Simple switches that do useful things

[dwinstonwood]

Hello all,
I've been noodling around in my Hoffman Plexi 6V6 that has his Hot mod wired "always on."
I have a hole in the front of the chassis (from the original Hot Switch) and some switches. I'd like to try some things that are more or less non-invasive.

First, I'm going to try this bypass cap switch as drawn up by sluckey, since it doesn't require surgery to the board, just attaching a single wire:

https://el34world.com/Forum/index.php?topic=15612.msg150776#msg150776

But, I'd like to hear other suggestions, too. For example, is there a tried and true way to change the value of a voltage divider shunt resistor using a basic switch? Stuff along those lines.

Thanks!

[Auke Jolman]

Hello dwinstonwood,

I don't know about the resistors although it would be easy to try out I guess.

What I've done is to make use of extra gain at the cathode follower (https://www.valvewizard.co.uk/dccf.html). I've got this on a relay that I can activate with a foodswitch. When engaged it gives just a bit more.

[dwinstonwood]

Thanks Auke!

What a coincidence, changing my CF to the Valve Wizard's values was the first thing I did. Actually, I think these values are also the same as the AC30's CF, except for me using a .68uF cap. I also added the anti-arcing circuit.

And, yes, it does add a good amount of distortion. It's very smooth clipping, too.

Here are my CF values:

[Auke Jolman]

Lowering the cathode resistor give's you more compression. But I was talking about bootstrapping for more gain. It is explained in the last paragraph.

[stratomaster]

Hello all,
I've been noodling around in my Hoffman Plexi 6V6 that has his Hot mod wired "always on."
I have a hole in the front of the chassis (from the original Hot Switch) and some switches. I'd like to try some things that are more or less non-invasive.

First, I'm going to try this bypass cap switch as drawn up by sluckey, since it doesn't require surgery to the board, just attaching a single wire:

https://el34world.com/Forum/index.php?topic=15612.msg150776#msg150776

But, I'd like to hear other suggestions, too. For example, is there a tried and true way to change the value of a voltage divider shunt resistor using a basic switch? Stuff along those lines.

Thanks!

I find it's quieter to short a 47k or so resistor in series with a cathode bypass cap you're wanting to switch in parallel.  Otherwise it's liable to pop the first time the switch is thrown after power up.

Edit: here's a photo of a switch I added to a Blues Jr to allow the choice of cathode bypass cap, 1μF or 11μF on the FAT switch.  So a switch that switches a switch.    I used a 220kΩ instead of the 47k--likely because I had that handy on the bench. 

[dwinstonwood]

Thanks Auke. I did read that section. It looks like he just splits the anode resistor in half, and connects the junction to the CF's cathode with a small cap. That's not too involved to try.

Thanks stratomaster. My plan was to put a 1m across the switch to stop it from popping. Would that work just as well as your method?

[stratomaster]

No.  Technically the way I do it the cathode cap is always in circuit, the series resistor just limits the current in that network. 

This way does affect triode bias somewhat, but it's usually negligible. 

Edit, just saw the jumper on the switch. Your way will work the same. Resistor value needs to be low enough to allow some current flow, but not so large as to significantly alter the parallel impedance calculation. 1M may be too high.

[dwinstonwood]

Thanks. Yes, that tiny black line on the switch is the jumper. I should make it the same color as the terminals. It's not clear.

[dwinstonwood]

...Resistor value needs to be low enough to allow some current flow, but not so large as to significantly alter the parallel impedance calculation. 1M may be too high.

I just found this thread. I'll start with a 100K:
https://el34world.com/Forum/index.php?topic=12471.0

[dwinstonwood]

Switch is assembled. I've got a complete set of ANOS tubes arriving tomorrow or Saturday. I'll install the switch, bias the tubes and see how I like it.

