF in the tremolo, also.

If you have pulled V3 (trem osc & CF) it could explain the difference in voltage. If V3 is still in circuit, try pulling it an see if this makes any difference to the hum.

Also check the grounding contacts on J4 and/or J2.

0.02p

Andy