Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: chocopower on April 30, 2014, 04:50:08 pm

Title: Identifying a unknown amp. Weird B+ and heaters wiring.
Post by: chocopower on April 30, 2014, 04:50:08 pm

Hi!


Time ago i get and old small tube amp with a EL84, EZ90 rect and a mullard OC71 preamp. Components and construction say it´s from 60´s-


First problem is figure out if the transformer is a 110vac or 220vac one. (here in spain is a usual problem because until mid 60´s we had 110v)


In that occasions i always put a 220/110 step down transformer in line so measuring heater voltages i can know what i have in hands. I always use too, a bulb limiter just for be more safe.
If i get +/- 6,3v i know is a 110v transformer.
If i get +/- 3,2v i know is a 220v power trafo and i can get out the step down transformer.


But, that baby is wiring in a different way.


Heaters are wire like in a Champ aa764. One of the tube heaters in the socket is grounded and the other is feed from the P.T. As the EZ90 use the same heater voltage, is wired the same way.
I know that in champs you can re-wire that into a more standard way. My amp have 2 solid copper green wires. One is for the heaters and the other is grounded.
I suppose that un-grounding that one and wiring the whole thing with the usual 2x100 ohm resistor will work. Isn´t?
Right now, with the step down transformer, i´m having 3.5 vac, so i think that this is a 220v transformer. I´m i right?



But the weird stuff come now.
The B+ secondaries are wired in the same way. To identical thin white wires come out from the transformer. One is grounded and the other goes to the EZ90 rectifier socket. The high voltage goes to pins 1/ 6 and the rectified out is in pin 7. So, in my low level knowledge, is a "half wave rectifier".
I think the reason is that the P.T. have no center tap. So, if i use a full wave diode bridge bridge, i should use both white wires a use the ground in the rect. bridge. Isn´t?


Thanks in advance.

Title: Re: Identifying a unknown amp. Weird B+ and heaters wiring.
Post by: eleventeen on April 30, 2014, 06:08:38 pm

It sounds to me like you have correctly figured out that you have a 220 VAC amp.


If that is the case, then moving the heater wires from (1 to ground and the other to each tube) to (2 to each tube) should be no problem; many people do this (in a Fender Champ, for example) expecting a large improvement in hum, but it usually does not produce much change. YES, it is likely you would have to add the 2 - 100 ohm resistors.


As for the B+, yes it sounds like you have a half-wave rectifier. Before I say something like "go ahead and change to a full wave" I would want to see the schematic.


The amp worked as an amp on 220 volts. I would tend to keep it the way it was.

Title: Re: Identifying a unknown amp. Weird B+ and heaters wiring.
Post by: sluckey on May 01, 2014, 05:30:05 pm

Quote from: drgonzonm on May 01, 2014, 04:50:25 pm

If you can find another EZ90, and a socket you can wire it for full rectification. 

How would you do that?

Title: Re: Identifying a unknown amp. Weird B+ and heaters wiring.
Post by: chocopower on May 01, 2014, 06:52:35 pm

i found that topic. It seems you can combine diodes and tube rectifiers.


http://el34world.com/Forum/index.php?topic=12908.0 (http://el34world.com/Forum/index.php?topic=12908.0)


Any way, i think i,m gonna use a full wave diode rect. bridge.