Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on December 02, 2010, 07:33:04 am
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OK, with parallel triodes and a common cathode resistor we use 1/2 the value for the cathode resistor. IOW if I want to bias parallel triodes as if each had its own 1.5K cathode resistor, I should use a 750 ohm shared cathode resistor.
What about a common plate resistor? Again, if I want the same bias point as if each triode had its own 100K plate resistor, should I use a 50K shared plate resistor?
Thanks for any insights,
Chip
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Yes. Current is doubled, so the plate R should be halved.
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Yes. Current is doubled, so the plate R should be halved.
Thanks. That's what I thought, but I keep seeing parallel triodes with a shared 100K plate resistor and haven't ever seen a shared 50K plate resistor. I really like the tone of a 12AX7 biased with a 220K plate resistor and 2K to 2.7K cathode resistor but was wondering if I was missing something.
Cheers,
Chip
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I would keep the 100k plate load resistor the same value.
I assume you want to use parallel triodes for a bit more gain.
Halving the plate load from 100k to 50k plate reduces the gain to about that
of using a single triode so not much to gain:)
And yes halve the cathode resistor if you want to bias at the same point as you say.
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I really like the tone of a 12AX7 biased with a 220K plate resistor and 2K to 2.7K cathode resistor but was wondering if I was missing something.
You can use 100K + 1K2 to 1K5 with // sections. The "compound" tube will behave more or less like a single triode loaded by 200K + 2K4 to 3K, *but* with twice the input capacitance - you may need to reduce the grid resistor(s) - and half the plate impedance.
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Yes. Current is doubled, so the plate R should be halved.
Yes, Ohm's Law applies everywhere. That's why it's the Law! :book1: :wink:
I really like the tone of a 12AX7 biased with a 220K plate resistor and 2K to 2.7K cathode resistor but was wondering if I was missing something.
Have you tried 3.3K? That sounds good too. Plate :: cathode resistor values jump right off the tube charts once you use a Load Line. Sounds like gobblety-gook at first. PRR & Buttery taught me how to use the tube charts -- there's a thread here somewhere. Basically: pick a supply voltage & plate resistor value. With that & Ohm's Law, draw a Load Line on the tube chart. It will be a straight line slanted down from the Y-axis (current) to the X-axis (voltage). Pick a point on the load line where the tube curves are most evenly spaced. Draw a horizontal line from that point to the Y-axis on the tube chart. Use Ohm's Law to compute the value of the cathode resistor. Given a 300V B+ supply w/ a 100K plate resistor this gives a 1.5K or so cathode resistor. A 220K plate resistor gives a 3.3K or so cathode resistor. Different cathode resistor values will cause the signal to distort or compress early, at your option, as the corresponding point on the Load Line is caused to move into different areas of the tube curves. (The foregoing figures assume a 12ax7 tube.)
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...I keep seeing parallel triodes with a shared 100K plate resistor and haven't ever seen a shared 50K plate resistor.
There's one right on our own back porch. Check out the Tweed Overdrive Special, V1B paralleled with V2A, shared 56K plate resistor. Although I've also seen a version of it with an 82K in that spot.
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FYL - thanks so much for pointing out the doubled input capacitance!
Boots - I stopped reading all of the TOS threads/posts after a while because there are 2 or 3 projects ahead of that one on my must-do list. Thanks for pointing out the compensated plate resistor.
Pleximan - maximizing gain isn't the goal for me. Minimizing noise in the first gain stage is. Parallel triodes with LED bias may be a workable alternative.
Cheers,
Chip
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Minimizing noise in the first gain stage is.
Some selected 5751s or 6072As can give much better results than 12AX7s. Or 6DJ8s. Or some triode-strapped pentodes.
You may also consider adding an input transformer or, gasp, a silion front-end.