guitar volume pot scratchy due to dc on tube input

[Colas LeGrippa]

Hi boys and gals,


I solved the problem by adding a .047 cap at the input. But why a 12ax7 would develop dc voltage on the control grids ? Is that the reason why on some gretsch amplifiers there are cap inputs ?




Colas

[shooter]

cap at the input

usually I expect an input cap to keep out AC noise, or unwanted frequencies.  What were you plugging in that had a DC component to it?

[jjasilli]

Is that the reason why on some gretsch amplifiers there are cap inputs ?
Can't be answered w/o a schematic.  There's a number of ways to bias a preamp tube.  We are most used to cathode (self) bias.  There are other bias methods which require a cap on G1.

But why a 12ax7 would develop dc voltage on the control grids ?
With fixed bias, it's obvious why there's DC on G1.  With self-bias, I think DC can appear on G1 due to a gassy tube, or fault in the amp.  In the latter case, this could be a serious shock hazard.  A series input cap would block the DC from leaving the amp.  Guitarnutz.com recommends putting a series cap in the guitar to keep DC out of the guitar in case the amp has no such protective cap.

[sluckey]

I solved the problem by adding a .047 cap at the input. But why a 12ax7 would develop dc voltage on the control grids ? Is that the reason why on some gretsch amplifiers there are cap inputs ?

That's a common issue with a VVR installed to control the entire amp. Is that the case with your amp?

Some amps put a cap on the input to block any dc voltage that may be on the input device. Others use a cap because they are required if the tube uses grid leak bias such as this Gretsch amp...

     http://el34world.com/charts/Schematics/files/gretsch/gretsch6150.pdf

[Colas LeGrippa]

Ok I must add some details here that I have omitted.
I was not talking about an amplifier but a preamp. The heaters of the preamp tube (12a,,,7) work on 12v regulated DC. I think this might have been something to do with the guitar scratchy pot. Is it possible that some of the dc voltage from the heaters jumps to the control grid of the 12ax7 ?

[sluckey]

Is it possible that some of the dc voltage from the heaters jumps to the control grid of the 12ax7 ?

Very unlikely.

[92Volts]

Besides being unlikely, voltage is never supposed to leak from heaters to grid. If it does, that's a faulty tube.

I've experienced this before and it was a bad 12AX7. Maybe gassy-- not sure, but switching tubes fixed it. I only read a couple volts with a DMM, but it caused a tingle when touching the guitar strings. So it might have been higher than measured, and I considered it potentially dangerous.

Anyways, an input cap is a workaround, but there's an underlying problem that should be fixed.

[Colas LeGrippa]

sure !

[Colas LeGrippa]

to resume, impossible to get dc voltage on a control grid of a 12a....7, fine thanks.
Then how come a guitar pot is scratchy when hooked up to a certain preamp, and NOT when hooked up to any other kind of device such as ss preamps, ss and tubes amplifiers ?


Colas

[sluckey]

to resume, impossible to get dc voltage on a control grid of a 12a....7, fine thanks.
Then how come a guitar pot is scratchy when hooked up to a certain preamp, and NOT when hooked up to any other kind of device such as ss preamps, ss and tubes amplifiers ?


Colas

Pay attention! Nobody said it's impossible.

In this short thread you have been offered two ways for dc to appear on the grid. And no one said it's "impossible" to leak from the filaments, just unlikely.

If you share that "certain preamp" schematic maybe we can spot a valid reason. Hell, it may even be fixed bias. But until you bare all, we can never know!

[shooter]

leak from heaters to grid.

I'm missing something, isn't filaments typically AC?

[sluckey]

leak from heaters to grid.

I'm missing something, isn't filaments typically AC?

see reply #4

[shooter]

 thx 

[Lauri]

But why a 12ax7 would develop dc voltage on the control grids ?

Grid current. Reasons for it can be: low plate voltage, high value grid leak resistor, gassy tube.

[Colas LeGrippa]

low plate voltage !!!!!!!!!!!!!!


We are talking about a 12VDC supply for the heaters and the plates. Sure the plates are starving !!!!!

[Colas LeGrippa]

I oughta find out why a 9VDC Ibanez ''tube king'' is non scratchy, while my own 12VDC ''no name'' is scratchy.(was scratchy....I should say).
A look at the schem might give me the answer (maybe I should change my name for Ibanez?? )


Colas (without coca).

[Colas LeGrippa]

on the tube king, the first half of the 12a.....7  is ss driven through a 100n cap. Anyway all the circuit b4 blocks the dc from reaching the guitar pot (if there is).

[92Volts]

We really need to see a schematic of the circuit you're experiencing problems with.

The schematic you posted "would" send DC voltage to the guitar because bias voltage is applied to the base (input) of transistor TR1 near the input, through resistor R15. Voltage is prevented from leaking back into the guitar by capacitor C8.

However, that bias technique is particular to the type of transistor used in that circuit. It has nothing to do with the tubes in that circuit, and it's not a good analogy of any other tube circuit I know of.

Both tubes used in that circuit are connected in a way that should not bleed voltage back onto the guitar... regardless of the circuit/capacitors between the tube and guitar. Yes, each tube is driven through 100n capacitors but this is needed to "protect" the tube from DC voltage before it in the circuit, not the other way around. The grid (input) of each tube section should be at ground (0 volts).

Again, a faulty tube could cause all sorts of trouble, even though there is no explanation for this is "normal" operation. Have you tried another tube to see if it fixed the problem?

