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El34 PP advice needed

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slav

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Joined 2010
#1
Hi to All
As my new amplifier is at the stage of quotes and pricing , I decided to give it another go and "mix" a little with the old one.
Amp is based on Jolida schematic as below:
View attachment 302b.pdf
PSU has been slightly improved :CLC filter ,bridge diodes paralled with caps, higher values of electrolytics , electrolytics paralled with PP caps , I used stepped lader attenuator and also added switchable PSE configuration.
I wonder if there is room for an improvement in pre- stage , phase splitter ( use different tubes , circuit , etc.) or output stage .
Any advice would be very welcome
Cheers
 
#2
The topology used by Jolida is a variation on the "classic" Mullard 5-20 theme. That's very well proven. The selection of a 12AT7 as the LTP phase splitter is excellent. I have reservations about a "totem pole" 12AX7 serving as the voltage amplifier. I'd like to see something with higher gm there. Also, put a 10M45S constant current sink (CCS) in the LTP's tail, instead of a resistor. Forced symmetry is SWEET.

BTW, watch out for excessive gain. "Gobs" of gain are not needed, when the 2 VRMS of a CD player are the driving signal.
 

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  • Mullard 5-20 Schematic.jpg
    Mullard 5-20 Schematic.jpg
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#3
Fairly standard circuit arrangement.

EDIT: Ah!! I see Eli reponded while I was still typing - seems we both agree about a current source for the driver.

If I were building it I would make the 12AT7 driver anode loads R7 and R8 the same value = 22K and replace the R6 resistor with a current source. A simple "Ring of 2" current source would be fine. Set current source for approx. 3mA.
C1, C3, C4 need to be good quality caps, I suggest Wima Polypropylene Film/Foil.

Cheers,
Ian
 
#5
There is an interesting cascade of knock-on effects in that amp. Adopting fixed bias means lower value grid leaks for the output. This means a lower impedance phase splitter is needed, hence 12AT7 instead of 12AX7. This reduces gain a little, so choosing a triode input (presumably for reduced noise) means a totem pole/SRPP can restore some of the lost gain. You can just about get away without a CCS tail for a 12AX7 PS, but 12AT7 needs one because of the lower mu.

One thing to watch when making changes to the 5-20 circuit, or derivations of it: the original had only marginal LF stability, with a subsonic peak. Changes can push you further down this road. The forward path through the amp has three LF roll-offs: R4/C1, R9/C3 (+ R21/C4), and the OPT. The original circuit had an LF shelf in the output stage cathodes, which might have helped or hindered a little, but fixed bias gets rid of this.
 
#6
Hi
Thanks for reply , excuse me ignorant question but I am not very familiar with CCS. As I understand one of the advanteges would be close to perfecr DC balance , what else . What does wait for me on the dark side?
 
#7
A CCS tail gives perfect AC balance, so you should get cancellation of even-order distortion. It does not affect DC balance, which is set by the grid voltages and the triode matching. You may get a little DC imbalance due to the grid current for the second PS triode causing a voltage drop across R10 (in the original 5-20 circuit).
 
#11
slav said:
How to built(calculate) CCS , where should I start ?
Click to expand...


If you want to use the IXYS 10M45S, it's pretty straightforward, you need to pick the value of the resistor to set the current, and use another resistor for a gate stopper to prevent oscillations.

You can also do a ring of two CCS in the tail, although I have no idea how to size the resistors, etc. There is a great summary of various ring of two CCSs in Morgan Jones' Valve Amplifiers-3rd edition.

Here are two very similar projects based on the Mullard 5-20 with CCS in the tail of the LTP.

http://www.diyaudio.com/forums/tubes-valves/156699-mullard-5-20-kt88-pp-blocks.html

http://www.diyaudio.com/forums/tubes-valves/161702-opus-5-0-modern-mullard.html
 

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#14
Hi
I am going to place an order from KandKaudio , they also do some CCS kits ,are they any good ?
Which one to choose :
- Compact Current Source Kit - up to 3 watts
or
- Large Cascode Current Source Kit -up to 7 watts

One more q. , are they directly connected between cathode and earth or I need extra Power Sup. ?
 
#15
Happy Easter everyone!!!!!!!!!!!
I am considering new OPT for my PP amplifier , and I am torn apart between Lundahl and Tribute .
Anyone has compared them?
How they differ?
Is there anything else worth considring in similiar price tag?
Cheers
 
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