need help w/ ampeg V4

[methodofcontrol]

my early 70's V4 makes popping noises. it seems to only happen when the volume is up. but once it starts it will continue intermittently even w/ the volume all the way down! none of the tone controls effects the popping. and i've had this problem for almost 2 years. i've opened up the amp on several occasions to change out componants here and there and try to diagnose the issue.
i've resoldered EVERYTHING on the two main boards hoping to fix a cold joint. and i've tried several known good tubes in all locations.
here's the weird thing:
when i use an attenuator for bedroom levels, i can dime the amp hard for 2 hours and not hear a peep. also, i have an ampeg mastervolume pedal for the back of the amp, and when i use that at bedroom levels i can dime the preamp and not hear a peep either.
does this mean i have a problem in the power section? is it arcing? where should i look? ack!

[Tom Phillips]

I worked on a V4 with a similar problem. It turned out to be the pre-amp tube sockets. Cleaning & re-tensioning helped but did not solve the problem. I finally replaced them. Try gently wiggeling the pre-amp tubes when the amp is doing the popping and see if it gets better, worse or makes the same type of popping sound. In the amp I was working on it was several sockets contributing to the problem.
This is just one idea. Hope it helps.
Tom

[Enzo]

HArd to say. Anything that affects the symptom is a powerful clue. COUld be an arc. Turn out all the room lights and watch the thing carefully for a tiny telltale blue spark somewhere. COuld be something inside a transformer even. If it continues with ALL controls at zero, yes it would seem to be in the power amp or in the power supply.

[Rob Mercure]

MOC:

when i use an attenuator for bedroom levels, i can dime the amp hard for 2 hours and not hear a peep. also, i have an ampeg mastervolume pedal for the back of the amp, and when i use that at bedroom levels i can dime the preamp and not hear a peep either.


I hope I'm misreading this - you're not using a power soak ("attenuator?") to dissipate the 100W+ of a V4 for "bedroom levels" are you? If so, and I assume that I'm wrong, the high plate voltages coupled with either a higher impedance - perhaps at certain frequencies - from the power soak or even a open in that unit can give you some really nice OT spikes that might be arcing.
But I probably am misinterpreting your post.

Rob

[Ray Ivers]

MOC,

I had this problem in an SVT once, it turned out to be one of the filter caps. In your amp, I'd suspect one of the two 40uF/500V caps; maybe with the amp or preamp dimed the voltage drops below the arcing threshold or something... anyway, just a WAG and maybe just something to throw your meter/'scope across and eliminate as a suspect.

Ray

[methodofcontrol]

hey enzo...you've helped me in the past, i know you know what you're talking about, so please follow this thread and chime in when you have some advice! thanks. and "yes". once the noise starts it continues even with all controls turned fully counter clockwise.
as for the "attenuator vs power soak" question from rob, i was using a weber mass. the amp was plugged into the mass and the mass was plugged into the cab. i had the amp dimed and the mass was attenuating nearly full on. if i remember correctly, i was able to speak over the volume i was playing at.
hey ray, regarding your SVT and the filter cap....how would i test one of those caps accurately w/ a multi meter? i don't have an O-scope at my disposal, but a friend owns one if i need it. but that would be a hassle, so if i can get by without, i'd prefer.

[Rob Mercure]

MoC,

Perhaps this isn't the absolutely worst way to use your amp but I really can't think of another! You're dissipating 100 watts plus in the output section and putting maximum stress/heat on the power supply to produce a miniscule audio output. You'd be much "kinder" to your V-4 if you were to use the preamp output to power a much smaller amp and then either remove the output tubes or lift the cathodes. As is, like I stated before, you're creating about the best situation for arcing between the output tube plate/OT primary leads and ground. Whether this is your problem or not I don't know but it really isn't the best way to use your amp IMHO.
But, what the hay, maybe we could make "lemonade" - I've got a V-4 for sale that I rebuilt a few years ago so keep me in mind if you wind up frying you OT!
There are numerous schematics floating around the web for low power amps using 6AK6s, 6G6s, and other small bottles - or a LM386 if you can stand SS "heresy." (and you could power a LM386 with a 9V battery and cobble it up on perf board if you aren't up for a construction project.).

