Tone King Imperial MKII - tube replacement advice

[Cpb2020]

Greetings and thank you in advance. My daughter's primary amp for the past 4+ years (rehearsal & gigging) has been a Tone King Imperial MKII.

About a week ago the kids had a gig and her setup was noisier than typical (buzzing). I figured it might be a pedal but it wasn't that bad so we went forward without using our backup plan.

Today was her first practice back at home and it started buzzing again. I went direct into the amp (bypassed the pedals) and the buzzing continued for a bit but then went away. I'm assuming that this is a potential tube issue? If so, which tube(s) would be the more likely culprit? I'm a former engineer comfortable with a multimeter and electronics, but I figured I'd start here with the experts before digging in.

Here are the tubes / functions for the TK Imperial. Thank you!

 

[tubedude]

Does this buzzing have a 120 hz component or is it just static like, with no real hum?

 

[Cpb2020]

Does this buzzing have a 120 hz component or is it just static like, with no real hum?

There was definitely a hum. Not just static.

 

[Askwhy]

If tubes are the suspect, try a good 12ax7 in place of each, one at a time, and see if that resolves it. Then try another set of power tubes. Then the rectifier. It's good to have spares anyway and that is really the only way to tell which tube it is or if you should start looking at something other than tubes.

 

[tubedude]

Possibly a bad ground or funky contact. I'd clean the jacks and tube pins.

 

[Milspec]

Noticeable hum leads me to think weak capacitor in the power supply rather than a tube.

 

[Cpb2020]

I’ve got a couple of replacement tubes on order to see if that is the cause. They should be here on Monday.

If I can find time before then I’ll check out the tube pins.

And if neither of those work I’ll look into the power supply.

Thanks for the suggestions!

 

[Askwhy]

What happened?

 

[Cpb2020]

What happened?

Ordered a set of tubes, replaced last night, and now good to go with no more buzzing or hum.

It appears to have been one of the 12AX7s in V1 or V2 position, I forget which.

 

[Cpb2020]

What happened?

And thanks for the advice!

 

[Wally]

I am late to the party here, but some hindsight might be useful.
If there was hum with all controls at zero on an amp, one starts to suspect the filter caps. That was not the case...so on to the tubes.
My search would have started by pulling tubes one at a time…starting at V1. When the noise goes away, the problem area has been identified. If a new tube in that one position cures the problem, then throw the offending tube away. If that does not cure the problem, one has to go inside the chassis…there is a problem in that area of the circuit.
As it is, you now have a handful of tube but you do not know which one/s is/are the problem. You could have had one bad power tube….or one bad preamp. You did not have an entire complement of tubes that need to be discarded, but now you do not know what to keep and what to throw away.
A systematic approach not only finds the problem more quickly, it also can save money.

 

[Cpb2020]

I am late to the party here, but some hindsight might be useful.
If there was hum with all controls at zero on an amp, one starts to suspect the filter caps. That was not the case...so on to the tubes.
My search would have started by pulling tubes one at a time…starting at V1. When the noise goes away, the problem area has been identified. If a new tube in that one position cures the problem, then throw the offending tube away. If that does not cure the problem, one has to go inside the chassis…there is a problem in that area of the circuit.
As it is, you now have a handful of tube but you do not know which one/s is/are the problem. You could have had one bad power tube….or one bad preamp. You did not have an entire complement of tubes that need to be discarded, but now you do not know what to keep and what to throw away.
A systematic approach not only finds the problem more quickly, it also can save money.

Thanks - I purchased an entire set of tubes and replaced them all, and the ones I took out will now serve as backups. I do have the offending tube set aside as the faulty one, as I tested for hum each time I replaced a tube - I just didn't recall if it was V1 or V2!

 

[Wally]

Alright, then! I suppose you could install the 12A-7s back in V1…one at a time. When you hear the hum, throw that tube away.

 

[Jack]

Old thread, but I have a similar problem with my TK Imperial MK II. Got mine in a trade and it has a noticeable hum at idle on the rhythm channel. Switch to the lead Channel and it’s totally silent. Tried replacing tubes in V1/V2 and PI tube with same result. Same result with attenuator bypassed. Anyone have any tips on what else I might check or is it time for a tech.?