Oahu Lap Steel Amp

[Guitarzan]

Hi there.
It's been quite a while since I posted anything here. I hope I'm not doing it wrong.
The extent of my experience is assembling a few amps from kits. The usual suspects like a few 5F1s and a couple of 5E3s. The kits are great for someone who can solder, and understand the safety measures you need to take before diving into one. So I recently acquired this Oahu Lap Steel and matching Amp in MOTS. The amp was a mess however, even though it looked OK at first glance. But 1 of the tube sockets was broken and a lot of things were changed under the hood by someone who probably shouldn't have been in there.
Anyway, the schematic shows a 47uf and a 16uf cap(s) where I would expect to see something like 16uf, 8uf, 8uf, or even 8uf, 8uf, 8uf, like the older champs had. This thing is probably never going to be all back in original condition so I wonder if I should do the power supply filtering like the old Champs or try and follow the schematics version? also there's a resistor on the schematic that is annotated a little strangely or at least outside of my experience. R3 says "1k2" at least I think that's a 2. Anyone know what's up there? Should I just put in a 1k and hope for the best? And finally C10 says it's a 100n. So is it a .1uf cap non-polarized in that spot?


Thanks very much in advance to anyone who takes the time to reply on these questions. I always stand humbled by the brain power on this board and generosity with which it's spread around. Seriously.

[shooter]

"1k2"

believe 1.2k
as for Ecaps, if they are old, change, swollen, oozing change
I'd stick with what "did" work until it works, then tinker if desired

[Guitarzan]

Oh yeah! I'll bet that's right. 1.2k that is.
So the old stuff all had to go. It had a waxy cap that couldn't be read anymore, resistors that were also no good, and the new caps someone before me attempted to install were just a mess. That plus one actually broken tube socket and one so boogered up I decided it was easier to replace than to try and suck up all of the solder and make sure the individual pins would make good contact. So for all intents and purposes it's a totally blank slate. That's the only reason I questioned the 47uf/450 and the 16uf/450.
If it had come to me working but in need of attention I would have tried to save more of it.

[sluckey]

That's not an original schematic. I doubt there were any 47µF or 22µF caps around in 1942. More likely 20, 40, or 50.

[Guitarzan]

How about that? Thanks for the heads up on that. I would have gone around forever thinking it was original. I suppose not being yellowed with age and stained with transformer goo and electrolyte should have tipped me off though.

[Guitarzan]

Hi guys.
So I'm trying to make a little progress on this thing and now I'm puzzled by the annotation again as it refers to the R6 resistor. It doesn't specify it but shouldn't that one be a 5-watt resistor? The Champ has a 470-ohm 5watt resistor there. Others seem to be higher wattage there as well. I'm sorry to be such a pest and a bit of a dolt to boot. I figure it's better to ask and get it right than to just plow into it and potentially release the smoke.

[shooter]

5W should be fine, once you get it working n do the math, might get by with 3w

[Guitarzan]

Thank you very much. I'm getting there so doggone slowly but at least I am actually making progress. I think I'm able to sit here a little longer now too. So I feel good about it on many different levels.

[Guitarzan]

One of the problems I have with schematics is determining when to go with components or even with wires where it looks like there are junctions all over the place.
For example, in this one we have R4, C3, one of the OT primary wires and one of the field coil wires going to G2 on the 6F6 which I believe is pin 4. Pin 6 on that tube isn't used so I can use it as a spare lug if #4 gets too busy but I guess what makes me worry about this is that the OT primary and field coil connection that ends up on pin 4 of the 6F6 looks like they should have just made that connection right at the speaker rather than bringing both wires all the way down to the tube sockets.


Oh, but the field coil acts as a choke for the power supply caps so both of those wires need to come down to the chassis.


I dunno, it just feels like it could be wrong. I wish I was better at interpreting these things and I vow to get better at it. I hate not being able to just look at it and fully understand what it means.

