5F6A build shorting/low voltage on power tubes

[drfunkphd]

So I tried to power up my latest project today - 5F6A Bassman.

I powered up with no power tubes with my light bulb limiter and no problems. As soon as I put power tubes in and powered up, my limiter is glowing bright and I noted a couple things:

- the voltage on the power tubes is low - between 225-230V biased at about 45 ma
- the preamp tube filaments all stop glowing as soon as I switch it from standby
- the pilot light dims when it is switched from standby
- the fuse doesn't blow, but my limiter indicates a short

I double and triple checked all the wiring before I powered up and everything matched the schematic and layout. I did make a couple of small changes to the classic circuit, though:
- made the bias adjustable by adding a 50k pot with 15k resistor in the bias circuit
- I am using a Weber Copper Cap rectifier
- the OT is a Heyboer 2/4/8 ohm multitap with the 8 ohm tap used only, the 2 and 4 ohm taps are not connected (but the wire ends were covered with shrink tubing and tape to prevent any shorting)

I am going to start troubleshooting it, but was hoping maybe someone would have some insight into what potential issues may be present to make the process a little easier. I know there's some really experienced folks on here and any input would be much appreciated.

[drfunkphd]

OK, update on the problem...no bias voltage to V5, but the plate voltage still reads about 275VDC. I pulled the output tubes and checked all my voltages throughout and everything tested fine. As soon as I put them back in, the voltages drop throughout and my limiter is telling me there's probably a short.

Nothing obvious I can see looking from the bias supply through to the power tubes and OT...guess I will drain the caps and replace the 220K resistors between the bias pot and the tubes, replace all the wire and/or resolder all the connections. Hopefully that solves the problem...

[Chuck H]

What is your heater voltage on the 6.3V wind? AC from one lead to the other. Do you have the 5V wind hooked up to a rectifier socket? What is that voltage?

Try disconnecting the PT HV wind and test for voltage. This should be as simple as unplugging the rectifier and testing the PT leads on the rectifier tube pins. No unsoldering needed. What is the voltage (ie:300-0-300)?

The heaters dying on the preamp tubes when the amp is switched off standby is very suspect. The standby is such a simple circuit that I can't imagine a way to wire it so that it disconnects the heaters in any position. This, to me, indicates that something is pulling masses of current (like a short) from the PT when the circuit is engaged (off standby). Since your plate volts are also low it seems that it's affecting both windings. This is strange.

I suspect the PT is shorted internally or you have an error in the wiring.

Is this a kit? Who's? (PM me with this one to avoid publicly naming names)

Chuck

[drfunkphd]

Got it solved. There was a bad solder joint and wire in the bias circuit that was causing some of the problems. The rest was actually an issue with my limiter...which I found by using it on another amp with no known problems.

Looking forward to putting my Bassman through some testing and seeing what she can do.