Fair warning: I'm not the most circuit-savy person, so feel free to roll eyes at my question.
I'm building a tube rectified amp which is a clone of a Decware SE84, but using headphone output transformers. The power transformer is a Hammond 272FX, which uses a center tap on the rectifier heater windings. I'm using said center tap as the B+ which is what the SE84 does.
The tranny is a 300-0-300 output. On the Decware (since sold) I measured 357VDC at the rect. center tap (B+), but on this one, it's 410VDC, which is nowhere near half of the stated output of the tranny. This puts the plate voltage on the output tubes 30+ volts too high. (These numbers are with the amp loaded.)
I should explain that Decware has a DIY circuit board version of this same amp, but uses diodes for rectification, and the spec's for the transformer for that amp is the same as the 272FX (minus of course the 5V output).
I'm building a tube rectified amp which is a clone of a Decware SE84, but using headphone output transformers. The power transformer is a Hammond 272FX, which uses a center tap on the rectifier heater windings. I'm using said center tap as the B+ which is what the SE84 does.
The tranny is a 300-0-300 output. On the Decware (since sold) I measured 357VDC at the rect. center tap (B+), but on this one, it's 410VDC, which is nowhere near half of the stated output of the tranny. This puts the plate voltage on the output tubes 30+ volts too high. (These numbers are with the amp loaded.)
I should explain that Decware has a DIY circuit board version of this same amp, but uses diodes for rectification, and the spec's for the transformer for that amp is the same as the 272FX (minus of course the 5V output).
Without a circuit diagram and posted node voltages the things I can think of are:
Are all the parts spec'd to handle the voltages and currents they are seeing? That would be my first concern.
Then,
10 to 15% variation on builds of an unregulated tube circuit aren't unusual.
Nevertheless:
Did you check all your cathode voltages? All where they should be?
Is your circuit drawing the Hammond's specified 173mA? if not the B+ can be higher.
The Hammond is rated for a 115VAC mains voltage. If yours is higher, the output will be too.
Just because the output voltages on the Hammond are nominally the same as on the original Decware power transformer, it doesn't necessarily follow that the output voltages will be the same in the same circuit.
Same goes for the primary winding on the output transformer if its resistance is lower than the original.
The expected output from the rectifier of a 300-0-300 winding into a capacitor input filter is roughly 420V minus the drop across the rectifier.
That's what I can think of. Good luck ! 'Hope it sounds good !
Are all the parts spec'd to handle the voltages and currents they are seeing? That would be my first concern.
Then,
10 to 15% variation on builds of an unregulated tube circuit aren't unusual.
Nevertheless:
Did you check all your cathode voltages? All where they should be?
Is your circuit drawing the Hammond's specified 173mA? if not the B+ can be higher.
The Hammond is rated for a 115VAC mains voltage. If yours is higher, the output will be too.
Just because the output voltages on the Hammond are nominally the same as on the original Decware power transformer, it doesn't necessarily follow that the output voltages will be the same in the same circuit.
Same goes for the primary winding on the output transformer if its resistance is lower than the original.
The expected output from the rectifier of a 300-0-300 winding into a capacitor input filter is roughly 420V minus the drop across the rectifier.
That's what I can think of. Good luck ! 'Hope it sounds good !
300 x 1.414 = 420v ish as Hearingspace said, I'd expect a 50v ish drop on a B+/HT rail when loaded, IE when the OP valves are working, or more if using a 5U4G . Check your voltages at the OP section and check and re-check your wiring and work in general, especially check your soldering joints, grounds etc.
The voltage can be tuned down easily with a series resistor , the one usually used for protecting the vacuum rectifier which anyway is missing in the schematic
The DECWARE SE84B was built with a Hammond 272FX power trafo. It also used a high forward drop 5Y3 rectifier.