Bias Circuit Problem

[mikeboone]

My bias tap is getting DC voltage when I turn on my standby switch. The bias voltage jumps to around -150VDC. The normal reference voltage is around -52VDC for this bias circuit.

I checked it with the tap lead out of circuit and it is getting about 440VDC when the SB switch is turned on. It has about 43VAC prior to turning on the SB switch.

This is a brand new MM FBFS-P40 power transformer.

FYI... This is a Single Channel (normal channel) clone of a Super Reverb - AB763 circuit.

There are no variations in the power supply from the original spec.

Any help would be great.

Thanks,
Mike

[tubeswell]

Could just be the start-up surge. Did you ever measure any surge before you replaced the PT?

[mikeboone]

It's actually there all the time. The 43VAC remains steady durning this time as well. I am going to change the rectifier tonight when I get home. I suspect that it is leaking DC voltage back to the AC side somehow.

The MM schematic shows the Bias Tap inside the windings of the B+. Thats the only avenue I can see for that high of a DC voltage being present on the bias tap.

I am by no means an expert and any other suggestions are welcome.

[tubeswell]

Hmmm..., is the bias diode the right way around? (It has to have the banded end pointing back towards the PT winding)

Are any of the bias caps the wrong way round? (I presume they are electrolytic - they need to be reverse biased - +ve end point to ground)

[mikeboone]

This is a straight up transformer replacement. I originally built this amp with a PT that I had in my shop which was spec'd for a lower current and voltage rating (325-0-235 @ 150mA) than the AB763 called for which is 370-0-370 200Ma.

I did it to see if my layout was going to sound and function correctly before spending more money on a PT, which it did. I then purchased a MM Tone Clone Super Reverb PT.

Everything was working prior to this swap. The amp was a little short on volume and the tone was not quite that of a Super Rev, but approaching. So in the interest of science and misery, I attempted to make it BETTER!!!

So, tonight... I looked at everything with a clear head. I took the SB switch out of the circuit. Now my rectifier Pin 8 is wired directly to the first node of the filter cap stages.

I checked the AC across 4 & 6 (red wires) and there is 749VAC, there is 5VAC across 8 & 2 (yellow wires).

With the Bias Tap out of the circuit it has 43VAC and 440VDC present (relative to ground). It makes no sense to me at all. I tried 2 other rectfiers and no change. A solid state and a 5y3. The original is a 5U4G.


Also there is no rectified voltage present at pin 8 on the rectifier

As an added note, the power tubes are not in circuit since there is no bias votage present during this troubleshooting exercise.

I am way confused.

Mikie needs help guys!!!

[tubeswell]

Are you saying you put in a PT with a higher AC voltage on the HT winding?

Is the bias supply tapped from the HT winding? That could be your answer. You might need to change the resistors in the bias supply to get the -ve bias voltage back to the appropriate level

[mikeboone]

That seems reasonable... But the 43VAC rectified and filtered should be around -60VDC which would give me sufficient range to tweak bias. The normal reference voltage in this ciruict is -52

I am a bit concerend that there is no rectified voltage at pin 8 and that there is 440VDC present on my bias tap.

[Amp Kat]

You don't have the yellow CT wire grounded do you ? If so unground it and make sure the red CT is grounded well. Other than that sounds like a wiring problem.

[mikeboone]

BINGO!!!

I forgot to attach the CT lug. I am such a Goob! Just did it - things are rolling along.

Thanks for the extra set of eyes my friend. I'll have to swing by and say hi when I am in BR again.

Thanks,
Mike