Bandersnatch said:
Well...I don't see why you'd jump to the conclusion that just because you need more it has to be global. RCA made a *VERY* nice set of 6V6 amps, and they used no global FB. They did run a lot of it back a stage to the driver cathodes from the final's anodes....So far that PPP 6V6 amp was the best sounding 'off the rack' amp I have ever heard.
The global loop is *NOT* required in any case. IMO, neither is a SS-like low output Z.
cheers,
Douglas
Can you post the schematic? I have some NOS RCA 6V6s measuring very strongly on the tube tester, that I want to put in good use given the opportunity.
hey-Hey!!!,
It is alphabetically arranged. Look to RCA.
http://www.freeinfosociety.com/electronics/schempage.php?cat=1
cheers,
Douglas
It is alphabetically arranged. Look to RCA.
http://www.freeinfosociety.com/electronics/schempage.php?cat=1
cheers,
Douglas
ray_moth said:
A very belated OOPS! from my side
When I agreed (my post #128) with Brandersnatch's remark that NFB was not required (as in good) for low damping factor; I fixed my attention on that, overlooking that pentodes were mentioned. Here I must agree with Ray.
At this stage: This thread is titled "Why do some people dislike UL?" It would appear that there is also a contract out on global feedback. Per se? WHY?
Is it because some designs fail to observe basics and consequently find themselves in trouble? It must be remembered that the highest degree of distortion reduction per lowest degree of total NFB takes place with global NFB (and also when all stages have equal distortion). The latter can be motification for local NFB round the power stage .... but then without including the output transformer in any loop as in the RCA designs? Sorry folks, that does not add up to best performance.
Yet again I hurry to add that I respect folks' hearing experiences. But I think it must by now be clear that that is not a general indication of exemplary performance. To tread very softly, the reason why certain amplifiers with quite audible distortion is liked, distinctly indicate that certain distortion is improving the programme, so .... This was dealt with elsewhere, so not to open it again here.
I am also still wondering, why pentodes instead of UL? May sound like a repeating CD - and perhaps somewhere in the past 140 comments here it did come up; I did not reread from the beginning. Why are we having this question (humour me and just remind me again, please
)?hey-Hey!!!,
I don't see the need to include the whole output TX in the FB loop. The RCA schematic drops the output Z of the finals. There seems to be no indictment for other open loop amps using finals with low output Z.
As to loop FB use, if I heard an amp that used it that I liked, I'd use it. While it may look good on paper, it doesn't seem to translate into Iron and valves.
cheers,
Douglas
I don't see the need to include the whole output TX in the FB loop. The RCA schematic drops the output Z of the finals. There seems to be no indictment for other open loop amps using finals with low output Z.
As to loop FB use, if I heard an amp that used it that I liked, I'd use it. While it may look good on paper, it doesn't seem to translate into Iron and valves.
cheers,
Douglas
I would like to ask the opposite: why UL instead of pentodes? Yes, UL is easier and you get all this power for "free." The price was madmen like Saul Marantz and Frank McIntosh. The only reason the watt race has ebbed out (I'm talking ss now) is that there's only so much power you can draw from a 115 and 230 V line. Makes me sick just thinking about it. I try not to.
OK, an honest and fair enough reply, Douglas.
Low impedance driving, by all means. And with respect to that I have no problem with the RCA or similar topologies. But that still assumes that the output transformer cannot contribute to distortion, and that is not correct. It does both at very low outputs (hysteresis) and at maximum output (B-H graph bending). It is difficult (and expensive!) to get an output transformer with below 1% contribution to distortion, as far as my experience goes.
As for hearing that it does not sound good .... I can also take a different stance to what I may have appeared to have done in my previous post. We drive loudspeakers with amplifiers, while we mostly test with 8 ohm load resistors. Often not even including half or double that and open circuit, let alone reactive loads. What would many design measurements look like then? For serious work one should at least have a dummy loudspeaker equivalent model. Mine now and then cut me down to size!
I can also say, if the design cannot cope, adverse effects reflected back to the input (per gNFB), can give a decent horror show. I have the disadvantage that I have not tested that many commercial amps with equivalent loudspeaker loads, but those were mostly quite acceptable. Where trouble showed, it was mostly in the supersonic area - spurious oscillation etc.
