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I have a friend who has a pair of VTL 80 monoblock amps. I take it to mean they are rated at 80 watts per channel. This is accomplished with a 12at7 voltage amp--->12at7 long tailed pair to a quad of PPP 6L6's. Bias point is 28ma per tube. It is claimed the amp runs AB1 tetrode mode but may be operated in triode mode at the same bias point with no changes and if done so will operate in class A.Does anyone else feel this is impossible...at least without exceeding the tube's rating?
Looking at a current production amp, the tiny triode, they claim 40 watts out of a quad of EL84's in PPP and if triode strapped that same quad of EL84's in PPP will deliver 25 watts. That might not quite as far fetched as the 6L6 amp above but it also seems unlikely to me.
So what do you guys think? False advertising? Operating points well above the tubes rating? That I am crazy and don't know how to read tube specs?
Follow Ups:
http://tdsl.duncanamps.com/show.php?des=6L6
Go here, 6L6 can get 55 watts push-pull per pair and you have 4 tubes per side. I would guess that they are running them conservatively and easily meeting spec.Triode would be less. However, from what I read in your post, they are not claiming 80W for triode mode.
The example you mention has the plate at 450 and the screen at 400. IIRC a standard 6L6 is rated for a maximum plate voltage of 350 and a maximum screen voltage of 250. So this is a prime example of what I think VTL did. Namely operate a tube beyond it's published maximum rating.But hey, I am ready to be proven wrong. Actually I'd like to be wrong as this is a friends amp who is having some run away tube problems from time to time.
The numbers you quoted are for the older 6L6/6L6G. The newer 6l6GC have much higher ratings: 500V plate, 450V screen, or 450V plate & screen in triode mode, according to GE. Most present day productions meet these specs.So 450V/400V are perfectly OK. Some guitar amps are a lot harder.
hi russ,
I don't think VTL uses "standard" = metal type 6L6 which indeed are rated @ 360V plate & 270V screen grid.
glass type 6L6GC are rated @ 500V plate & 450V screen grid, so this is a prime example of operating a tube well below it's published max. ratings.
so where's the problem ?
nicolas
.
VTL amps are often designed running the output tubes WAY over their screen and anode voltage ratings!I even confronted Luke Manley about this once at a hifi show, and he admitted it but said the tubes they supply are built to special standards and you HAVE to use their tubes in their amps.
To this I say B***S***!
If they have a special tube built, then it should have a special number and not cause techs all over problems when they re-tube a VTL using what would appear to be perfectly acceptable tubes.
I've gone as far as changing VTL's to cathode bias to try and maintain some reliability - it worked but lost some power.
then they would KNOW something wasn't kosher....call to VTL, DOH!!!, then recommend other amplifier to customer in future....
> you HAVE to use their tubes in their amps.Factory tube selection to insure best specs is one thing, but hand-picking so the tubes survive is quite another. That's a shameful way to design an amplifier. Considering the purpose of engineering is to solve problems, I'd say this doesn't qualfy.
Hi.Commercial brandname products are for sales just like any other business, be it "eningeering" design like an audio amp, or bakery like cheeseburger.
You're not wrong from your DIYer view point. But you think they care
about you for fixing their products which they definitely want to discourage? They are running a business, NOT a neighbourhood advice
centre working for nothing. Like it or not, what they are doing is legistimate. You'd do the same thing if you were an amp OEM.Of course, to protect their interest, a manufacturer will design their products so uniquely as to stam off copcats & design hackers.
Patenting their designs is the last coffin-nail.Good listening
If the amp would have been marked "6L6GC only" that would be one thing. But as far as we can tell it was shipped with 5881s. Obtaining info about these amps hasn't been easy. About all they will say is return it to the factory and we will be glad to take care of it....for a fee....but we can't tell you anything because of the high voltages involved. Gee all we wanted to know was if it was okay to use a 12BH7 instead of a 12AT7 for the long tailed pair and if this amp was able to operate in triode mode as it doesn't have a switch. The sole source of info seems to be VTL's little black book which is rather vauge and as Henry said seems to....shall we say....stretch reality a bit.It puts one in a difficult place to make a post like this....visions of lawyers and slander....so I am glad to have someone with knowledge and experience jump out there and state what appears to be the obvious with regards to voltages and ratings.
I will consider cathode bias as we can afford the reduction in power (but I do think the owner likes to check and fiddle with the bias). I don't suppose you know the current rating of the PT? It would be nice if I could drop a choke in front of the first cap and run triode strapped class A. The owner has just bought Lowthers. So if the PT can supply the current I am sure I can make it much better than what must be a close to class B amp. Hopefully the OPTs are good enough to make this a worthwhile undertaking (please tell me if I am crazy).
