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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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1st Apr 2008, 11:06 pm | #1 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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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 |
2nd Apr 2008, 12:23 am | #2 |
Moderator
Join Date: Jun 2003
Location: Oxford, UK
Posts: 27,785
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Re: HT current varying with audio
It sounds like an HT decoupling issue but I have no specific expertise with these sets.
Paul |
2nd Apr 2008, 2:27 am | #3 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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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. |
2nd Apr 2008, 8:59 am | #4 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,059
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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. |
2nd Apr 2008, 9:30 am | #5 |
Dekatron
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 5,997
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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. |
2nd Apr 2008, 9:49 am | #6 |
Dekatron
Join Date: Nov 2003
Location: Near Leeds, West Yorkshire, UK
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Re: HT current varying with audio
Agreed - the output stage has gone into Class AB!
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Mike. |
2nd Apr 2008, 10:00 am | #7 |
Heptode
Join Date: Jan 2007
Location: Parkes, NSW Australia
Posts: 877
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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 |