JMI AC15 plate resistor on V5 - why?

[fazeka]

Hi all,

I have access to an AC15 clone (JMI Amplification, 2007) that has a 47k plate resistor on pin 1 of V5. What's the purpose of this resistor? It's not on the original schematic.

Thanks,
Chris

[xtian]

You're saying there's a 47K resistor in your amp that's not shown on the schematic? OK, as you say, one end is connected to V5 pin1...where is the other end connected?

[fazeka]

You're saying there's a 47K resistor in your amp that's not shown on the schematic? OK, as you say, one end is connected to V5 pin1...where is the other end connected?

Pin 1 to one end of the 47k, other end of the 47k to junction of C19 and R31.

[sluckey]

You're saying there's a 47K resistor in your amp that's not shown on the schematic? OK, as you say, one end is connected to V5 pin1...where is the other end connected?

Pin 1 to one end of the 47k, other end of the 47k to junction of C19 and R31.

The 47K and R31 form a voltage divider to reduce the signal fed through C19 to the next grid. I don't know why someone needed to reduce the signal. My AC-15 only has a 150K in that spot.

[fazeka]

[ChopSauce]

The attached layout shows a bare wire instead of the 47k resistor. Might have been some confusion with R8 in position 31, same board, that has been responsible for this seemingly mistake?

Check this twice (or more) though.

[pdf64]

The AC30 has such a split load arrangement there, so perhaps the builder got mixed up https://el34world.com/charts/Schematics ... 301960.pdf

[fazeka]

The AC30 has such a split load arrangement there, so perhaps the builder got mixed up https://el34world.com/charts/Schematics ... 301960.pdf

Ah, that has GOT to be it. They were building both AC30 clones as well as AC15 clones.

So, this 47k in the JMI AC15 is a mistake and has no real benefit. Very good. Thank you!

[xtian]

So, this 47k in the JMI AC15 is a mistake and has no real benefit

"benefit" is a subjective judgement. But this resistor does have a FUNCTION, which is to reduce the output amplitude of that stage. So, if you wanted to increase available gain on your amp, you could replace that 47K resistor with a jumper wire--better yet, just tack in a jumper wire in parallel with the resistor to hear what it might sound like.

[fazeka]

So, this 47k in the JMI AC15 is a mistake and has no real benefit

"benefit" is a subjective judgement. But this resistor does have a FUNCTION, which is to reduce the output amplitude of that stage. So, if you wanted to increase available gain on your amp, you could replace that 47K resistor with a jumper wire--better yet, just tack in a jumper wire in parallel with the resistor to hear what it might sound like.

Sorry, let me rephrase my original statement. That resistor may have a function but is not part of the original design of the JMI AC15.

My goal is to get as close as possible circuit-wise to a JMI era Vox AC15.

[fazeka]

The 47K and R31 form a voltage divider to reduce the signal fed through C19 to the next grid. I don't know why someone needed to reduce the signal. My AC-15 only has a 150K in that spot.

Hi Steve,

I noticed in the same amp that pin 6 of the EF86 has a 100k plate resistor instead of 220k. I asked about this in another thread. I am thinking that they maybe reduced gain with the EF86 to get away from microphony that is common with the EF86. If true, then I guess the builder also wanted to bring the gain down accordingly in the vib/trem channel to balance the channels? I dunno, just a guess.

What do you think?

Cheers,
Chris

[sluckey]

Who knows what the builder had in mind. Anything I say is just a wild ass guess. I have a 220K in my amp. Works just fine.