SCHEMATIC QUESTION FOR OLD AMP

[nightfly61]

Can someone that can read schematics tell me please if this amp plugs into a wall with a regular 110 outlet plug? The power cord was cut from this old beast and all there is is a black cord going into it with a red, black(negative?) and yellow(grnd?) going in. This amp looks to be from the 1950s-ish & hasn't been turned on since at least 1981. All tubes tested "good". Thanks!...here it is...
http://www.miduho.com/jp/st/amp-kairo/rca/MI-9377/mi9377.gif

p.s...I bought an outlet plug for it...is it true that nowadays GREEN=GRND, BLACK is HOT(+) and WHITE=NEG.-?

 

[speakerman]

It is and old parallel/push-pull tube amp. Probably 50-60 watts. It looks like it has three ac taps for various line voltages such as 110, 115, 125 for example. What brand is it? It would be best to bring it up to voltage SLOWLY on a variac.

 

[nightfly61]

It is and old parallel/push-pull tube amp. Probably 50-60 watts. It looks like it has three ac taps for various line voltages such as 110, 115, 125 for example. What brand is it? It would be best to bring it up to voltage SLOWLY on a variac.

It's an RCA from an old movie theater. It says on the label on the inner chassis it's "105-125volts" "180 Watts" A.C. 50-60 CY (whatever CY is) ! I looked all over the web & couldn't find any others like it. Thanks.

 

[speakerman]

CY is for the original cycles per second which later became Hz (hertz) in honor of Heinrich Hertz http://en.wikipedia.org/wiki/Heinrich_Hertz . Use the center tap of the 3 tap choices as the "hot" lead and the other one as the neutral. Be sure to power it up slowly on a variable ac transformer as the power supply capacitors need to be slowly formed. I usually use my old Heathkit tube-regulated power supply to form the caps through a 10k series resistor with the amplifier not plugged in.

 

[nightfly61]

CY is for the original cycles per second which later became Hz (hertz) in honor of Heinrich Hertz http://en.wikipedia.org/wiki/Heinrich_Hertz . Use the center tap of the 3 tap choices as the "hot" lead and the other one as the neutral. Be sure to power it up slowly on a variable ac transformer as the power supply capacitors need to be slowly formed. I usually use my old Heathkit tube-regulated power supply to form the caps through a 10k series resistor with the amplifier not plugged in.

There are 3...red, yellow & black. Do you know which one's which on that old style?

Also, on the new cord is green=ground, black=hot & white=negative? Thanks.

 

[speakerman]

Follow the output of the fuse holder as it will feed the hot side of the power transformer. The schematic shows the yel/blk wire as the likely 117v hot wire connected to the transformer. The solid blk wire is the neutral side. Use an ohmmeter to test. You will be sure of the tap wires by getting a lower reading between all three of those as compared to measuring from the neutral wire (black on the schematic) to any of the tap wires.

I can't stress enough to power it up by means of a variable transformer (variac) when first testing it. You can watch the current draw as the voltage is slowly raised to see that all is kosher and the capacitors are not surged. Transformers and chokes can sometimes be bad also and short causing high current. Don't put over a 3amp fuse in the holder.

 

[nightfly61]

Okay...the red is going to a "patch bay" screw that says 6.3V and the yellow.black is going to one labeled 7.5.
I have an Eico 666 tube & transistor tester...could I use that as power supply? Thanks again very much!

 

[speakerman]

The amp looks like it has 1 terminal strip. The audio in is on TB1 screw 3 ground screw 2 hot. The speaker hooks to screw 7 minus screw 8 plus. AC power is screw 15 neutral (white power cord wire) 14 hot (black power cord wire). Hook the green power cord wire to the chassis. The schematic shows the AC in from the fuse (XF1) is connected to what is likely to be the correct power tap (117v).

I'm not sure what you would do with the tube tester for extracting power. They usually don't run at full B+ voltage. Another way to do it is to pull the 6L6 tubes and wire a 10k 10watt resistor in series with the top side (+) of C7 the 80uf power supply capacitor. You plug the amp in and let it run for 8 hrs. At the end of 8 hrs there should be very little voltage dropped across the 10k resistor if the capacitor reforms properly and is not leaking.
If all is well, unplug and short C7 to ground (across) to discharge it. Pull the 10k resistor out and connect back the way it was. Plug the tubes back in and see if it now works.

Here is an interesting video http://www.youtube.com/watch?v=o3faR_i6Oeo

The amp should sound pretty good although it is not a Williamson Ultra-linear circuit with transformer screen taps.

 

[Tom Wells]

I see internal jumpers being used in the VR tubes. Is this to keep the amp protected from a weisenheimer who would pull
them out to get "more power"?
If there's any major leakage in the cap, the VR tubes won't glow. But without the load of the 6L6s, won't
the 0C3 and the 0D3 go into full conduction and heat the 2500 ohm pretty good?
But if you pull them, C5 and C8 won't reform. So I guess they should be left in.
When the load and current is normal, the VR tubes should be pretty well lit up, in order that they have room to
conduct less as peaks are amplified. If they only light up a small spot, there's something else leaking DC.

As much power as this has on tap, and with no interstage transformer to muddy things up, it should sound great
at any sane level. You can even bypass the input transformer.
I see nothing in the circuit to intentionally roll off highs.
I bet that thing weighs 80 pounds, doesn't it?

Now don'cha wish ya had another for stereo?

 

[nightfly61]

I bet that thing weighs 80 pounds, doesn't it?

Now don'cha wish ya had another for stereo?

Yes it's a beast plus all the wiring is harnessed clean & tight which makes it hard to see where everything's going. That meter on the front...is that a VU meter to measure the sound or is it to measure power? It's cover plate's missing.

Yes, another would be cool for that stereo effect. but I'd have to invest in an audio rack & put a fan in there & could probably heat my house with 2 of 'em!

 

[speakerman]

Yes, another would be cool for that stereo effect. but I'd have to invest in an audio rack & put a fan in there & could probably heat my house with 2 of 'em!

That is why I am giving them up. I love the smooth sound of tube amps as I have had many. I now use a Crown K1 which sounds just as silky and has the dampening factor to kick butt and take names. I use a Bag End http://www.bagend.com/bagend/downloads/isub12_home.pdf for my sub-woofer. It's patent came from the Don Davis days also. It uses an integrator rather than a standard crossover to eliminate the phase shift problems to give realistic TIGHT bass. "Calling all elephants". 8Hz-92Hz with JBL LSR32 monitors for the mid-high. Add a tube pre-amp with Jensen transformers for computer interface with Apple I-Tunes/loss-less encoder on a 500 GIG HD Mac. Makes music fun and easy to manage.