SELMER CORVETTE 6V - Low B+

[smilan]

Hi, I have a Selmer Corvette amplifier and the B+ is very low, 60V at the junction of the 47K resistor and the primary side of the output transformer and 0.4V at the plate of the EL84.
When the wire that goes from the second 32uF cap to the primary side of the output transformer is disconnected from the transformer there is 220V.
All the electrolytics are new and I tried that with 3 EZ80 tubes.
The filament voltage seems to right at 6.32ACV.
Beside a faulty output transformer, there are some other things that might dropping the B+ so much?

[DummyLoad]

pull ALL the tubes, at the EZ80 socket, what is ACV readings on pins 1 & 7 referenced to ground?

[smilan]

pull ALL the tubes, at the EZ80 socket, what is ACV readings on pins 1 & 7 referenced to ground?

The AC voltage at pin 1 is 248V  and 264V at pin 7 (without the EZ80)

[smilan]

I made a test and replaced the EZ80 with two 1N4007 diodes with no change in the B+. When I replaced the primary side of the output transformer with a 5W 360R resistor the B+ raised back to 260V, so I replaced the diodes back to the EZ80 and with the 360R resistor the B+ stayed high. Do those tests confirm that the problem is the output transformer?

[AlNewman]

with the output tube pulled and no speaker, check DC resistance of your primary and secondary OT

[smilan]

I did the obvious thing and tested DC resistance from primary side to secondary side of the OT and there was a 364R between all taps, then I tried a different OT and the amp worked so It's pretty clear now that the original OT was faulty.
The original transformer marked "5000" it's probably the primary impedance. The original speaker is rated at 3 Ohms. This amp is rated as 6W. There are many 5W 5000K single ended output transformers and more expensive 8W like the Hammond P-T125CSE, but all of them are rated at 4 or 8 Ohms output impedance. I know it's wrong to connect a 3 ohms speaker to a 4 (or higher) ohms output, but does the difference between 3 and 4 is really that matter? 

[Latole]

I know it's wrong to connect a 3 ohms speaker to a 4 (or higher) ohms output, but does the difference between 3 and 4 is really that matter?
 

No , 3 or 4 ohms on a 4 ohms output it is safe.
3 ohms on a 8 or more ohms output ; never.

[smilan]

Thank you, another question I have is if I connect a 15W output transformer instead of the 6W transformer it might draw too much current from the power transformer and will overheat it?
Or will it be better to get a 5W replacement transformer?

[Esquirefreak]

No, the power tube idle dissipation determines how much current is drawn. Having an oversized OT just gives you more headroom before it starts to saturate.

/Max

[Latole]

No, the power tube idle dissipation determines how much current is drawn. Having an oversized OT just gives you more headroom before it starts to saturate.

/Max

Bigger cost more money and took more space

[tubeswell]

so you tried swapping out the EL84?

[smilan]

No, the power tube idle dissipation determines how much current is drawn. Having an oversized OT just gives you more headroom before it starts to saturate.

/Max

Bigger cost more money and took more space

This is correct, but since the original 6W has failed, wouldn't it be safer to go with the 8W?

[smilan]

so you tried swapping out the EL84?

I'm waiting for the new transformer to arrive, than I'll check the bias for the EL84, but with my temporary transformer it was dissipating 8.5W which is seems to be OK considering that the owner like it to brake early with his Strat...

[Latole]

No, the power tube idle dissipation determines how much current is drawn. Having an oversized OT just gives you more headroom before it starts to saturate.

/Max

El84  is a 5,7 watts tube. 6 watts OT is to close. I'll go for a 10 watts OT

Bigger cost more money and took more space

This is correct, but since the original 6W has failed, wouldn't it be safer to go with the 8W?