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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: RE: Bogen DB110 etc AGAIN posted by Caucasian Blackplate on January 29, 2016 at 18:54:16:
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.
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Follow Ups
- RE: Bogen DB110 etc AGAIN - ivan_terrible 01:21:15 01/30/16 (1)
- RE: Bogen DB110 etc AGAIN - ivan_terrible 22:13:01 02/07/16 (0)
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