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    Rebuilding my M125s - can I use this cap in the SCM module?

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    rebellovw


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    Post by rebellovw February 28th 2022, 10:26 am

    Update:

    Found something and also got the amps both whisper quiet.

    On Sunday I used my iphone crude decibel meter - and had 59 dB rebuilt amp and 34 dB old amp (meter at the woofer dust cover.) No HumX.

    I tried a bunch of things - HumX, wz68 rectifier SS tube, only two output tubes in different locations, etc and in the end I did the following:

    Two output tubes only - located in the rear row. Rectifier didn't matter - I'm using the GZ37. Bias at 500 mA. No HumX. Result: @35 dB. Silent! Now both amps are dead quiet - and I can live happily with this setup. I've always preferred the two output tube setup of these amps with my Klipsch (easier on the rectifier and leaves me tons of spare tubes and I can bias each tube individually.)

    What I found when I first turned on the rebuilt amp and went through the various start up tests - was with two output tubes in the front row - I couldn't get the (facing) left tube to bias at 500 mA. The floor was 700 mA. The other tube was in the double digits for the floor so it was easy to bring up to 500 mA. With four output tubes it wasn't a problem as I was able to go from 700 mA to 1A - the recommended setting. And the amps play just fine.

    Now with only two output tubes in the front row - I can't get the amp to bias - so I switched to the rear row - and now it bias' just fine. Both tubes floor is in the double digits for mA. Because of that - the two output tubes in the second row - I now have them biased at 500 mA - and the amp is dead silent.

    The output tube is circled below that has the floor of 700mA:

    Rebuilding my M125s - can I use this cap in the SCM module? - Page 2 TiXUj2p

    This socket has the extension of the heater wires - that go to the driver board. In addition I also created the long heater wire bridge from V4 that routes along the chassis (this was "adviced" to me - though the person advising went silent - so who knows - I could have misunderstood him) - I'm wondering if the long twisted heater wires are contributing.

    Anyhow - I'm perfectly content as is and don't really feel like messing with it any further - as who knows I might lose this nice peaceful setup. I think I should have simply kept the heater wires short and direct from V4 to V2 as everyone else does. Or perhaps this has nothing to do with it?

    Thanks

    PS:  this is how I orignialy had them wired - like everyone else:

    Rebuilding my M125s - can I use this cap in the SCM module? - Page 2 EKXDa4S

    Criticism I received was:
    - Twists not up to the tube socket - leads too long untwisted.
    - Heater wires over terminal strip and sensitive components - need to route along chassis.
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    Post by rebellovw February 28th 2022, 11:02 am

    UPDATE!! - It was the tube!! - someone on AK suggested to try the tubes that did bias fine -and sure enough the floor is super low on both right and left front - and I can now easily bring up the front both l&r to 500 mA. Problem solved!
    Seamus
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    Post by Seamus March 1st 2022, 11:15 am

    The original wiring was fine.
    There is so much misinformation about heater wiring which gets blamed for other faults.

    You may want to check the heater voltage with only two tubes, especially with the old power transformer on 120V or higher.
    With some tubes, raising the heater voltage 0.5vac doubles the hum injection.

    Overheated, tubes have shorter lives.
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    Post by rebellovw March 1st 2022, 4:39 pm

    Seamus wrote:The original wiring was fine.
    There is so much misinformation about heater wiring which gets blamed for other faults.

    You may want to check the heater voltage with only two tubes, especially with the old power transformer on 120V or higher.
    With some tubes, raising the heater voltage 0.5vac doubles the hum injection.

    Overheated, tubes have shorter lives.

    I've had much better luck with these amps w/only two output tubes than with four. I've been running them since 2010. When I had 4 tubes - I'd blow output tubes, rectifier tubes etc. I've gone many years w/o any troubles w/o only two output tubes.

    In fact - I just recently went back to four output tubes and it took about a month (very little use) for the amps to have a complete meltdown (the one amp I rebuilt.)

    I'm good with them now. Not to mention w/only two output tubes - I have tons of spares and in this day and age that is a good thing.

