Homepage Forums Guitar Discussion Guitar Checkink an old Ampeg and finding strange things…

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  • #24597
    riz
    Participant

    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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    • #74422
      1bassleft
      Participant

      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 😉

    • #74441
      riz
      Participant

      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).

    • #74465
      1bassleft
      Participant

      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.

    • #74411
      1bassleft
      Participant

      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.

    • #74406
      1bassleft
      Participant

      😆 , 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.

    • #74437
      riz
      Participant

      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… 🙂

    • #74409
      1bassleft
      Participant

      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.
      http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=110218453348&ssPageName=ADME:B:WNA:US:1123

    • #74455
      riz
      Participant

      THIS
      http://remmelt.com/amp/
      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…

    • #74458
      1bassleft
      Participant

      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.

    • #74426
      riz
      Participant

      Yeah, I need some holidays.
      I bought the two 15k resistors to build a cap discharge tool…

    • #74451
      myfoot
      Participant

      A 5 watt resistor could be a grid resistor . My jtm I think is 470 ohm 5 watt and the jmp50 (didn’t tell you about that one 1b 🙂 ) are 1k 5 watt.

      EDIT
      Just read the whole thread and it looks like it’s not a grid resistor .Sorry 🙁

    • #74416
      1bassleft
      Participant

      Yes, I’ve got a book full of A5 schems. That probably explains why I like Champs and 5E3s 😆 . I don’t remember seeing 15kOhm, either. What would be really good is if something like schematics heaven on the web had a facility to search + locate caps, resistors etc on a schem; a bit like the “search and replace” function in Word. That’d be a Godsend.

    • #74410
      riz
      Participant

      ok, I must be too tired really. I knew they had to be 1/2 watt… simply wans’t careful enough in that moment.
      and the other two resistors are not 15ohm but 15Kohm… Still no idea about what I had in mind when I placed the order. Weird. 5W seem to be a specific choice too. Who knows!
      As for the eyes… I can’t imagine how you can decipher that, my schem is printed A4 and still a nightmare to look at!
      mmm… those resistors… I need a check-up.

      (by the way… some OT rage: protools is good when it works, a nightmare when it doesn’t – damn. I hate it.)

    • #74482
      1bassleft
      Participant

      Yep, as the schem says, “all resisitors are 1/2W 10% unless otherwise stated”. As for your 15ohm resistors; sorry, I have no idea 😆 I know we talked a lot about 10/5W but I don’t recollect 15ohm and I can’t see them on my schem. HST, it’s printed on an A5 size piece of paper so I get funny eyes looking at it too hard…

    • #74418
      riz
      Participant

      Hi!
      Well the new parts arrived today. Obviously, I found out I ordered the wrong resistor for the bias circuit (1/4 watt – not 1/2 😥 ) – I’ll try to find that on monday. Hopefully, I’ll put them in the amp before next weekend, and let you know what happened.
      I bought a couple of 15ohm/5w resistors.. can you tell me WHY?!?!!?
      i FORGOT!
      ah, I’m too young to already behave like this.

    • #74407
      Tim
      Participant

      [quote=”1bassleft”] Let me (and Tim) know if you’re in the area again and we’ll make an effort to say hello.[/quote]

      Yeah, would be good. Maybe imbibe a beverage or two…

    • #74415
      1bassleft
      Participant

      If there’s no hum and the filter caps are neither bulging nor powdering out, then I’d keep them in. I’ve got 40+ year old amps with their original filter caps. Not bad, considering they were designed to last maybe 10. Let me (and Tim) know if you’re in the area again and we’ll make an effort to say hello.

      Best for now 🙂

    • #74408
      riz
      Participant

      I’ve actually been up there some months… Well it was Harrogate to be more precise. I’ll save some cash to come back some day…

      Talking about the amp… I ordered some parts yesterday: tubes, C9 (small cap), some resistors (including the 4 10ohm/5watters), the two electrolytics in the bias circuit, and that’s it for now. I’ll try to put everything back to specs and see what happens.
      I’ll change the filter caps later, since hum was really low and looking at them, they seemed fine (this is my average degree of accuracy 😕 )

      I’ll keep you updated!!!

    • #74449
      1bassleft
      Participant

      🙂 , riz. I have strange sleep patterns, all incompatible with my work. As you say, 7027s can be found at a higher price than 6L6GCs usually go for. I was getting mixed up with 8417s, which are even more expensive than NOS 6550s. With fresher glass and replaced caps (you can parallel caps, if that helps), it should sing. Again, putting up a message in “Miscellaneous American Amps” on Plexi might help you locate a source for the correct caps.

      I’m in the North West of England, midway between Manchester and Liverpool; either are easy enough for me to get to if you ever fly in 😀

    • #74431
      riz
      Participant

      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.

    • #74446
      1bassleft
      Participant

      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 😉 Time I got a pre-2am night.

    • #74371
      riz
      Participant

      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… 🙂 )

    • #74333
      1bassleft
      Participant

      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.

    • #74361
      riz
      Participant

      [quote]Much better to run the V4 clean-tube and use a pedal in front.[/quote]
      😀 😀 😀

      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!

    • #74339
      1bassleft
      Participant

      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.

    • #74301
      riz
      Participant

      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. 🙁

    • #74335
      riz
      Participant

      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!

    • #74315
      riz
      Participant

      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!!! 😕

    • #74323
      1bassleft
      Participant

      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.

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