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Thread: Basic amp topologies, what tranny's can do 'X'?

  1. #1
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    Basic amp topologies, what tranny's can do 'X'?

    I have AC-15 transformers.
    What type of amp is an AC15 ? [PP or SE?].
    What parameters are in place limiting the type of amp I can build using a HV, CT secondary [400/0/400] ?
    Any suggestions for basic tube amp understandings welcome, I've been looking though all the pages/links etc., but can't seem to find the basic stuff for....understanding the basics for designing something around the AC Tranny's.

  2. #2
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    AC15 is Push Pull, based on the type of output transformer. If you look at the schematic, the high voltage comes in at the centre tap and out to the power tube plates
    via the taps at each end of the primary OT winding.

    A single ended transformer would have the high voltage coming in at one end of the primary and out to the tube (usually one, but can be more) at the other end. There is no centre tap.

    I doubt very much that the AC15 power tranny is rated at 400-0-400 at the B+ secondaries, this would equate to a plate voltage in the region of over 500 volts, maybe 400VAC end to end?

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    That or maybe I need a 'trust-o-meter' for my trusty / cheepo 'ol 4 bukk DMM?
    The Vox PT, pulled, with 120AC going to primary, 'measures' 802VAC across the secondary, with center tap..near center, the MM sitting next to it on the bench [w/same input V] showed 847VAC secondary output [on DMM].
    ...according to this silly meter, [I should stick it in the wall plug for AC scale reference],[I didn't actually measure the primary supply... now thinking...re-thinking..] to see if it says 120 or so VAC there.
    Re-armed with the RCA Tube Manual...and better understanding of PP and SE topology [much thanks to MJWB !], I return to the gathering of info from reference data and various PP/SE schemata of AC / 5e3 and so on.

  4. #4
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    Quote Originally Posted by petemoore View Post
    That or maybe I need a 'trust-o-meter' for my trusty / cheepo 'ol 4 bukk DMM?
    The Vox PT, pulled, with 120AC going to primary, 'measures' 802VAC across the secondary, with center tap..near center, the MM sitting next to it on the bench [w/same input V] showed 847VAC secondary output [on DMM].
    ...according to this silly meter, [I should stick it in the wall plug for AC scale reference],[I didn't actually measure the primary supply... now thinking...re-thinking..] to see if it says 120 or so VAC there.
    Re-armed with the RCA Tube Manual...and better understanding of PP and SE topology [much thanks to MJWB !], I return to the gathering of info from reference data and various PP/SE schemata of AC / 5e3 and so on.
    the unloaded voltage will be higher than the voltage when in use. how much higher i dont know.

  5. #5
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    I want to agree with MWJB regarding those voltages being high but your measurements say otherwise. I would expect an AC15 PT to have multiple primary taps for running on 110, 120, 220, 240Vs. Are you sure you're applying the wall voltage across the correct leads? With the measurements you gave I'd expect you to end up with a B+ in excess of 550V. I'm afraid that level of plate voltage will turn any modern EL84 to toast in a short order of time. (I don't even think NOS Amperex or Mullard tubes could handle that voltage, it's greatly in excess of the EL84's rated max plate voltage of ~300V).

    I found a 1960s AC15 schematic on SchematicHeaven.com that shows secondary voltages of 300-0-300 rated for 120ma. The tap off the EZ81 rectifier is marked at 385VDC. The OT is wired up post-inductor, the schematic shows 310V at the EL84 plates.

    Assuming your PT is taken from a modern VOX reissue it's possible it only has a single 120V primary tap. If so it might be worthwhile to post some measurements of what you're getting on the fillament line/lines (the 90's versions seem to have switched to 5Y3 rectifiers with 5V filament lines). If you're 6.3V line shows up much more than 8-9VAC unloaded you may have a problem.

    If everything checks out ok with your PT but you still have the 800Vct secondary you could certainly make an amp out of it. Assuming the 120ma spec holds true you might build a PP EL34, KT66 or 6L6 output stage around it that was better able to handle the high B+ and might even run with the same OT. A single ended EL34 amplifier would be an option too but you'd need a different OT like a Hammond 125ESE.

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