PSU II voltage problem.. help!

[LSfarzo]

Hi all,
i got a PRE-PSU-PHONO Series II from WAD since 3 years ago together with a KAT6550. I'm very happy with them but only from the last year I'm start really using my phono since I bought a thorens 124 with a sumiko pearl and a linn basik arm...

The fact is that the phono ii sounds... bad!
The sound is closed, dummy, very inflated in the medium-bass section, dark and confused. My God!

I decided to tweak it just as I made with all the other components..but no great improvement came ! though I put Jensens PIO, carbon resist.. etc.

Then I decided to check if everything was fine and I discovered that my PSU gives, for the test point 3 217V with no load and 312V with load.. after some checking I discovered that the Test Point 1 (before the psu choke) is 307 V with no load and 332V with load! there seems to be a problem... the expected ones are 338/363 ! All the other test points that comes after Test point 3, for the phono II line are all lower, too much lower, i think, than the expected voltages!

I checked the tx and it was 247V ac, has expected.. so i checked the output of the D1/D4 bridge and found 307v/332v.. so it seems that the bridge diodes is the one to give the problem of the low voltage
Am I on the right path, in your opinion?

Have you got some suggestion with the kind of diodes I can use? I tweaked the kat by substituting the BR with the bridge discrete diodes and was very happyl. May I do the same on the PSU with the same bridge project (http://www.world-designs.co.uk/forum...ad.php?t=573)? or am I to use some different components?

Please, help.

luca

[John Caswell]

Hi Luca,
Your figures do not make sense as I would expect the voltages to be higher off load and lower on load, have you made a typo or a mistake in readings

John aka Dr John

[LSfarzo]

Originally Posted by John Caswell

Hi Luca,
Your figures do not make sense as I would expect the voltages to be higher off load and lower on load, have you made a typo or a mistake in readings

John aka Dr John

Hi John,

I apologize for having posted the message with errors.
I wrote it all at once becasue I start beiung a little worried about the question as I'm keeping on checking the other voltages around and I'm start wondering that there's something wrong..
Anyway the CORRECT (soory!!) voltages are:
LOAD Condition
1) Test pt.1 302V vs 338V expected (-36V, -10.6%)
2) Test pt. 2 327V vs 312V expected (-25V, 4.8%)
NO LOAD
1) Test pt.1 330V vs 363V expected (-33V, 9.09%)
2) Test pt. 2 322V vs 353V expected (-31V, 8.78%)

Anyway, though this voltages are into the 15% of difference that WAD states to be considered as "normal due to differences in mains voltages"..

Keeping on checking I found the following:

during my check on all points of my PRE-PHONO-PSU I found a strange result on the heater voltage supply coming from the PSU to the valve of phono and pre.
The instructions for the PSU report that, for test points 5 and 7, the voltage has to be (under load) 41V and 47.3 V respectively. My values are 40V and 46V, it seems to be fine.
The strange thing is that on the PSU board it is written that those 2 points should be at 0V and 6.3V ...

As the tabs that correspond to those test pts are tab 5,6,7,8 and as those tabs are routed to the cliffcon sockets that feed PHONO and PRE heater what I find is that the heaters of all the valves of the PHONO and PRE are supplied with a 40V or 46V voltage.
On the instruction manual of the PHONO it is stated that the heater of the ECC83 (pin 4 and 5) valves are expected to be feed ad 6.3V dc regulated from the PSU, and the instruction manual of the PRE states the same for the pin 4 and 5 of the 6922 valve. In my case I'm feeding those heaters with a 46V tension. I found on some 6922 and ECC83 that the heater volatge is expected to be 6.3 V

I guess if this voltage difference is regular or if there is some problem In the case it should be regular I guess if somebody would be so kind as to explain me why it is fine.

The last question is that while checking PHONO test pts I found that some tensions are, maybe, too much high:
1) Test pt 2, LHC V3, pin1 (anode), is 122V while the expected is 95V (+27V, +28.4%)
2) Test pt 5, RHC V2, pin 1 (anode), is 125.5V while the expected is 95V (+30V, +32.1%)
3) Test pt 7, LHC V3, pin 3 (cathode), is 0,79V while the expected is 1V (-0.21V, -21%)
4) Test pt 10, RHC V2, pin3 (cathode) is 0.754V while the expected is 1V (-0.246V, -24.6%)

I guess if those voltage differences (up to 32%) are normal of if there is something wrong and if it is so where I should start checking ..

I hope having been quite correct in reporting the voltages and I apologize for the lenght of the post but I did my best to report all the things I think could help some good sould who's willing to help me!

Luca

[John Caswell]

Hi Luca,
I have had a look at your voltages and offer the following comments:
The heater voltages are correct if you measure them directly across the pins of the valve itself, ie they are 6.2Volts. However if you measure them with the meter negative lead connected to earth then what you are measuring is the DC "offset" plus the heater voltage. The DC "offset" comes from the junction of R2/R3 in PSU II. This raises the heater voltage above earth approx 40v to help reduce the possibilty of heater cathode breakdown in V4, as its cathode is sitting at 120V above earth. So what you have measured is correct, on one side of the heater it is 40VDC and on the other side 40V+6.2V = 46.2VDC so the difference is 6.2V which is exactly what the valve requires.
All the other voltages whilst apparently outside what WAD originally stated are nothing to worry about. I think you will find that if you move valves around the voltage differences will follow them. It is very difficult to find valves where everything is exactly in the centre of tolerances and then you add the fact that you have two triodes in one valve this squares the problem.
Initially I suggest the you monve valves around to confirm what I say, then if it still sounds bad you need to look at possibly replacing valves (they are possibly 3 years old) or trying different makes and let us know how you get on. It is worth keeping a record of valves and voltages found

Regards

John aka Dr John

[LSfarzo]

Hi John,

I wish to thank you a lot for you precise and clear answer; it is really nice to know that there is someone (and maybe all the people on this forum!) who's willing to explain the theorical (and practical) things so nicely.

I will move the valves and the change is and let you know asap.

Bye
Luca