|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
73.239.123.14
In Reply to: RE: Bogen DB110 etc AGAIN posted by ivan_terrible on January 28, 2016 at 13:27:24
There's a decent amount of global feedback that will flatten out some of that loss of bandwidth in the driver stages.
Follow Ups:
Perhaps I should be a little clearer as to the basic circuitry.
I hope this is interesting.
Despite the obvious similarities in Transformers & output valves the DB10 is likely to sound quite radically different from the DB110 series.
As you can see, Bogen changed the designs of their AF amplifier and phase splitter stage from the late 40s/early 50s to use the cheaper mass produced 12AX7 or a combination of the 12AU7 and 12AX7 inside the 7247.
The 6SL7/ECC33 arrived from late 1940s to upgrade the low gain 6SN7/ECC32/5692
They struggled to contain microphony with those.
The 12AX7 is not an exact equivalent of the 6SL7,(see 5751) but I don't want to ignite the good old debate about what is different, octal v noval 6CG7 / 6FQ7 all over again!
You see there's a lot more to it than meets the eye!
However,- The result, pre 1950 octal DB10, was probably poor phase splitter/frequency resp balance because you can't come close to guaranteeing the 2 valve halves having the same gain.
Post mid 1950s, using cheaper noval valves, was a phase splitter with serious bias problems resulting in "surging" and an AF amp that had poor overhead, tending to non linear because it was being fed with microamps of current.
Global negative feedback was then used to compensate for the poor responses while not being very well coupled to the output.
We don't want to go there, that also involves redesign!
The 6V6 is a valve invented in 1936 originally as a budget 6L6.
We are in 2016, so it's 80 years old this year.
The 6V6 uses a very different G2 > A potential difference compared with a 6L6.
The G2 behaviour & characteristics of the 6V6/6L6 family are quite well known going from the gentle to the mental (in the 807).
In sonic terms, the 6L6/6V6 family has rarely been used for ultralinear or "distributed load" like the Philips/Mullard EL range from the EL37 on.
The main exception to this was the KT66 which has a redimensioned anode, and different spacings from the 6V6/6L6.
It's a fair proof the grid spacing/electron path is substantially different when the distributed load recommendation was for only 10-15% taps...
Coming back to the Bogen DB series.
As the A-A load of the ancient 6V6 is very high, the gain of the amplifier will depend more than ever on the reflected impedance of the speaker system & the internal impedances of the beam tetrode which are also really high. Ie. Damping.
It is probably the way it behaves that makes people believe it has a very benign quality of sound in the 21st century with "21st century speakers".
I would hazard a guess this is down to the relative lack of brightness.
It's suggested the unique distortion signatures come from the G2 spacing, voltage used and consquently the internal impedances and voltage current density/impedance which dictate the frequency behaviour inside the circuit.
(We also have to admit the modern "Kellogg/ loudspeaker. ie linear motor (apart from the Quad ESL type) is 95 years old.
The design has changed very little.
The only noticeable recent improvement being the change from Alnico to rare earth metal Neodymium.
Few people in reality can afford Neo based speakers, but it is those that more than likely have a better impedance curve)
Global negative feedback may well compensate and flatten the frequency response curve, but the phase shifts that come with it, especially with the 6V6 mean it's better to use good circuit design, with local feedback, than global, which can have unwelcome & unpredictable effects.
The DB10 & 110 DO use global feedback, in my opinion they do exactly what it says on the tin, but don't make up for the internal design problems.
Strange therefore the speakers of the 1930s and 40s were in any case not able to reach even close to the resolution of vinyl 78rpm, never mind 16bit CD.
The amount of distortion from the whole system was stratospheric and there was almost no treble after 4khz.
You wouldn't have noticed an amplifier with non linear frequency response and 2-4% THD for those very reasons.
What's the message this time*?
As I was coming back to the serious business of redesigning this Bogen amplifier, I happened to come across some quite wild claims for something similar, as follows:-
Where have I heard this before?
Back in 1943 of course!
