VTA board NFB tap question for Bob and Roy......

Bob or Roy.....I have installed my VTA board again, this is the 4th attempt, into a different amp. The amp uses an upgraded quad cap I got from Kevin, it has the old A-470's, it has an upgraded power supply. The VTA board was purchased from Roy already populated, it was the low gain version, I am running 12BH7A's from EH, I have made no changes to the VTA board.

My situation is that I cannot run the VTA board on a 16ohm tap. Last amp I ran this in I was forced to use the 8ohm tap. It ran great for a few months then the power supply burned out. That was also an upgraded power supply I got from Kevin and new A-470's also from Kevin. When the power supply went down nothing else was damaged, only the power supply burned out.

Right now this amp is running fine and sounds great, just like the other one did before the burn out of the PA-060. Is there anything wrong with running an 8ohm tap and could this have been what burned out the other power supply in my last amp I ran it in?

Thanks Guys for any help you could lend.

wolverine

I do not have a VTA board, so I don't have any SPECIFIC experience. However I have tuned many a tube amplifier's negative feedback loop and would suggest you look at what is happening throughout the amplifier with an oscilloscope. Taking the negative feedback from the 16 ohm tap will result in twice the feedback signal as from the 8 ohm tap and any tendency to oscillate will become worse, as a result. This usually happens at extreme high frequencies where phase shift results when the signal approaches the high frequency limit of the amplifier. It only takes one stage to do this; when a phase shift at 180 degrees is combined with negative feedback, you have a "perfect oscillator" that will pump out that high frequency at maximum power until something breaks.

GP49 wrote:I do not have a VTA board, so I don't have any SPECIFIC experience. However I have tuned many a tube amplifier's negative feedback loop and would suggest you look at what is happening throughout the amplifier with an oscilloscope. Taking the negative feedback from the 16 ohm tap will result in twice the feedback signal as from the 8 ohm tap and any tendency to oscillate will become worse, as a result. This usually happens at extreme high frequencies where phase shift results when the signal approaches the high frequency limit of the amplifier. It only takes one stage to do this; when a phase shift at 180 degrees is combined with negative feedback, you have a "perfect oscillator" that will pump out that high frequency at maximum power until something breaks.

That sounds like exactly what I am experiencing.....I do not have a scope. I have no problem with stock circuit boards using 7199's or the Triode board using the EF86's in that amp. The problem only occurs when I use the VTA board. It has done this since I got the board and I can only get it to stop when I use the 8 ohm tap. I have not made any changes to the board at all and I know darn well that Roy has sold tons of the these low gain boards with great success so I simply cannot understand why I seem to have this problem.

Thanks for the help....I appreciate any advice you guys can give me.

wolverine

Hi Wolverine,
you don't mention whether you are using an original transformer or the new
higher current replacement transformers. You also don't mention what you mean by using an "upgraded power supply"

Hi Wolverine,
you don't mention whether you are using an original transformer or the new
higher current replacement transformers. You also don't mention what you mean by using an "upgraded power supply"
My first thought would be that because you are using 12BH7s, those are drawing
twice the current of 12AU7s, and 3X as much current as the original 7199s.
Also you don't mention how much bias you have on the output tubes
(how much current they are using).
Having sold THOUSANDS of these PCBs over the past 20 years, I've yet to see any problems
with the design, but then I've only been offering the lower-gain version
with 12AU7/12BH7 option for around 3-4 years now.
For NFB changes using the 8 ohm tap . . .

(CORRECTION to previous post)

Divide the resistor by the square root of 16/8
Multiply the cap by the square root of 16/8
divide the 7.5K resistor by 1.4 and multiply the 220pf cap by 1.4 to get the correct NFB
with the 8 ohm tap this would give values of approximately 5.3K and 308pf,
using 5.1K and 300pf should work fine

tubes4hifi wrote:Hi Wolverine,
you don't mention whether you are using an original transformer or the new
higher current replacement transformers. You also don't mention what you mean by using an "upgraded power supply"

I am using new power transformers from Heyboer, the first one was from Kevin that got burned out.

