EL34 in place of a 6AS7G (used in the supply circuit as a pass tube). how?

[jarthel]

Originally posted by showdown
I'm sure someone can correct me if I'm wrong, but this is what I'm thinking:

last question: Do I still need R3 (20K 10W) as mentioned in the schematic?

thank you everyone for the help

[cogsncogs]

last question: Do I still need R3 (20K 10W) as mentioned in the schematic?

Yes, absolutely! Otherwise you would have all of your PS current across the zeners which would fry them! The resistor is neeed as you are taking the voltage drop across it as the voltage reference which is applied to the (control) grid.

Wayne

Edit: I was assuming you meant a short for R3. But still you need it as stated because you will need to have current across the resistor/zener diode string.

[cogsncogs]

jarthel

This may help - a picture:

Wayne

[jarthel]

I really appreciate the help.

thanks everyone

[ray_moth]

Firstly, jarthel, I apologise for using jargon, assuming you're familiar with pentodes. Suppressor is an abbreviated name for the suppressor grid, also called Grid 3. It's what makes the difference between tetrode and pentode.

Secondly, can I squeeze a quick question in here, please?
Do you guys believe that the EL34 can actually do the job as a regulator pass tube here, or will it incur too high a voltage drop between plate and cathode?

[jarthel]

Originally posted by ray_moth


Secondly, can I squeeze a quick question in here, please?
Do you guys believe that the EL34 can actually do the job as a regulator pass tube here, or will it incur too high a voltage drop between plate and cathode?

thank you asking the question.

I just believed it would work as the article was written by someone vastly more knowledgeable than myself.

[wrenchone]

If you are trying to sub an EL34 fpr a 6AS7 as a pass tube, one of the first things you want to check is the heater to cathode voltage rating. The 6AS7/6080 has a high filament to cathode breakdown rating - I don't recall the rating for the EL34 being anything special. If you have an isolated filament winding feeding the pass tube, all is well. If not, beware....

[tubetvr]

Do you guys believe that the EL34 can actually do the job as a regulator pass tube here, or will it incur too high a voltage drop between plate and cathode?

Good point!, with 250VAC giving a maximum raw DC of ~340V there will be only max 30V over the tube, a EL34 can only pass about 20mA at this voltage and then there is no margin for drops in line voltage.

I would recommend a higher voltage from the transformer and to build a better power supply with better regulation using at least a pass tube and a control tube, I have been very successful with a design similar to this http://www.bonavolta.ch/hobby/en/aud....htm#Tubes%201
although I use a 6BM8 with the triode as control tube and the pentode triode coupled as a pass element, I also use gas regulator tubes as reference.

Regards Hans

[jarthel]

Originally posted by wrenchone
If you have an isolated filament winding feeding the pass tube, all is well. If not, beware....

from what I understand, I should not worry if the EL34 has its own secondary and no other tube heater is connected to that secondary. is this correct?

looking at the datasheet, 6AS7 has 300V of heater-cathode voltage limit. While the 6CA7 has 100V limit.

I looked at Phillips and Mullard EL34 datasheet and I can't deduce the heater-cathode boltage.

[jarthel]

I won't be using a 500-0-500V xformer though. It'll only be a 280-0-280.