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Old 3rd Nov 2004, 8:41 pm   #41
hotbulb
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Default Re: Bush PB53 Output stage problems

That was quick work Dave

I don't think the low volume is going to be caused by the use of a TH4B instead of a TH4A. They are pretty much the same thing, the B superceding the A. I've just gone and done this swap in one of my sets and it made no difference to the output. I think it may be worth trying a different speaker though.
Glad the bits were useful to you

Regards

Philip T-E
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Old 3rd Nov 2004, 8:55 pm   #42
Radio_Dave
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Default Re: Bush PB53 Output stage problems

Hi Philip,

I think the TH4A has a anode voltage of 275V but the TH4B is only 250V. As this anode is connected to the HT of 270V I thaught the TH4A would be better? Anyhow I think it is the speaker too, using a resistor to bring the resistance down from 2000ohms to 600ohms must mean there isn't much power to energize the field

Many Thanks

David

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Old 4th Nov 2004, 11:23 am   #43
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Default Re: Bush PB53 Output stage problems

Quote:
Originally Posted by Radio_Dave
Hi Philip,

I think the TH4A has a anode voltage of 275V but the TH4B is only 250V. As this anode is connected to the HT of 270V I thaught the TH4A would be better?
Dave, I wouldn't get too obsessed about the anode voltages in sets. They can be subject to a fair bit of variation (+/-10%) without any discernible effect upon the set's operation.

Quote:
Anyhow I think it is the speaker too, using a resistor to bring the resistance down from 2000ohms to 600ohms must mean there isn't much power to energize the field
Well, you've got about 30mA flowing through the field, which is probably about 60% of what should be going through a 2k coil. Try connecting an external loudspeaker to the set and disconnecting the internal one; a car radio one is ideal as these seem to be 4 Ohm impedance.

I think the biggest loss you've got is not from the field coil providing insufficient magnetising current; it's the 2.7 Ohm resistor across the speech coil.

Last edited by Paul Stenning; 26th Dec 2004 at 4:33 pm.
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Old 4th Nov 2004, 11:37 am   #44
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Default Re: Bush PB53 Output stage problems

Hi

I've just noticed that the Bush service sheet (which I've been using) lists a TH4A and a HT of 270V but the Trader sheet has a TH4B and a HT of 245V . I'm looking at both circuit diagrams but I can't see what is making the 25V difference in voltage? I suppose I could change the resistor across the speaker field?

Quote:
Originally Posted by mjizycky
I think the biggest loss you've got is not from the field coil providing insufficient magnetising current; it's the 2.7 Ohm resistor across the speech coil.
Instead of the 2.7ohm resistor I'll try a 3 ohm PM speaker

Many Thanks

David

Last edited by Paul Stenning; 26th Dec 2004 at 4:34 pm.
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Old 4th Nov 2004, 1:34 pm   #45
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Default Re: Bush PB53 Output stage problems

OK I think I've cracked it : volume is very loud

I have changed the resistor scross the field and I now have 860 ohms, this is dropping 72V and the HT is now 252V which seems a better voltage for the anode of the TH4B

To the speech coil of the ME speaker I've added a 3 ohms PM speaker in series and got rid of that 2.7 ohms resistor

I measured the current drawn between the output transformer and the anode of the PenB4 and it is 62mA, I think this is ok?

The HT current is 83.7mA

Am I right in thinking the output stage is now OK? The only problems I have is that there seems to be a Hum coming from the speakers which is not affected by the volume, is this the resistor across the field? and although the cabinet is big I don't think I can fit two 8 " speakers in there

David
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Old 4th Nov 2004, 2:54 pm   #46
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Default Re: Bush PB53 Output stage problems

There is usually a 'hum-bucking' coil in series with the speech coil of the ME speaker. This should be connected in phase with the speech coil. If you are not sure which way round it is connected, just reverse the wires. The hum will either increase or reduce.

Failing that it might be the resistor you have connected across the field winding. The field winding acts as a smoothing choke. Fitting this resistor may be giving you the correct DC conditions in terms of HT voltage and current but it is probably upsetting the way the field coil works...in fact it may be almost neutralising the smoothing effect of the coil.


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Old 4th Nov 2004, 5:57 pm   #47
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Default Re: Bush PB53 Output stage problems

Hi Dave, output transformer connections are ususally only important if there is feedback from the transformer secondary. If it's wrong it HOWLS!. It could have a slight effect with the level of hum.
Have you tried removing the voice coil series resistor (3R?) and connecting the voice coil directly to the op trans. This mis match will do no harm in the short term and will probably double the volume. As was stated earlier, too much shunting of the field coil will weaken the field and again reduce volume.

Ed
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Old 5th Nov 2004, 9:02 am   #48
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Default Re: Bush PB53 Output stage problems

Hi Dave, the effect of altering the output impeadance is relatively small compared to the very great loss of the resistor in series with the speech coil. The theoretical best match for power transfer is for the load impeadance to equal the valve Ra. In audio amp design this is not always the case as a mis-match will give lower distortion. Try it and see, it will not cause damage.
Ed
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Old 5th Nov 2004, 2:13 pm   #49
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Default Re: Bush PB53 Output stage problems

I've tried lots of things with the speaker today. I found that an extra 3 ohms speaker in series with the speech coil gave the best results, but I can't mount two speakers in the cabinet

Using Ed's idea of removing the resistor in series with the speech coil only gave a very slight improvement . I therefore decided to put the resistor back and use it like that until I can find a better ME speaker

I've had the radio working for around an hour now and I think the PenB4 is running a bit hot? Is there anything I should check or does this valve just run hot?

Many Thanks

David
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Old 5th Nov 2004, 4:30 pm   #50
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Default Re: Bush PB53 Output stage problems

An output valve will run a lot hotter than other valves in the set, with the exception perhaps of the rectifier. Remember that the valve is drawing much greater current than any of the signal valves, so the heat generation will be greater. If the HT current drawn by the PenB4 is reasonable and the HT voltage is within parameters, then I wouldn't worry.
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