Author Topic: Mars 5-10 stereo - update  (Read 7207 times)

Offline handsome

Re: Mars 5-10 stereo - update
« Reply #75 on: February 06, 2020, 02:07:27 PM »
Neither option looks very good....you only have 11V of negative swing. So you could drive an EL84 but just and only if it is biased to that - but you will be at your limits so more distortion than necessary. A higher HT, a negative supply or less current will help and let you use a bigger anode resistors; ~17k is a low load for a 12AU7.

Sent from my SM-A750F using Tapatalk


Offline Mars

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Re: Mars 5-10 stereo - update
« Reply #76 on: February 06, 2020, 05:20:38 PM »
Thanks for the comment handsome...
Now I understand why Ampdog leans toward ECC88 (from comment somewhere, if memory serves me well :thinking:) Much better figures at 280V B+

I aimed for a 50% voltage drop across the triode, but 11k is as low as the anode resistor is allowed by the program.

ECC88


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Offline Mars

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Re: Mars 5-10 stereo - update
« Reply #77 on: February 07, 2020, 03:12:04 PM »
Here is a photo of the amp under discussion

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Offline fredeb

Re: Mars 5-10 stereo - update
« Reply #78 on: February 07, 2020, 05:04:51 PM »
That's a good looking amp Karel , certainly looks like it should have a bit of vooma for an EL84 PP .

How do things look when you double or triple the ecc82 anode resistor ? Do you mind doing that in the software so we can see the difference please .


That said , I'm definitely going to purchase tubecad , seems it's only $10
« Last Edit: February 07, 2020, 05:09:00 PM by fredeb »
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Offline Mars

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Re: Mars 5-10 stereo - update
« Reply #79 on: February 12, 2020, 11:34:31 AM »
As requested:



and report



As you can see, the 12AU7 does not perform well in this circuit. The easiest way to make this work is with a increased B+ or a negative voltage applied to the long tail resistor. 150V seems to be the ideal.



I think I can find a way of adding a negative 150V supply from the PSU without too much sweat. But using an ECC88 makes all problems go away, except you cannot use only tubes robbed from a guitar amplifier :sd:

Or, stick with guitar amp tubes and deal with the high sensitivity in another way:

12AX7 with original values as in Mullard 5-10

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Offline Qualityten

Re: Mars 5-10 stereo - update
« Reply #80 on: February 12, 2020, 06:11:36 PM »
Qualityten,

Yes, that is the official Stereo 20 schematic. It does not appear that a different schematic was issued after the 'recall' early in the 1960. In a brochure about Leak called "Firsts in High Fidelity"* by Stephen Spicer (p.154) the recall is briefly mentioned.

I am uncertain where I first learned about it. (I was then employed by the C.S.I.R. and several periodicals were in circulation for compulsory reading.) When H.J. Leak visited Cape Town 1965/66, a couple of us students  attended a demo talk and had tea with him afterwards,. but I do not think this was discussed then - not sure when the Stereo 20 was produced.

It appears no exception that maximum specifications for valves are often exceeded, particularly in guitar amps  (consequences are suffered afterwards.)

Picking up this point from Ampdog, I raised the question of when Leak changed the value of the grid leak resistors in the Stereo 20 on another forum and got this interesting reply:
  • Ray Whitcombe joined Leak in 1960 and one of his first tasks was to investigate failures of EL84s in TL12+ and Stereo 20 amplifiers, so that gives a very rough starting point.  The mains transformer and output transformers changed simultaneously as parts of the same modification, as did some other resistor and capacitor values. (The reduced HT and lower value grid leak resistors to prevent the run-aways, the 20% screen taps to restore the output power to be similar to the older design, and the component changes to accommodate the different gain of the new output stage).  Some Gold coloured amplifiers were built to the new design (mine was, and is dated 1963) and all Grey ones were.

Offline fredeb

Re: Mars 5-10 stereo - update
« Reply #81 on: February 12, 2020, 08:54:15 PM »
Thanks for indulging me Karel , it appears that many values would need to change . I almost purchased tubecad , but noticed the non 64-bit operating system warning , I suppose it wouldn't cost too much to setup a tubecad dedicated machine running 32-bit XP .

