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el84 amp

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M

mark_titano

Member
Joined 2004
Re: Thanks EC8010

kooltubes said:
Hi EC8010,

Thanks for the CSS circuit, I've just finished building the amp; it sounds good for a 5 watts'ish (PP EL84 triode connected).

It worked fine, the first time, without any hum and bang.
I had to tweak the current on the ECC99 to get a better sound; I am running the differential driver at about 21 ma pe tube, yielding to an anode voltage of approx 140 volts.

I am still learning.....oops...:eek: I haven't opened the scope box, as yet, but that my assignment for next week.
Cheers!:)
Click to expand...

Can you post the schematic of your amp please?
:D

Cheers

Mark
 
K

kooltubes

Member
Joined 2004
Hello Mark_titano and GeirW,

I've first built the Bevoy valley amp (schematic in Morgan jones book) Ecc88 (input) Ecc88 (concertina phase inverter) + traditional PP EL84... I couldn't fit the choke on the recycled chassis, for lack of space.
I didn't like the sound and got rid of the input stage and phase inverter and replaced them with a simple differential input/driver using ECC99 biased at 21ma per tube using, a CCS, courtesy of EC8010; the schematic is somewhere in the previous pages of this thread.
The anode resistors are 14.7k (for better or worse):confused:

As it is I am still experimenting with the CSS to find the best operating point that would suit my taste.

I am a newbie like you, and still at kindergarden :D
IMHO, the differential input/driver sounds much better than the concertina; meaning it has more substance.

I cannot take any credit for the design; this amp being some other people's original ideas...but I will keep you posted!
Cheers
 
EC8010

EC8010

Ex-Moderator
Joined 2003
kooltubes said:
Do I need to bypass the common cathode resistor of the output stage?
Click to expand...

That's a tricky question. If you don't, then the output stage tends to act like a differential pair and is self-balancing (only to a very limited extent because of the low value of the tail resistor). Unfortunately, when you go into Class B and one valve switches off, an unbiased shared cathode resistor causes increased distortion. If you replace the shared resistor with a CCS, the output stage only works sensibly in Class A. Typically, the first 5W of a push-pull EL84 is in Class A, and the rest is in Class B.

I'd bypass the resistor.
 
K

kooltubes

Member
Joined 2004
EL84 PP triode-connected

Hello,

I have got a biasing issue with the EL84 triode-connected in push pull
The output transformer is 9K

The data sheets do not seem to agree on the bias current for the EL84 triode-connected in push pull driven in class A.

What is the correct value? 48ma or 72 ma? Is it a shared cathode resistor?
Mullard suggest a 150R common unbypassed cathode resistor for a 10K output transformer. My transformer is 9K, how do I calculate the cathode resistor?
I am getting confused there but it's part of the fun.

For those interested in a DC coupled amp have a look at this:

http://www.wdehaan.demon.nl/el84dc.html

Cheers
 
planet10

planet10

frugal-phile™
Joined 2001
planet10-hifi.com
Paid Member
SY said:
Optimal idle current depends on the B+ and the plate to plate load. The RCA Tube Manual gives a great explanation of how-to-do-it.
Click to expand...

I finally found a copy of that (50 cents :)) and just read that section (and will have to read a few more times and then get out my triode connected EL84 curves)... info i've seen no-where else.

dave
 
K

kooltubes

Member
Joined 2004
Thanks Sy,
I have that book and never knew all I need was there. Now that I have learned to draw loadlines for output stage, I guess it's real fun!
Triode connected EL84, push pull; Output TX: 9K, tubes are biased @ 23 ma; I am using a shared bypassed cathode resistor.

BTW, what is the simple way to balance the voltages on the ECC99 differential input using EC8010 CCS design?
I have about 3 volts difference between the anode, since the grounded grid triode is fed from the input triode.
Cheers
 
M

mark_titano

Member
Joined 2004
I need help... :angel:
I think that I will go for one of these designs:

http://www.wdehaan.demon.nl/mono/mono/el84dc.html

http://www.artimusica.nl/wim/dc_coupled_amp.pdf

The question is...which one? The one that uses the ecc85+half ecc88 as constant current source seems pretty good, simple, provides a good stability and is easy to adjust (designer's words...).
I don't know the ecc85 at all, any feedback? Are they a good solution in a long-tailed pair driver stage?

Ouput transformers: suitable transformers are Amplimo VDV 8020, Hammond 1608, 1650f, some Sowters ecc. The schematic shows the amlimo but I'll buy an hammond 1608 because I want to spend not so much for this first attempt. The 1608 has 4-8-16 ohm taps, while the amplimo has only 5 ohm. Which tap must I use for the negative feedback with the hammond?

Power supply...with that design I'll go for dual mono amp, because seems good only for this purpose. I'm wrong? :)

Mark
 
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