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Old 15th January 2009, 10:16 PM   #1
drew1 is offline drew1  United States
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Default Basic Transformer Question - how do I know build possibilities?

Hi,
I'm very new to amp building, and have a very basic question that I can't seem to get answered... I've got a couple of non-functioning amps that I can take the transformers out of to make new amps.. How do i figure out which other schematics use the same sort of transformer(s) so I can know what amps I could make using these transformers?
For example, I've got a Blackheart bh5h 5 watt amp with a bad PCB. Rather than buying a point to point kit for this amp, i thought I could maybe build a different (better?) circuit and use the existing PT and OT. But I don't know which circuits are even possible.. The Blackheart is uses an el84; am i limited to el84 circuits or could I just build a tweed champ (6V6)? Any other recommendations? Here's a link to the Blackheart schematic: http://www.rowbi.myzen.co.uk/Blackhe...2020070611.pdf
The other amp is an old Carvin x100 100 watt amp. Ideally i'd like to use the transformers to build a Fender Bassman 5F6-A.. Is this possible? Schematic here: http://www.carvinmuseum.com/pdf/amps/xamp-revk.pdf
sorry for such basic questions, like I said i'm just starting out with this type of stuff.. Thanks for any info or recommendations..
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Old 15th January 2009, 10:41 PM   #2
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Hi

Basically you need to find out what secondary voltages you have available and the approximate power rating of the PT.
For the OPT the primary impedance and power rating is interesting.

When you have this you can choose any schematic that reasonably fits the bill regardless of what tubes are involved.
Probably most 6v6 circuits will be close enough.

/Olof
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Old 15th January 2009, 11:30 PM   #3
rjb is offline rjb  New Zealand
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Re output transformer.
The rated max plate dissipation for the EL34 is 12watts, that for the 6V6 14 watts, so close enough probably to assume either can be used as far as OP transformer power rating is concerned, as long as you don't run the 6V6 plate voltage any higher. (Note this is not audio power output).
The best match impedance for the single EL84 is about 8000 oms, and you can probably assume your transformer is for that.
However that of the 6V6 from memory is 5000 0hms, which is not a perfect match.
Although I prefer the 6V6 as being easier to obtain good results with, if you want to use this transformer but get the best possible quality, I would stick with the EL84 circuits.
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Old 16th January 2009, 12:31 AM   #4
drew1 is offline drew1  United States
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Thanks for your reply.. the PT on my blackheart has secondaries of 5v, 6.3v and 275v, which if I'm looking at everything correctly, is just fine for a 5f1 champ for example..
As far as the OPT, I'm still confused.. the primary says 0-5.2kohms, and the secondaries are 16, 8 and 4 ohms. It also says 75hz-7khz, which I'm not sure what that's referring to. Will this be suitable for a 5f1 champ as well? Any more info would help a lot.. Thanks.
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Old 16th January 2009, 12:32 AM   #5
SY is offline SY  United States
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Quote:
The rated max plate dissipation for the EL34 is 12watts
I suspect you mistyped- that's EL84.
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Old 16th January 2009, 03:24 AM   #6
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Drew-
About output transformers and guitar amps- you can get away with a bit of mismatch (more than a bit, often) especially if you are working with lower-powered amps.
Your Blackheart single-ended OT with the 5K primary should work fine for EL84 or 6V6 SE circuits. The PT from the Blackheart probably can't supply the heater and plate current for other power tubes like 6L6.

The Carvin OT primary impedance (for4x6L6)is probably on the low side for use with a pair of 6L6s like the Bassman uses, but you will probably get away with it.
(Measuring the impedance of an unknown output transformer (once you have it disconnected) is fairly easy, BTW).
If you are planning on using an 8ohm speaker with your Carvin re-build, you could just wire it up to the 4ohm taps- this would be equivalent to doubling the primary impedance (ie doubling the impedance the output tubes 'see'..)
Of course, you could use the Carvin 'iron' for a smaller PP amp project as well.
As you are just starting out, I'd recommend sticking to lower powered amps, which I've found to be more tolerant of parts mis-matches, etc. It's more fun to play an amp wide open, and you won't irritate the neighbours as much with a smaller amp.
The 75Hz-7kHz this refers to the frequency response of the OT- not really an issue for guitar amps, which generally use pretty small OTs by 'hifi' standards anyway.
You can get some good info by looking around
http://musicalpowersupplies.com/
as well as Duncan's Amp Pages.
Cheers
John
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Old 16th January 2009, 05:26 AM   #7
drew1 is offline drew1  United States
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Thanks John, good info..
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