M-125 Build

by Dahlberg Thu Jan 23, 2020 5:30 pm

Dahlberg: Posts : 132
Join date : 2016-09-27
Age : 62
Location : Sweden

Post n°51

Re: M-125 Build

by Dahlberg Thu Jan 23, 2020 5:30 pm

This truly is a beautiful build Smile

There are a few things that came up though. Triode mode ?
I guess that you did cut the groundplane traces on both sides of the input signal ground
connection, before connecting it again over the resistor on the pcb.
Or did you draw a dedicated wire from there to chassis ground that's not visible ?
There are two ground wires from the capasitor pcb to chassies ground right ?
I guess that you are building/buying rectifiers in sockets. If building, "Cree" and "Infineon"
have some nice silicon carbide 1200v ones I'm using that I like.

Really a nice build and thanks again for the inspiration Very Happy

Have you decided on what tubes to use ?

by KenGaler Fri Jan 24, 2020 11:09 am

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Post n°52

Re: M-125 Build

by KenGaler Fri Jan 24, 2020 11:09 am

No smoke so far

M-125 Build - Page 3 Top

by KenGaler Fri Jan 24, 2020 11:17 am

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Post n°53

Re: M-125 Build

by KenGaler Fri Jan 24, 2020 11:17 am

Dahlberg wrote:This truly is a beautiful build There are a few things that came up though. Triode mode ?
I guess that you did cut the groundplane traces on both sides of the input signal ground
connection, before connecting it again over the resistor on the pcb.
Or did you draw a dedicated wire from there to chassis ground that's not visible ?
There are two ground wires from the capasitor pcb to chassies ground right ?
I guess that you are building/buying rectifiers in sockets. If building, "Cree" and "Infineon"
have some nice silicon carbide 1200v ones I'm using that I like.

Really a nice build and thanks again for the inspiration

Have you decided on what tubes to use ?

Thank you

- No triode mode.
- The PCB is isolated from the chassis with nylon standoffs.
- I did wind up with two ground wires on the B+ filer PCB. I was going to wire the driver board directly but went to the audio GND instead. So, I used two wires to the audio GND instread, couldn't hurt.
- I went with a WZ68, for now.

- I was going to ask the forum about which tubes to use. Right now I'm using 6550WE I got from Bob. I've read through the threads but is there a conclusion with is best?

Ken

by cci1492 Fri Jan 24, 2020 12:14 pm

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Post n°54

Re: M-125 Build

by cci1492 Fri Jan 24, 2020 12:14 pm

Unique looking animal! I love it and that flux capacitor look :

M-125 Build - Page 3 Wired-top-left

Last edited by cci1492 on Fri Jan 24, 2020 6:47 pm; edited 1 time in total

by avi.inc Fri Jan 24, 2020 4:06 pm

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Post n°55

Copper Cap

by avi.inc Fri Jan 24, 2020 4:06 pm

KenGaler wrote:

Dahlberg wrote:This truly is a beautiful build There are a few things that came up though. Triode mode ?
I guess that you did cut the groundplane traces on both sides of the input signal ground
connection, before connecting it again over the resistor on the pcb.
Or did you draw a dedicated wire from there to chassis ground that's not visible ?
There are two ground wires from the capasitor pcb to chassies ground right ?
I guess that you are building/buying rectifiers in sockets. If building, "Cree" and "Infineon"
have some nice silicon carbide 1200v ones I'm using that I like.

Really a nice build and thanks again for the inspiration

Have you decided on what tubes to use ?

Thank you

- No triode mode.
- The PCB is isolated from the chassis with nylon standoffs.
- I did wind up with two ground wires on the B+ filer PCB. I was going to wire the driver board directly but went to the audio GND instead. So, I used two wires to the audio GND instread, couldn't hurt.
- I went with a WZ68, for now.

- I was going to ask the forum about which tubes to use. Right now I'm using 6550WE I got from Bob. I've read through the threads but is there a conclusion with is best?

