Power Tube Grid Mute Switch

XTRXTR

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After applying a three way DPDT to both cathodes of V1 on my 2204 bench prototype I am now getting serious pop/switch noise when the cathode bias switch is switch is engaged on any of the setting. V1b is clearly the louder of the the two. V1a I don't think is that offensive truthfully. I did not expect this on V1b. Until after I built it and tested it in circuit did I realize, of course the output then travels through V1a and gets amplified to a quite loud pop.

Anyway so I was thinking it is unlikely to remedy this without changing the architecture of the cathode switch, via adding capacitor and resistor, what some might call a pull down resistor etc.

Then I saw Robinette's Mute Switch and thought that looks promising. Similar to the type 3 Trainwreck MV single pot version it simply brings the two grids together to annihilate the signal. I know I had tried the pot to do this before as a PPIMV but had not done a switch.

Anyone here tried a Power Grid Mute Switch? I figure it sounds good on paper but don't know if a loud pop would ensue when opening or closing the switch. As long it is at idle it seems like a doable mute for engaging the input cathode switches and as a mute/standby between any other type of on stage quick change, broken guitar string, guitar change, other odds and ends.

It makes no sense if its going to pop since that is what I am trying to avoid.

Any thoughts? It's simple enough I could just try it and see but thought I would ask.
 

thetragichero

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1. some amps (silvertone, maybe?) used this switch on the power tube grids in place of the standby switch (frankly a lot better of an idea than switching dc)
2. the better idea is to re-engineer your cathode bypass cap switch. one of my builds has a three-way rotary switch for its 'bass' control that switches between cathode bypass caps on the single pentode gain stage. no annoying pops. i think i used 100k resistors between the caps and ground, the switch just shunts each respective resistor
 

XTRXTR

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I wonder if the switch type makes a difference, make before break or break before make? Also I had this done without much pop at all when I used an On Off ON and just two value changes. Then I thought hey get an ON ON ON DPDT and make a three way.

I'm just gonna jumper a clip over the power grids to see what kind of noise I'm dealing with. After the coupling caps @ the grid leaks... as per image from Robinette in case anyone is wondering;
1651641140821.png
Hmmm dual balance bias may make a difference in DC here. Dang it or I just turn down the MV volume and then back up, heard of placing a pot on the cathode too but this completely gets away from what I want, simple low med high switch on the cathode. Jumping the bias point of the characteristic -Vg curve lines on a 12AX7. Maybe two is enough.

I'll do some experiments and see what works. In general I'm switching LEDs on the cathode as per Merlin "LED acts like a capacitor" using various LED types for the VDC bias put on the cathode. It works awesome so I thought two and then three, but maybe I need to back up to Low and High. Adding a capacitor is redundant and a pull down resistor changes the Bias VDC (which is normally = to the Vf of the LED)

Too many ifs...I'll go back to basics and clip one at a time and see what happens. But the Grid Mute/Standby does seem better than a regular standby...on paper.
 

Jon Snell

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Why not use a 100k resistor, like R165, to balance the DC voltage.
Not sure what you want to do so I have included a top boost switch diagramme that reduces the noise you are speaking of.
Hope it helps.
 

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Pete Farrington

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Yes, as per above, you may be able to arrange the cathode mode options to get rid of much of the thump, though if the actual DC cathode resistor / cathode voltage is being changed, a bit of thump is inevitable.
 

Guitar Amp Tools

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Move the switch and place it between where the two cathodes (pin 8) on the power tubes are wired together and the power amp ground. When you open or close the switch it takes a second for the cathodes to charge/discharge and you get a soft ramp up and down and no thump at all. When the cathode is lifted the tube biases itself off.
 
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alpha al

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That's how the standby switch in a lot of 50's-70's Gibson amps worked. It's really an audio mute switch as opposed to a true standby.
 

XTRXTR

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Just to clarify this is a diagram of the cathode switch using various LEDs each with a different Vf that bias the tube triode. The cathode VDC jumps from a low 2.79, 1.8, 1.5 VDC. V1b the hi sensitivity cascades through the V1a so the pop is amplified.

On my 2204 the usual 2.7kR and 0.68uF cap provides approx. 1.9 VDC bias with a knee at about 110Hz
1651714656494.png
I don't see a creative way to get around this VDC POP other than muting the output tubes while switching. Sure I could turn down the MV but then I may not get it back to the stage setting I was on. I am open to suggestions.

As stated before Merlin points out:
"2: LED bias. Obviously this has the same effect as a bias resistor with a perfect bypass capacitor, so carries tonal considerations. The LED will slowly come on as the valve warms up."
And:
"By replacing the cathode resistor with an LED we eliminate the need for the large bypass capacitor, and obtain maximum gain and minimum output impedance at all frequencies in one fell swoop. A red LED will be suitable in most cases, providing about 1.6V bias, although other colours can be experimented with of course."

So this is the experiment I am involved with. I was thinking it was an easy way to see the setting visually and keep the knee very low and I wanted to hear this against another of my 2204s for comparison.

Perfect Bypass capacitor means to me a very low knee - Am I interpreting that wrong?
 

thetragichero

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yeah unfortunately switching the dc bias point is going to result in pops
by 'perfect bypass capacitor' merlin means that the -3db point (the 'knee' of the curve) would be at 0hz
 

Pete Farrington

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Maybe connect a resistor directly between cathode and 0V common, so that it’s never (momentarily) open circuit, eg 10k - 100k, to see if that helps?
 

