1939 Recording King resistor refurb

[wsscott]

A friend has asked me to refurb his 1939 Recording King amp, that was partially refurbed a few years ago.  All the caps and about 1/2 of the resistors were replaced.  It needs 3 new pots, and all of the remaining caps are about 30% out of spec, so I am going to replace them too.  This is around a 17 watt amp with dual 6L6's.  It's totally point to point wired.

I have a question about the best method to use in replacing the resistors. Some of them are pretty accessible and not connected at a joint with other components being wired in, like at a lug on a tube socket.  But there are others that are soldered together in a "bundle" of several components soldered together.

Is it better for some, or maybe all, of these replacement resistors to cut out the old resistor and use a J-hook to wire in and solder the new resistor leaving the original lead intact, or is it better to remove the old, original solder joint and resistors, and re-solder them all back together with a new solder joint?

[shooter]

what's the end goal?
you can get an 'ol classic runnin and call it a survivor car, or you can do a frame off restoration.

[wsscott]

To clean up the sound.

[sluckey]

Hi-rez pics please.

[wsscott]

Here are a couple of photos.

[WimWalther]

"Is it better to make a clean, workmanlike job of it, or just scab-on some new parts and call it good?"

Can you tell how I might feel about it? =)

Really, though. This thing is 85 years old, with a majority of parts failed, failing or already replaced. In many cases, the parts they used back then weren't nearly as stable as what we can get today.

I'd be considering a near shotgun-rebuild, saving only the best preserved and most stable parts.

[WimWalther]

Here are a couple of photos.

Nope, they don't make 'em like that no more!

[sluckey]

Here are a couple of photos.

Are those the smallest pics you have?   
Really, you ain't gonna hurt the looks of that amp. Just do what is easiest and call it done. 

[wsscott]

Agreed, the appearance is not salvageable.  I just figured if I was going to replace these components, I would at least want to give it a fighting chance to sound better with the new components.  Maybe it will sound better anyway with new components, regardless of how they are installed.

[WimWalther]

Maybe it will sound better anyway with new components, regardless of how they are installed.

The quality of workmanship shouldn't have much if any effect on the sonic qualities, so long as you make good solder joints.

[shooter]

I would at least want to give it a fighting chance to sound better

do ONE!! socket, test
do 2nd socket, TEST
...
the more you do at once the better chance Murphy comes along and ruins your day

[wsscott]

Good point Shooter.

[AlNewman]

Hopefully you can find an old schematic...  Normally those old ones will have diagrams with voltages at every pin, and that would be a place to start, assuming it's running safely atm. 

Normally I've found it's beneficial, if there's one bad part in a mess of spaghetti, to replace everything in the mess of spaghetti because likely it's going to need to come out anyways to replace the offending part.  On the other hand, if the parts read within spec on your meter, and they've been there for the last 90 years, they'll probably be ok.  I have yet to find a bad potentiometer from that era, but maybe I'm lucky.  I've found quite a few that needed several cleanings though.

Be wary of any original capacitors, as well as any of the large, low value resistors, as they are likely the first to shit the bed.  Normally the only capacitors I'll trust in an old radio is ceramics and micas, in that order. 

The hook up wires can often be the biggest problem in those old amps, the old rubber, plastic, fabric, whatever often gets so brittle that it just flakes off, and if you encounter that problem you'll be in for a fun and educational time. 

What is it doing that it doesn't sound right?

[wsscott]

Yes, I do have a schematic.  No voltages anywhere on it though. Its tone pot was making all kinds of scratchiness, hiss, hum and feedback. So I replaced that with a pot I happened to have, and that seems to have fixed that problem. But there’s still some low level hum. All 3 of the pots were way out of spec by at least 30%, and all of the caps and about 1/2 of the resistors have been replaced.  So I’m going to replace the remaining pots and the remaining resistors, and hopefully that will clean up the rest of the sound to some degree.

Thanks for your input.

[shooter]

some low level hum.

60hz or 120hz?

[wsscott]

60

[sluckey]

Does the PT heater winding have a center tap? If so, is it connected to chassis ground? If not, make an artificial center tap using two 100Ω 1/2W resistors and connect the junction of the resistors to chassis ground.

