Help needed changing from srpp5814a to srpp12bh7a

[georgehifi]

I need help to change the cathode resistors to the correct value, at the moment it`s set up for a 5814a srpp driving a 300b. Top section of the srpp plate is 310v naturally this get halved to 155v each section and cathode resitors on both sections are 4.3k I tried a 12bh7a in place of the 5814a and got a nice improvment, but i know it`s not working at it`s optional point, can anyone work out what to change the two cathode resistors to to give the 12bh7a it`s best operating point, someone suggested that i drop both of them to 3.3k Can anyone confirm this.
The circuit is 5963 straped (input tube.)
5814a srpp (pre driver)
300b (driver)
interstage transformer
805 (output tube)
Thanks George

Frank your the srpp king, what do you think?

 

[Burnedfingers]

George

Would you post the schematic for all to see?

Thanks
Joe

 

[georgehifi]

Joe i`ve emailed the circuit to you aol address in your profile, i have no idea how to post a circuit diagram, hope you can do it for me.
Cheers George

 

[fdegrove]

Hi,

George,

As I think Joe's pretty busy...If you mail me the circuit I'll try to post it here for you.

Cheers,😉

 

[planet10]

George,

What did you do the scematic in -- the picture embedded in the word doc was just a ghostly shawdow (put almost anything in Word and it gets pooched).

Here is a forum size pic thou... (print pdf out from word, RIP it in Photoshop, grag the top slider in the gamma almost to the bottom one and we get asomething that is at least representative.

dave

 

[georgehifi]

Thanks Dave, wish you could show a dumb ---- like me how to do that, btw i do`nt run any feedback on mine now, and my b+`s are a little different also, this amp guys has a killer sound, way beyond any expectations i had for it. And even with no feedback i`m still only 1db down at 20k. i`ve got the 12bh7a (tungsol black plate ) in at the moment, it`s a good sounding tube in this srpp stage, you were right Frank 12bh7`s are great, it not running at it`s best yet, over at AA Damir said i`ve probally got around 1.7ma of bais current on it, and that`s maybe to low to kick it up to 4.4ma change the 4.7k cathode resistors to 1.5k can you guys confirm this.
Thanks George

 

[Burnedfingers]

George

For some unknown reason I haven't gotten any mail from you. I see it is posted now for viewing.

Joe

 

[fdegrove]

Hi,

1.7ma of bais current on it, and that`s maybe to low to kick it up to 4.4ma change the 4.7k cathode resistors to 1.5k can you guys confirm this.

For a B+ of around 310 V I came up with 1K2 bias resistors.
So for a higher B+ as you're having 1K5 should do just fine.
It should also finally wake up that 300B driver tube...😀

Let us know what you think of it,😉

 

[georgehifi]

Yeah Frank, reporting back very nice, the 12bh7a (Tung-sol blackplate) is now at 300vat the top plate of the srpp which halves to 150v for each section, i now have 1.2k cathode resitors, which now have a voltage drop across them of 5.4v each which i equate to 4.5ma plate dissapation for each section being .675watt each section, do you concure.
One more question Frank do you think i have enough voltage swing on the 12bh7a for the 300b.
Also the input i`m using a rca 5963 blackplate it has about the same plate volts and current draw per section as the 12bh7.

Cheers George

 

[fdegrove]

Hi,

is now at 300vat the top plate of the srpp which halves to 150v for each section,

How did this happen? Did you change something else in the PS?

One more question Frank do you think i have enough voltage swing on the 12bh7a for the 300b.

Yes but the more B+ you have the more it can swing voltage, in theory at least.
There's of course a point where it'll start to distort the output.

Also the input i`m using a rca 5963 blackplate it has about the same plate volts and current draw per section as the 12bh7.

According to the Chen diagram it has even more B+ voltage than the SRPP stage, unless something got scrambled I don't understand how that can be.
I don't understand either why he felt he needed to put both sections in //.

.675watt each section, do you concure.

I'm pretty beat but didn't you calculate what the resistors are dissipating iso the plates?

Cheers,😉

 

[georgehifi]

quote:
is now at 300vat the top plate of the srpp which halves to 150v for each section,


How did this happen? Did you change something else in the PS?


Frank, is`nt it natural for the b+ to sag a little, especialy a choke regulated one with 3x the the extra current being pulled through it now?


quote:
Also the input i`m using a rca 5963 blackplate it has about the same plate volts and current draw per section as the 12bh7.


According to the Chen diagram it has even more B+ voltage than the SRPP stage, unless something got scrambled I don't understand how that can be.
I don't understand either why he felt he needed to put both sections in //.


There`s no way the front valve could have a higher voltage than the srpp stage it could only be less with r29 in the way.

Cheers George

 

[fdegrove]

Hi,

Frank, is`nt it natural for the b+ to sag a little, especialy a choke regulated one with 3x the the extra current being pulled through it now?

A little, yes...but I just now noticed you started out with 310V while I'd been looking at the voltages shown in the Chen schematic.

There`s no way the front valve could have a higher voltage than the srpp stage it could only be less with r29 in the way.

Precisely, why then do I read something that looks like 388V to me?

Anyways maybe I'm just misreading it but if that schem is anything to go by Mr. Chen seems to be running his amp at much higher B+ than you seem to.

Cheers, 😉