My first thought would be that because you are using 12BH7s, those are drawing
twice the current of 12AU7s, and 3X as much current as the original 7199s.
Also you don't mention how much bias you have on the output tubes
(how much current they are using).
I am biasing the VTA board at .35 at each tube, as soon as I try to get over that I get weird oscillations

Having sold THOUSANDS of these PCBs over the past 20 years, I've yet to see any problems
with the design, but then I've only been offering the lower-gain version
with 12AU7/12BH7 option for around 3-4 years now.
For NFB changes using the 8 ohm tap . . .

(CORRECTION to previous post)

Divide the resistor by the square root of 16/8
Which resistor are you referring to here?

Multiply the cap by the square root of 16/8
Huh? LOL....I am really sorry for my ignorance here on which caps and resistors you are talking about, I'll learn more as I continue to follow this forum I'm sure

divide the 7.5K resistor by 1.4 and multiply the 220pf cap by 1.4 to get the correct NFB
with the 8 ohm tap this would give values of approximately 5.3K and 308pf,
using 5.1K and 300pf should work fine

I really appreciate the help here, sorry for taking up so much of your time.
THANKS
wolverine

Also, the specs on the new replacement power supply were built to match the original except that the transformer is upgraded in that the stack laminations are much larger etc.

wolverine

From what I can make out here regarding the values you gave me Roy, comparing them to the information Bob sent me on the parts layouts,

1. I could try to change the R7 and R8 over to 5.1K resistor
2. Also change the caps at C11 and C12 over to 300pf
3. And by doing this I can safely run an 8ohm NFB tap instead of the 16ohm tap

I would query as to why to do this instead of solving the problem of why I cannot use a 16ohm tap as Roy designed the board. I've physically tried to install this board in several amps with the same results each time so I think my first step is to check all the values against the information Bob sent me. It is possible I suppose that one of the values is incorrect on the board somewhere, or perhaps there is a component that is not up to snuff.

Fun stuff nevertheless, Thanks to Roy and Bob for helping me try to decipher this problem, also to GP49 who is obviously very knowledgeable. Thanks for your time GP49, I certainly appreciate all the help you can offer me.

wolverine

I asked another engineering friend who is also an ST70 buff and repeat repeat customer, and he says he had experinenced a similar problem with a DIYtube driver board and SDS power supply combination. There may be some odd resonance/oscillation with the SDS style power supply when used in combo with other aftermarked mods other than the original old Dyna PC3.
He also suggested trying the 8 ohm feedback tap with changed NFB components.
That's about all I can recommend for now as I personally haven't seen or heard of a problem like this before.

tubes4hifi wrote:I asked another engineering friend who is also an ST70 buff and repeat repeat customer, and he says he had experinenced a similar problem with a DIYtube driver board and SDS power supply combination. There may be some odd resonance/oscillation with the SDS style power supply when used in combo with other aftermarked mods other than the original old Dyna PC3.
He also suggested trying the 8 ohm feedback tap with changed NFB components.
That's about all I can recommend for now as I personally haven't seen or heard of a problem like this before.

I'm using a quad cap I bought from Kevin, not an SDS board in this amp, I do however use an SDS board in two other amps, but I haven't tried it with the VTA board yet.

wolverine

Bob Latino wrote:Wolverine,

This is maybe a long shot BUT > have you tried the feedback line on the 16 ohm tap with a different set of speakers? I am just thinking that possibly somehow the load presented by your speakers to the amp is causing the amp to osscillate ?

You could try the amp with some other speakers on the 16 ohm tap and see if you still get the osscillation/resonance issue ??

Bob

I can run the board using the 8 ohm tap and hooking up to two very small Sony surround speakers I have that are for a television. The amp runs stable and sounds real nice, but those speakers are little single cone speakers and that just won't do LOL. I have been in contact with Roy, and he is about out of ideas too. He thinks there is some kind of interaction going on with the Klipsch speakers I am trying to use, but he can't really pin down the problem yet. I'm going to order some parts as Roy suggested and figure this puzzle out. I love a challenge like this and I am learning a ton in the process. Great fun!

Thanks Bob, you and Roy are the greatest! I love this forum, so many things for me to absorb.

wolverine

So here is what I have learned so far on the VTA board....

