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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: RE: DB110G (Bogen) 1950s. posted by ivan_terrible on July 2, 2024 at 02:56:07:
Some thoughts arrived since the last post... becoming a "blog"!
Conclusions:-
1/ The loctal x-BL71/etc is a great valve.
It stands severe violation - overvoltage, running flat out square wave testing, running the anode and screen at well over the recommended maximum diss - not a single sign of red plate or glowing screen grids like the OEM 6V6 btw.
The internal construction/glass seems well able to cope.
2/In detail Pa is quoted 11W. (plate dissipation).
Nothing bad happened static at 13.75W + 2W static on g2.
Under full load calculating back via the dropping resistor of 3k5, the screen showed a drop of 60V to 284V ie 17m/a.
17m/a is 4.9W for the 2 valves ie. 2.45W each.
The original figures for 6V6 state 13.5m/a at 285V for 2 valves max signal which is 3.85W or 1.92W each.
The 6V6 pulls just 4m/a at idle...0.5W ea.
This is to be expected with aligned grids.
3/ The drive requirements of a valve with gain 9.5ma/v compared with the original 6V6 of are very much lower.
It needs just 6.5v of drive to max compared with the 6V6 which needs 22.
In view of the much heavier screen grid currents of the pentode, the supply to that point needed reinforcement.
3/ CFB loop via the output transformer sec is around 6dB.
The original NFB loop gave about 15db of NFB.
The pentode has 50% lower THD (needs less NFB).
4/ OEM figures are WAY optimistic.
"With built in preamplifier giving 10 Watts output with only
0.65 % har monic distortion.
Frequency response: +/- 1 db, 15-50.000 cycles at 10 watts".
Valve manufacturer quotes max power output of 13.2W at 1.8% THD.
The 6V6 quotes 14W at 3.5% THD.
Distortion: was quoted 0.125%, 0.25%, and 0.65% harmonic at 1, 5, and 12 watts respectively; 14 watts peak
None were achievable here.
The IMD figures were no better originally between the 6V6-x-BL71 but after optimisation using CFB were up to 20dB better with the pentode.
This could not be checked with the 6V6 because the drive voltage requirements are more than 3x higher, so the effect of a 4v return loop would have substantially less effect to a -22V>-25V bias point.
I note the Bogen -25V voltage figures couldn't be met with the best 6V6 I could find.
FYI.
High Fidelity magazine, April 1955 quoted:-
" It is one of the best-sounding amplifiers we've heard in this general price class"
"Our checks on the power amplifier section showed low distortion at all power levels up to maximum, lower than in many amplifiers selling for a good deal more."
Our unit underperformed substantially.
After a lot of work we could get just short of 9V out clean into the load resistor.
This equates to around 10W, but with high losses of HF response.
Below is the much scratched out OEM diagram with loads of notes.
The next step is to test with a completely different (SCOTT) high quality transformer to see whether it's possible to improve on the BOGEN OPT frequency response.
This post is made possible by the generous support of people like you and our sponsors:
Follow Ups
- RE: DB110G (Bogen) 1950s. - ivan_terrible 08:28:21 07/02/24 (0)
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