Hard financial times and a lack of funds has forced me into building a GM70 amp

djnagle

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I got all geeked up about building the JE Lab 6SL7/2A3 amp but then I got a GREAT BOM for Redboy and realized that I did not have the money for all the iron and such.

At the end of last year Bauhausler (Ross) sold me all the iron, tubes, sockets, and choke I need to build a GM70 amp. I slapped my head last night realizing that I could build the amp with not much more money outlay.

So, once again, I am going in a differnt direction. Origanally I was thinking about a three stage amp, but since then I bought a preamp that has some gain to it, so now I am going with a two stage. 6SN7/GM70. I am using the 6SN7 because I have a new pair of them from Tongsol. Here is a pic of the amp chassis with the components I got from Ross more or less in the place they will end up.

I am making aluminum inside corner brackets tomorrow night and making the top plate next Tuesday night. I don't have a final Schem but hope to soon. With any luck I'd like to have this done by the end of Feb.

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I made progress tonight. Just got back from the shop. I made four corner brackts and a top plate. The corner brackets are aluminum and the top plate is 3/16 stainless steel. Here are some pix. I will punch out all the holes next week.
 

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The PT (the black thing) is a Bendix 1300v CT and the green thing is a Raytheon twin 5H choke. I am still pouring through schems to nail that down, but in the mean time I have access to a shop and can do the mechanicals. The OT are custom wond 5K SETs.

The top of the PT is flat with mount holes in each corner. I'd like to find some heat sink fins to mount on top......any ideas?

Of course there will be a Cap farm in front of the choke.
 
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Can output tx be facing the same way so close to eachother??? I don't really understand how that works.
 
I read the title of this thread, and my sarcastic inner child said "Oh, the horror! What a sacrifice!" :D

This may beat the pants off of any 2A3. Those big transmitter tubes HAVE to be VERY LINEAR. A very good thing for a single-ended amp.

And up to as much as FORTY WATTS in single-ended class A?! That's serious!

Regards,
Gordon.
 
Lookin sweet, Dennis! Coupling between output transformers isn't nearly at detrimental as between them and power iron. The outputs can be close, away from the power iron.
 
Here is the build of the base culled for a thread last summer. I was going to use it on a 807 PP amp but the opertunity came a long to get all these parts for Ross.

OK, thanks to Bauhausler, I have all the wood I need to build the base for my amp. I think we have covered all the exotic wood available. The only thing that is not in the picture is the progressive sized dowl rods, five colors of food coloring, and 1/2 inch schedule 80 PVC tubing.
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OK, here is what I got done yesterday. I will finish it off this week sometime. Once this block is all cured up, I will slice it into three 1" x 4" x 20" long boards that will be the front and sides to the amp base. For the back I am just maple.
This is the mold to build this up in. I will use tinted epoxy in between layers.
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This is the bottom piece of Paduk and everything will build on it.
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Here is the final design without the dovetail cuts here and there.
Here is an update: I've got all the dowl rod glued into the boards and will lay up the base stuff tomorrow.
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Hi All, the weather finally got warm enough to lay up all the bits for my base. Here are pics. With this done, I will have my woodworker start cutting and assembling the base next week. It did not go as planned. when I started pouring in the colored epoxy, the wood bits starting floating around and epoxy was gushing in every direction. I had to work fast as the epoxy was setting up as I worked to place all the wood layers in their proper places. The first picture shows the mold with all the bits in place and being smished down with a couple bricks and a heavy steel plate cut from the hull of a WW2 distroyer. The second pic was taken after my wife and I got back from dinner. It was set up enough to pull the bricks and steel off. Cheers.
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Here is what it looked like this morning. Ready to go off to the wood shop.
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I have the three sides cut and planed waiting to go together. I am using a piece of Purple Heart for the back piece. This pic does not do the boards just is. When you hold them up to the light, all the gaps that were filled with colored epoxy glow that color. So I am going to have some LEDs under the amp. They will go on when the power is turned on.
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Lookin sweet, Dennis! Coupling between output transformers isn't nearly at detrimental as between them and power iron. The outputs can be close, away from the power iron.

Thanks Brick, that is what I thought. Seeing as the PT is potted, I will just have to play with the orintation to see if there is any interference. Being potted thought, it might be alright. If there is a problem, I could build a very cool looking Fariday cage.:D
 
Still trying to decide on tube rec or SS rec for the B+. Any arguments one way or the other???
 
Unless you're wiling to go with something like a 866 mercury bulb, I'd think SS would be good. You'll want low voltage drop, just to avoid wasting a lot of heat in the power supply...

If you did want to go with a more "conventional" rectifier tube, you probably would need to use a couple of 5R4s. That's the only thing that comes to mind, that can handle 650VAC on the input...

But you gotta say... how cool would it be, with a couple of mercury bulbs flickering with the music? :D

Regards,
Gordon.
 
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I have been looking at the 3B28 which is a "safe" version of the 866a. I glows purple instead of blue like the 866a. It does not have Merc but, I think, Xenon gas. Here is a link. I gave two of these to Kegger last year thinking I would not use them. Maybe I'll have to be an Indian Giver:D

http://frank.pocnet.net/sheets/127/3/3B28.pdf
 
It's looking great, Dennis. A couple of points:

The OPT is 8K primary.
Gas rectifiers usually require a choke input filter (LcLc) which would limit the B+ to about 600V. I'd go with seriesed 1Kv 3A silicon diodes instead and a cap input filter. (1N5804).

I got over 35W out of those OPTs with GM70s but you have to go Class A2 which is very complicated.

At 10W output the distortion was under 1% and at 1W it was below the noise level.

6SN7 might not give enough gain. Set up in SRPP the gain of 6SN7 is about 18. The output tube gain is about 5. The transformer 'gain' is about 1/33, so the end-to-end gain of the whole circuit is about 3 or a little less. For 10 watts out into 8 Ohms the input voltage would be about 3VAC. Actually, that's doable with a good preamp that has lots of output swing. I like the idea of a 2-stage amp. It's simpler. The driver will have to swing over 200V peak to peak so it should have a fairly high supply voltage. The SRPP heater will probably have to be biased up in voltage to avoid exceeding the heater/cathode voltage rating of the upper triode.

(EDIT)

yes, the data sheet for that xenon rectifier says choke input. Also, you'd need 2.5V at 10A to feed the filaments of those.
 
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I sure do like the looks of the 3B28, but I've given up on it very quickly as I don't have the room. AND, I sure like things to be simple. I will look at how much gain my new pre amp is. It is the Maverick D1 that I will have at the SMAC meeting this Saturday and the gain figures they give in the site give it for headphones not the tube out preamp. I will look futher.

One of the main reasons I thought about the 6SN7 is that I have a new pair of them and money being so tight, I was looking at it from a cost standpoint. I know the 6SL7 has a higher gain...may that would be the one to use.

Speaking of being simple, I have a couple Dell laptop power supplies here and I am going to try to light up one of the GM70 right now. I will report back in 15 minutes.
 
Well, that was an interesting experiment. As soon as I plugged the wires from the filiments of the GM70 into the Dell 20v 4.5a power supply, a breaker in the power supply clicked and shut it down. Oh well.
 
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