DAC90 Suspect Output Valve - UK Vintage Radio Repair and Restoration Discussion Forum

coopzone · 6th Dec 2018, 2:36 pm

Hello,

I'm just revisiting a DAC90 I have. It's been working ok for months now, but with a very annoying whistle/feedback/oscillation just for a few seconds every so often. Almost not enough to bother with - it just puts my teeth on edge.

I've got the radio on the bench at the moment, just completed a re-measure of the resistors etc. voltages and joints in general. everything is ok.

During this i noticed the noise started for a few seconds then stopped as i was poking around the CL33 output valve. In fact if i tap the chassis near that end you get the noise for a few seconds (or less more like a 'ping' sound) then it's ok again. Having tapped all the components around the area the CL33 seems to be the cause. In fact if you tap the top class of the valve you get 'ping' and sometimes it develops into a continues sound ('howl') then you tap again and it stops.

The problem is i'm not 100% sure (95% maybe) that it's just this valve, so before I order a newer one I was hoping someone may be able to confirm this type of behaviour?

Derek

Herald1360 · 6th Dec 2018, 2:50 pm

Could be the valve is microphonic and the feedback is audio from the speaker. Try (carefully it's a live chassis set) an external speaker some way from the set and see if the problem persists. Is the valve bottle loose in its base?

Freya · 6th Dec 2018, 2:51 pm

Its going microphonic by the sounds of it.

coopzone · 6th Dec 2018, 3:05 pm

Just connected an external speaker and it seems better still goes ping but does not persist for seconds.

I've also noticed that when the valve is hot, after it's been on for 20 minutes or so, the issue is significantly less noticeable - more or less the same as using an external speaker you get a ping but no howl for a few seconds.

So now i just have to scrounge round for another valve to try in it's place - it would be the expensive one!

Derek

coopzone · 6th Dec 2018, 3:08 pm

Don't suppose you can fix this microphonic thing can you? no shoving it in the oven for an hour or other voodoo you can try?

Derek

RobRusbridge · 6th Dec 2018, 9:20 pm

CL33 is quite expensive. If you want a quick try-out, see if you've got a 20P3 in your box of old TV valves. Same pin-out as CL33, same heater current, small and cheap. Not a permanent replacement perhaps but you'd be unlucky to have a microphonic 20P3 as well as a microphonic CL33. It would prove a point, cheaply.

McMurdo · 6th Dec 2018, 9:24 pm

you see alot of microphonic valves in guitar combo amplifiers (ones with speakers built in). The 'ping' is one of those effects that once heard, you recognise instantly from then on. You can actually get it with other components too BTW, though rare.

Herald1360 · 6th Dec 2018, 10:51 pm

A UCL83 plus adaptor with heater shunt and maybe cathode resistor mod could be used or even an EL95 with heater ballast and similar changes.

A 12A6 has same base but would need heater series and shunt resistors and possible bias change.

Plenty of scope for bodgery err ingenuity I mean!

mark2collection · 6th Dec 2018, 11:00 pm

My black DAC90 had these symptoms.

Turned out the coating on one of the valves wasn't making contact with a fine wire protruding from it's base.

A teeny-tiny drop of conductive paint cured it.

Worth a look, from memory it was the valve between the I.F.T's.

Mark

julie_m · 7th Dec 2018, 2:52 am

Or a 35L6GT output valve from an "All American Five" set (35V @ 150mA), again with a heater shunt resistance (680Ω, 3W) to make up the extra 50mA.

Boater Sam · 7th Dec 2018, 3:51 am

Its usually not the output valve but the IF amp or even the mixer.

Check that pin 1 of these does connect with the coating on the valve. The aluminium foil wrap and grounded to pin one may work if the valves are shedding.

coopzone · 7th Dec 2018, 9:07 am

Thanks for the replies, I can get 20P3 cheaper than a CL33 but I can't find a data sheet for it? Does anyone have this sheet or know where I can find it? I can find 20P2 but not P3?

Derek

Nuvistor · 7th Dec 2018, 9:19 am

http://www.r-type.org/exhib/aaa0514.htm

Freya · 7th Dec 2018, 9:19 am

It's in the valve museum here
http://www.r-type.org/exhib/aaa0514.htm
Data at bottom of page

coopzone · 7th Dec 2018, 3:10 pm

20P3 seems to have a voltage rating of 20V for the heater so I assume I would need to add a series resistor (75ohm 5watt) to compensate?

