Preamp voltages

[LooseChange]

There have been a few mentions lately about the 150k (FET Simulator) dropping resistor on the preamp stage (B+5).

Few questions:

Did HAD purposely shoot for a certain voltage in all his designs?
What is the optimum voltages on the 4 preamp stages?
What have you noticed tone wise, when tweaking these voltages?

Thanks!

[Fischerman]

Are the preamp voltages that critical? I've heard some people (often the semi or not so experienced-with-ODS members) almost insist that you have to have Xvolts on the preamp stages but I've heard others (in some cases very-experienced-with-ODS members) treat them as non-critical and whatever they get, they get. In other words...they know the PT/power supply give them the right voltages to start with and then build the circuit and the voltages are close enough. Some guys have ~175vdc on some preamp stages and their amp sounds phenomenal and others shoot for exactly 200vdc and the amp doesn't sound that great. And besides, different preamp tubes (even among the same brand) yield slightly different voltages.

I guess I'm just a bit skeptical that your preamp voltages must be right near 200vdc in order for it to sound right (as some might have you believe).

[Bob-I]

IMHO the voltages are not so much critical as when things don't sound right they can trigger you to look for issues. I agree that some amps sound great with less the "perfect" voltages, but often when troubleshooting a poor sounding amp I'll find that a voltage is off because of something else wrong.

Often it's an incorrect bias on a tube, wrong value in the dropping string, marginal cap etc. The FET resistor is important because it changes the feel of the amp in a way the dropping string doesn't.

Scott's said it more than once, start with known good values and mod from there. 200V on the plates is a known good value but if your amp sounds good with different voltages, great.

[Fischerman]

I agree about troubleshooting.

Scott's said it more than once, start with known good values and mod from there. 200V on the plates is a known good value but if your amp sounds good with different voltages, great.

But see that's the thing...when you say 'known values' are you talking about the dropping string or the voltages? Because unless everyone starts with the same B+1 voltage you can't have both...either the preamp voltages have to be 'off' or the string has to change.

[Bob-I]

I agree about troubleshooting.

Scott's said it more than once, start with known good values and mod from there. 200V on the plates is a known good value but if your amp sounds good with different voltages, great.

But see that's the thing...when you say 'known values' are you talking about the dropping string or the voltages? Because unless everyone starts with the same B+1 voltage you can't have both...either the preamp voltages have to be 'off' or the string has to change.

Well, kinda both. IME if the dropping string is right, and the bias is right, the voltages will be pretty close. If they're not, something else is wrong, like a bad ground, bad tube, etc. We really should pretty much start with a similar B+, if we don't then I'd go by voltages first.

Bottom line is the tone.

[dogears]

True. There is a Dumble with 8.2K going to the PI. B+ started at about 470v. Rest of string is standard.

I agree about troubleshooting.

Scott's said it more than once, start with known good values and mod from there. 200V on the plates is a known good value but if your amp sounds good with different voltages, great.

But see that's the thing...when you say 'known values' are you talking about the dropping string or the voltages? Because unless everyone starts with the same B+1 voltage you can't have both...either the preamp voltages have to be 'off' or the string has to change.

[LooseChange]

Thanks for the info.
Forget about troubleshooting the amp vs' voltages. I'm talking about a working amp.

I built an ODS circuit (no hrm). I did not see the fet simulator resistor and thought my voltages looked high (I read something in another post a while back). I dropped the voltages by increasing the resistance in the dropping string between B+3 and B+4. The lowest plate voltage is now about 180.

I've been focusing on the 4 pramp stages because I have an effects loop and reverb mine.

Are there critical voltage levels for the PI?

Does it appear that HAD purposely shot for a certain voltage in all his preamp circuits?

[dogears]

You can certainly lower voltage by increasing the string resistance. This has an effect on the the caps ability to charge. It changes the charge time. Instead of doing that, you may want to try restoring the stock string and instead loading the last node. This dramatically effects the caps performance. There may (or may not) be a noticeably different feel, especially in the bass.

Thanks for the info.
Forget about troubleshooting the amp vs' voltages. I'm talking about a working amp.

I built an ODS circuit (no hrm). I did not see the fet simulator resistor and thought my voltages looked high (I read something in another post a while back). I dropped the voltages by increasing the resistance in the dropping string between B+3 and B+4. The lowest plate voltage is now about 180.

I've been focusing on the 4 pramp stages because I have an effects loop and reverb mine.

Are there critical voltage levels for the PI?

Does it appear that HAD purposely shot for a certain voltage in all his preamp circuits?

[Fischerman]

One thing I have noticed is that in some cases people have said that you should have 200vdc on all four triodes of V1 and V2 which implies that they be at the same voltages. But in most cases it's just not possible unless you carefully select tubes that yield those voltages. There's no way V1 can have the same voltages as V2 using the same plate/cathode resistors because there is current flowing through that last resistor in the dropping string so the supply voltage is lower...the 150k FET simulator resistor just increases this current and thus the voltage drop (unless you select your tubes to yield those voltages) making the voltages even further apart.

And if you're supposed to do it through tube selection that's an entirely different approach than trying to adjust the dropping string.

I'm almost sure I've read Gil not really putting much emphasis on the exact preamp voltages (within reason of course) and I think I read that heisthl always uses the same string and it always seems to work.

[dogears]

My two fave amps have the 15K/10K precision power string and have about 20V difference between V1 and V2. About 180V/200V

One thing I have noticed is that in some cases people have said that you should have 200vdc on all four triodes of V1 and V2 which implies that they be at the same voltages. But in most cases it's just not possible unless you carefully select tubes that yield those voltages. There's no way V1 can have the same voltages as V2 using the same plate/cathode resistors because there is current flowing through that last resistor in the dropping string so the supply voltage is lower...the 150k FET simulator resistor just increases this current and thus the voltage drop (unless you select your tubes to yield those voltages) making the voltages even further apart.

And if you're supposed to do it through tube selection that's an entirely different approach than trying to adjust the dropping string.

I'm almost sure I've read Gil not really putting much emphasis on the exact preamp voltages (within reason of course) and I think I read that heisthl always uses the same string and it always seems to work.