Resurrection of my WD88A XL

[systemx]

Current does not look too bad, given the low voltage. STPS1045D's arrived today, just waiting for the amp now. Glad I ordered this month given up and coming price increase.

[soulminer]

Hi Rich.

Yes, agreed that the voltage would drop again with a load in place. For some silly reason I assumed Nick was measuring without the valves fitted, and when there was no mention of a first cap I was just wondering if it was a RC or a CRC filter.

Also agreed that the 'extra' volts have probably gone astray across the diodes. When starting out with only 6.3v AC, you are sailing close to the wind trying to rectify and smooth that AC with a BR/CRC combo and still get your 6.3v. A nice 8 or 9 volt winding would be cool. That would allow any combo of rectification/smoothing/regulation then.

Good luck with the new transformer Nick. Hope all goes well.

Cheers,

[Richard]

Originally Posted by soulminer

When starting out with only 6.3v AC, you are sailing close to the wind trying to rectify and smooth that AC with a BR/CRC combo and still get your 6.3v. A nice 8 or 9 volt winding would be cool. That would allow any combo of rectification/smoothing/regulation then.

Yes indeed, this is mentioned in the FAQ; sometimes you can get 6.3V DC from the original 6.3V AC and sometimes you can't, due to load, supply and diode type. I'm sure WD88 is fine with the right diodes but it's easy to lose a volt or so;

With Kit88 the 6.3V AC was from the 9A heater winding of a torroidal mains TX - so pretty stiff. The DC load we took was just 0.6A. As standard the amp did manage 6.3V AC in and 6.3V DC out using an ordinary diode bridge. Using schottkies gave an extra 1V so burning this off with a 2R2 (2.2 x 0.6 = 1.3) left 6V DC for the heaters and gave good extra smoothing.

Member Paulo did indeed go the fully regulated way. It was all discussed on here at the time and, from memory, he ended up using a separate small 9V torroid for the supply to get the starting voltage as you say.

Rich

[Richard]

Originally Posted by systemx

Current does not look too bad, given the low voltage. STPS1045D's arrived today, just waiting for the amp now. Glad I ordered this month given up and coming price increase.

Hi Terry, What package are these, the site pic shows 3 legs and says TO220AB but the data sheet shows 2 legs TO220AC? Thanks, Rich

[systemx]

Morning Richard.
Yes the pic is wrong. they have just to legs. Knocked up a quick circuit last night to give 2.2A at 6.3V and the drop across the bridge was less than 0.2V so these will be fine in the amp.
When it comes to fitting them, I shall keep to the tag board and solder extensions to the diodes. I do not know how much clearence there is between the board and the amp bottom plate, but I'm sure I can get it all to fit!!.

Terry.

[petercom]

NickW, there's something odd going on, but neither Dr John nor I can work it out at a distance.

Even allowing for the voltage drop across the diodes of around 0.3V each (x2) you should be seeing 8v DC on the output of the bridge. So your DC output of 5.85V is clearly crazy.

Are you sure the diodes are all the correct way round? One of them the wrong way round will part cancel the rectification.

That's the only thing I can think of at the moment.

[NickW]

Originally Posted by petercom

NickW, there's something odd going on, but neither Dr John nor I can work it out at a distance.

Even allowing for the voltage drop across the diodes of around 0.3V each (x2) you should be seeing 8v DC on the output of the bridge. So your DC output of 5.85V is clearly crazy.

Are you sure the diodes are all the correct way round? One of them the wrong way round will part cancel the rectification.

That's the only thing I can think of at the moment.

Hi Peter,

I'll open her up tomorrow to double check...But she is singing her heart out right now as I type, and it is truly a lovely sound from my new speakers.

Does the AC voltage sound about right going into the BR?

many thanks, Nick

[Richard]

Originally Posted by petercom

NickW, there's something odd going on, but neither Dr John nor I can work it out at a distance.

Even allowing for the voltage drop across the diodes of around 0.3V each (x2) you should be seeing 8v DC on the output of the bridge. So your DC output of 5.85V is clearly crazy.

Are you sure the diodes are all the correct way round? One of them the wrong way round will part cancel the rectification.

That's the only thing I can think of at the moment.

Hi Peter,

Sorry if you've not followed my previous ramblings.

The switching threshold for germanium was indeed 0.3V and schottkies are also down around there. Silicon though, inc fast recovery, is around 0.7V.

That is only the start of conduction though. It is a steep characteristic but it increases with current.

The graphs below are from the manufacturer's data sheets. I've marked the Vf at 2A for the original 95SQ015 schottkies and the replacement UF5408. For the comparison both are at 25C, they will be lower at running temp, but I think you'll see where the missing volts have gone.




95SQ015 Vf 0.25V at 2A at 25C




UF5408 Vf 1.3V at 2A at 25C

[Richard]

Originally Posted by systemx

Morning Richard.
Yes the pic is wrong. they have just to legs. Knocked up a quick circuit last night to give 2.2A at 6.3V and the drop across the bridge was less than 0.2V so these will be fine in the amp.
When it comes to fitting them, I shall keep to the tag board and solder extensions to the diodes. I do not know how much clearence there is between the board and the amp bottom plate, but I'm sure I can get it all to fit!!.

Terry.

Thanks Terry, I had been wondering what to do with a third leg! That was a quick breadboarding. Some air between them will be a good idea, I see Nick's picture from the other thread shows the TO220 types for the HT standing up from the board so you should have space. Careful with the mounting tags as they are probably K too (sorry if you know that). Rich

[systemx]

Richard.
Yes I see there should be no problem with fitting the diodes. I must say thay I agree with you over the cap voltage issue, and have therefore decided to fit 35V 105 degree C ones. The 35V caps worked fine in the KEL84 mod and I see no reason not to fit the same ones in the 88. I mean the voltage and temp, not the value.
Terry.