Help I'm going crazy here 1965 bassman low dc at bridge output

[WillyK]

I have a 1965 Fender Bassman, that is buzzing like crazy through the speakers even with no tubes. Figure it has to be coming from the power supply. Check and found I was getting 320 volts to each input of the rectifier, but only 320 vdc output from the rectifier. I find one diode that was iffy, so I replace all of them with new 1n4007's Now Im getting 303vdc output and if I disconnect everything except for the diodes I get 299 vdc ? What the heck is going on here? Any help would be greatly appreciated Tell me Im an idiot, just point me in the right direction. thanks in advance. Willy

[loudthud]

If there is no capacitor at the output of the rectifier, you will not read the peak DC with a meter. The schematics I see like the AB165 have the stack of two caps before the Standby switch so unless they have been disconnected somehow, you should see 450VDC at the output of the rectifiers.

[Helmholtz]

^^^^^^^That.

Your meter reads the DC average.
DC average of full-wave rectified AC without any reservoir capacitance is 0.9 times ACV.

[Jazz P Bass]

If you have an output at the speakers with no tubes installed, I would suspect the OT.

[Helmholtz]

Without power tubes, how should any ripple/hum current enter the OT primary and get to the output?

[Chuck H]

Without power tubes, how should any ripple/hum current enter the OT primary and get to the output?

Well I suppose a short could do it? But if there were high DC voltage at the speaker jack I don't think "buzzing like crazy" would be the description

[Helmholtz]

Well I suppose a short could do it? But if there were high DC voltage at the speaker jack I don't think "buzzing like crazy" would be the description

A hard short between primary and secondary would load B+ with the DCR of one half-primary or lower.
A typical DCR of 50R would result in a DC current of 6A and a power consumption around 2kW even with a B+ of 300V.

Rather the "short" would have to develop a resistance in 1k range for a more reasonable current.

BTW, as long as the OT secondary is grounded there can't be high DCV at the speaker jack.

We need more information!

[Chuck H]

A hard short between primary and secondary would load B+ with the DCR of one half-primary or lower.
A typical DCR of 50R would result in a DC current of 6A and a power consumption around 2kW even with a B+ of 300V.

Rather the "short" would have to develop a resistance in 1k range for a more reasonable current.

BTW, as long as the OT secondary is grounded there can't be high DCV at the speaker jack.

We need more information!

Of course. I was being cavalier when I wrote that. There's a real safety concern here I think. According to the schematic this is a two prong AC cord amp with a death cap. This, if the original power cord wiring remains. If there is any sound coming out of speakers with no tubes in the amp there's a good chance the chassis is live! Indeed we need more information.

[Helmholtz]

If there is any sound coming out of speakers with no tubes in the amp there's a good chance the chassis is live! .

How would the speakers know?
For any sound there needs to be AC voltage across (not only on) the speaker terminals.
A live chassis won't cause a speaker current.


I actually think he didn't pull the power tubes

[Chuck H]

I have a 1965 Fender Bassman, that is buzzing like crazy through the speakers even with no tubes.

I guess this could be an abbreviated statement and he actually only pulled the PREAMP tubes. But if we take it at face value (no tubes) then only an AC voltage between the tip and ring on the output jack can make sound. While this doesn't seem like a likely scenario it is the only conclusion with the reports given. I agree that with only the OT secondary between tip and ring any AC from the power supply would have to be loaded catastrophically. I do understand that. But we really have no idea what's been done in the amp and we need more information.

[Chuck H]

FWIW here is the schematic for the model in question. Of course it may not apply to the actual amp.

[loudthud]

A short or carbon path at one of the output tube sockets between pins 2 and 3 would produce sound at the speaker if it didn't blow a fuse.