6550 5U4 valves

[KC Jones]

Good evening, it's been quite a long time since I have posted here but advice is needed.
I have a WAD 6550 KAT with Tungsol 6550, Mullard little thingys and Harma 5u4 rectifier(?) valves. One of the 5U4's has gone kaput and I need to buy another pair. I don't seem to be able to find any anywhere with a quick search of the internet. Now, I have read that rectifier valve makes, don't alter the sound in any way, which is good because I don't want to alter the sound of the 6550 in any way. Therefore would any old cheap make of 5U4 do the job, or is there a difference in sound between makes/price etc? Oh and does anybody know a good place to purchase from?I have only used 'Watford Valves' and ' HotRox' in the past, both were fine to deal with.

Any advice would be welcome.

[Richard]

Yes any 5U4 or Russian 5U3C will be fine. I liked the Svet Russian ones off eBay when I had my 5-20 amps such as,
http://www.ebay.co.uk/itm/5C3S-5U4G-...4AAMXQTZhR1x~g
or Watford and Hotrox will do similar coke bottles.
I would definitely do the balancing resistor mod also if you've not already done it,
http://www.world-designs.co.uk/forum...ead.php?t=3859

[bob orbell]

Hi KC, hope you are well, as Richard said, 5U3 rec. will be fine, as a regular who looks inside your 6550, I cant remember if I have done this mod for you if not, you know how to get to me. Always willing to help. BOB

[bob orbell]

Hot- Rox have 5U4 valves, Electro Harmonix for £13.95 each, or you could buy from a chap on ebay for the princely sum of £600.00 . BOB

[KC Jones]

Thanks Richard and Bob for your replies, I shall get some ordered soon.

Bob - All well here in Wellingborough /stafford. Hope you are fine too.
I'm not sure either about the mod, but when I have the amp up and running again, in a few weeks (Families!) I will pop it into you and you can take a look, if that's ok.

Many thanks -

Kevin

[bob orbell]

Look forward from hearing from you KC. BOB

[bikerhifinut]

If you are prepared to take a punt on eBay, there still seem to be some Russian vendors selling ex soviet miltary equivalents for very good prices. These are possibly better 5U4G than the Chinese stuff (Golden dragon etc) which i suspect from personal experience to be a rebadged inferior directly heated rect as mine do not like a lot of uF on the cathode. Some so called equivalents can only Tolerate 4uF on the cathode!
Has anybody here considered using a GZ34 in place of the 5U4G? Aside from the reduced Voltage drop which may mean using some high power rating anode resistors to drop the volts back down I can't see why not. And you get a nice gradual voltage rise into the PSU caps. A single GZ34 is plenty adequate for a pair of KT88/6550 so a pair should be fine for 4 x Kt88?

Andy.

[John Caswell]

I think the main snag with NOS GZ34 is price and the modern ones do not appear to be up to it.
As the 6550 amp has two 5U4s fitted it is fairly simple thing to replace them both with a pair of "bomb proof" 6D22S.
OK you will have to change the bases to a B9G type and arrange for the top cap to be wired in, but apart from that they run on 5V heater supply very well, are a nice slow warm up and more importantly are cheap! I use them in my GEC KT88/50 without any problems.

John

[bikerhifinut]

Thanks John, I was looking up alternative rects for my EL34 monos after an original Mullard GZ34 failed taking out the anode resistors as described in another thread and came to the conclusion that I may as well stick with GZ34, the JJ ones arent a bad price but I don't know how robust, but I tried a Golden Dragon new valve from a jumble i had in the bits box and sofar thats been fine running 450 to 480V at 160mA in one Mono so running well under its rating. I did notice the anode structure inside the Chinese and other modern valves seems a tad smaller in size so maybe that has issues with cooling and ratings? I think a lot of the issues with GZ34 come when running with vintage stuff like the bigger stereo leak amps where they are hammering the poor old GZ34 on its limits. I think thats why the designs only use fairly low value PSU capacitors to reduce ripple demand? I can see other issues too with subbing one for a 5U4, mainly the reduced forward voltage drop which would entail using some hefty anode resistors to bring the volts back down to the specified voltage. As to cost, the usual mainstream suppliers seem to sell 5U4G and variants for similar amounts as the GZ34, unless you get into the perilous waters of NOS where the sky seems to be the limit these days.