# Tech Help Please for this PA amp conversion...



## gerald guerrero (Oct 3, 2006)

HI. Can anyone help me with some tech advice. I have a friend who is going to help me convert a Tube PA to a guitar amp. It is a Lafayette model PA 675(a).We dont necessaraly need a schemetic, however my friend isnt certain about the Power tube configuration. There are 3 sockets , two of them are marked EL34. The third on the end in the pics,wothout any resistors tied to the socket is unmarked, and has seperate taps from the the EL34's directly out of the transformer. This doesnt appear to be a power tube, according to him. He cannot figure out waht it may be. It is definitly a socket similar to the two marked el 34 with seperate Power tansformer taps. Any help on this would be really appreciated. I think an EL34 PA amp would make a really great Guitar amp conversion, but we have reached an impasse with no available schematics. Ive searched just about everywhere. Here are the pics...


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## Wild Bill (May 3, 2006)

*"You must walk before you fly, grasshopper!" - Master Po*



gerald guerrero said:


> HI. Can anyone help me with some tech advice. I have a friend who is going to help me convert a Tube PA to a guitar amp. It is a Lafayette model PA 675(a).We dont necessaraly need a schemetic, however my friend isnt certain about the Power tube configuration. There are 3 sockets , two of them are marked EL34. The third on the end in the pics,wothout any resistors tied to the socket is unmarked, and has seperate taps from the the EL34's directly out of the transformer. This doesnt appear to be a power tube, according to him. He cannot figure out waht it may be. It is definitly a socket similar to the two marked el 34 with seperate Power tansformer taps. Any help on this would be really appreciated. I think an EL34 PA amp would make a really great Guitar amp conversion, but we have reached an impasse with no available schematics. Ive searched just about everywhere. Here are the pics...



Looks like this old amp is running two separate rectifier tubes, one for the pair of EL34's and a separate one for the rest of the preamp circuit tubes and maybe the screens of the EL34's. 

I have a REAL hard time with pictures! You have to try to peer under and over to see where wires go. A schematic is so quick and clean. Still, it's hard to get one sometimes. I use http://www.justradios.com a lot. $6-$7 via Paypal and there you go! Cheap!

I wouldn't try to keep much of the existing circuitry. Why? It'll be all wrong for a guitar amp and the parts are all old and many should be turfed. Especially the old filter caps. They dry up over the years even if the amp is in storage. To build a new amp and keep the old filter caps is like restoring a '57 Chevy and keeping the old brake shoes...

If it were my amp I would keep the two rectifiers, if that's indeed what's going on. If the one tube is feeding just the two EL34s in the output power section I'd feed the screens from the same rectifier tube. You could safely feed all the preamp tubes as well and just tape up the leads to the second rectifier tube if you want. They'd draw maybe around 10 ma all together so the extra draw on the first rectifier winding is mice nuts. Having separate supplies makes the amp much more "hifi" - not necessarily good for guitar tone but that's all taste.

I am a litte afraid for you, however. It doesn't sound like your friend has much experience either. The first thing that should be done is MEASURE the voltages coming out of all those wires from the power transformer! You should know how to safely do that as well 'cuz some of them supply several hundred volts that can in extreme cases stop your heart but ALWAYS hurt like a sonofagun!

Not to mention it might blow up something expensive to replace.

You'll find good stuff for newbies at http://www.geofex.com and also you should definitely google up Ampage, the ax84 project and http://www.aikenamps.com to find out how to safely get started. 

It's kinda hard to help you through msg posts. It's like helping a guy over the telephone to rebuild his car engine, and the guy has never popped the hood before! If you can find a good local mentor you can always bring questions back here and we can try to help as you go along.

Not meaning to sound patronizing or negative. I just don't want you to get hurt!


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## Ripper (Jul 1, 2006)

That socket is for your rectifier tube. Could be a 5y3, 5ar4 5au4 or the like.


