50 watt OT in 100 watt amp with 2 x power tubes removed

[harry]

Good afternoon,

I once again would like to draw on your expertise. Not so long ago you helped me re-build and modify a 50-watt tube PA head. It became my main amplifier for a year, so thank you for that!

I needed something a bit more powerful though, so I bought a Carlsbro TC100. It's a 4 x EL34 beast, and it sounded great until I blew it up by plugging it into an unknown cabinet (should have known better).

I saw a green flash, I am almost certain it is the output transformer. Here's the question: I have an OT suitable for a Marshall JTM-60. If I run the Carlsbro with two of the power tubes pulled, will I be able to use this OT? Obviously it won't be ideal, and as soon as I get the money, I will by the correct transformer, but as a temporary solution would this cause any harm?

Any comments/opinions would be much appreciated,

Thank you!

Harry

[MWJB]

Before you star ripping bits out, confirm that the OT is blown. First read dc resistance from each primary (plate wire) to the centre tap...what do you get?

Do you have a signal generator/low AC voltage source that you could apply the OT secondaries, to properly confirm turns ratio/OT condition.

Yes, if you only run 2 power tubes you can use a 60W OT (into the recommended speaker load).

[txstrat]

Originally Posted by harry

I have an OT suitable for a Marshall JTM-60. If I run the Carlsbro with two of the power tubes pulled, will I be able to use this OT? Obviously it won't be ideal, and as soon as I get the money, I will by the correct transformer, but as a temporary solution would this cause any harm?

Should be OK.
Watch for the impedance for the cabinet.

[harry]

Thanks for the suggestions. I will verify that it is really the OT before I do anything, probably by generating a sine-wave signal on my computer and measuring it with a meter the other side.

Will I have to adjust for the cabinet if I'm using a 60W OT; that is, the 8 ohm tap on the transformer will be 8 ohms? I know it's different when pulling tubes, but that would be when using the original 100 watt OT (because the extra two tubes alter the impedance)?

[MWJB]

With 2 x EL34 the 8ohm tap on the 60W OT will still want to see 8ohms.

When pulling 2 tubes in a 100W amp you effectively double the primary impedance, without selecting another secondary tap, this also means that the secondary impedance is doubled, hence the common suggestion of doubling speaker ohms when pulling 2 power tubes in a 100W amp.

Measure voltage with meters at BOTH sides of the OT, one meter on the secondaries (say half a volt), one meter on the primaries.

[Andy]

if you're using a transformer intended for two tubes, and you only have two tubes, you use the advertised impedance. not double, not half.

just for the sake of clarity.

[harry]

Thank you very much for the input.

MWJB, should I use two meters because they will have an effect on the input/output impedances?

The reason I ask is because I have two meters, but one is as cheap as they come, and the other is reasonable. What I would have done would have been to have measured the signal going in to the Tx, then measure the signal coming out and assume it would not change too much when I disconnected the meter.

[MWJB]

"What I would have done would have been to have measured the signal going in to the Tx, then measure the signal coming out and assume it would not change too much when I disconnected the meter." That would work, I just find it quicker to use 2 meters, use the cheap one on the primaries, the better meter on the secondaries wher you are applying your 0.5VAC.

It's the impedance of the OT that will skew the voltage reading, just don't assume because you have 0.5VAC selected on the voltage source's dial, that you actually have 0.5VAC at the secondary.