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Old 2nd Oct 2004, 1:12 pm   #1
Radio_Dave
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Default Bush PB53 Output stage problems

Hi

Oh dear, big problems My Bush PB53's output tranny is scrap, the speaker is incorrect and I have no output valve. There are several questions here but they do all relate too each other :

The output TX's primary should be 250R and the secondary should be 0.8R. I have a used RS output TX which seems to have the tappings I need but it is physically smaller than the original does this matter? My other option is another TX (of a simular size)with the correct secondary but the primary is 500R. Can I connect a resistor in parallel to bring it down to 250R?

The next problem is the speaker. The original should have a Speach coil of 2.8R, hum neutralizing coil 0.5R and the field coil 600R. The one fitted has a speech coil of 3.1R, hum neutralizing coil 0.1R and the field coil 2000R. Because the speech and hum neutralizing coils are in parrallel I think they will be ok? but the field coil is way out, again can I fit a resistor to bring this value down to the 600R needed

The output valve should be a PenB4, which is missing, I have sent a email to a supplier but I have'nt heard anything yet If I can't find the correct valve cannot be found and a substitute such as a PenA4 used will all the above values have to be changed or altered?

Any help gladly accepted

David
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Old 2nd Oct 2004, 2:01 pm   #2
paulsherwin
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Default Re: Bush PB53 Output stage problems

Hi David.

The DC resistance of the O/P transformer isn't too important. What matters is the impedence. You need to use a transfomer which has more or less the correct impedence for the valve anode load. In practice class A O/P valves are pretty tolerant of this so long as you check the current being drawn is correct and modify the cathode resistor if required. The worst that's likely to happen is you'll lose some volume.

The fact that the speech coil is slightly wrong is also not significant. It's the impedence that matters, not the DC resistance. These DC resistances both sound plausible for a nominal 3 ohm impedence speaker.

The hum and field coil values do seem wrong, so the speaker has probably been replaced at some time, but I wouldn't worry too much at this stage so long as it works. I can't see anything here that's going to cause any damage.

I can't suggest another O/P valve as sets of this period aren't my speciality, but subbing another valve should be OK if the characteristics are similar - again, you may need to adjust the cathode resistor.

HTH, Paul
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Old 2nd Oct 2004, 3:31 pm   #3
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Default Re: Bush PB53 Output stage problems

Thanks for the replies, I'll have to go away and find out what impedence is and how it relates to my problems then I'll be back with some more questions

Thanks

David
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Old 2nd Oct 2004, 7:47 pm   #4
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Default Re: Bush PB53 Output stage problems

Ok I'm back. I found lots of info about impedence on the web

Please let me know if I've done this right

I used a train set transformer with a 16VAC output and connected it to the primary of my reclaimed 500R/0.8R output tranny. Voltage on the primary was 17.14V and on the output 0.407V this means the turns ratio is 42.113:1. Now if I square this figure I get the impendance ratio of 1773.5:1.

Aparently if I multiply the speech coil resistance by 1.25 I will get the impedence of the speaker. The speech coil is 3.1R so this multiplied by 1.25 makes the speaker impedence near enough 4 ohms.

Now if I multiply these two numbers (1773.5 X 4) I have 7094 ohms which means I need to find a output valve with an anode load of 7094 ohms. Please tell me if this is right

Thanks

David
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Old 2nd Oct 2004, 9:15 pm   #5
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Default Re: Bush PB53 Output stage problems

Hi Dave.

Well done!!

Your calculations seem correct if I work it in reverse (the way I'm used to doing it with a known anode impedance) so 7094/4=1773.5 and then take the square root of that and it comes out to 42.11 which is the ratio :1. So you need to find a valve with an anode impedance of around 7000 ohms. In practice you could probably use anything from 6000 - 8000... it really isn't going to do any harm. Just make sure the current through the valve is not excessive.

Back to your original posting..you can't really take too much notice of DC resistances on transformers...it's the impedance thats important and it seems you've learnt a good way of finding it out.

With regard to the hum cancelling coil (hum bucking coil) I think this should be in SERIES with the speech coil but in reverse phase.

Good luck.

Rich.
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Old 2nd Oct 2004, 9:33 pm   #6
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Default Re: Bush PB53 Output stage problems

Hi

Thanks for checking . By some strange coincidence the radio came with a AC2/PenDD which has an anode load of 6700 ohms and an anode current of only 32mA. I think this will be a good choice to match my output TX? (Of coarse I wont use the diodes

Please explain what you mean by checking the current through the valve and how is the best way to do it

Many Thanks

David
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Old 2nd Oct 2004, 10:22 pm   #7
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Default Re: Bush PB53 Output stage problems

Dave.

