I measured my amp's power output - it doesn't seem right.

[Sonicboom]

Thanks, Chuck. Score one for you. I put the tubes from the SR in and rebiased. A diff. preamp tube killed the bleed through. I still have the clipping, which I'll trace from one end of the ckt. to the other. When I did it the first time, it looked similar on both channels all the way through. Perhaps I missed seeing something.

I don't want to take this down a rat hole, but you may be on to something w/the power supply. I agree with you about the voltages seeming low, especially in the PI, for today's line voltage. I also noticed that occasionally the pilot lamp on the faceplate flickers - only very occasionally and it is not dependent on the input signal or the vol. setting. However, if it is a weak PT, I should be able to see the plate voltage drop with a steady input signal, shouldn't I?

Looking at the signal coming off one of the plates, when it begins to clip (about 5 on the vol), the signal is about 300 vpp. Turning the amp up beyond that simply changes the duty cycle of the wave form from that of a nice sine wave to that of a square wave - like the amp wants to deliver a much higher amplitude signal but can't. Again, all this could be just a red herring. I'll look at it closer and try to post some photos of the trace with voltage, signal levels and vol. control settings noted. I'll also compare it to the SR.

Incidentally, the bias on the Winged C tubes I pulled was 45mA @ 460v, the same as what I've set their replacements to.

[MWJB]

"Incidentally, the bias on the Winged C tubes I pulled was 45mA @ 460v, the same as what I've set their replacements to."

Your amp is underbiased & won't sound like it ought to (you might like the sound, but hte amp will distort early), drop 10-15mA off that idle current, it won't be doing the tubes much good either, Winged C 6L6GC are not able to handle those kinds of dissipations. Dropping the idle current may also bring up your B+ to more typical levels.

The Fender clean W RMS test is conducted with the controls at 50% rotation, your distortion past "5" seems ball park. If using a RMS meter why are you still talking vpp? All the way through this thread you seem to be leaving out impoirtant aspects in terms of information. Take a deep breath, relax, collect your thoughts and list the dc voltages, idle current then VAC as described in the Fender schems.

You keep comparing to the SR, stop. It's a different amp, let's just focus on the amp in question.

Both channels share a cathode resistor at V1/V2 pin 8, this may be the source of some bleed accross channels, there may also simply be some imprint via unconnected wiring (I have seen an amp with the preamp circuitry disconnected from the PI that still made audible sound through the speaker). Some bleed through is not necessarily indicative of a malfunction.

[Chuck H]

Rectifier tubes typically either work or not. But perhaps swap rectifier tubes between the amps also just to see what happens. I do think they're supposed to be the same tube for either amp. Incidentally, if the rectifier tube in your VV is a 5y3 or some such that could be a problem.

If you could, measure actual DC plate voltages at idle, max clean and moderate clipping.

MWJB is right that your biased a bit hot. Your amp will produce the most power at it's ideal bias, which will certainly be cooler than that. But as mentioned, this will change the tone as well.

Have you checked to see what the bias current is on the SR?

If the power supply IS colapsing under the strain that doesn't mean it's the PT. In fact if your PT isn't getting almost too hot to touch it's probably not the PT. There are other components in the power supply. DC measurements throughout all the tubes would be helpful to us as we're trying to do a remote diagnosis.

[Sonicboom]

Again, thanks for all of your comments and help. I do appreciate how challenging it is working on this remotely.

MWJB,

I'll drop the bias back to 35mA as you suggest. Dropping the bias current will, indeed, raise B+ to the 470+ range. As for the VAC measurements and procedures you mentioned, you do realize that this is a 1964 VV? The schems from that era don't have any such AC voltage readings or procedure. Only DC are listed. I mention the SR because both amps are based on the same AB763 ckt. (except for parts of the output section) and are from the same era, meaning that measurements should be in the ballpark. I am comparing for significant differences, not exact matches. If I'm leaving out important info, please let me know what it is. All DC voltages are now within the 20% spec (most 5-10%) listed on the schem. measured with the controls at "5."

Chuck,

I swapped all tubes (including GZ34/5AR4 recto) with the SR - both amps use the exact same tubes. FWIW, the Winged Cs were in the SR, but I swapped them into the VV first to verify I didn't have bad power tubes. In the SR, they were biased in the same general area (45 mA) and had clean headroom to spare. As MWJB says, different amp = different animal. There is no read plating in the VV, and neither the PT nor OT ever gets more than slightly warm.

As requested, I'll go through the entire amp and re-verify/record the DC voltages in the three states mentioned. I'll also measure the DC levels on all tube pins. Stay tuned.

[MWJB]

"As for the VAC measurements and procedures you mentioned, you do realize that this is a 1964 VV? The schems from that era don't have any such AC voltage readings or procedure. Only DC are listed. I mention the SR because both amps are based on the same AB763 ckt."

At Fender.com there are schematics for AB763 amps, with AC voltages (65 Twin, 65 Super Reverb, 65 Deluxe). They probably won't match your amp volt for volt, but should be enough to highlight any serious discrepancies. The original 60's schematics aren't any more useful...in fact if the dc voltages matched the schematics I'd be very worried indeed.

[Sonicboom]

Got it. Thanks. I agree that the originals aren't worth much, but I was hesitant to use one for a RI because I thought they might be too far from the real thing to be helpful. I'll meas. and post the voltages.

[Sonicboom]

Okay, after fixing my bias meter that suddenly died, I measured the AC and DC voltages per the schematic of a SR RI. The ckt. isn't the same; the output section is a little diff. and B+ is lower. Anyway, see the attached file. The biased was set to 35mA at 475v - a bit cold, but good enough to work on the amp.

