ouch

riz · Posted: Thu Apr 21, 2005 9:08 pm Post subject: ouch

I'll try this days, did not have enough wire. In the meantime, I just came back from rehearsing and one of the two power tubes decided it was time to leave this world, showing its decision by glowing foolishly.
Why, everytime you sort something out, something else has always to go wrong?

Ok I'm being deliberately pessimistic, I'll go to bed now.
Thanks people, I'll keep you informed about the events.

Ah, one last thing.
I know that 6L6s can be used in place of 7027, but can the opposite be done?
Like putting a pair of 7027a's in my dear Fender?

G'night

riz · Posted: Thu Apr 21, 2005 9:16 pm Post subject: sorry

ehm... this should have been in the impedance... topic, but I pressed the wrong button.
So it was directed to Bass and so on...
Sorry, I'll never do that again Mr. Green

1bassleft · Posted: Fri Apr 22, 2005 12:57 am Post subject:

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

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 Shocked

riz · Posted: Fri Apr 22, 2005 8:23 am Post subject: I know...

the sad story in that in my town there is NO tech, besides one who is more like an old radio tech (and who looks a bit mad).
There is one shop some miles from here, but they are the same who did the bad job putting on those GTs that were not right.
I'm still looking for someone around here, but the point is that I need to replace those tubes soon because that's the only amp I have...

riz · Posted: Fri Apr 22, 2005 10:59 am Post subject: NOW I KNOW

well, i think I know.
Remember when I told you that I was given the wrong tubes?
Maybe here we have the solution.
GT 6L6B are in fact Sovtek 5881WXT, right?
My amp had Sovtek 6L6GC on it. 6L6GC were build to stand up to 450V (and here I have no idea of what that really means...), whereas 5881WXT have in their specs 360V max voltage.
Did I get the point, or was it just smoke in my eyes?
Have a nice day everybody, I'm going to work now!
Idea

1bassleft · Posted: Fri Apr 22, 2005 2:25 pm Post subject:

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 Embarassed

. 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) 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 · Posted: Fri Apr 22, 2005 6:44 pm Post subject: i'll check

Hi!
Guess what? Maybe (this word again) I found out that my specific amp actually CAN be rebiased through a pot. well: I found and watched well my amp schematics and layout (Bandmaster with ab763 circutry), and there is something that could be the symbol meaning a bias pot, but I do not know anything about schematics, so---mainly I'm guessing.
If any of you knows better, it is a black thick dot with a circle round it...

Bass, I'll check that shop--maybe there is one in Italy too, but they charge 6 euros to match the tubes... And maybe (again...) I found a tech, let's see what happens.
Thanks and see you later!

1bassleft · Posted: Sat Apr 23, 2005 12:36 am Post subject:

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 Smile

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... Rolling Eyes

1bassleft · Posted: Mon Apr 25, 2005 1:57 am Post subject:

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?

1bassleft · Posted: Tue May 03, 2005 12:18 am Post subject:

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?

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