Another problem with my ST-70 build.......Rectifier tube flashed and now dead Right Rear Tube......any ideas?

Hey Guys -

Well, I set the bias OK on the left channel this morning, but just hit a snag setting the RH channel. I put the RH power tubes in and powered up: the rectifier tube flashed, there was a hum and I pulled the power immediately. The fuse had blown, I guess right as I was pulling the power.

Nothing seemed wrong underneath. I thought it might be a bad power tube, so I put in a new fuse and plugged in the power tubes from the LH channel. It powered up fine and I set the bias on the front tube, but the rear one is dead. I know the tube is good and the amp is running: no blown fuse: but the right rear tube doesn't glow and there's no bias voltage.

Any ideas would be MUCH appreciated. I looked again at the solder joints etc and all looks good to me. Everything on the amp is new, even the tube sockets, except the tubes and output trannys. I tested the rectifier tube (NOS RCA) but couldn't check the JJ KT66 tubes.THANKS!!

Where are you?

a) About 30 minutes with a tube tester - the rectifier wants to cook for a bit before testing.
b) About an hour tracing _every_ connection against the schematic.
c) Checking for solder blobs and opportunistic shorts.
d) ESR + LCR meters on all the electrolytic caps.
e) Check for correct voltages at all test points.

Should solve the problem. Maybe 2 hours all-in unless the problem is found in step a).

I expect this is either an intermittently bad rectifier or assembly error. Or, by now, both.

Many thanks Peter - OK this is what I'll do first of all:

1: Go to my friend's house and get the rectifier tube on his tube tester, or borrow a replacement
2: Trace EVERY connection against schematic again
3: Check for solder problems

After this I'll get hold of the voltage test points and check those.I've come too far to give up now!!!!

B

Doctor Hugocat wrote:Many thanks Peter - OK this is what I'll do first of all:

1: Go to my friend's house and get the rectifier tube on his tube tester, or borrow a replacement
2: Trace EVERY connection against schematic again
3: Check for solder problems

After this I'll get hold of the voltage test points and check those.I've come too far to give up now!!!!

B

Bring *all* the tubes and test them while you are at it. On my big Hickok, I give rectifiers 5 minutes before testing as it has a robust filament power-supply. On the Simpson, 10 minutes. Power tubes need not cook so long, bit the typical 6CA7 does need some time on the typical tester. Make sure the line-adjust is at the proper mark!

Good luck with it!

Doctor Hugocat wrote:Hey Guys -

Well, I set the bias OK on the left channel this morning, but just hit a snag setting the RH channel. I put the RH power tubes in and powered up: the rectifier tube flashed, there was a hum and I pulled the power immediately. The fuse had blown, I guess right as I was pulling the power.

Nothing seemed wrong underneath. I thought it might be a bad power tube, so I put in a new fuse and plugged in the power tubes from the LH channel. It powered up fine and I set the bias on the front tube, but the rear one is dead. I know the tube is good and the amp is running: no blown fuse: but the right rear tube doesn't glow and there's no bias voltage.

Any ideas would be MUCH appreciated. I looked again at the solder joints etc and all looks good to me. Everything on the amp is new, even the tube sockets, except the tubes and output trannys. I tested the rectifier tube (NOS RCA) but couldn't check the JJ KT66 tubes.THANKS!!

Get one or two GZ34 ( 5ar4) and replace the broken one. Chances are huge
that this is the part that failed due to overcurrent. Once failed it cannot
be trusted upon.

Yes, it might be a powertube that is the cause, but start with replacing only the GZ34. If a new fails, then additional fault seeking procedures are necessary.

You might as well get GZ34 new production, they are cheaper to consume
and will last approximatly as well as expensive NOS.

Thanks Guys - my friend has agreed to lend me a brand new GZ34 so I'll re-check my work and try that for starters............

Doctor Hugocat wrote:Thanks Guys - my friend has agreed to lend me a brand new GZ34 so I'll re-check my work and try that for starters............

The key here is rechecking your work - even the best techs make mistakes, I try to limit mine to less than one-per-minute, and I consider myself to be a not-bad tech.

Just for giggles, and with no tubes in unit, check the transformer secondaries for the correct voltage

DO check each tube on your friend's tester, of only for shorts and gas. Many forms of shorts do not show up on a cold tube, so a VOM gets the mechanical shorts but not all the operational shorts.

Do also check the voltages at each of the filament pins at each tube socket. Again, just for giggles but sometimes you might discover a hidden fault.

Clean out everything and check all wiring, clearances and solder points. A neighbor down the block who works with jeweler's precision built a guitar amp that was driving him nuts, over several days. He brought me over to have a look. Within 30 seconds, I spotted a solder bridge in one of the 9-pin socket bases. Tiny solder bridge, but as I had fresh eyes, I saw it. He had overlooked it as he was constantly 'at' the amp. So, a 24 hour rest may be a good start.

And, allow all the time you need, be patient with yourself, you will figure it out.

Unless you are running a dual rectifier configuration... If the left channel plays when the right channel outputs tubes are pulled that doesn't sound like a failed rectifier or quad cap.

Have you pulled the rectifier and output tubes then compared voltages between the RR and LR sockets, pin-by-pin?

Have you rigged up a dim bulb?

Thanks guys -

j_beede, I'm hoping to do that today: check ALL the voltage points. Here goes!!

And the problem turned out to be bad power tubes. THANK YOU for all the help, this forum is awesome!! Now for some actual listening