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**Old Tele man** · 12-03-2007, 01:36 PM

..."typically" it's the plate current (Ip) that's "designed" for (about 1mA), with the plate resistor being "sized" to achieve the desired plate current...remember, the plate resistor (ra) is so much LARGER (Rp ~ 100K) than the nominal cathode bias resistor (Rk ~ 1.2-1.5K) that its' (Rp) value dominates.

...but, there is also the "rule-of-thumb" that Rp should be approximately 1.5-2 times greater than the tubes' dynamic plate resistance (rp); for example: 12AX7 has rp = 62.5K, so that'd be an rp-value between 93.8K and 125K.

**MWJB** · 12-03-2007, 01:41 PM

Most of these don't show what voltage is actually appearing on the preamp tube plate, as plate resistor values vary, it's not giving you a realistic guide.

The majority of guitar amps run between 150-250vdc on the preamp tube plate. Lower voltages sound darker/browner, higher voltages tend to sound brighter with better fidelity.

Later stages tend to be earlier in the power supply, with less voltage dropping stages from main B+. You might find that if you are getting plenty of "character" from the earlier, lower voltage stages, that higher voltages later in the circuit help prevent excessive distortion and help keep a degree of fidelity?

Why not build our preamp with different value preamp dropping resistors mounted on SPDTs so you can A/B ear test different values during development?

**chipaudette** · 12-03-2007, 01:44 PM

I hadn't heard these "typical" "rules" before. They're pretty interesting. What are the rules for a 12AT7? Are they different? What is the 12AT7's dynamic plate resistance?

But, back to the 12AX7, if one monkey'd with the plate voltage (and based on your comments, also monkey'd with the plate and cathode resistors in response), I could see how you could end up biasing the tube differently. I could see how you might manipulate the biasing to give it more (or less) headroom before clipping or otherwise going non-linear. I could maybe understand how to do that...though if someone had some experience with this kind of messing around, I'd love to hear their experience.

Beside the effect on the bias point, though, are there other impacts of the higher plate voltage? Does it affect how it sounds when it goes non-linear / clips? Does it affect how it compresses or if it makes it sound more/less buzzy when overdriven hard?

This is fun!

Thanks,

Chip

**chipaudette** · 12-03-2007, 01:55 PM

I agree that they rarely show the voltage at the plate. But, they almost all use either 100K plate resistors or 200K plate resistors (or 200K in one stage and 100K in the next). So, regardless of what the specific values are, some are being run quite "hot" and some quite "cold". That's got to have some impact on sound. They must be doing it on purpose.

I appreciate your comment about "dark/brown" versus "bright". Is this through your personal experience or is this (like so much in guitar amps) traditional knowledge? Not that one type of knowledge is better than another...I'm just curious how much people have played with this themselves.

In response to your comment about build it and try...I'm all for that recommendation. But, it affects what power supply I buy and how many step-down resistors and caps I buy. I've never built a power supply before and I'm worried about getting it all to work. The fewer parts the better.

So, if I can get people's comments on the effect of supply voltage, maybe I can pick a one early that is probably right for me. Currently, I'm kind of leaning to the 350V range since I see it a lot (only the Dual Rec seems to run 400V). But maybe I want to get freaky and go for only 200V or something. Eh, probably not...guitarists (myself included) are conservative by nature.

Chip

**MWJB** · 12-03-2007, 02:29 PM

"I appreciate your comment about "dark/brown" versus "bright". Is this through your personal experience or is this (like so much in guitar amps) traditional knowledge?" Both.

<350v range would look to be good. Let's take a common, "middle of the road" preamp triode, like Fender, with 100K plate resistor & 1.5K cathode resistor. A 12AX7 plate will show roughly 2/3 of the voltage applied to the plate resistor at the B+ node. So that would give you roughly 235-240v with 350v at the B+ node

A larger 220K plate resistor will darken down tone over the 100K. Perhaps preferable for the earliest stage? Very, very few amps use a plate resistor smaller than 100K for a 12AX7.

