67 Super Reverb ugly distortion

[earache]

Hello all

I’m looking at a 1967 Super Reverb that, when you play it hard or turn it up, it has ugly distortion.

It happens on both channels.

Sending the sound to a different speaker cab has no change.

I’ve cleaned and retensioned all tube sockets.

I’ve swapped in known-good tubes for all the preamp tubes, one at a time,no change.

While turning the Amp on and off during the preamp tube swaps, the rectifier arced and blew the fuse.

I’ve replaced the rectifier with a brand new Tube Doctor 5AR4.

Poked around inside the Amp with a chopstick while it was live, no apparent bad solder joints or broken/burnt components.

I’ve replaced the power tubes with brand new Sovtec 6L6GCs.

Bias of the new power tubes looks good at 37mA and is even at both tubes.

The Amp was serviced in 2011. It had the power caps replaced with a full complement of Spragues.

The bias cap was replaced then as well, and also the resisters on that little board.

The 100K resistors that tended to make that “sizzley bacon frying” sound were also all replaced at that time.

All of the other components are original.

Has anyone else in their experience had this situation?

I have to think that the problem is closer to the output...I haven’t tested the speaker jack very closely but I have cleaned the contacts on it and all jacks and pots

The Amp has been actively gigged since it had the service in 2011. It’s my sons amp so I know it was gigged locally and also been in and out of cars and clubs like 20 times a year.



http://ampwares.com/schematics/super_reverb_aa763.pdf

Thanks in advance for your thoughts

Earache

[Pedro Vecino]

I imagine you have both channels assigned to reverb and vibrato, right? It only has a 220K resistor connected at the PI input.
Measure resistors around this circuit: 82K / 100K / 2 x 1M/ 470R / 22K / 100R ...
An inactive triode in the 12AT7 it could also generate that problem. Measuring voltages on the plates could detect a severe imbalance.
If all is fine you could have a problem in the output transformer.

[Chuck H]

I’m looking at a 1967 Super Reverb that, when you play it hard or turn it up, it has ugly distortion.

I'm going to ask for clarity on "play it hard". Does this mean something like a hot input and/or heavy attack, but the amp isn't turned up loud? And, are we certain this problem never existed before and is not just being discovered now because of new playing circumstances? I'm wondering about a possible parasitic oscillation that may have always existed.

It happens on both channels.

So it's past the mix resistors following the preamps. EDIT: Pedro may have noticed a "reverb on both channels" mod. Please report on this. And on that note, can you tell us if any changes were made to the circuits immediately prior to the problem?

I’ve cleaned and retensioned all tube sockets....

Poked around inside the Amp with a chopstick while it was live, no apparent bad solder joints or broken/burnt components...

I have cleaned the contacts on it and all jacks and pots...

This is all well and good. But just to rule out mechanical causes further you should give the live amp a whack to see if it makes any noises.

Are other performance characteristics unchanged? When not "played hard" the amp sounds no different than it ever has? That is, no additional audio symptoms to report? Volume at setting is the same as before? etc.

If you have a signal generator, can the ugly distortion be recreated with a steady input signal? If so then a scope might be useful for tracing where in the circuit the anomalous distortion starts.

Is the distortion a seemingly mono tonal noise (crackle or fizzle) or does it seem to be sympathetic to frequency? An audio clip might be helpful.

[Helmholtz]

Best start with measuring/verifying tube voltages from the schematic and post results.

[pdf64]

Is the amp fitted with a grounded power cord, that has its earth wire properly bonded to the amp chassis?
The board ecaps are all original; maybe they all tested good when serviced but I'm rather sceptical of that
The power tube 1k5 grid stoppers look original, if so they've been cooked and flexed for many decades now (perhaps thereby developing invisible cracks from those stresses); hence my view is that they can't be considered suitable any longer for their mission critical role (of supplying bias to the power tubes).
The 100 ohm 'longways' resistor at the LTP's 0V return is a popular candidate for developing cracks and bad connections, due to the warping of laughably crap board substrate material used.

Note that the 'fx on both channels' mod used uses the channel's 2nd stage coupling caps in a reactive (rather than resistive) mixer; that results in a bass boost for the 'normal' channel, bass cut for the 'vib' channel, due to the differing cap values of those caps.

[earache]

Additional notes: after the replacement of the rectifier tube I initially turned the Amp on while connected thru a light bulb limiter. The bulb did not glow wildly at first, and it also dimmed as a minute went by, so I’m hoping no shorts in the power supply.

There is a 3 prong plug installed, with the ground lug crimped on and attached to one of the transformer bolts with a star washer underneath.

Chuck H: the onset of the distortion is more with heavy attack. It occurs on both channels, and after turning up past 4 on either volume knob. I have the chassis outside of the Amp presently, and when i bang it on the workbench it doesn’t create the noise.

I will now turn the Amp on again and take some measurements per the instructions I’ve been given. I will also check to see if the reverb on both channels mod is in place

[Helmholtz]

I will now turn the Amp on again and take some measurements per the instructions I’ve been given.

Just to make sure : Voltage measurements should be taken without the bulb limiter. And please also measure actual line voltage at the PT primary as well as heater voltage.

[earache]

All measurements taken with the amp plugged into the wall outlet, not the light bulb limiter

Plate P1 Plate P6
V1 279 V 284 V
V2 274 V 277 V
V3 437 V 436 V
V4 282 V 277 V
V5 *433 V * 386 V
V6 245 V 242 V

*V5 P1 calls for 280V but I measure 433V*

Also V5 P3 wants 2.5V but I see no voltage there
V5 P8 wants 170V but I see no voltage there either

Other oddities

The bias pot calls for -52V but I measure -45.7

Pedro V - resistor measurements follow. Any resistors not mentioned tested in spec (10%).

