5E3 using Hammond 291BEX

[Mick Bailey]

I'm getting very high voltages from a Hammond 291BEX in a new 5E3 build. This is the transformer recommended by a number of European suppliers (660v CT, 138mA) and whilst I was expecting it to be a bit on the high side, I wasn't expecting the B+ to be 514v and the 6.3v heater supply to come out as 9.4v ac. This is with tubes fitted and a JJ 5y3 rectifier.

On the variac it needs setting to 220v to get the heaters to measure 6.3v. Then the B+ is 455v. The Hammond primary is rated for 240v, which is my supply voltage.

Even used for its recommended Deluxe Reverb use it would still come out way too high.

Has anyone used one of these in the UK and measured the voltages?

[Jazz P Bass]

I would suggest that you contact Hammond.
Something is not right
http://www.hammondmfg.com/pdf/EDB290BEX.pdf
The datasheet clearly states '240V/50Hz' operation.

[Mick Bailey]

Just struck me to put a light bulb limiter in series with the variac - it glows too brightly. You're right -something is not right!

[g1]

Aside from what the variac says, what voltage do you measure coming in to the amp?

[g1]

By the way, I mentioned that because if you have a 220V variac and put 240V into it, you will be getting 240 out at the 220V setting. My variac is a 117V type, and when it is set to that the output will be what ever I put into it. So normally our wall voltage is about 124V and that is what I get out of the variac set on 117V.
Other things to double check are your meter and the battery in your meter. I can't think of an amp fault that would make the PT voltages go up.
Aside from that, it is possible you got a 220V transformer mislabeled as a 240V part, or a defective transformer. The data sheet shows what your unloaded voltages should read with the PT disconnected from the amp.

[Mick Bailey]

It's a 240v variac, though the output voltage is shown as a % of the input voltage, not actual - it has to be measured to be precise. The 120v is measured across the PT primary. Checked with another meter to be sure.

I've just taken the transformer out to take some measurements.

[Mick Bailey]

The transformer was perfect when tested out of the amp, so I reinstalled it with just the mains connected and the outputs were still all ok. The mains lead connects to a NOS terminal strip and I'd used a spare terminal on the end to locate the unused 50v bias tap. Probably a bad move anyhow. As soon as the bias tap was reconnected the voltages went off. There's a lot of leakage between the terminals and this looks like it was boosting the output. The fabric/phenolic insulation isn't insulating and some of the 240v was getting on to the bias tap. The resistance between terminals is all over the place but seems to hover around 500k, but this may collapse with mains voltage applied. Moisture absorption?

Must be less than 21mA leakage to ground (my mains trip leakage threshold) otherwise it would have tripped.

Got rid of the strip, heatshrink to tidy up the bias tap, and everything is spot on.

Voltages are now;

B+....366v
Heaters....6.4v
Rectifier....5.14v

[Enzo]

Some sort of contamination anyway.

My trick is to cook the phenolic with a heat gun to evaporate the crud. So far it has always worked, at least on Fender eyelet boards. (yes, I know eyelet boards are not phenolic) You can usually watch the stuff boiling off, much like the frost disappearing from your windshield when the defogger is on.