Question about tube amps power supply

[HamiltonPupo]

Quick question for all the experts out there. I am new to tube amp design. Why all tube amps schematic i saw have a resistor (usually 1k or 10k) on the power supply line IN BETWEEN preamp stages? because each of those have a capacitor to ground as well, my understanding was that they work as filters for the next stage. Am correct there? Or is it because of bias point setting? If that's the case, why is that? What happens if i dont use them? Meaning, what happens if i have all stages (in my case, two stages)?

thanks

[MWJB]

It's good practice to have a filter stage for the power tube plates, the power tube screens & the preamp & PI tubes (if only 2 tubes required...you don't really want more than 2 preamp tubes running from one filter cap. So for a simple amp (1 PI tube + 1 preamp tube), you are looking at 3 filter cap stages minimum.

Power tubes can run from anything up to 530vdc in a typical 50W amp, if that 530v was applied to a typical 12AX7 preamp tube plate resistor, the tube would be operating over voltage and probably sound awful.

In a low voltage amp the size of the power supply dropping resistors may be reduced if desired, as long as it does not take the PI/preamp tubes outside of their preferred voltage range.

What kind of B+ voltage, power tubes & how many preamp tubes are you envisaging?

[HamiltonPupo]

At this point i am only studding/working on the preamp section. So for now i have 2 12AX7 for preamp. Quick question, what is the PI tube? Would that be the first tube in the preamp? The one that would receive the direct signal from the guitar?
So my thinkings were right, those resistor/cap are for filtering...What should be the roll off frequencies for them?

[MWJB]

The input tube receives signal from the guitar, or rather the first half of the first 12ax7 does, the 2nd half of the first 12AX7 is usually the tone stack recovery tube, amplifying the output from the tone stack/volume control & sending it on to the PI (phase inverter) which splits the signal into 2 - one of which goes to one power tube, the other signal is "inverted" to 180 degrees relative to the first & feeds the 2nd power tube in a push-pull design (normally amps over 10W).

Don't think so much about the filter caps being tone shaping caps, the power tube plate supply cap will normally be 50-100uf, screen supply cap 20-50uf, same for the PI/preamp cap. The resistors are just a means to an end to meet the required voltage at each stage. Look at some proven designs & see if there is a common 12AX7 plate voltage that they use for the preamp & PI tubes.

If you are building your first amp, I very strongly recommend that you attempt to build a proven design, then even if you don't exactly understand what each component does, with a bit of common sense you will still have a useable amp at the end. A kit, with good instructions/support is highly recommended. There are lots of potential pitfalls for the novice builder...not trying to put you off, just trying to save you from building a several hundred dollar "doorstop"...like I and many others have done, before we knew better!

[HamiltonPupo]

Thanks a lot....
I see what you are saying now...Like i said, for now i am only plannign the preamp. As a matter of fact what i want to do is not use a tube power amp at all....I understand that it may sound really bad, i am just willing to experiment with it....I have some old parts here, so it really would not cost me much...Let's put this way, this is will be a test. If doesn't work, i am still happy with it...Again, thanks for the help....

[tedmich]

Thanks a lot....
I see what you are saying now...Like i said, for now i am only plannign the preamp. As a matter of fact what i want to do is not use a tube power amp at all....I understand that it may sound really bad, i am just willing to experiment with it....I have some old parts here, so it really would not cost me much...Let's put this way, this is will be a test. If doesn't work, i am still happy with it...Again, thanks for the help....

Duncan amps PS designer is a helpful program, and pretty accurate if you use current taps to account for load on different PS nodes.

Also Ax84 will be a great place both to find a Preamp design, build and trouble shoot it.

If you like the heavy check their lead II preamps:
AX84.com - The Cooperative Tube Guitar Amp Project

my fav tone:
AX84.com - The Cooperative Tube Guitar Amp Project

[Wilder Amplification]

I wouldn't advise running power tubes on a single rail supply at 530V. Reason being is that on these designs the screen voltage under zero signal conditions sits about 2-5 volts lower than the plate and most power tubes that are used in guitar amps have a screen voltage rating of around 450ish.

That being said, the idea behind having two preamp tube stages on one cap is that you typically want two stages that run out of phase with each other (i.e. two cascaded stages) on the same filter cap. This makes it so that only one of the two stages is pulling from the filter cap at any given time. This is done to reduce cross modulation distortion between the two stages.

Although a typical 4 input Marshall goes against this since the first two stages are mixed so they run in phase, originally the intent was never to have signal going to both the mixed stages at the same time. However most people who use these types of Marshalls tend to jump the inputs to mirror the signal to both stages so that they are both operating and pulling from the filter cap at the same time since those two front end stages operate in phase due to being mixed rather than cascaded.

