2 x el84 amp problems

[HBamps]

Hey All,
I built a 2xel84 amp 10 years ago and have gigged it, recorded several albums with it and just have loved it all along. It has become a big part of my sound. problem is, lately, it's been sounding pretty crappy when cranked. Still sounds good clean but when pushed it looses all it's sweetness. It as if it can't handle the signal and just kind of craps out where it used to sound smooth and compress in this great way before.

I would describe the overdrive as buzzy, raspy, fizzy and also has a unforgiving feel, like the compression time constant has decreased dramatically.

All the voltages, at idle look good, no components look burnt or bad. I am stuck and don't know where to look. I think I am crazy or has my ear changed that much that I now think it sounds bad???

Amp basically is EF86 -> tone control -> Vol -> no grid R -> LTPI -> Cut control -> cathode biased EL84s (180R). Plate voltage around 310V.

Could the OT be failing? Screen resistors drifting under load? beginning of filter cap failure?

Any advice on where to look? I am stumped.

Thanks!

[ChopSauce]

& what about tubes?

Really, I don't know but I read so often about tube failure, not so much about drifting transformers -

(just my 2 pico-cents)

[HBamps]

& what about tubes?

Really, I don't know but I read so often about tube failure, not so much about drifting transformers -

(just my 2 pico-cents)

I tried all new tubes, well almost all of them. I didn't swap the PI and crank it yet. I'll do that and report back.

Thanks!

[ChopSauce]

Might be caps too, then(?)

[Chuck H]

What is the nature of the build? Is it a modern type build with metal film resistors and PVC jacket wire? That is, made from the best, good stuff? Or is it a "vintage" style build with carbon comp resistors, paper in oil nos caps, etc. If it's using carbon comps there may be one or more drifted resistors that need to be weeded out. NOS caps are always more suspect of going leaky. If it's a modern build there still may be a leaky cap issue, but it's more rare with newer film caps. Still something to check for.

Otherwise, checking the bias is the first order of business. Since the amp sounded good before, and you're very familiar with it, it's easy to take you at your word that something is wrong. A schematic would help us understand the nature of the build. That can only help for future trouble shooting on the thread. Please provide one.

Since it's a 2Xel84 (and I have no schematic) I'll suppose it's cathode biased. So the bias is probably ok unless that resistor has drifted or that bypass cap has gone bad. So, since you tried new tubes and don't see anything else expressly wrong, did you try different speaker/s? Perhaps there is no problem with the amp and it's the speaker.

Otherwise we're down to opening it up and testing voltages, etc.

[HBamps]

What is the nature of the build? Is it a modern type build with metal film resistors and PVC jacket wire? That is, made from the best, good stuff? Or is it a "vintage" style build with carbon comp resistors, paper in oil nos caps, etc. If it's using carbon comps there may be one or more drifted resistors that need to be weeded out. NOS caps are always more suspect of going leaky. If it's a modern build there still may be a leaky cap issue, but it's more rare with newer film caps. Still something to check for.

Otherwise, checking the bias is the first order of business. Since the amp sounded good before, and you're very familiar with it, it's easy to take you at your word that something is wrong. A schematic would help us understand the nature of the build. That can only help for future trouble shooting on the thread. Please provide one.

Since it's a 2Xel84 (and I have no schematic) I'll suppose it's cathode biased. So the bias is probably ok unless that resistor has drifted or that bypass cap has gone bad. So, since you tried new tubes and don't see anything else expressly wrong, did you try different speaker/s? Perhaps there is no problem with the amp and it's the speaker.

Otherwise we're down to opening it up and testing voltages, etc.

Thanks for the thoughts. Trying to figure out how to post schem... but in the mean time.
As for the build, it's mostly carbon film resistors with a few carbon comps (grids and plates), metal oxide(?) for PS. Yes, it's a cathode biased pair of el84s (180R / 220u cathode).

Since an amp I built for a friend around the same time ended up back with me for repair for a couple blown filter caps, I just decided to replace them all in this amp. Many might consider this amp under-filtered but I think it sounds good that way. While I was in there, I added a 100k grid resistor on the PI input.

I tried that out today and the tone is much improved! The overdrive is much more held together and doesn't sound 'broken'. Even the clean tone is inspiring again, that voxy chime is back. At this point I don't know if it was the caps or the grid resistor but I suspect it was the caps. The original one were relatively cheap ones.