1. My low gain board purchased assembled from Roy did not work for me, trying it 4 different times in different amps.
2. I used a new power supply, a new quad cap, original A-470's(in two attempts), new A-470's twice (in two attempts)
3. 16ohm tap yielded oscillations as soon as bias began to rise every time
4. 8ohm tap worked for an extended period of time pushing my KG-4's, but a new power supply from Kevin burned out
5. 8ohm tap will still work and is stable pushing only two small surround speakers, single cones
6. 8ohm tap will not work on my Klipsch Heresey's at all
7. Suggestions made were to try varying resistor and caps for the NFB to tune it in
8. In each case an original circuit driver board works perfectly in every amp I have tried the VTA board in
9. A triode EF86 based board also works perfectly in every amp I have tried the VTA board in
10. The VTA board was not as spec'd according to the parts list Bob supplied in respect to caps on the board, which Roy acknowledges he sometimes makes some "changes" on assembled boards
11. Bob has been very helpful in trying to help me solve my problem, and if I buy a kit it will be from Bob or Kevin, not a question in my mind
12. Roy also has been very helpful, however did jump to some conclusions that the amps, not the board were the culprit, which one would expect of course
13. Roy has offered me another board, however with a fee applied to it, and only if my board can be resold, again understandable given the time it has taken me to get to this point
14. Roy would accept my amp to go over and charge me labor times at 40 to 50 dollars an hour depending on the problem, everyone needs to be paid for their time, it's only right

I have changed over the VTA board again to the Triode board, which is a great sounding board to me. The amp is running perfectly, all voltages are within specs, no problems with hum etc.

So my conclusion is the VTA board needs to be looked at for a problem within the board itself. I have to contend that their is some design flaw or component on the board that is causing me the issue with the type of speakers I use, e.g. Klipsh KG-4's or Klipsch Heresey's. Roy and Bob have a lot of these boards in the field performing well with many happy customers. Given the amount of time I have had the board, Roy's position is understandable in reference to switching the board for a fee or going over the amp himself and charging labor time.

I never tried to return the power supply to Kevin since it was a good 6 months before I got to the project and installed the board into an amp before the power supply went out. Kevin did offer to look into the problem himself however (I consider his service second to none), and I told him not to worry about it, I simply bought a new power supply from Heyboer that was built to the exact specs of the power supply Kevin sold me. Heyboer is right in the same town I live in and I thought any more problems would be easier handled locally rather then asking Kevin to deal with it. Thankfully all 4 of the power transformers I have purchased from Heyboer are performing perfectly and run cool all the time regardless of the tubes I have used or the boards I have tried.

So it was a great learning experience for me and I'm the type of person who doesn't get uptight about this sort of thing. If you're going to get into tube amps and change original parts to different circuitry you're going to have to learn a lot. I would use the VTA board again, but ONLY if I bought it as part of a kit from Bob. I wouldn't try to use the board as an add-on for an existing amp.

When the amp did work for a couple months and I was using with my KG-4's on the 8ohm tap it sounded very good. It was a different sound however then an original 7199 circuit board and also different from the EF86 Triode board. The sound was very clear and clean, almost to the point that it lost some of the "tube magic" if that makes sense LOL. At this point I prefer the Triode board myself, in my configuration of speakers.

I hope that Roy can use some of the information I gave him to help him in the future refine the low gain board for a wider application of speakers. Wasn't it Edison who said something like "I never discovered how to make a light bulb, I did discover several hundred ways you cannot make a light bulb however" LOL. Something like that anyway. Trial and error is the only real way that anything can be developed and it will always take some time and some failures to do so. We learn from each failure we make and it adds a bit more knowledge to the task, allowing for eventual total success. I am going to order some parts and experiment with the board as Roy suggested, this will further my understanding of the circuits involved and the interactions taking place. That can only be a good thing.

This has been a lot of fun guys, thanks to everyone.

wolverine

I don't know why I didn't think of this, but what I really want is another set of Gold Lion KT66's. So I put up my VTA board on EBAY and will use that money to get another set of the Gold Lion's. I really do love the sound of those tubes, best I have personally heard.

wolverine

Hi Wolverine - only just joined this forum but sorry to see you've had problems with the VTA board. As you rightfully say getting into tubes isn't an 'instant fix' or a plug and play route to instant sonic nirvana. One does encounter problems, and I applaud the way you have stayed level headed about the whole matter. I have to say though - and not wishing to step on anybodies toes - if I'd paid for a pre-assembled board which clearly didn't work correctly I would be quite annoyed at having to pay any additional charges at all. If it was a kit board I could understand, but pre-assembled must surely include some kind of guarantee?

