Amplifier principles for MRA5-L6

G1 refers to the "control" grid, which must always be negative with respect to cathode. In this case, it's on pin 3 of the 6Sk7. G2 is the "screen" grid on pin 6. G3 would be the "suppressor" grid.

In these simple amp designs that is correct. The 6SK7 (later replaced by a 6BJ6 or 6BA6) are called 'remote cutoff pentodes. They are designed for use in I-F frequency amplifiers and feature a wide operating curve. That is useful here where it is desired to have a smooth response to the DC control voltage in the AGC circuit. In 1993 I bought some "6SK7" tubes from ICC. They were obviously relabeled of some other types and their sharp curve was totally unusable in an AGC circuit.

Pin 2 on the chassis female, which I'll jump to ground, is pin 2 of the speaker plug's socket. Pins 1, 3 & 4 here are also on the socket.
J3 is the two prong speaker plug inside the cabinet.

I worded my previous response clumsily. I conflated two different points. the jump to enable B+ is on J2 @ pin 2 to ground.

on the mute-squelch, by removing the grounding connection, you mean physically disconnecting it. in terms of the J3, one can unground it just by unplugging it.

Correct, however, if you are working on the amp away from the mech a clip-lead jumper is necessary. Connected, the signal will control the AGC. with ground removed, the squelch will build up and lower the gain.

by "drive," you mean to power the amp with an input signal and speakers connected.

"Drive" here refers only to the signal input, but a load must also be connected to the power amp speaker terminals to avoid an inductive 'kickback" from the output transformer that might damage it, or the output tubes.

Perhaps my terminology was incorrect, because I've seen "Jones plug" in reference to a broad range of connections. I was thinking of a cable that would allow me to connect the amp to the receiver in order to power it on the bench, basically a four-prong extension cord. As it is, I'm sliding the whole thing in and out of WSR in the door each time.

How many of these things are you going to work-on? Years ago I built a test power supply for those old amps using a Jones plug and 6 volt 4 amp transformer.

If you are only going to work on the one you have now, you could just take the selection receiver out, jump the Play Control Relay contacts in the stepper box (to light the 5U4) and mate the two chassis on a table or bench.

I don't have any kind of signal generator to use in lieu of connecting the rca plug from the mech. is there a simple, lo-tech / lo cost signal generator available for this purpose? again, I'm looking to untether the bench and cabinet.

It is a magnetic phono input so a turntable equipped w/magnetic cart can be used. It won't be as loud as the redhead but it will work. You can plug a CD player or tuner, IPOD etc. But they must have a level control to avoid overloading the input and the sound will be way too bassy.

On the schematic, around the first section of the 12AX7 there are a number of circled AC readings-- 0.8, 17, 110, plus 0.24 in a box. The 0.8 seems to be for pin 2 and the 17 for pin 1. The 110 I'm not so sure about and I don't know what to make of the .24 in its rectangle.

The encircled 110 is an error on the MRA-5 print. That is a DC plate voltage and should in the open. I have no idea what the boxed number is about. All the pertinent voltages are accounted for there.

Rob

Guyz,
It would be my assumption that the encircled "110" should be un-encircled, making it a DC voltage of 110 at that point ? Ron Rich

Ok, next round. Been out of action awhile; happy to be back.

Please confirm that the statements are correct. Some have follow up questions.

Cathodes generally require a smooth dc current, except in cases of cathode followers, when they carry an ac current as well.

Control grid must not carry a dc current, but screen and suppressor grids can. Do they also carry AC?

Capacitors can serve as “filters” in two ways. One is by blocking passage of DC current, as the coupling caps do. The other is in shaping the sound, and occurs in concert with resistors. These capacitors are called “line signal” capacitors because of their purpose in the circuit, but they don’t operate differently from other capacitors.

Capacitors affect ac and dc differently in the sense that they block dc current but not ac. I understand that they serve a smoothing function as well. That makes sense with the dc, but would’t that be disastrous as far as the ac is concerned? Or is it that it “smooths” the voltage of the AC signal, but leaves the amplitude & frequency unaffected?

Signal ground and chassis ground are physically the same place— the chassis. Is it possible to ground one but not the other? I can’t find a clear pattern to the chassis and signal grounds on the schematic.

Specific questions:

With what does one replace resistor one, which has three connections and seems to appear twice on the schematic?

Why is there a dc voltage on both sides of c18, which appears to couple the two stages of the L? The voltage on one side is very small.

Are there two separate signal circuits through AVC, one from cathode follower stage of N to L, the other from volume control to 12AX7?

The notes on the schematic say that “Pin 1-3” on mute switch should be shorted when measuring AC. Does this mean pins one through three; that is, all of them? Or does it mean pin one of three; that is, pin #1 only?

As always, thank you to the wisemen.

I can only answer two of your questions--as I don't know tube theory --
R-1 shows up twice on the print as it is a "dual resistor pack", sometimes refereed to as a "standee". The common of both resistors are tied together inside the package, and appear as only one terminal on the outside. The values are as shown in the parts list.
The note says "WITH-- pins 1-3, P4 shorted" (together).
Ron Rich

So it's in effect two resistors. Are both 20W wire wound, or just the 27500 ohm one?

Note 7 on the schematic says that voltages should be read with pin 3 on J4 not grounded. (Presumably these are AC measurements, because note 4 says DC voltages are measured [i]with[/i} terminal 3 grounded.) But Note 8 says pins 1 & 3 should be shorted when measuring signal voltages. Doesn't that just ground pin three, contrary to note 7?

Just be be clear, when measuring DC voltages, there should be zero signal input. With the carriage in play position. does it matter if the signal input (to the amp) is disconnected, or does one let it "play" without a record?

To measure AC, according to note 8, one should introduce a 30 millivolt signal at 1KHz at the cartridge input. If i don't have a signal generator, can I use the signal generated by a record playing? I vaguely recall a discussion about a poor man's signal generator, which I'll try to find.

I'll miss Phonoland, and am very grateful to those who made it happen.

Well, yes, in effect it is, two 20 watt, resistors, should you need to replace one, or both parts--the construction is one single resistor tapped at 25000 ohms--just as the parts list says--
You need look at the center bottom of the schematic-see the "note" that says "see note 8" ?--well if you change that from saying "8", to "7", everything will make sense (yep--someone "messed up") Ron Rich

Cool. Got it.

T minus 5....