teaching VTA M-125 to sing

[bbqjoe]

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.

I agree.
I find it hard to believe Bob would be so offended as to ban someone who appears to be quite insightful and knowledgeable.

I was also a bit perplexed about the tube sockets.
Using KT88's or KT120's the tube bases do sit on the screws allowing the tube to rock, and also not allowing them to fully seat into the sockets.
(I can't help but wonder if poor connections with the pins weren't the source of my red plating woes.)
It's kind of too late to do anything about it now, but in a future build, I would do something differently.

I think if I were Bob, I would try building an amp using Ian's suggestions and mods, and revise the board and instruction manual if all was on par.
I'd probably send a few $$ Ian's way as well.

But that's just how I operate.

 

[Ian Eales]

(I can't help but wonder if poor connections with the pins weren't the source of my red plating woes.)

I don't think so, at least from what I observed.

The tubes are pretty secure.
After I completed the repairs & mods, I looked at all the tube pins in sockets from the bottom w magnifier and contact looked good.
​

Part of the plan, was felt dots, but the surface gets hottish, so they would likely migrate.
A second thought had been to replace the pan head with countersunk flat heads after drilling all the holes.

Since the amps are out of the way, I decided to fogedaboudit

 

[Lavane]

There is another AK member that used small felt dots. Cut them in half and put them around the socket. They seem to have stayed in place over the last few years. I don't know what he used to secure them though. If I ever decide to try a better set of KT-88's I'll probably use them. I've seen them in a package of 6 or more at hardware stores.

 

[trainbuftony]

Seems to me that the fact that the owner of the company scurried under a rock rather than address the obvious build issues tells a person everything they need to know. A reputable company is eager to hear criticism and tech reports from the field. It allows them to turn out a better product and often times they reward the effort with free goodies. Not this guy. If i owned an amp company and paid an assembler, i would want to know if that assembler was turning out unsafe work.

Call the M&M people about sub par candy, and they send you a new bag.

Thats how busines was once done.

 

[Drummerboy2]

I understand your position and would be upset about not getting what I paid for on the assembly of the amps which I agree was pretty poor. You should have gotten a refund for the bad job done. I just think there is two sides to a story and we don't know the other side and how things transpired as the emails or Dynaco Audio forum post were. I had an issue with my amp and brought it up to Bob attention and he was really cool about it and was resolved. That tech shouldn't be building amps for the public if that is the type of work he does.

My VTA-120 has been working great since I built it, only had one issue with the heater wire solder properly with connections on the bottom of the sockets. Problem was the wire that is supplied with the kits need higher temps for solder to adhere to it. never knew about the phase of the heaters phase being attach a certain way to sockets and always thought the twisting and keeping close to the chassis was good enough.

I do appreciate you sharing the issues you found and what you did to repair it.

 

[Erhard-Audio]

there is no excuse for bad service or support. We all have bad days, but those should not reflect against clients.

 

[kward]

The best SNR I’ve got out of a scratch build was about 88 dB. That was with a lot of brain effort in planning the layout and wiring. But I didn’t think about phasing of heater connections. Maybe that’s where that last 5 dB is hiding.

 

[Ian Eales]

The best SNR I’ve got out of a scratch build was about 88 dB. That was with a lot of brain effort in planning the layout and wiring. But I didn’t think about phasing of heater connections. Maybe that’s where that last 5 dB is hiding.

I don't think you'll get 5db from the heater phasing, at least not on an M-125 unless it is completely horked.
Anything approaching 90db is pretty darn good and I rather listen than have them on my bench again... ;-)

 

[Lavane]

I agree.
I find it hard to believe Bob would be so offended as to ban someone who appears to be quite insightful and knowledgeable.

I was also a bit perplexed about the tube sockets.
Using KT88's or KT120's the tube bases do sit on the screws allowing the tube to rock, and also not allowing them to fully seat into the sockets.
(I can't help but wonder if poor connections with the pins weren't the source of my red plating woes.)
It's kind of too late to do anything about it now, but in a future build, I would do something differently.

