1974 Super Reverb C.S.A Thermal Protector

[Leo_Gnardo]

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?

5AR4 draws 2 amps of heater current. 5U4, 3 amps. With a 5V winding that's 15 extra watts the PT has to deliver.

[Enzo]

The 5U4 might produce a lower voltage on the rectified B+, but the transformer won't care. But the 5U4 heater current is 3A from 5v while the 5AR4 is 1.9A from 5v. of course the original amp was designed for 5U4. But even so 5U4 would make the transformer warmer than the 5AR4.

I don't know how to determine the correct specs.

You look up your target part and see the data sheet. In the case of BYD33V, we find it is a 1400v diode with 1.3A current rating.

Now look at what you have, say 1N4001 or 1N5402. 1N4001 is only rated to 50v. And 1 amp. In this case 1A is probably enough, but 50v doesn;t even come close when we are talking 500v power supply. 1N5402 is in the 3A series, and 1N5402 in oparticular is only rated to 200v. Still way too low. I agree with above that a pair of 1N4007 in series in each spot would work fine.

[mozz]

Some chassis models had the 5ar4 as stock, others used the 5u4gb, there could be different transformers involved.

[Helmholtz]

that's 15 extra watts the PT has to deliver.

You surely meant extra 5 watts.


The 20V lower B+ with the 5U4 do not relieve the PT as long as the total current drawn stays the same. It just means that the 5U4 takes and has to dissipate more power than the 5AR4.

[Leo_Gnardo]

You surely meant extra 5 watts.

You surely are correct! Whoops... In any case it's hardly the sort of burden that would wreck a transformer.

[mozz]

Personally I have seen stock Fender (silverface) transformers that have been damaged by drawing more filament current than designed. El34 pair used in place of 6l6gc, thats 1.2 amps more than original, it shorted out the windings, was only putting out 5v unloaded.

[bobloblaws]

OK, some good info in the replies, but I'm still not sure how to determine whether I can be confident that everything is good to go. After playing through the amp at a very moderate volume ( I live in an apartment building) for 15 minutes or so I found the PT had become quite hot, but not so hot that I had to remove my hand. Even taking the higher filament current from the 5U4 into account, can we at least speculate that it is probably OK or is there still reason to be cautious (and go back to a 5AR4 for starters)?

[nosaj]

OK, some good info in the replies, but I'm still not sure how to determine whether I can be confident that everything is good to go. After playing through the amp at a very moderate volume ( I live in an apartment building) for 15 minutes or so I found the PT had become quite hot, but not so hot that I had to remove my hand. Even taking the higher filament current from the 5U4 into account, can we at least speculate that it is probably OK or is there still reason to be cautious (and go back to a 5AR4 for starters)?

How about at idle? Not playing but just being on for 15 minutes with no playing?
nosaj

[The Dude]

It's not uncommon for transformers to run on the warm side. That said, have you checked bias? Maybe you're idle current is high? What is the amp's total current draw at idle?

[Randall]

Why are we not using the rectifier that was original? Why throw an extra variable in the mix?

[bobloblaws]

How about at idle? Not playing but just being on for 15 minutes with no playing?
nosaj

About the same, maybe slightly less warm.

[bobloblaws]

It's not uncommon for transformers to run on the warm side. That said, have you checked bias? Maybe you're idle current is high? What is the amp's total current draw at idle?

The bias is actually quite low, about 20mA per tube. Unfortunately I don't know how to measure total current draw. Care to enlighten me?

[bobloblaws]

If I'm not mistaken this a 5U4 would have been original to this amp, not a 5AR4. That, and a 5U4 is what we had handy.

[Enzo]

It is normal for many PTs to run hot to the touch. If you can keep your hand on it, it is not too hot.

Mains draw? Use an AC current meter - an ammeter - in series with the mains power. One could use a metered outlet on the bench. Any well equipped shop should have a switchable outlet with a current meter, usually associated with a variac. But one could be easily made with a current meter and some temporary wiring. Most hand meters have AC Current scales these days. Clip such a meter in place of the fuse in the amp fuse holder. Or buy a Kill-A-Watt meter, they are not expensive, and just plug the amp into its outlet.

All these subtle things like which rectifier tube or the bias adjustment can have an effect on the heat of the transformer, but it is a small one. It won't be the difference between an overheating amp or a normal one.

[Leo_Gnardo]

If I'm not mistaken this a 5U4 would have been original to this amp, not a 5AR4. That, and a 5U4 is what we had handy.

Both rectifiers were installed at various times. I've seen far more 5AR4/GZ34 in Super Reverbs than 5U4. You may choose the rectifier that gives you the response you prefer. A solid state substitute for maximum power. Or even a 5Y3 if you want to generate less power and have an overall mellower tone. A couple other octal rectifiers not in common use could also be selected, 5R4 for instance. What's right is what makes your ears happy.