Super Reverb Distortion

[RightLurker]

I'm servicing my nephew's '67 silver face Super Reverb (AB763 circuit). In the interest of full disclosure, I'm a newbie/DIY'er/paint by numbers guy, but I'm learning as I go. I've serviced a half dozen amps so far, and things seem to have come out very well.

I replaced all the filter caps and electrolytics, a couple of plate resistors that were out of spec, checked and replaced the resistors as needed in the PI circuit, installed a new JJ GZ34, a matched set of JJ 6L6GCs, and a matched/balanced Phillips JAN 12AT7 phase inverter tube. All the other tubes in the amp seem fine, and substituting known good 12AX7s in the preamp section doesn't change anything.

I installed precision 1-ohm cathode resistors for setting the bias. The 6L6s are reading 452 volts on the plates and I have them running at 40 millivolts (milliamps) on the cathodes.

The amp sounds fantastic at lower volumes, and everything works as it should in both channels.

When I push the amp up to about 4 or 5 on the volume, however, the amp distorts on the lower notes in a way that almost sounds like an old Maestro Fuzztone unit. I don't know if this is what a Super Reverb of that vintage is supposed to sound like or not. It may just be what the speakers sound like when they start to break up. The speakers are the original CTS alnico units.

The only things I haven't checked are the coupling caps, but I don't want to mess with those old, blue molded caps unless they could be contributing to the distortion.

Any comments or suggestions from those who know what they're doing?

Thanks in advance.

[Firestorm]

Assuming it didn't sound like this before the overhaul, the first thing to check would be any component you replaced to be certain it is the correct value (and correct polarity for the electrolytics). What you describe sounds like the amp is underbiased, so next I'd check the new 1-ohm resistors to be sure they are reading accurately. Don't measure across the leads of the resistor: clip one lead to pin 8 and put the other lead on some nearby part of the chassis (like main ground where the PT center tap is soldered to the chassis.) See if you really have 1 ohm.

[RightLurker]

Thanks, Firestorm. I checked the resistors before (and after) they went in, and they are spot on. I am confident that the amp is biased correctly, and just for the heck of it I adjusted the bias to extremes while playing the amp - no improvement. I also played the amp through a 4 ohm, 2 x 12 cabinet and got the same result.

I should mention that this amp was a DISASTER when I got it. The filter caps were a mish-mash of circa-1975 TV-repair shop made in Mongolia junk. Whoever did the work put yellow stickers throughout the amp and noted that he "added extra filtering". He had rewired the rectifier to a solid state contraption, cut the 6.3 filament wires between the PT and pilot light in half, then spliced them back together and covered them with surgical tubing - I can't begin to explain what a mess this thing was. It didn't do much but hum like the dickens when you turned it on, so I have definitely brought the thing back from the dead.

I'm wondering if the main filter caps are too small for the job. As you know, the two main caps in an SR were 70 uf in series. I put in two 100uf Spragues, so I don't think that's the problem. (The other three caps under the filter cap cover were also replaced with Spragues.)

I think perhaps I just don't know what a properly functioning SR is supposed to sound like when it's turned up. I also realize my initial question was sort of stupid - like, "My amp doesn't sound like I think it should. Why?" But I appreciate the thoughts. This forum is great.

[skeezbo]

You've probably thought of this, but I'll just mention it: do you have a blown speaker? A blown 10" will sound pretty buzzy as you jack it up.

Skeezbo

[martin manning]

Is the distortion the same on both channels? If so, then you know it is in the common circuitry, from the PI on out, and you have verified that it's not the speakers. If you had leaky coupling caps that would cause problems at low volume too, so I doubt that is the case. Can you make a voltage table (voltage on each tube pin to ground) and post it? For the PI, measure the voltage across the 470 Ohm cathode resistor instead of pins 2 and 7 to ground.

[Bob-I]

I'm wondering if the main filter caps are too small for the job. As you know, the two main caps in an SR were 70 uf in series. I put in two 100uf Spragues, so I don't think that's the problem. (The other three caps under the filter cap cover were also replaced with Spragues.)

No, a Super should sound fine with the stock filters. If you went back to a tube rectifier then you do not want to up the filters any more, it'll blow the rectifier.

[Firestorm]

How big is the coupling cap going in to the PI? You said the amp came to you a "DISASTER." When amps get molested, that component is often part of the MO.

[RightLurker]

Thanks for all of the suggestions! The problem is in both channels. I will check to voltages and post them later today, as Martin suggests. (Martin was incredibly helpful with some issues I was having some time ago with a '62 Bassman.) Firestorm - the coupling cap going into the PI is the stock component.

I really should have taken some pictures of this amp when I first opened it up. I mentioned that someone had added a bunch of junk filtering caps. In order to get the cover to fit over the additional (and huge) caps, whoever it was built up a frame or wall made of silicone cement and electrical tape (actually I think it was Gorilla tape) and used longer screws to reach the chassis. It was incredible.

Thanks again, everyone.

[Firestorm]

If possible, a few photos of the chassis might help someone spot something. When you take your voltage readings, you might also test all of your circuit grounds, in particular the filter grounds inside the "dog house." (Discharge those puppies first). Since the amp was recapped inappropriately, the gounds might have been disrupted. The screens, PI and preamps share a ground (they shouldn't, but that's another story) and the wiring of the series cap "totem pole" and associated 220k resistors might have been goofed up by prior work.

[Roe]

I've experienced this problem on the reverb channel only. adding a grid resistor on the reverb driver fixed the problem in this case

[jjman]

The bass needs to be turned down to 2-3 when playing loud. If the problem is still there at that point I'd suspect oscillation. I corrected that in both of my (SF) Fenders by changing to shielded grid wires on the output tube(s.)

