Metropoulos Forum

Hi guys..

Tuesday last week when I turned my amp on in the evening I noticed out of nowhere there was a loud hum for a split second and then pop went the HT fuse. The amp was fine the previous evening and has been fine for a couple of years.

I took it to my local guy and we looked over it together. There was some flashover on the V7 output valve socket so we cleaned it up, put in a new fuse and tested it without valves - all seemed ok. We put in some sacrificial output valves and fired it up, all seemed ok again - plate voltages were a normal 480v. I ordered some new screen grid resistors and some new 1ohm cathode current resistors for all the output sockets just to be safe.

Yesterday I fitted the new resistors and fired it up with no valves. There was a pretty high plate voltage reading - 520v. I measured the mains voltage and it was at 255v. I chucked in the sacrificial output valves anyway and tested the voltages again.. 505v on the plates, 504v on the screens. I biased it up and had a play through it - all seemed good. But I thought at the back of my mind that voltage it too high, and unusual for my house.. but it has been quite some time since I've needed to check the voltages inside my amp.

Later on in the evening of yesterday I figured I'd just check the voltages again and see what I had. Same mains voltage. Flicked the standby on and noticed hum again (this time it wasn't there to begin with and then after 5 seconds the hum built up gradually louder), I caught it before I blew the fuse and went to bed to ponder over it.

This morning I cleaned all the sockets out, did a visual check of the wiring and circuit board.. no visible signs. I fired it up again and immediately - bam goes the fuse and V7 blew again - quite dramatically this time.

I'm going to take it over to my local amp guy again to have him give it a proper look over but I wondered if anybody has any ideas? The mains voltage is higher than it has been in the past, it's usually around 230-240v here but it's still (only just) within the 230v +/-6% that Northern Power Grid stipulate. Obviously 504v on the screens isn't good. Normally though the amp runs about 480v on the plates, 479v on the screens. So I doubt the high mains voltage is the cause. The bias supply maybe?

If you have had flash over between pins 3 and 2 then the socket is dead, pushing up daisies, shuffled off its electrical coil etc.

Cleaning up does NOT work.

If you have replaced the 1ohms, the screens and they are still ok you must replace the socket At those voltages I suggest ceramics to minimise this happening again (I guess you have grey Beltons in there now which are also pretty good.

If your house voltage is that variable it may be time to get a power regulator.

Where are you - Australia??

I'm in the UK, just outside Leeds. As I said, it's probably been at least 8 months since I've needed to check the voltages in my amp so I have no idea how long I've been getting a 255v mains supply. I switched energy suppliers about 6 months ago from a green supplier to one of the big six because the green supplier was extortionate in their charges so perhaps the 230v I'm used to is now 255v because of that switch.

So I wrongly assumed it had blown the HT fuse the second time round, but it turned out it was the mains fuse that went - still the same valve, V7 had blown - it actually had an air leak and glowed purple/blue on the valve tester. We went through quite a few checks this time.. primary and secondary OT measurements, checking all AC connections and voltages, checked the rectifier diodes, checked all DC voltages and so on.. nothing was wrong.

So now I'm on to my second set of sacrificial valves (a mismatched set of old JJ's), we fired it up on his Variac and set it to 240v mains on the mains. It read 490v on the plates, which is normal for this amp with that mains supply. We reckecked all the voltages and left it running for at least an hour, put a signal through it and all was fine.

Now the flashover was on the socket of V6 and only present the first time the valve/fuse went - not the second. I also have no idea if that was already present before the first valve/fuse blowing or as a result of it - remember it's V7 that blew as well, not V6 where the flashover was. The flashover was between the anode and screen. I will replace those sockets anyway, thanks for the recommendation.. do you have a particular set of ceramic sockets you like? Maybe send me a link?

I'm wondering whether it might be worth me putting in some kind of voltage dropper/regulator in to the amp itself.. just knock it down 25v or so with some zener diodes? I'm not sure how exactly, but I've heard of people doing that.

It sounds like your amp is running correct plate voltages when your input is stable. In the UK you should not be able to get 255vac at the wall (230+10% or -6%). I usually see 245 at home.
If your amp is picky then I would get a regulator of some sort before modding your amp:
https://sxpro.co.uk/furman-m10xe?gclid= ... gLSj_D_BwE

The way to add zeners is to stack them in series to the centre tap of your PT until you get to where you want to be, not my choice in this situation tho'.

Unless my meter is playing up, which is a consideration, I have been getting spikes up to 280v over the last week. I've had Northern Power Grid out to look at my supply, but they say what is coming in is a solid 244v, he checked for any issues with the neutral but nothing visible. I don't really know what to do about it...

I've had a look at that Furman, will it trip if it goes over a certain voltage? I found one from Tripp Lite that is a single output socket that you determine the over and undervoltage range and will trip if it strays outside that.

https://www.tripplite.com//products/product-detail

Of course, it doesn't filter the supply like the Furman does but I think that might be a little beyond my needs. See what you think.

Please tell me you're not using a slo-blo fuse for HT? Even with those wall voltage spikes I'd hope the HT fuse would trip before taking out the socket.

Still....sounds like your wall voltage is a real problem and a regulator would be a good idea. I hate the Furman stuff because most of their crap is $200 power strips.

Playing live, I've encountered a number of bars that have really dirty electricity, but that's more due to ground loops. I use a TRIPP LITE IS1000. It was under $300 and is a pure isolation transformer. It works so well for a clean line, we got another one to run our vocal PA. I'd recommend this over anything by Furman.

wdelaney72 wrote: ↑

Tue Mar 27, 2018 12:02 pm

Playing live, I've encountered a number of bars that have really dirty electricity, but that's more due to ground loops. I use a TRIPP LITE IS1000. It was under $300 and is a pure isolation transformer. It works so well for a clean line, we got another one to run our vocal PA. I'd recommend this over anything by Furman.

Looks like a useful bit of kit.
Wonder why you are getting those voltage spikes?

wdelaney72 wrote: ↑

Tue Mar 27, 2018 11:56 am

Please tell me you're not using a slo-blo fuse for HT?

If I didn't use a timed fuse on the HT it would most likely pop all the time because of current spikes when flipping the standby on surely?

you're in a tough position, then....using the slo-blo time delay fuses on HT is guarantee you're going to arc a power tube socket.

An other option would be to get a variac and keep your power a bit lower to add insurace for the spikes. tone compromise, though.

I'm a bit late, but just following this up with how things turned out - I don't know what was causing the voltage spikes, possibly duff readings on my part but it turned out that replacing the EL34 sockets did the trick as Neikeel suggested (thanks man).. I got a new quad of Beltons. Thanks for everyone's input on this.