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## gerald guerrero (Oct 3, 2006)

Well, According to onesponce from another board , it appears it could be a preamp out socket or something , since it is a PA amp. They usually use different sockets, but its possible that a regular tube socket was used. One thing I forgot to mention is that ther is alreadya rectifier tube in the preamp section. Its a 5Y3 , which someone mention is woefully inadequate for two EL34's. If I could only find this schematic, but ive looked everywhere, and noone has it.


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## Wild Bill (May 3, 2006)

*"That's what I mean..."*



gerald guerrero said:


> Well, According to onesponce from another board , it appears it could be a preamp out socket or something , since it is a PA amp. They usually use different sockets, but its possible that a regular tube socket was used. One thing I forgot to mention is that ther is alreadya rectifier tube in the preamp section. Its a 5Y3 , which someone mention is woefully inadequate for two EL34's. If I could only find this schematic, but ive looked everywhere, and noone has it.


This looks more and more like I'm right that this amp ran 2 rectifiers. The 5y3 would run the preamp stages and maybe the screens of the EL34s. There would be another rectifier like a 5U4, probably in the socket to the left of the 3 sockets in the upper left of your picture.

What's more, that's about all the connections to a rectifier tube anyway. A preamp stage will have a lot more stuff connected at the tube socket. You're not going to see coupling caps and resistors at a rectifier socket. There's really not much else a 4 wire socket could be...

It's not all that rare to find 2 rectifiers in a power amp. I've seen it in some Western Electrics, Bogens and Stromberg Carlsons.

Again, if you power up the amp with no tubes in the sockets and check with an AC voltmeter you'll KNOW! The rectifier sockets will have a pair of wires feeding them with a little bit over 5 vac on them. There will be another pair that gives a couple of hundred volts. The socket with the highest voltage will be for the EL34s but you can see that anyway. The centre tap of the output transformer will connect to the socket of its rectifier tube.

Don't mean to harp but if you don't have a schematic you really have no other choice but to measure the voltages. The chances of anybody reading these posts who recognises this amp and can advise you from memory is pretty slim. I'll bet 3 beer (Canadian!) that JustRadios can help with the schematic.


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## Scottone (Feb 10, 2006)

Wild Bill said:


> It's not all that rare to find 2 rectifiers in a power amp. I've seen it in some Western Electrics, Bogens and Stromberg Carlsons.
> 
> .


My Masco MA50 originally used 2 5U4's in parallel, although I only use one since I converted it to guitar amp use (I like the sag  )


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## Ripper (Jul 1, 2006)

I've got an old Harmon Cardon amp here that runs two 5u4s in it and I've seen plenty of amps with that run two. Take a look at a 57 fender twin. They run two as well.


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## gerald guerrero (Oct 3, 2006)

Here is the latest input ive had on this---- External power outlet is possible. But this beast is full-featured 
> already. And the pinout is too much like a rectifier.
> What's the power rating on this beast?
> Are "25V" and "8 ohms" the same wire? That would be 75 Watts.
> If it is 75 Watts, then it should have two rectifiers. One to make ~250V 
> for the preamps, and one just for the 600V-700V needed to claim 75W out 
> of a pair of EL34.
> There's a reason tube amps over 50W were rare before Silicon: one common 
> rectifier is not really enough to go larger. But you might squeeze just 
> the 75W output stage on one big rect, and use a cheaper (and shorter) 
> rect for everything else.
> Further clue: the filter cap to the right of the rectifier to the right 
> of the PT is a single unit isolated from chassis. It is probably in 
> series with another unit to get a >450V total rating.
> And I would assume that power caps of this vintage and brand ARE now 
> useless, you will have to replace them, it would be absurdly expensive 
> to find chassis-caps of the right specs. So you want a good trace-out of 
> the power chain, so you can plan a re-wire with common axial caps on 
> terminal strips (somewhere...).
> Trace the connections assuming it is a rectyfier. Two PT leads to rect 
> plates. Two more PT leads to rect heater. A connection from heater or 
> cathode to big power cap. Since this seems to be the larger rect, it 
> probably feeds the OT CT? Is the bottom of that cap grounded, or go to 
> "+" on another power cap?
> Remove all rectifiers. Take it outside and smoke-test. *Very 
> carefully[/i] get the AC voltage from ground to red leads on mystery 
> socket, and on 5Y4 socket. 500VAC red to ground suggests a ~600VDC 
> supply, 250VAC red to ground suggests a 300V supply for preamps.
> Don't bring up both rects and EL34s until you know where the bias 
> voltage comes from, and that it is ample and reliable. It is possible 
> the bias died, the power tubes sucked huge current, the main rect was 
> obviously cooked, someone took it away for replacement and the repair 
> was abandoned.
> Looking ahead: 75W from a pair of EL34 is really pushing. Maybe OK in PA 
> use. Might be over the top for big-venue guitar duty. And bottle 
> rectifiers are a pain, especially in super-amps. I'd be thinking silicon 
> rects (2,000V rating) and 6550 outputs. The 6550 will stand the abuse 
> and with the more efficient rectifiers you have 90+ Watts (whoopee). The 
> 6550 is a fatter bottle, this might force you to use the two outer 
> big-sockets, rewiring the rect socket for 6550. You might do this anyway 
> because the micanol sockets are dubious at this power and voltage, you 
> want to be ceramic.
Postscript...QUOTE_"I'll bet 3 beer (Canadian!) that JustRadios can help with the schematic."...Wild Bill, make that Three LABAtts Blue please, but only on a hot Texas day, and its got to be cold. I mean 32.01 degrees cold!! On the other hand, I will take a few Molson Golden/Moosehead anytime ! LOL


