1974 Super Reverb C.S.A Thermal Protector

[bobloblaws]

Some time ago I did some work on a friend's 1974 Super Reverb. The other day he told me it stopped working, no pilot light, nothing. We opened it up at his place and I checked continuity and found there was no continuity across the thermal circuit breaker (shown in attached photo) that was between the fuse and the power switch. I pulled up various Super Reverb schematics and didn't see any that showed such a device so I assumed it was not a mandatory item, nor had I come across one in any other Fender amp. We pulled out a soldering iron and I took it out of the circuit. We then slid the chassis back into the cab, hooked the speakers back up and fired it up. Everything seemed to work as normal. We turned it off and then tightened up the hardware and reattached the reverb tank. We turned it on again and got no sound. So now I have it at my house again. I looked through my resources and found the schematic I used the last time I worked on this amp. It does in fact have the "C.S.A Thermal Protector" indicated. The schematic is not a perfect match though as the amp does not have a master volume. Anyway, when I got it on the bench I started checking voltages. Basically I was seeing no output voltage from the 5AR4 rectifier tube that was installed. I checked it on my tube tester and it appears dead. I replaced the 5AR4 with a 5U4 that was handy and now have about 510VDC coming from the rectifier. However, I'm hesitant to run the amp in its current state since I'm not sure if that rectifier dying was just a fluke or because of what I did to the power circuit i.e. removing the C.S.A thermal protector.

So what do you guys think? Was it a mistake to bypass this thermal protector thing? Or should I throw caution to the wind and run with this 5U4 rectifier as is?





cbs_45w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

[The Dude]

C.S.A. is Canadian Standards Association. The amp probably came from Canada. As far as I know, the US amps didn't have that thermal protector (I could certainly be wrong). I wouldn't have a problem with bypassing it.

Edit: I should have looked before I typed. Stupid me. I see you're in Canada. Makes perfect sense then doesn't it?

[Justin Thomas]

At least the thermal protector isn't buried in the transformer... So, the amp worked until you tightened all the hardware, at which point you got no sound? I'd consider that odd... I'd check for looae/broken screws, something out of place, bare/chafed wires on lid of chassis, etc.

I'd also be xhecking filter caps, etc. Blowing rectifier tubes can be an indicator or a cause of a pretty bad situation...

Justin

[The Dude]

My best guess is that the rectifier went south causing the transformer to get hot hence blowing the thermal protector. If you do bypass it, make sure the main fuse is the correct value and keep an eye on the transformer temp for a while.

[g1]

Many of those have been bypassed (maybe most ).
I'm sure bypassing it did not cause any fault. You can probably get a coffee maker thermal fuse of similar temp. if you want, but consider that none of the US models had them, and they used the same transformer.
As The Dude mentioned, see that the PT does not overheat and you should be good to go.

[g1]

You might also consider the protection diode scheme which would probably have prevented this if it was due to a shorted rectifier.
As seen in the re-issue 65 Princeton Reverb.

[bobloblaws]

My best guess is that the rectifier went south causing the transformer to get hot hence blowing the thermal protector.

Possible, but it did work briefly after bypassing the thermal gadget. Maybe the rectifier was in the process of failing and still worked for a few seconds. My pal plugged a guitar in and strummed a few chords, I was preoccupied and I wasn't really paying attention to the quality of the tone at that juncture. Anyway, I will give it a shot and pay attention to the xfrmr temperature as suggested. How exactly would I know that it is overheating? Too hot to touch?

[bobloblaws]

My best guess is that the rectifier went south causing the transformer to get hot hence blowing the thermal protector. If you do bypass it, make sure the main fuse is the correct value and keep an eye on the transformer temp for a while.

As a matter of fact the PT is getting much hotter than the PT in my Twin Reverb. At what point should it be a cause for concern?

[bobloblaws]

You might also consider the protection diode scheme which would probably have prevented this if it was due to a shorted rectifier.
As seen in the re-issue 65 Princeton Reverb.

Would I want to use the same BYD33V diodes as in the Princeton? It doesn't appear to be a common item.

[eschertron]

Would I want to use the same BYD33V diodes as in the Princeton? It doesn't appear to be a common item.

Any diodes that would work as a "stand-alone" SS rectifier for this amp will work. (voltage, current ratings)

[bobloblaws]

I don't know how to determine the correct specs. I have some 1N5402 diodes and some IN4001's. would either of them suffice?

[g1]

The BYD33V are rated for 1400V. Your 1N4001's are probably good enough current wise, but you will have to use many in series on each side to meet the voltage spec.
Two 1N4007 on each side would be better.
As far as the temp. of the transformer goes, some of them run quite hot. Is your heater voltage holding up?

[eschertron]

I don't know how to determine the correct specs. I have some 1N5402 diodes and some IN4001's. would either of them suffice?

You'll want something with a higher reverse-voltage rating; 1N4007s are good to 1000V. You could string up a bunch (4 or 5) of the 1N5402s on each leg, but what a waste! In any case, for protection you need a voltage higher than the secondary peak AC swing. Provide a healthy margin.

edit: simulpost!

[mozz]

By using the 5u4gb you are making the power transformer even run hotter than it was when using the 5ar4.

[bobloblaws]

By using the 5u4gb you are making the power transformer even run hotter than it was when using the 5ar4.

Oh, I was under the understanding the 5U4 ran about 20V lower than the 5AR4. Or is that something different altogether? Or would that be the reason why the PT would run hotter?