Also, are you experiencing trouble with that exact circuit? If so, this is an issue with the SS circuit before the tube, not the tube itself. It sounds like you understand that but you haven't provided us enough info to tell you what or where the problem is.

[Colas LeGrippa]

my circuit is exactly like a typical amplifier first 12a....7, no more no less, in order to be transparent, with a 1m grid leak and 68k input. The power supply is separated.

[92Volts]

Can you check the voltage at the grounding point of the grid's 1M resistor? It should be equal to your circuit's overall ground voltage. In turn it should be equal to the ground/negative side of the guitar cable because those things should all be connected to each other, and to ground!

A bad solder joint, weak connection (via the chassis for example) or broken wire could cause this to float above ground.

To give a specific example, if both grid and cathode resistors are "grounded" to the same location but that location is poorly or not connected to actual ground, current would flow through the tube, out the cathode, and have nowhere to go except the grid and instrument.

Also, if you connect the grid leak resistor to the wrong end of the cathode resistor (aka connect it to the cathode) you'll get cathode voltage applied directly to the grid/instrument.

So there are some "easy" wiring errors (due to only 1 mistake) that could cause this. Have you ruled out a bad tube yet?

[Lauri]

low plate voltage !!!!!!!!!!!!!!


We are talking about a 12VDC supply for the heaters and the plates. Sure the plates are starving !!!!!

You can't use typical circuits that are designed to work with proper voltages with only 12V. 

You should read this
http://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

[pdf64]

...
You should read this
http://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

Wow, I hadn't come across that before.
Great info, thanks!

[92Volts]

You can't use typical circuits that are designed to work with proper voltages with only 12V. 

You should read this
http://valvewizard.co.uk/Triodes_at_low_voltages_Blencowe.pdf

Great article! And it sounds like it might be the explanation here.

Like intentional grid-leak bias, this effect would produce negative grid voltage.

I experienced positive voltage due to a bad tube, which I understand might have been gas or a hot grid. The polarity might confirm whether this is expected or indicates a faulty tube.

[Colas LeGrippa]

''Great article! And it sounds like it might be the explanation here. ''

Great article indeed. I really think this is the explanation and I surely will modify my preamp's circuit acordingly.

Colas LeGrippa

[jjasilli]

This is what I was pointing out 21 Replies ago.  The length, confusion & circularity of this thread is due to the failure & refusal of Colas to post a schematic in the first place.


I'm on a campaign to get posters to clearly state the facts up front.  (BTW: I still don't know if Colas actually has a 12V tube or not!)

[Colas LeGrippa]

jjasilli,  all the replies of the posts have been very interesting, I don't understand your reply, amigo.  I didn't refuse to post the schem of my circuit, I haven't drawn it yet and I probably won't, it is so simple it's stupid. Everybody here understands wat is a grid stopper, grid leak and 12V power supply so everybody figured it out but you ? If you wanna make your own rules, fine, I never fought the laws of this forum !!!!
Have a great day, take it easy man !
Colas

[sluckey]

We could have reached that understanding in one or two replies if you would have revealed all about your simple circuit, rather than dribble out little pieces of the puzzle spread out over a week. Be kind to us Colas. Just because you may not think a particular bit of info, such as plate supply is only 12V, is important, doesn't mean it isn't. I'm sure if you had known that low plate voltage causes your symptom, you would have never asked. We try to give you what you need. Do the same for us.

[Colas LeGrippa]

You're right Sluckey. (JJasilli, is that what you were trying to tell me ? ) Posting schems and all the infos available help a lot to pinpoint the problems we experiment. You guys always tried to help me a lot and maybe I have considered this forum more like a social club. Anyway it is true that I can't expect my mechanic to fix my car if I keep all the clues for myself and next time I'll share all of them with you.  Let's admit though that the discussion had been very interesting, because I kept some of the clues for me I presume ! 


Colas

[sluckey]

Let's admit though that the discussion had been very interesting, because I kept some of the clues for me I presume ! 

Maybe. I suppose it depends on which side of the discussion you are coming from. How would you feel if the answer would fit in a single sentence but you only got one word per day and had no idea how long the sentence was? Tell all. Be awesome! 

[Colas LeGrippa]

Ok Sluckey, don't add more than necessary, I think I get it now......


Thanks to all. The Valve Wizard article highlighted all we were ignoring on starved preamp tubes.
I love you all (almost all  ).


Colas.

[jjasilli]

You're right Sluckey. (JJasilli, is that what you were trying to tell me ? ) Posting schems and all the infos available help a lot to pinpoint the problems we experiment. You guys always tried to help me a lot and maybe I have considered this forum more like a social club. Anyway it is true that I can't expect my mechanic to fix my car if I keep all the clues for myself and next time I'll share all of them with you.  Let's admit though that the discussion had been very interesting, because I kept some of the clues for me I presume ! 


Colas

[Colas LeGrippa]

Jjasilli if I were you I wouldn't be so arrogant, Sluckey's the one who did the job, don't drink beer on it's behalf. You should have adopted a more constructive approach to reach your goal. I guess one needs certain qualities to become a moderator. I thought the discussion had ended up happily yesterday...

[jjasilli]

The beer clinking was meant from me to you.  It was your post that I quoted & responded to.

[Colas LeGrippa]

sorry buddy 

[jjasilli]

"Take # two" :   

[Colas LeGrippa]

Lol ! Now you're talking, at 4 o.clock, it's perfect !