Need help with my bassman 70

[zdmanz]

Turned on my silverface bassman 70 couple days ago and the output was very low. Turned every thing wide open and got a fuzzy sound but whisper quiet. Next day I thought I would mike it into another amp and see what it sounds like. Now I have more volume. I can put the volume on 10 and the master on 5 or 6 and it sounds great, just the sound I am looking for, and at a conversational level. The problem is something is not rite with this amp but I dont want to fix it becauce it if just perfect. I wonder it I will damage the amp by using it this way. You think I should take some voltage readings at the output trans? Thats the only component that would be very expensive to replace I think. Thanks

[Wally]

The amp has problems. Tech work is advisable. The OT may already be toasted,hence the low output. The OT may be okay, but something else is very wrong. There is no way that a 70-watt amp turned up that way should allow conversation!

[zdmanz]

Thanks Wally. I was afraid of that. Better look at it before something worse happens.

[Wally]

It would be a pity to lose a good OT due to a problem that is relatively simple and inexpensive to fix compared to replacing a transformer, wouldn't it? I wouldn't play it until it was serviced.

[dalandan]

Bring her to a tech. No point in risking electric shock if you need to go online to find out what went wrong.

better safe than sorry.

[jefrs]

On the bright side, a blown OT is generally accompanied by a nasty burnt insulation smell. Plus it is quite hard to blow the OT because there are fuses.

I suspect valve gone down or a blown resistor. There is another possibility that a valve or valves have become unseated, poor contact, ease them out and carefully push them back in.

[Wally]

Sometimes a failed OT doesn't yield much of a burnt insulation smell and will operate without blowing fuses. The output will be greatly diminished. I have seen this scenario in a '59 Bandmaster....no smell, just 8 watts of output. OTher times, the catastrophe is much more marked....stinkin' smell and no operation at all.
The OP will not know what is wrong with the amp until a tech gets a hands-on look at it, imho.

[celeste]

Originally Posted by Wally

Sometimes a failed OT doesn't yield much of a burnt insulation smell and will operate without blowing fuses. The output will be greatly diminished. I have seen this scenario in a '59 Bandmaster....no smell, just 8 watts of output. OTher times, the catastrophe is much more marked....stinkin' smell and no operation at all.
The OP will not know what is wrong with the amp until a tech gets a hands-on look at it, imho.

Yes, a lot depends on what killed the OT. If it is long term over current where the OT runs to hot of extended periods of time, then it makes a mess of the windings and potting and can get smelly. If it is over voltage that punched a hole in the insulation of a few windings and now you have in effect a shorted turn, there is usually no smell or other mess that goes with it. It looks good, it mesures well with an ohmmeter, but inductance will be low due th the shorted turn.

[jefrs]

Ahh, the smell of burnt insulation in the morning.

I agree that a technician needs to see it.

First find the fault. Yet I also opine OT are rather robust, you can determine the turns-ratio by connecting the primary directly across our 240VAC mains and measure the secondary voltage (don't try this at home). Even so OT do get hot and sometimes melt the old lacquer, continued use will produce an acrid burning, perhaps my sense of smell is more acute ;-)

I've more often seen lack of power due to other burnt components notably resistors and PCB tracks blown off. Capacitor failure is more gradual. But valves are consumables and they do blow and they can go low-gain when they go down, valve seats do get corroded or knocked. A burnt resistor can also produce a low-gain section.

One scenario I had was an amp with a snubbing cap in series with a resistor across the output anodes, the cap went short-circuit, the resistor tried to carry current, we got low-output as the bottles red-plated, the tell-tale here was the burnt smell of a very crispy looking resistor, the little cap looked fine until tested. Capacitor and resistor cost fourpence, pair of Mullards irreplaceable. Another scenario was just a loose bottle.

OP amp failure was sudden. No burning reported. Sound is fuzzy, Pre-amp gain has to be cranked right up but master at '5'. I suspect something loose or blown. I actually think it might be in a pre-amp section. Guessing really but something makes me think it's not the OT. I'm hoping it's not the OT because the Bassman-70 uses an ultra-linear.
Don't assume it was the OT.

One test the OP can do is re-seat the valves, swap the 7025 around, after that ...
I agree that a technician needs to see it.