Regarding your last post, why not get a quad of KT77s instead? I had a quad of JJ KT77s and - before one of them started sparking (!) - they sounded fabulous. Very similar to my Shuguang KT66Rs, but slightly more 'faster' sounding if that makes any sense? Lol! From what I have read of the new Gold Lion KT77s they sound just as good, but with reliability too (unlike my JJs). Might be worth a try - then again if you have found the sound you want might be worth buying an extra set as one never knows what tube availability will be like in the long run.

I loved the sound of the KT66Rs (R for Retro in this case from www.tube-shop.com) running a built from scratch ST70 (only original parts were the pair of output transformers) using original driver board design. I cannot understand those that say the bass sounds flabby and its too mellow/dated etc etc. Sounds fine built with a modern board and quality new components.

My ST70 is currently awaiting a slight rebuild (after too much tweaking, I want to tidy it up internally again). About 70% decided to go back to the modern copy of the original driver board (which I built using Kevin's Dynakits board, and using hand matched 2 watt Kiwames and also 3 watt metal oxides for the higher voltage items, and hand matched Arcotronics capacitors). The other 30% is still wondering about the VTA board though - from memory I recall I thought the VTA was impressive in a hi-fi sense, but didn't involve me with the music like the original board did. Hard to remember now it's been a few years since the ST70 was up and running. Then again I never got around to upgrading from the supplied orange drop caps, whereas my stock driver board used higher quality parts... Maybe I'll flip a coin and decide hahaha!

Anyways thank you for a great thread which has got the juices going for getting my ST70 back into business!

Best,

- John

Thanks for the thoughts John. I have to put myself in Roy's position in this case, it's been a long while since I purchased the board from him. It took me a long time to get to the project to begin with and I never anticipated any sort of problems like I encountered since I bought the board already populated. I did initially email tubes4hifi when I first had the problem, instructions were given to me to change the board over to a standard gain board. Simple to do really, replace a couple resistors and there you are.

I did find it odd that with a proven design and the parts and specifications of the board in place, that I would receive a board that had different parts in it. Roy said that sometimes when he populates boards he does a bit of tweaking and upgrading to the board in an experimental fashion. I was going to experiment with the board but I decided that my time has already been over allocated to the board, and after 4 attempts my time and money would be better spent in acquiring another set of the KT66's. I really do enjoy the sound of those Gold Lion tubes, great advice from Bob and others here to give them a shot, they are the ticket for me. I wish Roy only the very best of success going forward, but I won't be able to recommend the board to anyone given the problem I encountered. If purchased as part of a kit, it could very well be the best board around, but in my case simply failed to work as an add-on.

I agree with your take on the VTA also, the sound is very detailed, and there is no background noise at all. The sound did seem to take something away from the tube sound that I like so much. Almost as if the board was too detailed lol. In my case I believe Roy was correct in his thoughts on some sort of interaction taking place with the board and the speakers I was using, and I believe Roy can use some of the feedback he got from me in helping to further his board for a broader range of application in reference to speaker loads and types. Good things normally come out of set backs and hard work, and Roy has certainly worked hard at perfecting his board and his craft.

Best of luck on your ST-70. I love these amps. So simple but yet sound so great.

wolverine

wolverine wrote:I did find it odd that with a proven design and the parts and specifications of the board in place, that I would receive a board that had different parts in it. Roy said that sometimes when he populates boards he does a bit of tweaking and upgrading to the board in an experimental fashion.

