Help! I can't find the problem. Squealing!

[rcrmck]

Okay, so I got an old Valco amp on ebay. I went through it trying to put it in working order, and here is where I stand:
1. The model number doesn't come even close to the schematic for that model.
2. Someone recapped it, and who knows what else.
Here's the problem:
When I power it up, I get a couple high-frequency pops followed immediately by aloud squeal (volume or if something is plugged in doesn't matter). Swapped in the preamp and phase splitter tubes and no change. With one pair of the 6l6 tubes on the right, no change. But move those same tubes to the socket pair on the left and it works! With just a pair in the left side, everything seems to work and sound normal, except when I push the volume up almost all the way, and then I get a couple of the same type of high-frequency pops, but no squeal. I tried powering up without going to standby first, with the tubes on just the right side, and I can here the guitar volume build as the tubes warm up, then I get a slow building hum and 'pop-pop-Squeal'!

So I cleaned the sockets and moved all the wires to be sure nothing was touching the wrong place, and got no change. I'm thinking it must be a component somewhere related to the right side pair of 6l6 tube sockets.

What should I check next?

[J M Fahey]

COM - PU - TER - SAYS - NOT - E - NOUGH - DA - TA - BEEP !!

Post actual model, whatever schematic you have, if too different draw your own, some picture, etc.
Otherwise .....

[Chuck H]

Remove the NFB circuit. If that stops the squeal with all tubes in place then swap the OT leads that go to each pair of power tubes.

[Jecarroll7]

It sounds like you may have a couple of things going on with this amp.

The first thing I like to do is test all the tubes in my hickock tube testor just so I know I working with known good set. Be sure to retension the tube sockets so they are in their tight.

Then check all your supply voltage at the filter caps and tubes. Check for DC voltage, then swith to AC and check for ripple. Visually inspect for bad filter caps, etc.

Not very technical, but I'll pass along some good advise I received in a previous post. Check all your plate and grid resistors. Replace/reflow as necessary. Also, get a wooden stick (I use a standard size paint stick for mixing a gallon of paint) and with the amp on and the volume up start tapping all your resistors, caps and anything that might have a cold solder joint. With the exception of a few components around V1 just about everything should be relatively quiet when tapped. If it squeels replace or reflow the solder for that component or wire.

If you're comfortable its not a cold solder joint or burnt/broken/obviously bad component then pump an audio signal through the input (I use my iPod to play guitar tuning pitches) and scope the audio signal from input to V1 all they way through the amp. I usually probe each side of the coupling caps. A clean audio signal should be very smooth and rolling. If its clean on one side of the coupling cap and has sharp spikes on the other when you hear pops and static, etc., you know you're getting close to the problem.

Let us know what you find out, as JM said posting a schematic would be very helpful.

[g1]

The output tube layout is such that the 2 left and 2 right are push-pull pairs? I have seen this in newer amps but am more used to seeing the 2 inner or 2 outer arranged as push-pull pairs. If this is the case then you need to remove the two outer or two inner for proper function.
Otherwise you can ignore this post.

[rcrmck]

Woo Hoo! I got it! It was a bad coupling cap.
With all the mods someone did to that thing, it was tough figuring out what was what. Thanks you guys!

[MikeH]

Can you give a quick synopsis of what trouble shooting method you used to track down the bad cap? That would be very insightful. Thanks!

[rcrmck]

Well, nothing special. After I tried swapping the output transformer, I just started checking resistances from pin 5 working backwards toward the phase splitter. First the good side then the bad, back and forth. Then I got to the caps coming off the splitter, and didn't know how best to check them. So, I swapped them and the symptom changed to no-sound on one side. So I replaced both and sha---bam! it worked!
Thanks again!

[Enzo]

g-one, it matter what you mean by pairs. In a four tube push pull amp there are two tubes on either side of the transformer. We'll call them AA on the one side and BB on the other. What you describe is that you envision two "pairs" of tubes as AxxB and xABx. But the tubes are not really paired that way. There are simply two on a side, so AxBx is the same as AxxB same as any other one from A and one from B. So if you were trying to work on half a set of tubes, pull one of the A and one of the B. Inner or outer, one of each, doesn;t matter.

But when trying to troubleshoot, it is a different story. In troubleshooting, we are not trying to operate the amp reasonably, we are instead trying to find the source of some problem. Isolate the problem. In this case squealing. Now it sounded at first like his OT was out of phase, turned out not, but whatever. If you remove AA, leaving ooBB, or remove BB, leaving AAoo, the amp may not work RIGHT as an amp, but it still works enough to tell us something. His method was telling us something about the one side was causing the problem. By operating only AA or only BB "pair", we eliminate one whole side of the equation.


One of my favorite "poor man's" tube tester procedures is to take one power tube and stick it in one socket of a four banger and see if it works, then move it to the next, then the next, then the fourth socket. That tests teh sockets for function. Then pick one working socket and test each tube individually in it, again for function. That way, u8sing only the amp we have tested all four power tubes as well as the amp power tube circuitry.

[g1]

Thanks Enzo. I realize I did not make my point clear. In the past I have only seen output socket layouts that are AABB. Now I have seen an amp with the sockets laid out ABAB. In this new case, pulling the two left tubes would still leave a proper push-pull pair, to disable one half of the OT would require removal of 1&3 or 2&4. Perhaps this layout is very uncommon. But there are many amps I have not seen so I will no longer assume they are laid out AABB.
Unfortunately it makes online troubleshooting a little trickier unless you know the layout.

[Enzo]

Oh yes, I see some amps with four tubes in a sort of crooked square, I always have to look up the schematic to figure that out then.

Or the PV Classic 50/50 which has eight EL84s, four per channel. two rows of four. Top row/bottom row? Left four/right four? SOme odd interleaves nonsense? Had to look it up, left four/right four. Still don;t know whithin each four if the top two or the left two is one side of the PP.

[Chuck H]

The funny thing is that in any design anomaly someone thought they were pretty clever to set it up the way they did (guilty).

[g1]

Neglected to mention the amp in question, new Traynor YBA-2 & 2A. Now I see the new YBA300 has 12 6L6's, relays (with LED indicator) to disable any pair that has a failure, LED's for each pair to bias.
That's a lot of tubes.