S-5 Electronics K-12M (11MS8) 8 wpc PP Tube Amp Kit ($139) - Page 7 - diyAudio
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Old 11th March 2005, 01:52 AM   #61
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Quote:
Originally posted by chromal
a 100 Ohm 10w for balance
I've a couple 70 ohm 40(?) watt for sale... email me if you'd like detail.

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Old 14th March 2005, 08:10 AM   #62
chromal is offline chromal  United States
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Default Matching 11MS8s

So, I finally got in a batch of sixteen 11MS8 tubes and have been running them through a Hickok tester. Man, there's quite a wide variation in the tubes. I mean, this is stuff we already knew, but, wow. But, anyway, Of the sixteen, I have this continuum from which I can create seven pairs of tubes matched within about 100 micromhos of one another on the pentode. (Given the circuit topology, am I correct in thinking triode-section matching the 11MS8s is not particularly crucial for the K-12M?)

So far, I've seen two nearly-identical varieties of 11MS8s. Matsushita and Sylvania. These appear identical save that the Matsushita branded tubes in my possession have a slightly thicker support of the metal ring below the getter at the top. My Hickok doesn't seem to favor either brand. I've got mixed-brand pairs in my K-12M with no adverse affect. But, yeah, matching has definitely eliminated the glowing plates! S-5 really really needs to match the tubes they're selling. For the five minutes it takes, I'm sure it'll make a huge difference on the amp's life and the newbie builder's impression of tube amps' reliability.

If you'd like NOS matched-pentode 11MS8 pairs, contact me.
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Old 14th March 2005, 02:16 PM   #63
percy is offline percy  United States
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Default Does tube matching make sense ?

Chromal, I dont intend to discount your efforts by any means. It is better to use matched tubes and then no matching at all but these are just my thoughts on the subject..

Does tube matching really make sense ? There are other parameters - only two that I can think of right now - that affect the matching.

Although the tubes are matched before they are used, there is no guarantee that they will age at the same rate once they are in regular duty, hence at some point down the line they will mismatch again. What would you do then ? Especially when they are driven at or over their rating all the time!

I believe P-P transformers typically have different DCR on both primary windings. How would you account for that ?

What I am basically trying to say is that tubes that match perfectly on the tube tester, are not so forever, and might no longer be perfect matches in the circuit even on day one!

Secondly, what if one tube breaks down ? You need to find out one tube that matches the working tube or change the whole set!

To me it only makes sense to measure in-circuit voltage across the cathode resistor and adjust its value to achieve a dynamic match.
Especially for the S5 K-12M kit as it uses not-so-easy-to-find-in-bulk-quantities 11MS8 and to keep costs low (remember its a budget kit) I think S5 should modify the circuit to put adjustable bias resistors. That might be more cost and time efficient than matching tubes.

Just my opinion...
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Old 14th March 2005, 02:39 PM   #64
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Default Re: Does tube matching make sense ?

Quote:
Originally posted by percy
Chromal, I dont intend to discount your efforts by any means. It is better to use matched tubes and then no matching at all but these are just my thoughts on the subject..

Does tube matching really make sense ? There are other parameters - only two that I can think of right now - that affect the matching.

Although the tubes are matched before they are used, there is no guarantee that they will age at the same rate once they are in regular duty, hence at some point down the line they will mismatch again. What would you do then ? Especially when they are driven at or over their rating all the time!

I believe P-P transformers typically have different DCR on both primary windings. How would you account for that ?

What I am basically trying to say is that tubes that match perfectly on the tube tester, are not so forever, and might no longer be perfect matches in the circuit even on day one!

Secondly, what if one tube breaks down ? You need to find out one tube that matches the working tube or change the whole set!

To me it only makes sense to measure in-circuit voltage across the cathode resistor and adjust its value to achieve a dynamic match.
Especially for the S5 K-12M kit as it uses not-so-easy-to-find-in-bulk-quantities 11MS8 and to keep costs low (remember its a budget kit) I think S5 should modify the circuit to put adjustable bias resistors. That might be more cost and time efficient than matching tubes.

Just my opinion...
you are correct big daddy, current matching in circuit.
is far more important than mutual transconductance matching.

i have a kickass tube tester and only use it to find shorted tubes.
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Old 14th March 2005, 03:21 PM   #65
chromal is offline chromal  United States
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Default Re: Does tube matching make sense ?

Ah, it's good of you to pointing out pitfalls here. Though I hadn't considered this "solution" final, I feel a little more motivated to proceed with the circuit modifications I had been considering...

Quote:
Originally posted by percy
[B]Does tube matching really make sense ? There are other parameters - only two that I can think of right now - that affect the matching.

Although the tubes are matched before they are used, there is no guarantee that they will age at the same rate once they are in regular duty, hence at some point down the line they will mismatch again. What would you do then ? Especially when they are driven at or over their rating all the time![

I believe P-P transformers typically have different DCR on both primary windings. How would you account for that ?
Hmm. Well, I think my main goal was to come up with paired tubes that weren't so wildly mismatched that they'd destroy one another quickly with a glowing plate! But, yeah, your point that this is not a final solution to a circuit design problem is well-made.

Quote:
What I am basically trying to say is that tubes that match perfectly on the tube tester, are not so forever, and might no longer be perfect matches in the circuit even on day one!