Wolverive-
I'm looking at your board on ebay. Do you know what changes Roy made from the "stock" configuration?
Thanks,
Dave

I only know that he used 8 .22uf caps on the board. The parts list spec'd out 4 caps as .10uf and 4 caps as .22uf. I don't have a clue whether or not it makes a difference, I highly doubt it matters in the least really. My source of indigestion with the board is not being able to use it with a 16ohm tap and push my Klipsch speakers. I love the Klipsch speakers since they use horn loaded mids and highs, and at my age I need all the help I can get to hear the highs and mids LOL. My triode board has no problem along this line but it also doesn't allow me to bias the tubes individually as the VTA board does. I really like the idea of biasing each tube individually, makes rolling tubes a whole lot easier.

Hi Wolverine.

Wondering if this would help you?

http://www.dynakitparts.com/store/product.aspx?id=295

Looks like a very elegant/neat solution to adding two extra bias pots. I guess you'd need to ask them if it would work with your driver board, but I can't see there being a problem.

Cheers,

- John

here's a reply I got from the buyer of the old board on ebay . . .

"I was going to buy from your website, but then I found a VTA 12AU7 lo gain version for sale on ebay for $100 BIN with $8.00 shipping. It was actually built by you, according to the seller. I couldn't resist a unit built by the pro for such a fair price, so I took the plunge!

Anyway, to the real point here! You really did it! You nailed it! This board reminds me of the design for a VTL model 50/50 I had back in the early 1990's, only better!

I have owned Macintosh, Fisher, VTL, and others in my day. I have experienced Mark Levinson, Goldmund, and others. I know good audio! This VTA board is truly an audiophile experience!

Just wanted to let you know, I LOVE IT!"

~Raj

Excellent!.....Roy nailed it when he said that my particular setup was the problem with some sort of interaction with my main speakers. The Klipsch speakers simply didn't work. The sony surround speakers would work however, so it certainly was a case of my setup. I figured it had to be as much since Roy had an awful lot of these out in the field. Better for someone else to get to use the board rather then changing Roy's design by changing out parts on the board, I'm glad I didn't "experiment" with it LOL.
wolverine

I know this is an older post but I'm not sure I or possibly the original poster understand the wiring an use.

The feedback on the VTA low gain board with the spec'd resistors and caps should be hooked to the 8ohm tap.
(feedback on 8ohm tap not 16ohm, leave the feedback parts values as they are)

Your speakers can hook to any tap you want, the feedback tap makes no difference where you hook the speakers.
(put speakers on 4, 8, or 16ohm tap, leave feedback on the 8ohm tap)

And the VTA board should have nothing to do with a supply burning out, just unlucky circumstances he had there.

-

When I was deciding which version of the VTA board to use, eventually it became clear to me that "low gain" was the only way for me to go, with 103dB efficient Klipsch speakers. Kegger, I've read on AudioKarma about your changes to accomplish a low gain VTA board.
When I received my low gain board from Roy, I checked the values listed against ones you posted on AudioKarma. My problem is I can't identify which components specifically you've changed, because you talk a little too technically for me...
In any case, the values you specify and Roy's are not the same. In the end I decided these were the parts I had, and you guys must be going about the same goal in a little different way.
I can identify the feedback resistor, however, and it's a 7.5k resistor connected to the 16ohm tap on the schematic.
If I were to change this to a 12k on the 8ohm tap, what would I expect to happen?

The values I use now and the ones Roy uses are pretty close, I just tweak them
from time to time as the board gets more use, but the feedback was always meant
to come off the 8ohm tap and not the 16ohm tap even with the 7.5k resistor.
(the documentation got changed at one time but possibly not the schematic)..

The goal of the low gain was obviously to reduce gain, but also found less overall
feedback was "needed" as well an the 8ohm tap was used to do this and to make
the overall design stable under any load. (sounded better with bit less feedback to)

Anything from 7.5k to 15K is what I would suggest trying from the 8ohm tap, as u
increase that resistor you lower that feedback, gain will come back up a little and
the amp tends to have little more "life" if you will, going to large the amp will get a
little fuzzy or unfocused sounding, speakers play a roll to.

But yes I do suggest the 8ohm tap is the "correct" one to use for the low gain feedback..
(high gain board I use either 16ohm tap an 10k resistor or the 8ohm tap an 4.7k resistor)

Cheers,
Keg

If you had a scope that feedback "compensation cap" could be adjusted as well
but I've tested it with 7.5k to 15k on the 8ohm tap and it's still a fine value there.