So .... have we conflicting experiences - and is it only gNFB?? A psychic needed, please!
Regards.
Low impedance driving, by all means. And with respect to that I have no problem with the RCA or similar topologies. But that still assumes that the output transformer cannot contribute to distortion, and that is not correct. It does both at very low outputs (hysteresis) and at maximum output (B-H graph bending). It is difficult (and expensive!) to get an output transformer with below 1% contribution to distortion, as far as my experience goes.
As for hearing that it does not sound good .... I can also take a different stance to what I may have appeared to have done in my previous post. We drive loudspeakers with amplifiers, while we mostly test with 8 ohm load resistors. Often not even including half or double that and open circuit, let alone reactive loads. What would many design measurements look like then? For serious work one should at least have a dummy loudspeaker equivalent model. Mine now and then cut me down to size!
I can also say, if the design cannot cope, adverse effects reflected back to the input (per gNFB), can give a decent horror show. I have the disadvantage that I have not tested that many commercial amps with equivalent loudspeaker loads, but those were mostly quite acceptable. Where trouble showed, it was mostly in the supersonic area - spurious oscillation etc.
So .... have we conflicting experiences - and is it only gNFB?? A psychic needed, please!
Regards.
I think you nailed it- poor feedback design is sadly more common than good feedback design, and thus the reputation gets set into stone. The points you made (change in stability and distortion with load) are spot-on, and I'd also add overload recovery, which is MUCH harder to do properly when a feedback loop is involved.
Why pentode, you ask? I think I make a pretty good case for it in the Red Light District article, and the results are the pudding's proof. It's not the only way to go, but it's a valid design option with some strong advantages to counterbalance its disadvantages.
Why pentode, you ask? I think I make a pretty good case for it in the Red Light District article, and the results are the pudding's proof. It's not the only way to go, but it's a valid design option with some strong advantages to counterbalance its disadvantages.
phn said:
Another fair question, Phn (I was typing my previous post when yours came through and did not see it).
Let us rehash that: Borrom line: UL gives most of the advantages of both pentodes and triodes. Triode "good" characteristics: Low rp (internal resistance), relatively low distortion, just about no high order harmonic distortion. UL: Almost as low rp and distortion. Low rp also translates into relative little influence from varying and reactive loads (like our beloved loudspeakers). Disadvantage: The "inconvenience" of 2 extra taps on the output transformer. (My local winder does not even charge for that. Perhaps he likes me.)
Pentodes usually give at least twice the power output - but UL can do just about the same. Disadvantage: Pentodes have high rp, about 5 times the load impedance. They can give quite low distortion, but into a very narrowly defined load only. With a loudspeaker load they give about 5 times the distortion, and that often includes quite some high order harmonics - not so UL/triodes. (Although some members measured enough high order products from UL to bother them - wish we had some data on that.)
That be my background.
Regards.
I like Ultralinear BUT it is undoubtedly true that it suits some tubes better than others. One tube it absolutely does suit is the EL84.
EL84 Operational Data
Pentode Mode (Va 300V) 17W @ 3.18% THD
43% U/L (Va 300V) 11 watt @ 0.7% THD
Triode (Va 300V) 5.2W @ 2% THD
The Intermodulation Distortion figures follow the THD trends shown above.
That is, the EL84 is "better" in Ultralinear than in either triode or pentode modes.
When aiming for a zero or low global feedback design then that 0.7% THD is as valuable as the lowered Zout (compared to pentode mode) and 6 W is a small price to pay.
For comparison :
EL34 Operational Data (cathode biased - Mullard Data))
Pentode Mode 40W @ 5.1% THD
43% UL 34W @ 2.5% THD
Triode Mode 19W @ 1.8% THD
In this case it suggests Triode Mode may be the way to go for EL34.
I have also had good sucess with a number of 6V6 amp designs using 43% Ultralinear, but in every case I had to also employ balanced shunt feedback from the anodes (the Baby Huey scheme) plus about 3 or 4 dB of global feedback to arrive at "gloriousness". Well relative "gloriousness" - it did'nt match the zero global feedback, Ultralinear plus balanced shunt feedback EL84.
