Silverton Lap Steel amp

[mresistor]

Got what I think is a Kay Epitome Lap Steel amplifier from the late 40's in today.  The power cord had been cut back to almost a foot in length and what was left had several places of bare wires showing. The insulation was so brittle it was falling off.  So I took the chassis out and found a computer cord, snipped the EIC end of it off and replaced the power cord with grounded outlet. Much safer, and it was an non-intrusive and totally reversible mod.

[mresistor]

This one has a tube complement of a Raytheon 5Y3GT, GM6V6, Tung Sol 6SL7 metal envelope. It has a cap can with 2x 20 uf. And it has some interesting old components in side. I am going to copy the schematic, which is intact in the bottom of the cabinet,  by hand. It has an old 6 inch Rola with the transformer on it.

that's the new donor cord sitting on the PT.

[mresistor]

Guess this thread wouldn't be complete without a guts shot:

[mresistor]

It fired right up and sounds great. Probably could use a new 6V6 though. I tried one of my RCA metal envelope 6SJ7's in it and sounded the same as with the Tung Sol.

[John]

That old stuff is cool!

[HotBluePlates]

This one has a tube complement of a Raytheon 5Y3GT, GM6V6, Tung Sol 6SL7 metal envelope.

You sure you're supposed to have a 6SL7? The schematic appears to show a pentode, not a dual-triode.

[sluckey]

I suspect the tube is really a 6SJ7. I don't think there was ever a metal 6SL7.

     http://www.radiomuseum.org/tubes/tube_6sl7.html

[mresistor]

Sorry - it is a 6SJ7. But the Tung Sol that's in it is a 6SK7.  What's the difference between a "sharp cutoff" and a "remote cutoff" pentode? the SK is a sub for the SJ it appears.

[mresistor]

I drew out the schematic.  Model number letter hard to decipher, could be X or Y.

[HotBluePlates]

Looking at data sheets for both, the 6SK7 is probably close-enough with small signals. The stage may not bias optimally, but I'd have to try both tubes in an amp to see what happens.

What's the difference between a "sharp cutoff" and a "remote cutoff" pentode?

"Sharp cutoff" is like any preamp triode you've used. Except in this case with pentode gain and operating characteristics.

"Remote cutoff" is when a tube has varied spacing of its control grid; a portion is more closely-spaced than the rest of the grid. when the grid is made very negative, the close-spaced portion cuts off plate current. The widely-spaced portion still passes plate current, albeit much less. Thus, it is harder to drive these tubes to cut-off than regular tubes. There is also a fairly-defined area where the tube transitions from normal gain to low-gain operation below the transition.

Typically, remote cutoff stages are used fairly early in a radio receiver for automatic volume control. The signal at a later stage is rectified into a bias voltage that is applied to the AVC stage. Strong radio stations develop large late-stage signals, which result in large bias voltage to the AVC. Weak radio stations develop smaller late-stage signals and therefore smaller bias voltage to the AVC stage. Because the actual gain of the tube is less when the bias is large, this arrangement yields a stage with less gain for large signals than it has for small signals, and your radio can pick up a variety of stations with widely-varying signal strength without overloading the radio or fading out on weaker signals.

[mresistor]

Great info HPB - thanks. ;-p
 

[Willabe]

Typically, remote cutoff stages are used fairly early in a radio receiver for automatic volume control.

Could that be used in a guitar amp as a compressor?


              Brad     

[smackoj]

From looking at your pictures, this amp sure has the looks of an early Valco. Just curious if this Kay might be a re-badged Valco?

 

[mresistor]

smackoj - I am not 100% sure it is a Kay and I've seen posts and pics that call it both. Perhaps it is a Valco Lap Steel amp. Do you have any identifying data? It's kind of like a 48 Supro.

[mresistor]

Here's one with an amp chassis that is almost exactly like it. Its a Silvertone, bit this is a later '54 model it appears.

http://www.ebay.com/itm/Vintage-Circa-1954-Silvertone-Model-1301-Guitar-Tube-Amplifier-Sounds-Awesome-/281572799550?pt=LH_DefaultDomain_0&hash=item418f0bf43e


Yup the circuit is exactly like a Silvertone 1301. But which came first and why doesn't this amp have a manufacturers logo on it?
http://el34world.com/charts/Schematics/files/silvertone/silvertone_1301.pdf

[sluckey]

They look the same to me. Why do you think your amp is from the late '40s? See if you can find some mfg/date codes on the pot or the speaker.

[mresistor]

The chassis I had has the chassis mounts inside and this one I posted has the chassis lip outside.  I would check it out but is wasn't my amp and its gone now. I surmised it late 40's because of another post on the internet with an amp and cabinet exactly like this one. Could be early 50's, but it has the crinkle finish like the ones I saw that were earlier ones. The Silvertone also has a crimp stress relief on the power cord and the one I had was two wire with a knot in the wire inside for stress relief.

[HotBluePlates]

Typically, remote cutoff stages are used fairly early in a radio receiver for automatic volume control.

Could that be used in a guitar amp as a compressor?

Yes, and is largely the same concept as "vari-mu" tubes.

Close grid spacing (usually along with close placement of the grid to the cathode) leads to high-mu, wide grid spacing (usually along with distant placement of the grid to the cathode) leads to low-mu. So when the high-mu portion of the grid is cut off, the lower-mu portion is still going.

The drawback is you typically need a pretty large bias voltage to switch over from higher-to-lower gain, so you'll need to source that from a point later in the amp. You rectify that late-signal and filter it with a resistor/cap to make a d.c. voltage. You'll need to fiddle the cap and resistor values to adjust attack and release times.

I was once hot on this idea, then realized I didn't use compression much in pedal form and probably didn't have sufficient interest to develop the idea. There's very likely some examples on the 'net of others doing something similar, and of course you can always look at old tube compressors for ideas.

[Willabe]

Hmmmmm,   Thanks.



       Brad   

[PRR]

> Could that be used in a guitar amp as a compressor?

In radio AVC the signal frequency is far-far removed from the control frequency. Signal is usually half-Megacycle and control is tenth-second. The high-freq filters easily remove the control voltage from the signal. In audio compression the control voltage is a low audio frequency. Thump.

As you turn-down with large control voltage, current gets small and one half of the wave is clipped. In AM modulation, this does no harm: the tuned circuits round-out the other side the correct amount. In audio, we usually want both sides of the wave, and can't use narrow filters to fake it.

Audio AGC will usually require push-pull for distortion control and to cancel thump. This demands good matching between the push and the pull tube.

Since the signal and control are applied on the same grids different ways, you need an input transformer (or too-complicated equivalent). Since the output DC varies a LOT (often half the B+), you usually want an output transformer so the next stage isn't overwhelmed. Output signal is often small for low distortion and you want more gain after. And as said you need big control voltage so there's another amplifier (or a 2-purpose amp which limits many choices).

There was one, an Ampeg Bass amp? which used a push-pull pair in front of the power amp as a compressor. Maybe somebody knows what I am talking about.

The $49 boxes at the guitar store, or the Engineer's Thumb, do the job a lot easier.

[Willabe]

Ok, just wondering, thanks.   


         Brad