Asking for an opinion - Switchable Fixed Bias to Cathode Bias

[kagliostro]

Two options to achieve the same result, Fixed Bias to Cathode Bias





Which will be your choice ?

Many Thanks

Franco

[Jennings]

Aren’t these both the same, but just drawn slightly differently? Or maybe I missed something?

[Leevi]

I would choose the 2nd one. Actually I have never used the 1st one and do not know its behavior.

/Leevi

[Jennings]

Aren’t these electrically the same though in practice? I’d still build the second, if it were me, but that’s just because it’s the way I’d first think to draw and implement it. Or would the first be better in the rare event of a switch failure?

[sluckey]

Both circuits are ***FUNCTIONALLY*** identical, but the switch wiring is slightly different. I see no advantage of one over the other.

Here are a couple more circuits that accomplish the same thing...

[pdf64]

…

Here are a couple more circuits that accomplish the same thing...

The right hand one should be much more resilient to avoiding meltdown in the case of switch failure.
A DPDT switch could be paralleled up to further mitigate for switch failure.

I think that provided there’s greater than say 10k series resistance between a 50V bias tap off the HT winding, as per eg a 5F6a, it would be fine.
Not ok for most fixed bias black or silver panel models though, apart from the Princeton.

It may be worth noting that these mode switches often don’t give the results hoped for; amp circuits tend to work and sound best when operating in the mode they were designed for.
Amps designed for fixed bias may have too high a HT voltage / too low a load impedance, to give good results in cathode bias amp. Perhaps because it necessitates a cathode bias point too far from class A.

[kagliostro]

Many thanks to all you friends

I asked because I was thinking there may be something to keep in mind doing a choice also if they absolve the same function

Interesting the versions Sluckey submitted

Amps designed for fixed bias may have too high a HT voltage / too low a load impedance, to give good results in cathode bias amp. Perhaps because it necessitates a cathode bias point too far from class A

And .... yes, on this occasion the amp is an old Geloso G1 1020 A that is to be converted in a guitar amp, it uses a pair of el84 as power tubes with 330V of B+, a slightly high voltage for those tubes, we have think to solve the problem with a Gyrator that we (at DIYItalia) know to drop around 10V on that application

Lowering to around 320V the B+ I hope will permit to have both Bias options (Fixed and Cathode) without problems

From this


To this


Franco

[pdf64]

I think that may work out fine, sorry to raise a concern. 

[kagliostro]

 

[JPK]

How much different does Cathode VS fixed sound? I have no personal experience with this switching option.

[pdf64]

I suspect it’ll depend on the specifics in each case.
I’ve only tried it on one amp, a Vibrolux Reverb based build. I hoped that the lower than typical HT voltage of that model would help to get good results, but I wasn’t impressed and got rid of it.
Switching from fixed to cathode bias, the power output dropped about 25%, which wasn’t really noticeable apart from the bottom end losing a bit of oomph.
The overdriven tone wasn’t as fat, a bit reedy, perhaps due to the cathode voltage increasing a lot, causing crossover distortion.
I should have tried it with the speakers connected to a lower impedance tap, thereby changing the loading on the output valve from 4k to 8k. That should reduce the rise in cathode voltage, and reduce the max power output a little more.

[kagliostro]

I'm not a musician so I can't say how they sound different, only they do it

As the original circuit of the Geloso is a Fixed Bias and many guitar amps that uses el84 are Cathode Bias it was think to swap to the Cathode Bias alternative

Knowing that to have both the Bias versions on the amp is possible and ispiring to the Weber 6d30 Kit, we have think to have both options as to be able to have different tone responses

Franco

[sluckey]

I put the switch on my EL84 November Amp. Played with it a few days. Couldn't hear enough difference to matter to me. Remove the switch and never thought about it again. Until someone brings it up on the forum. 

Fun to dream about. Not much fun in reality.

[JPK]

Appreciate everyone's input. Interesting stuff. Wish I never sold my November. That was a very cool amp. Thought I was done building amps. Then I retired. Whups.

[brewdude]

Fun to dream about. Not much fun in reality.

I have found this to be true of most switching options.

[kagliostro]

I put the switch on my EL84 November Amp. Played with it a few days. Couldn't hear enough difference to matter to me. Remove the switch and never thought about it again.

So did you ended with the Cathode Bias (as I suppose) ?

Franco

[sluckey]

So did you ended with the Cathode Bias (as I suppose) ?

No. That amp was designed to be a fixed bias amp. I left it like that.

All my other EL84 amps are cathode biased.

[JPK]

I put the switch on my EL84 November Amp.

Wait I missed that. EL84? Was that not a 6V6 design like the October? Mine was actually an October not a November. The October Club 09.05.19 version.

[dude]

I put the switch in a Plexi 6V6 with EL34’s with a 30 watt OT. The fixed over saturated the OT when turn up too loud but at a loud enough volume just to over come a hard drummer, l like the cathode bias mode. Gives a nice blues sound, not as tight as fixed but gritty. I like the switch in this amp.
But l went through the switch this, switch that phase. Now, l rarely put switches in, maybe NFB but that’s about it. I’m itching to built the November, have several AO-43 chassis’s. But have Sluckey’s Tweed Deluxe with reverb scheduled next.
I’m a + for cathode to fixed bias, at least in the 6v6 Plexi. Cheers