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PostPosted: 26 Sep 2018, 03:12 
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Joined: 02 Mar 2009, 12:41
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Location: Vänersborg, Sweden
My place is filled with unfinished but started projects. There's a (propably) faulty 60W monoblock stuffed away in a closet, an advanced synthesizer and a 32-step sequenzer on the bed in our spare bedroom and 5 or 6 or 7 (or more) smaller amps ranging from 0.75 to 25W.

But one thing that is very disturbing is what's left of a Soviet 50W tube amp - the famous Priboi. Bought in Estonia for just 600 SEK (roughly 70 USD), at one point modified onto klass A quasi-triode 12W. Now it is emptied of all amp parts but for a HEAVY mains transformer and two HEAVY output transformers. By adding two OP tubes in each channel the amplifier was good for 75W.

I have had an idea for a very loooooooooong time to stuff some good powerful tubes into the chassis and re-install power in the amp using GU50, KT88, EL500/504/509, 6L6, EL34 ... whatever.

My problem: I am lazy or I have a tendency to act as if I am lazy.
The solution: Go for a kit.

I would need two separate boards or subchassis, approx 4" by 8" but I cannot find anything that matches my demands so there are two options in reality:
a) Design my own PCB (it's handy to use PCB's in this case) from a well-know schematic
b) Convert a kit for an SE-amp into a PP-amp.

I know the answer already but just in case I loose my head totally: What would it take to convert a kit (or at least the PCB from a kit) to make a PP-amp?
As an example I pick this one randomly: AliExpress 10W SE amp kit.

I have a lot of info and photos of my Priboi amp HERE.

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PostPosted: 26 Sep 2018, 14:59 
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Joined: 04 Jun 2008, 20:59
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Location: Arizona, USA
Hi, Yes you probably need to have your head examined, but then so do I. I re-purpose boards all the time. The only one that would need changing is probably the one with the output tubes. There are certainly connections for B+, heater and connections for the output of the tubes. The driver stage would need to be pretty much gutted and redone. You could use either a LTP or split configuration. A simpler way might be to get an inter stage driver transformer. Then just use one of the existing triode sections. The actual tube type might have to change. It probably now uses something like a 12AX7. Going to a 12AT7 type ought to work. If it uses NFB then you would have to be sure the phases are right and the amount might need to be adjusted. All in all possible.

Good listening
Bruce

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PostPosted: 26 Sep 2018, 16:38 
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Location: Vänersborg, Sweden
My judgement was that it should be possible to use two boards of this kind and rewire them for PP-operation. However an input/phasesplitter tube has to be added.
And just after I posted my question, I started looking for suitable material on my PC and found articles from Dutch journal Elektor Electronics, describing an EL34 amplifier based on the Williamsonschematics. The article contained full documentations with PCB design, BOMs etc etc and I even saw I hade copied the PCB layout in a CAD-program and just had to make some few changes in order to get all the Gerber files to have someone make me a set of PCBs.
I haven't made up my mind yet how to go forward. Maybe I redesign the schematics to use a more powerful tube like KT88/6550, but still the PCB is almost a perfect match for the amp chassis.

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PostPosted: 08 May 2019, 14:40 
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So I dropped the idea and while preparing for a DIY meeting, collecting stuff to sell and stuff to give away and stuff to keep I found I had at least three quartets of ... no, four quartets of 6L6 or similar. One set of Chinese, one set of Russian another 6P3S-E and a batch of CV1075.

When (or IF :confused: ) I finish the Aussie amp described in Silicon Chip a few years ago and another one with the input/phase splitter/driver cascode done to the SEPP recipe, I still have to batches left and one could be used to solve the issues that made me start this thread, make use of the heavy transformers in my old Soviet Priboi amp.

Well, I have come a few miles by ordering a PCB for a stereo EL34PP. It's a very simple amp - ECC83/12AX7 as input tube, a ECC82/12AU7 as phase splitter and driver. Fixed bias, but ....
The board looks like this: Ebay.

.... as often you lack a some info. Some figures on the enclosed schematic don't match with what's printed on the board and hey - what's the funny markings on the board - some extra components or functions not on the drawing. There are space (and holes) for two relays and after a lot of thinking I think the board is constructed to care for fixed as well as floating bias. The figures A and F indicates that.

So ordering the components I think I needed, I have managed to get this far:

Attachment:
IMG_7056.JPG


Missing the 0.1uF caps that actually are supposed to be 0.47uF and the 22K trimmers. Some electrolytics should be in my storages.

Next, I missed four octal PCB mounted sockets, got only three ... but had the novals :up: .
Being unwilling to pay $10 get the last $1.20 socket from Germany or Finland I cheated. I simply bent the solder tags flat and soldered them to the board using substantial amount of heat and solder.
Not seen on this photo:

Attachment:
IMG_7057.JPG


In due time I will modify the chassis, move some transformers around in order to squeeze the board in place. Hopefully I will then be able to use some of the power from this heavy piece.
Have to move the mains transformer to make way for the board.

Attachment:
Priboi-28.jpg


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PostPosted: 08 May 2019, 21:32 
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Location: Arizona, USA
Ugh, better you than me on this project. What did the supposed schematic look like? The board is probably used in a family of amps that would vary in B+ requirements and output tube types. Your octal sockets are special. Other than the original iron I would not mess with it....however I have done equally unusual things. It is probably why I have lost count of the proto type amps around here. Everything from 6GV8 to KT120s, PP and SE. Let us know how it comes out.

EDIT: The relays are probably for speaker muting during start up. Possibly a B+ delay as another thought.

Good listening
Bruce

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PostPosted: 09 May 2019, 03:14 
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Location: Vänersborg, Sweden
When I have had breakfast I will scan the schematics, but it is fairly simpel. I have built Peter Milletts line amp and headphone amp with the Korg Nu-tube and the PCBs were a dream. These "universal" PCBs I have ended up with, this one, the 'Dynaco' 6V6-PP and and a universal EL34-SE are a mess (more or less).

And if I haven't told you, my good old Soviet amp is a terrible mess much due to the rebuild many years ago and much fiddling since. I think the original amplifier was designed for 2 x 75W using parallelled power tubes (four 6R3S in each channel instead of two) and it would be nice to at at least get a third of that power.

But this is a "longer termed" project. I am fiddling with a bunch of synthesizers and modules and just have to get something to work.

Here's my iPad's version of the schematic:

Attachment:
el34.jpg


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