BUSH DAC90A output valve replacement. - UK Vintage Radio Repair and Restoration Discussion Forum

jesscorbin2006 · 16th Sep 2009, 9:35 am

I'm restoring a Bush DAC90A. The general impression is that the UL valve is short lived and unreliable for long term use. As this radio in my home will be getting regular use I don't want to be regularly buying new UL valves.

I am looking at using another output pentode ! I don't think this would be a major problem, but any help concerning suitable substitutes would be greatly appreciated. The reliability and longevity of the valve concerned are a major factor. Obviously not having to replace the output transformer and keeping the LS as original as possible is a major consideration. I doubt that the actual valve substitution would be such an horrendous prob?

Hmm time to get the valve data books out..
Thanks JC

julie_m · 16th Sep 2009, 11:12 am

UL84 should do the job. The base is wrong, but the ratings seem close enough and this valve is still reasonably-well available.

If originality is important to you, you could fashion an adaptor using the base of a scrap B8A valve and a noval socket.

kalee20 · 16th Sep 2009, 12:55 pm

The UL41 doesn't have the best reputation, no. But it does not die by failing emission - rather, by internal deposits which form on the glass inside and cause leakage between pins. Clearly, leakage anode-control grid or screen grid-control grid is bad news.

Good cooling rduces this massively. In the DAC90a the UL41 is under-run, which helps.

Make sure 'THAT' capacitor is replaced, as leakage in this will put positive voltage on the UL41 control grid, making it run hotter, causing the above problems.

The DAC90a uses one of the pins on the UL41 valveholder as a convenient wiring tag, for grid related circuitry. Isolate this pin! Because then, internal leakage to this pin is rendered irrelevant.

As ajs derby says, the UL84 should work. But if you have a working UL41 and you do the above, you should find it lasts ages. As far as I know, the UL41 in my own DAC90a has never been replaced.

paulsherwin · 16th Sep 2009, 1:07 pm

You have to remember that UL41s were in use at the height of the radio listening era. The radios would be used for hours and hours every day, much as TVs are used today. It's hardly surprising that the original 1950s UL41s fitted to these radios are getting a bit tired now.

The design was modified throughout the valve's life and changes were made to reduce the heater-cathode leak problem, which is the main reason they fail. Late production UL41s from the 60s are very tough indeed (you can spot them because they don't have an obvious glass collar at the bottom and look like a B9A valve with a locating pip.)

There's certainly no reason to change a UL41 if it is working well. If it needs replacing, a late production NOS UL41 will be fine if it can be obtained at a reasonable price (you can usually find them for £10-15.) A conversion to a UL84 is quite easy if you decide to do this - Tim Pullin describes one way to do it here.

Paul

Sideband · 16th Sep 2009, 1:37 pm

Paul's right. Another thing you have to remember is that the old paper capacitors were unreliable in the long term and my bet is that many UL41's failed before their time because of the audio coupling capacitor going leaky. Replacement capacitors of the time would have been just as unreliable in the long term.

Now of course, we have polyester caps which seem to last almost indefinately in these sorts of positions and in fact I replaced a UL41 in a DAC90A some 15 years ago for a friend who now lives in Ireland. At the same time, I replaced all the leaky old paper capacitors. It has just had a replacement UY41 rectifier (changed by a local collector out there) and he confirmed that the UL41 was still in good condition. The radio has fairly regular use.

I think that replacing the old waxies for modern polyester capacitors is greatly prolonging the lives of existing valves and new replacements will probably last many years longer than the originals simply because they are not going to be stressed after a few years due to leaky capacitors.

Rich.

jesscorbin2006 · 16th Sep 2009, 1:53 pm

OK seems the ideal is to go through all the preliminaries such as checking, cleaning and replaceing THAT cap ! plus the others as you guys have pointed out ! THANKYOU and then deciding where to go AFTER I have run the set with the UL41
I AM going to use Kalee's circuit re the mods made and will progress from there..
It won't be a quick project because of my health probs but the web WILL show the various stages 'before' after pics et .. as a writer writing up the stages won't be a prob and submissions from ANYONE on this site which WILL be linked to WILL be welcome !
What I hope will be the end product will be a deserving source of inspiration to those thinking of renovating one of these or simply as a good read..

geofy · 16th Sep 2009, 3:05 pm

The Mazda handbook gives as a substitution a 10P18 which has the 45 Volt heater.