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    Rec


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    Post by Rec March 1st 2022, 7:19 pm

    Interesting. Been following AK thread on the yellow sheet mod. Been working on my m125s to squeeze out last drop of noise on 3 wire. Just read heaters on these output tubes should be out of phase. I wonder if this is why two tubes are quieter than 4. I usually run ground lifted, been trying on and off to get them buzz free on 3 wire. They are silent shorted on 3 wire… I’ve swapped in many amps in this position and they are quiet. The m125 have a minor bzzz if I run 3 wire. I will be carrying the amps to the basement, worth one more shot… this is what I lose sleep over Smile

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    Post by rebellovw March 1st 2022, 10:46 pm

    Rec wrote:Interesting. Been following AK thread on the yellow sheet mod. Been working on my m125s to squeeze out last drop of noise on 3 wire. Just read heaters on these output tubes should be out of phase. I wonder if this is why two tubes are quieter than 4. I usually run ground lifted, been trying on and off to get them buzz free on 3 wire. They are silent shorted on 3 wire… I’ve swapped in many amps in this position and they are quiet. The m125 have a minor bzzz if I run 3 wire. I will be carrying the amps to the basement, worth one more shot… this is what I lose sleep over Smile

    Hey @rec - good the hear from you. Yeah my noise problem was simply a bad tube - I was using a tube that I basically fried but didn't realize it when I had the last meltdown. I'm now quiet on 4 as well as two output tubes with that tube out of the way.

    I was also wondering about the phase issue - as the new M125 instructions talks of polarity. Mine are the first iteration of these amps and there is no mention of polarity in the kit instructions for the heater wires - just hook them up - doesn't matter - so I "assume" with my older style trannies (I've heard they have been changed over the years) there is no polarity issue - hence it isn't needed. But I could be wrong.

    Anyhow - the amps sound wonderful now - dead quiet - no HumX and plugged directly into the power strip and into a Woods GFCI. I feel much safer now.

    I'm at a point where the HumX doesn't help.
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    Post by Rec March 2nd 2022, 7:41 am

    Can you share what the instructions say about heater phase? I’d guess they say to swap 2 and 7 across the tubes. The wiring picture in the instructions actually shows them crossed, text instructions did not. Thanks for this, only confirms my suspicion regarding my amp, especially if power tubes don’t match 100%. Will see today…

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    Post by rebellovw March 2nd 2022, 9:16 am

    Let me try to embed this for you @Rec

    *** THESE ARE OLD INSTRUCTIONS FOR REFERENCE ONLY. ALWAYS USE THE INSTRUCTIONS THAT CAME WITH YOUR KIT ***

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    Seamus
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    Post by Seamus March 2nd 2022, 10:44 am

    Tube heaters in the pairs in the same location should be wired IN Phase. e.g. v1&v2 in the M-125
    The reason for this is the output transform sees any heater hum component as a common mode signal, provided the tubes are well matched in terms of heater noise. Most modern tubes are superb.
    An additional pair of tubes can be wired with heaters out of phase as this will mitigate any hum in the tubes themselves thus reducing the OPT common mode signal.
    The inversion scheme should be followed for each additional pair of tubes with the caveats that circuit and layout particulars may require a different plan.

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    Post by rebellovw March 2nd 2022, 11:00 am

    Seamus wrote:Tube heaters in the pairs in the same location should be wired IN Phase. e.g. v1&v2 in the M-125
    The reason for this is the output transform sees any heater hum component as a common mode signal, provided the tubes are well matched in terms of heater noise. Most modern tubes are superb.
    An additional pair of tubes can be wired with heaters out of phase as this will mitigate any hum in the tubes themselves thus reducing the OPT common mode signal.
    The inversion scheme should be followed for each additional pair of tubes with the caveats that circuit and layout particulars may require a different plan.


    Thanks Seamus. Though my directions don't provide for that. The brown leads from the tranny to V4 pins 2 and 7 - that then bridge over to pins 2 & 7 on V2 - should the V4.2 go to V2.7 and V4.7 go to V2.2? Or should it be 7 goes to 7 and 2 goes to 2?

    I of course didn't do this at all. It would be anyones guess as to how it turned out as I just bridged it per the instructions.
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    Post by rebellovw March 8th 2022, 2:57 pm

    We'll my poor amp died Sunday at 11:23 AM.  RIP.

    I came in and all the tubes were dark.

    I found the slow-blo fuse blown.

    I suspected the GZ37 rectifier as it was involved in the major issue I had that started my rebuild - so I replaced the fuse, installed a W68 SS rectifier - and set the bias at the lowest level - both trim pots full clockwise.

    I turned on the amp - as the tubes started to glow - a very loud sound followed by 'BOOOM' followed by bad sound/noise - I had to kill the amp at the GFCI plug - fuse didn't trip this time.

