Another 5B6

[Phil_S]

For your viewing pleasure or misery....
http://home.comcast.net/~psymonds/5B6.html
Some notes:
Made brackets for OT, change from lay down to stand up.
Drilled OT wire holes in the wrong place...woops.
Blank chassis from Dirty Dawg.
Cardboard support on the bleed resistor...for spacing, forgot until I saw the pics
Extra turrets courtesy of a member here...forgot to put them in my order from the supplier...you know who you are, thanks.
0.25 sandblock in the filament supply reduces the old iron to provide 6.3V.
Still missing a #47 bulb.
Recycled parts include OT impedance switch, on/off sw, fuse holder and fuse, pilot assembly, filament wire (from an organ chassis, nice stuff).
You know what you are looking at. I don't have to explain.
I would have reversed (mirror) the build if I had the OT when I started.
OT and PT are both recycled. I'm anxious to see the B+. PT is supposed to be 700V, so I figure it will be more like 720V due to line voltage.
Haven't tried to fire it up just yet, so, as xtian says, it's still a perfect amp.

My favorite tool in this was the Shaviv deburring tool. http://www.ebay.com/itm/Shaviv-Deburrin ... 53f271f4e7
First time I had this for chassis work. Really amazing results. I can stick my pinky in a hole and .... nothing.

[Blackburn]

Awesome man! I really like the hi tech method of shielding that cap from that one resistor!

I actually just dismantled mine. It's going to be something else (can't make up my mind) so I can completely forget the tragedy I've suffered rebuilding it from rat's nest to board. Such a shame because I liked it a lot before. Placement of the pre in relation to the OT as well as all the wire making the leads exponentially longer. Oscillations that few have known... Oh well.

David

[kwijabo]

nice build.

pardon me for being what the french call le incompetent (home alone french lessons!), but what does that 0.25 do, and how is it set up - am i seeing it correctly that it is in series with one of the heater wires?

love that organ heater wire. let us know how it sounds when you fire it up!

[Colossal]

Great job Phil! I like your DIY transformer mounting bracket.

[JazzGuitarGimp]

Good grief man! That OT is STACKED!

[rp]

So Phil, lot of work for that OT, you didn't want to do a cut out w/ jigsaw, hand saw? Is it an old Scott OT?

Fire it up already!

[Phil_S]

The 1/4 ohm resistor on the filament leg... This is a matter of fairly simple toob math. In rounded numbers, with tubes in sockets, filament voltage was 7.0. The goal is to drop 0.7V. The load of the tubes is 2.4A. .07/2.4 = .29 ohms. That is actually darn close to the measured value of the resistor. It has to be placed between the PT and the first tube or it won't work right. Sure enough, it's just spot on for 6.3 at 122V on the line side. Too bad I forgot the pilot bulb was missing. I expect it will go down to about 6.2V, but still that is far better than 7.0. (Actually I think the meter read 6.99V!)

Jazz says, the OT is STACKED! I don't know what it was. The seller says it was PP 6L6 25W. There is a number on it: FT2386668. I am guessing 25W hi fi = 40W guitar. The turns ratio suggest it is about 4K to 4-8-16z. Solving for turns on a voltage test, I get 3658:4, 3457:8 and 4139:16. There is a 1 ohm tap as well that I capped and I thought it was odd to have a 1 ohm secondary -- maybe too low to test, or what? In any case, right or wrong, I judged it as OK for this circuit without further analysis.

There area couple of reasons I didn't want to cut the hole for a lay down unit. One was that I don't like to cut the hole, though I've got tools and skills to do an adequate job in aluminum. Another is that it helped me control placement. A third reason is that I don't like how much real estate gets consumed on the chassis when you do that. I wanted lots of room inside, which I got in spades.

Cutting the brackets for conversion was not that much work. I got a 3' bar of the aluminum at the home despot for about $3 or so. The one with the wire holes -- had to make it twice because I didn't quite understand about the screw holes for mounting to the chassis and didn't make the bracket wide enough. I actually had fun doing that, if you can believe it.

[rp]

...but what does that 0.25 do, and how is it set up - am i seeing it correctly that it is in series with one of the heater wires?

I had to do the exact same thing in a AX84 P1. First time I ever had my math work out perfectly. I HATE kluges like this, and hate those stupid Hammond 290 series w/ the 115/125 primaries.

[JazzGuitarGimp]

Huh, so.... cutting hole in chassis = no fun, whilst building a few brackets = fun!

[Phil_S]

It's all fun. I just didn't want to cut the hole!

