Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: pbman1953 on December 24, 2025, 11:34:13 am
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I've been on the forum talking about the Fender Super Twin's I've gone through.
The amp uses 6- 6L6 tubes. The RCA 6L6 has a draw of 9 amps for the heater. The heater total per side would be 27 amps .The amp sounds great with them. Some of my other amps have 6550's and I love them. Since I'm almost finished with this latest one, I wanted to see what a pair of 6550's sounded like in it and they sounded great.
The 6550's draw is 15 amps so I'm under the full current draw compared to 6l6's.
My question-
If I use 4-6550's am I going to kill my PT? I'd be 6 amps over, in total, of the 54 amp limit of the PT?
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6A @ 6.3Vac ~ 38W. That sounds like a bad idea to me. If you absolutely must have them then I'd suggest adding another small filament transformer, say at around 6A, to run just the preamp tubes and use the original one to run the power tubes and filament.
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All your numbers are off by a magnitude of 10X. A single 6L6 has a filament current of 0.9A, not 9A. Likewise, a single 6550 has a filament current of 1.5A, not 15A.
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All your numbers are off by a magnitude of 10X. A single 6L6 has a filament current of 0.9A, not 9A. Likewise, a single 6550 has a filament current of 1.5A, not 15A.
Ok so with 4 - 6650's the overage would be 6.0 a, compared to 5.4a with the 6l6 set. I don't have room for another transformer anyway.
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The current difference is only 0.6A. I would just try it. I doubt anything will suffer. I don't expect there will be any significant difference in tone. You may have to slightly modify the bias circuit.
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All your numbers are off by a magnitude of 10X. A single 6L6 has a filament current of 0.9A, not 9A. Likewise, a single 6550 has a filament current of 1.5A, not 15A.
Totally missed that. Gonna blame cedar fever and decaf.
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The current difference is only 0.6A. I would just try it. I doubt anything will suffer. I don't expect there will be any significant difference in tone. You may have to slightly modify the bias circuit.
Bias is all set to 28 watts just to test it
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OEM is three pair with a draw of 5.4A, you state a pair of 6550 @ 1.5A ea. so 3.0A total is not an issue. If your mean two pair of 6550, then that's 6.0A and may over-stress PT core and winding, but doubtfully so.
Note that two pair of 6500 can push 200W with Super Twin B+ and load.
If loaded with the 6550 and you measure 6VAC or lower across the filaments, with nominal AC mains voltage, you're probably overstressing the filament winding. Compare with 6L6 installed.
--Pete
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OEM is three pair with a draw of 5.4A, you state a pair of 6550 @ 1.5A ea. so 3.0A total is not an issue. If your mean two pair of 6550, then that's 6.0A and may over-stress PT core and winding, but doubtfully so.
Note that two pair of 6500 can push 200W with Super Twin B+ and load.
If loaded with the 6550 and you measure 6VAC or lower across the filaments, with nominal AC mains voltage, you're probably overstressing the filament winding. Compare with 6L6 installed.
--Pete
Rest assured , this was just a fact finding mission. In other amps that I had room for a FT, I did that. There is surface space for it but I have a set of 7581a tubes to use so I'll use them. I wanted to be sure the pair wouldn't do any arm
Thanks all!!