Safe to use 9mm pots in a tonestack in a tube amp?

[j_flanders]

Looking for the smallest possible knobs and pots, I came across a good suggestion here:
https://www.diystompboxes.com/smfforum/index.php?topic=121185.msg1137965#msg1137965

But can I can safely use 9mm pots for Treble, Mid and Bass Controls in a tube amp?

I'm modding a Blackheart Killer Ant:


Here's the schematic: http://www.sewatt.com/files/sewatt/BLACKHEART%20BH1%20SCHEMATIC.pdf

I want to replace the fixed resistors in the TMB tone stack and Master Volume with pots, but as you can see in the photo above there's very, very little room for pots or for knobs, so I thought about using those 9mm pots and micro knobs:

https://www.thonk.co.uk/shop/alpha-9mm-pots-vertical-t18/


https://www.thonk.co.uk/shop/micro-knobs/

Here's the datasheet for those pots:
https://www.thonk.co.uk/wp-content/uploads/2017/12/Alpha-9mm-Vertical-T18-RD901F-40-15K-B50K-0057.pdf

I read: Power rating 0.05 or 0.02W and also Max operating Voltage: 50V A.C. ?

[Steben]

50V ac is easily surpassed in Tube amps...

[antonis]

I read: Power rating 0.05 or 0.02W and also Max operating Voltage: 50V A.C. ?

I read (at schematic posted) All resistors of concern are rated at 1W..

[j_flanders]

50V ac is easily surpassed in Tube amps...

Ok, thanks, then they're out of the question.
I noticed Fender used a 9mm pot for volume control in my Chanpion600 which got me hoping but other tan 1M10A I can't say what its specs are.

I read: Power rating 0.05 or 0.02W and also Max operating Voltage: 50V A.C. ?

I read (at schematic posted) All resistors of concern are rated at 1W..

I think most of them are 'over specced'.  (According to this calculation it could be fine: https://el34world.com/Forum/index.php?topic=16784.msg166214#msg166214 )
The all 1 watt resistors was kinda part of their advertising:

it features completely overbuilt component specs, from 1W resistors in the signal path to double sided PCB with 2 oz traces. Even its diminutive cabinet, just over palm sized, is built to take a beating with 15 ply (18mm) void-free wood covered expertly in durable black tolex. Final touches include distinctive white piping accents and a salt & pepper cloth covered front baffle. 4, 8 and 16-ohm extension speaker jacks allow you to hook the Ant into virtually any cab you like, including a 4 X 12.

Anyway, I have 49mm between volume pot and input jack. In theory I should be able to get 3 x 16mm pots in there, but that's gonna be so tight.

Edit: Found this picture in Google, so it can be done:

[vigilante397]

Whether or not it is safe depends on the topography of the tone stack. If it's a tonestack after a cathode follower the voltage will be fairly low. Frankly I use 9mm pots for 200V preamps all the time, I just make sure the pots don't see 200V. Do you have a schematic you're working off of?

[PRR]

> But can I can safely use 9mm

What is the worst that can happen? £1.99 of smoke?

[j_flanders]

Do you have a schematic you're working off of?

http://www.sewatt.com/files/sewatt/BLACKHEART%20BH1%20SCHEMATIC.pdf

> But can I can safely use 9mm
What is the worst that can happen? £1.99 of smoke?

Having 3 holes in the faceplate with a spacing for whch no (suitable) pots exist.
But I guess I could try them before drilling holes.

[vigilante397]

So that schematic is using a cathode follower to drive the tone stack, and that's going to be plenty safe for 9mm pots. You have nothing to worry about there.

[j_flanders]

Great, thanks all for the help.
I'll give an update after ordering pots and knobs and doing the mods.

[mozz]

If there's no DC at the points you intend to use pots, size or rating is irrelevant.

[antonis]

If there's no DC at the points you intend to use pots, size or rating is irrelevant.

So, you can safely put a 22R 1/4W resistor into mains socket plugs..

[Mark Hammer]

I would think that manoeuverability of the knobs is of equal concern to whether the pots can manage the system voltages and such.  These are controls, after all.  If you can't easily and reliably adjust them to taste, and get your fingers in there to do so, they sort of stop being that.

And, unless there are factors that physically preclude it, there is no law that says all 3-knob tonestacks MUST be in a linear row.  A triangular arrangement (mids on top OR below treble and bass knobs) can provide a little more "finger space", while still occupying a small amount of lateral (i.e., left-to-right) space.

[mozz]

If there's no DC at the points you intend to use pots, size or rating is irrelevant.

So, you can safely put a 22R 1/4W resistor into mains socket plugs..

I'm talking about the original question about the tone stack, don't be giving advice on how to start fires. 

[j_flanders]

I received the pots and knobs last week. When testing they held up fine.
Both pots and knobs feel nice and good quality.

I eventually decided to not put the TMB controls on the front but on the back.
A wise choice as drilling holes equally spaced and on one nice line is something I can't seem to manage without a drill press.

On the front I only added a master volume.
On the back I also added 3 togle switches. The first two let me choose 3 different values for cathode resistor and bypass cap.
The third switch gives me 3 different bright caps.

Front:


Back:



This (pre)amp is essentially a single input 1959 super lead plexi going into a (single ended) 12AX7 as output tube.

1959 slp schematic:


Blackheart killer Ant schematic:
http://www.sewatt.com/files/sewatt/BLACKHEART%20BH1%20SCHEMATIC.pdf