Bassman 300 boat anchor

[drewl]

Jeez, who designed this monstrosity.
. Going after the SVT market?

Anyway, it appears the output trans is bad.
Need to check some things to confirm, but one of the 6550's was blown, lost vacuum.

With output tubes pulled I have signal up to the grids, all voltages present
As soon as one output tube is installed on either side it starts drawing extreme current/blows mains fuse.

So if it is the OT anyone know if replacements are available?

[Jazz P Bass]

What's the bias voltage look like at the output tube grids?

[drewl]

Bias voltage is around -47 volts present on all sockets and clean.

Powering the amp up with standby on, as soon as the tube starts conducting it draws excessive current.

Plate and screen supplies are also ripple free.

[The Dude]

I would do some quick testing on the transformer.
Is primary to secondary shorted?
Do you have equal resistance from center tap to either outside on the primary?

These things (as I recall) also have flyback diodes. Normally, since they go from OT to ground, the amp would blow fuses with or without tubes installed. I would still check them for leakage to be sure. OT's aren't cheap.

[drewl]

Good call, soon as I dig up the schematic tomorrow I'll see if anything else is causing the problem.
As I mentioned with no output tubes everything else is working and I have about 30ohms on each side of the primary.

If it needs an OT, I could possibly get by with just replacing the one bad 6550, I'd rather all six were replaced but you know how cheap some customers are.

[g1]

Here's the schematic:

Edit: Sorry, wrong boat anchor. See post #8 for correct schematic.

[mhuss]

That schematic looks familiar. :-)

I think he's talking about the new Bassman 300, not the old PS model...

[Tom Phillips]

...I think he's talking about the new Bassman 300, not the old PS model...

I agree. Also supported by the mention of Qty 6 6550s.
Attached is the schematic for the beast I believe that Drewl is working on. Bassman_300_PRO_FBA_schematic_Rev-D.pdf

[nsubulysses]

I believe this is also the same amp as the Sunn 300T. Is that true?

[Tom Phillips]

Based on the evidence it appears to me that the problem is one or more shorted power tubes. I.e. Fuse only blows with one or more power tubes installed and bias supply to the tube pins is reported to be OK.
If the OT was shorted primary to secondary then the fuse would blow without any power tubes installed.

[52 Bill]

Okay, I'll ask the dumb question here...

Did you test the other three power tubes for shorts? Just because they didn't lose vacuum doesn't make them good.

I would think that if the OT was bad, it would blow fuses even with the power tubes pulled.

Edit:I see that Tom beat me to it.

[g1]

I think he's talking about the new Bassman 300, not the old PS model...

I agree. Also supported by the mention of Qty 6 6550s.

I guess the word Bassman was a giveaway. But I didn't see that, all I saw was "boat anchor".

[drewl]

Okay, got lucky as OT is good.
Aside from the obviously blown 6550, two others were bad and one of my test tubes was bad.
Also one of the plate protection diodes was bad.

Found that after I pulled the output board then started going through the tubes to see which were good or bad.

Found a good pair and tested them in all six sockets.
New tubes are on the way.

[g1]

Not so good news for the boat.

[audiopete]

Yes the flyback diodes....like an Ampeg. Guess they worked If you are doing this work regularly I suggest a tube tester. First thing I do to an amp that comes in is test and label the tubes. Then you aren't swapping around unknown bad tubes and wasting time and pulling your hair out.