66 Super Reverb no so super sounding

[booj]

Clean as can be, like it just came out of the shipping box. Of course I told the guy he had to have a filter and bypass re-cap first thing, which he agreed to, so after that we tried it out. Also, we had a 69 Super right beside it to compare to.
Crap! Literally! Sounded like someone micing a crapper incident. the low frequecys at the point of clipping sounded like a combination of nails in a can filled with s^$t.
We tried the 69 SF out. Absolutely smooth and sweet on breakup.
So now the guy is asking me "What's causing that?" innocently with still trustfull eyes.
It had some old, not the same brand output tubes in it so I suggested new outputs first. Now it has TAD long bottles in it. When dI tried it again, it sounded better, but still had a not so sweet break-up sound.
I remembered getting similar sounds from a 65 reissue Twin Reverb at full output. I swapped out the output transformer and fixed it.... So I said "you need a new output transformer"
Now the guy wants to know how much and what kind and whats best, but I say no, no, lets try new coupling caps first and maybe even try another transformer before we decide, so he agrees.
I then changed out the 3 .1uf and 1 1000pf caps in the driver circuit.
That improved the breakup very slightly.
So, I've got the chassis on the bench and I'm comparing the two output transformers. this is a hosed proposition, since I can't connect the feedback resistor to the tranny that is not in the chassis, but I proceed doggedly on.
The difference is minimal and the breakup sound is still not sweet with either tranny.
Anybody got any ideas? I'm out of 'em.

[booj]

Oops, NOT so super sounding

[MWJB]

OK, you may well have checked the "obvious", but you haven't described that you have.

List dc voltages for power tube plates, screens & control grid, list plate & cathode voltages at the PI & preamp tubes.

What's your power tube plate current?

Whether you can hook up the nfb resistor or not isn't relevant, applying a small, measured AC voltage to the OT secondaries will help you determine turns ratio & show any shorted turns or uneven voltages between CT & primary ends.

Have you tried the tubes & speakers in the good sounding amp, in the bad amp to track down any differences there?

[Chuck H]

Do you have a scope? This could be a very helpful tool to see what's happening to waveform at different stages as the amp starts to get ugly. Also...

You may have a conductive circuit board. Those black boards do that and I don't want to count how many I've replaced because voltage present on the board was causing problems like bias shifts in the preamp. Try placing your black probe on the chassis and simply poke the board hard with the red one at different places on the board. Sometimes you can get a read like this and it's a fast diagnosis.

As MWJB said, some voltage measurements will help.

[booj]

I've measured the AC output of the driver stages, the input ac voltages on the power tubes and the plates of the power tubes at full output and they're all within 20% within each reading. tried a new driver tube but no change in sound. With new filter caps, bias cap, coupling caps and good readings on all the resistors as well as the ac voltage readings I'm suspecting no damaged parts.
The fiberboard WTF syndrome, I believe may not apply to this one. I checked a lot of points near the eylets where + voltage was present and got nada. (whew)
Here's a couple of pix to show the test area and the new caps.

[Chuck H]

I believe MWJB was asking about DC readings off the tube pins with the amp at idle, standby in "play" mode. Funny, I try never to say "standby off" or "standby on" because I feel it's too easy to misinterpret.

Good news on the board. I've dealt with it a few times and (whew) is right.

Could still be a leaky coupling cap throwing the bias on a preamp stage.

I don't know what testing AC at the driver and power tubes means exactly? Do you mean that you tested while injecting a signal at the input and then tested the AC voltage gain? DC tube pin readings will tell more about the operating conditions. If the amp is open it won't take long. A DMM, a pencil for notes and three minutes.

[Enzo]

20 volts of clean signal and 20 volts of distorted signal are both 20 volts of signal to a meter.

I also vote for DC voltage readings. Are ALL the power tube plates and screens getting B+? Are ALL grids getting proper bias?

Move back to the phase inverter, are BOTH plates getting B+?

Just because you stick a good tube in a socket doesn;t mean the elements tied to the socket are working.

[booj]

Sorry, yes I checked the DC voltages at all points guys. I guess I was just thinking if the amp puts out full power clean sine wave that the voltages would somehow have to be nominal. They are, but I could have mentioned that.
Where I'm going with this is do you think a new output transformer is looking like an option? I'm thinking now the only way to be absolutely sure is to swap out the tranny from the 69 and see if if fixes the 66.