[shooter]

I hate not being able to just look at it and fully understand what it means.

there's 2 schools for that;
the 1st is in E.E. where you do the math, get a deep understanding of the interactions limitations, etc
the 2nd, you play with a circuit using meters and scopes and some basic understanding, then after the 10th time through, all the Christmas lights come on 

[Guitarzan]

Sounds like one is the hard way, and the other is the harder way.
In my previous life I worked on all sorts of HVAC&R systems from the reach-in cooler at the corner bar to enormous combination rooftop units to a coal burning residential furnace out in the desolate open-range area of Eastern Colorado. So I'm comfortable with meters and tracking control wiring down to determine why something is or isn't happening when it should. But even at that I was largely schooled by the old-timers who took me under their wing and helped me along until I was able to go out on my own and (with hope) not kill myself or anyone else. I'm here so I guess I accomplished part of the objective.


So I gave the reservoir caps a home on a couple of tag strips and ran the field coil to them like the magnatone M197. The printed schematic I have shows the field coil hooked up like a choke in place of the 10K 2-watt resistor in this drawing. I don't know why there's a discrepancy between schematics unless it's just a matter of one version vs another.

[sluckey]

The two schematics you posted are clearly different amps. Better stay with the one and let the other one ride.

[Guitarzan]

I really only used it to sort of "borrow" the way they did the filter caps and field coil. Tomorrow, I'll draw the layout as-built and post it here. If you'd be so kind, and have some spare time perhaps you could look it over and see if it OK? I was gonna do that today, but by the time I got a couple of odds and ends tidied up I simply ran out of steam.


It originally had one wire to the heaters and the other side case-grounded. I put in a virtual center tap and twisted the heater wires tightly. What I wasn't sure about though (should read another of the million things) was whether to do the same thing on the 5-volt heater on 5Y3. I mean, I did use both wires from the P.T. twisted but with no virtual center tap. It would be a simple matter to add it though. I considered doing it but again, I just ran out of steam.

[sluckey]

whether to do the same thing on the 5-volt heater on 5Y3. I mean, I did use both wires from the P.T. twisted but with no virtual center tap. It would be a simple matter to add it though. I considered doing it but again, I just ran out of steam.

Absolutely not! There is B+ voltage on those filament wires. 

[Guitarzan]

Actually, that occurred to me as I was drawing the layout as built. I still have a hard time with the concept of high and low voltage, AC and DC sharing the same wire. So.. Big drum roll please. I hope this is at least close to being correct. I have a Variac and dim bulb tester which will be employed for the initial powering up of this thing. Fingers crossed.

[Guitarzan]

I'm trying to redo this with DIY Layout Creator but I've never used it before so it's awfully slow going.
If/when I ever get it done I'll post that.
In the mean time, sorry for the dismal penmanship.

[Guitarzan]

I hope I didn't say or do something to tick anyone off. It just got kinda quiet, it seems. 
Anyway, I took a picture of what I'm thinking/hoping is the finished product.
So with a 2 amp fuse and the light bulb current limiting device can I still blow the thing up if I have something really, grievously, wired wrong? I don't think I have anything that badly out of whack but I just don't have the confidence I used to have.
Please and thanks of course.


I don't know why the picture preview is upside down. It opens correctly though.

[Guitarzan]

Nothing smoked. That's the good news.
So I plugged it in to the Light bulb voltage limiter and first without any tubes I flipped the switch and nothing happened. So far so good. Then I inserted the 5Y3 into its socket and flipped it on again and in maybe 20-seconds, maybe a little less, the filament began to glow. Still so far so good.
Then I inserted the other 2 tubes and nothing happened. Or so it seemed. The reality of it is that the fuse blew. No fireworks or anything. The fuse just quietly checked out. I may have to borrow the fuse out of another amp to test any further since I only just noticed that the little box of five fuses is actually a little box of zero fuses. I'm putting through an order for some but that leaves me with wait until the fuses are delivered or borrow one form elsewhere to do anymore testing today. In the mean time I guess I'll look it over again to see if I can see where I went wrong.

[Guitarzan]

The value of that fuse was so lightly stamped in the metal end as to be practically invisible but with the help of a loupe I was able to make out "250ma" which made me feel a lot better. I dug through my stuff and found a 2a fuse and now the little amp works perfectly. It's totally devoid of any hum and sounds great.
I want to thank Sluckey and Shooter for their input. I truly appreciate your help. Of course it would have been nice if someone had told me why the replies suddenly stopped but that doesn't affect my appreciation for the assistance.
Kind regards to any and all who read this. TTYL
-d {8^)