If you want to check the total current that the valve is drawing, then just disconnect the cathode bias resistor at the valve base and connect your meter between the disconnected end of the resistor and the valve base contact (positive to valve base, negative to resistor). Set your meter to a suitable range (say 50mA) and it will indicate total valve current (made up of anode current and screen grid current).

If you just want to measure the anode current, then disconnect the HT feed to the primary of the output transformer, connect your meter (negative to transformer, positive to HT feed), set your meter to a suitable current range (say 50mA) and it will show the anode current only. BE VERY CAREFULL when doing this, first because of the high voltage and second, if the meter does not make proper contact, there is a danger that the valve will run without anode voltage and the screen grid will get red hot.

Don't worry! Just take your time and check your connections first.

Rich.
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Old 2nd Oct 2004, 10:31 pm   #8
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Default Re: Bush PB53 Output stage problems

David,

In answer to your first question the AC2/PenDD does seem a good match to your output tranny. The reason why you might want to check the current drawn through this valve if you use it is because your set was designed for a more powerful PenB4 which has a higher anode current; using a valve which draws less could cause a rise in overall HT voltage and could cause the valve to over-run. The chances are it will be OK but if you have your multimeter in the connected in the cathode of the output valve and the valve seems to draw excessive current then you can increase the value of the cathode resistor to compensate.

If the HT is noticably higher using the AC2/PenDD then you might need to reduce it before feeding the rest of the set.
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Old 2nd Oct 2004, 11:06 pm   #9
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Default Re: Bush PB53 Output stage problems

Thanks Guys for the help with this I think I've learnt alot tonight

I am still unsure what to do about the incorrect speaker field coil the original would have been 600R but the replacement is 2000R. I think this coil is acting as the H.T smoothing choke as well as to energize the speaker so wont this effect the H.T Voltage?

David
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Old 2nd Oct 2004, 11:34 pm   #10
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Default Re: Bush PB53 Output stage problems

Just to be clear - have you replaced the energised speaker or simply replaced the coil?

With your new coil and possibly replacing the output valve with one which takes less anode current you are altering circuit conditions; the set was designed with the PenB4 drawing sufficient anode current for the energising coil to activate the speaker - there is no guarantee that a valve with lower HT current requirements and a higher resistance coil will work effectively.

If you have to replace the speaker I'd be inclined to use a PM type and a choke in the place of the energising field coil for smoothing.
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Old 3rd Oct 2004, 12:20 am   #11
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Default Re: Bush PB53 Output stage problems

I recently brought this radio from a seller who had owned it since 1962. It was given to him as it was deemed an uneconomical repair by a local repair shop. This is when the radio recieved its different speaker, the AC2/PenDD, the SP41 to replace the TDD4 and the mains transformer

Unfortunately it does'nt end there for around ten years it was experimented on, went through college with its owner and generally got severely altered But the main thing is that apparently, so I've been told, it always worked until the demise of the output TX

So basically the speaker and AC2/PenDD have been used for this radio. The output TX I'm fitting seems to be a better match than what was fitted originally. Fitting a PM speaker would be awkward because of the size of the hole in the baffle board

Thanks

David
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Old 3rd Oct 2004, 11:36 am   #12
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Default Re: Bush PB53 Output stage problems

Blimey! It's well on the way to becoming a different set!

I suppose if the energising coil for the replacement speaker functions well on the HT current drawn by the AC2/PenDD and the other valves, then why bother to replace it - it probably makes sense to get the set working first in some fashion and then look at getting the working conditions close to the original.

It shouldn't be too difficult to find a PM speaker to replace the energised one if you wanted to whatever the diameter - if necessary, you could cut a hardboard 'sub baffle' to screw into the holes for the existing speaker, but with a different speaker attached to this baffle.
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Old 19th Oct 2004, 2:00 pm   #13
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Default Re: Bush PB53 Output stage problems

Hi

After over 2 weeks work I was finally ready to apply power today unfortunately the only good news is it didn't blow up!

The HT of this radio with the correct 600ohm speaker field is 270V. With my speaker field of 2000ohms I was hoping to get around 240V which would be ideal for the screen of the AC2/PenDD which I fitted in place of the PenB4. However the voltage at the resovoir cap is around 275V but the HT is only 190V .

