18-Watt TMB Issue

[dpm309]

I am having an issue with a loud, intermittent hum on the TMB channel of my 18-watt head build after it has been running for several minutes.  I built this amp about 6 months ago and has been working flawlessly with virtually no hum until about a week ago.  The normal channel seems to work fine.  I pulled the chassis and have it on my bench and have been able to recreate the hum but cannot seem to find the source by poking around with a chopstick.  All of the voltages are in line even when I am getting the loud hum.  I am guessing the problem lies in the preamp section of TMB channel but don't see anything that jumps out at me.  The input jacks are brand new Switchcraft shorting jacks and inserting a dummy load does not make a difference so I think I can rule out the jack as the source.  The hum gets louder when I turn up the gain and/or volume and sounds like when someone puts their thumb on the other end of a guitar cord plugged in (only louder).

I use this amp for recording and as a backup when playing out so I need to resolve this.  Any ideas on where to go?  I know these intermittent problems can be a pain to diagnose.

Thanks,
Dan

[phsyconoodler]

That could be any number of things.You built the amp so it's hard to say what could be amiss.
  A picture would help. What immediately comes to mind is simple stuff like tubes,bad grounds and cold solder joints.

[dpm309]

Thanks, I will post a schematic later today and a couple of shots of the amp guts.  I based the design off of Trinity's 18-watt TMB but made a few modifications like only 1 input jack per channel, and other mods to increase the gain and fatten up the TMB channel.  I will check again for cold solder joints etc.  Been running the amp for a couple of hours now and cannot get it to hum again.

Dan

[eleventeen]

"Been running the amp for a couple of hours now and cannot get it to hum again."

Heh, hate that.

Heater to cathode short...? Tube heats up in the cabinet....filament bends & touches the cathode....not so much on the bench? (all I can do is to toss out drive-by ideas, trust you understand) H to K shorts are pretty rare, but an intermittent problem....you shouldn't rule out anything. That's what sucks about them.

Also look at your solder joints with the best magnifying glass you can find. Sometimes a joint will look fine but have a heat-opened hairline crack. It DOES seem your problem is heat/time induced, no?

[dpm309]

11 Never experienced a H to K short but I will keep that in mind.  I have attached my modified schematic (my changes in blue) and a gut shot.  Keep in mind that this amp was made from an old Hi Octane 5W chassis that I gutted (never did like the sound of the HO) so there are a lot of rough scratches etc. due to having to re-drill for additional tubes and new transformers and other components.  I did use Doug's 18-watt PT and OT and they do sound great.  Also, had a brain fart and installed the turrets too far apart so I had to "splice" some of the resistors to span between the turrets.  Though they don't look pretty, they are crimped and soldered together.  Still have not gotten around to inspecting and re-soldering any potential cold joints.  I have a great magnifying lamp that lets me see anything inside of the amp.  Tubes are new but I will try swapping them around as well.

Thanks,

[dpm309]

Turns out you were on the right track 11.  Swapped out V2, and all is well.  Put it on my tube tester also and turned out that 1/2 of the AX7 was shorted out.  Still puzzled as to why the voltages on this tube were in the ballpark even when half of it were shorted.

Thanks for the help.

Dan