Deluded or daft? DAC10 anomaly?

[boater sam]

Please folk, tell me I am not cracking up.

Bush DAC10, working off the Bush manufacturers circuit etc. Not the trader sheet.

This set has plenty of output when injecting into the UL41 grid but next to nothing when injecting into the UBC41 grid, the set is totally deaf.

So I look for a reason why the triode is not amplifying. Valve swap produces the same deaf result. Anode voltage is low, cathode voltage is zero which suggested that the UBC41  cathode bypass cap was SC.

Not so, but there is a short cathode to chassis.

Looking at the diagram and the set I find that the braid on the screened wire to the top of the volume pot is grounded at both ends, on the pot and  effectively on the IF transformer can to chassis but also it is connected to the cathode of the triode of UBC41 removing any cathode bias!
As the cathode resistor and cap are shared with the UF41, there can be no bias on this valve either.

How did this ever work? Is the set wrong and the diagram also? Am I loosing my marbles?

[Mike Watterson]

The "earthy" end of the pot is connected to the shared cathode resistor of UF41 and UBC41. My DAC10 works well, but I've not checked how it's wired. If the valves are good and the grid resistors are high enough value it will work by "grid leak" bias method, the current flow creates a negative charge on the grids. You can ground a triode cathode preamp if there is maybe a 10 M Ohm grid resistor.

Obviously if your DAC10 is wired in error, it may work but the bias is wrong and gain would be low.

This is most obvious on a heptode/octode oscillator, where you can feed grid to a low pass filter (no diodes) and use the -8V approx to class AB bias a push-pull output as on the Philips Colette battery valve set.

[boater sam]

The "earthy" end of the pot is connected to the shared cathode resistor of UF41 and UBC41. My DAC10 works well, but I've not checked how it's wired. If the valves are good and the grid resistors are high enough value it will work by "grid leak" bias method, the current flow creates a negative charge on the grids. You can ground a triode cathode preamp if there is maybe a 10 M Ohm grid resistor.

Obviously if your DAC10 is wired in error, it may work but the bias is wrong and gain would be low.

This is most obvious on a heptode/octode oscillator, where you can feed grid to a low pass filter (no diodes) and use the -8V approx to class AB bias a push-pull output as on the Philips Colette battery valve set.

All true but the earthy end of the pot is earthed by a link onto the pot body, effectively shorting the 2 cathodes to ground.

[Mike Watterson]

Someone has made a mistake. The pot body can't be connected to the "earthy" end of the pot unless that wire below (to shared cathode resistor) is missing.

I looked at the photos I took of my chassis and cropped out this part.
I don't see a link to the pot casing.  It would make no sense.
   

There is no evidence of solder on the casing. There is no screen connected at the pot. The visible pale brown fat wire in the foreground is surely the "earthy" end. I suspect the visible yellow cap is C20 (Trader numbering).

Such a strange arrangement would be mentioned on Trader Sheet 1002.

Either someone got at it or it's an original mistake. Certainly it may have worked somewhat with a strong signal. R8 & R13 (Trader) might ensure the valves are not damaged, unlike shorting the cathode resistor on an output stage.

[boater sam]

Thank you Mike.

It all looks original, the pot is the same as yours in the picture, the wiring is the same, colours and layout.There is a single bare wire off the tag with the braid down to the side of the put can, again it looks like original as the wire is through the hole in the tag and the braid soldered on top.

Its very strange. I have another DAC10 somewhere. If I can find it I will have a look at that one.

The trader sheet doesn't show the screened lead but the maker's sheet details it exactly including the braid connected to the cathodes and not the chassis which again is unusual and I can see no reason for this rather than conventionally grounding the braid to the chassis.

Its working now with a low distorted output. The HT is low, only 146v with 50v on the anode of the UBF41 triode but there will be several caps leaking. I have changed the cathode cap ,it was toast, and the grid cap on the UL41 had a good leak, further I have yet to explore.

Thanks for the help.

Sam.

[Mike Watterson]

A screen on a wire on the DAC10 could be earthed at the IFT can, but should not connect to anything at the pot end. The earthy end of pot should not be linked to body on the DAC10. Certainly a screen on the audio of DAC10 could have originally been to the cathode resistor. But using both options is totally wrong.

In general internal screens in equipment should only connect at one end, not both. The only exception might be the signal and earth from a tag strip under a record turntable to the pre-amp or volume control. Maybe tape heads.

[boater sam]

Now this screen is not earthed at either end. The manufacturer's diagram is correct providing that the screen is not connected to the IF transformer or the metal body of the volume control. It is not clear in the circuit diagram. The incorrect ground link from the pot earthy end to the metal body has been removed enabling the cathodes to receive bias. The braid is now as per the diagram the link from the cathodes to the volume control track bottom end.

The screen is connected as shown to the cathodes of V4 and V5. I am wondering if the capacitance of the short screened cable plays some part in a feedback route to the cathode. I have never seen this before.

Further work on this set shows that every tubular wax capacitor is beyond use. They are all TCC caps. I did find that the modulation suppressor across the mains had been disconnected at one end. It has been replaced with an X2 cap. This is the only signs other than a new mains lead of any previous repairs to this set.

All the said caps have been replaced with modern film caps, its not a rare set and there is no point in restuffing the caps with more expensive poly caps.

It now works very well subject to the usual poor performance of the inside out Celestion loudspeaker. I have never found one of these that sounds as well as a conventional one.

One small crack in the top of the front case, where the handle is, has been repaired with superglue and a backing plastic re-enforcement glued inside. This repair will be almost invisible when the case is polished.

[Mike Watterson]

Glad you getting sorted. I don't think the screen is any sort of feedback, it's just more effective to cathode, as amplification is relative to the cathode, not chassis.
TCC are just as rubbish as Hunts, or any other paper dielectric. There are hermetically sealed paper caps in cans that 80 years later are not bad, but the TCC aluminium can types with two rubber bungs are not hermetically sealed.

[boater sam]

Another DAC10 now cleaned up, small crack in the handle of the front case repaired and hidden and the electronics sorted.
Reception is very good, tracking spot on.
All the buttons work and tune in properly.

Whilst finishing this off I realised that the IF transformers are much better than the DAC90 and DAC90A in that they have horizontal slugs in separate formers so they don't fall off the tuning screws.

[Mike Watterson]

While it's basically a similar design to DAC90a, I think the DAC10 with the Bakelite back instead of card on DAC90 or DAC90a and the three presets makes this a superior set to the DAC90a.
I've never understood why there is such a premium on the DAC90a and the Kolster Brands Toaster. In their day they were entry level sets. I think the DAC10 is nicer. I dodn't have original voltage bulbs, but they are the correct current rating with parallel ballast resistors adjusted. Perhaps very little brighter despite being higher voltage (6V rather than 3.5V) because they are under run due to ballast parallel resistors.