[RightLurker]

First, thanks for the tips, Firestorm. When I got the amp, I completely removed the mess under the dog house, thoroughly solder-sucked the eyelets, put in Sprague caps, and replaced the resistors as well. I'm fairly certain everything is right in that department, but I'll check it again - you never know, and, like I said, I'm fairly new at this.

I took the voltages. Here goes:

V1 12AX7: Pin 1: 274 V; Pin 2: 0 V; Pin 3: 2.45 V; Pin 6: 289 V; Pin 7: 0 V; Pin 8: 2.52 V.

V2 12AX7: Pin 1: 281 V; Pin 2: 0 V; Pin 3: 7.9 V; Pin 6: 280 V; Pin7: 0 V; Pin 8: 2.49 V.

V3 12AT7: Pin 1: 445 V; Pin 2: .06 V; Pin 3: 7.9 V; Pin 6 445 V; Pin 7: .06 V; Pin 8: 8 V.

V4 12AX7: Pin 1: 279 V; Pin 2: 0 V; Pin 3: 2.24 V; Pin 6: 292 V; Pin 7: .01 V; Pin 8: 2.2 V.

V5 12AX7: Pin 1: fluctuates between 130 and 396 V; Pin 2: fluctuates between -2.6 V and +1.9 V; Pin 3: fluctuates between +1 and +1.8 V; Pin 6: fluctuates between +325 and +391 V; Pin 7: fluctuates between -12 and +16 V; Pin 8: 0 V. (Note: the tremolo isn't working and I haven't started troubleshooting that circuit yet.)

V6 12AT7: Pin 1: 230 V; Pin 2: 69.7 V; Pin 3: 113.1 V; Pin 6: 238 V; Pin 7: 76.7 V; Pin 8: 112.9 V. Per Martin's suggestion, I took readings across the 470 ohm resistor. Reading directly across the resistor (one lead to the other) I got 2 volts. Reading from the tube side of the resistor to ground I got 113 volts; reading from the other side of the resistor to ground I got 111 volts - a difference of 2 volts. Looks right to me.

V7 6L6GC: Pin 3: 455 V; Pin 4: 453 V; Pin 5: -47.8 V; Pin 8: .038 V.

V8 6L6GC: Pin 3: 455 V; Pin 4: 454 V; Pin 5: -47.8V; Pin 8: .034 V.

V9 GZ34: Pin 2: 1 V; Pin 4: fluctuates between -4 and +2 V; Pin 6: fluctuates between -1 and +2.4 V; Pin 8: 457 V. The readings on pins 4 and 6 look ridiculous to me, but I took them numerous times using different meter range settings, with and without alligator clips on the pins, and no matter what, those are the readings I got.

When I was done taking the voltage readings I plugged a guitar in and the amp sounds really terrific and low to medium volume. And finally, I repeat that for all I know a Super Reverb is supposed to have that distortion I'm hearing when it's pushed.

I'll try and locate a digital camera, take some pictures and try and figure out how to make them appear in a post.

Thanks to all again.

[martin manning]

Looks to me like V2 pin 3 is a typo.

The funny readings on the rectifier pins 4 and 6 are because you need to measure AC voltage on those pins- they are the anodes. Pin 2 is the filament, and should have DC voltage similar to pin 8.

On V5, looks like maybe the tremolo oscillator is working, and it just isn't getting to the signal path. Is it switched off? If not turn it off and measure again; you should get stable readings. If you do, then I'd suspect that the opto-bug thingie is bad.

Everything looks pretty reasonable so far, but get the other voltages, and can you get the power supply voltage at each node? That will complete the picture. I've attached an Excel table- easier to read than the text listing n your post ;^)

[RightLurker]

Thanks, Martin. I rechecked the voltage on V2, pin 6, and it's definitely 280 volts. Pin 6 is a plate, correct? Isn't 280 volts in the ballpark?

With regard to the rectifier tube readings, I feel like a dolt. Obviously it rectifies AC to DC. I get 4 ACV on pin 2, 367 ACV on pin 4, and 393 ACV on pin 6.

You were also exactly right about V5, the vibrato tube. I switched the vibrato off, and presto, all the readings settled down and match the schematic numbers almost exactly.

I'd love to take readings at all the nodes, as you suggest, but I don't know how to do that yet - I don't know where the node points are. But I'll figure it out.

Finally, thanks for the spreadsheet, but when I click on the download button it says the Attachment on longer exists (?). There is a voltage spreadsheet on the Hoffman site, but I find it a bit confusing - it may still be in the developmental stage.

I'm really beginning to wonder if the distortion I'm hearing isn't speaker flab or some other speaker disorder. The SR has its 4 original CTS alnicos in it, and the 2 x 12 cabinet is from a 1962 Bassman - two original Oxfords wired for four ohms. The distortion is present through the 2 x 12 cab, but it starts at a slightly higher volume level and doesn't seem as severe as through the 4 x 12 set up in the SR cabinet.

Thank you again!!!

[Firestorm]

Nothing jumped out at me voltage-wise, so I'll go out on a limb. I think the onset of distortion at the 4-5 level is about where the signal gets big enough to drive the output tubes into cutoff. Below that, the output is still essentially running in Class A (at least by one definition) because the tubes are conducting for the whole signal. Above that level, if there's a problem with the PI or the output transformer, you'll get an extreme form of crossover distortion because part of the signal will be missing.

I don't see anything in the voltages to make me suspect the OT (though we're only looking at DC), so I'd suggest you have a look at the PI, specifically the grounded grid side. Particularly, double-check the .1 cap between the 1M and the junction of the 27K, 820R and 100R.