Super Reverb AB763

[lowell]

Hi this amp has a ss rectifier. The owner complains of early breakup and says it wasn't like that before. Not sure what "before" he means but someone else retubed it and possibly recapped it before it made its way to my bench.

In any case I've checked the bias on the power tubes, checked filter caps etc... All checks out just fine - that is the power tubes are all on and plate dissipation is at 14w p/tube. BUT, according to the schematic there should be 460vdc on the plates. I'm only getting around 395vdc. I then pulled the power tubes and it's still 395vdc. I then checked for the specified 340vac on the HT secondary and it's 311vac. I tried checking for excess AC on the first filter cap but I never get a good reading when doing this. Not sure why, my meter always jumps around, and yes my meter is set to ACv when measuring. So I'm think this is the PT. Any other suggestions?

[Enzo]

If there is 311 on the rectifiers, there is nothing a filter cap can do to bring up the remainder.

Monitor what is at the power tranny primary wires. Getting 120VAC there?

If your meter seems unstable, use clip wires to eliminate the possibility of probe position problems.


Pull the power tubes, does any of that come back up? Are heater and bias windings also lower than expected?


Meter the mains current, is there an excessive amount being drawn? Especially with power tubes removed?

[lowell]

Yes ~118vac on primary. I'll try that w/ the clips for the meter. My previously stated readings were with the power tubes out. Good call on monitoring the mains. I always forget that VERY useful test!! I'll check it on Tuesday.

[Chuck H]

I'm a little confused. You DID say that the plate voltage is 395 with the tubes in OR out, right? And that the tubes are dissapating 14 watts at idle?

As Enzo said, do check other PT secondaries.

[lowell]

The heater secondaries are at 6.5vac or 3.25vac to ground. So that seems fine. The bias also seems fine too at ~55vdc after the rectifier diode. The 395vdc is with power tubes in and it's 425vdc without power tubes in - still quite low. Oh and the amp is drawing 325ma AC when monitoring mains current with no power tubes in. 120v * .325ma = 39watts. ??

[Jazz P Bass]

Lowell, put the amp on a 2 ohm dummy load.
Inject a 1K, 100 mv test signal.
Tone controls at 12:00
While monitoring the output with a scope, turn up the volume until the nice clean sine wave starts to flatten.
Measure that voltage.
Square it & divide by 2.
These amps are what, 45 watts into 2 ohms.

[lowell]

I will do this test, however we know its not gonna be 45watts... being that plate voltage is 65v less than spec. AND the customer complains of early breakup.

[g1]

Is the SS rectifier stock, or has it been converted from a tube rectifier? What is the part # on the power transformer? What kind of AC voltage to you get at the HT winding if you disconnect it?
What we expect from an AB763 can be misleading. Some amps labelled as such are not. Some have been modded or have non-stock transformers.
Something has changed in this amp causing it to break up earlier than before. It could be due to the low B+ but it also possible that the B+ has been that way for years (non stock PT etc.).

[Jazz P Bass]

My thoughts at the moment.
Until the output test is done, the OP may be chasing windmills.
I wonder if the owner tried changing guitar cables.
Which leads to "did the owner get another guitar?"
And finally, maybe the speaker is suspect.

[Tom Phillips]

Lowell,
Based on the combined measurements that you have posted it seems that the PT is suspect.
You measured:
118VAC on the primary
311VAC on the HT secondary (I assume that you measured both halves so the HT secondary is 311-0-311. Correct?)
6.5VAC on the heater secondary.
If all these conditions exist at the same time then the PT isn’t right or the HT secondary is being severely loaded down. (In which case I’d expect the PT to be running Hot) The open circuit measurement and the PT part number stamped on the end bell suggested by g one will answer that question.

If we could see good gut photos it may be obvious what is going on here.
A stock AB763 would have a GZ-34 tube rectifier and a PT P/N125PSD with a 360-0-360V HT secondary.

This amp may have had an interesting history and a different chassis may have been swapped into the cab.

From experience I know you can’t always take it at face value when a customer says things like “it wasn't like that before.”
Let us know what you find.
Tom

[lowell]

I just did this test. I'm frustrated w/ the results as they make NO sense. I have the amp on a 4ohm dummy load (closest thing I have to 2ohms). I have a 1k 200mv signal going in, all tone controls up. When scoping the output across the 4ohm load I'm getting 50v p-p. 50^2/4=625watts ?? I'm doing something wrong here but not sure what it is. Is it RMS vs p-p? My scope is on 1v, my probe is on x10, and I have the wave at 5 "squares" p-p.

[Tom Phillips]

I just did this test. I'm frustrated w/ the results as they make NO sense. I have the amp on a 4ohm dummy load (closest thing I have to 2ohms). I have a 1k 200mv signal going in, all tone controls up. When scoping the output across the 4ohm load I'm getting 50v p-p. 50^2/4=625watts ?? I'm doing something wrong here but not sure what it is. Is it RMS vs p-p? My scope is on 1v, my probe is on x10, and I have the wave at 5 "squares" p-p.

To convert peak-to-peak voltage to Vrms you divide by 2.828
Your 50Vpp = 17.68 Vrms which calculates to be 78W into 4 Ohms.
This is still high for an AB763 Super Reverb even if you are driving into hard clipping.
I uspect that your scope may be out of cal or there is some other measurement error.

You could use your AC Voltmeter to measure the output voltage and see if that changes the measurement.
Leave the scope connected in parallel to monitor the output waveform and take the measurement at the max clean output signal level.

Still not sure if you really have an AB763. If it has a stock SS rectifier it may be one of the later 70W versions. Can you post photos? Does the 6L6 plate voltage still measure low?

[lowell]

Quick appology here. This is a Twin Reverb, not a Super Reverb. I misposted the title. I was also working on a Super at the time... my bad. In any case things that shouldn't really change anything we've discussed, aside from wattage.

So I used my meter to get RMS voltage and I got 64watts of clean power outa this thing. It's a non-master volume twin fyi. This still seems low right? Or maybe not I don't know the specifics of clean wattage vs rated wattage on these amps. The power transformer is F022756. Finally IME I've never known of a silver face Twin to have plate volts less than 400v+.

[Tom Phillips]

Now things make more sense.

I have seen more than one circa 1969 SF Twin Reverb that had the exact same problem you describe.
The 022756 PT is stock correct and although it is possible that your secondary voltage is below average for a 022756 PT, yours is not the only one with this problem. Read on…

For comparison, I made the following measurements on a 1969 Twin Reverb with the same problem. The measurements were taken after a tune up which included new power tubes.

All transformers were original Fender stock units.
With 120V line power
PT HV secondary = 315-0-315V
B+ rail = 403VDC
Clean power output = 72W into 4 Ohms

I think that the amp you have pretty much left the Fender factory in the current condition.
There have been other postings discussing the same issue. For example the one at
AMPAGE Archive: 69 twin reverb power transformer question

I have no explanation for the customer's report that it used to sound better but that could be due to many other things besides the low voltage PT.

Cheers,
Tom

[lowell]

Tom thanks very much for taking the time post that. Hmmm... that's weird! In any case mines 10watts less of clean power. Doubt anyone could HEAR that difference though. The amp has new filter caps and power tubes so those are all fine. Not sure what to tell him other than he could replace the PT with one that has a higher voltage HT winding.