A smaller cathode resistor will drop plate voltage and bias for a fuzzier tone (perhaps preferable for an early stage? I wouldn't recommend going below 820ohms). A larger cathode resistor will give more headroom & raise plate voltage.

Remove the bypass cap and you reduce gain & get more headroom still (maybe useful for later stages?). Larger bypass caps allow more lows, smaller bypass caps brighten things up. Typical values in Guitar amps are 0.68uf, to 25uf.

These are very useful methods of tweaking stages.

"In response to you comment about build it and try...I'm all for that recommendation. But, it affects what power supply I buy and how many step-down resistors and caps I buy. I've never built a power supply before and I'm worried about getting it all to work. The fewer parts the better." Resistors & caps are cheap compared to the time you will spend on this, it's unlikely you'll get through building this without some tweaking to taste. Even if you copy an existing design, differences in the output stage will probably mean some tweaking away from original values.

You'll learn a lot, if you don't run out of patience :-).

I wouldn't have more than 2x12AX7 to a filter stage.

**chipaudette** · 12-03-2007, 02:36 PM

Cool reply. Thanks.

With my Fender DRRI, I'm able to try some things like swapping plate resistors and changing the cathod configuraiton (resistor values and whether or not there's a cap). I've done some experimenting already and it's been instructive. And, I'll probably do a little more of it before I step up to a larger stand-alone pre-amp.

Looking at my schematic, I see that I do have a power supply rail at ~400V. Again, my amp currently pulls its pre-amp power supply off a ~260V rail. The 400V rail currently supplies the 6V6 power tubes.

It had occurred to me that, if I wanted to try higher voltages on a pre-amp tube, that I could pull one of my 12AX7's off the 260V rail and put it on the 400V rail. Is this a bad idea? Is it dangerous? Is it a problem that it would be sharing the rail with the power section?

Thanks,

Chip

**MWJB** · 12-03-2007, 02:52 PM

It would be simpler to reduce the value of the 10K power supply dropping resistors. Add values in parallel to find the final values you want.

**chipaudette** · 12-03-2007, 03:16 PM

I agree that it would be simpler to add a resistor in parallel to raise the voltage. But, I don't have any high-wattage resistors on-hand. I've only got 1/4 watt resistors. Everything else, I've got to do by mail-order. Ick.

Presumably, a 1/4W resistor isn't safe...is it?

Chip

**chocopower** · 12-03-2007, 03:18 PM

Time ago....

http://music-electronics-forum.com/s...?t=2713&page=2

I tested for some weeks i liked it... My next mod is wire a swicht in the cap can for diferent preamp voltages...

by

**MWJB** · 12-03-2007, 03:22 PM

"Presumably, a 1/4W resistor isn't safe...is it?" The stock resistors in RI Fenders burn up with some regularity, so...no, I wouldn't advise 1/4W.

**chipaudette** · 12-03-2007, 03:30 PM

I'm a little nervous about having 300-400V DC on a human-touchable switch. Is it safe?

Chip

**MWJB** · 12-03-2007, 03:37 PM

As safe as your "Standby" on your Deluxe.

**chipaudette** · 12-03-2007, 04:12 PM

Ah, but my stand-by switch is a big, robust, for-real switch. It makes me feel safe.

In my work on low-level signals, I'm usually too cheap for such big robust (expensive) switches and tend to use smaller mini-switches. They seem pretty good, but I don't really know their safety level.

In an application where I'm holding back a large DC voltage, using one of those mini-switches would make me more nervous...though maybe for no good reason. Maybe they're just as good.

If I were doing a switch on a high-voltage line like this, maybe I'd spend more and get a robust switch (if one would fit in one of my existing chassis holes). It's certainly a cool idea to be able to switch such elements into and out of circuit so that you can do a good, proper A/B comparison. Working like that is highly instructive.

Chip

**Paul P** · 12-03-2007, 09:29 PM

Or you could get yourself one of these :

http://cgi.ebay.com/Heathkit-Heath-Z...QQcmdZViewItem

Paul P

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