Tom Mitchell - Mitchell Pro 100

[xtian]

Anyone know how to contact Tom Mitchell these days? Does his book have contact info?

I just bought a Mitchell Pro 100 head on eBay, and thought Tom might like to know some folks are still interested in his amps.

[Bob Simpson]

Hi xtian.

I had to look... I have revision 1 from 1991, I think.
No contact info that I could see.
Thomas M Mitchell III turns up some hits and some present tense references.

Book published by Media Concepts, Norwalk, Ca...

Good luck.

Bob Simpson

[Cameron]

Anyone know how to contact Tom Mitchell is these days? Does his book have contact info?

I just bought a Mitchell Pro 100 head on eBay, and thought Tom might like to know some folks are still interested in his amps.

Tom Mitchell had nothing to do with Mitchell amps.

[xtian]

Oops. Who's behind Mitchell Amps then?

[M Fowler]

Not the same Tom Mitchell as the amp guru with the book but his name was also Tom Mitchell.

[ToneMerc]

Oops. Who's behind Mitchell Amps then?

Found the address, but nevermind, its in the posted ad

TM

[xtian]

Wow, this Mitchell Pro 100 is really cool. Normal or cascaded gain stages, mid boost switch, great clean-to-crunch tone and power.

This poor thing has been rape-aired a few times. Ugly solder joints and lead dress.

Non-adjustable fixed bias puts -50vdc on the screens, with 460 on the plates.

[xtian]

Can anyone ID the preamp tubes?

[Meat&Beer]

That's a 12AX7!

I kid, I kid. RFT, I do believe.

[Structo]

Looks like a pretty decent amp.

Can you tell how the half power switch is implemented?

[xtian]

Looks like a pretty decent amp.

Can you tell how the half power switch is implemented?

Yes, the switch lifts the ground for two of the power tubes' cathodes. No accommodation is made for output impedance.

[xtian]

I fixed some minor issues and gave the amp a good workout. Damn. Loud. And very fine.

I put a 1kHz sine at the input, and I can get 32v p-p at the output just before clipping (against a 4 ohm load), and it's a nice, sine-shaped wave, too. I found various formulae to convert to watts. Some say, p-p x 0.707 (RMS), but that gives me 128 watts. Some say p-p * 0.637 (average), and that gives me 104 watts, which seems likely, because this is a 100-watt amp.

However, then I found this paper: http://www.dytran.com/img/tech/a13.pdf
...which says average value is not peak-to-peak volage, but only peak voltage (half of p-p). But this gives me only 26 watts.

Is the method in bold actually correct and I really am measuring 104 clean watts?

[teemuk]

P = U^2 / R

Where, in your case, R=4 and U=32V/2=16V (peak voltage of one half wave).

I get 64 watts. Which, however, is only the peak power (remember, U was derived from sine's peak voltage). Average power for sinusoidal wave is 64W/2=32W.

---

So it seems that the amplifier, for some reason, isn't measuring even fully up to its quoted output power rating.

[xtian]

Wow, if that's the case (32 watts), I'd be floored to see it tuned up to 100 watts.

Bias is currently set to allow only 60% diss. per tube. (482 on plates, 22mA per tube).

[xtian]

Following up on this. I've made a lot of changes. Schematic attached is Boogie Mark I, with pink edits showing starting condition of Pro100, and blue edits showing my mods.

I changed V1b and V2a triodes to D-style clean values. Sounds good, but not significantly different than the original Fender style values.

Reducing NFB resistance to roughly 900R removed the last of the power-section hum that was bothering me; now it's nice and quiet and acceptable for home practicing. Really sounds great, in fact, and a beast when turned up, which hich is why I was surprised when...

I checked the bias and found only 8ma per tube, making about 32 watts total. Which of course matches what I found earlier in the thread when measuring the output sine wave with the scope.

So this amp is criminally underbiased, but sounds really good.