I'd go for the ideas you suggest:1/ triode strap the output tubes
2/ add a choke and hence convert the PS to choke input
Both steps will greatly increase the sonics, and the choke input filter will drop the B+ by 30% or so and give the tubes a little breathing space.
And then as Tom has suggested, you can up the running current into (near) Class A for a further sonic increase.
Their OPTs are more than OK - so it should end up sounding pretty good - and WAY better than it did!
Hi.Equipment OEMs & their authorized service centres earn revenues by fixing their products. Any 'unauthorized' service will NOT be welcome & even suffer warranty termination if done within warranty period. But, like it or not, this is legitimatem though from your view point, they are not doing you a favour. They are running a business, not some free neighbourhood consultancy centre.
In reverse situation, you would do the same thing to protect your
'intellectual property" & monetary interest.Besides, what you ask is a specific technical question involving swap of tubes not specified in the service manual. If I were the OEM itself, I would not make any comment to eliminate any legal consequences in case of loss & damage due to mishandling & improper
change of its original design.That's the real world to face. I'd not blame whoever. Just do it at your own 'risk'.
Good listening
.
> Any 'unauthorized' service will NOT be welcome & even suffer warranty termination if done within warranty period.I'm sure some companies do this, but it's a violation of consumer protection in the U.S. Manufacturers must prove not only that unauthorized repairs were performed, but that the parts/labor/processes caused damage or created liability. It is also not acceptable to U.S. courts, generally speaking, for a manufacturer to say that a unit which has merely been opened for inspection is automatically disqualified from remedies of warranty.
Hi.Who wants to go to the court for minor stuff as such, anyway.
Last week, I picked up a NAD SS intergrated amp from a chop shop for
8 bucks for scrapping its housing, which looked pretty mint for
building a phonostage & linestage in it.It save me the pain of constructing a housing with a brandname professional look. For busy guys like me, time worths as much as money.
Surprised to find the phonostage & linestage are still working OK.
So I called up NAD's local service centre for a service manual for that vintage model.Another surprise, they still got one & mailed it to me FREE. But when I find out they actually sent me the manual of another amp with the last digit of the model no. not same as mine, I called again for the right service manual.
This time, they said they did have it & asked for 13 bucks for a reprint.
I consider this NAD service centre is very responsible.
Good listening
If you add a choke, it will reduce the voltage and reduce the transformer current. So you can up the bias current to move further into class A. Of course you sacrifice power at the lower voltage, though that doesn't seem to be an issue. And you can add cathode resistors and still have variable bias (negative voltage will be much less - might have to tweak bias circuit values).12BH7 has considerably less gain than 12AT7, so open loop gain will be lower, effectively reducing the amount of feedback. Can't see how it would hurt to try it, but I don't know enough about the circuit to predict the results...
No, I was not trying to say the PPP 6L6 amp would do 80 watts in triode mode. What I was saying is it is claimed that it operates in class AB1 in tetrode mode but when switch to triode operation without a change in bias current it operates in pure class A. I was also just informed that the bias point is speced at 30 to 35 ma and not the 28ma I stated above. I don't know what the claimed wattage is in triode mode.But leaving that model alone and looking at the current tiny triode which has the specs at the website, I just don't see how one gets 25 watts out of a quad of PPP operated triode strapped EL84's. I would think 16 watts would be about the max possible. Even the claimed 40 watts in AB1 tetrode mode seems hard to buy without going over the tubes ratings.
This thread is not intended to slander anyone. Manley has a reputation and it speaks for itself. Instead I was looking to see if I had a misunderstanding or if the operating points might just be above normal maximum ratings. Plenty of guitar amps operated tubes above ratings.
> Even the claimed 40 watts in AB1 tetrode mode seems hard to buy without going over the tubes ratings.I think that's probably right, assuming you're willing to accept a number based on absolute maximum ratings. A pair will output 20W if the OPT is efficient and you really push.
I did say the 40 watts in tetrode "seems hard to buy without going over the tubes ratings" but stopped short of calling it impossible. However the 25 watts in triode does seem impossible to me. What are your thoughts on that? Of course it might be nothing more than a misprint on the web site.
I think I'd have to give that one a "maybe." They might just do it in AB1. Incidentally, do you know for certain the amp can't move into AB2? That would change everything, both pentode and triode modes. If I can make a suggestion, you might consider posing these questions directly to the manufacturer. The reply will probably resolve it, one way or the other.
Hi,Max anode dissipation for four 6L6GC is 4x30W, so total dissipation is 120W.
