Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: joesatch on March 22, 2022, 03:36:52 pm
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intersting circuit. The PT from this pull VK100, The amp uses DC heaters for all the tubes (4x6l6, 3x12ax7). Often the DC heaters on the power tubes causes problems because if a tube shorts it will blow this rectifier circuit . So i will wire it up to feed the power tubes with normal 7vac and run 12vdc heaters for the preamp tubes. Can i tap this to accomplish that?
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intersting circuit. The PT from this pull VK100, The amp uses DC heaters for all the tubes (4x6l6, 3x12ax7). Often the DC heaters on the power tubes causes problems because if a tube shorts it will blow this rectifier circuit . So i will wire it up to feed the power tubes with normal 7vac and run 12vdc heaters for the preamp tubes. Can i tap this to accomplish that?
So are you saying you will use another PT? The heater circuit in the VK100 has all 6L6 heaters wired in series (each 900mA) and then the 3 x 12AX7 heaters wired weirdly (in parallel it seems - i.e. another 900mA 6V heater string) on the end of that. (I think that's 30VDC for the whole heater supply but its doing my head in today) (see page 11 of the attached doc). That PT doesn't have a 6.3VAC winding
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... The PT from this pull ...
The First Rule of Conversion Club is you do not talk about Conversion Club.
But the Second Rule of Conversion Club is whatever you build from the Power & Output Transformers will look an awful lot like whatever circuit the original item used. That's because the Power & Output transformers were "right-size" for that circuit, and perhaps not much else.
So rather than pulling parts out to reuse somewhere else, assume the Power Supply & Output Section are almost set in stone. Then modify the preamp to be/do what you like. Along the way, you can tinker the preamp power supply some.
Any other path seems to lead to unhappiness.
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I will run a voltage divider off the 30v winding to get my 7vac for the filaments. Might run that winding into two paths one for ac and one for preamp dc heaters. well see.
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I will run a voltage divider off the 30v winding...
Its 30VDC - after its been rectified, so (simplistically) I'm estimating it will have been designed for 21.2VAC* @900mA prior to rectification.
*I see there's a 1R resistor (R213) in the heater PS circuit, which probably isn't dropping much. But any resistance you put in a 'voltage divider' to lower the output voltage to 6V will be in series (before VOut) and will eat up some power in dissipation. So you might not end up with a neat thing to run 900mA worth of 6V filament from
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Gotcha. Looks like I'll be buyin a filament supply and i'll use this 30v to run 12v DC heaters (and relays!)
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... DC heaters on the power tubes causes problems because if a tube shorts it will blow this rectifier circuit...
So don't use a lame rectifier. A 25A FWB is not a big investment.
(Yes, 1A 1N4003 on a 0.9+A load was stupid.)
...Can i tap this to accomplish that?
No. Maybe yes.
Show the interesting parts that you did not include to see what goes on, what can be done.
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Hmmm,
4 x 6L6 in series: 24v 900mA
in series with
3 x 12AX7 in 6v parallel: 6v 900mA
Tricky little drawing
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These amps are notorious for blowing the dc heater circuit. Consensus is the way they incorporated power tube dc heaters. Can i design it with some changes to make it more reliable? Any drawbacks to all dc heaters. Also, i will only run 2 6l6 on the new project.
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These amps are notorious for blowing the dc heater circuit. Consensus is the way they incorporated power tube dc heaters. Can i design it with some changes to make it more reliable? Any drawbacks to all dc heaters. Also, i will only run 2 6l6 on the new project.
as the man said, use something like this: https://www.taydaelectronics.com/diodes/bridge-rectifier/single-phase-bridge-rectifier-mic-35a-1000v.html
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The boogey voltage is really 37.8VDC.
And note the heap of 35V filter caps should have been 50V rating.
I think it was a mostly-good design until the penny pinchers went a couple pennies too far.
Yes, you could try to tease it apart and feed 6V or 12V from a switcher but on PCB that may not be easy.
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I'm ditching the pcb. Gonna build my "special 2204 circuit" on a turret board with a clean channel option.
This is what we got to work with.