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Lightning clone finished: ouch, high voltages!

7/19/2000 1:43 PM
Lightning clone finished: ouch, high voltages!
Just finished the guts of a Matchless Lightning clone that I finally got around to, and the thing sound great -- very much, well, like the Matchless Lightning I've still got (was going to sell that when this was completed, but with Matchless out of business and prices likely to keep going up I might as well just keep it).  
BUT: I need some suggestions on getting the voltages down just a little...  
The PT is a 280-0-280 from Douglas here in the UK (one they make as standard for Orange), and with a Sovtek 5AR4 I get 360v on pin 9 (screen grid) of the first EL84 and 379v on pin 9 of the second EL84 (379v & 380v respectively on pin 7, the plate). I've put some cheap, new, but unmatched (and visibly even slightly different) Sovtek EL84s in it for starters while still tweaking the thing, so as not to blow any of a handful of NOS Mullards I've saved up. It sounds great as is, but certainly running hot as these voltages.  
I tried it with an NOS 5Y3 pulled from my Champ, and that brought the screen grid voltages down to 320v and 321v -- a little too cool (the reading should be 348v I believe). I've got a Mullard GZ37 coming which I originally ordered from a UK supplier for another amp, then saw on the Angela sight that Steve says this is supposed to drop B+ by about 10-20v compared to a GZ34, so that might just do the trick.  
Any other idease to help me out?  
The only other thing was some fuse blowing: my Matchless (a 240v export model) carried a 1amp fuse when I bought it (used) but the back panel says "2amp slow blow". I put a 1a 250v fuse in my DIY clone for starters just to be safe, and that blew about 1 out of 3 or 4 times of switching on, right as I hit the switch (I've rigged it without standby, just power switch and slow warm-up via the rectifier as in the original). So far with the 2amp fuses I've started using it has been fine, but something tells me you generally halve the fuse amperage rating when you double the mains voltage for European-bound export amps (or did I dream that?). Am I taking a stupid chance with the 2amp fuse, or is that the most sensible sollution?  
Thanks in advance.  
7/19/2000 3:25 PM

Try a new SOVTEK 5Y3 instead.  
They're quite a bit stiffer then a NOS 5Y3GT and you'll see about 15vdc to 25vdc jump in B+.  
I use lots of them in the "under 20 watts" amp class.  
7/19/2000 4:25 PM

Thanks Bruce, I'll order one up.  
Between the Mullard GZ37 and a Sovtek 5Y3 it looks like I should be able to get the voltages there.  
Any thoughts out there on my fuse question?  
7/20/2000 3:04 AM
Steve A.

    What about adding a few 9.1v/5W zeners between the PT CT and the chassis ground? I really like how they work in the FWB power supplies I've been building and you did originally suggest using them with center-tapped PT's...  
    Since you may not know the exact B+ of an amp until it is actually up and running it might be a good idea to wire up a string of these to the PT CT and then connect the chassis ground to the junction which produces the desired voltages. You can also wire this up to a panel mounted switch ("high and low voltage")and at least with the FWB power supply this switching has been dead quiet. (I don't see why it should be any different with a full-wave non-bridge power supply.)  
    Thanks for the tips on using these cheap zeners (I believe that Mouser sells them for under 50 cents apiece if you buy a bunch...) When you first mentioned these (1999? 1998?) I figured that you were just being cheap [ ;) ], but I think that they are a lot more versatile than the 50 watt stud-mount power zeners recommended by the "experts"! And much cheaper, too!  
Steve Ahola
7/20/2000 4:55 AM

Oh yeah, a good supply of lower voltage 5 watt zeners is a must have for any decent "do-it-your selfer" dealing with various B+ voltages!  
Yeah, it has been a few years since I started promoting those inexpensive zeners but in the begining I got a lot of flack from guys who couldn't see how stringing a few little cheap 5 watt 9.1v zeners together could drop this much voltage and survive!  
Shoot, I've even used them in vintage AC30s to get the voltage down a little, and they pull quite a bit of current.  
I bet I've used more then a hundred of them in last couple years.  
I'm glad you are experimenting with them.  
The only reason I suggested the other 5Y3.. the SOVTEK 5Y3 is because they pass a few more volts and it's so easy to pop them in and out of the amp without taking the chassis out of the cabinet.  
7/20/2000 9:28 AM

Thanks guys --  
The chassis is still out of the cabinet (in fact, there IS no cab yet) so I might try linking in some of those zeners. There's plenty of room and easy access to get them between the PT CT and ground.  
1) How many should I use (eg to get the B+ down from 370 to 350)?  
2) Is there an approximate predictable voltage drop per-zener, or indeed is it 9.1v down per each you add on?  
3) I've never seen a zener in my life, but can order them from Mouser if there aren't any available here in the UK. BUT: is there a polarity to them, and if so, which direction do I mount them considering they're going to ground?  
I certainly like the idea of simply popping in a Sovtek 5Y3 and will definitely do that too, but it's nice to have the options. As a halfway house between the two, it's easy (for now!) to get those Mullard GZ37s at a good price, and I presume they're pretty sturdy rectifiers, so a zener or two plus that might be just right... but maybe I'm making it too confusing for myself.  
Well, thanks again.  
7/21/2000 2:09 AM
Steve A.

    After rereading Bruce's original post, you ought to try the Sovtek 5Y3 first... especially since they are in current production and are presumably inexpensive. (I hate it when the only solution is an exotic NOS tube, which is not the case here! ;)    )  
    But to answer your questions, the zeners are polarized and you would mount them with the cathode band going towards chassis ground. They will generate heat so try to keep the body at least an inch off the circuit board.  
    The NTE # for the 9.1v/5W zener is 5124A; I believe that the ECG # would be the same. Mouse sells them in bulk with the 1Nxxxx number (look in the index under "zeners"). If you are going to buy the NTE/ECG ones, you might want to get the 5125A's (which are 10.0v) since Mouser does not sell that voltage in bulk.  
    As for the actual voltage drop, in the negative leg of a FWB the voltage drop seems to match up with the zener voltage. I'm not sure if it would be the same with a non-bridge power supply.  
    I'm amazed at how quiet the switching is... but keep the leads short or they can contribute hum. And with the low voltages involved these could be hooked up to a relay (e.g., you could raise the amp's B+ when switching to the Clean channel or drop it when switching to the OD channel). You would have to check the bias voltage/current at each setting and maybe use one pole of the added relay to adjust the bias...  
    Thanks again to Bruce for telling us about these great little zeners! (If you do the math you can see that 5 watts is much more than enough for a 10v voltage drop in any amp up to 100w; SVT-ers need not apply!) The only drawback I see to these zeners (or any zeners) is that you *are* losing power from your PT; if you were to drop your B+ from 400vdc to 300vdc, I figure that you would be wasting 25% of the capacity of your PT... and radiating that wasted power as heat inside your chassis.  
--Good luck!  
Steve Ahola
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