Exploding bias supply cap (C17) on VTA board...

...well it would have exploded if I hadn't turned it off FAST - thank goodness for that magic warning smoke, and hissing noise, not to mention that weird stale smell!

The background: just (partially) rebuilt my ST70, and have installed my VTA board for a second time. I ended up preferring the stock board last time I did a head to head between the two, but this time I've used better quality film caps on the VTA board (Clarity Caps). Also bought 3 matched Mullard CV4024 valves so this should give the board a chance to shine this time.

All parts in the Dynaco were new when I first built it around 2 years back, except for the output transformers which are genuine Dynaco. Power transformer is the PA-521-S 240V, from Triode.

Had checked and triple checked and then checked again all the wiring against the VTA diagrams.

From the smoke that came out when I turned it on I thought the damage would be horrendous. However when I looked inside there was just a nasty smell and a little fluid around one of the caps: bias capacitor C17. It's bulging slightly, and was VERY hot.

This is the same 100uF 100V standard grade Black Gate cap I had used in the amp before (but then using a new Dynakits 7199 driver board). The other cap (C18) is fine. I'm assuming I either got unlucky and the re-used cap had a failure, or there is a more serious blunder with my work.

I've built kit amps before, but am still a bit of a novice when it comes to knowing what to measure. First of all, what's the most likely cause of failure for Cap C17 on the VTA driver board? I checked for shorts and polarity and all OK. Ditto the soldering too.

I have some other 100uF/100V caps I can use, but don't want to try those before I know what caused this.

Many thanks,

- John (UK).

And the Idiot of the week award goes too.....

OK folks - STUPID confession time!

I had BOTH C17 and C18 around the wrong way (so much for my saying I'd checked polarity !). I was working on the underside of the board and made one of those silly assumptions that both the negatives on the caps should naturally be connected to the negative/ground trace on the VTA board. If I'd only checked the polarity clearly marked on the TOP of the board...

DOH

Well, at least I've learned to be extra viligant in respect of cap. polarity. I got too cocky!!! Still I guess first fizzled cap in 10 years of DIY isn't toooo bad. Just embarressing!

Hope this saves anyone else from making the same mistake.

- John

EDIT: would this mistake have damaged the UF4007 diode? The amp was only turned on for about 20 seconds. Anything else I should check first before fitting new caps, firing her up again? Ta.

Hey Bob thanks for your reply.

I'm using an older copy of the instruction manual (2+ years old) and it's pretty clear in there too that one should triple check orientation of the caps.

I goofed!!

Anyways new caps installed - polarity checked - and ready for switch on again. Fingers crossed...

...Success!

Sounding fantastic after only 10 minutes. I don't know what was holding back the VTA last time I tried it, but those Clarity Caps seem to have made an improvement. Superb clarity, nice bass extension (it actually plays bass notes rather than waffles through them like many amps) and that valve delicacy on vocals. Very clear and articulate. I like!

Time for bed now alas, but looking forward to giving it a real workout tomorow

Cheers,

- John

OK well it's my first full day with the VTA board and I have to say, this time, it soundly bests the stock Dyna 7199 driver board (when I say stock in my case it was a dynakitparts repro. board with hand matched resistors and high quality film caps). The last time I tested the two the VTA board was using the stock components.

This time around the Orange Drops have been replaced with Clarity Cap ESA film caps, the onboard capacitance has been replaced with 4 x 47uF / 450 Panasonic CE/ED types. Internal wiring was also partially replaced with standard silver plated teflon insulated wire. When I first used the VTA board I was also using solid-core DNM speaker cable... That was 2+ years back. I have since realised the DNM isn't exactly a 'neutral' cable in many systems, and now use inexpensive but good quality Van Damme stranded speaker cable.

Not sure which of those alterations has made a change, but I'm hearing a brand new amp here. Sounds far better than I could have imagined. Loving the articulate bass response in particular. And it's sublime with vocals. Really teases out any/all detail in a recording, WITHOUT throwing it in one's face solid-state style if that makes any sense? If there was a song title to sum up the VTA board I'd select 'Glass Onion' by The Beatles; it just keeps peeling back layers in a song that you thought you knew well. This is a great amp for really seeing (or hearing!) into a recording.

I am glad at this result as I was starting to think I was going nuts preferring a modern stock 7199 driver board to the VTA. With my current setup I get all the warmth and midrange liquidity of the 7199 Dyna, but with modern levels of treble and bass extension.

Valves which sound best to me are currently x 3 Mullard CV4024 types (bargains - grab them while you can!) for the VTA board and x 4 Golden Dragon KT66-R 'Retro' types.

Sublime! Another firm thumbs aloft for Roy's driver board!