rcavictim said:
The fact: the Chinese 805A is designed for AF amplifier.There is no reason to accuse it for RF amplifier.The forum is diyaudio!
If you re-read my post,you'll find the Chinese 805A has a different characteristic from the original 805.The most important characteristic for AF amplifier is distortion.In 1970s,China improved the 805 tube which has a better characteristic than the American types.It can be driven more easily,therefore it can reduce the T.H.D largely.Moreover,the Chinese 805 has a good linearity at 75W-85W P.D. The original one's gird current is too large to be driven with low distortion.
About the safety,a tube without a top plate cap is better.
To Dave,
The 805 has a sudden change input impendance.It'll reflex to the primary impendance of interstage transfromer and cause the changeable load impendance for driven tube which will bring a large distortion.
Julien said:
I don`t see this as a characteristic of the 805 by itself but rather any tube driven into grid current. By design the 805 has to be driven in A2 (grid current) to get full power capability in SE. Any tube driven to A2 will have a kink in grid Z as the grid starts to draw current. Perhaps Julien means to compare the 805 to another well known tube of similar application, the 845 which is designed to work entirely in A1 and never goes into grid current.
The use of a step-down interstage xfmer is absolutely one way of dealing with the special grid drive needs of an A2 output stage. I employ it very successfully in the 809 SET amp shown in my avatar that uses an 809 in the final driven heavily in the A2 grid current regieme.
Sheldon,
Thanx for the support. I think Julien has financial interests in the sale of his favorite Chinese 805A and is therefore sensitive to any comments that look like they may be critical of it.
IMO the tube he is championing is NOT an 805 and should never have been given this type designation. To add to the confusion is that when vacuum tubes ruled the first time the numbering system included the use of `A` as a suffix and that DID NOT mean `Audio Application`.
I have some Shuguang (Chinese) 805`s with plate caps that are in fact 805 triodes and can be placed in any applications that call for an 805.
Thanx for the support. I think Julien has financial interests in the sale of his favorite Chinese 805A and is therefore sensitive to any comments that look like they may be critical of it.
IMO the tube he is championing is NOT an 805 and should never have been given this type designation. To add to the confusion is that when vacuum tubes ruled the first time the numbering system included the use of `A` as a suffix and that DID NOT mean `Audio Application`.
I have some Shuguang (Chinese) 805`s with plate caps that are in fact 805 triodes and can be placed in any applications that call for an 805.
Julien said:
My mistake. I thought you were saying was that there is something unique about the Shuguang 805 that makes it unsuitable for use with an IT. As rcavictim said, the 'standard' 805 can be used very well with an IT.
In fact, *any* transmitting tube needs a very low DC impedance to ground because there is always measurable grid current, even if the grid is never driven positive. That means either an IT, a grid choke (instead of a grid resistor), or direct coupling to a cathode follower. In the case of a tube like the 805 that is run with the grid positive during some of the signal swing there isn't much place for a grid choke, so it's either IT or direct coupling.
I haven't spent much time studying transmitters, but I'd bet my lunch money for a week that most of the working units actually built with 805's used interstage transformers.
-- Dave
rcavictim said:
What you've sad is disgraceful.I'm just a diyer.Do you have any evidence to prove your words?
You should apologize for your words.And I haven't said the 'A' was a symbol of `Audio Application`.I just know what the datasheet says.
A vacuum tube can be placed in any applications don't mean that they are the same!.How about the distortion,the frequency response and any more?Example:In most application,the 12AX7 can be replaced by 5751,also the 6SN7 and ECC32.Virtually, they are different.
Dave Cigna said:
If you want to get full power capability in 805 SE,any 805 has a probable that the range of driven voltage comes from negative to positive.
When the grid voltage is negative,the input impendance has a tendency to infinity;But when the grid voltage comes into the range of positive,the input impendance is very low.This changeable would be too hard for an interstage coupled stage to handle with low distortion.Another feature of 805 is a high nternal impedance vacuum tube.If you want to reduce the Zo(achieve a suitable DF to control loud speaker) by using negative feedback,the interstage transformer can be a trouble.
So the higher input impendance(the lower gird current) any 805 has,the better it will be.