    I think the transformers were likely shot after that first incident - one of them (the burning smell was bad - something didn't recover..).  I inspected the insides and the amp looks absolutely fine - I was expecting to see some blown caps, resistors etc - but they all look normal - no indication of any explosion or short. Even the insides of the new W68 looked fine as I've blown those in the past and when they blow the insides are all black and carbon coated.

    I'm going to shelf it for a while - I think I have to next:
    - Order new trannies - once I open up the old ones - hopefully I see some damage - I really need a smoking gun here.
    - Order a new quad cap
    - Test the remaining caps
    - Use a Bucking Transformer
    - Use a Timed startup module
    - Use my WS1 rectifier/w/thermistor

    I think that is about it.

    I'm bummed out - but at least the rebuild piece looks intact and not cause for any of the issues that I've currently experienced.

    Thanks,
    Dave_in_Va
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    Post by Dave_in_Va March 9th 2022, 9:50 am

    Just an opinion but I think I'd take it/send it to a tech and find out for sure what's up and go from there.

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    Post by rebellovw March 9th 2022, 10:35 am

    Dave_in_Va wrote:Just an opinion but I think I'd take it/send it to a tech and find out for sure what's up and go from there.

    Thanks Dave - that is what I was thinking too. There is a local guy that has posted some of his work on McIntosh stuff - so I'll interview him.

    Nice Interocitor you have.
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    Post by New2Tubez March 9th 2022, 10:38 am

    rebellovw wrote:We'll my poor amp died Sunday at 11:23 AM.  RIP.


    I suspected the GZ37 rectifier as it was involved in the major issue I had that started my rebuild - so I replaced the fuse, installed a W68 SS rectifier - and set the bias at the lowest level - both trim pots full clockwise.


    Isn't "full clockwise" all the way up?
    When I had individual bias pots on my ST120, I recall counterclockwise as lowering bias. It's been a couple years and have been using AB since then.

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    Post by rebellovw March 9th 2022, 10:52 am

    New2Tubez wrote:
    rebellovw wrote:We'll my poor amp died Sunday at 11:23 AM.  RIP.


    I suspected the GZ37 rectifier as it was involved in the major issue I had that started my rebuild - so I replaced the fuse, installed a W68 SS rectifier - and set the bias at the lowest level - both trim pots full clockwise.


    Isn't "full clockwise" all the way up?
    When I had individual bias pots on my ST120, I recall counterclockwise as lowering bias. It's been a couple years and have been using AB since then.

    Thanks - it sure felt like it was all the way up - and very counter intuitive to turn clockwise - but clockwise is correct - I tripled checked. And from the above startup install manual:

    "Initially turn the two bias adiusters on the driver board fully CLOCKWISE (to the riqht). This will start your bias out at its lowest point. Turn on the amp. Allow the amp to warm up for a minute and check to see that the two 65501KT88 tubes light up properly.

    Measure the bias on the output tub"
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    Post by rebellovw March 9th 2022, 10:56 am

    So the amp will get shelved until I find a tech that I can trust.  I will want some added safety measures installed too - like a couple of fuses in certain places and a timed delay board.  

    For now - I'm taking a break from tubes as not a good time to be into tube amps.

    I've ordered a Schiit Aegir - I'm going to try one with my Cornwalls and see how that goes - and also try one in bridged mode along with my other working amp to determine if I want to go to two Aegirs.  Or if I even like the Aegir - but I've read good things - keeping my fingers crossed.
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    Post by New2Tubez March 9th 2022, 11:50 am

    Found this video on the forum for biasing an ST120. He looks to be increasing bias by going clockwise. I'm not doubting what you're saying and the M125 pots may be configured differently.

    https://dynacotubeaudio.forumotion.com/t3043-vta-st-70-vta-st-120-and-vta-m-125-amplifier-information

    video:

    https://public.fotki.com/Rbertalotto/hifi-stereo-stuff/vta-dynaco-st-120/p1040049.html

    What's your wall current? I didn't see it mentioned in your post. If it's high, that will put an early end to a rectifier tube. Mine is on a variac besides using a Weber WS-1t. The AB has a delay built in. Used a TungSol 5AR4 prior to AB w/o issue.

    Erhard audio is the guy in AZ who now sells the AB board. He may be able to help you out w/ your issues.  He knows more than just the AB boards.

    If you haven't been on the Klipsch forums, they have good ideas, thoughts, opinions about amp matching, etc.