One other change I made was to move the two 470K resistors after the extra (Hot) gain stage around to create a 50% voltage divider. The combination of this and the new CF values sounds really good to my ears: the gain is lowered, but the CF can now soft clip more easily (according to the Valve wizard). It's nice, smooth overdrive, not hard or edgy sounding. But, not to everyone's taste.

The Hoffman Plexi 6V6 is a great design/project. Even I can experiment with it and hear what small changes can do. I really want to buy an oscilloscope now, and see what's happening to the signal. 

[pdf64]

No.  Technically the way I do it the cathode cap is always in circuit, the series resistor just limits the current in that network. 

This way does affect triode bias somewhat, but it's usually negligible.  …

How might bias (ie DC operating conditions) be affected?
I can see that if the anti thump resistor is much less than 10x the value of the cathode resistor, the unbypassed dynamic loadline may be skewed a bit.
But something in the range 10 to 100x the cathode resistor value should indeed have a negligible effect on the loadline.

[stratomaster]

That's what I was referring to.  You have to take into account being larger than the desired cathode resistor but not being so large as to allow popping.  Luckily that window is fairly large. 

[dwinstonwood]

A big thanks for all of the input!

I installed the switch and the (new) set of used tubes. I can't get onboard with "ANOS." It reminds of the "previously owned" phrase applied to European import cars.

Anyway, all three 12AX7's are Sylvania short gray plates; two have square getters. One was rebranded by Ampex, the tape recorder company. Supposedly Ampex selected quieter tubes for their recorders and mic preamps. Who knows? So, I put it in V1. 

The amp sounds great and is very quiet. There is zero pop on the switch. I still need to engrave a faceplate for it. I've attached a PDF of my schematic and layout, if anyone's interested. BTW, my hybrid Tele/Strat is supposed to have a 1961 look with its vintage white color and mint guard.

[dwinstonwood]

So, that is just a ridiculous amount of gain. I want to play stuff other than (electric) Revolution - although, it really does sound amazingly similar to that board distortion that Geoff Emerick supposedly let them get away with.

I'm going to change that second stage back to un-bypassed and put the switch on the CF driver bypass cap. I have the sound in my head, I don't know how to get there. Think "What is Life."

[stratomaster]

That 470k gridstopper on the 2nd stage is probably what you're hearing.  There should be good amounts of gain, but without the highs that are being snuffed by that gridstopper/triode capacitance filter it'll sound gained out beyond the point of clarity.  Replace that with a 10k.  Just wire one in parallel for testing before you change anything else.

[dwinstonwood]

Thanks stratomaster.

I ended up not going with the schematic my reply #13 (I did not use the 470K resistors between the 1st and 2nd stages, I left it stock).

Here's what I ended up with and it actually meets the requirements of my thread title.

Switching in the .1uF on the CF driver adds only a slight increase in perceptible volume. But, it fills out the high-mids/low-highs in a really nice way - not overly bright like a pF Bright switch on a volume pot.

I'm happy with the changes I made. They were simple (non-invasive) and for my type of playing very useful. 

My PI triodes are are about 8V out of balance, but some say this adds good harmonics? 

It's hard to get steady voltage readings in this old townhouse in the city. Readings will fluctuate up and down by a volt or two while I'm measuring (who knows what the neighbors are plugging in and turning on). So, the voltages on my schematic are the "steadiest" ones I could read.

Thanks again!

[stratomaster]

The 470k at the input of the volume pot after the first stage is extremely useful for taming the gain.  Experiment with keeping that one.

Also, you might find a small bright cap on the 470k/470k divider after the 2nd stage might get you to a similar place as your 0.1μF switched cathode cap. 

These two changes will free you up to put the switched cap back to 0.68μ for a lead boost.

[dwinstonwood]

Thanks, but I really like simplicity in amps. I'll keep the switch in for a while, and if it stays on (like the Hot Switch did) I'll remove the switch and wire the cap across the cathode resistor. The gain is where I want it, and it still cleans up easily with the guitar's volume pot.