5E3 Transformer ?s

[cooper]

Hay all,

I've been reading this board and other materials and I really want to not only build but truly understand the 5E3. Right now I am trying to learn a few things relating to transformers and was wondering if I could get some direction. If you feel so inclined, I would appreciate it a lot.

1) Power transformers: I understand how to ascertain amperage requirements, voltage taps, etc.; however, the overall voltage provided on a lot of 5E3 replacement power transformers seem way to high to me for some reason. I must be missing a lot!

I thought the max plate voltage for class A operation of a 6V6GT was 315V and to keep it under that. It was my impression that with a transformer giving 275V through a 5Y3GT - that would give me 310.75 volts??? "Inside Tube Amps" stated to convert transformer voltage by a "conversion factor" dependant on the rectifier type to arrive at voltage. For a 5Y3 this factor was listed at 1.13 giving me 275V x 1.13 = 310.75V. This part of arriving at the right power transformer voltage for m'tubes is confusing me!

Output transformer. OK. I think I can handle coming up with an appropriate power requirement, but... the impedance! I've read 6V6GT tubes can take between 5,000 and 10,000 ohms but, that's a big spread. What would the the difference mean to me between 5,000 and 10,000?

Thanks a lot for anything you guys can do.

Adam Cooper
www.blanketeer.net

[steve]

Hay all,

I've been reading this board and other materials and I really want to not only build but truly understand the 5E3. Right now I am trying to learn a few things relating to transformers and was wondering if I could get some direction. If you feel so inclined, I would appreciate it a lot.

1) Power transformers: I understand how to ascertain amperage requirements, voltage taps, etc.; however, the overall voltage provided on a lot of 5E3 replacement power transformers seem way to high to me for some reason. I must be missing a lot!

One thing to consider is that the voltage coming out of the wall in the late 1950s was less than today. My wall voltage averages about 125v.

Also, the 5Y3 rectifier has a lot of voltage drop due to the internal resistance of the tube: 50v@125mA. Modern SS rectifiers have about .7v drop, IIRC.

I thought the max plate voltage for class A operation of a 6V6GT was 315V and to keep it under that. It was my impression that with a transformer giving 275V through a 5Y3GT - that would give me 310.75 volts??? "Inside Tube Amps" stated to convert transformer voltage by a "conversion factor" dependant on the rectifier type to arrive at voltage. For a 5Y3 this factor was listed at 1.13 giving me 275V x 1.13 = 310.75V. This part of arriving at the right power transformer voltage for m'tubes is confusing me!

The 5E3 isn't a true Class A amp. It is biased hot towards Class A, but isn't really Class A. The specs for a 6V6 tube are pretty conservative. Later Fender Silverface amps had voltages in excess of 400v and the 6V6 tubes in them lasted for decades. I used to own a Jim Kelley FACS amp that had 470V+ on the plates. The 6V6 is a tough little tube.

The thing to keep in mind is that when the tubes draw current, the voltage goes down. So the 1.13 figure is about right.

Peak voltage is 1.41 x secondary voltage. 275 x 1.41 = 387.75. Once you consider the 5Y3 voltage drop and current draw from the power tubes, 310 sounds about right.

Output transformer. OK. I think I can handle coming up with an appropriate power requirement, but... the impedance! I've read 6V6GT tubes can take between 5,000 and 10,000 ohms but, that's a big spread. What would the the difference mean to me between 5,000 and 10,000?

Generally speaking, the lower the primary the greater the power but also the greater the distortion. With higher primaries there is less power but also less distortion. It is a tradeoff between maximum power out and an exceptable amount of distortion.

For guitar it is a good idea to find out what impedences were used in various classic circuits and use those as a guide. I believe the 5E3 OT has about an 8k primary. Later Blackface and Silverface Deluxes had lower primaries.

HTH,

steve

[cooper]

Steve,

Thank you very much, I really do appreciate it.

Sounds like around 8K impedence for output primary should be good but I can give or take a little with sonic considerations in mind. I think I'm settled there.

Back to the power transformer, very helpful knowledge. Do you know what the specs are for the original 5E3 transformer? From there I could look at the turns ratio and maybe adjust to accomodate for 120 power by reducing how much it steps up the voltage.

What are the transformer values folks are using for the 5E3? Did you make any mods to accomodate this transformer or that affected voltage in any way? Also, what brands/models have people found to be their favorites for the 5E3, and of course... why!? If you have it in you,,, I'd also like to know what plate voltage you're getting with whatever transformers you're using.

THANKS!!!

Adam
www.blanketeer.net

[steve]

Cooper,

Sounds like around 8K impedence for output primary should be good but I can give or take a little with sonic considerations in mind. I think I'm settled there.

Back to the power transformer, very helpful knowledge. Do you know what the specs are for the original 5E3 transformer? From there I could look at the turns ratio and maybe adjust to accomodate for 120 power by reducing how much it steps up the voltage.

I believe the original was 325-0-325 rated at 70-90mA, but am not 100% sure of that.

What are the transformer values folks are using for the 5E3? Did you make any mods to accomodate this transformer or that affected voltage in any way? Also, what brands/models have people found to be their favorites for the 5E3, and of course... why!? If you have it in you,,, I'd also like to know what plate voltage you're getting with whatever transformers you're using.

I scratch built a 5E3 a while back. I used a Hammond 125E for the OT, they're inexpensive and sound good. The 125E can be configured to reflect a variety of primary impedences. IIRC, I tried a few primary impedences but ended up setting mine to the setting closest to 8K as I liked it the best.

Right now the amp has a 5Y3 rectifier in it and 6K6 power tubes. The 125E is set to around 10K to reflect the higher plate resistance of the 6K6. FWIW, I like the sound of 6V6s better.

For the PT I used a NOS Thermidor unit. It is rated at 300-0-300@125mA. With a 5Y3 I get about 325V. Using a Groove Tubes SS plug in rectifier I get about 365V. I get somewhere in the middle using a 5V4 rectifer; can't remember the exact voltage.

One thing you might consider is a kit from Mission Amps. I know Bruce has been in many real 5E3s. The transformers in his kits are custom wound based on the data he has acquired from servicing real 5E3s. I've played a few and they sound fantastic.

Disclaimer: I know Bruce and have been to his shop a few times. However, I make no money off of his kits. I just think they are the best deal going on a 5E3 kit.

HTH,

steve

[cooper]

Steve,

Thank you for all of the helpful information! Really appreciate it.