Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: ncusack on March 10, 2011, 09:02:32 am

Title: New Project Ideas?
Post by: ncusack on March 10, 2011, 09:02:32 am

Hello,

Recently I picked up a transformer pulled from an old organ amp. It was originally powering a quartet of el84'2 along with 5 12ax7's. The Voltages recorded using a variac at 120VAC are as follows

590V w/ CT
35V w/ CT
7V no CT
6V no CT

I was thinking of doing something like this with it but with a pair of el34's instead.

(http://img.photobucket.com/albums/v173/00Chris00/My%20amps/36wtmblite.jpg)

I'm not sure if the voltages would be suitable. If this is a viable option please let me know or if you guys have other suggestions let me know.

Cheers

Neill

Title: Re: New Project Ideas?
Post by: phsyconoodler on March 11, 2011, 11:44:57 am

Yes it would work fine.No 5v tap for a rectifier but the EL34's don't need lower voltages.
  Just change the phase inverter and screen resistor values to a Plexi or 2204 and you will be fine.
It will have about 415v or so on the plates and you could make it fixed or cathode biased.It will kick ass.
 

Title: Re: New Project Ideas?
Post by: jojokeo on March 11, 2011, 11:55:31 am

Quote from: phsyconoodler on March 11, 2011, 11:44:57 am

No 5v tap for a rectifier 

Phsyc - I think those are likely unloaded readings so the 6V may be 5V and the 7V would be the heaters. Most if not all old organ amps used tube rectos.

Title: Re: New Project Ideas?
Post by: ncusack on March 11, 2011, 01:10:31 pm

jojo your right all the voltages here are unloaded measurements. When the circuit is built they should all drop a bit.

psych - by 2204 PI values do you mean 2x1M a 470R and 10K?

Title: Re: New Project Ideas?
Post by: phsyconoodler on March 11, 2011, 02:55:56 pm

Yes.The EL34 won't sound good with EL84 PI values.

  You can use negative feedback or better yet have a switch to lift it if you want a more open tone/

Good call on the unloaded voltages.Makes sense.However,you still might like to use a diode rectifier unless you cathode bias it and hit it with some current.Then the EL34's will sing nicely.