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View Full Version : Re: sophomore needs help w/ homebrew Vox AC10


Fred Nachbaur
July 29th 03, 03:59 PM
rp wrote:
> ***NOTE*** Please remove -nospam- from address when replying ***NOTE***
>
> 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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