JCM800 Presence

Marshall Amp Discussion

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Littlewyan
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JCM800 Presence

Post by Littlewyan »

The presence control in my JCM800 is a bit odd. It seems to have much more effect when I have the master volume turned down low. It still has some effect with the volume turned up but nowhere near as much, which is the level I expect as the JCM800 doesn't have much NFB.

Anyone else experienced this? Is it caused by the Pre PI Master Volume?
strelok
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Re: JCM800 Presence

Post by strelok »

Guessing its probably because when you turn the master up more the PI and output tubes are starting to distort. The NFB loop becomes less effective when that happens so as a result the presence knob will have less of an effect on the overall sound.
Bob S
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Re: JCM800 Presence

Post by Bob S »

What values are in your 800 feedback (Resistor, pot & cap) ?
Why Aye Man
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Littlewyan
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Re: JCM800 Presence

Post by Littlewyan »

Its got a 6V6 10W power amp so I had to change the FB resistor. 56K feedback resistor, 20K Pot, .1uf cap and a 4.7K resistor in parallel with the pot and cap.
Badside
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Re: JCM800 Presence

Post by Badside »

Littlewyan wrote:Its got a 6V6 10W power amp so I had to change the FB resistor. 56K feedback resistor, 20K Pot, .1uf cap and a 4.7K resistor in parallel with the pot and cap.
What are the resistor values in the PI? Is the bias resistor 470R or more? Tail 10k?
From what impedance tap are you taking the NFB feed? I like 56k on the 8 ohm tap, but I find it too much (aka too little resistance) on the 16 ohm tap, per example.

A pair of 6V6s will usually yield more like 12-15W in cathode bias, as much as 22W in fixed bias. What config are you using? Cathode bias or fixed? What's the output transformer's impedance? Voltage?

A 10W power amp would likely overdrive much earlier, meaning the volume range within which the Presence control has a marked effect is smaller.
Amplifiers built:
Marshall 2204 head with some mods
Low-power 2204 (cathode biased 6V6s)
Single-knob dual-6K6GT amps using a Wattkins uPCB
AC30 clone with Plexified preamp section
AX84 Firefly
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Littlewyan
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Re: JCM800 Presence

Post by Littlewyan »

Everything else in the PI is standard. Feedback taken from 8 Ohm tap. HT is only 340V with 8K OT, fixed bias. I measured the output, with the current valves its only 10W. Might get a bit more if I use different 6V6s. I calculated the feedback voltage divider so the amp has roughly the same NFB as a standard EL34 JCM800.

Has anyone got a JCM800 that they can try this on? Its not an issue, I just think its a bit weird that the Presence is more effective at lower master volume settings.
Bob S
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Re: JCM800 Presence

Post by Bob S »

I'd be tempted to use a 5k pot & maybe lose the 4k7.
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Colossal
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Re: JCM800 Presence

Post by Colossal »

Bob S wrote:I'd be tempted to use a 5k pot & maybe lose the 4k7.
I prefer the Superlead presence circuit. It has a more usable range that the JCM version. The caveat with the SL approach is that there is DC on the pot, causing some scratching when rotated. Not a big deal to me however.
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Littlewyan
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Re: JCM800 Presence

Post by Littlewyan »

I prefer that too but this amp is PCB so it isn't easy to change.

I just want to know if this behaviour is normal.
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