Mars 5-10 stereo - update

Mars

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Hi fredeb

It did work. I abandoned the idea when I had to change the cathode resistors to 390 ohm to run the tubes cooler and keep the GZ34 happier.

 

Mars

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I am looking at dropping the EF86 out of the circuit and replacing it with a triode. Have a look at this Stereo 20 design
LEAK-ST-20-EL84-PP-edt.png
 

Qualityten

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I've never heard a Leak Stereo 20, but I have read that the ECC83 in the first stage also has more gain than is needed.  Others will know much more. 

If this is not a problem, then one simple advantage is that ECC83 valves are easily obtainable and cheaper than EF86s.  I think such a SA-made Leak clone would be a popular product.  Maybe call it 'Sleek'!

The circuit drawing by Lilienthal Engineering is as beautiful as the physical layout of the Stereo 20.
 

fredeb

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Qualityten said:
I've never heard a Leak Stereo 20, but I have read that the ECC83 in the first stage also has more gain than is needed.  Others will know much more. 

If this is not a problem, then one simple advantage is that ECC83 valves are easily obtainable and cheaper than EF86s.  I think such a SA-made Leak clone would be a popular product.  Maybe call it 'Sleek'!

The circuit drawing by Lilienthal Engineering is as beautiful as the physical layout of the Stereo 20.

The Leak Stereo 20 does have such a neat layout .

06-leak-stereo-20.jpg


Lillienthal also has a revised schematic that changes the 2x ECC83/channel for 2x ECC82 .


"The LEAK ST-20 was introduced in 1958 and soon became one of the most popular amplifiers for domestic use in England. The similarity to the ?TL-12 point one? from 1948 is striking.  The ST-20 sounds good as it is in its basic form, but this favorite design of Harold Leak is not really one of my favorite designs ? I am here being polite. The thing that saves the circuit from fatal flaw is the generous use of feedback ? it measures fine when exposed to sinewaves. The input as well as the long tail phase inverter/driver are handled by ECC83?s. This is actually the Achilles heal of the entire design. Harold wanted a lot of gain in order to apply a lot of feedback. This would be perfectly alright if these high gain stages were good to begin with. But it is not so in this case or any other of the LEAK designs for that matter. The first stage ECC83 amplifier is rather mediocre, not much headroom ( 1,5 V bias ) and due to the high ri and plate load resistor , the HF roll off is low. In other words we are dealing with a ?slow and lazy?, yet sensitive and relatively noisy input stage. The longtail circuit, that serves as phase splitter and driver, provides further gain. The extreme high values of grid resistors are necessary in order to secure a reasonable adequate freq range as the z-out of the ECC83?s are high as well.

I am not a fan of the principle of designing amplifiers with poor freq range and relatively high distortion in order to ?win it back? by means of feedback. It is my experience that good amplifiers to begin with, partly needs less feedback and partly makes better advantage of eventual feedback.

The EL84?s needs some 8-10 V RMS in order to drive them to the full output. In case of an input signal of 1 V RMS ( Not an unreasonable estimate in our days ) , we are talking about a modest gain of 8-10.

Why not use that fact to our benefit ?



Modification of the LEAK ST-20.

LEAK-ST-20-modification-1.png


Above I have changed the ECC83?s to ECC82?s, the gain is still about 12 in the first stage and 10 in the long tail inverter ? giving a total gain of 120. This will allow quite a lot of feedback, should you wish. You may remove the 50u electrolytic at the cathode of the input, this will improve the linearity and headroom, at the price of a little gain of which we have plenty. You can implement all the modifications I have suggested or you can pick what you like.

Try these simple and easy modifications, just to get an idea about how much potential your old ST20, actually possesses. The capacitors between the stages in the original design are way too large. The 250nF that couples the long tail to the output stage results in an fn of about 0,5 Hz. This leads to excessive amplification of subsonic signals as well as capacitor blocking in case of transients. 5 to 10nF is plenty here. Same goes for the 100nF that couples the first stage to the paraphase. Change this one to a value between 1-5nF. This has the further advantage that capacitors of such small values are the best we can make. A glass or silvered mica will outperform even the best and most expensive ?audio grade? capacitor by quite a margin.

If you swap the ECC83?s in the long tail circuit to ECC82 as shown above, you may as well replace the EL84?s grid resistors from 1M to some 220k. This will increase the stability and life of the EL84?s. In this case you will need a capacitor between the paraphase and the EL84?s of about 47-100nF.

If you swap the first stage ECC83 to ECC82, you will need to change the 47k resistor in the power supply to some 4k7 or whatever you have between 3-10k. "

 

handsome

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while you in there you should drop that series resistor on the first tube's grid to about 1k as well. If you worried about RF add a 220-470pf cap from said grid to ground.
 

El Sid

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I'm certainly not disagreeing with handsome's suggestion of an ECC82, and i'm not going to provide you wuth the new component values  ;D,  but you may also want to consider an E88CC. It also has less gain than the ECC83 (u=33) but plenty enough in this application. I like it because it is super-linear, and can also easily ne used for DC coupling as it is quite happy with a low anode voltage. I used it as the pre/splitter combo in my EL84 concept amp and am very pleased with it.

No less a luminary than the esteemed Spiderman uses it in the VA Exclame in its 6922 guise, so it must have something going for it....
 