Cheers,
Ian
EL84 Operational Data
Pentode Mode (Va 300V) 17W @ 3.18% THD
43% U/L (Va 300V) 11 watt @ 0.7% THD
Triode (Va 300V) 5.2W @ 2% THD
The Intermodulation Distortion figures follow the THD trends shown above.
That is, the EL84 is "better" in Ultralinear than in either triode or pentode modes.
When aiming for a zero or low global feedback design then that 0.7% THD is as valuable as the lowered Zout (compared to pentode mode) and 6 W is a small price to pay.
For comparison :
EL34 Operational Data (cathode biased - Mullard Data))
Pentode Mode 40W @ 5.1% THD
43% UL 34W @ 2.5% THD
Triode Mode 19W @ 1.8% THD
In this case it suggests Triode Mode may be the way to go for EL34.
I have also had good sucess with a number of 6V6 amp designs using 43% Ultralinear, but in every case I had to also employ balanced shunt feedback from the anodes (the Baby Huey scheme) plus about 3 or 4 dB of global feedback to arrive at "gloriousness". Well relative "gloriousness" - it did'nt match the zero global feedback, Ultralinear plus balanced shunt feedback EL84.
Cheers,
Ian
All this blather about ultralinear makes me want to do another one, push-pull this time. I have a pair of the budget Edcor 8k, 10W transformers on hand, and some 6005s and 6CW5s I'm considering for output duty. I want to limit my options to stuff I already have. Of the two outputs mentioned here,which ones would you use?
wrenchone
I would use the 6CW5 rather than the 6005.
Higher gm, lower rp, higher Pa, larger Rg1 max value. This all helps in making a simpler circuit. The 6CW5 is the same as an EL86 and I seem to recall some EL86 circuits being posted here before - worth a search.
Also for your info and some ideas - look at EL84 version of the "Baby Huey" I posted on this forum. The high gm of the 6CW5 would make it ideal in that application. With that circuit and with the 6CW5 in Ultralinear mode it is highly likely that you will get away with zero global feedback. The high Rg1 max value also means that you could easily convert that circuit for fixed biased output tubes.
Cheers,
Ian
I would use the 6CW5 rather than the 6005.
Higher gm, lower rp, higher Pa, larger Rg1 max value. This all helps in making a simpler circuit. The 6CW5 is the same as an EL86 and I seem to recall some EL86 circuits being posted here before - worth a search.
Also for your info and some ideas - look at EL84 version of the "Baby Huey" I posted on this forum. The high gm of the 6CW5 would make it ideal in that application. With that circuit and with the 6CW5 in Ultralinear mode it is highly likely that you will get away with zero global feedback. The high Rg1 max value also means that you could easily convert that circuit for fixed biased output tubes.
Cheers,
Ian
gingertube said:
17/11=1.5454...
40/34=1.196
I.e. there are more snow on Tahoe lake than average temperature of patients of our local clinic...
No doubt THD on 1.6 less of power will be lower than on 1.1 less of power!
What THD in pentode mode of EL34 on 5.2W of output power?
What THD in pentode mode of EL84 on 19W of output power?
What load resistances were used in all cases? Why? What criteria they were optimized for?
Wavebourn,
Fair comment. The data was meant to be indicative to show that Ultralinear is ideal for the EL84 but might be less so for some other tubes.
Although the Langford-Smith paper which was referenced in the "Adjustable Distributed Load" thread suggests that the U/L "improvements" can not be accounted for by pure feedback theory alone.
Cheers,
Ian
Fair comment. The data was meant to be indicative to show that Ultralinear is ideal for the EL84 but might be less so for some other tubes.
Although the Langford-Smith paper which was referenced in the "Adjustable Distributed Load" thread suggests that the U/L "improvements" can not be accounted for by pure feedback theory alone.