It requires the fitting of a B9A socket and rewiring as the pin layout is obviously different.

There is a higher Anode resistance of 23K as opposed to UL41 3K, not sure just what the difference will make in practice.

But the pin layout is then the same for UL84 (45V heater) or

EL84 (6.3V h) even PL82 (16.5V h) if suitable resistor to maintain the 45 Volt drop or a separate heater supply for the output valve, with a resistor for the rest of the series chain to drop the 45 Volts no longer going to the OP heater.

The ra of the PL82 is only 4K, close to the UL41.

Geof

Leon Crampin · 16th Sep 2009, 3:39 pm

Be careful not to confuse ra (anode impedance) and Ra (load impedance). Ra is always < ra.

The output transformer is fragile in the DAC90A being wound with very fine wire. A higher value of anode current is not a good idea and biasing a valve such as the UL84 to a lower current gives non linearity - although you would not be likely to notice this much in such a low-fi application.

The UCL82 (pentode section) on an adapter is a better replacement - add about 100 Ohms extra bias in the adapter to optimise working. They are cheap and plentiful and therefore suited to heavy usage. You can always fit a UL41 when originality becomes important, but I would agree that the limited supplies of good UL41s are worth preserving.

Any replacement with significantly differing heater requirements makes the job too complex for any possible benefits.

A Mazda 10P13 is another possibility which will work as a straight replacement but I have not found the results to be fully satisfactory.

Leon.

richrussell · 16th Sep 2009, 3:51 pm

Another option is the CV1977 which is a direct replacement for the UL41, and more readily available. I've used a couple of these in my sets and they work well.

Rich.

JHGibson · 16th Sep 2009, 3:53 pm

If you increase the value of the UL41 cathode bias resistor by 50%, the plate current will be reduced and the valve will run cooler. The reduction in audio volume will not be great and the valve will last longer.
John.

paulsherwin · 16th Sep 2009, 4:17 pm

Quote:

Originally Posted by geofy

The Mazda handbook gives as a substitution a 10P18 which has the 45 Volt heater.

It requires the fitting of a B9A socket and rewiring as the pin layout is obviously different.

The standard 1960s serviceman repair was to remove the B8A socket, fit and rewire a B9A socket, fit a 10P18/UL84 and bump up the cathode resistor until the anode current was reasonable. Nowadays we would view this as a bit philistine, but it's easy enough to make the modification reversible as described in Tim Pullin's writeup which I linked to above.

As John says, it's certainly worth considering increasing the cathode resistor somewhat to reduce the anode current even below what was originally specified. This will reduce the maximum volume, but this probably won't be detectable and the valve will run cooler and last longer. There will be less stress on the output transformer too.

Paul

geofy · 16th Sep 2009, 4:33 pm

Quote:

Originally Posted by Leon Crampin

Be careful not to confuse ra (anode impedance) and Ra (load impedance). Ra is always < ra.

Leon.

Thanks Leon, Upper case always DC conditions. Lower case always AC conditions (valves and transistors), think that is correct.

Quote:

Originally Posted by geofy

The Mazda handbook gives as a substitution a 10P18 which has the 45 Volt heater.

Even the handbook just says check operating conditions, not much advice!

Steve_P · 16th Sep 2009, 4:33 pm

Or clear the leak on the UL41. It's normally caused by grot in the base. Why not do this...

https://www.vintage-radio.net/forum/...highlight=ul41

Also clear Pin 4. It is used as an anchor and can cause problems.

Cheers,

Steve P.

Sideband · 16th Sep 2009, 9:19 pm

Interesting! I'm going to take a leaky UL41 in to work and zap it on the insulation tester. It's variable up to 4kV so I can crank it up slowly. I'll post the results!