Ken

The WZ68 maynot be the best one to use you may want to try the WS-1

https://www.tedweber.com/wz68
https://www.tedweber.com/ws1

by Dahlberg Fri Jan 24, 2020 7:38 pm

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Post n°56

Re: M-125 Build

by Dahlberg Fri Jan 24, 2020 7:38 pm

I would say, build your own. Idea

With either this rectifier:
https://www.wolfspeed.com/media/downloads/77/C4D05120A.pdf

Or this one:
https://www.mouser.se/datasheet/2/196/Infineon-IDH02G120C5-DS-v02_01-EN-1226806.pdf

Sockets:
https://www.ebay.com/itm/8-Pin-Octal-Bakelite-Tube-Base-Socket-Saver-Adapter-for-6550-EL34-EL34-6N8P-10PC/223512174828?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

Protective capsulation, I'm quite sure you can manage that Smile

Edit: I'm using Tungsol KT-120 for power tubes and I have never adjusted bias yet. I went that way partly for longer
lifetime (seems to be working i guess) and even if I haven't tried anything else in their place I don't belive I'm way of Smile

Tungsol 12au7 in both additional positions, not sure if that's the optimal solution but they give a solid performance.
I have tried a few other ones and I might have to go over that again, should be fun Cool

by KenGaler Sat Jan 25, 2020 6:15 pm

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Post n°57

Re: M-125 Build

by KenGaler Sat Jan 25, 2020 6:15 pm

They are not working properly. I'm convinced it's not the wiring.

On both amps, when I get to about 800mV rms in they become unstable. The B+ dips at about 10Hz and the output collapses at the same rate.

I removed the power tubes and powered up at a low line to get the B+ to the same 475V. Both amps were able to output 105-110Vrms from the driver cleanly. And that was with about 1.75Vrms input, much higher than is normal. I believe this eliminates the driver section as a problem.

Both amps work as expected when just two tubes are used in either the front or back position. I'm getting about 50-60 watts out at max input with no problems.

It's only when all four tubes are installed and it's pushed a bit past what it can do with two tubes does it get flaky.

I thought it was the rectifier so I tried the 5AR4 (tube rectifier) and it does the same thing.

I'm using 6550WE power tubes from Bob. KT-120s from Tube Amp Doctor will be ordered on Monday but I doubt if they are the issue.

I don't think I made any significant electrical changes to the original.

Any thoughts? Anyone seen any unstable outputs at higher outputs? I'm starting to think that the feedback loop needs a tweak.

TIA

by **Bob Latino** Sat Jan 25, 2020 6:32 pm

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Post n°58

Re: M-125 Build

by Bob Latino Sat Jan 25, 2020 6:32 pm

KenGaler wrote:They are not working properly. I'm convinced it's not the wiring.

On both amps, when I get to about 800mV rms in they become unstable. The B+ dips at about 10Hz and the output collapses at the same rate.

I removed the power tubes and powered up at a low line to get the B+ to the same 475V. Both amps were able to output 105-110Vrms from the driver cleanly. And that was with about 1.75Vrms input, much higher than is normal. I believe this eliminates the driver section as a problem.

Both amps work as expected when just two tubes are used in either the front or back position. I'm getting about 50-60 watts out at max input with no problems.

It's only when all four tubes are installed and it's pushed a bit past what it can do with two tubes does it get flaky.

I thought it was the rectifier so I tried the 5AR4 (tube rectifier) and it does the same thing.

I'm using 6550WE power tubes from Bob. KT-120s from Tube Amp Doctor will be ordered on Monday but I doubt if they are the issue.

I don't think I made any significant electrical changes to the original.

Any thoughts? Anyone seen any unstable outputs at higher outputs? I'm starting to think that the feedback loop needs a tweak.

TIA

Ken,

When you alter the original M-125 layout and used a modified version of the original circuit, sometimes unexpected things can happen. The M-125's with the original circuit are quite stable.

I have doubts that the feedback line is at fault but you can experiment the feedback line. Roy and I have experimented with varying amounts of feedback with the VTA M-125 CCS driver circuit. Right now using the original circuit and component layout and the standard 7.5K feedback resistor, the amps are stable under all conditions. If you go to a lower value feedback resistor (which gives more feedback), when you hit about 3K the amp will sound a little harder and less "tubelike". If you increase the value of the feedback resistor to above about 10K (which gives less feedback), the bass will get a little "boomy" and IMHO less accurate to the music.

Bob

by audiobill Sun Jan 26, 2020 12:28 pm

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Post n°59

Re: M-125 Build

by audiobill Sun Jan 26, 2020 12:28 pm

Agree with Bob, try taking the negative feedback line (yellow out of output transformer) around the other side of amp, away from input wiring and see how it goes.

Bill

by pichacker Mon Jan 27, 2020 6:17 am

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Post n°60

Re: M-125 Build

by pichacker Mon Jan 27, 2020 6:17 am

Just a thought... As it's only when you are pushing it with a full compliment of tubes that you experience issues, could your B+ regulator be dropping out? Carefully put a DMM or 'scope (use an inline resistor to protect 'scope probe) and monitor B+ on loud passages...

Also how exactly is the "lift up ground" resistor connected in circuit?

Fantastic construction quality.

