50W HRM Build with an odd issue

[XB9R]

wondering if anyone has ever come across voltage on the choke frame/casing? Ive got this really weird issue, popping from speaker, and it changes tone. powered up a new hrm build a week or so ago, bulb limiter test was fine, tubes in voltages look ok, slightly hot but well within a 10% range. Continuity is good on all conections, ive been over this amp from both ends and all connections are spot on. Power tubes Biased up fine, PI trimmer bang on with V3A 6VDC higher than V3B. I pulled power tubes and popping stops, swaped them around and rebiased, this is where it got odd. This time i adjusted the bias with the tips of my fingers on the pot wedged between the choke and the power tubes trying very carefully not to get burnt from the heat coming off the 6L6s and touched the casing of the choke, well the popping stopped. Decided to get a reading off the choke casing to ground and the casing has just about 12VDC on it.  thats not normal is it? Could that be a grounding issue on the choke casing? Amp seems to play fine apart from the annoying pop. Any ideas? I dont want to just ground this thing if there is an underlying issue.

[Structo]

That is a weird one.

Is the choke new?

Try disconnecting the choke and measure resistance between wires and frame.

[ic-racer]

If the choke is attached to the chassis with metal screws it should be at the same potential. Is it getting insulated from your chassis paint or anodizing or from varnish on the choke feet?

[ToneMerc]

If the choke is attached to the chassis with metal screws it should be at the same potential.

I agree, there should be 12v on the chassis as well if this is truly the case.

TM

[Deric]

When you say "pop", is it a loud single pop or more like a constant ticking?

I've had a similar issue on an SLO clone. Turns out some SLO's had it from the factory. The issue was the choke not being connected to the chassis because the powder coat was too thick. On my build, I noticed the ticking stopped when I touched the standby switch. I removed the powder coat around the standby switch so the body of the switch made good contact with the chassis and the ticking went away.

Somebody here had the same issue with a D style build. Turned out to be the same thing.....

https://tubeamparchive.com/viewtopic.ph ... ing#110087

Good luck!

[XB9R]

If the choke is attached to the chassis with metal screws it should be at the same potential.

I agree, there should be 12v on the chassis as well if this is truly the case.

TM

Hey thanks guys for the quick follow up, havent had a chance to re-investigate this tonight, will try tomorrow evening and post results, in response to all above.
Yes choke and other iron is all new, chassis indeed powder coated, but has had coating ground back at relevant ground locations around the chassis. Have to assume not enough under the choke and yes its attached with metal screws. Will pull the choke out and both test the resistance between the connections and choke housing and grind back some more of the powder coating under there. Will pull the standby swtch as well and check this at the same time as i dont recall that powder coating being ground back in that spot.

Yes its a fairly consistant tick tick tick but occiasionally does change tempo and tone its like a tick tick tick pop or then a tick tick 3 4 tick pop, maybe i bulit an expensive metronome.

[Structo]

You can just sand away the powder coat on the inside of the chassis where the nuts are.

I like to use KEP nuts with lockwasher, or simply a star washer under the nut to lock it and also to bite into the chassis.

[ayan]

As others have already said, the casing should be at ground (chassis) potential. If it's not, make sure it is by sanding the finish as required. Furthermore, given this precedent, I would make sure everything that should be at ground voltage is: all mounting screws (for boards, tube sockets), pot casings, switch bushings, jacks, etc., etc. Otherwise you will probably be looking at trouble down the road.

Gil

wondering if anyone has ever come across voltage on the choke frame/casing? Ive got this really weird issue, popping from speaker, and it changes tone. powered up a new hrm build a week or so ago, bulb limiter test was fine, tubes in voltages look ok, slightly hot but well within a 10% range. Continuity is good on all conections, ive been over this amp from both ends and all connections are spot on. Power tubes Biased up fine, PI trimmer bang on with V3A 6VDC higher than V3B. I pulled power tubes and popping stops, swaped them around and rebiased, this is where it got odd. This time i adjusted the bias with the tips of my fingers on the pot wedged between the choke and the power tubes trying very carefully not to get burnt from the heat coming off the 6L6s and touched the casing of the choke, well the popping stopped. Decided to get a reading off the choke casing to ground and the casing has just about 12VDC on it.  thats not normal is it? Could that be a grounding issue on the choke casing? Amp seems to play fine apart from the annoying pop. Any ideas? I dont want to just ground this thing if there is an underlying issue.

[XB9R]

Resolved. Was a bung JJ out of the box, one of them had intermitant shorts. Bias voltage would sit at 35mA or so and then sky rocket to over 200mA and redplate up a storm. Replaced the power tubes and bingo shes away singing.