magnatone 280 help

[jjman]

Looks like each preamp gets fed from the same PS node (A). I'm having a hard time reading the scheme. I see 2 separate PIs (v5b, v6b) each feeding it's own pair of tube and each speaker separately. But I think the 2 channels are mixed together in the vicinity of those 3 2.2meg resistors? I would check them too.

[tubeswell]

So... my tube is conducting the way it should. 'Amplification', therefore, is presumably taking place. Yet... not even half the output of channel 2.

Stumped.

Have you checked the coupling cap coming off the V1 plate for channel 2 for DC leakage?

[martin manning]

So... I get around .6 on the cathode (pin 3).

I experimented with cathode resistors until I got the voltages that I wanted. Now my plates are very much in the ballpark, but the volume is still very weak from that channel.

I think you want to verify the 220k and 470R voltage divider feeding V1A's cathode (pin 3) and stick with those values. I believe the schematic actually says 0.8V on pin 3, BTW.

What voltages do you have now on V1 pins 1, 3, 6, and 8?

What voltages do you have on V3B pins 6, 7, and 8?

One of the big dropping resistors coming off the rectifier has drifted to 4.4 meg (I think it's supposed to be 3.3 meg), but I can't imagine that my trouble is coming from that. Second channel has the same voltage, but is loud and proud.

You mean supposed to be 3.3k, right?

Looks like each preamp gets fed from the same PS node (A). I'm having a hard time reading the scheme. I see 2 separate PIs (v5b, v6b) each feeding it's own pair of tube and each speaker separately. But I think the 2 channels are mixed together in the vicinity of those 3 2.2meg resistors? I would check them too.

The two channels are not mixed, but split- both preamps drive both power amps.

[Structo]

Yes this is a stereo amp.

[toneknob]

Ok... back at the bench.

first of all, by now I have swapped out just about every coupling cap in the preamp.

Next, I did go back to the original resistor values for the cathode on V1.

The voltages in question:

V1: pin 1 - 46V, pin 3 - 0.57V, pin 6 - 77V, pin 8 - 0V

V3: pin 1 - 46V, pin 3 - .001V, pin 6 - 250V

I know the problem is staring right at me...

Thanks again, folks. You're keeping me at my bench, and I'm learning a lot about this amp. Everything except how to make it work, that is-- ha!

[martin manning]

I was looking for V3b, that's pins 6, 7, and 8 on V3.

V1's plates (1 and 6) still don't look right- 46 and 71V when they should be 125 and 130.

What is the A power supply node voltage? I think you said 250V before? DO you see the same on the supply side of the Vi plate resistors?

[toneknob]

Ok,

V3B: pin 6 - 222V, pin 7 - 162V, pin 8 - 162V

The Voltage for A (after the dropping resistor that sits atop the cap can): 219V

The voltage on the supply side of the V1 plates is right there (217-218.5)

pretty curious voltage on V3B cathode?

[martin manning]

True. Cathode is too high and plate is too low. Are the 2M2 and 820K on V3B measuring close to those values?

[toneknob]

Ok...

The 2M2 resistor had drifted but the 820 was good.

Now the V3 pin 8 voltage is 86 and pin 6 is 91.

Pin 6 on V1 is unchanged.

???

[martin manning]

Now the V3 pin 8 voltage is 86 and pin 6 is 91.

Pin 6 on V1 is unchanged.

???

V3 pin 6 is 91? It was at least close at 222!? Can you check V3 6, 7, and 8 again? Is the 18k resistor at the cathode on spec?

Your supply voltage is a bit lower (30V) than the schematic. If anything I would expect it to be a bit high running on modern line voltage. I don't think that would explain V1's plates being >50V low. Any change to the sound from Ch 1?

[tubeswell]

Low plate voltage is a result of either low supply (B+) voltage or high tube current. The only other possibility I can think of is something stealing the plate current (like when the plate is DC-connected to another stage, or you have a short in the coupling cap which conducts current through to the ground parallel to the tube and thus pulls the plate voltage down).

So if the B+ voltage looks okay for that tube (and I didn't bother going back and reading through the whole thread to see what it was like sorry), and if you've swapped out the tubes already (to eliminate the possibility of a bad tube), and if you've eliminated leaky or shorted coupling caps, then check the plate resistor and the cathode resistor values. You might have a lower-than-usual cathode resistor value or a lower-than-usual plate resistor value, which makes the tube pass more current (and pulls the plate voltage down).