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Old 22nd October 2012, 04:46 AM   #1
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I am attempting to build a guitar amp using the 5F2A Princeton design.
I was wondering if I could use a Output Transformer that I salvaged from a R-R Tape recorder that had a single ended EL84 as the output tube.
The transformer doesn't have multiple taps so I'm assuming it is in the 4 to 8 Ω range as the tape recorder had the ability to connect a larger external speaker.
I would be using a single 6V6 output tube in the Fender design.
The RCA book lists the EL84(6BQ5) as have a Plate resistance of 38KΩ and a Load Ω of4500Ω.
Whereas the 6V6 is P.R= 50KΩ and L.R.=5500Ω
Judging by the size of the OT alone it looks like it could easily handle 5 Watts...
Is this crazy or do-able???
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Old 22nd October 2012, 04:55 AM   #2
kevinkr is offline kevinkr  United States
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Moved since this is guitar amplifier related question, and all such discussions belong in Instruments and Amps per forum policy. Note sub-headings in both I&A and Tubes/Valves...
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Old 24th October 2012, 05:18 AM   #3
kevinkr is offline kevinkr  United States
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This is doable, however you may find that this transformer is actually too good for the application as it probably has more linear core material than typical for a guitar amplifier transformer and less leakage inductance. Try it and see, you can always swap out the transformer later if the results are not to your liking. (Noting that the load reflected to the primary by the speaker probably varies more depending on speaker choice than the difference in winding ratios with any given speaker.)
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Old 30th October 2012, 02:44 PM   #4
Struth is offline Struth  Canada
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Hi Guys

In general, you can use 6V6 and EL-84 in the same circuits with the same OT values.

The only circuit difference required is the bias to set each tube to the desired idle point. In a cathode biased amp, Rk for EL-84 is about half or less the value needed for 6V6. Typically, 6V6 needs about 470-1W, where EL-84 is around 270-1W. These are starting values that work well with most tube samples.

As always, you should add a proper value screen resistor of at least 1k-1W flame-proof so the amp does not eat tubes.

When designing output stages using tube data, you do not need the tube's plate resistance. Traditional and modern design approaches do not need that particular piece of data for the design.

TUT3 has a 36-page 'Champ' chapter about SE design for guitar.

Have fun
Kevin O'Connor
londonpower.com
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Old 31st October 2012, 02:39 AM   #5
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Thanks guys. Thats great info.
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