Crate V32 OPT Primary Impedance?

Keg

Member
Messages
5
I'm a tech working on a Crate V32 that has a bad output transformer. I can normally test the transformer to calculate the primary impedance, but with the output transformer being bad it's not going to help. Going by the tube data with two EL84s in parallel push pull, the primary should be 4k. It's possible though that they might have went a little higher. I hate to change the tone of the amp too much. If anyone knows the primary impedance of the original transformer please let me know. Or even a replacement transformer.

Thanks,

Kennyg
 

HotBluePlates

Member
Messages
10,299
I'm a tech working on a Crate V32 ... Going by the tube data with two EL84s in parallel push pull, the primary should be 4k. ...
The Crate V32 schematic shows 4 output tubes, and I assume that's what you meant above as a Vox AC30 OT is typically a 4kΩ primary.

Crate's out of business, and buying an entire used V32 might only be $300-400. Grab a typical 4x EL84 OT from whoever and get the amp fixed.

... Crate V32 that has a bad output transformer. I can normally test the transformer to calculate the primary impedance, but with the output transformer being bad it's not going to help. ...
Bad how?

If there is a short or open in the primary, it probably affects one half-winding and not the other. Figure out which side is not way-wrong when you measure resistance from center-tap to EL84 plate lead. If the OT is still mounted in the amp, remove all output tubes. Apply some small voltage to the secondary and measure the voltage you get from center-tap to the plate-connection on the "good side" of the OT.

- At this point, you can go one of 2 ways:
1. Double the measured voltage, then calculate turns ratio and primary impedance as normal.
2. Calculate turns and impedance with the voltage measured; the actual plate-to-plate primary impedance will be 4x what you just calculated.​
 

gldtp99

Member
Messages
3,806
I've used many of these Classic Tone 40-18006 OT's in many builds/replacements.

With a 4k primary and 4/8/16 ohm output taps this would be a good match for the EL84 x 4 amp.

I think they are great sounding OT's and are reasonably priced----- also easy to source.

IMO, a good choice for a 30-40 watt amp running EL84 x 4, 6v6 x 4, or 6L6GC x 2:

http://www.classictone.net/40-18006.html
 

Keg

Member
Messages
5
The Crate V32 schematic shows 4 output tubes, and I assume that's what you meant above as a Vox AC30 OT is typically a 4kΩ primary.

Crate's out of business, and buying an entire used V32 might only be $300-400. Grab a typical 4x EL84 OT from whoever and get the amp fixed.



Bad how?

If there is a short or open in the primary, it probably affects one half-winding and not the other. Figure out which side is not way-wrong when you measure resistance from center-tap to EL84 plate lead. If the OT is still mounted in the amp, remove all output tubes. Apply some small voltage to the secondary and measure the voltage you get from center-tap to the plate-connection on the "good side" of the OT.

- At this point, you can go one of 2 ways:
1. Double the measured voltage, then calculate turns ratio and primary impedance as normal.
2. Calculate turns and impedance with the voltage measured; the actual plate-to-plate primary impedance will be 4x what you just calculated.​
I've already done all of that. I've calculated the impedance ratio and it's way off. Also I've applied a 28mv signal on the input and traced it through through the amp to the output tubes which there is 7.2v rms on the clean channel going into the output tubes close to what it shows on the schematic. I'm only getting 5v rms into an 8 ohm load out of the opt. I was going by the tube data showing the load resistance of 8k. Being in parallel would be 4k. But that doesn't mean the original opt was 4k. I was hoping someone might know what it was. I can purchase an output transformer for what ever impedance I need and will get it overrated as far as wattage for even better performance. A 4k primary would work but the tone could possibly be a little different than the original. The customer loved the way it sounded. Thanks for your reply.

Kennyg
 

Keg

Member
Messages
5
I've used many of these Classic Tone 40-18006 OT's in many builds/replacements.

With a 4k primary and 4/8/16 ohm output taps this would be a good match for the EL84 x 4 amp.

I think they are great sounding OT's and are reasonably priced----- also easy to source.

IMO, a good choice for a 30-40 watt amp running EL84 x 4, 6v6 x 4, or 6L6GC x 2:

http://www.classictone.net/40-18006.html
Thanks for the link. I'll check it out. I was looking at Edcors also. I've used them quite a few times in repairs and builds with good luck.

Kennyg
 

Keg

Member
Messages
5
I've used many of these Classic Tone 40-18006 OT's in many builds/replacements.

With a 4k primary and 4/8/16 ohm output taps this would be a good match for the EL84 x 4 amp.

I think they are great sounding OT's and are reasonably priced----- also easy to source.

IMO, a good choice for a 30-40 watt amp running EL84 x 4, 6v6 x 4, or 6L6GC x 2:

http://www.classictone.net/40-18006.html
Again, Thanks for the link. They are cheaper than Edcors. I just wished I knew what the original transformer was. It could have been 4.2k, or even 3.8k. Nobody probably knows though.

Kennyg
 

gldtp99

Member
Messages
3,806
Again, Thanks for the link. They are cheaper than Edcors. I just wished I knew what the original transformer was. It could have been 4.2k, or even 3.8k. Nobody probably knows though.

Kennyg
Yes, I like Edcor iron too.

I use more of their PT's than their OT's in the amps I do.

I have no idea what the exact specs of the Crate OT would have been----- just a general range of what I'd expect it to be.
 

Dave B

Exit... Dual Stage Left
Gold Supporting Member
Messages
1,773
The amp's designer, Obeid Khan, is a member here, with Crate and Ampeg design and production lineage at St. Louis Music during their nice resurgence of tube amps in both lines a decade or so ago. His TGP user ID is OKhan.

I'll activate the Bat Signal. He appears to still be an active member and participant here. If he sees this, maybe he'll recall what the component values are that you're looking for.

Paging @OKhan for technical assistance. :dude
 
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cxx

Member
Messages
156
I think the original transformers used in the vintage club series were made by classictone.
 

Keg

Member
Messages
5
The amp's designer, Obeid Khan, is a member here, with Crate and Ampeg design and production lineage at St. Louis Music during their nice resurgence of tube amps in both lineages a decade or so ago. His TGP user ID is OKhan.

I'll activate the Bat Signal. He appears to still be an active member and participant here. If he sees this, maybe he'll recall what the component values are that you're looking for.

Paging @OKhan for technical assistance. :dude
Thanks Dave, He would be the one to know.
 
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1,332
"It could have been 4.2k, or even 3.8k. Nobody probably knows though."

- and it doesn't make squat diddley difference anyway.
 
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1,332
use a 6l6gc o/t PP at around 4, 5k whatever, use anything like that , these are tube amps not digital devices, things don't have to be exact, in fact manufacturers often specified primary loads lower than optimum, sonics -wise, so that they could claim a higher output.
 




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