I have been reading a simular threed about low HT and I have tried a new rectifier to no avail and apart from a handful of mica caps around the IFT's every capacitor has been replaced with new ones

It is however worth mentioning that around 95% of the circuit had to be rebuilt and rewired. I have spent the last 3 days checking my work to make sure it follows the circuit diagram and I am confident it is OK. However it is possible that I have got something wrong :

Touching the top cap of the AC2/PenDD does produce crackles but apart from that nothing.

Any ideas gratefully accepted

David
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Old 19th Oct 2004, 5:23 pm   #14
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Default Re: Bush PB53 Output stage problems

Hi Dave, If the field coild resistance is too high and you are worried about reduced HT you can add a resistor in parallel to bring the equivalent value down, about 1K5 of suitable wattage should do. The train transformer may give poor HF response as it will be made with laminations suitable for 50Hz use. It may also saturate and produce more distortion as the transformer for a single ended output stage was usually air gapped in the core to prevent the (DC) valve anode current from doing this.
Best regards and good luck,

Ed
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Old 19th Oct 2004, 5:56 pm   #15
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Default Re: Bush PB53 Output stage problems

Hi Ed,

I didn't use a train set transformer to replace the output transformer, I used the train set transformer's 16VAC output to find the impendance of a spare output tranny I had. It is the only AC output I have, apart from the obvious

I thaught about putting a resistor in parallel with the field coil but the Bush PB51 is a very simular circuit to the PB53 but it uses a PenA4 output valve (which is simular to my AC2/PenDD) it has a field coil of 2000ohms and it has a HT of 240v so I don't understand why mine only gets to 190V

Regards

David
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Old 20th Oct 2004, 8:00 am   #16
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Default Re: Bush PB53 Output stage problems

Hi Dave, understood about the train transformer. I would have thought that the higher resistance (R) of the field coil as measured with an Avo would lead to a greater volt drop and was possibly used to reduce the voltage to the set. The reactance (X) is a measure of the inductance of the coil and will reduce the hum level by acting as a resistance to AC only. The resultant sum of these 2 is impeadance (Z). Try the resistor and see what happens.
Ed
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Old 21st Oct 2004, 10:54 am   #17
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Default Re: Bush PB53 Output stage problems

Hi

I have done as Ed suggested and tacked a 900ohm resistor across the speaker field. The resistance is now exactly 600ohms which is what it is supposed to be. However instead of a 270V H.T. I only have 220V .

Can I have some clues as to what could be giving the low H.T. As I said earlier I have checked the obvious but I am running out of ideas now

Thanks

David
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Old 21st Oct 2004, 11:23 am   #18
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Default Re: Bush PB53 Output stage problems

Dave, double check the rectifier. You could temporarily replace it with some silicon and a 220 ohm resistor and see what the difference is. It's possible that the rectifier you substituted is also bad (these are very old valves after all ).

If the HT is still down, then either the transformer AC output is low, or something is drawing excess HT current. You said earlier in the thread that the mains transformer isn't original, so this may just be delivering too little voltage. 270V HT isn't that usual and most transformers were designed to produce around 250V HT. You may be able to increase the available DC voltage by using silicon rectifiers and a small current limiting resistor (say 47 ohms). You'd obviously leave the rectifier valve in place for cosmetic reasons.

HTH, Paul
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Old 21st Oct 2004, 11:49 am   #19
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Default Re: Bush PB53 Output stage problems

Hi Paul,

I've tried a new rectifier and also one from another radio which I know is working fine and I still get a slightly higher voltage with the one I've got. I'm sure the transformer and rectifier are ok because I have 275V at the reservoir cap. 55V are being dropped across the speaker field

Many Thanks

David
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Old 21st Oct 2004, 1:42 pm   #20
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Default Re: Bush PB53 Output stage problems

Quote:
Originally Posted by Radio_Dave
I've tried a new rectifier and also one from another radio which I know is working fine and I still get a slightly higher voltage with the one I've got. I'm sure the transformer and rectifier are ok because I have 275V at the reservoir cap. 55V are being dropped across the speaker field
55 volts across 600 ohms indicates nearly 100ma (91.6mA ). This seems very high. If this is so, you need to find out what is drawing the excessive current

Good luck, Paul

Last edited by Paul Stenning; 6th Mar 2005 at 12:21 pm.
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