"It was found, too, that a really undistorted output of 4 watts was obtainable, and this was considered to be ample for nearly all domestic requirements."
"The World's Best Sounding Speakers and Amplifiers" 2012.
"My Dad's comments, on the phone, was on how good the sound was with the new 6V6 Ultralinear amp"...
""The 6V6 Ultralinear amplifier is superb, far better than I imagined! To say that the amplifier is amazing is an understatement! I cannot believe the detail that I am hearing in such a crisp manner."
""The 6V6 Ultralinear sound is really different from conventional amplifiers. There are a lot of details and it is not necessary to use treble and bass.....Pink Floyd sounds better than it was with top semiconductor systems before!"
So STOP here.
*People are claiming to hear things, made impossible by the simple fact male hearing frequency response decline making them impossible.
Men over 35-40, (short of a miracle) claiming they can tell the difference between the frequency response of an amplifier that is -10dB at 20khz and -2dB are simply lying, which means 99% of the "hi end" audio industry is built on hype.
By the time men are 55, their hearing is -30dB at 8khz.
NOT GOOD.
-30dB at 8khz is certainly approaching the noise floor of vinyl,(to where the music has been compressed anyway), to get around distortion of the cutting head, stylus system and surface noise.
I got the same claims made for top of the range mega bucks dual concentric Tannoy stuff, sounds terrible etc.
If it quacks like a duck, it looks like a duck, it lays duck eggs then it IS a duck!
To stretch the image a little more:-
Coloured duck eggs are coloured.
They might taste the same, compared with another less coloured egg from another farm, depending on what you've been smoking or drinking that day, (call that ego tripping).
How can you compare, when you have just had a cup of strong coffee or a cigar?
That's relativity folks!
I equate the "tube rolling" brigade and the acres of sites dedicated to the nuances of a "Telefunken this or Mullard that as equivalent to "rolling a spliff".
If it makes you happy, then by all means, but don't try to start me on it.
Can a 6V6 amplifier (using an 80 year old valve) really be made as good as the claims make out?
Here's where we can learn from history because the amplifier quoted on the post today above is doing exactly the same:-
"One interesting point is that with the same plate and screen grid voltages as used by Williamson for triode connection, the Osram valve data at the time stated an output of 50 watts for pentode connection of KT66's - a long way from 12.5 watts in triode configuration.
The solution was to convince the public that 12.5 watts was adequate for home hi-fi." !!
Coming back to my 1955 Bogen amplifiers my plan was not to go ultralinear in the output stage, but to clean up and go ultralinear in the INPUT stage.
This can be done really one of 2 ways,-
Either use a pentode for an input valve or a cascode stage.
A dual triode cascode stage IS a pentode, it just doesn't look like one.
"Ultralinear" is the quest to get a pentode to sound like a triode, or a triode to behave like a pentode, or somewhere in between if you believe Keroes, Hafler or Mullard and their "distributed load" data from the 1950s.
Mr. D.T.N. Williamson made a point of hating it.
He didn't make valves,(he worked for GEC0 and didn't wind transformers.
GEC was a direct competitor to the (dutch originated) Mullard & he might have been a tad anti American.
Who knows?
His stereo line up would be 4x KT66, & 4 x 6SN7/B65.
Read a bit, - http://www.oestex.com/tubes/williamson.htm
Mullard with their EL37 based EL34 offerings were substantially different from both the 6V6 family and the British made KT66.
Here at least the choice of methods & valves is wide open, but quoting on the subject of this post, would I do it with the following line up or something else?
"4 x 6V6, 2 x 6SN7, & 1 x 12AX7"
After all the Bogen DB110 comes with some of those onboard, the DB10 was roughly similar and putting a pair on a new chassis is a one off chance "to do it different".
I will go with the original Bogen output transformers as stated, which are NOT ultralinear, rather than "Real potted transformers wound in our own factory".
I might go for a cheap toroidal power transformer rather than an "oversized one", because I would fit TWO,- one for each channel.
The advantages of a pair of monoblocs on a single chassis become very clear..
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: