1971 Super Reverb Gut Shots

[alivegy]

I bought this 1971 Super Reverb in Seattle back in August of last year while I was in town for the Pearl Jam shows. It's obviously had some work done over the years, and I believe that one of the original CTS alnico's has been reconed. To me it looks like a workhorse amp that has been worked on over the years but the main tone caps are still original as well as all of the pots.

The pots are a bit scratchy so I'm going to clean them with some pot cleaner if I can. It also has a bit of a low hum when you take it off standby that is persistent at any volume. It came with a modern rectifier tube and JJ 6L6's, but the preamp tubes were all NOS RCA's in the vibrato channel and sylvania's in the normal. I'm probably going to swap the power tubes, rebias and then start rolling new preamp tubes in to see if one of them is causing the hum. I can't imagine someone parting with those tubes if they were in good shape.

Does anyone see anything in the gut shots that they think I should take care or be aware of? I'm currently comparing against the layout to see what may already have been changed.

 

[Blue Strat]

Is there only one photo? I can only find one.

Some components have been replaced...many are the usual suspects for noise or long term heat degradation (the ones on the power tube sockets). What I'm not seeing are the bias supply cap (should be way up top but the photo is cut off) and the power supply filter caps in the "pan" underneath the chassis. Those are all crucial and should have been replaced by now.

 

[Steppin' Wolfe]

Someone has serviced the amp to some extent. The electrolytics are fresh.The screen grid resistors are new.....but I would have used 5 watt resistors. Most if not all of the resistors in both preamps have been replaced.
The bias circuit is original....an addition of a bias voltage adjustment pot to the existing balancing circuit would be advisable.

 

[slider313]

It looks like the amp has been serviced. I would start with checking the power tubes for a mismatch as that would produce hum.

 

[zenas]

Bias looks like it was changed to a blackface style, only one wire to the board and the phase inverter looks like it was converted to the BF style.
The bias cap looks like it could be old, not sure.

 

[alivegy]

Thanks for looking guys, even though my title had a typo. Durp. The image should be a gallery of a few different images that I took.

I compared my amp to a few ab763 and aa270 layouts. It looks to my untrained eye like it has the blackface bias control, not the silverface hum balancer, as well as a blackface phase inverter setup. It also looks like they changed out most of the 100k resistors in the preamp, screen grid resistors, and the filter caps. I'm surprised the previous owner went to the trouble of blackfacing the amp but continued to use a 5u4 for the rectifier.

It has a 3 prong power cord installed but it looks like it was done in such a way that keeps the ground switch minus the death cap. Should I reinstall the 3 power cord the more accepted way by bypassing that switch completely?

Thanks again, everybody.

 

[zenas]

Never felt the need to go with a GZ34 in place of a 5u4 in my 73 SFSR. The GZ34 is more efficient and gives a bit more voltage, so probably more headroom, but by the volume point where you'd notice it you'd be talking to the cops anyway.
Besides good USA 5u4s are dirt cheap or free used and still cheap NOS. Real GZ34s are expensive.
Actually if it wasn't for the extra heater current draw I'd run 5u4s in my blackfaces too.

 

[alivegy]

I play in rehearsal spaces so I can usually get away with cranking the amp. I'll keep the 5u4 for a while and see if I like it.

 

[HotBluePlates]

... It also has a bit of a low hum when you take it off standby that is persistent at any volume. ... new preamp tubes in to see if one of them is causing the hum. ...

As @zenas said, the bias filter cap (shiny blue) looks like it could be an old-style cap, even if not original to the amp. A failed/failing bias cap can inject hum right to the output tube grids. If you can pull the phase inverter tube and still get the low hum, you'll know for sure it's due to the bias supply.

But also make a note of whether your hum rises/falls when you turn the Reverb knob. That's often a source of noise/hum in Fender amps with reverb. Of course, you could also be right about a humming tube.

 

[alivegy]

As @zenas said, the bias filter cap (shiny blue) looks like it could be an old-style cap, even if not original to the amp. A failed/failing bias cap can inject hum right to the output tube grids. If you can pull the phase inverter tube and still get the low hum, you'll know for sure it's due to the bias supply.

But also make a note of whether your hum rises/falls when you turn the Reverb knob. That's often a source of noise/hum in Fender amps with reverb. Of course, you could also be right about a humming tube.

That's a great trouble shooting tip. I'll make sure to pull the phase inverter before I do anything else, that would definitely be the easiest solution.

