WAD 300B PSE Voltage Problem

[Audionorth]

These monoblocks were my first kit project build almost 20 years ago. Clearly I am still a novice regarding electronic circuit understanding/ troubleshooting but these 300b PSE monoblocks have provided a lot of fabulous hours of enjoyment!

I have gone on to build many other audio kit projects, so I believe I'm appropriately safety conscious but I need assistance with troubleshooting . . .

I started experiencing some intermittent noise (scratchy sounds) from both amps so I decided to look under the hood and measure voltages after replacing a few capacitors. I replaced C1,C5,C6,C13, & C14.

The voltage from the secondary of (one of) the power transformers is 380-0-380 where it connects to the rectifier instead of 395-0-395v. AC input is 117.5 V AC with a Variac. The corresponding voltage is 50v down at "Point B" (approx 300v instead of 350v dc). I switched out the rectifier and then output tubes from the "other" monoblock with no change. The "other" monoblock measures a slightly low B+ of 405v instead of 425v but the other voltages check out. As in the "B through E" test points included with the original documentation are spot on. . .

Is this a transformer issue or another problem likely clear to other members but obviously not to me?

Thanks for your assistance!

Mike
Anchorage, Alaska

[bob orbell]

Mike, I am not familiar with your amplifiers but scratchy sounds are usually poor connections, the most likely being the valve pins, pull and clean all valves and see if that resolves the issue, as for your voltages I am sure someone will be along to advise you. Bob

[Chris]

It's strange that both amps have the same problem at the same time? What pre-amp are you using?

Chris

[Richard]

Hi Mike,

Does it do it on all sources selected, that is, it's not a source problem is it?

Worth trying new rectifier valves as both amps are noisy so swapping rects might have no effect if they're both noisy rects.

Unlikely to be the transformers. Small chance it is the chokes but unlikely both.

That leaves the PSU capacitors as prime suspects if they've not been changed for a few years, probably in descending order of likeliness, but good insurance to change them every few years anyway if they're being run regularly,
C7 and 8, C9, C10,
C11 and 12, C13 and 14, C15
C16

[Richard]

Originally Posted by bob orbell

Mike, I am not familiar with your amplifiers but scratchy sounds are usually poor connections, the most likely being the valve pins, pull and clean all valves and see if that resolves the issue, as for your voltages I am sure someone will be along to advise you. Bob

Yes, indeed, first step to try as Bob says


Your amps use 6AU6 input and I've had noise from dirty pins on those in 2 other WAD designs, particulary when using old NOS valves with oxidised pins. A glassfibre pcb pencil is good for cleaning oxide off, then a wash with alcohol.

[Audionorth]

Thanks for the responses. It will take a couple weeks for me to obtain and swap out some of the power supply caps needed but all the electrolytics are the originals (approx 20 years old) so I will recap the power supply.

I swapped out the preamp I was using and still had the intermittent noise (yes it is strange to have it with both amps - it's been going on for awhile - since it was intermittent I kept thinking it had disappeared . . .) so I will clean my valve pins and experiment with new valves (starting with the rectifiers). . . I certainly have enough lying around here to try

To recap (no pun intended): New power supply caps; clean all valve pins, try new valves if needed, clean valve sockets, and interconnect connections for good measure. . .

This will hopefully take care of the noise problem!

The bigger concern (to me) is the low voltage explained above in one of the amps. So the first step here is to replace the rectifier and PS caps and see if that resolves the voltage problem?

Thanks for the responses!!

[Audionorth]

I finally made the time to swap out the recommended electrolytic caps (mentioned in this thread) that were all originals from my initial build. The voltages snapped back in line and no noise!

I just finished listening to the amps all day with nothing but gratitude for all the help I've received from this forum over the years . . .

Back in business again! Hopefully I'll have another 15 plus years of listening from these amps.

Thank you!

-Mike

[Richard]

Hi Mike, that's nice to hear, great news!

With that sorted, I would go on to swap out C1 C5 and C6 - the cathode caps in both monoblocs.

We weren't looking at them based on your original problem but they will be equally aged and it will mean all the electrolytic caps are now new.

The ones on power valves in particular, such as your 300B valves, are known to fail from time to time and when/if the failure mode is going short the result is often catastrophic for the valve and possibly even the OPTX winding due to the increased current drawn.

Hopefully these would be relatively easy to get at and change on the tag strips

[Audionorth]

Hi Richard,

Thanks for the follow up. I did actually replace C1, C5 & C6.

It's always a relief when I've looked under the hood, made numerous changes, and everything works the first time it's fired up . . .

Again, thanks for the assistance!

-mike

[Richard]

Excellent Mike, glad you're ahead of me
Did you notice any difference after the new caps went in by the way (apart from loss of the obvious noise)?