It's my first experience with this: aging components slowly making an amp sound like crap but can be measured at idle. weird but glad I found it (at least I think I did).

Thanks for the help!

[Chuck H]

The "aging components" in that amp would be the filter caps and tubes. It's possible that CC resistors in high voltage or current applications could drift and should be checked for that. It's also possible that a cap could become leaky and skew the bias on a stage. This could also be checked for.

But you have already made changes to the fundamental circuit and reported satisfaction with the results. Since you were happy with the original circuit and observed it as failing this confuses me. Why not fix the original circuit. There isn't much to go wrong with it.

If what you want is to repair a beloved circuit and you have a clear bead (and intention) on what that means, that is one effort. If you are simply no longer satisfied by the original circuit and chose this time of it's aging to make random changes and choose new preferences, that is an entirely different matter. Just so you know, we here on the forum can get behind either. But it's VERY important that YOU are clear about it. Gong forward will be a lot easier if we know that we are attempting a repair, or attempting a refurb and upgrade. In other words... Making the amp sound like it did is one task. Making it sound like the ideal in your mind now is another. We CAN do either, but you need to be lucid on the matter or it becomes cumbersome.

[HBamps]

The "aging components" in that amp would be the filter caps and tubes. It's possible that CC resistors in high voltage or current applications could drift and should be checked for that. It's also possible that a cap could become leaky and skew the bias on a stage. This could also be checked for.

But you have already made changes to the fundamental circuit and reported satisfaction with the results. Since you were happy with the original circuit and observed it as failing this confuses me. Why not fix the original circuit. There isn't much to go wrong with it.

If what you want is to repair a beloved circuit and you have a clear bead (and intention) on what that means, that is one effort. If you are simply no longer satisfied by the original circuit and chose this time of it's aging to make random changes and choose new preferences, that is an entirely different matter. Just so you know, we here on the forum can get behind either. But it's VERY important that YOU are clear about it. Gong forward will be a lot easier if we know that we are attempting a repair, or attempting a refurb and upgrade. In other words... Making the amp sound like it did is one task. Making it sound like the ideal in your mind now is another. We CAN do either, but you need to be lucid on the matter or it becomes cumbersome.

Thanks Chuck, I get it. And yes, I just want to bring it back to the amp I have loved for years, but reading threads got me curious and I think some of you know how that goes.

[Chuck H]

To put one inquiry to rest, OT's don't fail gradually. They work as designed or they are broken. So that's either the issue, or not. And it can be tested. Easily. If replacing tubes didn't help (you did check the bias on the new tubes?) then there must be a problem with the circuit or the speaker/s. As for the circuit, I guess it's pretty simple. So if voltages check out (I mean REALLY check out) then I suppose you have to consider the speaker/s.

I will mention that ef86's, NOS or new, have a rep for inconsistency. It may serve you to try another over and above any replacement.

Also, ten years is a long time for some modern caps. Old ones have been known to be of lesser quality, but the truth is that most old amps worked fine for a very reasonable life of over twenty years. Some amps are sporting filters over sixty years old! "I" would never trust a cap that old, but enough old timers here have seen it work without issue and I can't ignore that. OTOH some newer caps fail in a relatively short time. Even a few extra ohms of impedance and a 20%uf shift could (should) change the tone noticeably. So, I would look to the HV filters for obvious signs like bulging seals and even test their ESR if possible, try yet another ef86, look for drifted carbon comps, check for leaky caps and make double sure the speaker still sounds like it should. For starters. If none of that pans out I would post voltages here.

[TimmyP1955]

I'd pull that 100k out just to see what happens. If it sounds worse, double check for leaking coupling caps. That resistor could reduce a leakage voltage enough to make a difference to the PI's performance.

[Chuck H]

Yeah, and that^^^^^^^^^^^^

IMHE it's a big no no to do mods or shotgun repairs whe trying to "fix" a problem because it often confuses the matter. The odds that a mod or blanket component replacement will fix an actual problem are slim. What your left with is an amp that is no longer original and harder to diagnose because of it. Not to mention the possibility of a bad, new part going in (which happens), a miswire or a cold solder joint from the new work confounding repairs to what is no longer a known circuit!?! Always fix the amp first. Then change it if you want. Never change a broken amp and then try to fix it. That actually seemed obtuse just writing it. But A LOT of people seem to do it.