Help with my old Ampeg

[Keven]

Hi guys,


i'm working on restoring a 1970 Ampeg v4 to its former glory, i'd juste like any amp guys here to confirm what i'm thinking


-the amp turns on and plays "fine", all tone controls and tonestack work fine and "do something in the amp"
- turning up the bass make the amp sputter a bit, i'm thinking maybe a cold solder somewhere...
-the real problem though, is there's a constant hum (60hz) and cracklepop, not too unlike the static you get from a dirty turntable needle. it pops with all controls turned off and nothing in the input jacks, so it's probably not in the preamp stage. the pops are pretty loud but if i turn the volume up, playing the amp is louder than the noisefloor.


this amp never had a cap job and has the multi-cap cans. i'm thinking first step is to change all the caps in the power supply to ensure there's even current and voltage and also the caps in the bias circuit as well.


when the amp pops, the power tubes glow brightly or even flash a little blue, this seems to tell me that the current is wonky so it goes back to my hypothesis that i need to do a cap job.


i checked the b+ and it's on spec at 525 and the current is around 20-25mA per tube and they seem within tolerance values, those are 7027 with 25w plate max so i can go as high as 30ma to bias them a bit hot at 70%.


with all these values, is it safe to assume the problem is the filtering and the DC current has too much ripple in it? i don't have an oscilloscope to check the ripple so i'm going a bit in the dark.


once a cap job is done on the supply and the bias is done, if i still get the same hum and noise, are we looking at a transformer problem?


Thanks for any help


here's a link to a schematic of my version if it's any help


http://www.ampegv4.com/images/schematics/V4.jpg


thanks guys!!!

[Dmoney]

You probably do need a cap job.

I returned a VT22 to its former glory a while ago which is essentially the same amp. You have to be careful with which multi section can caps to buy, and using ones with metal out parts can leave the OUTSIDE of the can have over 250 volts on it.

I got a selection of caps from http://www.fliptops.net/
I also own an ESR tester which made finding bad caps much easier. One of the contacts on the main reservoir cap in the VT22 had corroded in half and gone intermittent. I also had a failed resistor in the active mid circuit where that part was exposed to a high cathode voltage (a normal voltage in the amp). I also replaced the bias caps. The blue flash probably doesn't mean much, just that the amp is doing some work when that pop happens... which it is... it's amplifying it. If you have access to a tester that can test 7027's that would be a good thing to use. I think Tung Sol or JJ has just reissued the 7027 but I'd double check voltage spec as they probably just changed the pinout on a 6L6GC and labelled it 7027.

[Keven]

Thanks, that's what i was thinking


i'm actually replacing the multi-cans with single capacitors which i'll mount on the inside on terminal strips. i have good faith that once the cap job is done most of the troubles will be gone.


a funny thing! this amp was obviously modded before, the reverb unit has been ripped out, so the driver and recovery tubes go nowhere.


thanks for confirming my doubts!

[Dmoney]

Well at least after a cap job you'll be able to tell if anything else is up.

Shame the reverb is pulled. It sounds pretty good! although it requires a wacky tube. So does the preamp if I recall. The 6K11 triple triode? Ampeg have some interesting designs.

[Keven]

I did the cap job. the pops are gone but there's still a scratchy hiss. definitely comes from the 6k11 i wiggled the tube in the socket and eventually even though the tube was in the socket i got no hiss and a very weak tone. the noise doesn't get louder if i turn the volume up. so i ordered a new 12 pin socket and a 6AC10 tube which is a great low cost sub for the 6k11 (has 3 high gain triodes instead of one mid gain and 2 high gain)


once i change the socket and the tube i think i'll be good. i pulled the reverb tubes (6cg7 and 12ax7) since they really go nowhere at all with the reverb pulled.


i also ordered a new 12dw7 tube and the 12ax7 in there are good.


next step is to change the socket. let's hope it's dead quiet!


any pointers where the hiss comes from if it's not the 6k11? it does it with all the volumes off. can it possibly come from the power tubes? they're quite old but the bias and B+ show up okay on my tester...

[Dmoney]

Not sure about hiss. Could be some faulty resistor somewhere. might be hard to locate. I had trouble with a bad resistor in the cathode assembly of the 6K11 cathode follower stage that drives the active mid control. It had gone very high resistance, probably due to being exposed to the large voltage at the cathode. The dry signal from the amp wasn't really getting anywhere. If i cranked the reverb I got a lot of wet signal and no dry. Kind of sounded cool... but not correct.

