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Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

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Old 19th Apr 2018, 9:41 am   #21
Herald1360
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Default Re: Amplifier problem (record player)

Quote:
Originally Posted by Boom View Post
Ra is 230k for UL84 but Valve Museum doesn't give RA for UCL83. I got away with it for years though modding mates record players as they never were very loud until I upgraded/bodged them with a UCL83. I think that there is a couple of volts heater difference between the two valves also.
Ra is pretty much irrelevant here, RL is the useful value. (Though the datasheets are sometimes a bit vague about which is which!)

UL84 about 2.5k, UCL83 about 5k for ideal load. In practice, just using 2.5k on a UCL83 won't make a vast difference. UCL82 also about 5k but would give more grunt than the UCL83.
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Old 19th Apr 2018, 10:44 am   #22
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Default Re: Amplifier problem (record player)

Ken...everyone has been asking about the cathode voltage of the UL84 but so far there has been no response. Measure it across the 330 ohm resistor/cathode decoupler. Otherwise this thread is likely to veer off and get very confusing.
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Old 19th Apr 2018, 6:32 pm   #23
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Default Re: Amplifier problem (record player)

Hello John

Had chance to measure cathode volts, and this is 17.8V and yes the stylus is the correct one for the cartridge, I did check to see if the wires are on the cartridge ok and yes they are, I even disconnected the cartridge to see if this made any difference to the pickup hum by touching but no difference, I have got a feeling the valve may not be drawing the right current. I haven't got a spare UL84 so have to get one.

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Old 19th Apr 2018, 7:13 pm   #24
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Default Re: Amplifier problem (record player)

Recheck the value of the cathode resistor.
Cheers
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Old 19th Apr 2018, 7:21 pm   #25
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Default Re: Amplifier problem (record player)

17.8V across 330 ohms is 54mA which seems too high. Should be nearer 12V. Short of checking to make sure a silly mistake hasn't been made (there aren't many to make) then a new valve is probably the next option. The fact that the current is high suggests a fault with the valve (internal leak possibly). If the HT was low, the current would be lower and in any case, it isn't as you measured 248.

If the valve was low emission, the HT would be higher and the cathode volts would probably be much lower as it would be drawing much less current. So as there is low output and a high current as evidenced by the high cathode voltage, it only leaves an inter electrode leak within the valve.

Make sure the cathode resistor IS 330 ohms (orange, orange brown, silver or gold) by measuring it. If it's gone high resistance or an incorrect (higher) value fitted then the cathode voltage may be higher because of this and the valve will give low output.
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Old 20th Apr 2018, 8:09 am   #26
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Default Re: Amplifier problem (record player)

Thank you sideband

Yes, I did check the cathode resistor and it is 330 Ohms, so as you suggest and what I had thought the valve its self is the only option. I will get another one and then get back to everyone with the result.

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Old 20th Apr 2018, 12:23 pm   #27
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Default Re: Amplifier problem (record player)

Don't necessarily jump to condemn the valve. A faulty capacitor across the cathode resistor can cause loss of gain without apparent disturbance to DC conditions. What happens is as the valve conducts better (on the crest of the input signal), more current flows through the cathode resistor and the voltage on the cathode rises. This makes the grid more negative, which causes the valve to conduct less. Basically it ends up fighting against itself. With a big capacitor across this resistor, any increased current through the valve flows into this capacitor first; the capacitance is sufficient to prevent the voltage across it from changing much. Exactly the opposite happens on the trough; the valve conducts less, but the capacitor maintains a steady cathode voltage.

This capacitor was nearly always 25V, 25µF in most of the EL84 and UL84/UY85 record players. 22µF is the nearest modern value, but 33 or even 47µF would work fine if that's nearer to hand.
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Old 20th Apr 2018, 4:42 pm   #28
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Default Re: Amplifier problem (record player)

The function Of the cathode capacitor is understood, it is the high voltage reading across the cathode resistor which gives cause for concern, as has been discussed in the previous posts.
Do you have any views on the high cathode voltage?
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Old 20th Apr 2018, 4:45 pm   #29
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Default Re: Amplifier problem (record player)

It should be possible to calculate the anode and screen currents given the info in post #1.

Screen grid current looks low to me.
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Old 20th Apr 2018, 5:14 pm   #30
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Default Re: Amplifier problem (record player)

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Originally Posted by John10b View Post
... Do you have any views on the high cathode voltage?
EL84s, and so I suppose UL84s too, can be very prone to going gassy which leads to the control grid voltage rising and the cathode current going high as a consequence. I see that the OP says in the first post that the grid is at 0V. The question then becomes 'How close to 0V ?'. Even a volt or two positive can make a significant difference to the cathode current in this relatively high transconductance valve.

Cheers,

GJ
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Old 20th Apr 2018, 5:46 pm   #31
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Default Re: Amplifier problem (record player)

This is getting interesting and a little puzzling. In your first and subsequent posts you say words to the effect “ changed all the components........it was working till I change the STYLUS ...”
Let’s stand back for a moment and recap.
1. It was working to start, you say.
2. You changed the stylus ( not cartridge).
3. Then it became faulty.
4. You changed all the components.
5. Cathode voltage is rather too high at +17volts.
6. Only faint buzz when you touch control grid.

So what caused it to become faulty after you changed the stylus?
I presume you didn’t disturb any part of the amp.
It is quite common for an amp to go u/s after switching on/off/on.
Did you make an error when you “changed all the components “.

Considering all that’s been said, I would double check every thing you have done and if you are satisfied no mistakes were made I would pop in a new valve.
Cheers
John
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Old Yesterday, 5:25 pm   #32
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Default Re: Amplifier problem (record player)

Hi folks

Problem now solved, I replaced the valve and sounds pretty good

Thanks everyone for your thoughts and contributions

Best wishes

Ken
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Old Yesterday, 7:27 pm   #33
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Default Re: Amplifier problem (record player)

Presumably the UL84?
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Old Yesterday, 8:55 pm   #34
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Default Re: Amplifier problem (record player)

Quote:
Originally Posted by Radio Tech View Post
Hi folks

Problem now solved, I replaced the valve and sounds pretty good

Thanks everyone for your thoughts and contributions

Best wishes

Ken
So can you now measure the cathode voltage and tell us what it is?
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