Fried BF Bassman Heater CT!

[Justin Thomas]

Hi everyone,

I've been a lurker here for years and have learned tons, but don't post too often.

I've cooked my 67 Bassman's (AB165) heater CT while working on the rectifiers the other night. After getting the board back in (tight fit) I turned the amp on and the smoke came out. Powered down, and noticed that the Heater CT was melted. (I don't think this is the first time this has happened, as the CT wire was degraded when I bought the amp, so I taped it up.) Upon inspection, the other windings were okay but the CT wire broke right off.

I checked the chassis heater wiring for shorts, after disconnecting the PT windings; found that one of the pilot light's tabs had spun around and grounded itself on the bracket holding it. Moved that tab back, and no more shorts. I've ohmed out the windings on the PT and pulled the bells off; everything seems okay besides the obvious...

I found this by R.G.:
"What often kills power transformers is a low-voltage fault. This produces a huge current but the change in primary current is not large enough to blow the primary fuse. The local current does burn out the wires it's on, though. A shorted tube heater, or a shorted filament wire to chassis or the other side of the heater winding will do this neatly."

I can check the rest of the PT for voltages; if everything else seems okay and since I have to have this thing rewound/replaced anyway, should I try usng 100-ohm resistors first? Also, has anyone replaced one of those pilot light brackets, and are they all "adjustable?" R.G. also had some advice on preventing future incidents that I'll be using...

Thanks in advance,

Justin Thomas

[Tom Phillips]

...should I try usng 100-ohm resistors first?

That's what I do after I inspect the PT and insulate the CT stub that's left over.

Also, has anyone replaced one of those pilot light brackets, and are they all "adjustable?" R.G. also had some advice on preventing future incidents that I'll be using...

I don't replace the original style. Many other bulb holders do not fit behind the slanted front panel. At least not the ones that use nice glass jewel lenses. I don't know what RG suggested but I solder the inner contact to the body of the bulb holder to prevent rotation. Then I bend the rear tab toward the back so that it won't short out if it should rotate at a later date.

Cheers,
Tom

[Justin Thomas]

Hey Tom,

Thanks for the reply. I'll be firing up the amp as soon as I get a chance, may not be till the weekend. I'll keep everyone posted.

R.G. was talking about fusing the secondary leads so the tranny doesn't cook itself totally to death before the mains fuse blows. I'll try out the solder trick you mentioned; would putting a dab of hot- or super-glue on the fiber insulators to help stiffen them help, or is that stuff conductive? Worst case, if the PT needs to be rewound/replaced, I'm not too worried about originality/collectability; it's had a few holes drilled in the chassis already (NFB switch - noise switch). I just really love the sound of this amp - thick and really sweet, would be great for slide, even if it's not a highly regarded circuit.

Justin

[Tom Phillips]

...would putting a dab of hot- or super-glue on the fiber insulators to help stiffen them help, or is that stuff conductive?

I don't think that the hot glue is conductive but I don't think it will hold in that location because of the heat and the small contact area.

Fuses won't hurt. The amps sold in Canada use fuses in every secondary line IIRC.
The PT often survives when the heater secondary CT melts because the CT lead is usually a lighter gauge wire than the other heater leads. The wire essentially acts as a fuse. Not the best thing to happen but at least it doesn't usually destroy the transformer.

[Justin Thomas]

Update!

Just wanted to check in and say I got the heaters fixed, everything seems okay after about two hours of playing time last night. Thanks to Tom in particular and the Forum at large for all the help. Now I've just got to find out why the Bass channel passes a very small amount of signal even with the volume and tone knobs down, and why the channels won't jump. It seems to be wired pretty stock, no funny business...

Justin Thomas

[Tom Phillips]

Justin,
Glad to hear that the PT is OK.

... Now I've just got to find out why the Bass channel passes a very small amount of signal even with the volume and tone knobs down

That's pretty common because the pots are not perfect so the volume pot doesn't always read zero Ohms when all the way down. There can also be bleed thru in other circuitry , even thru the power supply of the caps are old. Anyway...why is this a concern?

... and why the channels won't jump.

What do you mean by that?

Tom

[Justin Thomas]

Hey Tom,

I figure the signal bleed is just me thinking "it's not supposed to do that..." and that's why I'm curious. It is specific to the Bass channel, and I know the first input jack is a little dodgy, so I'll check all that out, along with the volume pot. The "channel jumping" means bridging the channels to use both at the same time. If I put in the jumper cable the volume knobs on the second channel will increase the noise, but not the signal. It should be getting thicker like my Bassman 100, since this circuit shows two gain stages per channel and the 1/2 triode is AFTER the channels are summed.

The NFB on the plates of the power tubes has been removed; the terminal strip is still in the chassis. So it's not totally original. I'm slow to work on or modify this amp without lots of consideration because it just sounds so darn good. So progress will probably be slow until January when I won't be so hurried.

BTW great avatar, Tom!

Justin