 

[pdf64]

The hum could be due to mismatched power tubes; hence the point of the bias balance pot, as that would allow idle hum from mismatched tubes to be dialed out

 

[alivegy]

The hum could be due to mismatched power tubes; hence the point of the bias balance pot, as that would allow idle hum from mismatched tubes to be dialed out

I've seen people do a hybrid approach where they add a second pot so that they can set overall bias and then balance the two tubes. Best of both worlds. For now I'll pull the PI to see what happens. I also have a set of Sylvania 6l6 and a couple tung sol's from a 65 bassman that I used to have that I can swap in so that I can see how much of the noise is tube related.

 

[pdf64]

...I'm surprised the previous owner went to the trouble of blackfacing the amp but continued to use a 5u4 for the rectifier...

Consider that the SF’s PT may have been specified in conjunction with the chosen rectifier to achieve the intended HT voltage characteristics, in the same way that the BF’s PT was. So if you want to ‘BF’ the rectifier then perhaps also ‘BF’ the PT.

 

[Philo]

I

Consider that the SF’s PT may have been specified in conjunction with the chosen rectifier to achieve the intended HT voltage characteristics, in the same way that the BF’s PT was. So if you want to ‘BF’ the rectifier then perhaps also ‘BF’ the PT.

I think this is right. When fender went to the 5U4 they also changed the power transformer specs. You could try a Weber 5R4 copper cap rectifier if you wanted to lower the voltage a little more.

 

[alivegy]

I bought a JAN Phillips 5u4 to replace the current new production rectifier tube. It should last forever and I can think of better ways to spend $200. Thanks for all the info guys.

 

[alivegy]

I worked a bit on the amp over the weekend cleaning it up in general, and replacing some bent hardware. I also hosed down all of the pots inside with deoxit d5 which fixed some pops and scratchiness and they are nice and quiet now. I found that the original reverb input cable was a bit frayed and the cable on the pedal of the on/off switch itself had a short that was preventing the reverb from working properly. I replaced the input cable and I'm just not using the pedal until I have a chance to fix it as the reverb defaults to on, and the tremelo defaults to off as is standard on most fender amps.

In the process of doing this I pulled the tubes and put them back in a couple of times, in doing this the hum has gone away and the amp is pretty quite now. I have a feeling the hum may have been coming from the phase inverter tube itself as its likely that it got swapped around with the other 12at7 when I was putting them back in, or another tube in the line for that matter. Other than cleaning out the pots that's really the only thing that changed electronically. The reverb itself adds quite a bit of hiss as you turn it up so I wouldn't be surprised if that 12at7 just needs to be replaced.

Anyway I still need to roll some preamp tubes through it to see if any of the ones it came with are problematic and I have some power tubes and a NOS rectifier to swap in as well. When biasing a super reverb, do you assume a 30 watt plate dissipation of the 6l6, or the 22.5 watts from it being designated a 45 watt amp? I'm guessing the 30 watts, but I just want to be sure as I'm used to biasing el34's.

 

[jcs]

The CTS alnicos in my 71 SFSR had the dreaded issue with the 'cone edge doping' that hardens...the speaker will work but will not sound right and the cone will warp.

Cool amps....mine has a SS rectifier mod so the PT runs very cool and its a real powerhouse but also sounds very good at low volumes...replaced speakers, RCA Blackplate 6L6GC at medium current draw (55%) with different preamp tubes i audition from my stash.

Congrats, killer amps imo!

 

[dani_boy79]

Consider that the SF’s PT may have been specified in conjunction with the chosen rectifier to achieve the intended HT voltage characteristics, in the same way that the BF’s PT was. So if you want to ‘BF’ the rectifier then perhaps also ‘BF’ the PT.

So true. My '76 SF Deluxe Reverb had a GZ34 in the Rec. And the B+ was 458VDC! Even after swapping it to a NOS 5U4GB, the B+ was still around 430ish VDC. Having that high voltage for so many years wore the JJ 6v6s tubes out, lol

 

[terickson]

Congrats on a killer amp! I picked up a 70 last year that has made me re-examine any prejudice I may have had towards “brown blob” caps...
I don’t know about that bias cap, mine was a big silver Mallory.....easy enough to change it if there’s any doubt.

 

[dbts808]

So true. My '76 SF Deluxe Reverb had a GZ34 in the Rec. And the B+ was 458VDC! Even after swapping it to a NOS 5U4GB, the B+ was still around 430ish VDC. Having that high voltage for so many years wore the JJ 6v6s tubes out, lol

JJ 6v6’s are touted as being able to handle up to 500 volts. Other 6v6’s might have trouble with 430 volts but probably not JJ’s.
I have RCA 6L6’s in my Silverface Deluxe Reverb due to the stress its high voltages puts on regular 6v6’s, but I just ordered my first pair of JJ 6v6’s to see how they will cope.