Pete Farrington

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dual balance bias may make a difference in DC here
Yes, much of a difference in control grid voltage between the push pull sides will cause a thump in itself.
Why not mute the signal earlier on, eg at master vol, by shorting it out, ie to 0V common?
Is there an fx loop? If so, use of a momentary type footswitch pedal to do that may be convenient.
 

XTRXTR

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Yes, much of a difference in control grid voltage between the push pull sides will cause a thump in itself.
Why not mute the signal earlier on, eg at master vol, by shorting it out, ie to 0V common?
Is there an fx loop? If so, use of a momentary type footswitch pedal to do that may be convenient.
Yes, Yes, and Yes Thanks, sometimes you can't see the easy stuff.

Thank you for confirming the dual bias balance DC difference.
The Mute at MV is a great Idea.
There is an FX Loop too.

Then the logical Mute point while FX loop in place is through the FX unit. The unit doesn't have a mute per se but does mute when going into the Guitar Tuning circuit and that is what I have used in the past when switching guitars etc.

When FX is not in circuit via the return input switch jack then a Mute could still be used at that point where the send meets the return or before the PI, at least for now while I am prototyping the amp.

Awesome, solved, thanks again Pete.
 

XTRXTR

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yeah unfortunately switching the dc bias point is going to result in pops
by 'perfect bypass capacitor' merlin means that the -3db point (the 'knee' of the curve) would be at 0hz
Thanks for confirming dc pops and the 0Hz knee Merlin points out.
I won't know whether this works out in its full working condition until at least fully prototyped but I can tell you the bass response from the guitar is great for super cleans and overdrive.
 

XTRXTR

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I think I got some bad switches. The concept works when I clip in the mute but once I solder in the switch it pops just touching it, and as I say this I'm wondering if I have DC on the MV from a leaky cap. The MV is scratchy when turning... So I have what feels like cascading issues. Ugh I planned to rebuild this after I tested everything maybe it's time to just build it.
 

thetragichero

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if you're willing to rethink your switch and add a bunch of extra components you could look into adding a temporary mute circuit (i'd do it right after the stage you're changing the cathode voltage to). here's a bunch of reading material: https://sound-au.com/articles/muting.html
might also wish to google jfet mute switch
 

XTRXTR

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Well I figured out that I did not have the problem I thought.

As it turns out, the solid core wire I was using to connect the temp board to the old board, through a PCB ground plane on the old board broke loose.

Every time I touched the new board it was stressing this connection.

I have an LED feeding the DCCF B+ connection and it was losing light when I even slightly touched the board under test, thats when it clicked.

I pulled the power tubes and DMM probed the B+4 point and then chopsticked the new board and surrounding area. At some point I touched the ground point and visually saw it move up and down in the through hole. The voltage rose and the LED turned off, a clear indication it stopped conducting as the ground was lifted.

Fixed that and played the guitar. The new LEDs on the V1a cathode did NOT have what I was expecting either.

The Low switch is actually a 39kR, Med is a 2.73 Vf Blue 10mm LED, High is a 1.5 Vf Red 5mm LED. On low the amp works as expected but the Med...while providing a less -Vg bias point -2.73VDC it also has the effect of allowing all frequencies to wrap around the fundamental. I couldn't give you the math on it but I'm sure its the capacitive effect of the LED. As Merlin pointed out its a perfect bypass cap so everything from 0Hz when first picked and then ridiculous highs come through as the note is sustained.

I haven't verified with a scope but it acts like an oscillating feedback loop. As the note fades all I hear is high end squeal.

I was trying to combine the three way cathode switch from Robinette with Merlin's idea of using LEDs to change the the -Vg bias point.

On the bright side I got past the pop. Now I will go back to just resistors on the three way cathode switch for V1a. Rob used an On Off On where the 39k was the center position, and then 10k and 4k was achieved with other resistors in parallel to the 39k. But I will use an ON ON ON with the resistor values in series. It worked before so I expect it to again, I just want a low, med, hi, toggle instead of the Regular, Low, High toggle as Rob posted on his site.

V1b three way LED switch works fine with ON ON ON using Blue, Red, and IR(emitter) LEDs 2.73, 1.8, and 1.14 VDC respectively.

Set backs and progress... Life right?
 

XTRXTR

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Wall Voltage yesterday 119.5 VAC, today 125.2 VAC, perhaps a switchable bucking transformer would be useful as a DIY. Plate Voltage 486 VDC highest I've seen. Ran a check on 5881s bias, about 56% not accounting for screens - so - cool bias still.

I replaced the LEDs on V1a with 39k, 10k and 2k and that solved the oscillating issue described above. I think I need to tweak the values now to get a more linear feel from low, med, to high. Right now the VDC is changed from 3.9, 3, and 1.8 but it doesn't feel like a big change from low to med and then med to high. So I will play around with clipping in parallel some values until I get what I want then replace the resistor on the ON ON ON switch. Maybe start with higher resistor on low then something like 8k for med and 1.5k on high. ...We'll see tomorrow. Overall pleased with the progress now.

The pop is dramatically lower and I am using a make shift alligator clip from the MV wiper to ground as a temp mute which works great. Definitely adding a mute switch in for situations that arise unforeseen.
 
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