I think if I were Bob, I would try building an amp using Ian's suggestions and mods, and revise the board and instruction manual if all was on par.
I'd probably send a few $$ Ian's way as well.

But that's just how I operate.

I didn't have any problems with pin connection when I used KT-88's, but after heating up, cooling down, they would sit a bit tilted. My OCD could not abide.

 

[pjrengineer]

Curious if anyone knows how the kits are since this info came to light. I was just thinking of buying a kit but now I don’t want to have to read a website and a instruction manual to fix problems from the build.

 

[Legrace]

I incorporated one of the recommendations, ie adding the autobiasing boards. Latino has a negative view of these, but they are the only reason I still have these amps. Solved my red plating problems, bias is finally stable, amps sound great now. Great advice.

 

[Lavane]

The boards are no longer available for the VTA kits. There is a better board now with some protection added if I'm reading it right. I know of one other company using the new boards. I built a ST-70 without the bias board. They were a new thing at the time and I didn't trust them.

 

[Lavane]

Curious if anyone knows how the kits are since this info came to light. I was just thinking of buying a kit but now I don’t want to have to read a website and a instruction manual to fix problems from the build.

I built a pair of the M-125's a few months ago and have had zero problems so far. The instructions are very good. When it comes to wiring these amplifiers you do need to follow wire routing per the instructions. Wire routing is very important with these amps. Both amps are dead quiet with no hum. I did make a couple changes with mine. I put the power switch on the back using the hole meant for the RCA jack, and put the RCA jack on the front in place of the switch. I just didn't like those AC wires to the switch running all the way to the front and wiring from the input running all the way to the back. The way I did it both wire runs are very short and less chance of injecting noise. You do have to drill the hole in the rear out a bit to fit the switch though. I use a wifi activated power strip to turn them on and off, leaving the power switches on. You can use those Bluetooth remote outlets also, they work great too.

 

[GordonW]

Someone was just not paying attention when those amps were initially assembled.

Getting the polarity of heaters correct, so that the tubes can cancel their heater-induced hum properly, is something that I am very conscious of... it's disappointing to see a "pro built" amp with those sort of errors. And leaving pinched wires, and wires that are on the verge of breaking off- that's also quite disappointing.

I also believe in over-kill when it comes to insulating bare component lead wires that have to pass near other terminals. How much time or money does it take, to slip a little piece of heat shrink on a cap or resistor lead, so it can't contact a nearby metal object? Heck, I even go to the extent of color-coding such added heat-shrink, so that it matches RETMA/EIA color codes (red=B+, orange= screens, yellow= cathodes, green= grids, and so on)... it makes servicing the amp much more straight-forward later, if needed.

I can stomach the occasional loose bolt or stripped terminal- those things can happen in shipping, or due to a mechanically defective part that isn't caught. But most of the other problems, above, are solely in the lap of the original assembler. Especially in this case- this is not a one-off design that's being built from scratch every time- this is a standardized amp that has been produced in reasonably large numbers, ostensibly the same, for quite a long time. There should be a standard reference, to make sure that these things are built the same way, every time- without error.

Regards,
Gordon.

 

[GordonW]

The best SNR I’ve got out of a scratch build was about 88 dB. That was with a lot of brain effort in planning the layout and wiring. But I didn’t think about phasing of heater connections. Maybe that’s where that last 5 dB is hiding.

88dB is better than most source material. CDs are theoretically like 96dB- but that's if everything is perfect in the mastering process.

Regards,
Gordon.

 

[GordonW]

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.

Actually, in a PP amp, the heaters should be IDENTICALLY WIRED in their phase, for the two tubes on the opposite sides of a PP pair. In other words- say, you have a gray and a brown heater wire from your PT, connecting to a PP pair of 6L6, EL34, KT88 or such- if you connect the gray wire to pin 2 and the brown to pin 7 of one output tube, it should be gray to 2 and brown to 7 for ALL of them.