[Keven]

Well! it seems like it's alive, now i'm down to really tracking down every problem.


i think i need to rework the lead-dress in there, some wires not really crossing how they should


also, i changed the socket and the 6k11 tube. it works, but i feel like something is fragile in that board, i'll probably replace the caps and resistors just to make sure, but it sure is starting to work ALOT better than when i got it. when i wiggle the 6k11 tube or just tap the board a bit (with a wooden stick!!) there's some popping, but it only appears when i physically want it to, not on its own.


the power tranny is humming a bit too, since all the power caps were changed am i looking at new diodes too?


strange thing, when i work on the amp i work on it vertically as it's heavy to turn around and i have access to both sides of the circuits. i noticed a small liquid leak coming from probably the OT, is that bad? it's not leaking when horizontal and it only leaks when it's been vertical for like a day or so...


thanks for your advice and opinions Dmoney, you were the guy i wanted to talk to about this!

[Keven]

i played with it proper now. it can take a good beating. i ran it with all 4 power tubes, 3 are in the 20mA range and one is in the 40mA range. 40mA is 70% for 7027A's so i'm biased cold and unbalanced. could that be why i get a farty bass? maybe.


after letting it heat up and work at about quarter to half volume playing a fretless bass tuned in B with flats, it starts to pop back on its own, but it's manageable. i wiggle the 6ac10 and with enough effort it comes back again but not for long.


i turned off the amp for about 2 hours. turned it back on again, same thing, after about an hour, there's some popping. i think a cap is failing in the board holding the 6ac10 so i'm going to change all caps. do you suggest resistors too?


here are a few pics of the job thus far in order









the absence of reverb connections near c212

the smaller board holds the 6ac10/6k11

replacing the caps

job mostly done!

[Dmoney]

I think the amp I had was from 1974 are really most of the resistor were ok. I'd double check plate resistors or any resistor with a large voltage across them. If you think a part is loose near a valve then id try to poke the individual part and see if i could make something happen. Check your preamp valve grids for DC voltage maybe to rule out leaky coupling caps. I don't think you have a problem with a rectifier diode but the unbalance in the output valves might be inducing some hum. Might be worth swapping bias supply caps.

I had an advantage when I worked on mine in that I own a peak atlas ESR tester. I have their LCR tester too which is awesome, but I wish it would tell me if a coupling cap is leaky out of circuit.

[Keven]

I'm actually replacing most of the caps in the 6k11 board, just preventive and because i don't have the toolings to positively rule out a faulty cap


also upping the screen grids to 1k instead of 470, it's good practice apparently and i'm putting a new quad of 7581A which are pretty much the best i could find that holds up 530V


i'll check the preamp grids like you said though as that could be the problem, but tapping on parts and caps on the 6k11 board definitely unleashed the fury. also playing the amp for more than 1h definitely makes the problem more present.


i'm waiting for parts now so this amp is probably on the bench for another 2 weeks. easy does it, but it'll be well done!

[Keven]

Alright! another part done


changed the screens and rebuilt the 6k11 board. also completely pulled every component part of the reverb circuit and cut the traces leading to the heaters of those tubes.


i improved the grounding, replaced a cooked resistor. now the amp is dead quiet and free of popping


there's a light hiss and some hum. probably due to the tired power tubes. i also installed a bias pot to make sure i can switch between every tube i want to.


the one thing that's bugging me is that there's some interference between channel 1 and 2. there's no way i can play channel 1 while channel's 2 volume is even lightly raised, i get a deafening hum that makes the amp unplayable. same thing for plugged into 2 while playing into 1.


Now, in the schematic, each channel has its own input stage with a dedicated 12ax7. pulling one of the two shuts down the channel and fixes the problem.


have any idea what i should look at for this interference? lead dressing is one thing i haven't tried with the amp humming, but it's also 7am here XD


it's nearly fixed, and with a guitar there's some glorious overdrive to be had, with a power attenuator, because otherwise HOLY cr@p this is loud....

[Keven]

More updates!


i replaced nearly all the coupling caps between stages, really improved the interference between both channels. there is still some interference so maybe it's the anode resistors too. at this rate it's worth changing them, so much work has been done on the amp it'd be a waste to leave failing components in there that are worth only a couple of cents...


now i really need new power tubes to positively rule them out for the added hum and the overall "pumping" compression i get occasionally.


i pretty much changed every single capacitor in there haha.


it'll be a glorious amp once back to full health, tried it again yesterday swapping the preamp tubes, man that thing gets some sick overdrive. even through a 4x10 it's heavy heavy heavy.


i'll get a quick and nasty clip tonight if possible. :)