let me fix this bias

[Gluca]

Ciao

my next amp is going to have the grids running at fixed bias. I saw quite a number of solutions for that, but at the end of the day I believe I will ask a couple of zeners+trimmpots to take care of the bias.

Any comments/advises/suggestions/recommendations/etc... before I print the PCB?

Gianluca

HAPPY NEW YEAR!!!!

[edl]

Hi,

I don't like to overcomplicate such things.
I'm using the following circut:

Regards,

[Giaime]

Originally posted by Gluca
Ciao

my next amp is going to have the grids running at fixed bias. I saw quite a number of solutions for that, but at the end of the day I believe I will ask a couple of zeners+trimmpots to take care of the bias.

Any comments/advises/suggestions/recommendations/etc... before I print the PCB?

Gianluca

HAPPY NEW YEAR!!!!

Sorry Gianluca if I ask a silly question,

but why you need zeners and stabilized voltage if you regulate through the trimpot? I mean, no need to bring the input voltage to what you expect, a little turn of the trimpot could counteract! You intend to do this to counteract line voltage variations? But if you do so, you should also regulate plate and screen voltage for the output tubes... IMHO it's useful to let the bias supply track the variations of the line voltage, if you don't regulate the plate/screen voltage. It might not track perfectly, but it's still a protection from overvoltages.

[Alastair E]

Originally posted by Giaime


Sorry Gianluca if I ask a silly question,

but why you need zeners and stabilized voltage if you regulate through the trimpot? I mean, no need to bring the input voltage to what you expect, a little turn of the trimpot could counteract! You intend to do this to counteract line voltage variations? But if you do so, you should also regulate plate and screen voltage for the output tubes... IMHO it's useful to let the bias supply track the variations of the line voltage, if you don't regulate the plate/screen voltage. It might not track perfectly, but it's still a protection from overvoltages.

I quite agree--To stabilise the Bias Supply, means that under differing line-voltage variations, this leads to bias drift from the setting you originally set.

I have a similar problem, but caused by a different scenario--My current OTL Circlotron, has the O/P tube supplies derived by a voltage doubler, from a 0-55, 0-55V toroid.

My Bias supply is by a conventional bridge-rec, C.L.C supply of around -170V.

Due to the 'magnifying' effect of the voltage doubler, by bias can vary by as much as 20mA over the 'peak' electricity demand periods, falling during the peak periods and climbing during later periods of less demand!--Its just a bread-board so doesnt really matter much, Ill take care of it on the proper build.

[Tom Bavis]

If you regulate bias, you must regulate screen voltage too. If all are unregulated, idle current drops as line voltage rises, since control grid has higher gain than screen. Since plate voltage goes up and current goes down, dissipation is relatively constant. If bias only is regulated, idle current goes up with voltage. If screen and bias are regulated, idle current will be almost constant with change in line voltage - plate voltage change has little effect.

[Johan Potgieter]

Gluca,

If you have not done so already, also look under the thread "EL34 schematic confusion". Quite a number of hints there.

Regards.

[Gluca]

Thanks all.

I will use triodes ...

Frankly speaking I am not such a fan for fixed bias at power stages. Probably I will mix fix bias with a smaller cathode resistor or most probably I will use just the simple auto-bias ... I do like simple solutions too and each time I see my current schematics with fixed bias ... well each time I raise an eyebrown.

I'll prepare the PCB btw just to see how things are going to be.

Ciao
Gianluca

[Giaime]

Hi Gianluca

cathode bias is simple and sufficient when you have B+ voltage to spare. If you have to use a specific PT, maybe that's a little low on B+, you'll gain some power using fixed bias, but at the expense of needed rebiasing every power tube change.

Happy new year to you too!

[Onra]

Hi,
if you really intend to build up this circuit, please reverse D4.
Onra