The Red Light District - another PP EL84 amp

[jackinnj]

I am brain-challenged for PCB design, so that will have to be someone else. I think Jack Walton did a design a few years back, but it was too expensive because of the size.

The bigger version keeps getting pushed back in my queue. I wish I had more lab time...

I had the design in Ultiboard, didn't seem much interest. Lemme see what I can work up again.

 

[SY]

At idle, it will be well below that, perhaps 10-20mA- remember, this reg is only for the screens.

 

[skal]

ok thats cool but what voltage should i be seeing at the cathode of the final's ?

Forgot to mention i am using a 6H*6 instead of the el84 seem to be similar, i think maybe i'am wrong neebie ere .

i'am seeing about +9.38v at the cathodes 9.6v across 1 resistor on the led array , screen reg voltage 290v and plate current is 8.26mA .

P.S

ok ,i have removed the led array from the cathode , and put in place a positive voltage supply of around 7v or so on the cathode , plate went up to 44mA and screen voltage holding at to 290 , could the the led array be at fault here seems it could may i'am wrong

Forgot to mention i am using a 6H*6 instead of the el84 seem to be similar, i think maybe i'am wrong neebie ere

cheers
den

 

[SY]

I don't know what a 6H*6 is, but you'll have to look at the curves to see how to bias them correctly. Do you have a link to a datasheet?

 

[skal]

Sy


http://frank.pocnet.net/sheets/135/6/6HB6.pdf

cheer

DEN

 

[SY]

OK, that's a VERY different tube than an EL84. It looks like you'd want 4-5 volts on the cathode. So maybe three LEDs in series. I don't know what idle current is optimum for that tube, so you'll want to experiment a bit. You'll probably get less power.

 

[skal]

Ok, will try that and all order some el84 in the week ,


cheers for the help

Den

 

[skal]

Ok, that did not work , voltage's around the same, so i think i am going to build a new led array with the ones i got left and see what happens then..


Den

 

[SY]

You can test the arrays if you have a power supply. Put a resistor in series sized to run 20mA or so, and see if they light up. For example, if you have a 48V supply available and your array is 5 LEDs in series, the drop across the LEDs will be about 8.5V, so the series resistor should be R = V/I = (48 - 8.5)/.02 = 2k or thereabouts.

Remember that here, unlike Zeners, we forward bias!

 

[sporc]

Go to https://oshpark.com/shared_projects/rkU0U4LT pay them $17 and in about 3 weeks you'll get 3 of these at your door. This is untested, but it is hard to imagine what could go wrong.

That's exactly what I was looking for. Thanks.

 

[skal]

Hi Sy

i think i have made it work, it has something to do with the grounds from my power to the ground in the centre of the 250k resistor , when psu to centre of the 250k is connected a small amount current is taken by the plate , but when you remove the ground from 250k divider the current start to flow to the plate .COULD IT MY POWER SUPPLY it is an old Radford power pack 300v@100mA..


Still learning
cheers
den

 

[dsavitsk]

That's exactly what I was looking for. Thanks.

Note that I used the little 7mm (generally 1/8W) resistors to save space. In small values, they are getting a little tricky to source, but these should work: ERO-S2PHF4R30 Panasonic | Mouser

 

[Doug in Indy]

Hi All
I got my Edcor iron yesterday, and made a top plate. LED arrays and Maida regulators are made.
Noob question: are the 47K resistors on the EL84's really grid stoppers? Should they be carbon comp. or metal film. I know to use cc on the inout tube's grid stoppers.
Thanks
Doug

 

[6L6]

Yes, they are gridstoppers. The resistor does the damping, so in theory composition doesn't matter, but tradition would use carbon.

Personally, I'd use whatever I have in my box.

 

[Doug in Indy]

Thanks 6L6!
Doug

 

[Doug in Indy]

Hi All,
Yesterday I set up the 400 V supply to the 12AT7. 350 V Txr to Bridge to CLC (100 uf/9 H/100uf) into a 40K dummy load. I was getting 450 volts, so I put a 3300 ohm resistor in after the bridge. This brought the voltage down to 390.

My problem is when I model this in PSUD2, it takes 5 seconds to reach voltage. See attached photo. Is this OK, or what should I change.

Thanks,
Doug

 

[SY]

That's absolutely no problem. You could even use a tube rectifier, if you're feeling particularly perverse.

 

[Doug in Indy]

Thanks SY
No tube rectifier. It's getting pretty crowded down there already!
Doug

 

[SY]

I should add that the main thing is to not have the screen come up to voltage before the plate does, otherwise, the screen will think it's a plate and start drawing excessive current. And there's no issue I can find with having the plate voltage higher than normal- in fact, I've found some advantage to that.

 

[Doug in Indy]

Sorry, SY, I don't follow. Are you talking about the first stage or the second? (Assume I'm Kofi's other brother Darryl.)