Fender Pro Reverb Rectifier dead, too much current

[Cheagor]

Dear EEVBloggers,

I'm trying to fix my 73' Fender Pro Reverb. Here's the story:

Got it for little money from a friend, who couldn't fix it. It turned on, but the fuse was blown. Because I suspected a dead rectifier tube, I changed it. During heat up time, all was fine, but as soon as I flipped the switch of the standby, one of the powertubes started glowing violet very strongly, and the rectifier tube heated up very quickly, arced and then died.

I suspect that that the rectifier was somehow shorted to the powertube which resulted in excessive amounts of current draw.

What would your suggestion be on how to find the problem quickly?

Best,
Phil

[alsetalokin4017]

The answer depends a lot on your experience level and your workbench. Do you have at least a variac and a DMM?

"It turned on but the fuse was blown." If the fuse was blown when you got it, how did it "turn on"? Did you bypass the blown fuse somehow in order to get to the point where you blew the new rectifier tube? Or would the rectifier have blown even with an intact fuse of the correct value in place?

There is a general "flowchart" to proceed in these cases but where you enter the process depends on your capabilities, pretty much.

[Edwin G. Pettis]

Sounds like a bad electrolytic cap on the rectifier tube output, something is definitely bad, not necessarily shorted.  With something this old you often need to replace the electrolytic caps especially if they have been sitting for years off.

[Chris56000]

Hi!

This is a classic symptom of a valve with air in it – in this case one of the output (power) valves, which will present almost a dead short across the h.t. supply from the rectifier valve and potentially destroy it!

Throw out the o/p (power) valve that was glowing violet – it is useless – and obtain a new or known good used replacement of the same type, and you'll also need to get a new rectifier valve of the indirectly heated type – most likely 5R4 or GZ34, but if a the original was 5Y3 then you can use 5Z4 to replace it.

I'm assuming the amplifier uses octal (8 pin valves) here!

Obtain a copy of the Circuit Diagram (schematic) and get a good dvm if you've not already got one · a cheap Uni–T will be adequate for valved equipment!

In order to help prevent costly damage to the mains transformer, please connect a 60W incandescent household lamp of your usual mains supply voltage in series with one of mains input leads to the amplifier before you test it again, and also wire one 1N4007 diode in series with each anode of the rectifier valve – to do this, locate the thick high–voltage a.c. h.t. secondary leads from the mains transformer – often red in colour – these will connect to tags 4 (a') and 6 (a") of the rectifier valve. Disconnect these leads from pins 4 and 6 and reconnect them to pins 3 and 5 (these are spare tags), then connect the marked (band) end of each diode to tags 4 and 6 of the original rectifier – the anodes (non–marked ends) of the diodes connect to tags 3 snd 5 on the rectifier holder together with the transformer leads you disconnected from pins 4 and 6. These added diodes protect the mains transformer and reservoir/smoothing capacitors from costly damage if the rectifier valve flashes over – I do NOT recommend using the diodes alone in place of an original rectifier valve, as the original smoothing and decoupling capacitors were rated for the more gradual rise of voltage obtained from a valve rectifier!

I have years of experience of thermionic valved equipment and will be keeping an eye on this thread to offer help if need be!

Chris Williams

[Cheagor]

Very useful answers so far. Thanks a bunch!

It should be mentioned, that the caps don't look like the originals but actually look like modern axial aluminium electrolytics. I checked them and they are fine. (Checked with capacitor meter and curve tracer)

Just for my understanding: What would be the advantage of changing the 5U4GB that is mentioned in the schematic for an indirectly heated one?

[Chris56000]

Hi!

There are two advantages:–

1) With directly heated valves like the 5U4 and 5Y3, they are operative almost at once as soon as filament and anode voltage is applied, which results in a both a very high unloaded h.t. supply voltage and a large surge current into the reservoir and smoothing capacitors, and neither the very high unloaded voltage nor the current are good for old components! The indirectly heated valves like GZ34 and 5Z4 warm up at roughly the same rate as the other valves in the amplifier, and therefore the other valves will start to take h.t. current whilst the rectifier is still warming up – the result is the h.t. voltage rises to rated value much more slowly and smoothly, which reduces the stress on the reservoir/smoothing capacitors and also the mains transformer, greatly prolonging their life.

2) An indirectly heated rectifier valve has a more robust electrode structure, and short circuits between anode and cathode due to electrode sag are much less likely to occur with the modern indirectly–heated type or rectifier valve.

The purple glow you saw in your output valve is caused by the ionisation of the unwanted gas/air atoms that had entered the valve envelope from some sort of unwanted crack, etc. When a gas–filled valve conducts, the current rises to a very high value by means of a mechanism called "gas amplification", in an audio power stage, the d.c. resistances from the h.t.+ line to anode and cathode to chassis–earth are very low, (there are no cathode–bias resistors as the Fender Reverb Pro has    –50V negative G.B. rectified from a tap on the mains–transformer H.T. secondary) so a defective valve with gas in it will draw many times the rated current of the power supply, this is what caused your rectifier to rapidly overheat and fail. You should therefore replace BOTH the o/p and rectifier valves, then feed the amplifier by a lamp limiter as I explained earlier!

Chris Williams

[Cheagor]

Your Advice is godsent! Thanks so much!

[Old Printer]

Check out Uncle Doug on YouTube for a bunch of vids on tube amp theory and repair, particularly Fender. He is a bit of a windy old timer but with a bit a patience you will learn a ton. I use ClipGrab to download them as MP3's and listen to them on my rides to and from work everyday.