El34 PP advice needed

slav · 13th April 2011, 09:18 PM

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:
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

Eli Duttman · 13th April 2011, 11:22 PM

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.

gingertube · 13th April 2011, 11:31 PM

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

slav · 14th April 2011, 04:01 PM

Would it be wrong to use just one CCS set at 6mA and -90V ?
Where can I find schematic for one?
Best Regards

DF96 · 14th April 2011, 06:51 PM

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.

slav · 14th April 2011, 07:27 PM

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?

DF96 · 14th April 2011, 07:33 PM

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).

slav · 14th April 2011, 07:38 PM

Any specific CCS circuit worth to look at?
Do I need to make couple 3mA CCS's per triode or one 6mA?
Cheers

DF96 · 14th April 2011, 07:48 PM

One CCS, replacing R6 the tail resistor.

slav · 14th April 2011, 09:09 PM

How to built(calculate) CCS , where should I start ?

13th April 2011, 09:18 PM	#1
slav diyAudio Member Join Date: Mar 2010	El34 PP advice needed 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: 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

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13th April 2011, 11:31 PM	#3
gingertube diyAudio Member Join Date: Oct 2004 Location: Adelaide South Oz	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

14th April 2011, 04:01 PM	#4
slav diyAudio Member Join Date: Mar 2010	Would it be wrong to use just one CCS set at 6mA and -90V ? Where can I find schematic for one? Best Regards

14th April 2011, 06:51 PM	#5
DF96 diyAudio Member Join Date: May 2007	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.

14th April 2011, 07:27 PM	#6
slav diyAudio Member Join Date: Mar 2010	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?

14th April 2011, 07:33 PM	#7
DF96 diyAudio Member Join Date: May 2007	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).

14th April 2011, 07:38 PM	#8
slav diyAudio Member Join Date: Mar 2010	Any specific CCS circuit worth to look at? Do I need to make couple 3mA CCS's per triode or one 6mA? Cheers

14th April 2011, 07:48 PM	#9
DF96 diyAudio Member Join Date: May 2007	One CCS, replacing R6 the tail resistor.

14th April 2011, 09:09 PM	#10
slav diyAudio Member Join Date: Mar 2010	How to built(calculate) CCS , where should I start ?