Oahu Amp? Tube Arraingement

[plexi50]

I have this old i believe early 1940's Oahu tube amp. It is missing a tube. I have no clue what model it may be. The Oahu logo is missing from the grill cloth. I was told that sometimes they sprayed the logo on the grill cloth and that it just faded away over the years. It has a field coil speaker. I have not removed the chassis yet to see if there is a schematic under it

[plexi50]

I found a 1936 capacitor date. Looks to be a mess inside. Someone has already been chopping

[HotBluePlates]

The other 3 tubes are??

The one on the far left is almost certainly the input tube, given the placement of the jacks and the fact it is screened with a shield and has a top-cap.

The one on the far right looks like your rectifier.

Since you're missing the tube next to the rectifier, it is probably the output tube for this circuit. Makes me wonder if it was a 6V6, as the rectifier was replaced at least since the 70's, and someone might have swiped the output tube to use in another amp.

[plexi50]

This is a 6C5G tube next to the top hat input tube. Is this one of two power tubes?

http://frank.pocnet.net/sheets/127/6/6C5.pdf

[plexi50]

Upon use of my glasses i found the tube type stamped on the sockets. Tube 80 is a full wave rectifier

[Ritchie200]

I was just going to say it may very well be for one of the early big bottle 6L6's.  Many of the early juke boxes have the same tube layout and spaghetti mess you have!  To make things even more frustrating, that thing is probably as quiet as a church mouse - unlike my meticulously laid out builds that vibrate off my bench!

Jim

[plexi50]

Well i put a 6L6 in it and turned it on. No sound. I suppose i have to learn more about the field coil speaker to know if it is good or wired properly

[Willabe]

Looks like the 6C5 has an internal shield for noise.


          Brad          

[kagliostro]

Nice Amp

K

[sluckey]

I suppose i have to learn more about the field coil speaker to know if it is good or wired properly

The field coil is just a coil of wire around an iron core. In this case it makes an electromagnet (instead of a permanent magnet we see in modern speakers). The coil will have some resistance. Probably a few thousand ohms. Not 100K!

The field coil was usually connected into the power supply. Sometimes in the B+ line (kinda like a choke), sometimes between the PT CT and ground (to develop a negative voltage).

Since the field coil is integral to the speaker and the speaker cone is usually rotten, the usual way to deal with it is to measure the dc resistance and replace with a high wattage resistor of equal value. And replace the speaker with a modern one.

[PRR]

> learn more about the

Bah. Is there Voltage in it? 6L6 plate-hole is the obvious place to check. If it aint getting Volts, nothing is happy.

One thing about old speakers: the OT is on the speaker. Often there is a plug from speaker/OT to the chassis. With field-coil and OT, it also carries B+ from rect to speaker and back to 6L6. Like most plugs, it works better if it is plugged-in (Stan's Rule #1.)

The thing in the perf-can with a top-cap is a tube. Likely your First Stage. Would be amusing to know what type.

[plexi50]

Likely your First Stage. Would be amusing to know what type.

First tube stage is a 6C8.  I am getting 2.5 vdc on pin #3 and 400vdc on pin #4 of the 6L6 socket
I got a flicker of 407 VDC on pin #3 for a second but can not repeat it. Talk about probable cold solder or crusty tube pins

I quess i have to pull the chassis and check all the power supply caps and take it from there. It's a mess in there*

[eleventeen]

Interesting...somewhat...how the 6C8 (the shielded tube) is branded "Cunningham" (must go back to the late 30's, maybe wartime, as this brand was the predecessor to RCA. Meanwhile...the 80 rectifier...has the much later late-60's-70's blockish "RCA" logo.

I definitely don't enjoy working on stuff that old. Lotsa times you look at it and wonder "how did this ever work?"...but obviously, it did. Once you delve in, though, not so bad. They just look intimidating at first because you're thinking "By rights, I should strip down to the chassis & replace every single part in this".

