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In Reply to: RE: Bogen DB110 etc AGAIN posted by ivan_terrible on February 9, 2016 at 11:09:20:
INPUT STAGE decisions.
Here's the next stage in the planning:-
I posted the original AF amp part of the circuitry some time ago here.
This is where the river really divides, doing things differently, very differently.
You read the zillions of acres of worthless speculation or commentary on what round, flat, triangular, plates, colour base, colour plate, "holy grails" (whatever Parsifal or Lohengrin was supposed to do in there).
Then reviews of distortion and 1000s of pages of supposed tweaks....
https://tavishdesign.com/pages/audio-tube-noise-measurments
http://www.fourwater.com/files/hist6sn7.txt
You have reviews of "MUST HAVES" in the latest inaudible circuit change to a 6SN7, then see prices to make anyone dizzy...
Fashion is fashion. Sheep always make the same basic noises.
The big big decision here is whether to go down the octal route for "old time's sake", go down the "loctal" path which has very limited choices..
Tread a noval route to prove you can make them sound good, or the do-decal TV valve route to squeeze it all in nice and tight?
I lost interest in the end
If doing food audio has to involve reviewing the totally contradictory ego trips of 1000s then there's 99% b..llox going on out there.
So, I decided it was well overdue to go back to using a pentode for the input valve.
I can almost hear the screams of horror.
It will sound like a "pentode" it will be bright and brash.
It will be noisy and take up too many valve holders to be able to get all the wiring right, the holders have to isolated and mounted on rubber!
hum will pour out the speakers and the whole chassis will become a big rattly microphone!
What of course it won't have is Miller effect roll off!
The choice looks like this:-
It didn't take a great deal of research, because there's not many candidates for the valve, and what there is goes back to the 30s, with basically the same stuff as found in the EF86.
I post a load of pictures as usual.
Luckily someone summed a nice lot of it here:-
http://mike.wepoco.com/Home/retro-geekery/reference/small-signal-audio-pentodes
Broskie came up with some really smart stuff too:-
http://www.tubecad.com/articles_2002/RIAA_Preamps_Part_2/page2.html
It was exactly what my dad from his Mullard days said it did.
Reduce noise, increase gain and get rid of all the Gremlins.
Here's more on it in opamps:-
http://www.linear.com/solutions/5657
I'll start with the humblest of all TV valves PF86 and UF86.
They don't cost a lot!
I maybe wanted to have a go at using the 6BR7/6BS7 alternative, but all these pentodes have known issues with hum, noise and microphony.
Here's the original amplifiers which are going to be torn apart + the tone controls and preamps all to be scrapped.
OCTAL base model.
Noval base model which clearly has a much bigger output transformer than the Bogen HE model (not the DB10).
That's a serious size transfomer for the original 10-12 watts.
I wonder what it will do if forced to deliver double that with the Russian stuff from the last post?
So as you can visualise the transformation,-
Destination base cheap chassis, as you can see not a lot of room.
The valve rectifier gets binned for a solid state one of course.
It appears flagrantly obvious the tube count won't allow the EF/PF/UF86 for a stereo amp.
Octal based circuits look like something out of the ark, because they would involve using the "full metal jacket" top cap, 6J7 or 6SJ7.
One could be tempted to use 2 Octal valves like 6SN7, but again it would cramp all the options unless using them as a cascode.
Cascode circuits ARE in fact pentodes in disguise.
The answer coming to the rescue lies yet again in the USSR in Siberia.
This time from NEVZ.
An perfect equivalent of the EF86, but ultra low noise and almost zero microphony, as it's wired ended, 165m/a heater so 0.660A for 4.
Runs on low voltages.
(ideal for valve mics)
I'm going to wire 2, 3,or 4 in parallel for each channel (8 total) as I get them for 0.75 EURO each.
This drops the impedance of that stage and the noise.
They will go UNDER the chassis, well away from stray magnetic fields hidden away unnoticed.
That leaves 1 valve for each channel and 1 spare centre socket which looks like may end up as the phase splitter, driver for Left & Right channels.
The clever part is how to squeeze 2 channels of audio into the 2 other valves.
Will it be compactron 3 triodes or just Noval duals?
That very much depends on the final design.
The more triodes the merrier, so for each channels it could be as many as 4, or maybe 3.
I wanted to throw a good dose of local feedback into there in the form of ultralinear mode on the pentode, deliberately NOT opting to run it as a triode.
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Follow Ups
- RE: Bogen DB110 etc AGAIN - ivan_terrible 13:43:17 02/09/16 (0)
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