UL41 grid (on DAC90A) - how high is 'too high'? - UK Vintage Radio Repair and Restoration Discussion Forum

TimS · 9th Aug 2007, 3:50 pm

How high a grid voltage on here is 'too high'? Mine is +0.3V initially, but does increase to about +0.5V after a few minutes use.

There is a hum - quite noticeable when not tuned in or with the volume down - that is vastly reduced by grounding the UL41 grid.

-- Tim.

paulsherwin · 9th Aug 2007, 4:21 pm

That grid voltage is fine for a UL41 with a few miles on the clock.

Grounding the o/p valve grid will kill hum from many causes so it's not telling you very much.

Paul

TimS · 9th Aug 2007, 4:58 pm

Thanks, and good to hear given that there appears to be not that many UL41s left.

Just a UY41 rectifier to buy then (no grey area here - the present one doesn't do anything, whereas the UY41 swapped from my Bush AC11 does!) - a fairly straightforward set to fix.

-- Tim.

Sideband · 9th Aug 2007, 6:34 pm

As Paul says, a slight +ve grid voltage with respect to chassis is OK because it will still be sufficiently -ve with respect to cathode. I would be inclined to monitor this voltage over an hour or so to make sure it doesn't go too high though. If it stays at it's present level, that's fine but if it creeps up to over a volt or so then I would look around for a better valve. The output transformers are rather delicate and don't take kindly to excess current.

Rich.

TimS · 9th Aug 2007, 11:31 pm

Have checked it now over 1.5 hrs. Although it rose from 0.3V to 0.5V within 20 minutes it hasn't risen since.

Sideband · 10th Aug 2007, 9:26 am

I'd say you are OK then

.

Rich.

Studio263 · 10th Aug 2007, 4:37 pm

The important voltage is of course that between the (control) grid and the cathode, this is where the bias is. You will probably find the cathode voltage has risen by a similar amount too so the valve will be in roughly the same part of it's characteristic. You could always try reducing the value of the grid leak resistor (assuming that it is still the correct value to start with) if the valve has got a bit leaky inside.

ppppenguin · 10th Aug 2007, 4:47 pm

Worth remembering that an ordinary analogue meter will give a rather optimistic view of the grid voltage since its internal resistance on a low voltage range will be rather smaller than the grid leak resistor. A DVM or VVM with its high input impedance will give a true reading.

9th Aug 2007, 3:50 pm	#1
TimS Pentode Join Date: Jun 2005 Location: Forres, Moray, UK. Posts: 194	UL41 grid (on DAC90A) - how high is 'too high'? How high a grid voltage on here is 'too high'? Mine is +0.3V initially, but does increase to about +0.5V after a few minutes use. There is a hum - quite noticeable when not tuned in or with the volume down - that is vastly reduced by grounding the UL41 grid. -- Tim. Last edited by TimS; 9th Aug 2007 at 4:02 pm.

9th Aug 2007, 4:21 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,936	Re: UL41 grid (on DAC90A) - how high is 'too high'? That grid voltage is fine for a UL41 with a few miles on the clock. Grounding the o/p valve grid will kill hum from many causes so it's not telling you very much. Paul

9th Aug 2007, 4:58 pm	#3
TimS Pentode Join Date: Jun 2005 Location: Forres, Moray, UK. Posts: 194	Re: UL41 grid (on DAC90A) - how high is 'too high'? Thanks, and good to hear given that there appears to be not that many UL41s left. Just a UY41 rectifier to buy then (no grey area here - the present one doesn't do anything, whereas the UY41 swapped from my Bush AC11 does!) - a fairly straightforward set to fix. -- Tim.

9th Aug 2007, 6:34 pm	#4
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: UL41 grid (on DAC90A) - how high is 'too high'? As Paul says, a slight +ve grid voltage with respect to chassis is OK because it will still be sufficiently -ve with respect to cathode. I would be inclined to monitor this voltage over an hour or so to make sure it doesn't go too high though. If it stays at it's present level, that's fine but if it creeps up to over a volt or so then I would look around for a better valve. The output transformers are rather delicate and don't take kindly to excess current. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

9th Aug 2007, 11:31 pm	#5
TimS Pentode Join Date: Jun 2005 Location: Forres, Moray, UK. Posts: 194	Re: UL41 grid (on DAC90A) - how high is 'too high'? Have checked it now over 1.5 hrs. Although it rose from 0.3V to 0.5V within 20 minutes it hasn't risen since.

10th Aug 2007, 9:26 am	#6
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,567	Re: UL41 grid (on DAC90A) - how high is 'too high'? I'd say you are OK then . Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

10th Aug 2007, 4:37 pm	#7
Studio263 Octode Join Date: Apr 2007 Location: Hampshire, UK. Posts: 1,576	Re: UL41 grid (on DAC90A) - how high is 'too high'? The important voltage is of course that between the (control) grid and the cathode, this is where the bias is. You will probably find the cathode voltage has risen by a similar amount too so the valve will be in roughly the same part of it's characteristic. You could always try reducing the value of the grid leak resistor (assuming that it is still the correct value to start with) if the valve has got a bit leaky inside.

10th Aug 2007, 4:47 pm	#8
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: UL41 grid (on DAC90A) - how high is 'too high'? Worth remembering that an ordinary analogue meter will give a rather optimistic view of the grid voltage since its internal resistance on a low voltage range will be rather smaller than the grid leak resistor. A DVM or VVM with its high input impedance will give a true reading.