1568a filter caps

[1568A]

I was blowing fuses in my 1568a and someone suggested the rectifier tube or filter caps. When I changed the rectifier tube, the amp ran for about five minutes. Then the power seemed to shut off to the RT while the power remained to the output tubes? Can I presume that the filter caps are the next to be changed?

What kind do I need and where can I get them?

Thanks,
Gary

[Richard C.]

Gary,
The only way to troubleshoot this amp is to take voltage readings, and a few resistance readings. Do you have a decent multimeter, and are you able to take these readings. It's got a 400V B+, so it is really going to hurt if you get across it. I downloaded the schematic (thanks to Altecunofficial web site) and can help you look for the problems.
I like to work on tube type equipment, but this thing is heavy and I live south of Atlanta, GA. Shipping would be costly. Unless this has been refurbished lately, I would replace the filter caps: C9, C10, probably C8. Than I would replace the coupling caps: C6, C7, & C11. You need to check T2 secondary AC V.(maybe 800V center tapped). Set the Bias pot (P2) to -30V, and than check both output tube's plate V's. (about 390V). If you get that far, check all the other tube voltages, repair any problems you find and you are about there. The resistors may be out of tolerance, and on, and on. These high voltage filter caps are getting harder to find, but many electronic catalogs have them.

This is what is REALLY involved in troubleshooting this amp. It may sound like a lot, but guessing just won't work. If the filter caps are very leaky (enough to blow a fuse), than chances are the amp needs some tuning up.
Hope this helps,
Richard C.

[airboss]

Hello Again:
Well, ditto the foregoing but......If I read your post right you say that power continued to the power tubes but is absent on the 5U4? Do you mean by that: The fuse didn't blow, The filaments are hot on the 6CA7's. The 5U4 is dark?

If this is the case, it sounds on the face of it like maybe you got a bad (gassy?) 5U4. If it is dark, you will not find the DC voltages that were described in the previous post. The AC voltages should be present though.

I whole hearteldly agree with the full checkout already described, but, in the meantime get a new rectifier tube maybe. Shorted filter caps will most likely cause arcing in the 5U4 as the filaments come up to temp. A shorted 5u4 will cause the fuse to blow. Shorted caps are not likely to cause the filaments of the 5u4 to go dark immediately.

Good luck
Dan Hall
Fairbanks

[Richard C.]

Gary,
If the heater in the 5U4 goes out and the remaining tubes stay glowing, you probably need to work on the 5U4 tube socket. Those very large heaters just don't burn out. If you have heaters, than the power trans is most likely good. A burned out high voltage winding causes...well, smoke and burning. These heavy duty rectifier tubes usually don't arc internally. In your amp, a shorted (very leaky) C10 will cause so much current to flow that the plates in the 5U4 will glow red hot!,and THAN they will get gassy. A rectifier will still work gassy. They will take some punishment! If C9B is shorted, it takes out R19. If C9A is shorted, it takes out R18 & R19. Did you download the schematic from the "unofficialaltec" web site yet? If you need help with this, get a friend that knows tube type troubleshooting (high voltage dangers)
Good luck,
Richard C.

[1568A]

Thanks a lot for the advice! I downloaded the schematics and will get my DMM and take some readings. I will also try another rectifier tube.

Should I try one of my GZ34/5AR4s this time?

I will let you guys know what I find out.

Thanks again!
Gary

[Richard C.]

Gary,
The 5U4GB would be the prefered type. It can handle more current which means greater reliabality. It is better than the 5AR4 or the 5R4. If you have already changed it out, I really doubt that it is the problem. Are all the heaters lighting up? B+ readings? burnt parts?
Let me know...
Richard C.