Rob

[methodofcontrol]

thanks for your concerns rob, but i only ever used a weber mass on my V4 just that one time. i plugged it in to the mass so i could check all the voltages in one of my trouble shooting attempts. i borrowed the mass from a friend. i didn't find any issues, so i figured...what the hell!? i'll plug in a cab and start playing until the noise happens. i knew that the noise typically happened at louder volumes, so i cranked the amp and let the mass control the volume in the room because at the time it was near midnight and my neighbors hate my amps enough already. i'm not a fan of attenuators. nothing bothers me more than the players that hoarde 10 vintage 100w plexis and then keep the volumes on 1 and use a pedal anyway. i'm a firm believer in having the amps that you need for your stlye. i feel that the people that own 100w amps and are always looking for a way to make them quiet should really be playing through 15w amps in the first place. but anyway..... sorry for the rant.

[Ray Ivers]

MOC,

I was actually able to hear the popping sound directly from the cap - kind of a faint pap! pap! sound coming from inside the can. Hmmm, finding it with just a meter... it's hard enough with a 'scope as a large section of the B+ rail can be affected, but maybe you'll get lucky if you connect your meter across each cap while the popping is happening, and see if the meter has a harder time 'locking in' on a particular DC voltage reading on one cap than another (also note the reading itself, and compare it with the cap voltage rating - sometimes stock caps are marginal, sometimes they've been replaced with lower-voltage ones, etc.) - you might also try setting the meter on AC volts to look for spiking. FWIW, IMO an amp this old that's exhibiting symptoms of filter-cap failure is a prime candidate for a "shotgun" replacement of all those caps.

I've heard of cases where arcing E-caps "healed" themselves - in which case you'll probably never really trust the amp again - and also the more-likely dead-short (possibly accompanied by catastrophic collateral damage), in which case you'll kick yourself for not changing them out sooner.

Ray

[Enzo]

You can't check a cap for breakdown with a meter. You can test to see if the cap is shorted or something, but what a cap does at a couple volts like your meter presents says very little about what the cap will do when there is 450-500 volts across it. The scope is the tool for looking at the B+ rails.

[methodofcontrol]

hey enzo..
when the noise gets going, it continues even w/ all control turned to zero. and would indicate that the problem is in the power amp or power supply, right? what should i look at/replace first?
assuming i don't have access to a scope. if i wait for my friend and i to get together to use his scope, i could be an old man before then. but if there is a simple test to do w/ a scope that would save me time/money/effort/etc....then i'll start harassing my friend immediately for some assistance. i have to warn you though, i don't know how to use a scope at all. and my friend taught himself howto use it, so he's a novice at best.

[Enzo]

I don't have an order of approach here, I'd be doing what my guts told me were it in front of me. I'd scope the B+ and look for little dropouts in the voltage level. I'd also remain aware that if the power tubes are involved, that impulse represwents a current draw that would also show up on the B+.

I would take a .047 or .1 cap and ground one end. Then on the free end, I'd connect a clip lead. Then with that clip lead I would "ground" the grids of the power tubes to see if it would attenuate any noise signal. I'd yank the phase splitter tube to see of the nopise was coming into the power stage through it.

[Amp Kat]

Since your not exactly sure where the problem is you may want to at least try isolating the problem to either preamp or poweramp. Some great advice from great techs on this thread and have you visually inspected those tube sockets well for burnt arch marks ? You can take the send or better preamp out if present and send it to the front or return to another amp which disconnects your power amp and see if it's coming from the preamp section or the power amp section. If it does it on both them I would think of Ray's suggestion and get the scope out. Much easier to troubleshoot if you know exactly the section where the problem is.

[methodofcontrol]

the problem w/ the amp is just so intermittant it's really hard to nail it. i've had this problem for almost 2 years now, and i've opened up the amp countless times. whenever i'm feeling ambitious about fixing it, i'll turn it on and wait for the noise to happen and of course...it never does. and then when i just want to play...the noise will happen and i won't have time to figure it out.
it's a vicious cycle. i'm just going to have to button down and attack it w/ everything i've got.

[Ray Ivers]

MOC,

If it's a filter-cap problem, it will probably be more likely to happen at high AC line voltages and/or high temperatures; if you had a Variac with a 140V range, you could adjust your AC line voltage to 125VAC or so with the amp on (or use a lightly-loaded AC outlet near the breaker panel), and let it run for a while.

Ray