    PS, I'm not trying to sell anyone on the AB board. I just put it out there as this is my particular amp config and others will differ.
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    Post by Dave_in_Va March 9th 2022, 12:05 pm

    "Nice Interocitor you have."

    Thanks. When I first picked it up it didn't quite work correctly and I couldn't figure it out (Yes, I checked the tubes).
    So I sent it to the only tech I could find qualified to work on the things. Unfortunately I had to ship it via UPS Sub-light so I won't be getting it back for 365,000 years :-(

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    Post by rebellovw March 9th 2022, 12:06 pm

    New2Tubez wrote:Found this video on the forum for biasing an ST120. He looks to be increasing bias by going clockwise. I'm not doubting what you're saying and the M125 pots may be configured differently.

    https://dynacotubeaudio.forumotion.com/t3043-vta-st-70-vta-st-120-and-vta-m-125-amplifier-information

    video:

    https://public.fotki.com/Rbertalotto/hifi-stereo-stuff/vta-dynaco-st-120/p1040049.html

    What's your wall current? I didn't see it mentioned in your post. If it's high, that will put an early end to a rectifier tube. Mine is on a variac besides using a Weber WS-1t. The AB has a delay built in. Used a TungSol 5AR4 prior to AB w/o issue.

    Erhard audio is the guy in AZ who now sells the AB board. He may be able to help you out w/ your issues.  He knows more than just the AB boards.

    If you haven't been on the Klipsch forums, they have good ideas, thoughts, opinions about amp matching, etc.

    PS, I'm not trying to sell anyone on the AB board. I just put it out there as this is my particular amp config and others will differ.

    Thanks really appreciate it.  Yes my wall current is high - 122.  The wall current was never a concern when I bought the amps - definitely something learned over the years and now documented.  So yes - I have to adopt to that with a bucking transformer should I ever use these amps again.

    Good to know Erhard is in AZ - I might reach out - the AB boards are nice - but I just want the amps to work like they used to and I only use two output tubes so AB isn't absolutely necessary but the fact that it makes sure the bias is good in realtime is nothing to sneeze at.  It is probably on my list if I ever rebuild.  I've heard nothing but good things about the latest version and I have reached out to Holger a couple of times - I just flip flip - but now glad I didn't pursue as I don't want to spend any more $$ on these until I have a plan.

    Thanks again!

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    Post by rebellovw March 12th 2022, 9:37 pm

    Update:  I decided to take a chance with a local tech - so i dropped off the amp yesterday and he called me this morning:

    Shorted tube, blown resistor on the quad cap - a resistor I reused and didn't replace.

    Looks like all my tubes are testing poor - even the driver tubes which I've used for 10 years - so very old - time to get rid of all the tubes and replace - I just hope I have enough replacement tubes that test well.

    I'm installing the time delay startup board (on order) and will use the WS1/T SS Rect.   Also in the cards is a bucking transformer as my home voltage tested over 125v today and also around 123 using a kill-o-watt.   So Hammond 166N12B on order.

    Hopefully that is all the tech finds - but we will see.  Also a cap on the driver board - tall front cap he wants to replace as it has visual indication to him it's time to replace.
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    Post by LeGrace March 13th 2022, 6:45 am

    Are you planning to use two transformer, one for each amp? Its only rated 4 amp. Also secondary is 12v versus 6.3v or 5v often suggested for these devices in some build guides. For mine I went with a 16 amp rating and 6.3 volt center tap, Hammond 167U6. This one is clearly overkill rating wise, but had the form factor I wanted for external mounting and the price differential I recall was minor. Even then it gets warm. At only 4 amp and if internal mounted a single 166N12B I'd be concerned about how hot it will get.
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    Post by rebellovw March 13th 2022, 11:06 am

    LeGrace wrote:Are you planning to use two transformer, one for each amp? Its only rated 4 amp. Also secondary is 12v versus 6.3v or 5v often suggested for these devices in some build guides. For mine I went with a 16 amp rating and 6.3 volt center tap, Hammond 167U6. This one is clearly overkill rating wise, but had the form factor I wanted for external mounting and the price differential I recall was minor. Even then it gets warm. At only 4 amp and if internal mounted a single 166N12B I'd be concerned about how hot it will get.

    Yeah - just the one for now - I am only running two output tubes per amp ("what do I look like I'm made of money?" hahaahaaa) so I "assume" that will work.