[Chuck H]

Well, if the amp is making full power and the voltage readings all spec I would say there's nothing wrong with the OT that's in there. Unless it's a replacement unit of questionable merits. If it's stock, as it would seem by the amp description, then I don't think changing the OT will help. Your own tests so far concluded that the amp still has the same problem with the other OT hooked up (granted slightly differently).

It would be good if you could look at whats happening at each stage with a scope.

Did you happen to plug the 66 into the 69's cabinet to see if it's a speaker issue?

I'd still be anxious to see voltage readings from every pin in the amp.

[booj]

Guys I'm getting frustrated. I've posted a lenghly replay twice only to be kicked off the forum and have to re-log in, only to lose my post. I'm going to swap out the two Super output trannys, then go from there. Thanks

[Mandopicker]

Have you tried changing the phase inverter tube? That tube gets a lot of abuse...certainly if its an old or original tube. I'd try everything else, even the preamp tubes, before considering it's a blown OT. They are generally pretty robust.

Just my 2 cents.

[pdf64]

You need to confirm that it's not the speakers - plug the 69 cab to the 66 amp, and the 66 cab to the 69 amp, and see if the weakness follows the cab or the amp.
You might even have some open circuit speakers - as they're in parallel, unless you check, it's not obvious.
Sorry if your report of having done that got zapped as described in post #11 - I've found that if youre writing a long post, it's best to compose it externally in wordpad etc.

[booj]

This time I pre-wrote the response in a text file just in case I get bumped out of the forum and have to repost again!
I have swapped out the speakers, using the 69 speakers with the 66 amp. The 66 has 3 ceramic Fender speakers and 1 alnico. (Your guess is as good as mine!) The 69 has 4 alnico Fender speakers. The 66 sounded noticably better with the 69 speakers but still had the same problem. Obnoxious sounding lows. Not smooth. Kind of gravelly.
I have also swapped out the driver with no noticable effect.
Ok, let me clarify. I have a scope to see the perfect sine wave at maximum output with no crossover distortion. The tubes are idling at about 15 watts each. When I measured the AC voltages, it was with about a 300 HZ signal injected into the input. Then I turned up the volume until the numbers didn't go up any more, so the measurements were all made I'm sure in square wave mode with everything pushed all to hell. Reasoning? I just wanted to see how high the numbers went. I also wanted to see if there was a big difference in the voltages between the two sides. There wasn't.
Everybody's jonesin' for DC measurements so here they are
Driver: Pin 2 is the amp input and pin 7 the feedback input.
Pin 1 - 273.1
Pin 2 - 68.7
Pin 3 and 8 - 113.5
Pin 6 - 258.9
Pin 7 - 74.8

Output closest to the driver:
Pin 3 - 479.3
Pin 4 - 478.3
Pin 5 - (-51.83)

Output next to rectifier
Pin 3 - 477.9
Pin 4 - 476.8
Pin 5 - (-51.79)

The output waveform looks perfectly normal for what its worth at full output and it seems to go into clipping without any issues. It just does not sound good when pushed with a guitar input.
I said in the first post that I fixed a Re-issue '65 Twin Reverb with a similar problem by changing the output transformer. Has anybody had a similar experience?

[MWJB]

Does the problem affect both channels? Can we have the preamp tube voltages?

Your OT can be easily & quickly tested by applying the AC voltage to the secondary as suggested before. Google determining turns ratio.

You seem to want the OT to be the problem, so sub it, only takes 30 mins. But I think that this might be clouding your judgement.

What plate current do you have at each power tube, you have given us the neg voltage & saif 15W/tube, but not indicated if there is a significant mismatch in current between them.

[andyjazz]

I'm working on a Super Reverb right now. It's a 1971. Chief complaint: The vibrato was "clicking." I found a tutorial about this issue and after replacing the opto-coupler I tried some of the things described and got it working fine. I also put in new ceramic tube sockets for the power tubes (The old ones were in bad shape - one was cracked) and replaced a couple of what I thought were noisy filter caps-the same ones indicated on your photo. The result was a tone that was not unpleasant but not that sweet Fender tone you expect from a Super. I have since replaced the old filter caps - the sweet tone is back but I have some distortion going on. There are two .022 200v caps that look like little june bugs, I think they are on the circuit for the driver tube. I believe these are suspect. I don't know if this will help. Anyway these Supers are kind of finicky.