double the secondary current capability of a CT Power Tranny

[Mikka Grytviken]

Hi,

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 HT have around 35VA.



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 in CT 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.
HT 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
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
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 R / C 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.



Nota : Don't take care about the current and power values on this picture because they are wrong.

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.



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
Understan 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.



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.

[loudthud]

I tested and took few mesurements on the 369GX PT.
HT coils R => 175/165 ohms,
.........

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)

Wouldn't the resistance of the parallel coils be (175*165)/(175+165)= 86.2 ohms ?

The primary and core are still sized for 50VA. You haven't done anything to increase the VA capability of those.

[Wilder Amplification]

The primary and core are still sized for 50VA. You haven't done anything to increase the VA capability of those.

+1. It's not just about the coils. The iron has to be sized to be able to transform more power so the VA rating stays the same.

[Steve Conner]

I think I calculated that the theoretical power boost you get this way, with a rectifier load, is about 20%. The secondary current rating is increased by 41% (not doubled, it's RMS) but the primary is unchanged, so you have to cut it in half again.

Still worth it, if you're sure that it won't stress the insulation too much (the wires that used to join at the CT may lie next to each other with only the enamel for insulation)

[Mikka Grytviken]

Wouldn't the resistance of the parallel coils be (175*165)/(175+165)= 86.2 ohms ?

Yes you are right in part, in fact there is an error in what I expose you because we have to hold account the impedance of the primary. I present the thing through a shortcut that is in itself false.

The correct maths are :

In CT:

Zt = Rs + (Us / Up x Rs) = 170 + (225 / 230 x 170) = 336 Ohms

25 / 336 = 74.4mA
74.4 x 225 x 2 = 33.5VA

With parallel winding:

170 / 2 = 85 ohms (because of the two windings in parallel)

Zt = 85 + (225 / 230 x 85) = 168 Ohms

25 / 168 = 147mA AC
0.147 x 225 = 33VA

The primary and core are still sized for 50VA. You haven't done anything to increase the VA capability of those.

I never said that I was changing or increasing the transformer power !
Every nuance is on this point.

I ask no more power from the transformer, the calculations also shows very clearly the only available current is doubled thanks to the modification of the impedance reflected to secondary, and this by wiring the two secondary windings in parallel. Thus we no longer work with a voltage of 225V x 2, but only 225V and a total impedance globally divided by two.

This is something that opens up very interesting perspectives in terms of performance.
So when we do not necessarily need a CT transformer , it is possible to save on power transformer and thus its price.

In my case it would have taken in center tap transformer with a secondary high voltage around 65VA as seen for example on a AC15 or Tweed Deluxe. But when I release the need for a secondary in Center Point for not using Valve Rectifier, I could do without a high-voltage secondary of a power of 65VA, and so I settle for 33VA transformer available on my récup simply making this modification.

Since November 2011 my EL84 push-pull amp works very well with its 50VA 369GX Hammond Transformer.

[loudthud]

Since November 2011 my EL84 push-pull amp works very well with its 50VA 369GX Hammond Transformer.

You must have a time machine. Tell me who wins the NBA championship so I can get a bet down

[Mikka Grytviken]

lol

Yeah, I am living in the future !


Sorry, I meant 2010 of course.