On the layout drawing, above and between V1 and V2, the two 1500 resistors measure 2000 and 1900

Above V3 the 2200 resistor reads 2600 and the 820 measures 1200

Above V4, left to right, 2700 reads 3200, 820 reads 1100, 820K reads 175K, 470 K reads 175K, 3M3 measures 3M7

Above V6, the 82K reads 93K

Chuck H:
"Are other performance characteristics unchanged? When not "played hard" the amp sounds no different than it ever has? That is, no additional audio symptoms to report? Volume at setting is the same as before? etc." The ability to produce loud volume seems unaffected. The amp sounds really good at 2 on the volume.

"If you have a signal generator, can the ugly distortion be recreated with a steady input signal? If so then a scope might be useful for tracing where in the circuit the anomalous distortion starts." I am going to try to get a signal generator going. I will need some direction on where and how to take the scope measurements.

"Is the distortion a seemingly mono tonal noise (crackle or fizzle) or does it seem to be sympathetic to frequency? An audio clip might be helpful."
Audio clips coming soon, after I return the chassis to the case.

[Chuck H]

Well, vibrato not working might not seem like a related symptom. And maybe it's not. But it's still a performance issue that was unreported. I assume the tremolo doesn't work.

It's odd though that you would have no conduction in either triode of V5 since they're separated circuits. Including their grounds. It's either a remarkable coincidence that both cathode circuits are compromised or that tubes filament is bad.

We had a case here not long ago where someone BF amp was behaving strange (ugly distortion, etc.) and it turned out that the ground connection for the PI and the second vibrato triode was bad. The amp was still working because the two circuits were still grounded through the NFB circuit and OT secondary.

[vintagekiki]

I’m looking at a 1967 Super Reverb that, when you play it hard or turn it up, it has ugly distortion.

It happens on both channels.

http://ampwares.com/schematics/super_reverb_aa763.pdf

Ugly distortion on both channels indicates a fault in the output stage (12AT7 and 6L6GC) and (or) power supply (GZ34 and some elco (20u / 525V).
To begin, replace the GZ34 and both 6L6GC with new tubes and check the resistors in the g2 output tubes (470 Ohm/ 2W)
Adjust the bias to -52VDC or 25mA each 6L6GC tube. If you do not get bias -52VDC check (replace) the parts in the bias circuit, (20u/ 70V, diode, 470 Ohm/ 1W)
Standby switch to on. Measure the voltage before and after GZ34 (360VAC / + 465VDC). If the AC voltage (360VAC) is in the range but the DC voltage (+ 465VDC) is low, replace both elco before the standby switch (20u / 525V), they are most likely to dry or lose value.
Measure the voltages in voltage point B (+ 460V) and D (+ 410V). If the voltage deviates significantly, check the resistors (1k and 4.7k) in the filter circuit.

If all of the above is OK, the speakers are OK, but the sound is still ugly distortion on both channels test the output transformer TR3.
The fastest way to test the output transformer TR3 is to measure the transmission ratio between the primary and the secondary, with known AC voltage at the secondary (5 ... 10 VAC), does it exist symmetric AC voltage between the 6L6GC anodes relative to the mean terminal of the primary of the output transformer.

Edit 190929
Questions
- When FSR has ugly distortion, do you play it without pedals only guitar (clean) or through some pedals (effects).
- After the "general" service in 2011, is it the FSR was put in proper condition and how it worked then.
- When it appeared and what preceded the ugly distortion.

[J M Fahey]

Save a lot of time and SCOPE IT.

[Helmholtz]

V5 P8 wants 170V but I see no voltage there either

Acc. to schematic V5 P8 should be 6.4V. This refers to a 56k cathode resistor. Later versions with a 100k cathode resistor have a cathode voltage of 12V to 17V.

170V at P8 seems impossible (would cause the 25V ecap to explode).

The plate P6 voltage seems normal and indicates that the second tremolo triode is conducting (otherwise plate voltage would be around 460V). But that means that there must be some volts at cathode P8. Could you please repeat this measurement?

[Chuck H]

Actually the schematic (AB763) DOES say 170V. And would seem to be correct from what I've seen in CF type circuits. But it would only go up that high when the tube is conducting and that's dependent on the oscillator. The earlier version schematic (AA763) does say 6.4V and much later schematics (mater volume models) say 12V or 13V. I'm guessing that this a measure taken when the tube is not conducting. And you're right that the 25V cap should pop. I guess I never noticed this anomaly in the circuit before and I must admit that I don't understand it.

EDIT: The AB568 schematic is illegible and the AA1069 and AA270 schematics say 417V for V5 pin 8 !?! Which is, of course impossible as that's higher than the plate voltage. However, it's probably that the voltages (plate vs. cathode) could have been measured under different circumstances of conduction and some sort of peak was measured? It looks wrong to me.

[vintagekiki]

About measurement



When specifying measured values, be based on the original version schematic, not the earlier or later version schematic.
Specifying various measured values from earlier or later version schematic, lead go to nothing and the thread loses its original meaning.

[earache]

Hello all

I apologize - I think I inadvertently posted the wrong schematic. I linked to and I think have been referring to AA763 when it should have been AB763.

https://www.thetubestore.com/lib/the...-Schematic.pdf

I think it’s AB 763 because it has grid stoppers, I haven’t looked too closely at any other details yet.

Vintagekiki - as I wrote in an earlier post, both power tubes and rectifier are now brand new and all measurements taken above were with the new components in place