Julien said:
Exactly what words did I say that you think are disgraceful and should be supported by evidence presumably to make them not disgraceful?
Julien said:
I didn`t say that it was you that gave these tubes you are championing the `A` suffix. Obviously it is given by the manufacturer of this new vacuum tube offering. Suggesting the `A` might be meant to mean `audio version`because the rest of the name definitely needs some additional identification to warn potential users that it is not an 805, was MY guess. The manufacturer obviously isn`t afraid of not conforming to the type designation standards, so why should they not also be expected to dream up any convenient meaning for an `A` suffix at the same time?
If I have a piece of gear that calls for a tube, let`s say a broadcast transmitter made in 1955 and I place an order for some new 805`s to get it back on the air, if I get sent the 805A tubes you like I won`t be able to use them. Why not? Because it isn`t an 805.
Julien said:
If I have an amp that calls for a 12AX7 and I replace it with a 12AX7A, I can be sure that the tube will fit in the socket without wiring changes and the amp will work just fine.
Just to be clear. I am not blaming you in any way for what I believe to be an error in calling the tube you like an 805A. How could I, it wasn`t your doing. Furthermore, I`ve never said that it (your tube) was not a great sounding audio tube (I have not heard one so cannot form any opinion). The only thing I have done is point out how it is not an 805.
Julien said:
Julien,
I agree in a general that that the points you make above are valid, but I think you are being overly critical of the concept of employing an IT to drive an A2 triode stage.
Of course as you say the grid Z goes low when it goes positive and starts to draw current. I made the same comment earlier referring to this as a `kink` in the impedance curve. It is general practise to use a step down IT to handle this challenging grid drive situation. The drive capabilty of the driver including the transformer should be such that it can source 10 times as much current as the output tube grid will demand when in the positive regieme in order to keep the detrimental changing load current effects of the positive grid to an insignificant percentage of the drive current available. This deals with the distortion issue.
Low impedance does not eliminate the introduction of phase shift which you also mention can become a problem with amplifiers employing large amounts of global NFB to reduce output Z and improve loudspeaker DF. The answer is less global FB and careful use of local NFB in the output stage itself (if really needed). One can also choose to employ speakers that are inherently better self damped with SET amps and thus stay away from high levels of NFB entirely. Horn loudspeakers are probably the best example of highly self damped speakers and also highly efficient. No wonder they seem to find popular pairing with low power SET amps. A good synergistic relationship!
If you are using low efficiency speakers that demand high power and plenty of amplifier derived damping factor, a SET is not the first amp of your choice anyhow. Go for P-P KT88`s and keep parallelling them until the bloodflow from your ears reaches the desired level, or better yet, go with sand.
I was misquoted as stating that the regular 805 responded well to IT coupling. In fact I made a comment only of an amp I`ve made using IT with an 809 to great success. I used only a small amount of global NFB to tame it and it is completely stable. I have no experience with a IT coupled 805. The only 805 amp I have uses direct cathode coupling from a 300B stage and I had no part of the audio circuit design. That said, I don`t see why a good step down IT based circuit could not be made to drive a standard 805 in a sweet way.
TugaTweaker said:I would like to use them in thisbut the pentodes 6J8P , 6SJ7 , 6H8 , etc
never heard of them . Why not the usual 12AX7 , or 6SN7 ?
Nothing scary there to me. The input stage needs lots of voltage gain because the 2nd stage is a less than unity cathode follower. You can use any of several input pentodes. I think I`d use the EF37A Mullard because it is a premium tube and I have some NIB (Quad of England used them too). Otherwise a 6J7 (also with grid cap) or 6SJ7 or the red marvel 5693 (no cap) will work.
2nd stage (which is wired as a triode) is easy 6L6. Options are 6CA7/EL34, KT77, KT66, 807.
The output tube you have.
That fancy audio power level display circuit using a 6E2 eye tube in the bottom right corner of the schematic can be eliminated.
The rectifier to form B+ delay is easily swapped as well. 5AR4 for example.
The pwr supply called -B2 290 volts is a positive supply for the driver stages. Confusing since the + symbol is used in the other spots.
Good luck.
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