BTW ... 6N1P , could be an affordable replacement for ECC88 now that 6N23P and the -EV version are becoming so expensive .
...evolution is the gradual development and stratification of progressive series of wholes, stretching from the inorganic beginnings to the highest level of spiritual creation.
Jan Smuts

Offline Ampdog

Re: Mars 5-10 stereo - update
« Reply #82 on: February 13, 2020, 07:34:22 AM »
Basics again (OT, but at least Karel gets the exposure :)!

In voltage amplifiers one stays away from load resistors close to the valve value of rp .  Rl = rp is the condition for maximum power transfer (not required in voltage amplifiers), but also for maximum distortion. Thus for say 12AU7, rather higher than 15K, for pentodes either <220K or >1M (the latter becoming impractical for other reasons).  For ECC83, rather >39K etc. (But as always with electronics, not quite that simple: rp under specific operating conditions; rp changes significantly with current).

Qualityten:
Thanks. Yes, easy to comment after the fact, but the man who originally reckoned that >470K is safe EL84 grid resistors with that grid construction (evident from the high gm):  :o

But again: Back to topic (Karel is most indulgent!)
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline Mars

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Re: Mars 5-10 stereo - update
« Reply #83 on: February 18, 2020, 10:17:30 AM »
  • the 20% screen taps to restore the output power to be similar to the older design
    [/quote]

    I am intrigued by this quote. Maybe the Mars 5-10 design should return to 43% screen taps to reduce the gain of the amp, and reduce distortion simultaneously.

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Offline Qualityten

Re: Mars 5-10 stereo - update
« Reply #84 on: February 18, 2020, 02:30:32 PM »
The excess gain is in the first stage, with the EF86, so I don't think the different taps would affect that issue.  However, 20% screen taps are recommended as optimal, rather than the more usual 43%.  I don't know enough theory to know why!

Offline handsome

Re: Mars 5-10 stereo - update
« Reply #85 on: February 18, 2020, 08:17:34 PM »
To reduce gain: strap the ef86 as a triode and unbypass its cathode resistor

Offline Qualityten

Re: Mars 5-10 stereo - update
« Reply #86 on: February 19, 2020, 03:38:17 PM »
I have had good results with triode strapping EF86s in my Verdik Qualityten 5-10 amps, which otherwise follow the classic Mullard circuit exactly. 

I followed the guidance in this thread on the UK Vintage Radio Repair and Restoration forum. https://www.vintage-radio.net/forum/showthread.php?t=99399. See posts #7 and #16 for circuits.

Offline Ampdog

Re: Mars 5-10 stereo - update
« Reply #87 on: February 19, 2020, 07:40:05 PM »
To reduce gain: strap the ef86 as a triode and unbypass its cathode resistor

. . . . and consequently reduce the  feedback resistor accordingly - otherwise the above steps will merely be countered by decreasing the NFB, without causing a notable reduction in gain.
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline handsome

Re: Mars 5-10 stereo - update
« Reply #88 on: February 19, 2020, 09:02:53 PM »
Whoops! forgot about the feedback <blush> you are, as ever, quite correct, Boss - judicious diddling with feedback resistors, compensation networks and inviting a square wave to the party would be in order then

Offline Qualityten

Re: Mars 5-10 stereo - update
« Reply #89 on: February 22, 2020, 12:28:07 PM »
It so happens that the same question was asked in a post on Audiokarma.  A knowledgeable member there commented as below.  This helps explain handsome's advice to remove the cathode bypass cap:

'The cathode bypass cap will increase open loop gain. Amps that apply a lot of feedback may need more open loop gain for a given input sensitivity. I’d try it without cathode bypass. See if you can live with the input sensitivity given the amount of feedback you want to apply.  Also, if you triode strap, you may need to rebias the EF86, meaning you may need to change the value of the cathode resistor to give you the bias point you want. Since the EF86 is direct coupled to the phase inverter, you do want the EF86 plate voltage the same as in the original circuit where it was pentode wired.'