Harmony 415 Power Transformer

[foscoe944]

I recently bought an old Harmony 415 in great shape. It's almost all original except for the power transformer and the 5Y3 tube was replaced with diodes. The transformer that is in it doesn't have a 5V output for the 5y3 heater and I am hoping to put the 5Y3 back in it, so I'm looking for a replacement transformer. Can anyone tell me if this one would work?

https://www.amplifiedparts.com/products ... nder-champ

This amp is about 15-18 watts has 2 x 12AX7, 2 x EL34, a 6AU6 and the 5Y3. On the schematic I'm looking at the 310V listed as plate voltage in the upper right and thinking that this transformer can supply 325V which may be as close as I can get and I should still be able to get the tubes biased correctly. Am I figuring this correctly?

Harmony 415 Schematic.pdf

[Helmholtz]

Do you rather mean EL84s?

[foscoe944]

Do you rather mean EL84s?

Oops, typo. I absolutely do, thanks.

[sluckey]

That PT specs look good. Will it physically fit?

[stephen_w_keller]

I recently bought an old Harmony 415 in great shape. It's almost all original except for the power transformer and the 5Y3 tube was replaced with diodes. The transformer that is in it doesn't have a 5V output for the 5y3 heater and I am hoping to put the 5Y3 back in it, so I'm looking for a replacement transformer. Can anyone tell me if this one would work?

https://www.amplifiedparts.com/products ... nder-champ

This amp is about 15-18 watts has 2 x 12AX7, 2 x EL34, a 6AU6 and the 5Y3. On the schematic I'm looking at the 310V listed as plate voltage in the upper right and thinking that this transformer can supply 325V which may be as close as I can get and I should still be able to get the tubes biased correctly. Am I figuring this correctly?

Harmony 415 Schematic.pdf

I think you'll want a bit beefier unit. The 6.3 volt filament winding supplies 2.5 amps and your tube complement draws 2.42 amps. That's awfully close to the limit of that winding. Likewise the high-voltage winding is going to be overtaxed with a pair of el84 tubes in push-pull. That output stage is going to pull somewhere in the neighborhood of 110-115 milliamps. I would look for something like this to give the power tranny some room to breathe: https://www.hammfg.com/files/parts/pdf/ ... 1697661948

[Astronomicum]

I think you'll want a bit beefier unit. The 6.3 volt filament winding supplies 2.5 amps and your tube complement draws 2.42 amps. That's awfully close to the limit of that winding. Likewise the high-voltage winding is going to be overtaxed with a pair of el84 tubes in push-pull. That output stage is going to pull somewhere in the neighborhood of 110-115 milliamps.

The closer to the current spec of the filament winding, the closer to the target voltage of 6.3V which is what you want. Based on the schematic, 11V across the 150 ohm cathode resistor is 73mA. 27 mA left for the screens and preamp which is plenty. Specs look good like Sluckey said.

[Stevem]

In that schematic the first power supply node is showing 320 volts, so I think the plates will be more like 315 .

[foscoe944]

Thanks everyone! I'll go with the one I found. The Hammond doesn't have enough voltage on the high voltage secondary and I'd rather be a little over than a little under. Plus, as sluckey asked, it also does physically fit, it's actually a perfect fit to the original holes.

[didit]

Hello --

You may want to at least consider a Hammond "Princeton" replacement as it also ought to be bolt pattern compatible. There is broad confidence that they somewhat over-engineer their iron, and it would be good guess that this amp design is a bit stressful for this. The fact that this amp doesn't have the original is plausible evidence of this. Just a thought.

..

[sluckey]

You may want to at least consider a Hammond "Princeton" replacement

His chosen PT is for a Bronco, Champ, Tweed 5F1 Champ, Princeton, Princeton Reverb, or Vibro Champ.

[didit]

You may want to at least consider a Hammond "Princeton" replacement

His chosen PT is for a Bronco, Champ, Tweed 5F1 Champ, Princeton, Princeton Reverb, or Vibro Champ.

Fully understood but thanks Steve for clarifying, as it's the over engineering I was calling out ..

[stephen_w_keller]

The closer to the current spec of the filament winding, the closer to the target voltage of 6.3V which is what you want. Based on the schematic, 11V across the 150 ohm cathode resistor is 73mA. 27 mA left for the screens and preamp which is plenty. Specs look good like Sluckey said.

I'm guessing that 73mA on the plates is the no-signal bias. This looks like a class AB push-pull stage. I reckon it's going pull a lot more current when driven to full-signal levels.

[Stevem]

You could always just make up some L brackets and mount the PT on the cab bottom and then extend the leads.

I have done this many times on these bargain bin amp to save them and it also helps with the 60hz noise floor.

[sluckey]

The closer to the current spec of the filament winding, the closer to the target voltage of 6.3V which is what you want. Based on the schematic, 11V across the 150 ohm cathode resistor is 73mA. 27 mA left for the screens and preamp which is plenty. Specs look good like Sluckey said.

I'm guessing that 73mA on the plates is the no-signal bias. This looks like a class AB push-pull stage. I reckon it's going pull a lot more current when driven to full-signal levels.

These cathode biased circuits usually run at 100% dissipation. That means the idle current is about as hot as full drive.

[foscoe944]

For what it's worth, I actually ended up going with this one:

https://shop.amppartsdirect.com/product ... 3a03&_ss=r

I chose it after reading that the Heyboer's were constructed more faithfully to the transformers used back at the time this amp was made. I really just want the amp to perform as originally designed. Everything else is completely original on it, speakers, caps, and even a few of the tubes say Harmony on them.