• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

teaching VTA M-125 to sing

IEales

Member
2017-08-20 5:24 pm
As built by VTA, my M-125 mono-blocks were very noisy, ≈55db unweighted.

Before_n_After.jpg


After fixing build and design errors, the amps quietened almost 40db to ≈93db unweighted.

I suspect many DIY and retro designs may suffer similar problems.
In an effort to help others with similar problems, complete instructions:
ieLogical VTA M-125
 
Off topic maybe but I had to share this. Years ago I purchased a driver board kit for one of my Dynaco stereo 70's from them and put it together and it worked fine with the exception that someone didn't do their homework when it came to calculating the wattage of a few resistors. These particular resistors were running at max current. I first noticed they changed color a little which was enough for me to take readings and do the math. I haven't purchased a thing from them sense and when I mentioned the problem on their forum I was banned.
 
Thanks IEales.

I usually visit the your site and have been emails couple times for questions. Can you add the "Update date" on top of the page whenever there is new mods or tweeting on this amp.

Yours has auto-bias pbc, what is the different without it?

In your modification schematic, what are the value of W and V for resistor and cap? Do you have part# of bridge rectifier or can 1N4007 be used? Can I apply this ground house to other amps as well?
 
Thanks IEales.

I usually visit the your site and have been emails couple times for questions. Can you add the "Update date" on top of the page whenever there is new mods or tweeting on this amp.

Yours has auto-bias pbc, what is the different without it?

In your modification schematic, what are the value of W and V for resistor and cap? Do you have part# of bridge rectifier or can 1N4007 be used? Can I apply this ground house to other amps as well?
The page : ieLogical VTA M-125 is broken, it only shows top part and no working links( using firefox / Macos )
 

IEales

Member
2017-08-20 5:24 pm
Can you add the "Update date" on top of the page

Done. Click to jump to the relevant section.

Yours has auto-bias pbc, what is the different without it?
Without it, bias drifts continuously with varying line voltage and amp temperature. With it, bias is rock solid and monitored so any failure is immediately indicated, although almost impossible to see in the M-125. All tubes are always the same value and easily adjustable to other values.

In your modification schematic, what are the value of W and V for resistor and cap?
For the Loop Breaker? Links to the parts are in ieLogical VTA M-125
2x 50A diode bridge
10 Ohm 5W-Resistor WireWound
10uF 400V

can 1N4007 be used?
NO!!! A 1N4007 is 1A. You must be able to conduct the ENTIRE A/C line current to ensure the breaker trips.

Can I apply this ground house to other amps as well?
Provided you FULLY understand what you are doing. If you are not well skilled in tube amps, you could create something LETHAL!!
 

IEales

Member
2017-08-20 5:24 pm
The page : ieLogical VTA M-125 is broken, it only shows top part and no working links( using firefox / Macos )

I'm sorry but I don't have a mac or Firefox. ieLogical is not a commercial site, but it is plain HTML with a bit of php and javascript. In 10 years, this is the first time anyone has said there is a problem. If you PM with an image of what you can see, I could have a look.

I'm retired and only test on the Windows version I'm running on Edge & Chrome, now basically the same thing.

I quit commercial web development in the late 90s when there were 27 flavors of Windows OS and browser versions, plus Linux, apple, Netscape, etc.
When the browser checks exceeded the markup, I knew it was time to move on.
Never regretted it.
 
Last edited:

IEales

Member
2017-08-20 5:24 pm
lab on the cheap

test results on the M-125 with admittedly tinker toy tools from August 2020: lab on the cheap

"If it measures good and sounds bad, it is bad. If it sounds good and measures bad, you've measured the wrong thing." –– Daniel R. von Recklinghausen, HHScott

The M-125's sound G R E A T !!
Built properly, they are an incredibly good value.
 
I'm sorry but I don't have a mac or Firefox. ieLogical is not a commercial site, but it is plain HTML with a bit of php and javascript. In 10 years, this is the first time anyone has said there is a problem. If you PM with an image of what you can see, I could have a look.

I'm retired and only test on the Windows version I'm running on Edge & Chrome, now basically the same thing.

I quit commercial web development in the late 90s when there were 27 flavors of Windows OS and browser versions, plus Linux, apple, Netscape, etc.
When the browser checks exceeded the markup, I knew it was time to move on.
Never regretted it.
Found a solution; if i browse in fullscreen a second scrollbar appears. The web editor obviously assumes things about screen size ( error !)


Thanks for the write-up. Good reading.
 
Ahhh.
Post # 1 looks like B+ Capacitor input filters, combined with poor attention to B+ ground loops.
Generally, solid state rectifiers make the supply stiffer, and also generally increase the amplitude of the transient current.

And of course, those B+ secondary current transients can also appear on the filament secondaries too.
Interference on the filament wiring to anything other than an output tube can also cause trouble.

I love Choke Input B+ filters.
I use them whenever possible, especially since I use solid state diode rectification.
Does anybody want to take a guess why I love choke input B+ filters?
 

IEales

Member
2017-08-20 5:24 pm
ST-120 Improvements

IF my factory built M-125s worked and were quiet, I would have just plugged them in and enjoyed them.

There are some common issues:
  • Smallish main capacitance SCM
  • 16Ω tap feedback
  • output fault OPT protection
  • chassis as earth safety and 0vA
  • multi point 0vA
  • bias noise
  • pent/triode mode
  • unstable LM-334
  • under rated driver electrolytics
  • A/C power wire run with DC
  • general wire layout

I'm a firm believe in if it ain't broke, don't fix it.
OTOH, I love making things the best they can be.
After a lifetime in electronics, I had the knowledge and skill, albeit a bit rusty, to right the wrongs.

Would I recommend reworking a quiet, functional ST-120?
NO