Grid leak resistor master volume question - 5F1

[camdenh]

Hey all! Pretty much a rookie here. You’ve been amazing with helping me out thus far so I thought I’d pop another question in.

I have a 5F1 as my tester amp and I’m looking to add a Master Volume — on the schematic/layout R9 is the 220k Grid Leak resistor — I used alligator clips and simply clipped a 250k potentiometer on either side of the 220k Grid Leak resistor, creating a “Master Volume.” Unless I am mistaken, this would mean the MV pot is “wired/clipped” in parallel to the 220k GL resistor.

I’ve read online that some folks say to leave the GL Resistor there in place and some say to remove it.

Is there an issue with leaving the 220k GL resistor there? The issue I can think of is that if this is indeed in parallel, would I be dropping the GL resistor value to 30k (250k pot - 220k GL resistor = 30k, much lower than stock value of 220k) — and, if so, is that an issue?

Thanks in advance and I look forward to learning from your replies!

[Helmholtz]

Schematic?

[camdenh]

Helmholtz yessir - it’s at the link below

https://images.app.goo.gl/CAr746Tz7ycbU16L8

thanks!

[Helmholtz]

Is there an issue with leaving the 220k GL resistor there? The issue I can think of is that if this is indeed in parallel, would I be dropping the GL resistor value to 30k (250k pot - 220k GL resistor = 30k, much lower than stock value of 220k

Your calculation doesn't make sense. Look up the formula for paralleling resistors.

You can leave the grid leak in place but it should be wired from pot wiper (=connected to grid) to ground.
The resultant grid leak resistance will be around 120k.
The lower value will somewhat reduce gain.

A better solution would be a 470k pot with a 470k resistor from wiper/grid to ground. This results in a total resistance of 470k/2 = 235k at max. volume.

[camdenh]

Helmholtz ah yes - you’re right about the formula. I currently have COVID so a little head fog.

So at present the way it’s wired in would yield ~120k ohms. Silly error on my end before.

is there a specific benefit to keep the GL resistor closer to the stock 220k? I’m not sure “why” it matters but I’m sure it does.

thanks again!

***edit*** my bad - again with the head fog. You already said it would reduce gain. Thanks again.

[Helmholtz]

Total resistance of the assembly acts as a load to V1B. Lower resistance means lower gain.
The resistor from grid /wiper to ground makes sure there is always a grid leak path to ground, protecting the power tube in case of a bad pot with an intermittent wiper contact.

[camdenh]

The resistor from grid /wiper to ground makes sure there is always a grid leak path to ground, protecting the power tube in case of a bad pot with an intermittent wiper contact.

All worked out perfectly as the node I had the original 220k resistor in also has a wire going to the ground bus so I put the wiper in that node and voila! Thanks again for your help. Super informative and deeply appreciated.

as a side note - I think I liked the 220k resistor in there! I have a 470k in there now (with a 500k pot - closest I had to 470k as per your recommendation) and there’s a lot more bite! I think I’ll work out how to put in a 220k on a switch so I can go back and forth.

thanks again

[Jon Snell]

If I get your idea, do you mean the 220k on the control grid of the 6V6, R9. If you do then you will screw around with the negative feedback loop.
It already has a master volume control, why add another I wonder. It is labeled 'Volume Pot' on your schematic.

[Chuck H]

the node I had the original 220k resistor in also has a wire going to the ground bus so I put the wiper in that node and voila!

Something about this doesn't sound right. Can you please draw up something to show the actual circuit you have wired in there?

[Chuck H]

If I get your idea, do you mean the 220k on the control grid of the 6V6, R9. If you do then you will screw around with the negative feedback loop.
It already has a master volume control, why add another I wonder. It is labeled 'Volume Pot' on your schematic.

In the schematic the "volume pot" is still between the triode amplification stages so any clipping of V1b is mitigated. For most sensibilities this doesn't constitute a "master volume". A master volume, in typical vernacular, would be a circuit where all available gain and clipping available from the preamp is tapped and voltage divided.

But I agree about the feedback loop. Using a master as described above would place it within the NFB loop reducing NFB as the master volume is turned down. There may be stability consequences or, at the very least, undesirable tonal consequences.

[Helmholtz]

It depends on how you define the function of V1B. Is it part of the preamp or rather the power tube driver and thus part of the power amp?
I tend to consider all tubes within the global NFB loop part of the power amp.

A MV after V1B has the same drawbacks as a PPIMV in a PP amp with NFB, as it messes with the NFB.
And in both cases an additional gain stage is used for extra distortion at low level..
I don't think, though, it will cause instability because it just lowers loop gain. Lead dress might be critical, though.
Wires to the pot should be shielded.

[camdenh]

Something about this doesn't sound right. Can you please draw up something to show the actual circuit you have wired in there?

yessir I will work on that today - thanks!