A "National" BBU is on my bench at the moment. It is an unlovely thing ( the radio, not the bench ),having been converted from 2.5 volts to 6.3 volts by simply adding a smaller 6.3 volt transformer to the chassis and disconnecting the 2.5 volt winding.A 42 has replaced the 2A5 and the 2A7 is now a 6A7 while the 58 has given way to a 6U7 and so on.
But checking the original circuit on our NZVRS Circuit file I notice that the 2A5 plate and screen and output transformer are not connected to H.T. at all.So the 2A5 must have had an exceptionally long service life! A trap for young players.
And it must be conceded that the 171kHz i.f. frequency proved a trap for a slightly older player too, drifting the signal generator from 400 to 500 kHz and back and wondering why there was no response in the signal tracer ! Ah well,we're on the right track now.
Good spotting - but t'is not the only error in that circuit from memory - I stumbled across this very thing a couple of years ago while restoring my Rolls BBR (same circuit - there was a BBU - Ultimate and National, BBR - Rolls and Golden Knight, BBS - Skyscraper, and BBC - Courier, all the same BB'x' chassis' which they annoyingly gave different codes to and painted different colours)
If I recall correctly, the second smoothing cap in the power supply is shown wrong - its either to ground but it isn't, or its shown not to ground but it is... one or t'other... I'll see if I can find the amended copy I did...
Should the 58 not have been replaced with a 6D6?
oh - I put the amended circuit on the model page for all the BBx models I have catalogued - I don't have the National though - could you take a photo of it for the site? The details are here: www.vintageradio.co.nz/model/rolls/bbr
And yep, second smoothing cap is shown connected to the field coil when it actually goes to the chassis.
There are no personal problems that can't be solved with the liberal application of high explosives
The un-loveliness went on.The original field winding was on the centre tap of the power transformer to earth via a 375 ohm resistor but this had been modified such that the field is now in series with the H.T. to the screen and O.P.T. of the 42 and the smoothing caps connected in the usual way for this type of setup.
But, yes ,I did notice that quirk where the caps went to either side of the field in passing, tho' I simply thought "Oh that's a funny way of doing it".
And ,yes, a 6D6 would have been logical in that no socket change would be required but the 6U7 is there, as is a 6B8 in place of the original double diode triode. Overall I get the feeling someone used the valves they had.However the set plays, through my bench test field and speaker .Did I forget to mention this set came to me with no speaker?And of course the Candohm has several gaps.
But it does have the original knobs ,no burns on the dial face, a very refinishable cabinet and I have found a splendid 8 inch e.m. speaker with the correct 2500 ohm field.So that all the bits are there to make this a restoration competition winner.Have I talked it up enough? Any takers?You will want a concave 3 7/8 inch dial glass. I will take a photo or two before it leaves the bench. Oh, and the chassis is finished in an attractive shade of intermittently conductive silver. Is there a colour chart name for this colour ?
All the best.
Just checked out the link, Steve.Wonderfully accurate and helpful as always. Thanks.
Last Edit: Jul 14, 2020 14:51:01 GMT 12 by trombone
I would check how the 42 output tube is now biased, as they have removed the field coil from the CT via the 375 ohm resistor to ground? this would have given you around neg 18 to 20 volts for biasing the grid of the 42, someone must have changed the 42 to "cathode biasing" now?
Thanks, Richard for the heads up on the bias on the 42. Yes , the person who did the "conversion" was short on valves but not on technical knowledge. When I tipped it upside down I had not seen the circuit and so the cathode resistor etc. and everything generally looked original except for the odd valves. And as I worked my way backwards from the 42 most circuit values seemed "Normal".