Effects of plate voltages?

[Doug]

First off, I'm a layman or aspire to be,
I've been experimenting with lowering plate resistor values im my marshall to say 47K and raising cathode resistors to like 3.3k then using a bypass cap. this is a two channel marshall 2205 from 1983. I was using a triode calculator on "adams amps" website and according to the program the output gain would be the same. Plate voltages have went from around 140v to 230v. The amp sounds much improved, maybe less distortion, but less buzzy.
Can anyone shed some light on the theory behind this?
I was trying to emulate voltages from the early marshalls. My el34 plates went from 480v to about 465v, still higher than most jcm800s.
Does raising the cathode values present problems at some point?

[satsloader]

Randall Aiken has some good tech info on his site

http://www.aikenamps.com/

Look up "Designing Common-Cathode Triode Amplifiers", it will most likely answer your questions..

Cheers,

Bill

[MWJB]

"Can anyone shed some light on the theory behind this?" - both mods raise plate voltage & rebias the tube cleaner. Higher plate voltages = less grit & grind, more high end.

"Does raising the cathode values present problems at some point?" - at some point the tone may become sterile/brittle. In extreme cases a very large cathode resistor may push plate voltage beyond "design max" (usually 300vdc), if not necessarily beyond what your average 12AX7 can take. Not many amps deliberately run a first stage preamp plate much over 250-270vdc.

[Chuck H]

It's better to raise the preamp plate voltages at the B+ rail. Doing it with the plate R and cathode R changes the tone of the amp too much. Both methods change the bias of the tube, but the change is less drastic when you raise voltage at the rail. You would also get an increase in gain instead of a reduction.

Raising the voltage gives more gain and headroom. More headroom generally means "cleaner", but with multiple stages of amplification the extra gain becomes more significant. You would still get less buzz by raising voltage at the B+ rail. For those Marshalls I like between 190 and 200 for the first gain stage. The stage with the 10k cathode R then ends up with ALOT of voltage on it. But that cathode R can be reduced for even more gain, more nominal voltage and even less buzz. Big tone improvement.

Chuck

[Doug]

B+ rail voltage

So am I reducing the 10k resistor values between the filter caps? I considered this but went ahead tweaking here and there at the various stages. Upping the b+ rail seems a lot smarter way to go about it. Maybe a 4.7k replacing one or both 10k?

[Chuck H]

"So am I reducing the 10k resistor values between the filter caps?"

Yes.

You can get about 35 more volts on V1 by changing those resistors to 4.7k. So if you had 140 volts you could raise it to 175 without any modification to the preamp...Not bad.

Try this... Return the amp to stock, then do the B+ rail mod. Drop the cathode R for the stage that uses a 10k (V2a I think) to 1k and drop the plate resistor to 100k. Then change the coupling cap after V1a from .022 to 2200p. Less buzz/ more gain. If you want even more gain you can then experiment with cathode bypass caps on any unbypassed stages. Use values from 1uf to 22uf. This may create so much gain that you'll need to attenuate other areas. Like making the first coupling cap even smaller, like 1000p. And adding top end bleeder circuits to stabilize the amp.

You mentioned that with the mods you did, Vp for V1 went from 140 to 230...140 seems low to me looking at the schem. With the EL34s at 470 (or so) volts, a choke and a pair of 10k resistors you should have about 380 volts above the plate resistor for V1a (according to Duncan PSU with appropriate current draw installed). The way that stage is biased you should have between 175 and 185 Vp for V1a. I could be missing something though.

Chuck

[otto]

Doug, in the original schematic the 3 first stages have plate resistor of 100k, 200k, 200k.
In the first one there is no need of headroom because amplitude of the signal is low. I've taken it to 150k to have the gain I wanted (adding a bypass cap gave too much gain). Soldano goes to 220k in this stage.
I've lowered the value of the 2nd and the 3rd plate resistor to 150k but I don't go lower to not require too much power from tubes, to not shorten their life.

[Earl]

Just like to make a staement here. I couldn't speak to what any of these mods sound like..but I'm sure they are ear tested "good".

The point I'd like to make is that the Plate Load resistance plays the role of setting the stage gain, as in - Av = (mu*Ra)/(Ra+ra).

Ra is the Plate Load (including what it's driving downstream), and ra is the tube's plate resistance.

The Cathode resistor sets the tubes bias, and Class of operation. By shifting the Plate Load resistor, you essentially alter the gain of the stage, and by changing the cathode resistor, you move the bias point around.

Just sayin' .. :-)

[Guitarist]

Chuck,
with throttling in the mF like that would you be moving away from the huge Engl type tones we're hearing from Scandanavia. Those Engls I see have .047 throughout. Is that more poweramp and speaker dependant to get some thick whomp without the woof that wimpy OT might provide?

I'm no expert, but wish my HiWatt shell turned 3 channel had more low end. Conversely it seems it is nice to have more sizzling high end with lower gain setting as well. Yet too often that treble bypass on the gain knob doesn't quite cut it.

[Chuck H]

With a multi stage preamp creating the distortion (three gain stages or more) you wouldn't lose any bottom end using a smaller coupling cap in the first gain stage. You might lose some of the lower end distortion and harmonics, but thats the point. If you drive too much low end all the way trough a multi stage preamp your asking for mush and buzz. Not that thats wrong for everyone, but it's wrong for most. With a simpler one or two stage type preamp you'll certainly lose some bottom end if you reduce the first stage coupling cap. But, if you plan on overdriving the PI and power tubes you once again have multiple stages generating the distortion. It all comes out in the wash. So, if you want to create all of your distortion with the preamp and have the use of a master volume to control the final output, your probably better off using smaller value coupling caps in the early preamp stages. If you plan on cranking the amp ALL the time to get your tone then your probably still better off trimming the first stage coupling cap at least a little. Another but, if you want a good clean tone you need to amplify the whole guitars tone early on by using larger coupling caps. All this is why we have channel switching amps. No amp will go from great clean tones to great distortion tones on the same channel at the same volume level. The Tranwreck Express comes close. But you need to use your guitar volume control for the cleans and I personally hate the non dynamic sounding attenuation of loading down the pickup. Plus, you must run the amp at full output all the time. So quiet venues and noodling in your apartement are out.

It all comes down to modding or building an amp for what YOU need it to do without asking it to do too much. Building an amp that is equally good at everything is easy. Because of all the comprimises it will also be equally bad at everything. Building an amp that is actually good at everything is very hard. So it's usually best to be specific in your needs from a particular design.

Chuck