    It will have a 4amp fast blow fuse - so if I'm blowing fuses - then I'll add another/rethink it. I'll also use an on/off switch so that it is not running all the time.

    But yeah after I ordered it - I was like - ohh I got two amps... but I think (from what little I know and the thread at AK) that one amp is at @140-150VA and this transformer will be good for 480VA - so I think it should work out.

    Thanks!
    JohnC
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    Post by JohnC March 13th 2022, 2:08 pm

    rebellovw wrote:
    LeGrace wrote:Are you planning to use two transformer, one for each amp? Its only rated 4 amp. Also secondary is 12v versus 6.3v or 5v often suggested for these devices in some build guides. For mine I went with a 16 amp rating and 6.3 volt center tap, Hammond 167U6. This one is clearly overkill rating wise, but had the form factor I wanted for external mounting and the price differential I recall was minor. Even then it gets warm. At only 4 amp and if internal mounted a single 166N12B I'd be concerned about how hot it will get.

    Yeah - just the one for now - I am only running two output tubes per amp ("what do I look like I'm made of money?" hahaahaaa) so I "assume" that will work.

    It will have a 4amp fast blow fuse - so if I'm blowing fuses - then I'll add another/rethink it.  I'll also use an on/off switch so that it is not running all the time.  

    But yeah after I ordered it - I was like - ohh I got two amps...  but I think (from what little I know and the thread at AK) that one amp is at @140-150VA and this transformer will be good for 480VA - so I think it should work out.

    Thanks!

    My mains voltage varies between 122 and 125 volts and for a bucking transformer, I am using the 166N12B protected by a 5 amp SB fuse.
    I power my M-125s with 4 tubes each, a small DAC and SP-14 pre-amp without issue. The transformer will get warm to the touch but nothing excessive.
    Output voltage from the transformer is 114 to 115 volts and I am seeing a load of 4.50 amps.
    I bias the M-125s at .45 volts.
    I think you'll be good.

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    Post by rebellovw March 13th 2022, 3:06 pm

    JohnC wrote:
    rebellovw wrote:
    LeGrace wrote:Are you planning to use two transformer, one for each amp? Its only rated 4 amp. Also secondary is 12v versus 6.3v or 5v often suggested for these devices in some build guides. For mine I went with a 16 amp rating and 6.3 volt center tap, Hammond 167U6. This one is clearly overkill rating wise, but had the form factor I wanted for external mounting and the price differential I recall was minor. Even then it gets warm. At only 4 amp and if internal mounted a single 166N12B I'd be concerned about how hot it will get.

    Yeah - just the one for now - I am only running two output tubes per amp ("what do I look like I'm made of money?" hahaahaaa) so I "assume" that will work.

    It will have a 4amp fast blow fuse - so if I'm blowing fuses - then I'll add another/rethink it.  I'll also use an on/off switch so that it is not running all the time.  

    But yeah after I ordered it - I was like - ohh I got two amps...  but I think (from what little I know and the thread at AK) that one amp is at @140-150VA and this transformer will be good for 480VA - so I think it should work out.

    Thanks!

    My mains voltage varies between 122 and 125 volts and for a bucking transformer, I am using the 166N12B protected by a 5 amp SB fuse.
    I power my M-125s with 4 tubes each, a small DAC and SP-14 pre-amp without issue. The transformer will get warm to the touch but nothing excessive.
    Output voltage from the transformer is 114 to 115 volts and I am seeing a load of 4.50 amps.
    I bias the M-125s at .45 volts.
    I think you'll be good.

    That is really great to hear - thanks John!
    avatar
    rebellovw


    Posts : 73
    Join date : 2022-01-23

    Rebuilding my M125s - can I use this cap in the SCM module? - Page 2 Empty Re: Rebuilding my M125s - can I use this cap in the SCM module?

    Post by rebellovw March 22nd 2022, 1:07 pm

    Added some toggle condoms to the working amp - as it always gave me the creeps to turn these off when there is a massive failure.

    Easy Peasy - just screwed right on.

    Rebuilding my M125s - can I use this cap in the SCM module? - Page 2 P308vUI


    Condoms (other colors available):

    https://www.amazon.com/dp/B07TYYQ9XQ?psc=1&ref=ppx_yo2_dt_b_product_details

    Sponsored content


    Rebuilding my M125s - can I use this cap in the SCM module? - Page 2 Empty Re: Rebuilding my M125s - can I use this cap in the SCM module?

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