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Checkink an old Ampeg and finding strange things...
Submitted by riz on Tue, 01/08/2008 - 20:49.
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!
Nice to see you again, riz. Here are my limited thoughtsd at this time of night:
1) My first thought was that this was some modification to convert from 7027As to EL34s or 6550s but what you describe is not like that at all. No idea. What are the power tubes, though? You say different brands but they are all the same type, aren't they?
2) Yep, that should be the bias circuit. Now, I've read that Ampeg's cathode bias had resistors that ought to have their Wattage doubled and caps that should have their voltage doubled but not, of course, the actual resistance increased. Seems odd and, although Ampegs are designed to run clean, I don't like this mod and would return the 100k back to 75k.
3) Not sure, but some attempt to reduce hum by a previous owner?
You'll get a better idea of things by asking in the "miscellaneous American amps" category on Plexi Palace. Or directly PM "methodofcontrol" over there. I'm sure he's worked on numerous V4s.
Hi bass!
Strange things, as always.
I thought about a conversion too when I looked at it, but first, the resistors's placement doesn't make much sense and second, it was made on just one socket...
I think I could just take them off and see. They are not good for bias testing either... The resistors that control plate and grid are ok, so nothing bad should happen. Don't know why on the schematic the value for R39, 40, 45, 46 it says "PR1" instead of 4.7 like mine... Some schems say 10ohm instead, I think ampeg liked to mix things up a lot.
Thinking about it, the two new electrolytics could be a substitution for C19 - but I have to check this afternoon.
I'll put the bias resistor back to specs, but the real question is (something I always get confused about): more resistace=cleaner, or the opposite? Maybe the amp was running too "cold" and the overdrive I was hearing was some kind of result of that?
I'll try to contact methodofcontrol immediately (yes I know, I'm at work now... but I've got nothing to do!)
PS I just noticed I misspelled the title of this topic... that's sad.
have anice day!!! :?
ok,
one issue sorted out just now.
Quoting Chill from http://vintageamps.com/plexiboard/viewtopic.php?t=24638 :
IIRC, there are three filter cap cans and one mounted on a clip inside. Two cans are 70/40/40, and the third is 40/40/40, the one mounted on a clip inside is a pair of 20uF's.
The two unknown caps should be those - their 20% tolerance seems way too loose in any case.
I'll wait for further help on the other issues.
Bye!
By the way, a little question:
what's a safe (well, as safe as it can be) to discharge capacitors? And to test them?
Always got confused on this subject, maybe because of my not-so-perfect english, maybe because writers tend to assume that you at least have SOME experience... and I have less than that. Bought a couple of old tube books and the one wrote by Pittman, but they're not so exaustive either. :(
Hello again, riz,
Lets work in reverse order. Here's what I think is the best advice I've seen for safely discharging filter caps. Get yourself some stiff but flexible wire and solder one end to a crocodile clip. Solder the other end to a 100k resistor. Solder another piece of the wire to the other end of the resistor. Now use shrink-wrap or good insulating tape to cover all bare metal except the crocodile clip and the end of the other wire.
Attach the clip to the chassis, hold the insulated part and touch the bare wire to each of the caps in turn. The stored charge will be safely discharged to ground (the 100k avoids sparking that would happen if just wire were used).
Next, this thread (http://vintageamps.com/plexiboard/viewtopic.php?t=53558&highlight=ampeg&sid=57b121a36f9ab5473f1dd74079d0d636) has a discussion on checking caps to see if they are working. It's actually an Ampeg V4 thread.
20% tolerance does seem pretty wide, even for those days. Modern components (except valves, unfortunately) have much tighter tolerances.
The other possible for the pin resistors, for bias measurement, also occurred to me. Doesn't make sense to use that value, though; if you're using Ohm's Law then you want a nice easy value like 1, 10 or whatever - not 47. As Baron Machinemann says on Plexi Palace, someone's been messing and returning to stock is the best idea.
I have Ampeg schematics for the VT22/V4 and the V4B. Here's what I read:
VT22/V4 - R39, R40, R45 and R46 = 10ohm/5W
V4B - R39, R40, R45 and R46 = PR1/5W
I would fit 10/5W resistors in these positions myself.
If the bias resistor change runs the tubes too cold, you will get early distortion. Maybe this is what the modifier wanted, but this kind of distortion is quite harsh and unpleasant, IMO. Much better to run the V4 clean-tube and use a pedal in front.
:D :D :D
That's what I had on my mind...
Well I found some schematics that were suggesting 10ohm too and not 4,7 like the ones on my amp. I'll change them too.
I hope I have enough time next week to start doing something, I have to buy new tubes too and wait for them to arrive... Italy's the worst place to live in, if you like tubes. Rare techs, rare parts, expensive everything... Sad.
Meanwhile I'll check all other values around the amp with the schem in my hand, to see if they are ok.
I'll keep you updated.
Have a good night!
Italy is an unfortunate place to live sometimes. I have seen tubes on eBay from Italian sellers. The tube price might be nice, but the Italian postage charges make all but the largest orders uneconomical.
My VT22/V4 schematic is Ampeg's own, dated 06/1970. Let me know if you want my help to confirm other values. This schematic is for 7027A power tubes. Does yours have them? Even if the amp were modified for EL34s or 6550s, those 10ohm resistors remain unchanged.
For EL34 conversion, R41, 42, 47 and 48 are changed from 470/1W to 1k/5W and R49's value is adjusted to correct bias. For 6550 conversion, the same changes are made and, in addition, R50 is changed from 56k to 82k.
At this point, I imagine the guy started something that he never took to an end.
At the moment everything else looks to be ok, since it's as well hard to understand what the previous owner changed, and what was simply modified by the factory.
One thing is curious: on the pc board there are R59 and C22 marked, but no component is there and I can't find them on the schem... Could it be a factory revision?
At this point I thing I'll change the bias caps too. I still have to test the diodes (do you know which type they are?).
About the two mistery resistors:
Can it be that they are there to take the end of the filament wiring to ground?
The hum balance variable resistor was disconnected.
Not that I know what this is made for... Just guessing.
(by the way, I'll put 7027s for now, like it was supposed to be... :) )
I hope you find the 7027s - are they there already or have EL34s or 6550s been fitted? The schems I have make no mention of R59 nor C22. My two schems stop at R57 and C20. Maybe they were put on the PCB for future versions but my schems are so tiny they make my eyes go funny. I'll double-check for them tomorrow.
I'll try to answer the other bits (if I can) after some much-needed sleep :wink: Time I got a pre-2am night.
Hi Bass!
Are you going to sleep so late for work or for fun? :)
Ok I'll mind my own business...
When I bought the amp off ebay it came with some clearly unmatched 7027s.
Maybe being really different in brand and supposed previous usage, they could contribute to cause the early breakup I mentioned and - just in the last period before I started using only my other amp - too many lows. Or, that was mixed with the too high bias, Or, with those resistors being 4.7 and not 10ohm... or everything together.
Anyway, I found the 7027, they are quite easily available at the moment, even if they're a bit pricier compared to 6L6s. I'm planning to buy tubes & bits at banzaieffects.com. The biggest trouble is related to the fact that caps are hard to find if you're looking for odd values like those found in ampegs, and for a non-expert, trying to find parts on sites like Mouser or rs-components.it (similar) is a nightmare, when I search for a cap value I receive something like a zillion results!!!
I'm going to sleep too now, my alarm is set at 7:30, and tomorrow's going to be a pretty long day.
Thanks sooo much for your help!
PS by the way... where exactly do you live in the UK? I'll be glad to come back there and offer a couple of pints.
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