Oahu Publishing Company lap steel with in-case amp 1941

[printer2]

Question.

I'm trying to trace the HT rail. Starting at rectifier, goes first to choke, out of choke to voice coil on speaker, out from coil to screens.

Is this valid?

Yes it is.

[xtian]

Thanks!

I snipped out the old e-caps, and confirmed the power transformer is working, at least when unloaded.

Headed out for spring break, so won't be able to get it working for a while.

[johnnyreece]

That box is in near pristine condition. Sell it as a collectors item on eBay. Oh, I guess you are ethically bound to return it to the owner. Ask if you can keep it.

Am I the only one thinking of gutting the box, stuffing e-caps in, and reusing the old leads? I think it would be cool if you could make it look as stock as possible. I mean, it's looked that way almost 75 years...would be cool if it could make it to 100 still rocking its original look.

[xtian]

Am I the only one thinking of gutting the box, stuffing e-caps in, and reusing the old leads?

I like the idea, but THAT box was in there to replace an even OLDER e-cap. Yes, this thing had a cap job back in the third century!

[printer2]

Am I the only one thinking of gutting the box, stuffing e-caps in, and reusing the old leads?

I like the idea, but THAT box was in there to replace an even OLDER e-cap. Yes, this thing had a cap job back in the third century!

Maybe not. I gutted an old radio from the 30's with the same style of capacitor box.

Stuffing old capacitor with new ones is not uncommon.

[Leo_Gnardo]

That power supply setup tells us it's a field coil speaker. The coil acts as a huge choke, good power supply smoothing. Olden dayz amp manuf's ran B+ to the screens, ran those tubes wide open and no regard for the screens. You could opt to put resistors in series with the screens and probably buy a bit of output tube longevity.

Spring break? Who ever heard of such a thing. Time for that around here too but it's still dipping below 10F at night and a foot of hard packed snow still on the ground. This ain't like any spring I ever seen. Well it's likely the local drunks won't be St. Pat's partying like usual out on my lawn in the wee hours. Or if they do, they'll be stuck out there way past sunrise. Too bad for them.

[xtian]

I got the amp fixed!

But what do I do if the speaker is bad? With a Telecaster plugged in, it sounds ok on the higher notes, but the open low E, no matter how quietly strummed, makes a bad buzz. Gently pressing on the speaker cone causes the signal to drop out completely, like there's an open connection. I don't see anything on the speaker coil I can wiggle or tighten or adjust.

Can a field coil speaker be replaced these days?

[billyz]

Fieldcoil speakers can be reconed. Or replaced with a permanent magnet speaker. Your amp appears to then have two chokes. The output transformer is on the speaker and the fieldcoil is also a choke. you could replace the fieldcoil with a resistor , maybe 1k 2w, or even 470 ohm 2watt, etc. are there two more leads from the OT to the voicecoil ? not pictured ?

[johnnyreece]

What are the specs? I don't know much about field coil speakers, but I've got a handful of them at home. I got a bunch of PA speakers/boxes from an old school, and they're loaded with FC speakers. Not sure what kind of shape they're in, but I know some of them worked at the time.

[xtian]

I could only get the speaker to distort when playing my guitar thru the amp. When playing the lap steel, it's acceptable. Client will be by this morning to check it out. I'll let you know if we decide to replace the speaker, but think it's unlikely.

[xtian]

It's back. Owner complains "it sounds bad". I plug it in, and no sound. With a guitar cable plugged into the amp, but cable's other end free, when I touch the tip, I get some great arcing inside a 6V6.

As Jana just pointed out, as if by synchronicity,

Some possible insights into screen voltage being higher than plate voltage.

This happens with some amps that use smaller bottle tubes (6V6, etc.).

I have seen it in some fender amps.

Why?

Ohms law will give a simple explanation for it. The small bottle amps typically have an OT transformer that is uses smaller gauge wire for the primary windings. As a result, there will be a higher DCR in the primary. If you measure the DCR of a 50 watt Marshall OT, for example, it will be ~ 40 ohms or so. If you measure the DCR of the typical 6V6 OT with an 8K primary, the DCR can be somewhere in the range of 100 to 300 ohms (making some guesses here based on old memories--don't quote me).

Then, take into account that the screens on a small bottle usually draw much less current that a big bottle, there is very little voltage drop across the choke.

Because of the DCR of the OT primary, the voltage drop across the primary of the OT is sometimes more than the voltage drop across the choke! So, what you wind up with is a screen voltage that is higher than the plates. It is the nature of the beast. What you can do, which is sounds like you already have, is to increase the resistance between the plate node and the screen node (choke plus resistor, etc.).

I would not try to adjust the screen voltage with the individual screen resistors. If you do that, then you are messing with the compression/feel of the amp as well as changing voltages. Chose the screen resistor value based on the "feel" you want (larger value, more compression--I would stick with 1K). Then adjust the choke/resistor value to get the voltage you want.

This has been the monthly drive-by post by Jana.

This amp has screens at the SAME node as the OT CT, and they are about 10 volts higher than the plates.

[Colossal]

Those cool old orange Aerovox electrolytics were found in Pre-war Gibson amps too. Usually a whopping 10uF in the size of a block of butter.

[mandopicker]

Love it. I restored a Supro 60 from 1939 of a similar look. I had Weber recone the speaker and they did a phenomenal job. Worth every cent.

This is not my image, but I found it freely on the web when I researched the schematic for the restoration. Mine is in similar condition to the one in the photo.

Fully restored with new Electrolytics and a good speaker, the amp sounds great.