Dynaco VTA ST-70 Negative Feedback & SS Rectifier Experimentation - Improving THD+N Performance

[@NovaluxStereophonic]

Follow up experimentation w/ 50mA bias can be viewed HERE: kzbin.info/www/bejne/bZ6nnnl3a953brs

[@insurrectionindustries1706]

I’ve been waiting for this one!

[@gregraynard781]

You were more precise about when it reached 1% than your averaging of thd+n so you missed that the solid state rectifier was actually less distortion in every case! Another great video!

[@NovaluxStereophonic]

I will have to go back and look at that. Thank you for that observation. So far the comments are indicating that I will need a follow up to this video.

[@Tinybench]

Fantastic video! I loved it! You are the best!!! Thank you very much! I would have liked an audio test along with the different combinations. Just your subjective opinion about the sound. Maybe a couple of high-quality small monitors in your bench or maybe you can rig something to be able to test them with headphones. This additional testing would be subjective, and not realistic, but interesting in my opinion. 👏👏👏🙂👍💥💥💥

[@K5HJ]

When you were unable to reach the proper bias current you had distortion dominated by the second harmonics due to crossover distortion. Changing the bias adjustment range improved that considerably.

[@yellow_noise3]

From my knowledge, deeper feedback (3.5k) means less distortion, and narrower bandwidth. Less feddback=wider bandwidth. This is probably reason of compromise with 7.5k resistor in feedback. This also heavily depends on output transformers which are (their quality) most important element of tube amplifier. This also leads to the conclusion, that without listen test, these evaluations are worthless. Simply do not give valuable information about the sound. Less distortion with less bandwidth can sound worse than bit more distortion with better bandwidth. Especially here, in tube amp…. Great video and lot of good information to think about…..!

[@welderfixer]

Thanks for this video. I sure would like to own/build a good tube amp someday. The research you have done here is a bit over my head, but darn interesting and educational. The quality of work you do is top shelf. As I have said before, I am envious of your fantastic bench. Please keep you videos coming. I wish I had the funds to send my 3 failing NAD amps to you for repair/updates. Sir, If you have any suggestions on the repair of my pair NAD 214 and a CI9060 amps I'd be grateful. They all make a crackling sound at certain frequencies after warming up. I do some board repairs in welding machines and I hope I can learn from repairing my cherished amps. Thanks a million and please have a great day, Kevin

[@gregf9160]

Fascinating video! Though 13db of NF, especially in a tube amp, seems a _tad_ excessive? I'm surprised, but results are results. _Love_ the moustache, btw, you have the type of face that can carry it off and I'm quite envious 🤗

[@dloorkour1256]

Isn't the original around 20db of feedback? I think I'm remembering that right. I know the VTA is on the low side.It's kind of fashionable these days to have it low and get that "tubey" sound.

[@SuperMcgenius]

What I’ve learned from this video, is that I have a lot to learn. I think my brain is overheating. What biased setting should I be looking for my Brain

[@Tuberhead1000]

Did note any differences in 2nd vs 3rd order harmonic relative levels between the SS and tube rectifier? It seemed at one point, the 2nd increase a bit relative to the 3rd even as the 3rd order decreased overall.

[@peterhakanson6797]

In my VYA-70 2k as feedback resistor was optimal. bias 50mA was maximal for the GZ34 but if power allowed bias od 60mA was better

[@dloorkour1256]

Did you try the standard 50 mA bias? That's the original design. I don't see any issue running that with JJ EL34s or KT77s, but I do use a bucking transformer so the input voltage is around 115 to 117 V AC. It would be interesting to see measurements comparing distortion with 50mA vs. 40mA. I think the 40mA was only based on reducing the chance of runaway bias, not testing. Thank you for sharing your test results!

[@NovaluxStereophonic]

@@dloorkour1256 I just did a bunch of digging on this, not sure why Bob Latino is advocating 40mA. I just tested at 50mA and the distortion results are drastically improved. Makes it to rated power 35W both channels driven with only .5% THD+N I am going to try to get a follow up video going shortly. Thank you for the comment. I had tunnel vision on this one with the 40mA setting form the manual which runs the output section in AB, original design appears to be biased into class A operation.

[@dloorkour1256]

@@NovaluxStereophonic Nice to know, thank you! Looking forward to the follow-up.

[@dloorkour1256]

@@NovaluxStereophonic I'm pretty sure it's still AB, but both tubes are on at a bigger range of current.

[@defec1]

​@@NovaluxStereophonic Originally this board was marketed and maybe designed by Roy Mottram and the .40 was claimed to greatly extend tube life. I believe David Gillespie pointed out in his fantastic Dynaco ST-70 Baseline Testing paper that .50 was a conservative rating "With a matched tube set, this is 50 ma per tube, with about 45 ma of it conducted by the plate. With 410 volts across the plate and cathode then, this equates to 18.45 watts, which as a percentage of the very conservative Design Center 25 watt plate dissipation rating for these tubes, equates to operating them at only 73.8% of their rated dissipation level. Even when operating the amplifier from a 122 volt line and still l using the recommended Biaset level, the dissipation only climbs to 19.4 watts, for 77.6% of rated capability. These are both very conservative levels to operate these tubes at -- particularly when you consider it is being judged against the Design Center Rating value" Anyway, maybe compare .45 vs .50 bias with the VTA board.

[@NovaluxStereophonic]

Follow up posted. Thank you for the idea. It made a huge difference: kzbin.info/www/bejne/bZ6nnnl3a953brs

[@russbutton9347]

Instead of just pulling an article from the VTA forum, have you tried directly contacting Bob Latino with what youʻve found? Going right to the source might be a good move.

[@dloorkour1256]

He's retired from selling these anyway. I don't think it was his design either.

[@cathiestubes2820]

To my ears, running more fb, with lower distortion yields a "harder" sound, and the opposite effect with less fb. This can be important with the type of speakers you are running. Horn speakers make this quality very evident and less sensitive speakers may tolerate more fb. Lower distortion characteristic vs less fb can also be a factor with the type of music you like. So many variables, it can make you chase your tail.

[@dloorkour1256]

I've played with several different drivers. I think the low feedback circuits can sound nice initially but eventually get tiresome. Above a certain amount of feedback, the circuit had better be properly tuned or you will get ringing or oscillation, but when they are, they can sound great.

[@WA2JPU]

Hi Ben , you are measuring bias current by reading voltage across a 10 ohm resistor. Can this technique be used in other amps simply by permanently adding a series 10 ohm resistor? Thanks ..

[@NovaluxStereophonic]

Yes, this is a common modification that is used a lot on tube receivers when an individual bias adjustment circuit is added. By choosing a low wattage resistor it can also serve as overcurrent protection (like a fuse for the cathode).

[@WA2JPU]

Thank You Ben ! I've been replacing fuses with an in-line amp meter to set biases on my own gear , but never again 😅 ! What a great idea. I wish that I had learned this trick years ago. Thanks for your great work and for sharing your knowledge.

[@visualnetwork3773]

You need larger filter capacitors for the SSD's.

[@NovaluxStereophonic]

@@visualnetwork3773 sorry, I don’t quite understand what you are suggesting.

[@phillefever1934]

Even with the high quality 5AR4’s of yesteryear, the Stereo 70 had an inadequate power supply. With modern tube rectifiers the performance will always suffer. In 2024 there is no reason to have the inherent inferiority of a tube rectifier! I’ve been running my Stereo 70’s SS since the 80’s. Only time I ever use a tube rectifier in a tube amplifier is when I intentionally want a soft supply, I.e. a guitar amp where I want the rail sag. A HiFi amp is always better with a stiffer supply.