Author Topic: Mars 5-10 stereo - update  (Read 6863 times)

Offline Qualityten

Re: Mars 5-10 stereo - update
« Reply #45 on: November 16, 2019, 08:49:18 PM »
Another dof question: is the varnish applied externally, or is the whole transformer dipped?

Offline fredeb

Re: Mars 5-10 stereo - update
« Reply #46 on: November 17, 2019, 12:19:57 AM »
Another dof question: is the varnish applied externally, or is the whole transformer dipped?

If I recall correctly , Karel dips the entire transformer in varnish and then hangs it out to dry .
...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 #47 on: November 17, 2019, 07:14:49 PM »
If I may barge/chirp in with my way of applying varnish:

I have the privilege of access to a vacuum pot, courtesy of another member. Thus I am capable of vacuum impregnation - enough suitable varnish to cover the transformer (plus some; some varnish is going to be 'sucked' into the windings!)

Where not available, one can leave the transformer in enough varnish overnight, or perhaps better, apply lavishly with a brush on both ends of the winding. This can be time consuming; don't do both sides simultaneously or the bottom side varnish will flow out all over the show.  Again one leaves until varnish on a particular side has at least gelled. One may also heat up the transformer (BEFORE application!) to drive out some air plus causing the varnish to gel quicker. But disadvantage: If gelling rapidly less varnish will flow in/impregnate.

So in absence of a vacuuming facility I would take the time-consuming route (which I did in the past) of applying varnish, 'thrusting' into layers (if layer=-wound, which I still prefer) with a fairly stiff brush, leaving varnish side up for gravitational ingress, varnish some more, etc. until there is fair certainty that penetration was maximal or has gelled, then flip side etc. etc.  Patience - - but a reasonable degree of penetration can be reached by taking time.

This my way-of-doing-things.  Karel?
Audio must be the only branch of engineering where lack of basics' knowledge is considered a superior form of wisdom. (Anon)

Offline El Sid

Re: Mars 5-10 stereo - update
« Reply #48 on: November 17, 2019, 07:21:34 PM »
Another dof question: is the varnish applied externally, or is the whole transformer dipped?

Back off dude! I ask the dof questions around here....

Offline Mars

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Re: Mars 5-10 stereo - update
« Reply #49 on: January 27, 2020, 06:33:11 PM »
Hi All

The Mars 5-10 rides again. I have a kit amplifier a client built on the table for fine tuning. I was requested to look at the "hum" that was louder in one speaker than the other, and it moves to the other speaker when the amp is moved!

First I combed through the circuit and cleaned up bad solder joints. Next I added a virtual earth to the filament secondary, and lifted it to 30V DC above ground. That fixed the humm/buzz.

I recently built an amp with EL84 push-pull with a dual choke PSU. It did not have a tube rectifier, but did have an incredible bass response. Thinking back about how this circuit differed from the 5-10 I could list a few points that might or might not have improved bass response:

  • dual choke C-L-C-L pentode output section
  • 0.22uF 400V coupling caps
  • 1000uF 16V low ESR cathode bypass capacitors
  • recalculated phase inverter load resistors

So I started doing mods and listened to the sound, especially the bass response after each mod. I was hoping one of the interventions alone would shift the bass response to a more prominent position in the sound spectrum.

The easiest was to insert a 8 Henry choke between the EL84 anode and screen grid PSU nodes. That did improve bass and the sound in general.

Next I increased coupling capacitors to 0.22uF 400V. Bass improved, so another good idea.

Then I did the cathode bypass capacitors. 1000uF 16V all around. Bass improved again.

I had a look at TubeCAD calculator. I typed in the circuit values (47k anode load resistors/22k cathode long tail/260V supply). TubeCAD did not like this arrangement. It suggested I rather try 15k and 18k load resistors and a 9k long tail. I used a 10k long tail and 33k//27k and 33k//39k anode resistors. Bass does sound better? Or am my ears lying? Let me leave it on for a while. Voltages measures safe.

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Offline Qualityten

Re: Mars 5-10 stereo - update
« Reply #50 on: January 28, 2020, 10:13:56 AM »
Interesting post, thanks.

I have wondered for a while if 0.1uF is too low a value for the coupling caps. Maybe bigger caps were too big and/or expensive back in the late 50s? Perhaps that affected the size of the cathode bypass caps too?