[plexi50]

I have 397 VDC from the chassis umbilical cord feeding the small transformer mounted to the speaker. I have 3.50 VDC coming out of the opposite lead wire of the transformer as well as any other speaker leads/

The blue wire from the speaker coil needs to return that + - 397VDC to feed the 6L6 power tube plate

[eleventeen]

Sure sounds like a bad OT primary. Continuity test > swap in another. (at the risk of stating the obvious)

If you determine that your repair path is to swap in another OT...I would definitely NOT be surprised if some fault in the amp caused the OT primary to act as a fuse, eg; to open up. Shorted 'Lytic on the cold side of that OT and there you go. Especially when I consider that in 5-tube radios, there was that dual-section electrolytic cap, I think it was a 20-40/150 (don't quote me) that was the failure item in probably 75% of them. Thus, unless you proceed directly to replacing all the 'lytics (no doubt the best plan) IMHO it would be smart to provide some sort of fusing in the B+ so you don't smoke the new one. Could splice in a length of really fine wire pulled out of a length of stranded wire. Once I get past the lamp-limiter stage of testing out a brand new (to me) amp on my repair bench, I usually solder a short piece of fine wire across the fuseholder (and remove the fuse!) so that I am not blowing piles of fuses while working things out. Fuses ain't cheap any more.

[plexi50]

I removed the wires from the OT and measured,it is open. The speaker reads 1 ohm but the voice coil is shot and rubbing badly. The bias resistor is cooked. I think i am reading the color banding right (red-brown-red) as 212r. But it reads 340r. You know that smell of the early 1900's just makes me want to go back to that time. The great depression doesnt sit well but i think i could have worked for fender and hoarded up a bunch of 54 teles and strats. Lets go!!!

[sluckey]

I think i am reading the color banding right (red-brown-red) as 212r. But it reads 340r.

red-brown-red would be 2100Ω. That's a non standard value. I suspect the cooking has changed the colors a bit.

[eleventeen]

"IMHO it would be smart to provide some sort of fusing in the B+ so you don't smoke the new one."

Incidentally, this was meant to be a comment about >>while you work on it<<, not part of the permanent circuit once you're done and have fixed the issue.

Yep, all those '54 Strats!

I remember when I first started dealing guitars, circa 1970. I could have filled up a garage with Strats. They and Les Paul Jrs. were $150 and $175 and I could have bought as many as you wanted.

[plexi50]

The fused OT is a good thing to do. I do that in any of my personal builds. Id rather replace a fuse than an OT. But even that sometimes wont stop a cheap OT with one foot already in the grave.

2200r. Im gona blame the discoloration this time on the bias resistor. But when working with resistors in ohms i have been known to not slow down,stop and think. 2200 it is. Id like to hear what it sounds like if i can round up an 8" field coil speaker.

[PRR]

Looking at RCA 1937 data for 6L6, I'd guess the 300V 4.5K condition at 55mA plate current for 6.5W out.

Adding G2 and little-tube current, say 60mA.

You observe 397V un-loaded. Yah know, I'd pencil a 1K field-coil resistance. 300V at the 6L6, plus a few volts lost in OT, plus 60mA*1K= 60V dropped in FC, is maybe 370V at the first cap, loaded, which is reasonable for 397V un-loaded. Or at least you are likely to run cooler than 300V at 6L6, and a little low is a reasonable target in view of this amp's age.

The 1K resistor dissipates 3.6W, so really should be a 10W part, with lots of cooling air. Since the chassis is already packed, you might have to shot-gun a band-aid tin and mount the FC-replacement off on/near the speaker.

But wait. What's that other "choke" under the chassis?

There's lots of other newer parts. At least two electrolytics and a film-cap. Also I see a wax-cap which is surely past its freshness date. Some care taken: the sleeve on the (surely rotted) line-cord to power switch is a nice touch (new cord would be nicer).

I hate to see historic amps scraped-clear. But between hasty assembly, decades of patches, and the ravages of time, this may really be ready for a deep re-build. Lot of painstaking work. Good winter project.

[plexi50]

I turned it back over to the owner. He didnt want to sink the money into it right now. Times are tough for many so i understand that. Was still nice to see it and go through it.