[fyl]

Why all tube amps schematic i saw have a resistor (usually 1k or 10k) on the power supply line IN BETWEEN preamp stages? because each of those have a capacitor to ground as well, my understanding was that they work as filters for the next stage.

Each stage or section should have it's own decoupled power supply node, either a resistor + capacitor or choke + capacitor. Filtering is much better, the amp is less noisy and more stable if the R * C time constants are well choosen.

Series R's in the power supply also serve as voltage droppers, but usually aren't significant for low current stages - U = R * I, with R typically at 4K7 to 10K and I below 3 mA, U will be 30 V max.

Most commercial amps use very poor filtering topologies for cost reasons.

[HamiltonPupo]

I did take a look on some of marshal preamps, but i dont understant how they can run in phase. Being that (as far as i know) every stage forms a inverting amplifier....

[fyl]

Cathode followers are non inverting...

[Wilder Amplification]

I did take a look on some of marshal preamps, but i dont understant how they can run in phase. Being that (as far as i know) every stage forms a inverting amplifier....

Every GAIN stage forms an inverting amplifier.

What I mean by they "operate out of phase" is that the two stages on the filter cap are drawing plate current at opposite times. Let's say you have two cascaded stages. When the input signal hits stage 1 and swings the grid positive, this causes plate current on stage 1 to INCREASE, causing the plate to become less positive as more negative electrons strike the plate, which is translated by the coupling cap as a negative swing since the DC plate voltage gets blocked by the coupling cap.

This negative swing drives the grid of stage 2 negative, which reduces plate current on stage 2 as it pushes stage 2 toward cutoff. Hence you have 2 stages operating out of phase (plate current of one stage is increasing while plate current of the driven stage decreases and vice versa).

Yes, the OUTPUT signal of a cathode follower is in phase with its grid signal. However, when the stage driving it draws more plate current, it pushes the grid of the CF stage less positive, which causes plate current in the CF stage to decrease, so the CF stage also operates out of phase with its driving stage.

On a 4 input Marshall, when you mirror the signal to both channels, the grids of these two stages are both being driven positive at the same time, hence both stages operate in phase and pull current from the filter cap at the same time.

[MWJB]

"I wouldn't advise running power tubes on a single rail supply at 530V. Reason being is that on these designs the screen voltage under zero signal conditions sits about 2-5 volts lower than the plate and most power tubes that are used in guitar amps have a screen voltage rating of around 450ish." Plenty of typical 50W style guitar amps run 500v+, normally with 470ohm screen grid resistors on 6L6s that drop about a volt (or 1K on EL34/6550), yes there are 35/40 amps that run <450V, but it is not a design limit in the real world.

If 450v on the screens was a practical limit there wouldn't be any BF Twins, Super Reverbs, 5F6A bassmans, Blues Devilles, brown tolex Pros/Supers/Bandmasters/Concerts/bassmans....and so on.... & so on....& so on...

...in reality they are perfectly reliable & references for classic guitar tones.

[HamiltonPupo]

Thanks .. I understand what you mean now....So let's say for a preamp, if i have the first 2 stages running on ONE filter cap, and a third stage and the CT running on a second one, i would good right?

[Wilder Amplification]

Plenty of typical 50W style guitar amps run 500v+, normally with 470ohm screen grid resistors on 6L6s that drop about a volt (or 1K on EL34/6550), yes there are 35/40 amps that run <450V, but it is not a design limit in the real world.

If 450v on the screens was a practical limit there wouldn't be any BF Twins, Super Reverbs, 5F6A bassmans, Blues Devilles, brown tolex Pros/Supers/Bandmasters/Concerts/bassmans....and so on.... & so on....& so on...

...in reality they are perfectly reliable & references for classic guitar tones.

And this may very well have been fine with the tubes that were being produced around the time these amps were made. However, as you've probably read elsewhere, current production tubes have been getting a bad rap for their lack of reliability as compared to the tubes of yesteryear and overtaxing the screens may very well be the reason why. Tubes of yesteryear/NOS tubes may very well be much more tolerant of being ran under those conditions, but I wouldn't trust anything current production running at the ragged edge of their limits or in an amplifier where the limits are being exceeded.

Of course there's really no way to prove with 100% conclusive results that the reliability of NOS tubes is superior to that of current production tubes, but that does seem to be the going trend. And yes it also calls into question how the people who have problems with CP tubes are running and biasing their tubes so this leaves some unknown variables out of the mix. That and you can never predict how long one particular set of tubes will last under said conditions as compared to another set. But the assessment of NOS tubes exhibiting superior reliability under those conditions as compared to that of CP tubes does seem to be the going trend.

Then again it could be people just making up reasons to justify spending ungodly amounts of $$$ on NOS tubes when they could be using CP tubes with superior results.