rp wrote:
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Hello,

I used to own a real Vox AC10 and it sounded lovely so I built one as a
head, point to point to see if I could capture any of the loveliness.
FYI used Hammonds: OT is PT-1608 10W 8K; 6CA4 was changed to 5Y3; Plate
(7) 290V, Screen grid (9) 297V. Bias 10.3V 36mA. Schematic:
http://www.voxamps.co.uk/support/circuits/ac10_2.jpg

That is a rather strange circuit... it uses a long-tail-pair phase
inverter, but with not a very long tail, so the response of the two
halves of the push-pull pair won't be equal. This asymmetrical response
might be part of what gives the design its "sound."

The off-side of the phase inverter is also used as the input for the
tremolo (Vox somewhat incorrectly calls it "vibrato") input, after a
four-stage (!) high-pass filter. Interesting...

So far it runs fine but I'm getting about 7 more volts on the screens
than on the plates. The screen grid V should be the same as the plates
according to the schematic: 305V. This amp's output stage is wired
different than others I've seen. Screens and plates are off the same B+
i.e. the same cap, and screens
connect to the OT center tap. OT primaries go to plates (EL84 pin 7)
through 100 ohm resistors, no resistors on the screens. I'm guessing the
EL84's are triode wired here.Correct?

No, they're just wired as any ordinary pentodes, with the screen at a
fixed voltage. In triode connection the screens would be connected to
the plates.

The DC current flowing through the output transformer and the plate
snubber resistors will indeed cause a slight voltage drop. Whoever drew
up that schematic should probably have been a bit more careful. ;-)

It's bad for the screens to be above the plate, right? Or does it not
matter here in this design? I'm guessing again that the internal
resistance of the Hammond OT is different than the original Vox one and
it is dropping some voltage? What else could it be? Unfortunately I
never wrote down the voltages of the original AC10 - would have been
nice to have.

Not to worry. A few volts either way is not going to make a difference,
especially in the relatively tame 300 volt region. This technique of
connecting the screens to raw B+ at the center-tap of the OPT is quite
common in other gear (radios, TVs, stereos) in addition to musical
instrument amps.

- So, do I leave it alone and it's ok to have the screens a few volts
above the plates?

Yes, and yes.

- Or do I have to fix this? And how do I do it? Do I have to hunt down a
nicer transformer? Mercury Magnetics makes a replica but I don't want to
spend any more $ on this right now.

Even with a perfect transformer (zero DC resistance) which doesn't exist
anyway, there will still be a drop due to those 100 ohm resistors.

Don't worry. Be happy. Enjoy the sound.

Cheers,
Fred
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