Checkink an old Ampeg and finding strange things...

Hi everyone!
Yes I've been away for a long time.
Anyway, I often have some funny problems to tell, and here's the last one.
After a long time, a month ago I decided to switch my old Ampeg V4 on again. Sadly, switching the stanby off caused immediately bad noises (thumps, wobbles, and so on), and after few seconds, plate on a power tube started to glow. Switched the amp off in a rush and left it there for some time.
Today I decided to open the beast, to see if I could find something clearly broken-burnt-dead, but did not find anything like that. I found three strange things instead (assuming I know the previous owner had made some work on the amp, and the power tubes were really old and from different brands-ages):
1) on one power tube (the one that started to glow actually) there were 2 47ohm resistors wired from pin 7 to pin 8, and from pin 8 to pin 2. No trace of this on the schem, so no idea what it was made for.
2) in what I think to be the bias section, there are two resistors, R49 and R50 (respectively, 75k and 56k). The first one is around 100k instead: I was wondering if this could cause early breakup (not something the V4 was originally famous for)
3) there are two extra electrolytic caps in there, wired in series, linked to one end to one pin of the main filter capacitor, and on the other somewhere else I could not find (didn't want to take the amp completely apart for now). The value is unknown since it's covered by the body of the caps, in a place hard to reach.
Does anyone have any ideas?!?
I'm at the point were I can track parts from an amp to the schem now, but these things are pretty obscure!

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Oh, I remember that bit, riz. I suggested the croc-clip, wire, 100k res, more wire (all touchy bits insulated of course) as a cap-discharger.

is where the value for the resistors came out (as far as I can remember - the whole thing is still foggy)

anyway, let's talk about serious stuff:
I put the new bits in the amp, new tubes, switched the beast on, and after just a couple of crakles and pops (and a heart attack on my side) it LIVED!
yes, it's working. loud as hell, it is. sounding like nothing else, yes.
great amp, worth every pound (and it's a lot of 'em!)

question: it's still crunching a bit early (volume knob set on the first notch on the scale). power tubes look ealthy, lit, with just a little shade of blue, and don't change their color much when I play hard, so I guess they're running pretty clean. maybe the pre tube is getting old (it's a GE, don't know the age of it)? texas specials on my tele pushing the preamp too hard?
and... how does the whole tone control work?!? still wondering...

riz, if the distortion's coming in early and slightly unpleasant, perhaps the bias has been set a bit cold? That blue will be residual gas in the tubes fluorescing; nothing to worry about. I usually find that GE 12AX7s are cleaner and tighter than Mullards and Dutch. I actually prefer them for bass. You could always get a replacement and try it in V1 to see if there's a difference. If you don't want the expense and risk of old-stock, I quite like the Chinese ones made in Lizhou and sold by PM, amongst others. They're slightly harsh on the treble, but otherwise reasonable on the ear.

EDIT: I just looked at this auction by chance and the Zenith on the left looks like a GE to me. Good numbers and balanced triodes, which may just make a difference. If nothing else, balanced triodes are good in theory for the PI. Being rebrands, there may not be a lot of interest in this auction and if you get both delivered for $20 I'd say that's a pretty good buy.

Hi Bass,
thanks for telling me about the auction!
I made some experiments yesterday, turning up and down the tone knobs and with the sensitivity switch... and the result is:
there's no unwanted distortion! I was just understanding the sens. switch to be a mere attenuation pad, but as stated by the manual too, it's a tool to control distortion - and yes, it works.
Everything else is perfect.
At this point, I wonder how it could sound with 6550's... :)

:lol: , I would never have suggested the sensitivity switch in a million years; partly because I didn't even realize it had one. Normally, I would assume (as you did) that this was just for padding high-output pickups or, on a modern amp, for active preamp equipped instruments. Nice to know everything is going smoothly.

If usual Ampeg history is to go by, converting to 6550s is a better option than EL34s but, for now, I'd keep to 7027s seeing as the amp is set up for them and they're still obtainable. 6550s are expensive, even modern makes, compared to the 6L6 and you might find that a good pair of 7027s is no worse in price.

Hey, guess what riz?

Over on Plexi Palace, I mentioned that I was considering auctioning up my old 60W Laney head and a US member has offered a straight swap for an Ampeg B25. Sounds good to me, so I'll be joining you in 7027-land soon if all goes ahead. Incidentally, I was researching the B25 and I came across a forum thread where a user said he'd swapped 7027s for 6550s and his advice was DON'T.

Update. I've just been informed that a previous owner had already done the 6550 conversion. I'll see how it is and maybe (but unlikely) revert back to 7027s. If the 6550s are General Electric, I probably will leave it as is.

Hi Bass!
Lucky you, a B25 would just fit in my bedroom.
But I have no Laney to swap...
Well, I haven't heard my V4 with 6550's yet, but yesterday I tried it with a '65 Mustang plugged in, and now I have to buy one (trying those things is never a good idea) - well, V4 IS clean (and sounds sooooo good).
I'm thinking about taking texas specials off my tele and putting something cleaner back in... Or I'll just buy a Mustang. And keep my tele hot like it is now.
Help please...
This was just to say that with the right guitar the amp already is clean, 6550's could be something not necessary, since overdrive/crunch is developed mainly in the preamp (this is my experience, at least).

In my case, the B25 is intended to be used as a valve bass head. It does look to be designed as a low-distortion circuit, so guitarists might shun it unless fronted by a pedal. I plan to split my bass output so that the dry signal goes into the Ampeg and FX signal goes into the Marshall. Back on the Ampeg, my latest brainwave is to have the swapper remove the new 6550s, send them to Terry, and post the head to me glassless. Terry can then send me a pair of GE 6550s drawing the same current as the whatevers for a straight replacement.

That's the theory. See if it works :wink:

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