Cathode / Fixed Bias switch confusion

[WRC34]

Happy Sunday everybody!

I'm kind of cruising through a little mod to an homebrew experimental amp here and adding a cathode/fixed bias switch. I dunno if any of you guys are familiar with the Hoffman EL34 world boards but a big wig over there named "sluckey" who has mucho knowledge and experience has posted docs including a schematic & layout of how to wire said switch using a big ol' SPDT.

Does he hang out here under another name? Sluckey can you hear me?

I've got it wired accurately and it is (kind of) working - it's allowing me to switch between fixed and cathode bias, and it sounds good but there is one issue - the bias pot is not longer working. I can turn the pot all the way in either direction without any change in mA/mV reading.

Prior to the mod the amp was exclusively fixed bias and the bias pot worked perfectly - and was tested just before adding the switch & cathode bias resistor/cap.

The only thing coming to mind in my slightly hurried state (sneaking this in before heading to work!) is perhaps I've wired the incorrect lugs in the switch? It's a big Carling SPDT switch with one lug on one side and two on the other. I've got the side with the single lug wired to ground and the other side has one lug wired to terminal 3 of the bias pot and the other lug has one wire going to the cathodes of the output tubes (through the pre-existing bias test point setup) and the other is connected to the "positive" side of the cathode bias resistor/bypass cap.

I know it's tough without photos or a schematic but anyone else run into this issue??

Thanks & hope y'all are well!!

ps - yes this amp gets it's bias voltage from pin 6 of the rectifier tube a la JTM 45 style, not a separate wire/winding on the PT

here's a link to said documents:
http://sluckeyamps.com/november/november.pdf

[WRC34]

thx in advance to any suggestions!!

[pdf64]

Sluckey’s wig is a fair size here too
In fixed mode, is there (negative) bias voltage on the trimmer?

[sluckey]

I've got the side with the single lug wired to ground

Deleted a lot of words!

Sounds like it's wired correctly. Need to see your actual schematic to say for sure. This SPDT switch will not work with all bias circuits. Some circuits require a DPDT.

As to your bias pot not working, well it's not supposed to work when in cathode biased mode, but it should work just fine in fixed bias mode. Here's a layout showing the Carling SPDT switch.

[WRC34]

Sluckey! Thank you so much for the sheer poetry you have bestowed upon me (I'm talking about the layout with the Carling SPDT illustration). I have learned a lot from your posts on the Hoffman boards. I am honored!

I was fortunate enough to catch a glimpse of what you wrote earlier this afternoon. It was cool. I was psyched to read it again in less hurried circumstances, but alas! I suppose I'd trade that for the layout you attached because a picture is worth waaayyy more than a thousand words in this instance.

Pete your wig ain't too small either my friend, you've been responding to my posts/helping me from the beginning

Indeed I do have the switch wired incorrectly. The tab that should be wired to the bias test points ground and the cathode resistor/cap I have grounded. Will re-wire correctly for maximum shreddification. I searched fruitlessly prior to wiring up the switch then just decided to take a guess, I guess...

Interestingly enough, I believe it did allow me to plug in a guitar and play in both positions, and it sounded pretty good! I think plate voltage was around 355 on one side (I assumed that to be cathode bias) and jumped to 389 on the other side (so I assumed that was fixed) although I know that the current way allows Slimer to haunt the amp's card catalog so I'll refrain for powering it up again until I've 'fixed it'.

Will report back with results, thank you gentlemen!!

[pdf64]

I think that the single pole switch method is fine if it can be ensured that the switch won’t be flipped while the amp is energised. The issue being that it will take a second or so for the fixed bias output to charge up, ie the output valves will have an initial ‘no bias’ shock, which seems like something it would be preferable to avoid.

Hence if the goal is to instantly flip between modes whilst the amp is operational, a dual pole switch arrangement would seem to be a better option.
eg

[WRC34]

Ok! Got it wired up exactly as Sluckey’s layout with the illustration of the Carling SPDT switch. Bias pot is now working fine and plate voltage between cathode/fixed is not as dramatic - around 375 in cathode bias and 390 fixed.

No chance for a sound test but these are encouraging results & numbers!

This amp is a sort of home-bound Frankenstein so I’m ok with the switch only ever being thrown with the amp powered off. The switch was added to the back panel of the amp so the chance of flipping it mistakenly while in use is very small.

However, going forward if I’m going to add this option on an amp for sale having it be more “idiot proof” is of major importance. Pete your drawing with the DPDT switch is awesome, thank you! I’ll be using it in the next build.

Hoping to record some quick sound tests tomorrow in both modes and post here

Thanks again guys, more soon

[WRC34]

OK!

Finally got a chance to test “at volume”

https://youtu.be/a6GX9vdGXHY

In the video above, I’m using a semi-matched pair of 6V6s. Measurements below are from a separate pair very tightly matched pair of 6V6s:

Fixed bias
Plate 337VDC
Screen 330VDC
tube 1 current 24.0 mA
tube 2 current 23.9 mA
Approx 16.5 watts output

Cathode bias
Plate to cathode 349VDC
Cathode to ground 15.7VDC (8.02 chassis)
Resistor 246 ohms
Approx 22 watts dissipation/11 watts per 6V6

The difference was less dramatic than I expected, but I’m happy to have done this ‘mod’ nonetheless. Thanks again Sluckey and pdf64!!

[sluckey]

The difference was less dramatic than I expected, but I’m happy to have done this ‘mod’ nonetheless.

I felt the same. I've had no need for that function since that one and only amp.

[Colossal]

The difference was less dramatic than I expected, but I’m happy to have done this ‘mod’ nonetheless.

Which mode do you prefer?

[pdf64]

...
Fixed bias
...
Approx 16.5 watts output
...

Just to note, although the total combined idle dissipation is an output of sorts (ie heat), to avoid confusion with audio power output, it might be best not to describe it as such.

[WRC34]

Colossal - I think cathode bias would be my choice, but I like the fixed quite a bit as well.

pdf64 - Thanks for the notation! Would the calculation for cathode bias output be considered inaccurate for audio output watts too?

I'm going to mess around with different tubes and see if I can get the amp to sound better. For sure one of the 6SL7s I had in there when making the demo has gone microphonic as your gin & tonic

[pdf64]

Audio power output is one thing, idle anode dissipation is another.
In fixed bias AB they’re pretty much unrelated, with cathode bias AB they’re somewhat tied together, but certainly they can’t be equated.
In class A, audio power can’t be higher than about half the total idle anode dissipation, but with a push pull amp, careful design and testing is required to be sure that it isn’t actually operating in AB.

[Colossal]

Colossal - I think cathode bias would be my choice, but I like the fixed quite a bit as well.

I'm going to mess around with different tubes and see if I can get the amp to sound better. For sure one of the 6SL7s I had in there when making the demo has gone microphonic as your gin & tonic

Sounds great, man. I like the throaty midrange. Did you use new production 6SL7s? What size bypass cap did you use to bias your power tubes?

[CraigGa]

Sounds great either way but I love how it's loud enough to mess with your camera