Adding bias pot to a Traynor YRM-1 (and Screen R question)

[g-man]

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[eleventeen]

I think you have the right general idea, although I think the range of your pot (meaning the voltage range of the wiper terminal) isn't going to give you quite as much adjustment as you'd imagine.

What is that number, by the way? What bias voltage range would you like to achieve?

In very round numbers, you have about 83 volts thru 90K, just under 1 ma through the voltage divider consisting of the three resistors. Thus the 10K pot is going to allow about a ten volt adjustment range, pretty much in the middle +/-5 volts. Let's say the middle is -41 volts, so your range would be  -36 volts to -46 volts. If "spec" is -36 volts, then you'd be able to go no hotter, but substantially colder than stock. Is that what you want?

What I am saying is that as drawn, the range of your pot is shaded towards the cold (negative, output of the diode) side rather than the hot (ground) side because you aren't changing the 39K resistor. What would you like the voltage coming from the pot to be if it were dead center?

I don't think you are anywhere near running too much current thru those parts, by the way, at ~~1 ma. You have .05 watts through those divider resistors which is nothing, assuming they are 1/2 watt or even 1/4 watt.

[DummyLoad]

you have the right idea, however, maybe you'll need to change the 39K to a 33K or 36K to center wiper of 10K pot at around -36V.

interesting reverb driver.

somewhat more legible original schematic attached...

--pete

[g-man]

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[scrimpus]

I did a similar mod to a YSR-1. I reduced the 39k to 33k and used a 50k pot. I left the 47k alone. This give me just enough range to use EL34 or 6L6s.

[g-man]

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[sluckey]

Both tubes share a common 470Ω/10W screen resistor. If I were to replace that resistor with individual resistors, I'd use the 1K/10W that I had on hand, or buy some 1K/5W wire wound.

[g-man]

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[sluckey]

I have seen some arguments that screen resistors are needed after the dropping resistor (the 470K shared you mention) because the screen node filter cap could still cause current spikes under certain situations. At least that's how I understood it when I read it. People seem to think the newer production EL34's need that extra protection on the screens?

I'm not talking about the 470Ω/10W dropping resistor R42 that's located between B+ nodes A and B and is shown on the schematic near the PT and bridge rectifier. I'm talking about the other 470Ω/10W (maybe R43???) that's located just under V6 on the schematic. That is the shared screen resistor. I don't know about any 'current spikes under certain conditions'. I'd still use the 1K/10W I had on hand. Since you will be replacing a 470Ω/10W, you only have to find room to add one more resistor.

[g-man]

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[g-man]

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