On the other hand the 20P4 is a 38V heater ~35V but the anode is connected to the top cap. May try one of these - if I can find one at the right price.

Derek

cathoderay57 · 8th Dec 2018, 11:03 pm

Are you sure it's the CL33 that has gone microphonic? It can be surprising how far along the chassis tapping vibration can travel. Does it still happen with the volume control at minimum? If not then I would suspect the EBC33 as a more likely candidate for microphony. Cheers, Jerry

coopzone · 10th Dec 2018, 12:04 pm

I finally decided to risk it a got another CL33 from ebay.

It works! in fact the (newer) valve checks out on my home made valve tester, it's a little low on emissions but well up in the 88% mark - so not bad.

Picture of the old (left) and newer (right) attached. A bit of a difference !

Derek

ms660 · 10th Dec 2018, 12:49 pm

That valve looks like a Bentley Acoustic re-rolled special....A 20P3?

Lawrence

coopzone · 10th Dec 2018, 1:49 pm

It's printed as a CL33, but you can't see that very well in the photo. It is Bentley and it does have a round label with acoustic in the writing. No other mods are present to support the lower heater voltage. So it's either re-worked to be 35v or all the other valves have been run over voltage for some time!

Either way it's no good now except as a quick test valve, you only have to knock it and it's howling away

Derek

ms660 · 10th Dec 2018, 1:50 pm

It looks like a re-rolled 20P3 to me, Bentley weren't all that fussed.

Lawrence.

6th Dec 2018, 2:36 pm	#1
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	DAC90 Suspect Output Valve Hello, I'm just revisiting a DAC90 I have. It's been working ok for months now, but with a very annoying whistle/feedback/oscillation just for a few seconds every so often. Almost not enough to bother with - it just puts my teeth on edge. I've got the radio on the bench at the moment, just completed a re-measure of the resistors etc. voltages and joints in general. everything is ok. During this i noticed the noise started for a few seconds then stopped as i was poking around the CL33 output valve. In fact if i tap the chassis near that end you get the noise for a few seconds (or less more like a 'ping' sound) then it's ok again. Having tapped all the components around the area the CL33 seems to be the cause. In fact if you tap the top class of the valve you get 'ping' and sometimes it develops into a continues sound ('howl') then you tap again and it stops. The problem is i'm not 100% sure (95% maybe) that it's just this valve, so before I order a newer one I was hoping someone may be able to confirm this type of behaviour? Derek

6th Dec 2018, 2:50 pm	#2
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Dac90 Suspect Output Valve Could be the valve is microphonic and the feedback is audio from the speaker. Try (carefully it's a live chassis set) an external speaker some way from the set and see if the problem persists. Is the valve bottle loose in its base? __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

6th Dec 2018, 2:51 pm	#3
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: Dac90 Suspect Output Valve Its going microphonic by the sounds of it. __________________ Stephen _________"It`s only an old telly" ___

6th Dec 2018, 3:05 pm	#4
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: Dac90 Suspect Output Valve Just connected an external speaker and it seems better still goes ping but does not persist for seconds. I've also noticed that when the valve is hot, after it's been on for 20 minutes or so, the issue is significantly less noticeable - more or less the same as using an external speaker you get a ping but no howl for a few seconds. So now i just have to scrounge round for another valve to try in it's place - it would be the expensive one! Derek

6th Dec 2018, 3:08 pm	#5
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: Dac90 Suspect Output Valve Don't suppose you can fix this microphonic thing can you? no shoving it in the oven for an hour or other voodoo you can try? Derek

6th Dec 2018, 9:20 pm	#6
RobRusbridge Retired Dormant Member Join Date: Apr 2018 Location: St. Austell, Cornwall, UK. Posts: 67	Re: DAC90 Suspect Output Valve CL33 is quite expensive. If you want a quick try-out, see if you've got a 20P3 in your box of old TV valves. Same pin-out as CL33, same heater current, small and cheap. Not a permanent replacement perhaps but you'd be unlucky to have a microphonic 20P3 as well as a microphonic CL33. It would prove a point, cheaply.

