Resurrection of my WD88A XL

[NickW]

Originally Posted by John Caswell

Puzzled of Wokingham here.
Will investigate further and get back. 4.8v is outside the mfrs 6.3V ±10% limit by a fair way.

John - Dr John

Hi John,

Peter told me that WD supply these new diodes as standard now, so presumably others may have the same problem?

Thanks for the help.

Nick

[Richard]

Running at 4.8V may cause lower emission and cathode poisoning in the longer term. When I get a mo I'll put an ecc83 in the valve tester and see if there's a measurable performance change running different heater voltages.

I think Nick's low voltage is explained by the various Vf (forward voltage drop) figures for these diodes of which UF5408 is not good. We are only using them for a few volts output so the voltage lost in Vf makes a big % difference. Even the standard 1N5408 is better but a schottky replacement such as the STPS1045D suggested is best if a pi filter is wanted.

Here are the data sheets. It's not easy to compare Vf between diodes directly as If and other factors are involved but compare Vf for the first 2, which is Nick's scenario, and we may see what's happening,

95SQ015

http://docs-europe.electrocomponents...6b80063533.pdf

UF5408

http://docs-europe.electrocomponents...6b800a8fae.pdf

STPS1045D

http://docs-europe.electrocomponents...6b8047f21a.pdf

1N5408

http://docs-europe.electrocomponents...6b800265a3.pdf


Rich

[Richard]

Hi All,

I checked a NOS boxed Sylvania 12AX7WA (ecc83) at different heater voltages this evening. It tested near identical on both anodes,

At 6.3V heaters

Anodes 1 and 2
2 mA/V slope and 1.1 mA current

At 5V heaters

Anodes 1 and 2
1.5 mA/V slope and 0.9 mA current

At 4V heaters

Anodes 1 and 2
1.3 mA/V slope and 0.6 mA current

I then checked a Trigon branded NOS boxed ecc83 of european manufacture which confirmed this by testing almost identically.

There's certainly a difference in current and slope with lower heater voltage, comparable to testing an old weak valve against a new one. Whether the sound will translate that way I don't know.

Rich

[NickW]

Hi Richard,

Many thanks for all the info.

I can't honestly tell whether the sound is affected by not running at 6V, mainly because I only had the amp running at home with new speakers for 2 evenings. The old speakers were £20 from Argos and ONLY used for testing in case anything went badly wrong on switch-on.

So with only 2 evenings on 6V heaters and the new speakers (before the circuit burnt out), and 2 weeks of time-off for repair, I can't say whether the sound is different or not.

I think its time to contact Peter and see what WD advise.

many thanks, Nick

[Richard]

Hi Nick,

I understand, let us know how it goes.

Rich

[petercom]

Dr John has built plenty of WD88VA XL amps with these diodes and reported no problem with the design or components!

I've asked NickW to check the AC voltage and report back.

[NickW]

Originally Posted by petercom

Dr John has built plenty of WD88VA XL amps with these diodes and reported no problem with the design or components!

I've asked NickW to check the AC voltage and report back.

Hi Peter,

Ok I've measured the voltages around the bridge as requested using a "calibrated" Fluke Multimeter.

The results dance around a little bit depending on point of contact on a joint, and pressure - but here goes:

1. AC output of the Mains Tx measured at the tag board across the two red wires = 6.415V

2. AC output of the Mains Tx measured after the in-line fuse = 6.370V

3. DC output of the bridge measured across the brown/grey wires = 5.850V

4. DC output of the bridge measured after the 0.47 ohm resistor in the filter = 5.026V. Note this 0.2V more than I measured the other day - so it is varying a little.

Hope this helps?

Many thanks, Nick

P.S. The HT, all cathode and anode voltages all measure to spec, so I'm pretty sure the Fluke multimeter isn't playing up.

[soulminer]

Wouldn't the first cap of the filter after the BR normally bump the voltage up from 5.85 to probably about 7 or 8 volts? Do you have a CRC filter directly after the BR or just an RC filter?

[NickW]

Originally Posted by soulminer

Wouldn't the first cap of the filter after the BR normally bump the voltage up from 5.85 to probably about 7 or 8 volts? Do you have a CRC filter directly after the BR or just an RC filter?

Hi Soulminer,

The XL kit has a CRC filter after the BR.

The volts I quoted are simply those I measured. So...

5.850V DC going into the first Decoupling cap.

5.026V DC coming out of the CRC filter and on to the valve heaters.

And thus 0.824V dropped across the 0.47 ohm filter resistor.

This of course equates to a heater current of 0.824/0.47 = 1.75 A

Cheers, Nick

[Richard]

Originally Posted by soulminer

Wouldn't the first cap of the filter after the BR normally bump the voltage up from 5.85 to probably about 7 or 8 volts? Do you have a CRC filter directly after the BR or just an RC filter?

Hi Lee,

It would without a load; it would bump it up to the peak of the AC going into the bridge, 6.37V rms, so around 8V, less the voltage lost in the diodes.

In practice though, once we apply the load, the DC voltage we end up with depends on the impedance of the TX, the load, and (in this case due to the low voltages involved) the Vf of the diodes themselves.

Given that the Standard and XL amps, presumably, had the correct DC voltage before then the reason will be the diodes.

Rich