Qualityten

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See the modification to the Leak Stereo 20 here: http://lilienthalengineering.com/100-amplifiers-chapter-1/100-amplifiers-part-4.  Just scroll down for the discussion.
 

charles

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My advise. Leave the Leak Stereo 20 as it is.

It sounds very good and it is a good design.

The day will come that somebody wants to modify a Leak 12 point one and we will call it.........................
 

Mars

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Hi Everyone

I am rebuilding a Mars 5-10 for a client that wants the gain to tamed. I'll be posting some photos below. But first some technical details.

I had some good results using a split load inverter to bring down the gain to a more use-able level. Initially I used the ECC82 with good results. Studying the matter I came upon these articles that helped me along the path of improving the EL84 amplifier.

https://www.angelfire.com/electronic/funwithtubes/more_phase_inverter.html

This article made me realise how good the ECC88 is as a split load inverter. It also discourage me to use the ECC88 for the voltage amplifier section.

http://www.valvewizard.co.uk/cathodyne.pdf

More of the same, but an example of using a single ECC88 for both voltage amplification and phase inverting (DC coupled)

For this build I will stay with the EF86 on input duties. Here is the schematic:

q7YlOez.jpg





 

Mars

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Here is the schematic as it was built over the weekend.

Some values were left out:

  • R27 and R28 should be 47 ohm 5 watt
  • R7 and C7 left out for the moment till square wave tests
  • R6 was chosen at 33k. (39k or 47k might be better)
[*]R11 was chosen as 4K7. (2K2 might be better)
[/list]

pQ2fmZ1.jpg
 

Mars

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Here is progress on the rebuild I am currently busy with:

ormoWoT.jpg


PI filter in place. Earth bar across the power amp section in place.
 

Mars

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Update on circuit:

R11 kept at 4K7 ohm
C3 enlarged to 270uF 400V (220uF should be good too)
R6 changed back to 68K (improves split load inverter load match)
C12 47pF

I took some voltages:

B+ 310V
UUdjICi.jpg


+/- 12V across 390 ohm cathode resistors

1HHhQih.jpg


6.3V filament voltage with 230V AC mains

gjCDyTY.jpg


282V on phase inverter

u7Hfxuw.jpg


+/- 70V across phase inverter anode load resistor

vj06cAp.jpg


+/- 70V across split load inverter cathode resistor

mFamyxz.jpg


+/- 140V across phase inverter triode

R1likcz.jpg


156V on EF86

AErkh2X.jpg


65V on EF86 anode

eCwMdp8.jpg





 

Ampdog

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With due respect to to Charles, the circuits of both the Leak 2020 and TL12 are such general circuits that I do not view the suggested changes  so much as changes to the particular circuits, rather than ideas pointing the way forward at the hand of said circuits for further designs. Many amplifiers  followed these topologies, the Mullard 510 and 520 to name just two.

That said, the shown 2020 schematic still indicates the EL84 grid resistors as 1M, an error for which Leak paid dearly. This value falls outside the maximum allowed grid return values of 470K, only quoted as such in later data sheets. (It was found that values exceeding that caused grid run-away conditions with certain brands, as a result of which dozens of 2020's had to be recalled. Here some valve manufacturers themselves were initially in error!).
 

Mars

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Hi Ari

Thanks for the comment. I am only using one half of the 6N1P. I still need a bit of gain to lower the output impedance with global negative feedback, therefor the high gain of EF86 pentode works fine.

Great discussion here: https://www.tube-town.net/cms/?Info/How_To_Use_a_EF86

I am going to try the triode mode soon.

This is the triode configuration I want to try:

Figure%203.jpg


But first I want to try out pentode mode on the EL84's with this particular set of OPT.
 

fredeb

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Hi Karel

That triode mode for EF86 you show above seems like an excellent idea . Such low distortion negates the need for NFB - good one !

 

Ampdog

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fredeb said:
Hi Karel

That triode mode for EF86 you show above seems like an excellent idea . Such low distortion negates the need for NFB - good one !

Hi Fredeb!

Careful here! The distortion figure given is just for the input and driver part! The main distortion comes from the power valves and output transformer!

Then again I am not too sure why everybody wishes to go for as little NFB as possible. Yes: Too much NFB increases the possibility of instability, but other than that there is no disadvantage. one must remember the effect of NFB on damping factor and valve differences. The designer has absolute control over the effects of NFB. (I hope that ancient lore like 'feedback sounds bad' and such is not still entertained.)
 
 

Qualityten

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I have been thinking about optimal NFB in my build of Karel's earlier 5-10 design, using ECF82s combining first stage amplification and phase splitting duties.  There his schematic calls for a 100K NFB resistor, with no capacitor bypass.  As 100K is much more than I am used to seeing in this place, I wonder whether this means that there is little NFB being applied?

In my particular build I am using 8K Vortexion output transformers intended for an EL84 valve amplifier and I can see oscillation or ringing when viewed on a scope with a 10KHz square wave.  As shown in Figure 5 example (d) in: https://www.rfcafe.com/references/radio-news/practical-techniques-square-wave-testing-july-1957-radio-tv-news.htm.  Should I add a capacitor in parallel with the 100K to control this?

Thanks for this interesting thread.
 
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