NAD (kind of.....): ~'64-65 AC10

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Re: NAD (kind of.....): ~'64-65 AC10

Post by murky »

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.
Thanks for the knowledge bomb!

Curious - the old AC15s, and the couple of clones I have here, all have 68k grid stoppers on the inputs (with 1m to ground). Wondering how that relates/reconciles to your comment on the EF86's input capacitance, and RF issues/potential damage?

Yeah, the extra plug in the unused socket works a treat. Here's my pidgeon attempt at the maths behind it - am I in the ballpark?:
  • Stock (220+220), no extra plug: 220k + 180k (1m and 220k parallel) = 45% of the signal going through
  • Stock (220+220), extra plug in unused socket: 220k + 1m = 82%
  • Modded (68+220), plugged into 68k socket, no extra plug: 68k + 180k = 73%
  • Modded (68+220), plugged into 68k socket, extra plug in unused socket: 68k + 1m = 94%
  • Modded (68+220), plugged into 220k socket, no extra plug: 220k + 63.67k = 33%

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sizzlingbadger
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Re: NAD (kind of.....): ~'64-65 AC10

Post by sizzlingbadger »

Yep that looks about right...

The larger grid stopper on the EF86 is technically correct, however in reality you can use a smaller resistor because the socket is usually grounded with nothing plugged in or has a "guitar's resistance" plugged into it so its unlikely to cause any issues. There is usually other "stray capacitance" involved in the layout to. Some amp makers (Ken Fisher) omitted the grid stoppers in their designs and they don't appear to have had any problems. I wouldn't worry about it too much ;-)
Tube amp and guitar tones straight from 1958… amazing how believable the sounds were back then, even without the modellers...

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Re: NAD (kind of.....): ~'64-65 AC10

Post by murky »

sizzlingbadger wrote: Tue Jun 15, 2021 3:42 pm Yep that looks about right...

The larger grid stopper on the EF86 is technically correct, however in reality you can use a smaller resistor because the socket is usually grounded with nothing plugged in or has a "guitar's resistance" plugged into it so its unlikely to cause any issues. There is usually other "stray capacitance" involved in the layout to. Some amp makers (Ken Fisher) omitted the grid stoppers in their designs and they don't appear to have had any problems. I wouldn't worry about it too much ;-)
:thumbup:

What cracks me up is, all these years, I could have just plugged a jack into the unused socket and been entirely happy...... :oops:

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Re: NAD (kind of.....): ~'64-65 AC10

Post by sizzlingbadger »

murky wrote: Tue Jun 15, 2021 4:15 pm
sizzlingbadger wrote: Tue Jun 15, 2021 3:42 pm Yep that looks about right...

The larger grid stopper on the EF86 is technically correct, however in reality you can use a smaller resistor because the socket is usually grounded with nothing plugged in or has a "guitar's resistance" plugged into it so its unlikely to cause any issues. There is usually other "stray capacitance" involved in the layout to. Some amp makers (Ken Fisher) omitted the grid stoppers in their designs and they don't appear to have had any problems. I wouldn't worry about it too much ;-)
:thumbup:

What cracks me up is, all these years, I could have just plugged a jack into the unused socket and been entirely happy...... :oops:
I just looked at the schematic the other day after reading your post and worked it out, I have never played about with one of these old amps :winky:
Tube amp and guitar tones straight from 1958… amazing how believable the sounds were back then, even without the modellers...

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