DAC90A distorted output

[Skywave]

Not my usual area, but one recently came my way at a price that I couldn't say no to.
Fortunately, the case is in good condition and it came with the original mains plug + lead. An initial exam. showed that the 'usual suspects' required replacing - and were promptly replaced. Having done that, the audio output was then found to be distorted on full volume - and that is because the main HT line from the cathode of the rect. is low at about 140 v. The 10 kΩ series dropper / filter is O.K.; the filter dual cap. has been replaced. Suspecting a tired UY41, I tried replacing it with a salvage 'pulled' one: no change. Tried another: nope - still too low. I reckon I need a good 'NOS' one - which I don't have.

Any chance someone might have a UY41 that they would care to sell to me? And a couple of 3.15 v. 0.15 amp. dial bulbs, too, please?

Incidentally. I have read David's Guide to the DAC90A and have obtained the Trader sheet.

Al. / May 7, 2013 //

[camallison]

Google is your friend - there are some on Fleabay at £6.25 +PP that are NOS.

Colin

[Eddystonepete]

Hi Al it could be the UL41 taking too much current and pulling down the HT. UL41s are famous for leaking G1 to G2 puting pos. on G1. Also look at the spare pin (cant remember which one) which Bush used as an anchor point. Cut the wires off the valveholder pin.
Peter

[Skywave]

Thank you Peter.
The UL41: I haven't checked out the 'unused / spare pin' yet, but the d.c. voltage at grid 1 is zero - mind you, it wasn't prior to changing the coupling cap.! Its un-bypassed cathode resistor is 150 Ω: there is about 6v. quiescent across it: that gives a standing current of about 40 mA, which sounds about right for a single-ended class-A O/P stage.
I hooked the 'scope up to the anode of the UL41. With the volume turned well up, I could see that although the tops of the speech waveform were fairly clean, the bottom of the waveform was clearly flattened, strongly suggestive that the valve has insufficient anode volts: i.e. insufficient HT.

Al.

[Eddystonepete]

Hi Al, OK red herring but its worth mentioning Bush practice of using what seem to be spare pins on valve bases as convenient wiring points. OK on some valves but pins 3 and 4 on a UL41 are internal connections and shouldnt be connected to anything. Some Bush models used this and caused pos to be applied to G1. If you find 3 or 4 connected on a UL41, its probably wise to snip the tag (not the valve pin!) to prevent future probs. Agree with you diagnosis about low HT, try a silicon diode with a series resistor across the UY41?

Peter