--*


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## Wild Bill (May 3, 2006)

*"Close enough for rock and roll!"*

I could quibble about a point or two but you've covered enough to be not only in the ball park but covering the right bases.

The two rectifier examples posted by others refer to 5U4s in parallel. This is done when you're talking over 100 watts and you are running a bit low on the voltage side. This means you need more current (Power in Watts=Voltage x Current) and might exceed the current capacity of just one 5U4. From what I can see of the wires in your picture this doesn't seem to be the case.

Lots of Traynors ran 70 watts from a pair of EL34's - more if you're talking clipped and not still clean. They ran plate voltages of maybe 525-530 vdc. 'Course, they also ran to fans. ANY power tube appreciates a cooling fan! Power ratings are confusing 'cuz it depends if they're measured by a guitar tech, a hifi tech or a marketing "suit". Traynor never liked to give power levels in their advertising and if they did were always on the conservative side. You just knew they were loud as hell and didn't care! 

Be careful if you decide to go with silicon diodes in the rectifier circuit. A 5U4 tube rectifier has a voltage drop of maybe 30 volts at a low idle current draw and 50 volts when you're cranking the amp full boogie. A silicon power diode has a drop of may be 1.2-1.4 volts. So your plate voltage would take a big jump. Everything in the amp would have to be able to handle that jump.

If you're trying for more power than the original design you have to make some more "guesstimates". Will the output transformer be able to handle it? You've got a few factors in your favour here. First, unlike guitar amp transformers PA amps tended to overrate their trannies a bit. Their products were designed to be installed in poorly ventilated closets in banquet halls and work for years with little or no maintenance. The trannies will have some "fudge factor" available. Also, PA trannies usually ran to bigger output transformers to give good low end bass response for music than those in lead guitar amps. Lower frequencies need more iron 'cuz bass notes involve more energy. This means that if you intend to never ask any power handling of frequencies lower than low E on a lead guitar you can get away with at least 30% more power happening.

Odds are the power trannie will be good for a bit more current. You'll know when you finally get the amp running. If you wail the amp for a few hours put your hand on the power transformer and see if you can keep it there for at least 15 seconds. If you can't then the trannie's likely to cave sooner or later.

Real HIFI OTs like Hammonds are routinely pushed to over double their power rating and have rarely been known to fail.

What is your approach here, Geraldo? Are you intending to build a classic Marshall or Fender circuit, which means gutting out everything but the trannies? Or are you looking to leave big parts of the circuit intact and just make some mods to tweak things for guitar purposes?

If you wanna just tweak you really should get a schematic so that the rest of us can more easily tell what's going on. If you copy a vintage design we can all easily google a schematic and be on the same page.


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