Class A one could under ideal conditions get 60W out in pentode (tetrode) mode, and 30W under ideal conditions triode connected.
Going class AB, or even AB2 or B2, one could get a lot more power out of them (say 70% and 40% instead of class A max 50% and 25%).mvh /Pär
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Me fail english, thats unpossible!
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Hi,Aha, that makes sense with hindsight :-)
mvh /Pär
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Me fail english, thats unpossible!
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I think they mean the idle plate current is the same in triode mode. That is not the same as claiming the maximum power output is the same (which is indeed unlikely.) With the same load and idle current, I would expect the tubes to clip sooner in triode mode as the grids go positive. As long as the tubes aren't driven to that point, it is plausible that they might not cutoff at the other end of the load line, thus operating class A. But the power output would be about half the beam-tetrode maximum.
IIRC, the most power you can get out of a quad of triodes in ideal Class A, push-pull operation without exceeding their ratings is the value of a single tube's plate dissipation. This would be 30W. It presumes the tubes are perfectly and ideally matched to their load, that the bias point is exactly right, and that the tubes are perfectly linear. in practice, the Class A power output will be lower. But there is a caveat. Especially if you run the tubes at high plate voltage and low bias current (as VTL does), you can get this situation where, due to nonlinearity at low currents, the tubes never quite cut off, no matter how hard you drive them. On the other hand, they don't contribute any useful power in this part of the operating region and may actually become net power sinks. But under the strict definition of the term, the output stage is still operating Class A.I looked into this back around 1993 and discussed it on rec.audio.tubes. This was early in my tube education, when the "VTL Book" was one of my main sources of information and I was still trying to sort out a lot of new (to me) information. I was quite frustrated at the time because what I was seeing in the book didn't add up. Knowing that everything I say becomes part of the public record, I don't want to go so far as to say anyone is lying. But I will say that it appeared at the time that VTL was interpreting "Class A" in a way that contradicted my understanding of the term.
I propose that a practical and conservative definition of Class A requires not only that the tubes never cut off throughout the AC cycle, but that they remain in the substantially linear part of their characteristic curves, and deliver meaningful power to the load at all times.
And it looks like about 35W would be the limit for a quad in PPPT, and less (probably about 25W) if limited to class A operation. So I agree with with Henry's comments (except I have no experience with VTL, so no comment on his 2nd paagraph.)
without looking at VTL's claims, redefining Class A seems to be a particularily unregulated arena. Class A for 1 W, and AB1 to 500W is a continual favorite.I'd just call it a stretching of the term, and for the original intent/meaning of the term such things could be interpreted as Marketing. Like how can you be certain a salesman is lying? his lips are moving.
cheers,
Fred
Hi.I tend to disagree with you basing on VTL's 20 years in building tube amps, & 20 years in the recording industries prior to that.
Its flagship tube monoblock delivers earthquaking 1,250W using 24x6550 power tetrodes (on AB1) consuming stagging 1,800W power.
I believe you were talking about its baby model: TT-25, which is rated for 45W/4R tetrode (using 1x12AT7 & 4xEL-84 AB1), & 25W/8R triode, consuming 160W.
Looking at its power efficiency ratio, I believe the rating is proper. Nothing smells fishy to me.
BTW, VTL is a 'rare' tube maker that impresses me, by adding a tetrode/triode O/P mode switch to most (if not all) of its power amps. This is a very sensible feature for triode sound lovers, like me.
One time in an audio show years back, VTL was demo'ing its power monoblocks in triode mode, which sounded pretty nice & attracted me to it.
The sales guy there switched it back to tetrode mode to "make it sound more gut". I told him the previous triode mode sounded much much better. He said no, there was no difference.
So he put me to test in front of all the audience inside the crowded room, by switching back & forth tetrode/triode mode. I got it right all the time - triode sounded better even with my back facing the speakers !
Good listening
cheap-Jack
June 16, 2005.
Try asking his ex, Eve-Anna Manley.........and her stuff, loosely based on his, is fabulous stuff......
If VTL states 80-watts triode, that can not be correct per any manufactures data sheets. I see no issue with 80-watts tetrode & 30-watts triode.Another interesting item is the use of a 12AT7 in the high cost VTL amps. I have a copy of the VTL amps published by the VTL design engineers that explain their circuits in great detail. This book seems to disappeared. I really enjoyed the VTL 807 amp discussion. Anyways, who would use a lowly 12AT7 in a 'best quality' circuit? Obviously, the engineers are not new to audio design.
Does VTL claim their amp will output 80W in triode mode?
You are right in your thinking.
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