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Thread: Boosting filament Ampere rating

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    Boosting filament Ampere rating

    Howdy folks, i scored a sweet old home made tube bass amp from the late 70's (guessing) 4x6l6gc's a 5751 input and a 12au7 P.I.
    The Output transformer is a hammond 1650T Massive 20lb iron that is rated for 120W rms. The Pt is my guess a 278cx 800v CT @ 530ma (this Pt is 6" wide from bell to bell). 6.3V @ 6A This is my issue. I want to reuse the transformers for a bigger tube amp.
    4xKt88 or 4xEL34 . I need 7A of filament current (4xel34's and 2x12ax7's is 6.6A...better but still makes me alittle nervous). I know hammonds are conservatively rated but adding an addition 1A seems like a real gamble. I have a vintage 60's hammond filament transformer 266L6 - 6.3v@2A sitting on the shelf. Can i wire this transformer parallel with the PT filaments to boost the current rating or will this cause balancing issues?
    im tempted to just wire it up with el34's and see where we sit. my goal was to use it in a hillbish Matamp GT120 clone chassis for 4xel34's

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    Senior Member ThermionicScott's Avatar
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    You wouldn't want to wire the filament transformer in parallel with the larger PT's filament winding. Just use the extra filament transformer to power the preamp filaments by themselves. Easy-peasy.

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    Ya i didnt think running them in parallel would work well i just wanted to double check. But If i use 4xKT88's it will put the current on the PT at 6.4A thats prob ok for the transformer?

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    Senior Member ThermionicScott's Avatar
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    Let's see... my first thought was to wire one of the output tube sockets to the filament transformer, but then you'd be over its limit.

    I suppose you could try the old PT with 4 KT88s on it and measure how warm the PT gets and whether the filament voltage sags under 5.7V. I think the best practice would be to get your hands on a 2.5A filament transformer and wire one of the output tubes to it so that you are well in the clear for all uses.

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    Bent Member Chuck H's Avatar
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    That filament winding will be fine. Better than fine actually. Many transformers, Hammond included, rate the filament winding for voltage @ current. That means that it is designed to give the desired 6.3V at the specified current. Run at anything lower than 6A and your filaments will be OVER 6.3V. In other words, those aren't max ratings, they're operational characteristics. Since modern wall voltages run higher than when that PT was made you will probably be glad for any sag that might work to correct an otherwise higher filament voltage.

    This has been my experience with Hammonds. They probably wouldn't make an off the shelf PT for four big bottles and then limit it's use to one tube type. That would be inconsistent with their product line. But don't take my word for it. Do check with a long burn in to see that the PT doesn't get overly hot. And have a contingency plan for the auxiliary filament transformer when you plan the build. That way you won't need to retrofit it in there if it comes up.

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    Lifetime Member Enzo's Avatar
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    I agree there is probably no real issue using it for the extra half an amp. 10% over spec is no big.

    However, I have to wonder how you came up with that spec. You don't mention a make and model PT, only the OT. And you GUESSED at the high voltage ratings. SO is the 6A rating on the heater voltage also a guess?

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    Quote Originally Posted by ThisLifeILead View Post
    (guessing) 4x6l6gc's a 5751 input and a 12au7 P.I.
    I want to reuse the transformers for a bigger tube amp.
    4xKt88 or 4xEL34 .
    im tempted to just wire it up with el34's and see where we sit. my goal was to use it in a hillbish Matamp GT120 clone chassis for 4xel34's
    Just a little side note here. If you are guessing that it had 4 6L6s and you want a "bigger" amp, I'm assuming more power. EL34s are rated for 25W whereas 6L6GCs are 30W tubes. You will get less power from the EL34s than 6L6s. So the extra heater current drawn by them is non-productive. The KT88s make sense.

    I'm in agreement with Enzo. I think it's best to determine the actual capacity of the filament winding instead of guessing. And in in addition to agree with Chuck, hook up the amp how you want it for a long enough time to get a heater voltage reading. If you're at 6.3v, you're fine. Even a little low by .1v or .2v is OK. Hammonds are over-built, so should take some small amount of extra current draw as Enzo stated.

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    Supporting Member loudthud's Avatar
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    The PT in a Traynor YBA-3 is generally considered to be an OEM version of the Hammond 278CX. Early versions of the YBA-3 used a quad of 7027A plus three 12AX7. After about 1968 or 9 the YBA-3 came with a quad of EL34. I re-built a YBA-3 to use 6550s. The heater Voltage was on the low side, about 6.1VAC at the first power tube. I replaced the long feed wires from the pilot light to the first power tube with #16 teflon wire and that brought the Voltage up to 6.3VAC. The PT runs warm (not hot) and benefits from the fan cooling standard on that amp.

    If Hammond is listening they really need to up the heater current to 8A on the 278CX. With the high power available from the HV secondary, you need more heater power to light up the tubes.

    http://www.hammondmfg.com/pdf/EDB278CX.pdf

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by loudthud View Post
    The PT in a Traynor YBA-3 is generally considered to be an OEM version of the Hammond 278CX. Early versions of the YBA-3 used a quad of 7027A plus three 12AX7. After about 1968 or 9 the YBA-3 came with a quad of EL34. I re-built a YBA-3 to use 6550s. The heater Voltage was on the low side, about 6.1VAC at the first power tube. I replaced the long feed wires from the pilot light to the first power tube with #16 teflon wire and that brought the Voltage up to 6.3VAC. The PT runs warm (not hot) and benefits from the fan cooling standard on that amp.

    If Hammond is listening they really need to up the heater current to 8A on the 278CX. With the high power available from the HV secondary, you need more heater power to light up the tubes.

    http://www.hammondmfg.com/pdf/EDB278CX.pdf
    Well... They could bump the rating. But they probably know that the transformer will survive even if it's hotter than you'd think is alright Of course you're right. I'm just making some fun. Hammonds are Tough (< capitol T).

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    "Take two placebos, works twice as well." Enzo

    "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

    "Being born on third base and thinking that you must have hit a triple is pure delusion!" Steve A

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