AC30 Schematic -- I Can't Believe my Eyes... 160ma ?!?!

[Steve Conner]

The AC30 is a cathode biased amp, the cathode voltage is the "bias conditions". I thought 20V was awfully high, 13 or 14 is more like it. But bear in mind that while increasing the cathode resistor reduces the idle current, it doesn't reduce the max signal current. (At least not to a first approximation: it will reduce it slightly by reducing the effective screen voltage.) The bigger the Rk, the bigger the difference between Vk at idle and Vk at full signal.

Yes, the HT winding wire was made thinner to limit peak current for tube rectifiers. Also saved money on copper. :-)

[bob p]

> The AC30 is a cathode biased amp, the cathode voltage is the "bias conditions"

Still on the first cup of coffee...

My point was to say that how hot you bias the amp really matters. There are guys out there who report +20V under load. The only answer could be that their bias conditions are significantly different.

Prior to starting this thread I had done some envelope math related to implementing Chuck's Zener thingy. IIRc my simple one-iteration model suggested 12.5V under load. If that modelling is accurate then the 13V zener isn't helpful. But now I'm hijacking my own thread...

[pdf64]

Sorry for bad info, on checking, the cathode voltage of nearly 20V is with a 68 ohm cathode resistor (well, 270 ohm per tube).
Pete

[pdf64]

The lamination stack on my Woden PT (662318) measures 5 x 4.25 x 1.5".
The total B+ winding resistance is 120 ohms
My notes indicate that with a GZ34 and cathode resistor of 50 ohms, the B+ winding Vac was
No GZ34 - 586
Static - 560
max sine - 559
max sqr - 554

Vk static - 9.5Vdc
max sine - 10.2
max sqr - 12.5

Given the above, my feeling is that the PT seems well able to cope with the application.

However, the Hammond replacement http://www.bluebellaudio.com/EDB290NX.pdf seems to have a larger stack (4.39 x 5.26 x ?) and lower B+ winding resistance (68 ohms).
Pete

[Ted]

A point worth making is that the position of the mains voltage selector makes a difference - the amp I was looking at was set to 245V and the mains round our way is usually about 235.

The amp showed signs of drops of wax melting out of the transformer - they can go on like this for years before the transformer lets go.

I'd say that maybe one in four of the 1960s AC30s I've looked at has either needed its transformer replaced, or had already had it done.

[Gingertube]

Power Transformers really have an overriding specification which is its total VA rating. This will be speced at a particular running temperature (often 60 degrees C) . Running 50% above its total VA rating it will get hotter than thnat running temperature.

Then each secondary winding will have a current rating. You can get away with overloading any secondary winding up to say 50% oe even more so long as you have lighter loads on other secondaries such that you are still withing the total VA rating.

Its a bit hard to read but I think the 5V heater for the rectifier tube says 3 Amps, The GZ34 (5AR4) draws only 1.9 Amps, there is 5.5 VA saved which you can use as additional B+ winding current draw (another 20 mA ). Could'nt see the 6.3V winding rating but it may be similar.

This is not ideal and if building from scratch I would'nt recommend it if you are within the total VA spec then its not really a reliability issue.

Cheers,
Ian