WAD 300B PP Problem - help please (high volume rustling sound)

[tomtidswell]

Dear all,

I built a PP clone 3 years ago which has given sterling service, but has now developed problems.

The initial problem was a popping, quickly followed by a burst capacitor (either C6 or C7 - rated at 100microF at 200V), presumably either due to a capacitor fault or voltage overload exceeding the 95 V specified. The GZ37 glowed too brightly and I switched off just before the capacitor burst (with smoke).

I took apart the 300B, and decided to rebuild the tag strip boards on the amplifier side (having checked that the voltages from the mains transformer were appropriate and the solid state rectifier delivering power to the capacitors C9/10 which had about 600V across them. The reason for the rebuild was an intermittent hum (worse on tapping the chassis) that had developed in the last year and this was found to be a loose connection to the interstage transformer, but on the other channel to the side that the C6 or 7 capacitor blew.

After the rebuild the amp started up well, and played music for 20 minutes on both channels, then developed a loud rustling noise (sounded like near DC switched on and off randomly through the speakers at near full volume) and I have switched off. No excess glowing valves this time, and no capacitor smoke.

The problem is, I don't know where to restart the fault finding and daren't switch back on until I know. I wonder if it could be one of the 6072 or 5687 valves (which appear normal) - if they throw random voltage swings to the interstage then this could change the voltages across the C6/7 caps causing the initial fault and capacitor blowing, but does seem to be an intermittent fault as they were passing through music for 20 minutes (after the rebuild) before tonight's fault.

Any help would be appreciated. Best wishes - Tom

[John Caswell]

HI Tom,
I suggest initially that you have a very close look at the tagboards that you rebuilt and thoroughly check all connections and terminations to make sure that you have not made any mistakes/poor connections.
C6/7 failing is usually due to a 300B failing, causing it to draw excess current through the associated "cathode" resistor thereby raising the voltage above the limit set on the capacitor. The fact that the GZ37 glowed to brightly would tend to suggest this
Then report back here.

John

[tomtidswell]

Originally Posted by John Caswell

HI Tom,
I suggest initially that you have a very close look at the tagboards that you rebuilt and thoroughly check all connections and terminations to make sure that you have not made any mistakes/poor connections.
C6/7 failing is usually due to a 300B failing, causing it to draw excess current through the associated "cathode" resistor thereby raising the voltage above the limit set on the capacitor. The fact that the GZ37 glowed to brightly would tend to suggest this
Then report back here.

John

Thanks John,

I will do so over the following week then report back.

Tom

[david]

Originally Posted by John Caswell

HI Tom,
I suggest initially that you have a very close look at the tagboards that you rebuilt and thoroughly check all connections and terminations to make sure that you have not made any mistakes/poor connections.
C6/7 failing is usually due to a 300B failing, causing it to draw excess current through the associated "cathode" resistor thereby raising the voltage above the limit set on the capacitor. The fact that the GZ37 glowed to brightly would tend to suggest this
Then report back here.

John

Agree with all John said except to add the cathode bypass caps can sometimes fail on their own, causing excess current to flow thru the rectifier and associated 300b.

[tomtidswell]

Dear John (and David),

Thank-you for the sage advice.

The problem (after the rebuild) was a poorly soldered connection between R4 and the battery mod (visibly looked fine of course). This explained the amp working for 20 mins before the rustling sound.

Having thoroughly checked the circuit (as per John's advice) - this was one of the last 2 joints tested (see Sod's Law - Home DIY Electronics Section).

The 300B is now up and running again and sounds wonderful. I replaced all of the C6/C7 caps and I hope that one blew at random, as cheaper to replace than a faulty 300B. Having gone from the 300B to my reserve amp (Quad 405) then back again, it really highlighted the quality of this amp.

