Amplifier problem (record player)
 

The record player I have which has no make or model number has an amplifier that employs the ul84 and uy85, quite a simple circuit by any standard, however, this has hardly any output. I have changed the components but to no avail, when I looked at the details of the output valve the voltages on the grid, screen and anode are very different to those measured, I am getting 260V from the rectifier on one side of the output transformer, anode side reads 248V grid sits at 0V and screen around 241V cathode resistor 330 Ohms and screen resistor 15K, I believe the valve its self is ok and so is the needle and cartridge. any ideas folks.

Ken

Re: amplifier problem (record player)
 

What is the voltage on the cathode (pin 3), should be be about +3volts.
Have you tried touching the control grid (pin 2 ), you should get a healthy buzz.
If you do get a good buzz it would indicate a faulty cartridge.
Cheers
John

Re: amplifier problem (record player)
 

Is the cathode resistor bypassed by a large cap? If so it would be worth checking, since if dud it would allow some negative feedback which would reduce the gain. You say the cartridge is OK, but I wonder how you established that. A single valve amp like that would require a high output from the cartridge.

Re: amplifier problem (record player)
 

The voltages shown for the valve characteristics will be MAXIMUM values....not much use when comparing them in a simple amplifier like this one. What is the cathode voltage? From that, it's easy to work out how much current the valve is drawing by using Ohms law. It will probably be somewhere between 9V - 12V in a circuit like this.
As others suggested, touch the control grid (pin 2....hold one end of a .01uF capacitor or similar and touch the other end to the grid....you should get a loud hum). If you do the amplifier is OK and you need to check the cartridge and wiring.

Re: amplifier problem (record player)
 

The fault you describe is nearly always a cartridge problem. But do continue with your voltage measurment by all means, so as to eliminate the amplifier.
If at the end of the day you do have to replace the cartridge, then beware as the options will be quite expensive - and probably cost more than the value of the player per se. We can advise later. Good luck with your tests!

Re: Amplifier problem (record player)
 

Hello everyone,

I have touched the control grid only to obtain a fairly weak hum even at full volume, but not measured the cathode voltage, and it does have an electrolytic cap 47uf 40V. The strange thing is though, before I replaced the stylus it did play very loud so I am not sure as to what has happened.

But if it is the cartridge as Edward suggests then it will have to sit on the shelf until something comes along.

Best wishes

Ken

Re: Amplifier problem (record player)
 

Are you saying it was loud (enough) before you replaced the stylus, and was too quiet after that? If so, then I would suspect stylus or cartridge problems.

Do you still have the old stylus? If so, put it back in and see if it is loud enough then. It's always possible the new stylus is the wrong type or something, and not coupling to the rest of the cartridge properly.

When you replaced the stylus, did you disturb anything else (did you remove the cartridge from the arm, for example)?

Re: Amplifier problem (record player)
 

You might have disturbed one of the 2 cartridge connector wires. What type of cartridge is it?

Re: Amplifier problem (record player)
 

I like things you can measure. As has been previously mentioned, check the cathode voltage. You'll soon know whether the valve is OK.

Re: Amplifier problem (record player)
 

Measuring the cathode voltage is such a simple test. As I said above, a voltage between 9 and 12 volts will equate to a current of between 27 and 36 milliamps (measured across the 330 ohm resistor). It will tell us if the valve is drawing sufficient current

Re: Amplifier problem (record player)
 

Not making sense! unless coincidentally you incurred a fault, you say it worked ok before you changed the stylus, and you only get a weak “hum” when touching the control grid, and you have not measured the cathode voltage.
Let’s try to establish one thing at a time.
Measure the cathode voltage, previous posts have indicated what to expect.
If you get no voltage reading it would suggest amp fault.
Cheers
John

Re: Amplifier problem (record player)
 

I'd go with John on this. If the fault came up when you changed the stylus it is possible that the stylus isn't sitting on the cantilever probably so won't generate output from the cartridge. Grab a loupe and have a close look.

Re: Amplifier problem (record player)
 

You might well get very little hum from just a UL84. It has very low gain when used like this. I used to rip them out and rebuild the amps using a UCL83 so it had a pre-amp, as a UL84 on its own is so quiet. It isn't hard to do.

Re: Amplifier problem (record player)
 

So as to make your recommendation clearer, one assumes just using the triode section of the UCL83.

Re: Amplifier problem (record player)
 

No use both sections. Wire the UCL83 pentode up in place of the UL84 then add the UCL83 triode to the circuit to give a bit of gain. It will only take a few extra components to do it.

Re: Amplifier problem (record player)
 

I'd agree a weak hum is all that could be expected. The UL84 is fairly sensitive as an output valve, but even so, it needs a few volts of audio for any sort of volume.

The voltage readings all sound reasonable.

Can you feed in some audio from something else - a battery-powered CD player, radio, MP3 player - from its headphone output? You can listen to see if it sounds distorted or not. If OK, then I'd question the cartridge, the volume control, or maybe the leads between cartridge and amplifier.

Re: Amplifier problem (record player)
 

That makes good sense. The UCL82 is an option, but it's pentode anode load, without checking, may not match that of the UL84. The UCL83 can make a sweet sound and is loud enough.

Re: Amplifier problem (record player)
 

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.

Re: Amplifier problem (record player)
 

Ra is in the valve data sheets, Ra for the UCL83 is at least twice that of a UL84 for similar Va/Vg2 conditions so far as I can make out.

Lawrence.

Re: Amplifier problem (record player)
 

What is the cathode voltage ?
Cheers
John