Rich.

kalee20 · 17th Sep 2009, 5:36 pm

Please do so Rich - on the appropriate thread of course! - I'd be most interested to see how others get on.

I think that (with the exception of the use of pin 4 as an anchoring point), the DAC90a gives the UL41 a fairly benign life, so they should last a good long time.

G4XWDJim · 17th Sep 2009, 6:07 pm

The 10P13 is a direct plug in replacement I believe. I don't have a data book handy to back this up I'm afraid so those with books handy may find it worth checking out.

Jim

Leon Crampin · 17th Sep 2009, 7:57 pm

The 10P13 has differing characteristics. It is capable of a lower maximum anode dissipation than the UL41 and is designed to run at a lower anode current (about 30 mA from memory).

I found that when fitted to a DAC90A, the results were poor because on this chassis the screen of the output valve runs at an unusually low voltage - about 90V, again from memory.

The resulting very low anode current gave poor output - not surprisingly, and non-optimal load matching. There were also hum problems with a few 10P13s I tried. The UCL82 works far more effectively.

Leon.

mickjjo · 17th Sep 2009, 8:25 pm

The UL46 is a direct substitute for the UL41, It is claimed to have better internal insulation for TV use but is not that common. Could be cheaper if fewer people are looking out for them though.

.

Regards, Mick.

ronbryan · 17th Sep 2009, 8:38 pm

The UL46 does have different base connections to the UL41, so a bit of re-wiring would be required.

Ron

mickjjo · 17th Sep 2009, 9:15 pm

Indeed the Valve Museum website does give different base connections for the UL41 and UL46:-

http://www.r-type.org/exhib/aan0001.htm

But confusingly my Mullard Maintenance Manual on page 231 under the entry for UL46 states,"For ratings, characteristics, base connections and dimensions, see type UL41"

. I'n sure I have at least one set with a UL46 in it and I never did any rewiring.....

.

Regards, Mick.

16th Sep 2009, 9:35 am	#1
jesscorbin2006 Retired Dormant Member Join Date: Sep 2009 Location: manchester Posts: 9	BUSH DAC90A output valve replacement. I'm restoring a Bush DAC90A. The general impression is that the UL valve is short lived and unreliable for long term use. As this radio in my home will be getting regular use I don't want to be regularly buying new UL valves. I am looking at using another output pentode ! I don't think this would be a major problem, but any help concerning suitable substitutes would be greatly appreciated. The reliability and longevity of the valve concerned are a major factor. Obviously not having to replace the output transformer and keeping the LS as original as possible is a major consideration. I doubt that the actual valve substitution would be such an horrendous prob? Hmm time to get the valve data books out.. Thanks JC

16th Sep 2009, 11:12 am	#2
julie_m Dekatron Join Date: May 2008 Location: Derby, UK. Posts: 7,735	Re: BUSH DAC90A output valve replacement. UL84 should do the job. The base is wrong, but the ratings seem close enough and this valve is still reasonably-well available. If originality is important to you, you could fashion an adaptor using the base of a scrap B8A valve and a noval socket. __________________ If I have seen further than others, it is because I was standing on a pile of failed experiments.

16th Sep 2009, 12:55 pm	#3
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: BUSH DAC90A output valve replacement. The UL41 doesn't have the best reputation, no. But it does not die by failing emission - rather, by internal deposits which form on the glass inside and cause leakage between pins. Clearly, leakage anode-control grid or screen grid-control grid is bad news. Good cooling rduces this massively. In the DAC90a the UL41 is under-run, which helps. Make sure 'THAT' capacitor is replaced, as leakage in this will put positive voltage on the UL41 control grid, making it run hotter, causing the above problems. The DAC90a uses one of the pins on the UL41 valveholder as a convenient wiring tag, for grid related circuitry. Isolate this pin! Because then, internal leakage to this pin is rendered irrelevant. As ajs derby says, the UL84 should work. But if you have a working UL41 and you do the above, you should find it lasts ages. As far as I know, the UL41 in my own DAC90a has never been replaced. Last edited by kalee20; 16th Sep 2009 at 1:10 pm.

