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Old 1st Apr 2008, 11:06 pm   #1
petervk2mlg
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Default HT current varying with audio (DKE38).

This is mosyt likely a very silly question, but here goes anyway.
I have a battery version of the German DKE38 Volksempfänger.
I've noticed that the HT current varies up and down in time with the audio. Now I know I've also seen this with transistor radios with push pull output transistors, (increase the volume and the current increases) but I don't think I've seen it on other battery valves sets.
Can someone explain what is going on?
To my mind perhaps the audio peaks and troughs are varying the bias on the output valve and thus its plate current is also varying in synch with this.
Peter
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Old 2nd Apr 2008, 12:23 am   #2
paulsherwin
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Default Re: HT current varying with audio

It sounds like an HT decoupling issue but I have no specific expertise with these sets.

Paul
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Old 2nd Apr 2008, 2:27 am   #3
petervk2mlg
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Default Re: HT current varying with audio

Here is a copy of the circuit.
Perhaps when running off batteries as would have been originally the case,there isn't the need for decoupling.
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File Type: pdf dke38b.pdf (73.6 KB, 94 views)
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Old 2nd Apr 2008, 8:59 am   #4
kalee20
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Default Re: HT current varying with audio

It sounds like a case of wrong bias on the output valve.

If the HT current is increasing with volume, there's too much bias (the positive audio half-cycles increase anode current, but the negative ones just take the valve round the 'bottom bend' and no more). So, the net result is an increase in current.

If the HT current is decreasing with volume, the bias is not enough. The negative audio half-cycles will give a nice linear decrease in anode current, but the positive ones will take the valve into grid current with clipping of the waveform. So, the net result is a decrease in current.

In either case, you'll get some distortion.
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Old 2nd Apr 2008, 9:30 am   #5
PJL
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Default Re: HT current varying with audio

I think Kalee20 has explained the problem very well but it's worth mentioning two possible causes:

1) Low emissions on the output valve would have the same effect as low bias by shifting the characteristics.
2) A leaking audio coupling capacitor could cause a high grid voltage.
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Old 2nd Apr 2008, 9:49 am   #6
Mike Phelan
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Default Re: HT current varying with audio

Agreed - the output stage has gone into Class AB!
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Old 2nd Apr 2008, 10:00 am   #7
petervk2mlg
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Default Re: HT current varying with audio

Many thanks.
I think the bias explanation has hit the nail on the head.
My set has had the socket changed and a KL4 valve fitted.
The previous owner also fitted a 9 volt bias battery clipped to the chassis.
This is too much bias for the KL4. I'll reduce the bias and check on the B+ current.
Peter
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