DC at both tone controls ?

[HaroldBrooks]

Just noticed there are no caps between the plate on the 6J5 and the first stage of the 12AX7, so DC appears at both the Treble and Bass controls. Is this normal on some amps ?

I always thought you should avoid running high voltage DC to any pot on any amp ? On this amp there is DC from the plates at the tone controls, I measured it as 120vdc (Bass pot), and 152vdc (treble pot).

What gives ?

Thanks !

[Dave H]

It's not normal as far as I know and they could have easily avoided it by changing the order of the series components.

[Helmholtz]

Pot noise is caused by DC current not voltage. As there is no DC current through the pots, I see no need for changes.

[Chuck H]

Pot noise is caused by DC current not voltage. As there is no DC current through the pots, I see no need for changes.

Except that a shorted pot, which isn't an uncommon failure, could create a dangerous condition for the player and/or the amp. Easy solution is a .1 cap between the plate and the pot on both circuits.

[Helmholtz]

Except that a shorted pot, which isn't an uncommon failure, could create a dangerous condition for the player and/or the amp. Easy solution is a .1 cap between the plate and the pot on both circuits.

I never had a shorted pot, but a short between track and the (metal) shaft could be a safety issue.

It suffices the reverse the order of caps and rheostats - no need for additional caps.

[Chuck H]

It suffices the reverse the order of caps and rheostats - no need for additional caps.

Of course. I agree. But depending on how the amp is built and wired it may simply be easier to install additional caps. No other reason to do it.

[R.G.]

I think I'd say that a different way.

In this circuit, the pot is DC blocked at the wiper and has one end open. There can be no voltage >difference< across the pot, as it all floats at the same DC voltage as one end. Therefore, there can be no DC voltage across the wiper contact, which is where moving the pot shaft causes little interruptions and therefore noise. To DC, this pot is a monode, therefore can't have any voltage difference and therefore can't have any DC current flow. So the question of whether it's the current or the voltage that makes noise is moot.

[Helmholtz]

So the question of whether it's the current or the voltage that makes noise is moot.

Who raised this question?.

All I said is that it takes a DC current through the pot to make it noisy. It's a conditio sine qua non.

Of course any current through a resistor (or contact) causes a voltage drop and any voltage across a resistor means that there is current flowing through the resistor, acc. to Ohm's law.

The noise signal itself is a fluctuating voltage.

[Enzo]

Some people do not understand the difference between voltage ON a part and voltage THROUGH a part, so it bore further explaining.


A internal pot shorting to a metal shaft? The metal shaft sits in a metal bushing that is bolted to the chassis. NO hazard.

[Chuck H]

Some people do not understand the difference between voltage ON a part and voltage THROUGH a part, so it bore further explaining.


A internal pot shorting to a metal shaft? The metal shaft sits in a metal bushing that is bolted to the chassis. NO hazard.

Understood. Still wouldn't do it that way

[HaroldBrooks]

Ok, just to make myself feel better, I added .047uf caps before tone controls. Hey, I just needed something to solder, and I realize there was little worry at this point.

I got another plesent surprise when I swapped out the "fully baked" 2 watt 5.6k ohm carbon resistor, that is between the first and second B+ (just before the screen grids), with two paralleled 5.6k ohm 5 watt metal oxide resistors.


Now it's changed 2.8k ohms and the screen voltage jumped from 315vdc to ~ 360vdc. Close to the 375vdc plate voltage.

The amp is now nastier than ever when driven hard, and it has taken on a new character that I like a lot. It seems to happen when you introduce just the right amount of Bass into the signal chain at high gain. You bend notes at just the right rate with your finger close to the fret, and the note "Howls" is the best I can describe it. The note starts off with a treble bite, and on decay, it loses the treble and becomes a good deal duller. I found this is more likely to happen when you have everything biased hot and with high plate and screen voltages. Cool effect, and it comes right from the amp, your fingers, and the guitar, no pedals required !

The great David Gilmour get's this howl on many notes in many of his solos. Here's one of the notes, starting at 4:39 and lasting just two seconds. https://www.youtube.com/watch?v=cpbbuaIA3Ds

Nice when an vintage amp can do this, and the ancient Zenith Hi-Fi amp now does it, and for very little Money !

[pdf64]

Just to note that increasing the g2 voltage will increase plate current under all conditions, idle all the way up to fully overdriven. So more current drawn from the PT and through the OT.
Of course with cathode bias, the cathode voltage will increase somewhat and so counteract that plate current increase a little.

[R.G.]

Who raised this question?.

All I said is that it takes a DC current through the pot to make it noisy. It's a conditio sine qua non.

Of course any current through a resistor (or contact) causes a voltage drop and any voltage across a resistor means that there is current flowing through the resistor, acc. to Ohm's law.

The noise signal itself is a fluctuating voltage.

All I said was OK, fine, it doesn't matter. That's not a question.

[HaroldBrooks]

Just to note that increasing the g2 voltage will increase plate current under all conditions, idle all the way up to fully overdriven. So more current drawn from the PT and through the OT.
Of course with cathode bias, the cathode voltage will increase somewhat and so counteract that plate current increase a little.

Thanks for pointing that out pdf64, I didn't think of it... Until I saw one tube red-plating ever so slightly, so I upped the shared power tube cathode resistor from 220 to 370ohms, and all is well now. I did this right after the change to the screen voltage, so I could test if I mucked something up or not.

As a matter of fact, at that new bias the two 6V6 tubes are closer in wattage dissipation and I feel more comfortable running them more equally, but regardless of that, the amp sounds great, and noticeably better as a result of raising the G2 voltage. I was suspicious that dropping the ohms on the B+ node resistor might make more noise or create some type of oscillation, but as the say the proof of the pudding is in the eating, and nothing bad happened so far.

I was also a bit concerned that I was pushing the original NOS tubes too high with plate and screen voltage, so I did a search and found a Fender Reverb running 6V6s at 415 volts on both plates and grids ! So I dropped the issue, but added two 470ohm grid resistors (one for each tube) as a bit of insurance. All my amps now run with diode protection on the rectifier, and Varistors on the plates to the center tap.

Thanks for your continued help.

I'm learning a lot from you guys.