1969 Deluxe Reverb blowing filter cap

[Murdrak]

Hi, this is my first post here and I really hope you guys can help me. I have very little experience with vintage amps and here in Mexico there is little acces to information and parts. I recently bought a 1969 Silverface Deluxe Reverb with the intention of doing a full restoration. When I got the amp it was working but sounding like crap. It had all of the original tubes except for 1 of the 6v6s and the GZ34 rectifier. When I took it out it became apparent that at some point the PT blew and somebody changed it, also the 2 16uF 450v caps that are before the standby switch were changed for some huge 8uf 600v Vibrolux caps that someone probably got from an old radio. I had all the transformers rewound (the one that was changed before was rewound to specs found at mojotone.com) and bought all new tubes. I couldnt get good caps here in Mexico so I ordered some frome mojotone.com but decided to buy some cheap 22uF 450v caps just to try the amp before the good ones get here from the States. The problem is that when I turn the amp on (still using the old tubes and without the speaker plugged) the rectifier blinks 2 or 3 times and after about 40 seconds one of those caps thats before the standby switch gets really hot and just blows. This happened with 3 different caps, so i can rule out deffective new caps. I measured DC voltage right after the rectifier and it reads 398v. What do you think could be the problem?? Now Im afraid to try the new rectifier tube and also know that if i get those new expensive caps from mojotone (40 dllrs just for shipping) they will blow in 1 minute. What should I do?? Please help!! You think it is a problem with the rectifier? Should i try the new one????

[Jazz P Bass]

#1: if the power tubes are installed, so must be the speaker.
Do not operate the amp without a spaeker.
#2: Are you sure that you have the caps polarity correct?
#3: you may have a faulty rectifier.

[Murdrak]

Hi, thanks for your reply. I am sure the cap's polarity is correct. Is it still important to have the speaker plugged if Im not oppening the standby switch? I just did a AC test and realized that the rectifier is sending almost 100v of AC... Does this mean that the rect is faulty??

[52 Bill]

Yes, the 100 ac volts is causing the cap to blow. Try a new rectifier and a new cap.

[Murdrak]

I took out the power tubes and tried the new rectifier. The DC reding went up a little bit to 415v but Im still getting a AC reading of 95v. Should I try new caps or will they just blow again? Maybe this cheap metal caps can't take as much ripple current as film. How can i measure ac current after the rectifier? I was thinking of getting a new cap and lifring the + sideto measure current (amps)... Would this work??

[Steve Conner]

If you look at the output of a rectifier without any filter capacitor connected to it, you will see quite a large AC voltage. That's perfectly OK.

At this point I would be building a light bulb limiter/dim bulb tester (google it), also checking that I didn't wire the capacitors backwards, and that the entire batch of caps aren't 40 year old "NOS" junk, or Chinese fakes that aren't actually 450V rated.

Test all those things, and we can go from there.

If you want to measure current, yes, you can set your meter to "amps" and interpose it between the rectifier cathode and the "+" terminal of the cap.

[Murdrak]

Thanks Steve I'll get myself one of those light bulb limiters as soon as possible.... I feel really stupid... The caps were inversed in polarity! Now i placed some new ones the right way and they dont blow. Now i get hum comming though the speaker and no signal from the guitar. The volume knob has no effect on the hum, the stanby switch takes it away. When i plug the amp with both the on and stanby switches open it starts quiet and the hum starts comming as the tubes get hot. The DC voltage reading comming out of the rectifier now reads 500v!! And you were right, the AC voltage goes away when the caps are in place (i guess thats what they are for)... I found it strange that when i open the stanby switch the reading goes down from 500v to 175v.. Could this mean that there is something shorted? Bad power tubes? Bad OT?? Thanks!!

[g1]

Sounds like you have no bias. This will cause the output tubes to conduct too much current and drop your B+. If you replaced the cap in the bias circuit it must have it's positive end to ground. Remove the power tubes and check pin 5's for negative DC volts, should be somewhere around -35 to -40 volts.

[Murdrak]

I just took a reading of -29v DC at pin 5 without power tubes. I didn't change the bias cap, you think that could be the problem? The hum at the speaker is exactly the same without power tubes.

[52 Bill]

There is a bias control that will raise or lower the bias voltage to pin 5 on the output tube sockets. Minus 29 volts is a little low. Try measuring the voltage at the bias pot. If it is low on the supply side, the bias filter cap could be bad.

[Enzo]

Here is a tip. Measure that -29vDC, now flip your meter to AC volts and measure the same place. You SHOULD get about zero AC volts, if you get a substantial AC voltage, your bias filter cap is probably dead. And like Bill warned, remember the bias cap needs to have the positive end grounded, because it is a negative supply.

You just found out bad filter caps leave an AC reading on your B+. The same process can happen on the bias supply.

[Murdrak]

Thanks guys this is really helpful. I changed the bias cap and adjusted the bias pot to a reading of -35v. I did this before reading Enzo's advice so I didn't measure for ac voltage whith the old cap. With the new one it goes from 20 to 0v in regular intervals (i guess its normal). This helped alot with the hum but i still get no signal from the guitar and the hum is still not affected by the volume pot. Any more ideas??

[Murdrak]

Changing the bias cap also helped a little with the voltage drop when i open the standby switch but still it drops to 200v

[Steve Conner]

First of all, when you say "open" you mean "close", right? As in, complete the circuit and let current flow.

So, when you close the standby switch, the B+ voltage drops too low. Either the amp circuit is drawing too much current, or the power supply can't provide enough.

Does the B+ voltage drop even when the power tubes are removed? If it does, the OT is most likely shorting to its core, or the tube sockets are burnt, or you miswired something seriously.

If the B+ does not drop when the standby switch is closed with no power tubes, then it's more likely that the power tubes are toast or the bias supply is faulty. Or you destroyed your rectifier tube when you installed the caps backwards.

With the new one it goes from 20 to 0v in regular intervals (i guess its normal)

I'm not sure what you mean, but no, it doesn't sound at all normal. The bias voltage should hold steady, except it may drop slightly when the standby switch is closed.

[Enzo]

You OPEN a valve to let water flow, but you CLOSE a switch to make electric current flow. Darn strange, that.


In the water valve, the path for the water is always there, but the valve blocks the way when closed. IN electrical circuits, we break the circuit path to turn it off. Analogy might be this. A crossing gate at a railroad track doesn;t interrupt or move the road, it just blocks the way. But a drawbridge actually moves the road and breaks the pathway. Electrical switches are like the drawbridge.