Tweaking a preamp pentode

[g-man]

Greetings,

I have an old PA amp that uses 6SJ7 preamp tubes that I’m tweaking for guitar use. It has three essentially identical channels. The plate, screen and cathode resistor values and voltages are shown below. The screen voltages are quite low and I would like to bring those up to about 80-90V range (based on other forum posts I have seen). Is it just a matter of reducing the screen resistors until hitting the desired voltage? Or is there something more that needs to be done? I’m a hobby builder but not much of a tech or designer. Thanks.

[Helmholtz]

Yes, lowering the screen resistor value will increase screen voltage and gain.
But higher screen voltage also means more plate current which in turn lowers the plate voltage.
You might want to lower the plate resistor to get the same plate voltage as before.
What's your B+?

This article should help: http://valvewizard.co.uk/pentode.html

[g-man]

Thanks. B+ at that node is 226. From the node prior to it, there is a 22K dropping resistor followed by a 150K plate resistor to each preamp plate (each one has now drifted up to around 160K). So after reducing the 1.5M screen resistor, I could then change the 150K plate resistor as needed to get the plate voltage back to approximately 145V, correct?

[Helmholtz]

So after reducing the 1.5M screen resistor, I could then change the 150K plate resistor as needed to get the plate voltage back to approximately 145V, correct?

That's the idea, yes.

[mozz]

Try 100k/220k and 1 meg. Also what is the value of the screen cap? You can have 1 channel grid leak bias. Beware those tubes sometimes are microphonic.

[g-man]

Thanks guys. The screen cap is 0.1uF. I may lower it on at least one channel to reduce the low end a bit.

[Helmholtz]

Bear in mind that the screen dropper and the cap form a power supply filter.
Lowering either may cause 120Hz ripple hum.
To cut bass use a smaller cathode cap.

[mozz]

Valve wizard

"The Screen Bypass Capacitor
The screen bypass capacitor serves a similar purpose as the cathode bypass capacitor: it holds the screen voltage steady to prevent internal negative feedback from reducing gain. However, in the same way as for the cathode, the capacitor can be made small in value to boost higher frequencies, or omitted completely to reduce gain and increase headroom. Many traditional guitar amp designs do not include this capacitor at all.
In general, the screen bypass capacitor has a greater effect on the gain of the pentode than the cathode bypass capacitor. The screen bypass capacitor should always be connected to the cathode rather than to ground. Yes, I know a lot of circuits connect it to ground and get away with it, but it is bad practice.

The screen bypass capacitor can be chosen according to:
Cg2 = 1 / (2 pi f Rg2)
If we want to pass all audio frequencies without any loss of gain then we might set f to 10Hz or therabouts: <
Cg2 = 1 / (2 pi × 10 × 390000) = 40.8nF
A more common standard value is 47nF.
"

[Helmholtz]

Valve wizard....

Yes, I know, but lower resistor or lower cap value means lower PSRR.
You might get away if your B+ is very clean.
Otherwise its better to lower cathode or coupling caps.

[J M Fahey]

1) I for one do not trust your measurements

What are you using to measure screen voltage?
On what scale?
Brand and model please, not just "a Multimeter"

I suspect your actual screen voltage is way higher.
Around 75V or so, go figure.

Why?
You are measuring from a 1.6 M resistor.
IF your multimeter has 1M input impedance, it makes a voltage divider with that resistor, which causes about 2.5X attenuation.

Multiply each "measured" value by 2.5 and you´ll get the real one. Which is perfectly fine, do not mess with it.
Besides, mutiplication factor may be even higher, since the screen IS also pulling some current, in parallel with multimeter input impedance, so voltage divider action is even worse..

2)

The screen voltages are quite low and I would like to bring those up to about 80-90V range (based on other forum posts I have seen).

Oh, the World Famous "You Tube University"
Uncle Doug, The Guitologist, The Gear Page, Harmony Central, etc.
You can safely ignore all those and many more.

Your screen voltages are actually fine. Do not mess with them.

3)

Is it just a matter of reducing the screen resistors until hitting the desired voltage?

Not an approved design method, but worst is that, *suppose you mess with values until you measure 80-90V
The instant you remove Multimeter probes, screen voltage will shoot sky high.

4)

Or is there something more that needs to be done?

Best MOD/tweak is "leave as is".

Or, as ENZO used to say: "keep fixing it until it´s broken"

5) the elephant in the room is that that pentode stage is working PERFECTLY, why touch it?

225V +B?

140V on plates?

What´s wrong with that?

6) bonus points:

So after reducing the 1.5M screen resistor, I could then change the 150K plate resistor as needed to get the plate voltage back to approximately 145V, correct?

Throwing values at random is not the proper way to design anything.
Use curves and graphs printed on datasheets, that´s what they are there for.
Or better, leave as is, no good reason to change anything.

The screen cap is 0.1uF. I may lower it on at least one channel to reduce the low end a bit.

That cap is a ripple reducer and a screen decoupler from Audio signal, not a way to EQ a gain stage.

Try cathode, input and output capacitors instead.

Valve Wizard, same as Gerald Weber, is just a slight notch above average You Tube/Forum University.

The "wizard" bit should already be a red flag.

[g-man]

Thanks again for the comments.

My meters are Crenova 890Z with 10M input impedance, and Meterman 34XR with 9.1M impedance. I was using the Crenova.

One of the posts was from TAG forum which ended up somewhere around 80V screen and 140V plate.
https://ampgarage.com/forum/viewtopic.php?t=30019

The amp probably is working fine for it’s original intended purpose of amplifying a 1950s microphone, but it’s a little lame for guitar use IMHO. I’m not knowledgeable enough to design anything, that’s why I’m asking the more experienced people for the advice.

[John_H]

Try cathode, input and output capacitors instead.

That's where I would start.

[J M Fahey]

Thanks again for the comments.

My meters are Crenova 890Z with 10M input impedance, and Meterman 34XR with 9.1M impedance. I was using the Crenova.

One of the posts was from TAG forum which ended up somewhere around 80V screen and 140V plate.
https://ampgarage.com/forum/viewtopic.php?t=30019

The amp probably is working fine for it’s original intended purpose of amplifying a 1950s microphone, but it’s a little lame for guitar use IMHO. I’m not knowledgeable enough to design anything, that’s why I’m asking the more experienced people for the advice.

Sure enough, but voltages are fine, so nothing to do there.
Best proof is 140V plate voltage, perfect for 220V something supply. FWIW Fender/Marshall/etc. often show some 160V plate for 250V supply. Same thing.

You may optimize EQ for guitar, of course, maybe tweak gain, either increase for more distortion (if circuit allows it) or attenuate it to have more headroom.

Please post full amp schematic for better suggestions.

If not available, hand draw it, it should take one or two boring rainy Sunday afternoons at most

If unsure about some value, post what you see, we may guess it right with some background.