6G6 Bassman - Is my transformer dead?

[plumcrazyfx]

Got a 6G6 that was non-functioning.  Had a broken rectifier pin, so I figured that was the issue.  Did a cap job, new dropping resistors, new rectifier socket, etc.  I also installed adjustable bias.  Light bulb limiter indicated no shorts but although getting plate voltage (about 500v) I was getting like 4v on the bias.  Started poking around with the continuity tester on the DVM and the bias winding indicates it is grounded.  Checked under the bias board to make sure there wasn't a connection to the ground wire, no.  Checked where the wire comes out of the transformer to make sure there is no cut in the covering allowing it to touch the bell end, no.  Is my bias wining probably dead?  Transformer replacement the only solution? 

[sluckey]

Disconnect the bias tap from the board and measure AC volts between dangling wire and chassis. What have you?

If the bias winding is dead you can do a simple mod to obtain bias voltage from the HT winding like the Princeton Reverb or several Marshall amps. Here's my adaptation of the Marshall bias circuit into an AB763 amp...

     http://sluckeyamps.com/phoenix/phoenix.pdf

[plumcrazyfx]

I thought about that bias method after posting.  I was fixing this one to sell on since I have a couple other Blonde Bassmans.  I check out the bias voltage while hanging.  I don't know if a prospective buyer looks worse at the bias mod or the replacement transformer.  Looks like Mercury charges 200 for their replacement.

[pdf64]

For the price of changing a resistor and moving a couple of wires, how about getting the amp working? Then you can sell it and let the new owner make the decision.

[PRR]

The bias winding is a fraction of the main winding. It may be under 50 Ohms. The "continuity" function is very likely to call that "continuity". Is there a number? With tubes out, is there any AC Volts on that winding?

[plumcrazyfx]

Thanks for the info, I'll investigate further.  Hope it can be saved, the set of iron dates to the 40th week of 61.

[plumcrazyfx]

Tested the bias winding by itself no tubes in amp- AC volts 55

[tubeswell]

Tested the bias winding by itself no tubes in amp- AC volts 55

So with no bulb limiter and the bias circuit hooked up*, how many VDC have you got on the 6L6 grid pins?


(* first make sure you have the bias diode(s) reverse-biased and the bias filter cap polarity is +ve to ground)

[plumcrazyfx]

I'll check.  Didn't replace original bias diode.

[sluckey]

I suggest you pull the output tubes until you have a healthy negative voltage (approx -56V) on pin 5 of each socket.

[plumcrazyfx]

I know it's been a while so I thought I'd close the loop since I hate when a thread is pointing me in a direction and ends before I find out how the guy fixed the amp.


Finally got back to this after working on 3-4 other amps.  This morning I rechecked all of my work, reheated all of the solder joints in the bias board, replaced the bias diode and, just in case, replaced the 0.047uf coupling caps.  Now idling at 40ma at 450v on the 6L6s.  Sound good; a little hum - which isn't too bad considering I kept the 20uf filters when I recapped it.