I found in my stock a Hammond 369GX tranny that I used years ago for my first SE amp.
The datasheet says it's a 50VA tranny with center tap 225/225V 65mADC HT.
The heater is given for 6,3V / 2,5AAC so maths say around 15VA ... and by the way I know the HV secondary have around 35VA.
[img:576:509]https://lh5.googleusercontent.com/-gp8l ... exions.JPG[/img]
Ok, it's very simple until now.
Today, SE is nomore my kind of tube amp and I'm looking for between ten and twenty watts push-pull amp.
It's easy to see that 35VA are not enough for a 6V6 or EL84 15W push-pull.
However the center tap don't help to use the total power habilities hide in power trannies.
Center tap was used because of the rectifier design but today if we are not against to use silicium diodes we better have to use a full wave design.
I tested and took few mesurements on the 369GX PT.
HV coils R => 175/165 ohms, U=240V (no charge, no consumption) for a mains of 230V with a 240V primary wiring.
(240 / 230 ) x 240 = 250V
250 - 225 = 25V => 25 / 225 = 11% of regulation ratio.
175 + 165 = 335 Ohms
25 / 335 = 74.6mA AC
0.0746 x 450 = 33.5VA
It's closed to what I assumed above.
So I can say, if I can separate the two secondary coils, and wire them in // then the new HT caracteristics will be :
335 / 2 = 167.5 Ohms (becaus of the // coils)
25 / 167.5 = 150mA AC
0.150 x 225 = 33.5VA.
Yeah !
At the first vue it's seem to be a good value for a EL84 / 6V6 PP amp.
I can assume that :
UA+ = 225 x 1,4 = 315V with C / R filtering stage.
Cool, that seems to be a good voltage value for a kind of AC15 power stage !
So I started to work on my 369GX PT modifications.
I opened it and do that things.
[img:576:492]https://lh6.googleusercontent.com/-IfoJ ... 20Mods.JPG[/img]
That was not easy.
Cut the insolating cartboard was not the more delicate things.
I found very quickly de center tap. I desolder The red/yellow wire and the two distinct coil wires were able to be separated and at this point it was very difficult to resolder the red/yellow wire on one of both of them and the other one with a orange wire for differencing them. But it was done without all break. After that I insulated and fixed them.
I tested my mod and it seems to work well.
[img:640:480]https://lh3.googleusercontent.com/-oBu5 ... 5%2525.JPG[/img]
I give you the result of the tests.
You remember that the primary is wiring for a mains voltage of 240V
The secondary is in // with a nominal voltage of 225V
I would like to precise that the ambient Temperature is 28°C, because I live in a tropical erea.
The first test was done with a 25W bulb.
Mains Voltage : 235VAC
HV Voltage : 228VAC
The theorical Iac = 25 / 228 = 110mA
Iac measurement was done with my oscilloscope and a 1 Ohm 1/2W 1% resistor.
I read on the screen a voltage of 319mV peak to peak.
Irms = (319 / 2) x ( V^2 / 2 ) = 113mA
Understand V^ by square root.
No signifient temperature elevation on the tranny.
The second test was done with a 60W bulb.
Mains voltage : 236V
-At the begining : HV=210V
-After 15 mn : HV=209V / Tranny T° = 40°C / I can touch and keep my hand on the tranny
-After 20mn : HV=208V / T° = 45°C / I can touche and keep my hand on the tranny.
-After 25mn : HV=206V / T° = 50°C / I can touch and keep my hand on the tranny but I feel it's pretty Hot.
-After 30mn : HV=205V / T° = 55°C / I can touch the tranny but it's hard for me to keep my hand on it more than 15s !
The theorical rms ac current was around 289mA (it's a mean).
Peak current is :
Ip = 289 x 1,4142135 ... = 410mA
here is the load line of the transformer I have drawn from the measurements I made.
[img:512:467]https://lh4.googleusercontent.com/-6nH2 ... ine_05.JPG[/img]
I just woul like to precise that a EL84 Push pull amp is working today with the tranny since November 2010.
So the rest of that story is for an other thread
Best regards.
