Amp only has sound when caps are draining

[blueamberol]

I see something scary. There's a metal washer on the left-most screw for the circuit board that looks like it's about 1mm away from the HT connection. That's going to want to arc.

Thanks, I will locate a smaller washer!

[Phil_S]

Most big box home improvement stores, Ace, True Value, etc., carry bins of small parts. Look for a nylon washer. If not try the plumbing department and look for nylon, plastic, or rubber.

[blueamberol]

I'd like to see a couple of shots of the PT end of the amp showing all the PT leads and where they go. The bias supply looks odd: where there should be a reversed diode, there appears to be a 1/2W resistor (and a toasted one at that). The PT wire going to the 1W 470R (I think) looks red/yellow to me, rather than red/blue. If red/yellow, that is the center tap. Can you make out the colors?

Could this have been deliberate? If that is the center tap and the bias diode was eliminated, this might have been an attempt at back biasing.

So I found info about the clipped center tap. Apparently it is a practice to eliminate the center taps and place two 100ohm resisters off of the pilot/heaters to ground. This is exactly what has been done.

The wire colors seem to have faded to a point where they are really hard to determine, but based on the above, it would appear that it is the correct bias tap wire.

[ampgeek]

That applies ONLY to the filament CT connection.

The HV CT should remain connected to ground.

Good luck,
Dave O.

[blueamberol]

the resiter on the bias board is brand new (470-2W) specs only call for 1W. All of the diodes have been tested and are functioning correctly.

I may just go out out on a limb and drop the $$ for a new transformer.

Seems like it's a pretty strong limb at this point.

Does anyone have opinions about the 100ohm reststors off of the pilot/heater thing?

[blueamberol]

That applies ONLY to the filament CT connection.

The HV CT should remain connected to ground.

Good luck,
Dave O.

OK, so they cut two wires off, but only should have removed the filament CT?

[ampgeek]

The 100 ohm gig is commonly used for filament secondary windings that don't have a center tap.

I have found no benefit to that over simply grounding the filament winding CT in cases where a CT is available.

They can be used beneficially in instances where elevated voltage heater supplies (e.g., voltage divider off of the B+ supply) are implemented in both CT and non-CT filament secondaries configurations.

Note that you should never use both the filament supply CT and the 100 ohm referencing approach. It is one...or the other.

Good luck,
Dave O.

[ampgeek]

I am not clear on what the "second wire" was connected to before being cut from the grounding point.

However, from what I can ferret out of all of this, it sure seems like the HV seconday CT needs to be connected to ground.

Hoping that someone else who is following this a little more closely will confirm that suspicion.

Cheers,
Dave O.

[blueamberol]

`ok, so i'm going to replace the PT, and while i'm at it, i want to rebuild the bias/rectifier board and replace the bias pot. the only thing that isn't listed on the schematics, is the diode type. I've seen some blackface twin reverb clones that use a 1A 1000V diode, but I can't find anything referring to the bandmaster. looks like options are 1000v 800v 600v...Thoughts?

[eniam rognab]

use a 1A 1000V diode, but I can't find anything referring to the bandmaster. looks like options are 1000v 800v 600v...Thoughts?

1N4007: http://www.diodes.com/datasheets/ds28002.pdf
for $0.02 here: http://www.taydaelectronics.com/1n4007- ... diode.html spend a buck and get 50 of em

UF4007: http://www.fairchildsemi.com/ds/UF/UF4007.pdf

[gui_tarzan]

If you have a RadioShack nearby you can get several 1N4007 diodes in a pack for a couple bucks. Ordering them online means you have to pay shipping.

Replace all seven of the diodes with fresh ones. If that banded component is actually a diode and not an old resistor it looks like it's much older than the amp. I'd replace it just to be safe.

[gui_tarzan]

Does anyone have opinions about the 100ohm reststors off of the pilot/heater thing?

Yeah, put the right transformer in there and forget about the faux center tap.

[sluckey]

OK.

[blueamberol]

Hey everybody!

I replaced the power transformer with a Classictone 40-18005, replaced all the components on the rectifier board, and some of the resisters that were far off spec, and it's working great. I'm using the taps to ground as opposed to the 100ohm resister thing. I also created a ground hub on a separate chassis hole instead of the transformer lug like it was using before. Now I'm off to look for some nice NOS tubes for it.

Thank you to everyone who took the time to help with this!

[martin manning]

Great, and bonus points for not using the transformer bolts for grounds!