[shooter]

^^^^what he said
guessing it's a SE PA?
if so;
gator-clip a big-ish E-cap across the cathode R, ~~~47uF

[wsscott]

The guy who worked on it before, and did a lot of work that looks pretty good, replaced the PT with a toroidal transformer, and replaced what I think is called a Field Coil Speaker "output transformer" with a standard 8 ohm OT.

Here's the schematic if you're interested.

[shooter]

so no fil CT and cathode biased PP
artificial Fil CT and a big cap on the 200 ohm'r n the hum should drop
The neon on the heaters might be a cause for hum'n, not sure though

[sluckey]

Show us the tube side of the chassis. Please, no more postage size pics.

[wsscott]

I'm trying to send some photos, but having problems with the size.

[wsscott]

Don't know why it won't upload.   RK101.jpeg

[sluckey]

When you open the attachment window read what's written below it. There's a filesize limit. A single file cannot exceed 2048KB (that's 2Meg).

[wsscott]

I know. It's 1.8M

[wsscott]

See if this one works.

[wsscott]

Here's #2.

[wsscott]

And the last, #3.

[PRR]

> Here's the schematic...

Shows two 6N7. One of your bottles is clearly a 6SC7, and another may not be a 6N7 either?? A 6SC7 in the phase inverter with 6N7 resistors will be very unbalanced.

Have tubes been changed? One bad tube spoils the pot. We had someone was going to re-build his Twin but finally noticed that one 6L6 was just dead (so, not as much as a good Champ).

30% tolerance in pot makes little difference. (New pots may be almost as far off.) Volume controls vary, that's how they work. Same for the tone control which may be less useful than we expect today (more like a static-cut for long-range AM radio). All small resistors can be more than 20% out no big deal. Caps too, but 50% out starts to change things.

Shooter may be seeing a different schematic. I see CT and no 'neon' on heater-- that's a curlicue filament lamp like a Fender.

That parts-replacement style makes me shudder- that's how I would do it, very un-original. I suppose the amplifier can't see its own guts but the people around it might believe it better if it looked like Old Work.

[shooter]

I see CT and no 'neon' on heater

i see the same thing......now 

[WimWalther]

Here is the symbol for a neon lamp, along with the symbol for a cold-cathode gas regulator tube. These are, in fact, very similar devices.

Note that each symbol incorporates a solid "dot" within the envelope, signifying the presence of a gas fill. You won't see this element on any standard incandescent lamp symbol, as they are low-vacuum devices.

[wsscott]

As for the tubes:
2- 6J7's
1- 6SC7
1- 6SC7 MG
2-6L6 GC's
1- 5V4G

I don't know when they were last changed.

I think the Fuse/Power Light holder was changed when the 3 prong lead was installed.  The other 2 bulbs are incandescent to illuminate the pots.

All the caps and about 1/2 of all the resistors were already changed.

My replacement of the tone pot really cleaned it up a lot.  Now just some low level hum.

The Mic Volume pot and circuit has a lot of gain as expected.

The Guitar jacks and circuit all seem to have a normal level of gain.

So it may be that this is as good as it's going to get.  At the end of the day, the only thing "original" about this amp will be the chassis and the cabinet!

[wsscott]

Would you check the bias on this amp like you would on any other cathode biased amp, or is there anything wonky about this circuit that would affect the bias or biasing procedure?

[shooter]

yep, voltage drop across the PA cathode resistor divided by the resistor value, the divided again since it's shared between the 2 PA tubes = tube current.
the PA B+ you should be able to get at The tap created when the field coil was replaced.  at the PA tube gets sketchy if you flinch, twitch and short pins

[WimWalther]

At the end of the day, the only thing "original" about this amp will be the chassis and the cabinet!

Was this amplifier ever owned by a guy named Theseus? He also owned a ship that you might have heard of. 

[wsscott]

From my days of studying Latin and mythology, I do remember the guy Theseus.  He had problems with minotaurs as well as ships!!  Lucky he wasn't fooling around with tube amps!!!

