“Poco” Riz. I get by in French, Spanish and Italian. The “Terra” guess came from my limited Latin.

I’m glad to hear the problem was an inexpensive fix. A microphonic valve doesn’t harm the circuit; it just sounds horrible if it’s the first stage. 12AX7 or ECC83 type valves can be replaced easily, without any need for technical knowledge. The only thing required is a respect for the dangers of valve amps. There are a number of suppliers who can guarantee their preamp tubes to be free from microphonics. The replaced tube can be kept as a spare for the phase-inverter, where microphonics don’t matter.

Incidentally, don’t replace those filter capacitors just because they are old. They can last for decades more than their supposed life-expectancy and (bizarrely) many collectors like to see the original caps. A visua inspection of those capacitors is your best bet. If they are bubbling around the seal, or you see any traces of white powder, get rid of them soon. When they do explode, the mess is quite nasty. If they look in good condition, keep playing. My amp’s caps turned nasty because I just wasn’t using the amp much.

Riz, you want me talk technical Italian? 🙂 “Earth” is spot-on for us English, what the Americans would call “ground”. I wonder if your tech would call it “terra”? – just a guess.

I would be very interested to know if your problem also affects the phase inverter (in a Bandmaster, this should be a 12AT7 but it may have been changed for a 12AX7 with no ill consequences). If the 6L6 tubes are burnt by the vib channel, but not the PI 12AT7, then this narrows down the culprit even further. Your tech does indeed sound like he knows his stuff. I think you have put your amp n good hands. Best of luck for a speedy, cheap return from Behringerland. 😀

Riz, I would enlist that mad bloke with the valve radios you mentioned. Otherwise, you’re chewing tubes and that’s costing you money. Also, to locate the problem involves running the amp on power, tapping with a bit of metal and listening for pops (or lack of). There is always, I MUST STRESS, the risk of popping your red, pumpy thing in the left-centre of your chest.

Briefly, though, it sounds like your normal channel is OK, the transformers are OK and the main filter caps are OK. Something in the vib circuit is sending an uncontrolled signal to the power tubes but that’s as far as I can speculate. At least the likely culprit has been narrowed down. An honest tech should not charge too much to fix this – more than €100 would surprise me and more than €150 would have me reaching for my shotgun.

Welcome to groupiedom, Riz. I remember that hell. The other contributors are OK – “Jammin’ Hot” doesn’t qualify for services – but favours you have to perform for the moderators… 😳

Back to your Bandmaster. Pity you missed the headsup. I know those boxes – I have loads of them. They never made tubes, just rebranded them. All four of those GCs (whether labelled Fender or not) were made by RCA in USA and are the de facto tubes for those Fenders. I have finally had time to check through some schems. The 6G7 and previous Bandmasters appear to be fixed bias, whereas the later ones (including yours) have the pot. Near to the black circle, you’ll see a variable resistor (zigzag lines with an arrow pointing to it) and “10k”. I’m not an amp tech or electrical genius, but unless I’m hallucinating that’s your bias pot. It helps to explain that site’s “Fixed bias, adjustable bias” comment – they were covering the whole production era.

You’re right, Riz, a blue/violet glow is not something wrong per se. Causes can be electron bombardment and natural fluorescence, a small amount of inert gas. As long as the plates are not reddening, then things may be all well. It’s impossible for me to say without seeing. Use this site:
http://members.aol.com/larrysb/blue_glow.html
to guide you.

Riz, I have found a superb quad of 6L6GCs on ebayUK. These will make your Fender sing. Can you PM me, or I PM you?

This is a ‘bump’, to remind myself and others. I’ve also had a rotten weekend and not checked the schems. Has anyone else had a look in the meantime?

Riz, I’ve got through a tough week, so I’ve only had the chance for a quick look. HST, I am definitely NOT a schematics expert. It’s all hieroglyphics and makes my head go fuzzy. I have to look at it slowly, and carefully. I know that many others can read schems like a newspaper, so please chime in 🙂

The AB763 info lead me to http://www.ampwares.com/ffg/bandmaster_bf.html and that enigmatic comment, “Fixed Bias, bias adjustment pot”

That is what I call an oxymoron.

I’m now having a quick look at the schematic: http://www.ampwares.com/ffg/schem/bandmaster_ab763_schem.gif and now Mrs Oz has returned to fill my ear with “teaching” stuff. Suspending… 🙄

Riz, you’re absolutely right. Sorry, I don’t use the 6L6 and I’ve had a hectic week, but I should have thought of this 😳 . The 6L6GC can handle a 450V plate voltage whereas the 6L6GT is nowhere near. That rotten shop ought to give you your money back.

If you can’t easily find a tech, you may want to consider Groove Tubes or (better, and based in England so post won’t cost much) http://www.watfordvalves.com and their “Harma” range.

Both sell valves graded from 1-10. The idea is that, once the amp is set up for these tubes, then replacements can simply be bought of the same grade. No rebias necessary. A fixed-bias amp is just set for an average value but I’m not too happy with that. Far preferable to optimize the bias and change resistor values if necessary. With Harmas and Groove Tubes, this would only have to be done once.

Riz, we can chase a thread; it adds to the fun 😉

Decent 6L6 tubes can be used instead of 7027s (that’s a good bit of searching, BTW) so yes, you can do the reverse, in theory. I say “in theory” because the 7027 is a terribly difficult power tube to find. The 6L6 (even the old stock) is much easier to get hold of.

I don’t mean to patronize, but you can’t just pull out a set of power tubes and replace them with another set. Glowing tubes are one possible outcome, along with crap sound and other things burning. Different/replacement power tubes = rebias (and make sure the replacements are matched for current draw).

A rebias can be done on ANY valve amp, and should. A “fixed bias” amp doesn’t accept any tubes, it just involves a slightly more complicated rebias than pot-turning. Suddenly switching from GE or Mullard to Svet, Sovtek or JJ will require serious re-bias or the new tubes will frazzle. Techs don’t charge very much for this, so why not use them? Also, poking around in valve amps can quite easily kill you. Even when switched off and disconnected from the mains socket. Just thought I’d mention it 😯