Steve

by KenGaler Tue Jan 28, 2020 4:29 pm

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Post n°61

Re: M-125 Build

by KenGaler Tue Jan 28, 2020 4:29 pm

Bob,

I'm about out of things to check. Both have a problem where when the input is brought up to near 1vrms they start to hiccup. I don't think it has anything to do with the layout because it's much better than the original. It's better but not that it had to be. There is an analog common that is 10 ohm from the chassis. The resistance from the supply to the common is low single digit mOhms and nearly a zero inductance.

I changed the feedback wire back to a wire (from a shielded cable) and ran it directly. That made no difference.

I, and others, have sanity checked the wiring and we're sure it's wired correctly.

Questions:
1) In your specs you mentioned 1Vrms in = 125W out. That implies a gain of about 31. My gain is 19 on both. The gain is the same when just two power tubes are installed. Does that give us a hint of what may be going on? Could a low gain be screwing up the stability?

2) I removed the power tubes and measured the output of the driver board. I adjusted the AC in for a 475Vdc B+. The drivers were happy with even 2Vrms input where they both output about 105Vrms. Is this correct?

3) What is the nominal target B+ voltage?

4) It's starting to smell like the output transformers. I made measurements on the windings but because their inductance is so high I don't think the readings are reliable. Have you ever had trouble with the output transformers?

This is a pic of the output while it's hiccuping. It cycles at about 10Hz. Anybody have an idea of what could cause this?

Also, a pic of how they are wired now.

Thank you

Ken

M-125 Build - Page 3 Num1-Unstable

by cci1492 Tue Jan 28, 2020 5:36 pm

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Post n°62

Re: M-125 Build

by cci1492 Tue Jan 28, 2020 5:36 pm

Can you bypass the B+ delay circuit just as a test?

by audiobill Tue Jan 28, 2020 8:06 pm

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Post n°63

Re: M-125 Build

by audiobill Tue Jan 28, 2020 8:06 pm

" I don't think it has anything to do with the layout because it's much better than the original."

Ken, hard to imagine that you could so confidently conclude this when, after all, your amp doesn't work when many, many other builders have few, if any problems by carefully following Bob's carefully thought out and well written instructions. For example, why have you wired the resistors to the terminal strips when you've opted not to use the triode/pentode switch? Burnt smelling transformers? Suggests you just may be overstating your expertise.

Having personally built dozens of these without a hitch, I recommend you disconnect the autobias circuits and whatever the additional board is, rewire the amp to closely follow Bob's instructions, including lead dress, and at least get the amp working in base form before adding your "upgrades".

You may wish to obtain Morgan Jones' texts "Valve Amplifiers" and "Building Valve Amplifiers" before attempting to roll your own once you get them working.

Best,

Audiobill

by KenGaler Wed Jan 29, 2020 10:32 am

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Post n°64

Re: M-125 Build

by KenGaler Wed Jan 29, 2020 10:32 am

cci1492 wrote:Can you bypass the B+ delay circuit just as a test?

I did look at the input to the rectifier with a scope diff probe while during the hiccup and it wasn't moving much at all. Shorting the relay is esay and I'll give it try. Thanks

by KenGaler Wed Jan 29, 2020 11:09 am

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KenGaler: Posts : 76
Join date : 2019-10-11

Post n°65

Re: M-125 Build

by KenGaler Wed Jan 29, 2020 11:09 am

audiobill wrote:" I don't think it has anything to do with the layout because it's much better than the original."

Ken, hard to imagine that you could so confidently conclude this when, after all, your amp doesn't work when many, many other builders have few, if any problems by carefully following Bob's carefully thought out and well written instructions. For example, why have you wired the resistors to the terminal strips when you've opted not to use the triode/pentode switch? Burnt smelling transformers? Suggests you just may be overstating your expertise.

Having personally built dozens of these without a hitch, I recommend you disconnect the autobias circuits and whatever the additional board is, rewire the amp to closely follow Bob's instructions, including lead dress, and at least get the amp working in base form before adding your "upgrades".

You may wish to obtain Morgan Jones' texts "Valve Amplifiers" and "Building Valve Amplifiers" before attempting to roll your own once you get them working.

Best,

Audiobill

Until I find the problem I'm not ruling out anything. But, with over 40 years of electronics design including power, audio, embedded, etc. I can evaluate a good layout. I'm also getting input from 3 other EEs with at least my experience. I know that we could be missing something with the layout but I want to rule out the more likely problems first.

