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Not usually one for NAD/NGD's: But a bit of a revisit, and a-ha moment, has motivated me to share my hard-on…..

Purchased this '64-65 AC10 about 10 years ago. It had a visit to Clarry prior to my tenure. Everything was "fine" on it (with reference to the schematic), but it never really sounded like I would have expected. Quite clean (crunched a bit - in a “Beatles” sort of way - when cranked), and a little like the tone control was wound down. Also - and this drove me nuts - I couldn’t hear “much” difference between Hi/Low inputs on each channel. "Maybe that’s just how it was" I thought……"maybe my ears were just s#*t". I’m not a pedal/clean guy, so somewhat ashamedly, it just sat in the corner……for a long, long, time……

Anyway, pulled finger the other day and decided it was time to do something about it.

Wired her up for an attenuator - helped a bit, but still a bit muffled.

Did some research and poking (non-conductive chopstick), and realised that both inputs, on both channels, had the same pretty circle colour thingies from the jack tips (I now know these to be the original 220k resistors). Ha ha dumb arse - no wonder the Hi/Low inputs sounded similar! Turns out Vox did this to “neuter” the AC10 so it sounded “less” than the AC15. 2 x 40c 68k resistors later and she sounds GLORIOUS! Every bit as good as her older brother (an AC30T also from ’64), and now getting some serious catch-up play time......

Anyway, lesson learned: trust your ears, and do something about it sooner rather than later…..

Have you had a chance to compare it to a modern one?

Hell yes! Nice work.

AC10 - on my list!

Loooooove it

Oh man, that is radballs!!!

robthemac wrote: ↑Sun Jun 13, 2021 5:56 pm Have you had a chance to compare it to a modern one?

Ironically, it was looking at AC10's as a grab and carry that prompted me to get this sorted....

I haven't compared to a modern AC10. I have owned an AC4HW, AC15HW, (and borrowed CC and C1 series 30's) which I extensively compared to my '64 AC30T and JMJ30 and concluded:
- most (i.e. 80-90%) of the magic is in the old speakers - if you have good ones!
- the other ~20% is in the output transformer - that grindy modulation when dimed, and the smell of burning wax.....
- the electrics can sound close, if they're the same schematic using the same tubes etc (e.g. the JMJ and AC30T sound very similar through the same speakers).
- a 12AX7 preamp will never sound like an EF86.
- you absolutely need a switching system to compare properly - the minute you have to unplug/plug etc, you lose your reference point.

That amp looks amazing congrats

Oh man this is so epic, I have the modern one and it’s an exceptional sounding amp, would love to compare with an original!! Are they 10” speakers?

jeremyb wrote: ↑Sun Jun 13, 2021 8:22 pm Oh man this is so epic, I have the modern one and it’s an exceptional sounding amp, would love to compare with an original!! Are they 10” speakers?

Yeah. 10" Elacs. They only used them for a short time - apparently they would blow (what didn't back then....!) - I can see why: amp's neutered, have to turn right up to get the crunch, but it's still pretty clean so the cone is moving a lot.....

68K grid stoppers are pretty standard on 12AX7 valves, however, the input capacitance on EF86's is about 10 times less so that is why the grid stopper has to be a larger value, 150K is about as low (cut off around 100Khz) as you want to go otherwise you risk RF issues, these can potentially damage the amp without you even hearing them.

When you have the guitar plugged into the Normal jack try plugging in another jack plug (not connecting to anything else) into the other Normal socket and see what happens.

sizzlingbadger wrote: ↑Mon Jun 14, 2021 12:48 pm 68K grid stoppers are pretty standard on 12AX7 valves, however, the input capacitance on EF86's is about 10 times less so that is why the grid stopper has to be a larger value, 150K is about as low (cut off around 100Khz) as you want to go otherwise you risk RF issues, these can potentially damage the amp without you even hearing them.

When you have the guitar plugged into the Normal jack try plugging in another jack plug (not connecting to anything else) into the other Normal socket and see what happens.

I would have thought the 1Mohm resistor was the grid stopper? The 220Kohm would be in series with the input signal and reducing that to 68K would be allow much more signal to get to the grid without negative effects.

The 1M is the Grid leak resistor, it goes between the grid and ground. Grid stoppers go in series with the input and the grid.

The AC10 has two inputs for each channel, each input has a grid stopper that goes to ground when no plug is inserted. When you plug in one lead the grid stopper is in series with the grid. However the other grid stopper is still grounded so they become a "potential divider" essentially halving your input signal (Marshall low input works like this). If you put another plug into the other socket it lifts the grid stopper from ground and you now get the full signal.

The Marshall High/Low inputs is a much better way to do it, it wouldn't take much to change yours, just copy the JTM45 input wiring.

So cool

sizzlingbadger wrote: ↑Mon Jun 14, 2021 2:44 pm The 1M is the Grid leak resistor, it goes between the grid and ground. Grid stoppers go in series with the input and the grid.

The AC10 has two inputs for each channel, each input has a grid stopper that goes to ground when no plug is inserted. When you plug in one lead the grid stopper is in series with the grid. However the other grid stopper is still grounded so they become a "potential divider" essentially halving your input signal (Marshall low input works like this). If you put another plug into the other socket it lifts the grid stopper from ground and you now get the full signal.

The Marshall High/Low inputs is a much better way to do it, it wouldn't take much to change yours, just copy the JTM45 input wiring.

respect