[Phil_S]

I found the EIA code and date on the OT: 831844
831= Better Coil & Transformer
According to this http://www.triadmagnetics.com/catalog_t ... egoryId=94 Better Coil was absorbed by Triad in the 1970's.

Look here: http://forums.fender.com/viewtopic.php?f=12&t=66503 maybe it was made in 1968 and maybe it really is a guitar OT?

[rp]

I would have gone for one from Best Coil & Transformer, but I'm sure Better is good too.

[Phil_S]

I fired it up. Good news and bad news. The good news is that voltages seem reasonable and the dim bulb tester says there are no shorts. Probing the grid makes a nice pop, so it is passing a signal.
B+ at the rectifier 365

6L6 plates 347
6L6 screens 292
6L6 cathodes 23

6SL7 plate#2 125
6SL7 cathode#2 3.43
6SL7 grid#2 3.0 (uhg!) pin 1, see the layout

6SL7 plate#1 159
6SL7 cathode#1 1.92

6SC7, plates are bridged/parallel 106
cathode 1.475

That grid with 3V it runs through around to the bottom side of the .1 caps. That's not the big problem. I see 3v on all three resistors on the far side of the board (2x 250k and one 6800), but pin 5 of the 6L6 appears to be at 0v. I was cut short on voltage readings...see below. This is my first build with a paraphase inverter. Maybe this is how it should be...voltage on the second grid? Any comment on this is welcome.

It's arcing where the B+ connects to the board. I'm showing 3 views of it. This is where the red wire connects to the board at the corner. That is also where the OT CT connects. I used 2 pairs of turrets to hold two caps in parallel. There is a 560K bleed resistor across each cap, making 270K total. There sparking at that corner turret. I am not sure where the other end of the spark is going as I shut it down as soon as I saw it and then I tried again and saw it again. No more power until I do something.

Behind the turret, you can see the bolt for the board. I can remove that and leave 3 mounting bolts. It will be fine. Maybe I should? That is what's nearby going to ground. There is a nylon standoff that I can goop with some black Permatex to hold it in place. You think maybe I can just goop it with some silicone to insulate it from that turret?

I have a bad feeling about separating those bleed resistors. I couldn't find a 2W 220K -- this is what I could find. Anyone think that's a problem?

Opinions and help will be appreciated!

Phil

[Phil_S]

OK, I solved the arcing problem. If you look at the middle picture, at the 2nd turret from the corner, you can see a wire lead not wrapped around the turret properly. I clipped it. Problem solved.

On to the preamp section. If you look at the layout, near the volume pot, the three resistors 250K-6.8K-250K that attach to the bottom side of the .1 caps... There is 3VDC on pin 1 of the 6SL7 that follows to the junction of the .1/250K and is also present at the grid stopper and at pin 3 of the 6L6. I think this should be no volts at idle.

Then, there are the preamp voltages. Plate voltage at the 6SC6 is 108. At the 6SL7 it is 159 at pin 5 and 75 at pin 2. I believe the low voltage at pin 2 is because there is 3V on the grid of that section. Is this a leaky cap? How do I check this?

Thanks.

Phil

[ToneMerc]

OK, I solved the arcing problem. If you look at the middle picture, at the 2nd turret from the corner, you can see a wire lead not wrapped around the turret properly. I clipped it. Problem solved.

On to the preamp section. If you look at the layout, near the volume pot, the three resistors 250K-6.8K-250K that attach to the bottom side of the .1 caps... There is 3VDC on pin 1 of the 6SL7 that follows to the junction of the .1/250K and is also present at the grid stopper and at pin 3 of the 6L6. I think this should be no volts at idle.

Then, there are the preamp voltages. Plate voltage at the 6SC6 is 108. At the 6SL7 it is 159 at pin 5 and 75 at pin 2. I believe the low voltage at pin 2 is because there is 3V on the grid of that section. Is this a leaky cap? How do I check this?

Thanks.

Phil

Phil, looking at your layout I see a couple of errors mainly centererd around the 6SL7; pin 6 R/C cathode network should not be connected to the ground rail, it should be elevated by the 25 ohm resistor above ground potential. Also, the wire pre the 6.8K NFB resistor is shunting all your FB to ground, that wire should connect between the 6.8K and 25R, with only the tail of the 25R connected to ground.


I changed the 6SC7 resistor from 250K to 180K and that increased voltages from 135ish to 160V+ range.

https://tubeamparchive.com/download/file.php?id=26956

TM