This means that the noise will come out of each tube in the SAME phase- that, combined with the push-pull nature of the output transformer (one tube is in opposite phase to the other, relative to the speaker out), means that the noise will be cancelled out. Common-mode cancellation, in effect.

The exception is a PPP with a multiple of 4 output tubes per channel, such as the M125- in this case, treating the tubes as two PP pairs per channel, and counter-phasing the two pairs (one "positive" phase tube and one "negative" phase tube pair wired gray-2 and brown-7, and the other positive phase tube and negative phase tube pair wired brown-2 and gray-7) can further cancel out the residual common-mode noise.

BTW: I did a variation of this, on the parallel-single-ended 6L6GC amps that I built with the Slagle transformers- I wired one output tube, DELIBERATLY, in opposite heater phase to the other. Since in a single-ended amp, multiple tubes are directly in parallel (same phase on the plates), the opposing heater noise should cancel out...

Regards,
Gordon.

 

[Lavane]

Someone was just not paying attention when those amps were initially assembled.

Getting the polarity of heaters correct, so that the tubes can cancel their heater-induced hum properly, is something that I am very conscious of... it's disappointing to see a "pro built" amp with those sort of errors. And leaving pinched wires, and wires that are on the verge of breaking off- that's also quite disappointing.

I also believe in over-kill when it comes to insulating bare component lead wires that have to pass near other terminals. How much time or money does it take, to slip a little piece of heat shrink on a cap or resistor lead, so it can't contact a nearby metal object? Heck, I even go to the extent of color-coding such added heat-shrink, so that it matches RETMA/EIA color codes (red=B+, orange= screens, yellow= cathodes, green= grids, and so on)... it makes servicing the amp much more straight-forward later, if needed.

I can stomach the occasional loose bolt or stripped terminal- those things can happen in shipping, or due to a mechanically defective part that isn't caught. But most of the other problems, above, are solely in the lap of the original assembler. Especially in this case- this is not a one-off design that's being built from scratch every time- this is a standardized amp that has been produced in reasonably large numbers, ostensibly the same, for quite a long time. There should be a standard reference, to make sure that these things are built the same way, every time- without error.

Regards,
Gordon.

I would defenetly buy the kit over the assembled amplifier. This isnt the first time I've read about assembly problems buying pre-built. That bias board was was a bad idea to me. When I ordered the kits I noticed they weren't being sold anymore. I was going to request kits without them if they were still being offered. But if assembled per the instructions, they are nice amplifiers.

 

[Erhard-Audio]

The boards are no longer available for the VTA kits. There is a better board now with some protection added if I'm reading it right. I know of one other company using the new boards. I built a ST-70 without the bias board. They were a new thing at the time and I didn't trust them.

The auto bias modules, where the pcb's are shaped to correctly fit the VTA ST-70, ST120 and M125's chassis' are available from us. We are the USA and Canada authorized reseller/agent for these auto bias modules.
Here is the link to our page, https://www.erhard-audio.com/AutoBias.html. The modules for the ST70/120 are called AB-Q-ST70/ST120 and for the M125 are called AB-Q-M125

 

[Erhard-Audio]

I would defenetly buy the kit over the assembled amplifier. This isnt the first time I've read about assembly problems buying pre-built. That bias board was was a bad idea to me. When I ordered the kits I noticed they weren't being sold anymore. I was going to request kits without them if they were still being offered. But if assembled per the instructions, they are nice amplifiers.

there is absolutely nothing wrong with the auto bias modules. I use them in ALL my custom builds and I have sold many units for the VTA ST70/120 and M125's. They initially got a bad wrap because of misunderstanding and incorrect implementation. In order to ensure trouble free operation, all auto bias modules now have the isolation transformer installed on the auto bias pcb, to isolate the 6.3VAC filament supply used to power the auto bias module.

 

[Erhard-Audio]

They are different now though aren't they? I noticed an added relay.

no, that is the isolation transformer for the 6.3VAC, as mentioned in my post #39 above. The auto bias circuit has not been changed at all, as it is a top and reliable performer. I would not use them in my builds, or sell/represent them, if I had any doubts!