Now it's working again, the AudioNote kits 300B monoblocs are (unfortunately) now off the Christmas list (interestingly these appear to be what Andy Grove designed after making the designs for the WAD 300B with Noel - slightly upgraded transformer coupled amps, but with PSE rather than PP arrangement for the 300Bs; and the C-core transformers look rather lovely - well to my eyes, not my wife's).

Thanks for all your help - very much appreciated.

Best wishes

Tom

[tomtidswell]

I should just add that the Audionote Kit would probably account for the next 10 birthdays and Christmases combined, given the price.

Cheers

Tom

[John Caswell]

Tom,
Have you fitted the batteries in holders?
If so see if you can get the tagged type and solder them in place instead (A sticky base and Tyrap or two work wonders) a much improved method, as I have found that since the lead free directive battery contacts seems be prone to poor contact in their holders.

John

[tomtidswell]

Originally Posted by John Caswell

Tom,
Have you fitted the batteries in holders?
If so see if you can get the tagged type and solder them in place instead (A sticky base and Tyrap or two work wonders) a much improved method, as I have found that since the lead free directive battery contacts seems be prone to poor contact in their holders.

John

Dear John,

Yes the batteries are the tagged types and directly soldered, so there have been no problems there (and two spares in case the NiCads become unavailable in future, but the current ones are still holding 1.35 V each after 3 years of use).

I still haven't tracked down the problem that causes the slight buzzing (mains hum, but slightly distorted, but only barely audible at 2m if doing late night low level listening) - this measures at 5mV across the speaker terminals (nominally 4 Ohm output).

The 300B's appear to be behaving themselves, so hopefully it was a dodgy capacitor that blew (causing the rectifier to glow brighter).

Thanks (and any other suggestions for improvements gratefully received)

Tom

[Richard]

Hi Tom, I had an original WAD version of this amp for a couple of years and rebuilt parts of it to eradicate hum. Some things worth checking are,

1) That the signal ground is "lifted" inside the chassis, which is solidly bonded to mains earth, by 10R. This is easily checked by a meter probe on the outer shell of an RCA input and the other probe on chassis earth looking for 10R. The original earthing arrangement on the WAD drawing shows signal ground and chassis connected solidly together and mains earth connected via a 10R resistor. This is clearly wrong and potentially dangerous. My (secondhand) amp was a sorry mix of the two.

2) Look at the grounding priorities (relative impedances) inside the amp and give high current parts such as the 300B cathode connections their own wires to your main ground point (psu caps). That way they will not cause hum in the sensitive input stage.

3) Check that AC heater hum is being cancelled in the output transformers. Both 300B valves in a pair should have their heaters wired in phase. This is a bit tricky as there are 4 separate windings all the same colour white iirc. I eventually hit on the idea of connecting together one wire from each heater in a pair of valves with a croc lead and measuring the AC voltage on the other 2 wires in the pair (a bit like connecting batteries in series or parallel). Take out the valves inc GZ37 and be extremely careful of the ss ht of course when probing and connecting with croc clips. Have a search of the forum if you have time.

4) Swap out the first stage batteries for cathode resistors (the batteries may be noisy). If you've not done this you might want to try this original arrangement and also without cathode bypass caps. I tried all 3, all clearly different, the resistor alone gives a noticably less sensitive input.

[tomtidswell]

Thanks Richard,

Currently reading 7mV on the left channel (high from my reading) and 4mV on the right channel (probably about right, but could be reduced further).

1. Signal grounds (and speaker outputs) are all lifted by 10 Ohms to the Chassis Ground.

2. I will look at the grounding priorities - it makes sense to wire the 300B cathode grounds directly to ground (rather than via the tag strip board).

3. AC Heaters - will also try - I think swapping the heaters on one Valve would do the same job (either to increase hum or reduce it).

4. Will also try an input cathode resistor or R/C to bypass the battery mods - makes sense that this could be the source of hum.

I will get back and post on my results. Have been driving the amp hard today - all is working (best it's ever been really, and it was always very good).

Best wishes

Tom