Ampeg B-15 voltages off

[Garydean]

I'm working on an early 60's B-15. Pin 8 on the rectifier reads -.8dc. Pin 2 reads 5VAC. Pin 4 296, pin 6 403. All other dc voltage readings in the amp are less <1. Heaters are all correct +AC.

Any ideas on what could be the problem?

Thanks,
Gary

[52 Bill]

Bad rectifier tube?

Pull the tube and measure the ac voltages at pin 4 and pin 6, they should be about equal.
Measure the ac filament voltage across pin 2 and pin 8, there should be about 5vac there.

[Garydean]

Thanks Bill. With the tube out, 4 and 6 reads 296 and 403 respectively. AC across 2 and 8 reads 5.2

thank you

there are 45.6vac coming thru the standby switch.

[52 Bill]

There's a 100 volt difference in the secondary halves of the power transformer. I'd check into that first.

I think the standby switch grounds the center tap on that amp, check it and also the rectifier tube socket for signs of arcing, etc. Take resistance readings of the transformer high voltage secondaries, how far off are the two sides?

You can also try unsoldering the two red ac leads from the board and see what the ac voltages are right out of the transformer, to see if the problem is on the board or in the transformer.

[Garydean]

I measured 56 ohms from one end to the center tap and the other end to center tap is 62 ohms, and 118 ohms from end to end. The secondaries connect directly to the pins. Sounds like a transformer issue?

the amp came in with a bad rectifier tube. Could a failed rectifier take out a PT?

[Garydean]

Really obvious....but, the chassis and the cab need to be connected via the xlr style plug for correct voltages to appear?

[52 Bill]

56 ohms from one end to the center tap and the other end to center tap is 62 ohms, and 118 ohms from end to end
Those resistance readings don't sound that far off, did you check the tube socket itself?

the chassis and the cab need to be connected via the xlr style plug for correct voltages to appear?
Yes, there is a grounding circuit in the speaker cabinet that shuts off the power tubes, so that you can't run the amp without a load.

[Garydean]

Thanks Bill for stickin with me. As I was reading the schematic it occurred to me that the 4 pin plug needs to be plugged in. The standby grounds through the xlr cord going to the speaker.

thanks again.

[J Martin]

The way I would check that in my shop would be to install a solid state rectifier module and bring it up from 0 on a variac that has an ammeter. If it starts drawing excessive current, you have time to shut it down. Its probably a bad filter cap at that point. A good replacement for that 30/600v first cap is a Solen 33/630v poly cap. There is room and its neater than the caps in series solution.

[Garydean]

great info J. Thanks for the pointers.

Gary

[J Martin]

In addition, if you need a new power transformer, I have found the one from Mojo Musical supply to be the best fit. Its pricey, but its potted in the correct size can and slides right in. I have also bought one from Mercury Magnetics which was not potted (they wanted more money for that) and was running overvoltage. They gave me an argument when I complained. Believe me when I say potting one of those yourself is not the way to go. I had transformer wax and tar all over my kitchen last time I did one. Just pay the money.