Magnatone 450 amp biasing

[jpcar]

The Magnatone 450 guitar amp seems not particular loud so I’m checking the power tubes bias.
The plate voltages and B+ are considerably lower than the schematic states. (See schematic attached)
For plate voltage I'm getting 340v and the schematic says 410v. I tested the plate current and I'm getting around 7mA. Seems very low.

I’m adjusting the 15k resistor (circled in green on the schematic)
Here’s the results of gradually increasing that resistor and lowering the neg voltage going to the grids.


Resistor | Voltage going to grid | Plate current | Plate voltage

15k -23v 7mA 355v

27k -18v 13mA 325v

37k -14.7v 16mA 313v

54k -11v 21mA 300v

81k -8.6v 24mA 290v


Should I change that 68k resistor going to ground? So these two resistors are like a ratio and affect each other. I should probably go back to lowering the first resistor (green circled one) and then lowering the one to ground too.

I’m trying to get the bias to be something close to the middle (according to the 6973 data sheet)

Plate Voltage 350v
Plate current 44mA
Control Grid -22v

But the more I higher the Plate current, the lower the Plate voltage goes.

Am I going about this the right way?

Thanks for any help!

[Enzo]

Why are we fussing with the bias? YOu have substantially low B+ and the tube current is already low. That means the tube is not dragging it down. It seems to me you have a fundamental power supply issue.

The more tube current you draw, the more it drags down the B+ voltage, that is how it works.

Pull the power tubes, now what B+ do you read at the plate pins of the empty sockets? Have you tried a new rectifier tube?

[Jon Snell]

Increasing or decreasing the bias voltage does not increase the power output.
The anode voltage will reduce as the anode current increases due to the valve rectifier and transformer windings.

[Helmholtz]

I’m trying to get the bias to be something close to the middle (according to the 6973 data sheet)

Plate Voltage 350v
Plate current 44mA
Control Grid -22v

You misread the datasheet. Recommended zero-signal plate current is 2x29mA@350V or 2x25mA@400V.

In addition to what Enzo said:

What heater voltage do you measure? What is your mains voltage?

[jpcar]

Thanks! this is all really helpful.

I was suspicious of the low B+ voltage.
So, the B+ without the power tubes is 365v, and with power tubes is 355v.
Plate pins are a little lower than the B+, maybe 5-10v lower.

The high voltage from the transformer to the rectifier (2 red wires) read:
without rectifier or power tubes: 750vac
with rectifier, no power tubes: 715vac
with rectifier and power tubes: 690vac

Heater voltage with all tubes in: 6vac

Line voltage is 110vac

I checked the 6CA4 rectifier in the Hickok tester. Seems fine.
I don't have another rectifier at hand but I'll order one and new power tubes. All the tubes look like they have been in the amp for many years and the power transformer is original. The capacitor can has been replaced to 40,40,20uF instead of 40,30,20uF.

Thanks for spotting my 6973 datasheet error - zero-signal plate current shown is for both tubes!

[Helmholtz]

Thanks! this is all really helpful.


The high voltage from the transformer to the rectifier (2 red wires) read:
without rectifier or power tubes: 750vac
with rectifier, no power tubes: 715vac
with rectifier and power tubes: 690vac

The drop between "without rectifier or power tubes: 750vac" and "with rectifier, no power tubes: 715vac" looks like a leaky reservoir (first filter) cap.

[jpcar]

The drop between "without rectifier or power tubes: 750vac" and "with rectifier, no power tubes: 715vac" looks like a leaky reservoir (first filter) cap.

You nailed it. Thanks!
Bad first filter cap. Now the B+ is 380v, more much in the range to get this bias right.
I foolishly didn’t suspect that cap because it had been replaced in a can. I had tested for AC with a voltmeter across it but there was 0v. Maybe that method doesn’t always work. The amp did have a slight hum (independent of volume control) which has now gone. That was another clue I missed!

[g1]

380VDC where there should be 410 still seems low, but 110VAC out of the wall is low so that probably accounts for it. I didn't know line voltage got that poor. We are consistently about 124V here.

[jpcar]

380VDC where there should be 410 still seems low, but 110VAC out of the wall is low so that probably accounts for it. I didn't know line voltage got that poor. We are consistently about 124V here.

Actually, my current limiter light bulb was sponging up some of the line voltage. Without the limiter it's more like 117vac and the B+ is more like 410vdc.

[Enzo]

Please always tell us when your voltage readings were taken while on the bulb. Bulbed voltage readings are largely useless. We were trying to cure your low voltage when all the while it was due to the bulb.

[jpcar]

Sorry, my bad.
I thought that bulb limiters are often used when working on amps for safety. I didn't realize the voltage drop would be significant. In this case that first filter cap was definitely a culprit for the low voltage. In future I'll watch for that now that I know!
Thanks everyone.

[Enzo]

A bulb limiter is there to protect an amp that would otherwise be lowing fuses. Once an amp gets stable and not blowing fuses, get rid of the bulb as it affects operation.

[jpcar]

Ok, great to know.
Thanks!