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teaching VTA M-125 to sing

Ian Eales

Ian Eales

New Member
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:
http://ielogical.com/Audio/VTA_M-125.php
 
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Wow, that's a lot of good info in there. Ouch for the Bob Latino build quality...

I didn't know that AC heater wiring have a polarity (in phase, out phase) and that would add more noise. I'll have to dig in there.
 
question about this

  • • brown heater 6.3VAC CT is connected to transformer case and thus Audio GND, bypassing heater CT AC coupling
is the CT connected to the trafo case internally, or do you mean that the lead is external to the transformer, but connected in the wrong place?

and this

  • • power tube sockets should use flat head screws or be atop the chassis to preclude KT-88 rocking
I've used a lot of standard pan head screws with bottom mounted octal sockets and not had issues, but the socket top stuck above the chassis further than the screw head. Are the sockets used in this not of that sort, or is it just a thick enough chassis that the socket doesn't poke up far enough to clear the screw heads?



I'll have to check the trafo polarity thing. Its not something I ever considered looking at. When you say less noise, thats less noise as measured at the output of your "sniffer" choke?


surprised the bias noise is that noticeable. Many old amps had half wave supplies but between the low current draw and the fact it goes in common mode, the residual noise was generally a non-issue. What was the original circuit, a single cap input filter? I'd figure even a simple CRC would be enough to make it quiet enough.

I own only one power amp that has a full wave bias supply though, my Pilot SA-260. I have a couple of integrated amps that are full wave, but only because they also used DC tube heaters and the bias supply is tapped from that.

I always like seeing detailed posts like this from experienced people. I find it instructional and gives me ideas on how to improve my own stuff.
 
I can answere the socket question. The sockets on my ST70 sit low enough that
tubes with thicker bases do in fact sit on the thin pan head screws. If I remember right, Skizo used small felt pads on the chassis around the sockets to address this with his ST120. It isn't a problem with the KT77's I'm using, but the KT88's I tried did sit on the screw heads.
 
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I didn't know that AC heater wiring have a polarity (in phase, out phase) and that would add more noise. I'll have to dig in there.
Click to expand...
In a bipolar amp, the tubes push/pull the DC relative to the transformer center tap. If there is any hum noise from the heaters, wiring the tube heater pairs out of phase the noise is cancelled.
In the case of the M-125 used in half-power mode, which I did not measure, one should use sockets that have the same heater polarity so that any heater induced noise is the same on both ends of the output transformer, resulting in no current through the transformer.

is the CT connected to the trafo case internally,
Click to expand...
The brown 6.3v CT is tied to the transformer case

When you say less noise, thats less noise as measured at the output of your "sniffer" choke?
Click to expand...
yes. Back before polarized plugs were de rigueur, significant noise improvements could be made from the line cord orientation.
surprised the bias noise is that noticeable. Many old amps had half wave supplies but between the low current draw and the fact it goes in common mode, the residual noise was generally a non-issue. What was the original circuit, a single cap input filter? I'd figure even a simple CRC would be enough to make it quiet enough.
Click to expand...
The bias noise is an artifact with the AB-Q AutoBias. The original circuit has an 1.6Hz LP, but none with the AB-Q. Changing to 100R and moving the connection gives a 16Hz LP.
 
This is news to me.
In all the amps I have worked on, all the power tubes had regular twisted heater wires in whatever phase random order.

I never thought at the need, for each PP power tubes, to have the heater wired in opposite phase to each other.

Have you measured the effect of implementing that wiring schema in an actual amp? I am guessing you had obviously.
Thanks for sharing.
 
same honestly, I never gave it any thought since it seems like the folks who built it didn't either. I don't recall ever running across mention of doing this, though it makes sense in a push-pull amp. Back to that common mode noise cancelling thing,
 
93 db is impressive. My M125`s must definitely suffer from some of the described issues as no way are they anywhere near this level of performance. Nice to know that they can be improved on. Thanks for posting this most informative guide, it is bookmarked.
 
I just read the whole article. What I've read on this forum has lead me to the conclusion that the Latino Kit is not as well thought out as the builder may have intended. At least one other AK member had a runaway bias problem (probably due to lead dress issues) which we never heard the solution to. Another common theme is the Rectifier tube is tremendously over worked, people seek out the industrial version of the 5AR4 and it lasts for months instead of weeks, but at what cost? This article didn't come across as an intent to flame, but more of a here is what to actually expect... We need a lot more of that so that consumers can make the best choices without falling victim to advertising lies. I'm an amateur amp designer and I know that the M 125 needs 2 5AR4s if it's going to be tube redtified, and 3 probably wouldn't hurt. I didn't see anywhere in the article a full throttle distortion test. Those are always insightful, maybe incluse B+ sag measurements at no signal and at full power...
Great fun.
 
Brice said:
This is news to me.
In all the amps I have worked on, all the power tubes had regular twisted heater wires in whatever phase random order.
I never thought at the need, for each PP power tubes, to have the heater wired in opposite phase to each other.
Have you measured the effect of implementing that wiring schema in an actual amp? I am guessing you had obviously.
Click to expand...
It was a long time ago.
Conceptually, a PP amp works by changing the current through the OP transformer halves. One side shuts off, the other draws more. If the heaters are in phase, any noise is common to both ends. If the heaters are out of phase, then any heater induced noise is a small inverted signal.
Single tubes per phase want the heaters in phase
Dual tubes per phase want paired ±tubes heated in phase and the other pair inverted so any heater noise cancels at the plates.
 
bbqjoe said:
Ouch indeed.
I was thinking about the 125's as a future build.
Now I'm not so sure.
Click to expand...
I'd buy the kits in a heartbeat. I love the sound of the amps.
I didn't buy the kits because I'm nearly 70 with less steady hands, can't see as close and I gave away all my test gear when I left LA.
 
If you're building it, it would be real easy to apply Ian's corrections as you go. No re-work required.
 
trainbuftony said:
I didn't see anywhere in the article a full throttle distortion test. Those are always insightful, maybe incluse B+ sag measurements at no signal and at full power...
Great fun.
Click to expand...
I gave away all my test gear when I left LA. Had to buy a cheap & cheerful PC based scope, load resistors, & make misc bits to get this far.
It's fun now, but there were a few WTF?!?!? during the project.
Also, now the missus thinks I'll "never quit..."
 
I think instead of banning you for posting these findings it would make more sense to say Thank you and consider your findings in future builds.
 
It does look like Bob needs to find another assembler. Who ever is building these isn't up to the task. It doesn't take much to check all bolts and look over all the solder joints and connections.
 
well it looks as though you got your M125's all sorted, good to hear. A lot of the issues you reported are basic tube amp building knowledge and implementation. It certainly is not rocket science, just neat wiring and paying attention to the overall assembly.
You should get many years of enjoyment out of them!
 
tonyg88 said:
I think instead of banning you for posting these findings it would make more sense to say Thank you and consider your findings in future builds.
Click to expand...

Lavane said:
It does look like Bob needs to find another assembler. Who ever is building these isn't up to the task. It doesn't take much to check all bolts and look over all the solder joints and connections.
Click to expand...

Others elsewhere have indicated that Mr. Latino does not brook any criticism. His lack of response to obvious build flaw speaks volumes.
I was sort of half expecting an offer to refund build costs, but as my Gramps was oft to say "Him that expecteth bugger all is seldom disappointed"

I offered to tell him how I effected the improvement, but he did not respond. Likely because it is absolute proof of my initial position that the build was defective.
I therefore created the web page to help others get the most out of the product.
I may be old school, but either do it right or don't do it.
 
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