Offline Mars

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Re: Mars 5-10 stereo - update
« Reply #51 on: January 28, 2020, 04:22:05 PM »
Thanks for the reply Qualityten.

If I look at the maths, I get these results: (The load resistor after the phase inverter coupling capacitor is 470k Ohm in this circuit.)

That means that (using Xc=1/(2*PI*f*C)   f   for 0.1uF is 3.4Hz and for 0.22uF  1.5Hz

Not a big difference on paper.

So why did the ancients use 0.1uF caps rather than 0.22uF or 0.33uF? My guess was that the speakers could not reflect those low notes we hear in modern music. Another issue would be the stability of the circuit with large amounts of negative feedback (as in the original 5-10 circuit). Maybe someone else can comment?

Ampdog has mentioned before how large cathode bypass capacitors can have the effect of fixed bias; thus less bias fluctuation during music playback.
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Offline Qualityten

Re: Mars 5-10 stereo - update
« Reply #52 on: January 29, 2020, 01:35:42 AM »
Thanks Karel. I need to go back in a time machine and do an elec eng course at a time when I still remember my school science and maths. What is Xc in your formula, and why are the load resistors increased from 4k7 to 470k in your circuit?

Your last point is intriguing too!

Offline fredeb

Re: Mars 5-10 stereo - update
« Reply #53 on: January 29, 2020, 05:57:46 AM »
Thanks Karel. I need to go back in a time machine and do an elec eng course at a time when I still remember my school science and maths. What is Xc in your formula, and why are the load resistors increased from 4k7 to 470k in your circuit?

Your last point is intriguing too!

Look here : http://sim.okawa-denshi.jp/en/CRhikeisan.htm
...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 Qualityten

Re: Mars 5-10 stereo - update
« Reply #54 on: January 29, 2020, 07:38:44 AM »
My turn to thank you, fredeb.

Online Jacobus

Re: Mars 5-10 stereo - update
« Reply #55 on: January 29, 2020, 07:48:43 AM »

So why did the ancients use 0.1uF caps rather than 0.22uF or 0.33uF? My guess was that the speakers could not reflect those low notes we hear in modern music. Another issue would be the stability of the circuit with large amounts of negative feedback (as in the original 5-10 circuit). Maybe someone else can comment?


Reading through this give a possible reason.

https://www.aikenamps.com/index.php/what-is-blocking-distortion

Offline Mars

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Re: Mars 5-10 stereo - update
« Reply #56 on: January 29, 2020, 02:29:03 PM »
Hi Qualitten

The resistor value you need to use is the grid leak for the following EL84.



R=470k
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Offline fredeb

Re: Mars 5-10 stereo - update
« Reply #57 on: January 29, 2020, 10:45:57 PM »
Hi Qualitten

The resistor value you need to use is the grid leak for the following EL84.



R=470k

Hey Karel , I'm interested to know whether 470k is the max permissible value allowed for that resistor in an EL84 amp ? As this this value already appears to be high . And just guessing , but I suppose the higher this value is , the easier it is to drive the EL84 ?
« Last Edit: January 29, 2020, 10:50:24 PM by fredeb »
...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 Mars

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Re: Mars 5-10 stereo - update
« Reply #58 on: January 30, 2020, 07:14:23 PM »
fredeb, the data pages gives 1M as max value for cathode bias. Yes, the higher the grid leak resistor value the easier the load on the phase inverter. But with a medium mu tube like the ECC82, lower values could be tried.

I have also found that the lower the grid leak resistor value, the better the output tube sounds; with transformer drive being the best sounding. So I generally go as low as I can.

I tried 220k value for grid lead in this amplifier, and the results seems to be better. So I am staying with it for the moment.
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Offline fredeb

Re: Mars 5-10 stereo - update
« Reply #59 on: January 30, 2020, 07:46:14 PM »
fredeb, the data pages gives 1M as max value for cathode bias. Yes, the higher the grid leak resistor value the easier the load on the phase inverter. But with a medium mu tube like the ECC82, lower values could be tried.

I have also found that the lower the grid leak resistor value, the better the output tube sounds; with transformer drive being the best sounding. So I generally go as low as I can.

I tried 220k value for grid lead in this amplifier, and the results seems to be better. So I am staying with it for the moment.

Thanks for the explanation Karel , appreciated man !  :thumbs:
...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