36 Watt with EL-34

klingo · Post by **klingo** » Sat 10/25/08 11:10 am

i've said in the first page of this topic that the long tail pair PI with ecc83 100K/plates & 47K/cathode have a 200K'ish output impedance(more than the plate resistor

)
redoing the math i have ~70k (is that correct?)
i hate to let my BS floating on the world wide web so next time don't hesitate:

Post by **zaphod_phil** » Sat 10/25/08 1:06 pm

That sounds a lot more reasonable to me....

sub · Post by **sub** » Sat 11/08/08 9:49 pm

Guys,

I'm happy, because i got all parts and stuffs to 36W EL34 build.
What i want is a EF86 Lite with EL34 power stage.
...but i'm not sure about the PI coupling caps value....
What you recommend? .01uF (normal 18W value) will be good? ...or maybe bigger?
The other PI components are: 1M/470R/1M with 15K (or 22K) tail resistor, 5K6 grid stoppers.. other components are same than in any 18W PI....

....and if i'm not wrong my B+ voltage will be around 400-410V, but i want around 300-320V at the PI filter cap (similar than my 18W) Can i replace the 8K2 resistor (between the 2nd and 3rd filter caps) with a bigger one?

Any suggestion would be appreciated.

Post by **zaphod_phil** » Sat 11/08/08 10:53 pm

Keep the PI coupling cap the same, or you may get "flubby" bass. And yes, you can change the value of that resistor to get the voltage you need.

sub · Post by **sub** » Sun 11/09/08 5:14 pm

zaphod_phil wrote:Keep the PI coupling cap the same, or you may get "flubby" bass. And yes, you can change the value of that resistor to get the voltage you need.

Thank you very much Phil!

sub · Post by **sub** » Fri 01/08/10 6:06 pm

Sorry because i resuscitate this old topic...

I built this amp (EF86, EL34 36W) with my old (34 years old) Drake 784-139, and a 50W PT (300-0-300)...
First time i wasn't happy with the result:
- too loud (not big problem)

- too hot, not only the EL34s but the chassis was also too hot

(PT and OT was cold, both worked well)
- a little "rattle" sound (yeah i thought the EF86... but EL34 also was dodgy)
- Vox style tone switch was useless to me
All in all, i rebuilt the amp ... replaced EF86 with 12AX7. This was big mistake, i got a almost clean amp

and i replaced the tone switch with a 18W style tone pot.
Then i put back the EF86 (18W tone pot stayed). Replaced the power tube cathode resistor (was 330R < this is why it was very hot...) to 460R, put shock mounted EF86 socket... etc.
The amp is much better... i will try few other things to replace, and i think i will be happy.....
Sorry for the long post

Here are few pics:

sub · Post by **sub** » Fri 01/08/10 6:07 pm

...more pics:

sub · Post by **sub** » Wed 01/13/10 4:21 pm

Guys, please help me to calculate anode dissipation.
I was able to calculate my 18W lite plate dissipation (with your help), but i don't really understand the methode, and this amp is a little more complicated (for example: separate power tube cathode res.)
Few things are temporary in my amp and i would like to finish these things.

My voltages are:
(all voltages are measured respect to ground)

B+ = 396V
no choke, i have a 1K 10W resistor between the first and secound filter cap.
voltage after 1K res = 373V

V4(EL34)
P1 = 0V (to ground)
P3 = 394V (plate)
P4 = 361V (screen) (1K screen res)
P8 = 27V (cathode)

V3(EL34)
P1 = 0V (to ground)
P3 = 393V
P4 = 364V
P8 = 33V

The two EL34 were well matched, but i think the 330R cathode resistor was too low, and the tubes wear out fairly ... (this happend with my other set EL34). The cathode res are 460R now (330+130)

voltage on the PI/pre filter = 309V

V2(PI)
P1 = 193V
P2 = 22V
P3-8 = 35V
P6 = 190V
P7 = 22V

V1(EF86)
P1 = 104V
P2-7 = 0V
P3-8 = 2.6V
P6 = 90V

Any help would be appreciated.

Post by **zaphod_phil** » Fri 01/15/10 12:13 am

You also need to measure the voltage *across* the screen grid resistors, so you can work out the screen current. That will help to calculate the dissipation correctly.

sub · Post by **sub** » Fri 01/15/10 11:20 am

Thank you Phil!

Voltage across screen grid resistors:
V3 = 9V
V4 = 12V

sub · Post by **sub** » Fri 01/15/10 11:53 am

Ok, I have a try at calculation

Cathode Current Ik = Vk / Rk
V3: 33V/460R = 0.072
V4: 27V/460R = 0.059

Screen Current Is = dVs / Rs
V3: 9V/1000R = 0.009
V4: 12/1000R = 0.012

Plate Current Ip = Ik – Is
V3: 0.072 - 0.009 = 0.063
V4: 0.059 - 0.012 = 0.047

Plate Dissipation Pp = (Vp-dVs)* Ip
V3: (393V - 9V) * 0,063 = 24.19
V4: (394V - 12V) * 0.047 =17,95

If I not wrong EL34 is a 25W tube, so:
V3: 24,19/25 = 0.97 i.e. 97%
V4: 17.95/25 =0.72 i.e. 72%

Is my calculate good?

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36 Watt with EL-34