The DC voltages are in the ballpark, but the AC voltages are off - way off in the PI & output sections. As part of this exercise, I noticed that B+ drops from 475 to 455 over the course of an hour as the amp/PT heats up. The PT never gets too hot to touch, but it's definitely warmer than the OT, which is only slightly warm.

These AC measurements were with a clean signal - vol. at 5. The two sets I'll post next will be at first sign of clipping and full clipping. I can also take photos of wave forms if that is off interest at some point.

Suggestions?

[MarkusBass]

I wonder why you are making measurements with tremolo circuit turned on and speed set to 10.
In simplar amp I was fixing lately (Dual Showman) there was one incorrect valve in the preamp. Someone thought that putting a different type of valve was a good idea but it was not. Have you checked the valves?

Mark

[MWJB]

Pin 8 off the rectifier is 5v lower than the power tube plates?

I'm at a loss as to how your voltage gain at V1 & V2 could be 2.5 times greater than the other BF Fenders at x100, rather than x42/43?!

35mA @ 475 is NOT cold, it's hot...or about as hot as you want to go for "normal" operation & tone. Note the Fender schem specifies 30-35mA.

What happens to yourpower tube idle current & negative voltage at pin 5 as the power tube plate voltage drops to 455v?

Your 20vdc difference betwen V6 pins 1 & 6 should be looked at, I'd replace the PI plate resistors.

Tremolo should be switched off should it not, according to test procedure? Speed control at full CW rotation.

Your AC input signal is 1000Hz? The amp has been serviced recently with good electrolytic caps at preamp cathode bypass, bias circuit & power supply in the dog house?

Even so, the voltage gain from PI plate to power tube plate is only in the region of x2. Power down, pull the power tubes & measure the dc resistance from each plate wire to OT Centre tap. Then apply 0.5VAC to the speaker jack plug (measured), now record the AC generated from the OT primary end wires at the 6L6 plates & also measure AC from each 6L6 plate to centre tap.

[MWJB]

Actually your PI grid voltage is pretty well spot on according to the Fender schem, with 20mVAC at Normal channel input the SR schem shows 2.4VAC pk-pk and you have 2.6VAC pk-pk. So this kind of makes the readings at V1a/V2a plate suspect?

Any chance we can see some pics of the VV? V1/V2 circuit & socket wiring?

[Sonicboom]

I measured the voltages with the tremolo on due to misinterpreting the notes on the schem. I remeasured the DC voltages and found that pin 1 of V5 is 465v unless the tremolo is on; then, it drops to ~250v. Also, pin 7 measures 0 v unless the tremolo is on, in which case it varies with the rate knob from 11v to 19v.

Rect. voltage difference to plate is probably due to the times at which I measured each - B+ may have dropped in the meantime. Remeasuring it, rect. voltage is always 2-3v higher than plate.

Per the attached photos, all wiring/parts on this amp are orig. except the main filter caps (changed a few months ago).

Tubes came from a working/screaming loud '66 SR. All are of the correct type. I'll recheck the preamp tube sockets to ensure that the connections are tight. Power tube sockets are tight.

I'll check the OT as advised. I'll have to improvise on the AC power source. I only have a square wave gen. or a guitar and eBow (my sine wave gen).

EDIT: Don't ask about the blob of white gorilla snot on the cap. It was there when I bought the amp.

[Sonicboom]

The DC resistance of the OT primary is 67 ohms from plate to center and 84 ohms from opposite plate to center - 146 ohms end to end. Secondary is not measurable. What should the primary DC resistance be?

I didn't have much luck measuring the AC voltage on the plates when running a signal into the speaker jack. The low impedance loaded the generator such that I could not get any meaningful measurements. Is there any other way to test this? FWIW, DC resistance from primary to gnd. shows open.

PI plate resistors measure 86.8K and 106K.

I'm hoping to get a break at some point.

[MWJB]

Your difference in OT primary DCRs is worrying. It should be fairly close from one side to the other, not 20% different. I would suspect the OT.

Secondary DCR will probably be half an ohm or so, meter may well default.

Without a AC signal source part substitution (OT) would be the next step...you don't have to use 0.5VAC exactly but you want to keep the voltage fairly low (a few volts) so that you're not in the hundreds of volts on the primary side. Have a crack with the square wave generator, this may still highlight any difference between the primary windings.


"PI plate resistors measure 86.8K and 106K." That's nice...but you still have 20v discrepancy between the 2 plates, probably borderline but it's more usual to see 10 or so volts difference.

[MWJB]

Oh, and those brown card covered Mallory cathode bypass caps need to come out, as does the bias electrolytic if not changed. The VV schem shows a brownface style wiring on the main filters, this isn't really good enough, so a pic of the dog house caps would be good too.

[Sonicboom]

Sometimes the answer you seek the most is the one you least want to hear. Dispensing with the philosophy, the OT is partially cooked - med. to med. well by my guess. Now, a moment of silence for the resale value of my amp...

I cross-cabled the VV OT to the working BFSR - same result, low signal level and output. To verify, I ran the VV in its current state into the SR OT. I measured 45+ watts output. Mercury Magnetics will test the xfmr. for free, so I was thinking about doing that - not that there is much doubt left. I am curious to see what they find though.

Thanks to all who have spent time helping with this. I greatly appreciate your comments and the lessons you've taught me. Now, I'm going to go drown my sorrows in beer.