[Arran]

Hello Everyone;
  I picked up a British made Bush DAC 90A today, it was the second generation in a series of post war AC/DC radios made by Bush Radio. In any event it is a series string set, much like an AA5, but unlike an AA5 the audio output tube runs at 190 VDC. What I was wondering is if it would make sense to rewire the set to include a voltage doubler for the plate/anode voltages whilst operating the tube heaters from the 120 volt power line (string adds up to 116.6 volts not including the pilot lamps)? https://www.radio-workshop.co.uk/service...dac90a.pdf 
  When I'm asking about a voltage doubler is it possible to use the UY41, a half wave tube rectifier, as the first of the diodes, whilst using something like a silicon 1N4007 as the second diode? 
Regards
Arran

[morzh]

Arran


Depends on your load. It is a Greinacher scheme, so maybe it does not matter as the voltages are simply added to each other and if one is lower than the other, so what, it simply is losing a few volts due to higher impedance of the tube.
This said, unless you want to keep the radio AC/DC and use that tube's filament for that reason, you could use a cap to trim the votage to the load of one less tube.
Or you could use the tube's filament, but then not use the rest of it, and use 2x1N4007.

Also, look up my Telefunken thread from a year ago: there is a thing about the common going to one of the lines, without which there will be hum.
I also experimented with the input and output caps; their ratio will determin the output voltage.

https://philcoradio.com/phorum/showthrea...telefunken

[Arran]

Mike;
  Thanks for reminding me of your Telefunken thread, I had almost forgotten about that. One thing I will do is check the tubes and see if the UY41 is any good, if not then double silicon diodes it is. I was mostly thinking of using it as it will offer a controlled rise in the B+ voltage rather then an instant on before the rest of the heaters have a chance to warm up. The UY41 only consumes 31 volts at 100 ma, I'm not sure how close I could get with a capacitive dropper, should I chose to eliminate it but maybe? You bring up a good point about the ground reference, I've heard this is also a reason NOT to retrofit a bridge rectifier into an AC/DC set that had a half wave selenium rectifier in it originally, and I think your thread confirms this, at least with regard to that Telefunken.
Regards
Arran

[morzh]

Also as I found out, the bridge does not bring voltage high enough (well, I knew the formulae, but then with filter cap it depends on the load also).
The doubler works real well, plus, yes, it lets the negative wire through which makes it the common point.
And I like the fact that simply changing the caps ratio you can variate the voltage fairly widely.


Oh...and if you want that comtrolled rise, you then probably should make the tube D2 (the horizontal) diode.

[Arran]

Hello Everyone;
I finally got around to taking the chassis out of this set today, vacuumed a large accumulation of dust off of it, and from the inside of the cabinet. The good news is that the output transformer, the IF transformers, and the oscilator coil test good. Most of the resistors are good, I think maybe two had drifted somewhat, even the big wire wound dropper tested good. According to the date printed on the filter condenser can it was likely manufactured in the fall of 1956, this model certainly had a long run.
The spider is loose on the Celestion made speaker, which is apparently a common issue with the ones in these sets, so I vacuumed up the dust on both sides of the cone, and on the basket, to prepare for regluing. I will need to shim around the voice coil with plastic strips, but hopefully will not have to remove the cone to do this.
Unfortunately a good percentage of the steel brackets of the dial mechanism, and the back plate behind the plastic dial diffuser, are rusty. Ironically the chassis itself is made out of sheet aluminum, but not this backing plate which does nothing but hold the dial lamps and a plastic diffuser. Apparently Bush Radio didn't believe in plating their hardware, every other screw into the Bakelite cabinet was also rusty, so I needed to squirt each screw with WD40 to get them out. It's very odd because I have never encountered this issue with machine screws in a North American Bakelite cabinet, did the Bakelite itself cause them to corrode?
I still need to test the tubes, I only have one tube tester with a socket for testing Rimlock tubes. They were a Phillips invention, they have eight pins, with a locating stud on the side of the base, sort of a transitional glass tube type between the octal and Phillips Loctal style tubes, and the seven and nine pin tubes Phillips used up until the end of the tube era. Hopefully they test good or I will have to put this set into drydock for a while, and won't get to try out a voltage doubler scheme. The heater string adds up to 116.6 volts, plus two 3.5 volt dial lamps, with a 75 Ohm shunt resistor in parallel with them.
Regards
Arran

[Arran]

Hello Everyone;
  I started cleaning up parts of this radio, started on the knobs, they are sort of a three piece affair, they have a plastic ring that is pressed onto the back as a skirt, and a polished metal ring inlayed into a groove in the face of each knob. The function is also engraved into the face outside of the metal ring, on off volume, and tubing, this was filled with paint but I can't tell what colour. I don't know why they even bothered with the clear plastic skirt rings, behind those is a felt washer so you can barely notice them, but I will put the rings back on regardless. Next up is washing, and polishing up the Bakelite cabinet, and unearthing the one tube tester I have that will test Phillips Rimlock tubes
Regards
Arran

[Arran]

Hello All;
I found this interesting thread on a British radio forum about the Bush DAC-90, and modifications one fellow made. Some of the mods make sense, others not so much, like adding a 220 uf cap as a cathode bypass on the UBC41 first audio tube, assuming this was not in error. Most of the North American AC/DC sets don't even have a cathode bias resistor on the first audio tube at all, it goes directly to the B-. What I find odd is that of all the mods he made he didn't add a cathode bypass capacitor on the UL41 power output tube in parallel with the 150 Ohm cathode bias resistor there, nor did he change the B- wiring to make it into a floating chassis set, give the high line voltage they use there.


https://www.vintage-radio.net/forum/show...hp?t=35784

https://www.vintage-radio.net/forum/atta...1230506945

Regards
Arran