Magnatone 280 speaker oscillating with no signal...and weak vibrato

[christarak]

Greetings.
I have a Magnatone 280. It seems to have 2 Vibrato problems which may or may not be related.
Problem 1: With no input signal the speaker cone moves in & out at Vibrato speed. Here are some clues:
No Vibrato:
Bias to 6973 power tube is -23.2VDC (schematic shows -24VDC)
Plate is 367V (schematic shows 375v)
.
With Vibrato:
Bias is -22V to -24.6v
Plate: 382 to 354v (with no input signal, which would account for the cone excursions)
Question: I don’t think the cone should move with no input signal, should it? Any ideas? The V10 and V11 power valves have been replaced with EH6973’s the V8 and V9 are old RCA’s

Problem 2: (This may not be a problem, as I have no sense of what voltages I should see)
Valve V6: pin 1 oscillates between 107v and 230v
Wiper of Intensity pot: oscillates between +30v and -35v
Question: Is this a high enough sweep of voltages to obtain the legendary Magnatone 280 tremolo?

In a Feb 14, 2011 post, member guitarcapo suggested changing “all the blue .01 uF and .02 uF electrolytic (does he mean blue molded caps) caps on the left side of the amp on that rail near the vibrato controls”. I haven’t done this yet, just in case the oscillator is working OK.

Thanks again,
Chris

[Enzo]

How about a schematic?

If the trem is a bias wiggler, then sure I would not be surprised to see the cone moving.

I don't know what V6 is, but if it is the trem oscillator, then sure there would be a large swing on the plate, and a large swing on the depth pot.

Is it enough sweep? I don't know, you tell us, do you get a good trem effect?

[christarak]

How about a schematic?

If the trem is a bias wiggler, then sure I would not be surprised to see the cone moving.

I don't know what V6 is, but if it is the trem oscillator, then sure there would be a large swing on the plate, and a large swing on the depth pot.

Is it enough sweep? I don't know, you tell us, do you get a good trem effect?

Hi Enzo.

The trem looks like a bias wiggler to me. The vibrato is present, but very weak.

I have tried to attach a schematic, but my ineptness never fails to amaze me. I wonder if this link will work: magnatone 280a amplifier schematic Download page :: Schematics Unlimited

[Mick Bailey]

This isn't a bias wiggler trem. It's is a vibrato circuit that uses varistors as a variable-impedance element to shift the phase of the signal. The bias on your output tubes is varying because there's a signal on the grid, the same with the shifting plate voltages. V6 is the PI and it looks to be the same thing - an input signal on the grid is causing the varying plate voltage.

I may be a long way off on this, but my instinct is to check the 0.002uf coupling caps that connect to V3 grids for DC leakage. These should block the LF modulation signal and if they're passing DC would cause a varying control signal to appear on V3's grids. Easy to scope the grid to see if there's a waveform present there.

My advice is to only replace failed components. Replacing good parts introduces additional risks. Also, be sure not to damage the varistors in any way.

[Tom Phillips]

Your schematic attachment worked well.
Can you post a series of photos of the amp including gut shots so we can see what condition it is in? That will also help us to determine what previous work, if any, has been done to the amp.
Cheers,
Tom

[J M Fahey]

Chiming in to say same as Mick Bailey: Magnatones are THE original "Stereo Chorus" amplifiers, later replaced by SS Roland Jazz Chorus and other clones.
True frequency/phase shifting Vibrato, using (inherently non linear) Varistors as modulation elements.

Since they are 2 pin devices, desirable Audio and unwanted modulation signal mix in the same Varistor, only way to get rid of modulation signal is to balance the thing, so it cancels out.

Notice that modulation networks including varistors are driven simultaneusly from both ends with out of phase signals, see the 8k2 resistors in cathode and plate.

It´s an incredible achievement that they could null out *most* of the modulation signal, personally I wouldn´t mess with it because there is a small chance of improvement and a lot of making things worse.

[christarak]

This isn't a bias wiggler trem. It's is a vibrato circuit that uses varistors as a variable-impedance element to shift the phase of the signal. The bias on your output tubes is varying because there's a signal on the grid, the same with the shifting plate voltages. V6 is the PI and it looks to be the same thing - an input signal on the grid is causing the varying plate voltage.

I may be a long way off on this, but my instinct is to check the 0.002uf coupling caps that connect to V3 grids for DC leakage. These should block the LF modulation signal and if they're passing DC would cause a varying control signal to appear on V3's grids. Easy to scope the grid to see if there's a waveform present there.

My advice is to only replace failed components. Replacing good parts introduces additional risks. Also, be sure not to damage the varistors in any way.

Thank you Mick. I will check out the coupling caps.

[christarak]

Your schematic attachment worked well.
Can you post a series of photos of the amp including gut shots so we can see what condition it is in? That will also help us to determine what previous work, if any, has been done to the amp.
Cheers,
Tom

Thank you Tom. Let's see if these photos work....

[christarak]

Chiming in to say same as Mick Bailey: Magnatones are THE original "Stereo Chorus" amplifiers, later replaced by SS Roland Jazz Chorus and other clones.
True frequency/phase shifting Vibrato, using (inherently non linear) Varistors as modulation elements.

Since they are 2 pin devices, desirable Audio and unwanted modulation signal mix in the same Varistor, only way to get rid of modulation signal is to balance the thing, so it cancels out.

Notice that modulation networks including varistors are driven simultaneusly from both ends with out of phase signals, see the 8k2 resistors in cathode and plate.

It´s an incredible achievement that they could null out *most* of the modulation signal, personally I wouldn´t mess with it because there is a small chance of improvement and a lot of making things worse.

I appreciate your help Juan. I am not sure if you are recommending (a) I check to see that I have a balanced (but phase inverted) from V3B and adjust 8K2 accordingly; or (b) to not mess with the vibrato for fear of making it worse.

The truth is the vibrato is so weak, the customer won't be happy. The speaker cone moves in and out at the Vibrato speed way more than your ears can actually hear the vibrato.

[christarak]

Thank you Mick. I will check out the coupling caps.

Mick can you have a look at V4B? Is that doing any "bias wiggling" into V3A&B? If so, that comes after the 0.002 coupling caps. What do you think, my learned friend?