UY85 UL84 Amplifier
 

Although this article deals with modifying a UL84, UY85 record
player amplifier for more gain and improved quality it still
contains some useful information on amplifiers using the original
line up and may be of use to newbies.
The article is very informative and, as usual, the antics of Dick and Smithy
the Radio Constructors intrepid service engineers, make it is an amusing read.

The full article is on the website under the "In your workshop" heading,
there is also another UY85 UL84 amplifier in the "G.A. French" section.

http://www.radioconstructors.info/

Geoff

Re: UY85 UL84 Amplifier
 

I used to modify these UL84 amps with a UCL82, it had the advantage that no extra base was required or a heater transformer.

PS the UCL86 was the better valve but was always a little hard to find!

Re: UY85 UL84 Amplifier
 

Interesting, though I would have thought a more logical approach would have been a UCL82- no need for extra valveholder nor heater transformer and the extra 5V needed by its heater would probably be of no real account split between the two valves, and the slightly lower HT current requirements would help the UY85 cope with lower heater volts a bit. In fact if there were space round the motor winding, a few extra turns of insulated wire could be added to give that extra 5V anyway.

Edit.... Beaten to the draw again!

Re: UY85 UL84 Amplifier
 

Yes, an interesting article. But In Your Workshop articles always were.

Two slight inelegances are the (necessary) addition of a heater transformer, and the unused ECC83 section. But then Dick and Smithy were using what they had to hand.

Re: UY85 UL84 Amplifier
 

Hi,

Agree interesting article, as kalee20 said they always were.

A young trainee at work recently wanted some background on switcher power supplies and I gave him a copy of the In Your Workshop, which describes the circuit and action of the BRC3000 power supply - this was exactly what he wanted as it was all discrete without a control IC.

Seeing this has me thinking about converting my old Dansette Major to ECL86, which has an EL84, this is one of the units that uses a 6.3V filament transformer. It will mean hacking around the PCB but it may be worth it.

Regards.
Terry.

Re: UY85 UL84 Amplifier
 

I once had a copy of this magazine with this article in it.

In 1990, I had a go at carrying out this upgrade to a Fidelity record player. To eliminate the need for another valve base, I replaced the UY85 valve with a 1N5408 diode and increased the anode resistor from 130 ohm to 270 ohm. The 90v heater tapping from the motor required dropping to 45v for the UL84's heaters. I used a 470 ohm 7 watt wirewound resistor for this.

The ECC83 valve was fitted into the holder vacated by the UY85. The improvement to the sound was amazing, but the original speaker could not cope with the extra power. I should have changed the speaker. A 4 ohm car speaker would have been ideal. Eventually, I restored the player back to its original state.

Re: UY85 UL84 Amplifier
 

Hi,
I wonder if a UCC85 could be pressed into service at AF? It might simplify the heater arrangements and use some of that wasted heat.
Just an idea.
Cheers, Pete:wave:

Re: UY85 UL84 Amplifier
 

Hi,

I don't think that I have seen a UCC85. Is it similar to a ECC83, but with a higher voltage heater?
It was valves that first got me interested in electronics when I was 10 years old, after being given an old BRC 2014 record player. Unfortunately, I don't know a great big lot about the workings of valves, as they were not covered on the electronics courses that I have done.

Another possibility with the modified circuit, would be to change the UL84 to an EL84 enabling both heaters to be powered from an 0-6v mains transformer.

Re: UY85 UL84 Amplifier
 

If they (the ECC85/UCC85)are anything like the ECC88 etc when used in an audio role, they can be extremely microphonic. Some are worse than others.

Bill

Re: UY85 UL84 Amplifier
 

Hi,
The UCC85, along with the ECC85 & PCC85 were (was? I can never be sure) designed for use in the front end of VHF receivers as an RF amplifier/oscillator-mixer and has a 100mA 26volt heater. I don't know about how microphonic it might be, alas.
An EL84 should work in place of a UL84, given the correct heater supply, but I think other characteristics may be different besides the heater.
Cheers, Pete:wave:

Re: UY85 UL84 Amplifier
 

Hi,

Was there an exact equivalent to the ECC83, but with a higher voltage heater in the 'U' series?

I have read elsewhere that the EL84 had a slightly higher supply voltage, (the UL84 is rated at 250v absolute max). However, comparing the amplifier circuit used in the Dansette Tempo for example, in comparison to a Fidelity HF35 there is not much difference. The main one is that the Tempo has no supply dropper resistor on the metal rectifier anode (it would need one if replaced with a diode due to the lower voltage drop). The HF35 had a 130 or 150 ohm connected to the UY85 anode. The resistor between the HT smoothing electrolytics is 12k in the Tempo and 10K on the HF35.

