ouch

riz · Posted: Mon May 09, 2005 5:06 pm Post subject: late!

Hi bass, sorry I didn't reply but I've been out of town for study causes last week and could not reach a computer.
Thanks for your help anyway!
In the meantime I've bought a couple of teslas for a really cheap price, put them on and now I'm waiting to bring my darling to a tech to set the bias right. Just for a test I tried to switch the amp on but there was something not completely right with the colour, so I preferred not to insist and wait.
Just one thing came to my mind: isn't it that when they are ffirst used valves do make some bluish/reddish colour? I mean, don't they need some time to settle, while they are strange looking?
Uhm, one day I'll know this and stop annoying people with silly questions...
Have a nice day (evening)!

riz · Posted: Mon May 09, 2005 5:11 pm Post subject: am I becoming somehow different?

I just noticed I've been named "groupie" in this forum...
Does that mean I have to wear short skirts and no underpants and do something weird with you people?

Shocked

1bassleft · Posted: Wed May 11, 2005 1:12 am Post subject:

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... Embarassed

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 · Posted: Wed May 11, 2005 2:49 pm Post subject: not only blue!

well, besides the other GT tubes I was given, that were glowing blue ANd orange on the inside, the new ones have some blue and some orange on what I believeto be the plate (if that's the grey alloy-ish thing inside the glass). Anyway, the amp is being given some check-up by this tech, hoping he knows what he's doing. Thank god at least there is the pot, this shuold make things easier.
Again, thank for your help bass! I thought the pot was that black round spot, but considering I know NOTHING about schematics I really needed your help.
I'll let you know if the tech was able to sort things out.
Bye!

riz · Posted: Fri May 20, 2005 11:41 pm Post subject: argh

hi again!
sad times have come.
well it came out that on my trustful bandmaster there IS something wrong.
this means:
when only the first (normal) channel is used, everything seems to be fine.
but, when the vibrato channel is used, this brings the power valves to glowing.
this means that:
1 the final stage is allright Very Happy

2 the first channel is allright Very Happy

3 the vibrato channel is not Sad

at least following some logic thie is what the whole issue look to me.
not knowing anything about how things work inside amps, i imagine that this could be a simple problem, like some resistor gone nuts or stuff like that. having referred the problem to a tech service on the phone, they told me it seems like current is not controlled in its path from chann. 2 to final stage.
has anyone an indea of what's going on?
Sad

1bassleft · Posted: Sat May 21, 2005 12:57 am Post subject:

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.

riz · Posted: Sat May 21, 2005 1:04 pm Post subject: maybe..

... we're close to the solution.
i was remembering what you told me about frying my heart off, that's why i popped the valves in and brought everything to a shop to try if it worked, so if something had to explode it would have been far from me. fortunately no one got injured in the process.
more to the point, i spoke on the telephone with this guy who finally gave me the impression to know what he was talking about. he told me that, as far as he can understand without seeing the amp, should be something wrong with the vibrato. he told that the vibrato affects the bias circuit somehow, or works through it, and something must be "earth" or whatever its called in english (this language is not mine, i don't know what it means in italian, in foreign languages it becomes a nightmare). result: easy and cheap fixing!!!
and my happiness, finally.
of course, thanks bass for your support &help!

1bassleft · Posted: Sun May 22, 2005 1:48 am Post subject:

Riz, you want me talk technical Italian? Smile

"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. Very Happy

riz · Posted: Thu Jul 07, 2005 4:03 pm Post subject: yeah!

hi bass!
just posted a topic, anyway:
yes the man is a good tech. he does that as an hobby and is really passionate nd prepared. just charged me 20 euros for everything, even explained me how to check bias.
he did not tell me anything about the phase inverter, we only noticed that one valve was microphonic, but it probably was the vibrato one so i did not care, since i do not use it.
i'm downloading some manuals to start learning some eectronics now, it looks interesting but having to learn in english is pretty hard.
one thing: what would a nice and darksounding distortion pedal be, that could be played through the bandmaster, apart from ts9 and od808's? tried a rat but it simply was not the case.
P.s: "terra" was right, that's why I tried with "earth". do you speak italian?

1bassleft · Posted: Sun Jul 10, 2005 1:03 am Post subject:

"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.

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