Cheers,
Ian
Johan Potgieter said:
As an audio hack I like triodes, not pentodes. Pentodes are harder to deal with, if so only because they have one leg more to deal with. I modelled an amp using pentode output in spice and had the problems you described. Spice may not translate 100 pct to the real world, but close enough. Triodes are very load insensitive. 16Ohms, 8Ohms, no load, the triode still looked good. The pentode was all over the place. Screw this.
I still have this fascination with pentodes. Perhaps simply because people dislike them and because they are harder to deal with--the old, "if somebody says it can't be done, all the more reasons to do it."
I expressed one of my issues with UL in my previous post. As mentioned earlier, I have an issue with the nestled feedback. And one thing that has made me take note is that UL seems to not have been used in pro gear. I haven't seen all pro amps. But so far I have seen none using UL. And I rather put my trust in the engineers at Telefunken than any home audio manufacturer. And which Leak amps did BBC use? The triode-connected TL/12 and pentode-connected TL/25A. I trust BBC's judgement a lot more than I trust the average hi-fi buyer's judgement, you know the ones UL was sold to.
Of course, now somebody will counter and say pro audio and the engineers at Telefunken are conservative and hold on to the old and proven. Just like the old and proven missile guidance system Telefunken devised in the 1930s? The simple fact is that the people who bought the Telefunken V 69 were no pushovers. At the same time I'm sure none of them questioned the judgement of the engineers at Telefunken. If the engineers at Telefunken had said that UL is the way to go, their customers would have said, "Whatever, I'll have a dozen." But they didn't.
For me UL remains little more than a just another marketing stunt.
I'm getting very negative here. But the thread is about dislike and not like.
Correction. I saw a pro amp with UL yesterday: http://cgi.ebay.com/RIM-tube-monobl...ryZ50593QQssPageNameZWDVWQQrdZ1QQcmdZViewItem
And UL might have been used in the Klangfilm backup cinema amps and other such amps as well.
And UL might have been used in the Klangfilm backup cinema amps and other such amps as well.
gingertube said:
Careful here, Ian,
You are showing that at 19W in triode mode the distortion is 1.8%. But for UL mode at 19W the distortion would be 1.4% (taking a linear ratio, which is normally not far from reality). This beats the triode mode. Also, the triode mode will then be at its maximum, while UL will still have somewhat higher to go.
Anatoliy,
I respect what you are doing, but your post was a little cryptic for me. I did not quite get the "apples/oranges" comparison that you seem to indicate. Yes, Ian's data gave relative distortion, nothing else - he did not claim to give the whole picture. One accepts that what he tested would have been somewhere near optimum for all; it is not that anybody has anything to sell here. Kindly see my next post (to phn).
Regards.
from phn: The simple fact is that the people who bought the Telefunken V 69 were no pushovers. At the same time I'm sure none of them questioned the judgement of the engineers at Telefunken.
I'm sorry, but I put not questioning engineers in the 'most foolish' category. The idea that some group of engineers has done something beyond question gets my 'most ridiculous' classification. One should always look at things like why, and because...and the like.
For example, explain the Bottlehead exclusive use of the three-legged BJT current regulator. Only some of it is based in the engineering choices in case you need a hint.
cheers,
Douglas
I'm sorry, but I put not questioning engineers in the 'most foolish' category. The idea that some group of engineers has done something beyond question gets my 'most ridiculous' classification. One should always look at things like why, and because...and the like.
For example, explain the Bottlehead exclusive use of the three-legged BJT current regulator. Only some of it is based in the engineering choices in case you need a hint.
cheers,
Douglas
Not so fast Johan. Check the operating conditions. The UL numbers use a higher impedance OPT (6k vs. 5k) and if I'm not mistaken the triode example is being driven grid-positive. (70 ma into 440 ohms is 30.8 volts grid bias, Vin grid-grid is 48 volts rms = 48/2*1.4 grid-ground = ~34 V 0-p.) Mullard chose operating points to generate those results. How else could UL, with less feedback around the output, have less distortion otherwise? Lies, dang lies and spec sheets? I'ld be more interested in Dout well away from non-linear areas, say 1 or 2 watts.
Sorry for the hit and run but back to trying launch new studios!
Sorry for the hit and run but back to trying launch new studios!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.