16th Sep 2009, 1:07 pm	#4
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,957	Re: BUSH DAC90A output valve replacement. You have to remember that UL41s were in use at the height of the radio listening era. The radios would be used for hours and hours every day, much as TVs are used today. It's hardly surprising that the original 1950s UL41s fitted to these radios are getting a bit tired now. The design was modified throughout the valve's life and changes were made to reduce the heater-cathode leak problem, which is the main reason they fail. Late production UL41s from the 60s are very tough indeed (you can spot them because they don't have an obvious glass collar at the bottom and look like a B9A valve with a locating pip.) There's certainly no reason to change a UL41 if it is working well. If it needs replacing, a late production NOS UL41 will be fine if it can be obtained at a reasonable price (you can usually find them for £10-15.) A conversion to a UL84 is quite easy if you decide to do this - Tim Pullin describes one way to do it here. Paul

16th Sep 2009, 1:37 pm	#5
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,577	Re: BUSH DAC90A output valve replacement. Paul's right. Another thing you have to remember is that the old paper capacitors were unreliable in the long term and my bet is that many UL41's failed before their time because of the audio coupling capacitor going leaky. Replacement capacitors of the time would have been just as unreliable in the long term. Now of course, we have polyester caps which seem to last almost indefinately in these sorts of positions and in fact I replaced a UL41 in a DAC90A some 15 years ago for a friend who now lives in Ireland. At the same time, I replaced all the leaky old paper capacitors. It has just had a replacement UY41 rectifier (changed by a local collector out there) and he confirmed that the UL41 was still in good condition. The radio has fairly regular use. I think that replacing the old waxies for modern polyester capacitors is greatly prolonging the lives of existing valves and new replacements will probably last many years longer than the originals simply because they are not going to be stressed after a few years due to leaky capacitors. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

16th Sep 2009, 1:53 pm	#6
jesscorbin2006 Retired Dormant Member Join Date: Sep 2009 Location: manchester Posts: 9	Re: BUSH DAC90A output valve replacement. OK seems the ideal is to go through all the preliminaries such as checking, cleaning and replaceing THAT cap ! plus the others as you guys have pointed out ! THANKYOU and then deciding where to go AFTER I have run the set with the UL41 I AM going to use Kalee's circuit re the mods made and will progress from there.. It won't be a quick project because of my health probs but the web WILL show the various stages 'before' after pics et .. as a writer writing up the stages won't be a prob and submissions from ANYONE on this site which WILL be linked to WILL be welcome ! What I hope will be the end product will be a deserving source of inspiration to those thinking of renovating one of these or simply as a good read..

16th Sep 2009, 3:05 pm	#7
geofy Retired Dormant Member Join Date: Jun 2006 Posts: 1,798	Re: BUSH DAC90A output valve replacement. The Mazda handbook gives as a substitution a 10P18 which has the 45 Volt heater. It requires the fitting of a B9A socket and rewiring as the pin layout is obviously different. There is a higher Anode resistance of 23K as opposed to UL41 3K, not sure just what the difference will make in practice. But the pin layout is then the same for UL84 (45V heater) or EL84 (6.3V h) even PL82 (16.5V h) if suitable resistor to maintain the 45 Volt drop or a separate heater supply for the output valve, with a resistor for the rest of the series chain to drop the 45 Volts no longer going to the OP heater. The ra of the PL82 is only 4K, close to the UL41. Geof

16th Sep 2009, 3:39 pm	#8
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,873	Re: BUSH DAC90A output valve replacement. Be careful not to confuse ra (anode impedance) and Ra (load impedance). Ra is always < ra. The output transformer is fragile in the DAC90A being wound with very fine wire. A higher value of anode current is not a good idea and biasing a valve such as the UL84 to a lower current gives non linearity - although you would not be likely to notice this much in such a low-fi application. The UCL82 (pentode section) on an adapter is a better replacement - add about 100 Ohms extra bias in the adapter to optimise working. They are cheap and plentiful and therefore suited to heavy usage. You can always fit a UL41 when originality becomes important, but I would agree that the limited supplies of good UL41s are worth preserving. Any replacement with significantly differing heater requirements makes the job too complex for any possible benefits. A Mazda 10P13 is another possibility which will work as a straight replacement but I have not found the results to be fully satisfactory. Leon.