The PCB in the Tempo was used on other Dansette models. It also has holes for another valve holder. This leads me to believe that the same PCB was also used in models using UY85 and UL84 valves.

Re: UY85 UL84 Amplifier
 

Bearing in mind the above posts, and the original mod used only half of an ECC83 anyway and added a valveholder, the neatest way would be replace the UL84 with a UCL82 or UCL86 (no extra valveholder added at all, no change to heater supply really).

This is what Murphyv310 and Herald1360 suggested.

Using a UCC85 rather than an ECC83 has the advantage only of keeping to 100mA heater chain. But that's no good if the chain was running from an 80V tap on the motor, as the 3 valves would probably be under-run.

Changing to an EL84 and provisioning for the heater for this is starting to get to the point where it would ve better to start from scratch! And no the EL84 and UL84 are not equivalent apart from heater - the UL84 is optimised for lower anode voltage.

I don't believe there is a 100mA equivalent of the ECC83, however the ECC83 can be operated from 150mA chain and a little diddy autotransformer would easily stretch this to 100mA (with effectively 19V drop).

Re: UY85 UL84 Amplifier
 

It is of interest to note that the BSR deck motor heater supply tapping provided 90V. It was common on the UY85 UL84 amps to use this for the heater supply. I have come across a number of record players where the UL84 heater filament has gone open circuit. I believe that this may have been caused by the extra 7 volts applied across the heaters without a resistor to drop the heater chain to the correct 83v.

The Fidelity HF25 was unusual in that it used a BSR motor that was rated at 155v simply wired in series with the valve heaters.

The UY85 UCL82 combination would require an 88v heater supply which is near enough to the 90v tapping on the standard 230v 1960s BSR motors. However, would changing from a UL84 valve to a UCL82 require a different output transformer?

Re: UY85 UL84 Amplifier
 

Hi,
No. I think the same transformer will do for either valve.
Cheers, Pete:wave:

Re: UY85 UL84 Amplifier
 

I used to have a Dansette Capri, which used the UY85 UCL82 line up, but I seem to remember that the output transformer had an extra tapping on the primary winding. The Bush RP60 appears identical and it would be useful if I had access to a circuit diagram for reference.

Re: UY85 UL84 Amplifier
 

That was probably for smoothing. A portion of the primary is effectively wound opposed to the rest, DC wise, and provides a measure of extra smoothing for the HT line.

Re: UY85 UL84 Amplifier
 

Not quite - the smoothing of the HT line is relatively unaffected. What it does is neutralise the hum in the loudspeaker as a result of feeding the output valve from the (relatively rippley) reservoir capacitor.

Re: UY85 UL84 Amplifier
 

Yes, that's a better way of describing the effect. I've never really analysed it, just accepted that it works!

Re: UY85 UL84 Amplifier
 

I have just picked up a couple of vintage record players at a car boot sale. One is a Fidelity from the early 1970s with a solid state amp using a TBA800 amp IC. It needs the deck relubricating.

The other player is more interesting. It is made by Alba and has a BSR UA25 deck. There are two valves inside. One is a UF89 and an unmarked one which looks like a UL84. When first tried it had low sound due to a faulty X3M cartridge. I fitted an Acos GP91-3SC which I had to hand which cured the fault. The wiring on the turtable leaves a lot to be desired. The white heater tapping wire is soldered to a spare tag on the PU strip with a 300 ohm wirewound resistor, then another wire connects it to the valve heaters. There are also pick up isolation capacitors crudely placed on another tagstrip under the deck. I am going to tidy all this up and fit new capacitors for safety reasons.

Re: UY85 UL84 Amplifier
 

The Bush RP60 amplifier (UCL82 and UY85) is interesting because it takes its negative feedback directly from the pentode's cathode resistor, on the primary side of the speaker transformer. This means the secondary side is isolated from the live chassis, and allows for the provision of external speaker and tape recorder sockets (the former switched to disconnect the internal speaker). (It also means that any non-linearities in the transformer are not compensated for; however, this is mitigated by the presence of bass and treble controls -- and in any case is no worse than a UL84 running open-loop.)

I won't say too much more about it; the circuit diagram is available from the link "up top" for a very reasonable £1.99, or on Paul's excellent DVD.

For the experimentally-inclined, it should work, albeit with a little less efficiency, with a UL84 speaker transformer -- the hum cancelling winding is not strictly necessary. The UCL82's heater is rated 50V rather than the 45V of the UL84; but the actual error is only 6%, thanks to the 38V heater of the UY85.