6th Dec 2018, 9:24 pm	#7
McMurdo Dekatron Join Date: Jun 2009 Location: Staffordshire Moorlands, UK. Posts: 5,269	Re: DAC90 Suspect Output Valve you see alot of microphonic valves in guitar combo amplifiers (ones with speakers built in). The 'ping' is one of those effects that once heard, you recognise instantly from then on. You can actually get it with other components too BTW, though rare. __________________ Kevin

6th Dec 2018, 10:51 pm	#8
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: DAC90 Suspect Output Valve A UCL83 plus adaptor with heater shunt and maybe cathode resistor mod could be used or even an EL95 with heater ballast and similar changes. A 12A6 has same base but would need heater series and shunt resistors and possible bias change. Plenty of scope for bodgery err ingenuity I mean! __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

6th Dec 2018, 11:00 pm	#9
mark2collection Hexode Join Date: Jul 2010 Location: Royal Berkshire, UK. Posts: 470	Re: DAC90 Suspect Output Valve My black DAC90 had these symptoms. Turned out the coating on one of the valves wasn't making contact with a fine wire protruding from it's base. A teeny-tiny drop of conductive paint cured it. Worth a look, from memory it was the valve between the I.F.T's. Mark

7th Dec 2018, 2:52 am	#10
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: DAC90 Suspect Output Valve Or a 35L6GT output valve from an "All American Five" set (35V @ 150mA), again with a heater shunt resistance (680Ω, 3W) to make up the extra 50mA. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

7th Dec 2018, 3:51 am	#11
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: DAC90 Suspect Output Valve Its usually not the output valve but the IF amp or even the mixer. Check that pin 1 of these does connect with the coating on the valve. The aluminium foil wrap and grounded to pin one may work if the valves are shedding.

7th Dec 2018, 9:07 am	#12
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: DAC90 Suspect Output Valve Thanks for the replies, I can get 20P3 cheaper than a CL33 but I can't find a data sheet for it? Does anyone have this sheet or know where I can find it? I can find 20P2 but not P3? Derek

7th Dec 2018, 9:19 am	#13
Nuvistor Dekatron Join Date: Aug 2013 Location: Wigan, Greater Manchester, UK. Posts: 9,431	Re: DAC90 Suspect Output Valve http://www.r-type.org/exhib/aaa0514.htm __________________ Frank

7th Dec 2018, 9:19 am	#14
Freya Octode Join Date: Jan 2009 Location: Falmouth, Cornwall, UK. Posts: 1,989	Re: DAC90 Suspect Output Valve It's in the valve museum here http://www.r-type.org/exhib/aaa0514.htm Data at bottom of page __________________ Stephen _________"It`s only an old telly" ___

7th Dec 2018, 3:10 pm	#15
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: DAC90 Suspect Output Valve 20P3 seems to have a voltage rating of 20V for the heater so I assume I would need to add a series resistor (75ohm 5watt) to compensate? On the other hand the 20P4 is a 38V heater ~35V but the anode is connected to the top cap. May try one of these - if I can find one at the right price. Derek

8th Dec 2018, 11:03 pm	#16
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,382	Re: DAC90 Suspect Output Valve Are you sure it's the CL33 that has gone microphonic? It can be surprising how far along the chassis tapping vibration can travel. Does it still happen with the volume control at minimum? If not then I would suspect the EBC33 as a more likely candidate for microphony. Cheers, Jerry

10th Dec 2018, 12:04 pm	#17
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: DAC90 Suspect Output Valve I finally decided to risk it a got another CL33 from ebay. It works! in fact the (newer) valve checks out on my home made valve tester, it's a little low on emissions but well up in the 88% mark - so not bad. Picture of the old (left) and newer (right) attached. A bit of a difference ! Derek Attached Thumbnails

10th Dec 2018, 12:49 pm	#18
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: DAC90 Suspect Output Valve That valve looks like a Bentley Acoustic re-rolled special....A 20P3? Lawrence Last edited by ms660; 10th Dec 2018 at 12:56 pm.

10th Dec 2018, 1:49 pm	#19
coopzone Hexode Join Date: Jan 2018 Location: Cannock, Staffordshire, UK. Posts: 268	Re: DAC90 Suspect Output Valve It's printed as a CL33, but you can't see that very well in the photo. It is Bentley and it does have a round label with acoustic in the writing. No other mods are present to support the lower heater voltage. So it's either re-worked to be 35v or all the other valves have been run over voltage for some time! Either way it's no good now except as a quick test valve, you only have to knock it and it's howling away Derek

10th Dec 2018, 1:50 pm	#20
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: DAC90 Suspect Output Valve It looks like a re-rolled 20P3 to me, Bentley weren't all that fussed. Lawrence.