16th Sep 2009, 3:51 pm	#9
richrussell Heptode Join Date: Jul 2008 Location: Selby, North Yorkshire, UK. Posts: 979	Re: BUSH DAC90A output valve replacement. Another option is the CV1977 which is a direct replacement for the UL41, and more readily available. I've used a couple of these in my sets and they work well. Rich.

16th Sep 2009, 3:53 pm	#10
JHGibson Retired Dormant Member Join Date: Apr 2003 Location: Salt Spring Island, BC, Canada Posts: 368	Re: BUSH DAC90A output valve replacement. If you increase the value of the UL41 cathode bias resistor by 50%, the plate current will be reduced and the valve will run cooler. The reduction in audio volume will not be great and the valve will last longer. John.

16th Sep 2009, 4:33 pm	#13
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: BUSH DAC90A output valve replacement. Or clear the leak on the UL41. It's normally caused by grot in the base. Why not do this... https://www.vintage-radio.net/forum/...highlight=ul41 Also clear Pin 4. It is used as an anchor and can cause problems. Cheers, Steve P. __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

16th Sep 2009, 9:19 pm	#14
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,577	Re: BUSH DAC90A output valve replacement. Interesting! I'm going to take a leaky UL41 in to work and zap it on the insulation tester. It's variable up to 4kV so I can crank it up slowly. I'll post the results! Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

17th Sep 2009, 5:36 pm	#15
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,087	Re: BUSH DAC90A output valve replacement. Please do so Rich - on the appropriate thread of course! - I'd be most interested to see how others get on. I think that (with the exception of the use of pin 4 as an anchoring point), the DAC90a gives the UL41 a fairly benign life, so they should last a good long time.

17th Sep 2009, 6:07 pm	#16
G4XWDJim Octode Join Date: Oct 2008 Location: Littlehampton, West Sussex, UK. Posts: 1,465	Re: BUSH DAC90A output valve replacement. The 10P13 is a direct plug in replacement I believe. I don't have a data book handy to back this up I'm afraid so those with books handy may find it worth checking out. Jim

17th Sep 2009, 7:57 pm	#17
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,873	Re: BUSH DAC90A output valve replacement. The 10P13 has differing characteristics. It is capable of a lower maximum anode dissipation than the UL41 and is designed to run at a lower anode current (about 30 mA from memory). I found that when fitted to a DAC90A, the results were poor because on this chassis the screen of the output valve runs at an unusually low voltage - about 90V, again from memory. The resulting very low anode current gave poor output - not surprisingly, and non-optimal load matching. There were also hum problems with a few 10P13s I tried. The UCL82 works far more effectively. Leon.

17th Sep 2009, 8:25 pm	#18
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: BUSH DAC90A output valve replacement. The UL46 is a direct substitute for the UL41, It is claimed to have better internal insulation for TV use but is not that common. Could be cheaper if fewer people are looking out for them though. . Regards, Mick.

17th Sep 2009, 8:38 pm	#19
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: BUSH DAC90A output valve replacement. The UL46 does have different base connections to the UL41, so a bit of re-wiring would be required. Ron

17th Sep 2009, 9:15 pm	#20
mickjjo Rest in Peace Join Date: Jul 2004 Location: Dartford, Kent, UK. Posts: 1,661	Re: BUSH DAC90A output valve replacement. Indeed the Valve Museum website does give different base connections for the UL41 and UL46:- http://www.r-type.org/exhib/aan0001.htm But confusingly my Mullard Maintenance Manual on page 231 under the entry for UL46 states,"For ratings, characteristics, base connections and dimensions, see type UL41" . I'n sure I have at least one set with a UL46 in it and I never did any rewiring..... . Regards, Mick.