Amp making guitar pot scratch?

[duriehill]

Hi all

I finished a rocket build a while back and I've notice that when I use this amp (compared with my other amps) that it makes the volume pot on my guitars scratchy when turning up or down.

Why would this be? DC?

Thanks!

[pdf64]

Try a known good tube in the 1st V1 slot, maybe one from an amp that doesn't have the problem.

[xtian]

Yeah, DC. I found some 12ax7 let dc thru the grid. Swapping tubes fixed it.

[Blackburn]

It's not a tube problem. In fact, it's not a problem at all. It's common in amps that run V1's plate voltage pretty low. I believe the grid goes slightly positive and bleeds DC onto the guitar's volume pot.

Edit: I guess tubes may differ with regard to the amount of DC. Haven't tried that. A swap eliminated it completely, Xtian?

[JazzGuitarGimp]

When I put the vvr into my 5E3 build, I was told that I might need a coupling capacitor at the input grid, depending on how low I wanted to go with the vvr. Of course, I wanted to go as low as possible So I put the cap in and I've never had any issues with it.

[duriehill]

Thanks - sounds like it was intentional after reading this post (very interesting):
https://tubeamparchive.com/viewtopic.ph ... eamp+tubes

I'm getting -180mV which seems like a bit much though, so will try some tube rolling. The problem is I only have modern tubes to hand. When I first set up my rocket I settled on JJ ECC83 in v1 and v2 - they seemed to be the quietest when comparing with the modern gold lion, tungsol and LPS.

Any recommendations for modern tubes in these positions in a rocket? I read maybe a Svetlana Winged C could be worth a try. Also, do you think -180mV is ok!?

[Blackburn]

I posted this in that thread, but in case one doesn't reach that page.... Here's a quote by Hogy:

--------------------------------------------------------------------------------
"You can't forget that Ken absolutely loved AC30s.

I have had the good fortune of owning a couple of particularly great sounding AC30s, and I can tell you that if you find a good one, it's as good as a Rocket in terms of tone. In fact, for certain things, it might be better.

I think what Ken wanted in the Rocket was an amp that sounded consistently as good as the greatest vintage AC30s. Very few of those actually do.

So take a killer AC30, optimize the layout, correct some issues like excessive ghost noting and flubby bass response, "trainwreck" the circuit by careful parts selection, and there you are.

Note that Ken didn't just up the cap values in the power supply, but completely changed the design from a radial supply to a totem pole style. This is one reason the voltages in the preamp are what they are, and that is deliberate.

This circuit has its quirks, too. A properly running Rocket leaks a small amount of DC onto the guitar's volume pot, making it scratchy. Doesn't happen with all preamp tubes, but interestingly more often than not with the ones Ken liked best in V1, Bugle Boy ECC83s."
----------------------------------------------------------------------------------

There you have it.

[xtian]

Very interesting quote from Hogy, Blackburn. Thx. Yes, in my case, swapping tubes completely cured the issue, but now I can't even remember which amp it was!

[sliberty]

The Rocket input consists of a 1M resistor to ground, and a 33K resistor in series with the grid. This is effectively the same input as a Fender. Why would it be any more likely to leak DC to the guitar?

[JazzGuitarGimp]

The Rocket input consists of a 1M resistor to ground, and a 33K resistor in series with the grid. This is effectively the same input as a Fender. Why would it be any more likely to leak DC to the guitar?

The lower the plate voltage, the more likely the grid will leak DCV.

[duriehill]

Just tried a few tubes I had which show difference in DC, taken pin 7 v1:

Gold Lion standard: open -31mV, closed -0.7mV
Electro Harmonix 12AX7EH: open -128mV, closed -9.5
JJ ECC83s: open -88mV, closed -3mV
Sovtek LPS: open -64mV, closed -3mV
Tungsol reissue: didn't record but similar to LPS.

I had a JJ installed where I noticed the DC, I've now installed the Gold Lion so there's no scratching on the guitar pots now.

[beasleybodyshop]

I posted this in that thread, but in case one doesn't reach that page.... Here's a quote by Hogy:

--------------------------------------------------------------------------------
"You can't forget that Ken absolutely loved AC30s.

I have had the good fortune of owning a couple of particularly great sounding AC30s, and I can tell you that if you find a good one, it's as good as a Rocket in terms of tone. In fact, for certain things, it might be better.

I think what Ken wanted in the Rocket was an amp that sounded consistently as good as the greatest vintage AC30s. Very few of those actually do.

So take a killer AC30, optimize the layout, correct some issues like excessive ghost noting and flubby bass response, "trainwreck" the circuit by careful parts selection, and there you are.

Note that Ken didn't just up the cap values in the power supply, but completely changed the design from a radial supply to a totem pole style. This is one reason the voltages in the preamp are what they are, and that is deliberate.

This circuit has its quirks, too. A properly running Rocket leaks a small amount of DC onto the guitar's volume pot, making it scratchy. Doesn't happen with all preamp tubes, but interestingly more often than not with the ones Ken liked best in V1, Bugle Boy ECC83s."
----------------------------------------------------------------------------------

There you have it.

Blackburn, could you elaborate on what you think Hogy meant by changing the design from radial to totem pole?

[matt h]

(deleted)

[beasleybodyshop]

Not blackburn, but a "radial" design is another word for a parallel node power supply. The Matchless ac30-clones (actually almost all of the matchless vox-descended amps) keep this parallel node bit.

The "totem" (a terrible word) in this case refers to a serial power supply node. This is, uh, the ridiculously common one that most fendermarshalleveryoneelse tends to use for everything. With each successive stage, you get better and stiffer filtering. Better hum reduction, more control over voltages, and (and this is a big one for vox-like amps), much less chance of motorboating!

Thanks Matt, this makes more sense to me now.

[Cliff Schecht]

The Rocket input consists of a 1M resistor to ground, and a 33K resistor in series with the grid. This is effectively the same input as a Fender. Why would it be any more likely to leak DC to the guitar?

The lower the plate voltage, the more likely the grid will leak DCV.

I've never heard of this. Not saying it's untrue by any means but under what mechanism does this happen?

Also why not place a DC blocking cap at the input? Somewhere around 0.022uF places your cutoff a decade below where the guitar even picks up (7Hz cutoff for an ~80Hz signal). I've done this on plenty of amps and it doesn't ever hurt the sound but it has stopped amps from shocking themselves (when someone is playing guitar and singing through multiple amps on the same power circuit).