Secondly, what if one tube breaks down ? You need to find out one tube that matches the working tube or change the whole set!
Why do manufacturers bother with matched pairs/quads at all? Is there more going into their matching than simple micromho comparisons?

If I lost a matched pair because a tube failed or changed drastically, wouldn't I simply drop another pair in, re-measure the removed tube, and add it to the spare pool for possible future utility?

Quote:
To me it only makes sense to measure in-circuit voltage across the cathode resistor and adjust its value to achieve a dynamic match.
Especially for the S5 K-12M kit as it uses not-so-easy-to-find-in-bulk-quantities 11MS8 and to keep costs low (remember its a budget kit) I think S5 should modify the circuit to put adjustable bias resistors. That might be more cost and time efficient than matching tubes.
Even if we set the operating point such that each tube is drawing the same current on zero-signal, what's to assure that the current flow through each tube will match over the range of the control grid voltage swing?

In the exisitng circuit, measuring the voltage across the pair's shared cathode resister doesn't say much about the current through a specific tube...
Perhaps a socketed resister arrangement or wirewound pot should be placed between each cathode and ground. I've been thinking about this ever since I assembled the kit. Adjusting the resistance between each cathode and ground seems like it would be better. It would have been nice if S-5 had gone that route in their design, though providing a $10 wirewound pot for each tube would have added 30% to the price of the kit!

One of the things that's really sort of giving me pause in all this is my general lack of knowledge about the tube. The info out there seems minimal at best, and I have yet to come across something as simple as a grid/plate curve chart.
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Old 14th March 2005, 03:46 PM   #66
percy is offline percy  United States
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Default Re: Re: Does tube matching make sense ?

Quote:
Originally posted by chromal
Why do manufacturers bother with matched pairs/quads at all? Is there more going into their matching than simple micromho comparisons?

If I lost a matched pair because a tube failed or changed drastically, wouldn't I simply drop another pair in, re-measure the removed tube, and add it to the spare pool for possible future utility?


or in the seperate pot/resistor arrangement, you could just change one tube and save half the cost and might still be better off than using a "matched pair". Not to mention vendors charge - I think its about $5-$6 per tube for matching - so you are paying a premium there which is included in the cost.

Quote:
Even if we set the operating point such that each tube is drawing the same current on zero-signal, what's to assure that the current flow through each tube will match over the range of the control grid voltage swing?
an expert would have to answer that, although I believe once you have the operating point covered, it should not be a problem, atleast with tubes of the same brand.

Quote:
In the exisitng circuit, measuring the voltage across the pair's shared cathode resister doesn't say much about the current through a specific tube...
Ofcourse not. I meant seperate/individual resistors for each tube.

Quote:
Perhaps a socketed resister arrangement...
thats a good idea if cost is a bar.

Quote:
The info out there seems minimal at best, and I have yet to come across something as simple as a grid/plate curve chart.
Create one
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Old 14th March 2005, 05:35 PM   #67
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Default Re: Does tube matching make sense ?

Quote:
Originally posted by percy
I believe P-P transformers typically have different DCR on both primary windings. How would you account for that ?
An interesting article on minimizing that (requires 4 OPTs for stereo thou)

http://www.pearl-hifi.com/06_Lit_Arc...hPull_OPTs.pdf

Quote:
Secondly, what if one tube breaks down ? You need to find out one tube that matches the working tube or change the whole set!
Yep, you have to replace the pair... or if you have an all-wheel drive car, all 4 (i have 2 sets of 3 good tires if anyone is interested).

Depending on how deep you are into their lifespan, you are unlikely to find somethng close enuff anyway.

dave
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Old 20th March 2005, 09:48 PM   #68
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Default Capacitor Upgrades

I finally got around do doing some more mods on the K-12M and I am very glad I did.

As suggested by Blackie, I removed the DC blocking input capacitors. This has turned out to be the biggest improvement thus far. The best sounding capacitors are clearly no capacitors at all. Removal of these caps really reduced midrange and tweeter hiss.

I upgraded the four coupling capacitors to film and foil. This also had a big impact on further improving the sound. The sound with the film and foil caps is more dynamic. Also, all mid and high frequency is gone .. the amp is dead quiet now.

I added some polystyrene caps to the electolytic PSU capacitors. This also had a positive effect. On this kit, small changes to the PSU seem to have a big impact on sound.

Finally, I added some cheap tube dampers. These also helped the sound slightly. However, I am thinking that tube coolers (which will also provide some damping) may be a better upgrade to invest in over dampers.

Anyway, you can read more on my website.
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Old 23rd March 2005, 02:21 AM   #69
james_b is offline james_b  United States
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Default volume pot in shunt mode

GG, since you already removed the DC blocking caps you may be interested in this thread on the Decware forum on running the volume pot in shunt mode with an added resistor

http://www.decware.com/cgi-bin/yabb/...num=1110965291
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Old 27th March 2005, 08:29 AM   #70
bhd812 is offline bhd812  United States
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Hello everyone I am new here to the forums, I am mostly on the headfi.org...yeah I am a headphone guy.

anyway I bought this kit last week and fell in love with it.

I have another kit coming in this week so I can run these as mono's, and I already know how to do it.

the gear I am using the kit in is a Meridian G08 cdp and NORH 6.6 speakers (93db senn)...

any thing else I should be looking into as mods?

thank you and its glad to be here, I can see myself here for a long time.
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