[shooter]

Latin and mythology

I wouldn't want someone speaking a dead language and telling tall tails working on amps either

[wsscott]

Well I found the label on the toroidal transformer and it's a Antek AS 2T350.  Attached is the spec sheet.  Doesn't appear to have a CT.  So, sounds like putting in the 100 ohm resistors might be a solution, at least in part, for the hum.  Unless the other guy working on this did it in some other way.  But I haven't seen any 100 ohm resistors on the heaters anywhere.

[shooter]

just follow the wires, the wires tell all.  check each socket, fil pins, usually a convenient place to add R's
verify both windings are used, or not, make note to self

[WimWalther]

just follow the wires, the wires tell all.  check each socket, fil pins, usually a convenient place to add R's
verify both windings are used, or not, make note to self

You can also measure resistance from a 6.3V filament pin to ground. If it's 50-100R, you have the resistors (or a pot) somewhere. If it measures open, then not.

[Platefire]

I've been kind of watching this project. I'm still wondering what this amp was originally built to amplify---lap steel--PA amp --or what? 

[wsscott]

It's got 3 inputs in one channel for guitar.  Then a separate channel that's hi impedance for a microphone.

It was a National Dobro schematic and the amp was made by Supro/Valco.

Maybe it was for harmonica vs. vocal on the mic.

I can't imagine 3 guitars playing into 1 amp.

That makes me ask the question "Why do amps, like Fenders et al, have a channel that is clean with no effects, and then another channel for Tremolo/Vibrato/Reverb?

If you don't want the trem/vibrato or reverb, just turn it off.

What's the reasoning?  I'm sure someone out here knows!

[wsscott]

I hooked up my dim bulb current limiter to the amp just to see if there were any shorts/problems, etc.  I had not done it previously and it was played with no problems, and I haven't made any changes to any of the components other than replacing the tone pot.  I also hooked up my signal generator for input and connected the output transformer to a speaker.  I plugged in the amp into the current limiter, and turned it and the signal generator on.  The bulb briefly lit at a low level and went out, then as the amp warmed up the tone signal came through clearly, but also the light bulb started illuminating.  It's a 150 Watt bulb.  Wasn't at full brightness for a bulb of that wattage, but I'd say maybe 50%.

Is this a sign of trouble?  I thought the bulb should only briefly light up and then go out.

[Platefire]

Well as an ole timer I can testify about two, three or more plugging into the same amp. Not every picker had a amp in the 40's, 50's and even into the 60's. Just for one instance I recall playing for a dance and our main sound system was my Gretsch 6164 Variety. It had a bass channel and normal channel. The bass, rhythm, lead guitar and vocal mike was all plugged into this amp. We had an un-miked drummer. Yes I understand the need for all those inputs perzactly!

[wsscott]

That's great!  At almost 75, I guess I'm too young. 

I've got the Gretsch Variety Plus 6165 amp that I got when I was in high school, around 1965-1966.  I've also got a Gretsch Country Gentleman that I got about the same time.   I've replaced 1 filter cap, and that's it.  Still going strong.  Plus it's got true vibrato.  It has the normal and bass channel like yours too.

[shooter]

but I'd say maybe 50%.

Would you check the bias

voltage drop across the PA cathode resistor divided by the resistor value

keep the bulb in-circuit, measure then post the current

[PRR]

...I can't imagine 3 guitars playing into 1 amp....

In 1939, that's how they did it. Maybe in film-clips you see an amp per player, but they only made films of "good" bands, not working-class bands. And the Depression was still going on.

FWIW (maybe not much), in 1960s the Beach Boys had Sunn(?) or Fender(?) build one amp for the whole band, and vocals too? They did any gig they could get and needed to keep things simple.

[wsscott]

Cathode resistor is 229.3 and the Cathode voltage is 15.22 VDC so the bias is 6.63 ma.  Pretty low?

[wsscott]

Sorry, or is it 66.3 ma?

[PRR]

> or is it 66.3 ma?

Don't use long fancy numbers. Is 15/230 any easier?

When a problem exceeds my mind, I use tools. Slide-rule won't help you on this point, but any 4-banger or even the Windows Calc will do it.

(Ugh. WinCALC "Digit grouping" only groups to the left, not the right of the decimal point.)