UCL83 substitution for UL84

[awc]

I very rarely venture into the magical world of innovation, but I need to try and put a bit more "oomph" into an amp on a small valve record player to cope with a lower output cartridge, by substituting the UL84 for a UCL83

I have found a circuit for this, but of course the UCL83 only has as heater voltage of 38 against the 45 of the UL84.

If I put a 70 ohm 1 Watt resistor in parallel with the heater of the UCL83, am I right is assuming this will drop the surplus 7 volts?

I would mention that the heater voltage for the UCL83 and the UY85 is from a tap on the gram motor.

There are of course other issues such as the output transformer ratio being adrift, but I'll do a "suck it and see" on that one first.

Thanks

Alex

[ukcol]

Originally Posted by awc

If I put a 70 ohm 1 Watt resistor in parallel with the heater of the UCL83, am I right is assuming this will drop the surplus 7 volts?

Hi Alex,

Your resistor will need to be in SERIES with the heater chain to drop the extra 7 volts. 68 ohms is the nearest preferred value and although a 1 watt resistor will be enough to dissipate the 0.7 watts, I would use a wirewound type of perhaps 2.5 watts.

Colin M

[awc]

Hi Colin

Many thanks for your reply, you are right of course, slapped wrist! I guess two out of three wasn't bad. Seriously though, at least my calculation of the resistor value and wattage was right. As teacher used to say, must try harder!

Thanks again

Alex

[peter_sol]

The UCL83 and UCL82 have higher RA requirements so may not sound very good unless the output transformer is also changed but I look forward to your opinion if you go ahead with the mod.

[Herald1360]

It'd be a bit more work, but maybe change the UY85 for a UF89 + heater resistor and fit a 1N4007 rectifier? You'd get more grunt that way and the output TX would still be right. Also enough gain to fit decent tone controls if desired.

If it's only for more gain rather than full blown output it will be fine. Thinking about it, if that's true why not turn the volume up?

[awc]

Hi all

I'm changing the circuit to that of the rather grandly named Champion "Debutante" Model 888 from the R&TS Book. I already discovered an error (it says pin 7 on the UY85 is the cathode, it's not it's pin 3) I'm hoping the rest of the circuit is ok!

Alex

[Tim]

Why a UCL83? Do you have lots of them? I would have thought an UCL82 is a closer match, heater wise.

[awc]

Hi Tim

Two reasons why I propose to use an ULC83 rather than an 82:

1. I have a few UCL83 and no UCL82 to hand.

2. The circuits available in the service books all show a UCL83 circuit rather than an 82

It did getting me thinking (as far as such extreme activity is possible these days!) whether BSR made custom wound motors with a tap to suit say a UY85 and UL84 (90 volts) or the combination of a UY85 and UCL83 (83 volts) or whether the gram makers just went for it and hoped a few volts overrun on the latter combination didn't matter.

Work on conversion is progressing and I will report back when I've tried it out.

Alex

[awc]

Hi all

I have completed the rewiring to change the circuit to suit an UCL83.

As previous discussed, I have added a 70 ohm resistor in the heater circuit by connecting the white wire from the gram motor to said resistor, the other end thereof going to pin 4 on the UY85 then via pin 5 to pin 4 on the UCL83, pin 5 being connected to chassis. The black wire from the motor goes to chassis and the brown to live and the 220 ohm resistor to the anode on the UY85

Result? On switch on (and off again quickly) the UCL83 lights up like a light bulb!

Why?

Alex

[ukcol]

Hi Alex

With both valves out power up and measure the voltage on the 70 ohm resistor, it should be approx 83 volts AC.

If that is OK, double check your heater wiring.

Finally check that the UY85 heater is not short circuit.

It is worth mentioning that "flare up" at switch on, in series heater chains, can be quite normal provided the flare dies away very quickly.

Colin M

[awc]

Hi Colin

The AC across the motor winding from white to black reads 95 volts, valves out.

Interestingly, the reading through the 68 ohm resistor to black still reads 95 volts.

DC resistance of the UY85 is 52 ohms and the UCL83 59 ohms. Putting both back in their holders and reading across both to chassis reads as expected 111 ohms, so there shouldn't be a problem there. I guess I will have to try them again and leave the amp switched on a little longer to see if the flare dies down

Alex

[julie_m]

It will read 95 volts on the other side of a resistor that goes nowhere; hardly any current is flowing through it, so hardly any voltage is dropped across it. The input resistance of the voltmeter (10 MΩ for a digital meter, 20 k Ω/V for an AVO 8 depending on what range is selected) is much higher than 68 ohms, so most of the voltage appears across that.

If something else was drawing current through the 68 ohm resistor, it would drop 0.068 V per mA.

[awc]

Well, I have completed the conversion work for this amp and can report that the UCL83 seems to be perfectly happy running through an output transformer designed for a UL84. The overall volume from the amp has, as perhaps to be expected, been increased to an acceptable, if not over loud, volume level using one of the current generic stereo ceramic cartridges which give an output of about 350mv.

The only real problem encountered was with the smoothing/reservoir cap. This seemed fine after playing two sides of an LP (say 40 minutes) so I pressed on and played a few more LP,s (about 2 hrs). When I checked the cap again however, it was extremely hot, indeed much to hot to touch. I guess another LP or two and it would have gone off bang!

[Stylo N M]

Hi there,

I would say that it would be unlikely that the can capacitor would have got much hotter, it was probably about as hot as it was going to get, it isn't unusual for that capacitor to be hot to the touch, as the valves will heat everything up, but at the same time the heat will dissipate.

Check that the voltage selector is set at the highest setting, the situation will be made worse if it is set on only 220V and the effect may be the need to rewind. I really notice a heck of a difference, when my Disc Jockey Majors are set on 220V, they really do run hot when set on that. It might well of been OK 50 years ago, but I always use the 245V setting now. Is there plenty of ventilation? You could find a discrete part of the player and drill some reasonably largish holes say the diameter of a new one pence piec, and insert a brass grommet to neaten the outside of the hole like on the side of the Bush RP40 (although they look like hard plastic). You could also use those very tiny black cotton bobbins, those ones that you get in the pound shops and get a load of them on a card. They only have about a yard of thread on each one and there's a hole in the middle. Cut one end off and push that into the hole. You could drop the heater voltage a volt or two, it shouldn't make that much difference to the sound ouput, a valve will still emit sufficient electrons even when partly lit.

Paul, you've done a good job well done!

[awc]

Hi

I decided to act on the side of caution and replace the can with a couple of 33ufs, and it sounds quite good (well for a small basic gram anyway!) although the cartridge only just gives enough output for the job. At least 78 styli are available for a few quid although changing from one to another is a bit of a pain and nowhere as easy as a flip over BSR!

Alex