Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Joel on August 28, 2022, 04:19:07 am
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Hi All,
I've been toying with the idea of playing around with a MOSFET based phase inverter for a while, I've seen these referred to as "FET-a-Dyne".
I have an AX84 "Clean" Preamp mated to a 5W SE power amp stage as my donor amp. I have a couple of "standard" amps, this one is my test bed.
* I do miss the old AX84. I'm glad I copied all of their Core range building block schematics.
I like the Clean Preamp, and I want to mate it up to a 5E3 style cathode biased power amp stage. Rather than add another preamp tube as a phase inverter, I'm thinking a FET-a-Dyne might be just the ticket. I had great success using a power MOSFET as my Tremolo oscillator in my Hoffman Single Channel AB763. https://el34world.com/Forum/index.php?topic=26928.msg295792#msg295792 (https://el34world.com/Forum/index.php?topic=26928.msg295792#msg295792). I plan to use the same FDPF12N50NZ device.
I've attached a schematic of my proposed idea along with some documents I used as references.
* I'm away from home, so I can't remember one or two specifics of the "as built" AX84Clean Preamp, but it's close.
* Can only add 4 docs, so no 5E3 schematic - it's pretty well a standard so I don't think there's any need. And my schematic has the Hoffman 5E3 power section verbatim anyways.
Is there is anyone out there who has successfully used a FET as a Cathodyne before, I'd appreciate your thoughts. For the brains trust, thoughts? Opinions? I'd appreciate any input you might have.
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This: https://el34world.com/Forum/index.php?topic=11428.msg105566#msg105566
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You have my schematic there, I have used regular HV mosfets a few times with the circuit as shown. One mess fitted into a 5F1 chassis using 12AQ5's.
(https://i.imgur.com/4dTUjkg.jpg?1)
Ready to drop into an amp (minus 51k resistors or coupling caps), also the mosfets plug into standard connectors which might make using in tube circuits easier.
(https://i.imgur.com/wULIlDT.jpg?1)
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Thanks for the input!
I've seen some of these circuits using 47k, and some with 100k? Any particular reason?
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The higher value resistors go hand in hand with higher voltages for output tubes that need a greater swing in voltage to drive them to full output.
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I would think 'in proportion to the grid resistors'.
Or: "I got a heap of 68k".
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Thanks for the input!
I've seen some of these circuits using 47k, and some with 100k? Any particular reason?
What ^PRR said^
The size of the load resistors on the PI acts in conjunction with the size of the grid (leak) resistors on the output tubes to form a 'source/load voltage divider' that attenuates the signal.
Higher value* load resistors for a given amount of grid leak resistance** = more signal attenuation, and vice versa. So 47k load resistors work 'better' with 250k grid leak resistors at preserving the signal (than 100k load resistors would)
*Up to a point. The bigger the load resistance gets, the more it squashes the gradient of the driving tube's load line down into an ineffective operating region. (In this case we're talking about a MOSFET PI/driver, but the principle is the same)
** The determining factor for grid leak resistance size is mainly the type (and number) of output tube(s). Output tubes don't like 'too much' grid leak resistance (and more powerful output tubes prefer smaller grid leak resistances - to counteract excessive grid current build-up).
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Updated schematic.
Preamp section now reflects its current incarnation.
I forgot to add the MV to the 1st version. Due to the FET bias voltage divider network, a second coupling cap is added for the MV pot to keep it DC free.
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It's alive!
I call this one the Woke Heretic.
Woke because its 100% recycled. Mostly recycled from the previous amp that lived in this box or whatever parts I could quickly find in my junk boxes. Even the hookup wire is recycled. There are one or two minor value variations from the schematic due to strict adherence to the Woke 100% recycled philosophy.
Heretic because of the Fet-a-dyne phase splitter and diode rectifier. It's not a pure "Toob amp"! You should be able to see my aborted attempt at an amplified zener in the top left of the pics. It was in supposed to drop the B+ by 50V but it introduced very loud buzzing. Perhaps it's due to using two 24V zeners in series instead of one? It's not really needed fortunately.
The Fet-a-dyne works extremely well. Very happy with it. Recommended.
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Thanks for sharing! I love innovative ideas. IF you can post some sound clips, I'd love to hear them.
With respect, Tubenit
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+1, would love to hear this!
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Full disclosure, this amp is no longer 100% recycled!!!
I bought three 5W 15V zeners to do the zener trick and drop the B+ down a bit. So, its now only 99.9% recycled.
I also upgraded the speaker to a Fender 10" Alnico Blue I bought from a music gear repair place closing down sale that I discussed here, https://el34world.com/Forum/index.php?topic=29626.msg326082#msg326082 (https://el34world.com/Forum/index.php?topic=29626.msg326082#msg326082). The speaker was brand new in box. But since I bought it secondhand, I'm counting it as recycled! Sounds absolutely magic.
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I would love to hear this! I also love the concept. I will build mine soon after i finish my SE build! Thanks for sharing this.
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Completed an upgrade of the Woke Heretic today. I converted it into a 4x10 combo cab.
In keeping with the concept, I bought some more 10" speakers second hand over the last half of this year. It now has 3 Alnico and one ceramic speakers. 1 x Fender blue alnico, 1 x Eminence 1028K alnico, 1 x Music Man 10" alnico, and one Fender V30 ceramic (included as I haven't found any 10"alnicos for cheap yet).
Cab is oz pine with merbau racing stripes and 1/2" ply baffles. Must admit to having to buy the merbau new from the local hardware...
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That is one beautiful cabinet! I really like it. Thanks for sharing.
PLEASE share some sound clips. I'd love to hear what it sounds like?
With respect, Tubenit