I didn't install the resistors used for the Triode mode. Are you referring to the black resistors from the tubes to that terminal? Those are the 10 ohm current sense resistors for measuring the bias current. It was a long shot but I did try changing it back to the original configuration where the cathodes are tied together on each side with their single 10 ohm resistor. That didn't make a difference at all.

I don't have an autobias. That circuit on the bottom left is just a 2V voltmeter with a high input impedance. It's hard to imagine that being a problem but it's worth a look. I'll disconnect it and see.

Pichacker: I have a high voltage diff probe for the scope and have been looking at the B+. It seems clean and doesn't move more than expected. That "lift-from-ground" resistor is a ground loop breaker. I've never done that before on commercial equipment because UL would never approve it but I do know someone that has on a M125 and it works fine. See section 9 here https://sound-au.com/earthing.htm#s6. I will bypass that as a check also just to be sure.

I really appreciate all of your input. We'll get to the bottom of this eventually.

Thank you

by KenGaler Wed Feb 05, 2020 2:38 pm

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Post n°66

Re: M-125 Build

by KenGaler Wed Feb 05, 2020 2:38 pm

Short update: Changing to KT-120s seems to have fixed it. 120W+ out and no hiccuping. We're going to do some more investigation and see if we can get the 6550s to work also.

by Dahlberg Wed Feb 05, 2020 2:54 pm

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Post n°67

Re: M-125 Build

by Dahlberg Wed Feb 05, 2020 2:54 pm

From the shematics in the manual it says 500v B+ and the 6550 shouldn't have a problem with 475v B+ https://www.jj-electronic.com/en/6550

by KenGaler Wed Feb 05, 2020 3:35 pm

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Post n°68

Re: M-125 Build

by KenGaler Wed Feb 05, 2020 3:35 pm

Dahlberg wrote:From the shematics in the manual it says 500v B+ and the 6550 shouldn't have a problem with 475v B+ https://www.jj-electronic.com/en/6550

I'm getting from 475 to 525 depending if I use the WZ68 or the WS1. I don't think that's it. Our working theory right now is that the plates and G2s need to be snubbed.

by KenGaler Thu Feb 06, 2020 1:18 pm

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Post n°69

Re: M-125 Build

by KenGaler Thu Feb 06, 2020 1:18 pm

Can anyone confirm firsthand that the M125 can output 125W into a dummy load using 6550s? TIA

by KenGaler Mon Feb 10, 2020 10:56 am

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Post n°70

Re: M-125 Build

by KenGaler Mon Feb 10, 2020 10:56 am

Got it! Full power from 6550s with no hiccuping.

My friend Chris figured it out with his Yoda-like tube skills even 450 miles away in CT. The plates and G2s needed to be snubbed. All transformers have some leakage inductance no matter how well they are made. That is the uncoupled inductance. If you short the output transformers secondary, on a perfect transformer the primary would also be shorted and have zero inductance. On any practical transformer the coupling is not perfect and the primary will have some small amount of inductance. This inductance can have some resonant effect with plate and G2 and causes ringing in the 10s of MHz region. This was casing the instability. For reasons I'll never know or care to know, the 6550s were more prone to the ringing than the KT120s.

On some older designs schematics you can see 47 ohms resistors in series with the plates and G2s to solve this. The problem with that is that they dissipate significant power. To get the same effect with less power I made a R-L parallel combination by wrapping a 47 ohm 2w Carbon Film resistor with about 27 turns of 28ga wire. I didn't bother to count the turns on all of them but just filled the center section with wire. The inductance turns out to be about 1uH. At DC the R/XL combination is 90mOhms. It rises to about 10%, or 4.7 Ohms @ 600KHz. 23.5 Ohms at 3.5MHz. Somewhere between 3.5 and 10MHz is flattens out to 47 Ohms.

This could be a layout issue as suggested. If the wiring has longer runs, such as is the case when having a triode/pentode mode switch on the far side of the chassis, there could have been enough parasitics to hide the problem. It would take way too much investigation to find out for sure.

The initial testing looks very good. 96db S/N @ 125W. This will be a hectic week so next weekend I'll take a bunch of measurements and post the results. Let me know if there is a measurement you want to see.

M-125 Build - Page 3 Inductors

M-125 Build - Page 3 Finals-wiring-with-inductors

by Dahlberg Mon Feb 10, 2020 7:12 pm

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Post n°71

Re: M-125 Build

by Dahlberg Mon Feb 10, 2020 7:12 pm

Congratulations Very Happy

Since I'm using the KT-120's it would be interessting to
see if the solution you came up with is worth doing for me as well.

I might skip the UL/triode switch and use the space for a transformer
balanced input with the xlr input through the vacant switch hole.