Issue With Bassman 5F6A Build

[Guitarnut]

Well, I have the speakers wired up and plugged in. I added the remaining tubes and when I hit the switch on the DBT, the light glows full intensity. So, I have a short in one of the gain stages or PI most likely.

Is it safe to add the first preamp tube and power up, then another until I find the stage that's causing the issue?

Peace,
Mark

[Guitarnut]

I just checked voltages in the 3 preamp sockets. The only DC I get is B+ on pins 1 & 6 on the PI. The heaters voltages are good on all 3 sockets. The voltages in the power tube sockets are good as well.

Should there be other measurable voltages in the preamp sockets with only the rectifier installed?

Peace,
Mark

[martin manning]

Have you tried powering it up again with the rectifier and the power tubes? If that still looks good, insert the PI, and so on.

[Guitarnut]

Have you tried powering it up again with the rectifier and the power tubes? If that still looks good, insert the PI, and so on.

Okay, I'll give it a try.

Thanks!

[martin manning]

You should see B+ on all pins 1 and 6. If not then something is open or shorted to ground

[Guitarnut]

Have you tried powering it up again with the rectifier and the power tubes? If that still looks good, insert the PI, and so on.

I just tried the above and it seems to be fine with the PI...the DBT glows slightly but no where near full. Then the 12ax7 the bulb glows just slightly brighter. And with the 12ay7, it's about the same.

I think when I first tried with all tubes I over reacted in switching it off. The DBT does come up full when the tubes are cold but dims down to a glow after about 5 secs.

Bothers me that I didn't meter B+ on the other 2 sockets. Where should I start tracking it down?

Peace,
Mark

[martin manning]

So you have all tubes in and the bulb glowing slightly, but no DC on pins 1 and 6 of V1 or V2? Check the positive end of the 8uF cap on the board and both ends of the 10k 2W next to it, and the junction of the two 100k near V1.

[Guitarnut]

So you have all tubes in and the bulb glowing slightly, but no DC on pins 1 and 6 of V1 or V2? Check the positive end of the 8uF cap on the board and both ends of the 10k 2W next to it, and the junction of the two 100k near V1.

I think the 10K is the issue. I have B+ at the turret with the jumper from the PI direction but nothing at the other side, closest to sockets, of the 10K, + end of the cap or the junction of the 100Ks. Bad R or bad solder joint?

Peace,
Mark

[martin manning]

See if you can measure 10k across the resistor on its leads. If so then try relowing the joints. Be sure to drain the caps first.

[Guitarnut]

See if you can measure 10k across the resistor on its leads. If so then try relowing the joints. Be sure to drain the caps first.

I'm religious about the filter caps.

Thanks for sticking with me and helping me track things down.

It was a bad solder joint. The 10K is fine.

I now have:
440V at the top of the 10K
424V at the bottom and + end of cap
422V at the junction of the 100Ks
423V on pin 6 V2
248V on pin 1 V2...other side of 100K resistor on back of socket.
419V on pin 1 & 6 of V1

I'm going to try with all tubes again.

Peace,
Mark

[JazzGuitarGimp]

If the voltages above were taken with the preamps tubes removed, then there looks to be a problem with the wiring at V2, pin 1.

[Guitarnut]

If the voltages above were taken with the preamps tubes removed, then there looks to be a problem with the wiring at V2, pin 1.

They were taken without the tubes.

It's only connected by a 100K between pin 6 and 1 then jumped to pin 7. I'll double check the R value installed.

[Guitarnut]

I checked for a short between pin 1 & 2 on V2 and it shows continuity. I powered up again and pin 2 shows the same voltage as pin 1. Is this normal or do I have a short somewhere?

Peace,
Mark

[martin manning]

Pin 2 should read zero volts. Definitely a problem there. Look for a solder bridge or strand of wire shorting those two pins.

[Reeltarded]

Close up picture of what is going on in that part of the amp would be good about now! It's simple. You are really close.