Hum in MV Twin Reverb

[earache]

The previous two bias measurements are now -55 and -58. The amp is quiet as far as noise..but it seems that it should be louder. Of course I can't turn it up all the way because I am using a single 12" 8 ohm speaker as the load, it's my shop speaker.

[Chuck H]

If poking that pin causes the amp to hum you should reflow the solder on that pin lug. If that doesn't help you should clean any oxide from the tube pin and re tension that socket hole if possible. You can usually poke a straightened out paper clip end in there and gently push the contacts in so they squeeze the tube pin better. If the socket continues to give you trouble it should be replaced. It might be a good idea to just clean all the tube pins and re tension all the power tube sockets completely. If a grid pin loses bias it can blow up that tube.

[earache]

So...I cleaned and resoldered the offending ground pin on the output tube. It was still a little iffy. I swapped tubes around until I found one that seemed to fit best/made the least noise. I can get the amp to run almost completely silent now, save for some "frying bacon" pops and crackles at start up that go away after a minute or two.
I still think that this amp should be WAY louder than it is. It should just about melt my test speaker, but I can turn it way up without it getting too loud.

[Jazz P Bass]

The amp is rated at 135 watts into a 4 ohm load.

Measure the output voltage at the speaker with the amp cranked.
Volts ac.

P=V x V / R

[earache]

So....just clip the leads of my DMM across the speaker, crank the amp up full (master volume too) and just wail away with a guitar and read meter?? I think I at least need a dummy load to do this test..probably a signal generator aswell...or am I missing something?

[potatofarmer]

Double check the "external speaker" jack; it needs to be isolated from the chassis. They used those compressed felt (paper?) washers for this, one with a 'shoulder' and one flat. If the sleeve of the external jack is contacting the chassis, that's no good.

Fender did something clever/stupid there depending on your perspective - the external speaker jack is in series with the main jack. When only the main jack is used, the 4 ohm secondary tap is connected. When both are used, the two jacks are in series and the 8 ohm secondary tap is connected. When only the external jack is used, the 8 ohm secondary tap is connected. So you can use 1x 4 ohm cab (the internal speakers), 2x 4 ohm cabinets, or 1x 8 ohm cabinet.

They put a lot of faith in those little fiber washers.

[J M Fahey]

You miss nothing, that's the basic procedure

That said, it's far from perfect (although it's a rough approximation) because:

* signal is not defined nor constant level: you are plucking at a guitar with varying pick pressure, guitar notes have a strong attack and then die away, you are playing random frequencies, the works, so instead of a human driven guitar it's way more trusted and repeatable to have an oscillator, which can produce a fixed frequency, fixed level sinewave as long as it has power applied.

* speaker impedance varies a lot, so it's better to use a load resistor.

* rated power is measured at *just* reaching clipping .
A human+guitar signal will jump up/down all the time, an oscillator will be stable and allow precise setting.

* You must also have some means to know you just reached clipping , a scope is best.

That said, I often have to do that in the field, generally onstage at some faraway City, so my "no Lab available" method is to:

1) I always carry some test frequency MP3 in my MP3 player , phone or notebook
Download Audio Tone Files

2) speakers are close to rated impedance between 200 and 400 Hz, so using 250Hz or 400/440Hz straight into the speakers is acceptable.

In fact 400/440Hz are slightly optimistic because there speakers are already above normal, so derate that measure by 10% to 20% .

3) if you rise that tone volume sloooowwwllllyyyyyy , specially the lower ones (200/250 Hz) you will notice the point where sound goes from flute like to reedy or buzzy .

Go up/down around that point and you'll get very close to finding onset of clipping .

Now in SS amps that point is very defined and relatively easy to notice, I often amaze friends and customers by describing waveforms with my back towards the scope , but tubes are somewhat more difficult, because there's a "grey area" between clean and dirty .

That said, when clipping, background buzz jumps, so that also helps notice the clipping point.

So in a nutshell:
* use speakers as load
* use a 250Hz tone if available, or a 440Hz one but then derate by 10/20%
* use your calibrated Ear-O-Meter distortion analyzer to detect onset of clipping.
* measure AC volts with any meter, at such low frequencies precision is good.
* do the required Math
* post here

[earache]

Mr Fahey

That is an Awesome and helpful post! I appreciate the time you put into it, the explanations are very clear. I just need to gather all the equipment to do this test as described. Will take a few days.

[earache]

Mr Fahey and all

I am looking at ordering a dummy load (4 ohm x 200W) to test the output as outlined above and to have for the future.

In the mean time, I have a new set of 6L6's that I have swapped into the amp....and still the output is low. I am using a different speaker set to test the amp, 2 x12" speakers that are in an old 100W Lab Series amp I have handy.

Other than an occasional crackle, the amp is quiet...way too quiet! turned up full it is only somewhat distorted but not loud.

I have been checking all the voltages in the amp and found a couple strange readings, all around V5 and V6. The bias reads near what is listed on the schem, just 3 or 4 volts low.

V5 = Pins 3 and 8 expect +12 V but I have no reading there. Pin 1 expects +250V but there is +389
V6 = Pins 3 and 8 expect +110 but there is only +61. Pin 6 expects +280 but there is +387

Before I swap out these tubes, what components would cause this problem? Would it be bad tubes or a fault in the supply circuit(s) leading to them?

[g1]

Have you tried a different tube in V6 position? Or maybe bad connections at the socket?
You can ignore V5 for now, it is for tremolo only and the voltages depend on whether it is turned on or off, in any case, it should not cause the problem you have.

[Chuck H]

It sounds like you have a bunch of bad tubes and/or a bunch of bad grounds and/or drifted components and/or leaky coupling caps. Any of these things could cause the problems you're having. Check for DC under operating conditions at pins 2 and 7 of the questionable circuits. If you don't find anything over half a volt (check meter default) try replacing those tubes.

[earache]

I followed up on your post about the shoulder washers and I have a question. I unbolted the external speaker jack, pulled it away from the chassis (without disconnecting any wires etc ), and I could still measure continuity between the chassis and the sleeve of the jack.

Why the washers then?

[Chuck H]

The jack "-" terminal is grounded where they want it grounded. Which may not be at the chassis where it pokes through for access to your speaker jack. It can be more important than you think. There may be no obvious difference WRT chassis to you and your meter, but to a small signal it can matter.

[pdf64]

The sleeve of the socket seems to be connected to secondary tap of the OT; OT secondaries have a very low resistance, hence the continuity.
Don't assume that because there's a low resistance that there can't be a significant signal voltage.

[earache]

Double check the "external speaker" jack; it needs to be isolated from the chassis. They used those compressed felt (paper?) washers for this, one with a 'shoulder' and one flat. If the sleeve of the external jack is contacting the chassis, that's no good.

Fender did something clever/stupid there depending on your perspective - the external speaker jack is in series with the main jack. When only the main jack is used, the 4 ohm secondary tap is connected. When both are used, the two jacks are in series and the 8 ohm secondary tap is connected. When only the external jack is used, the 8 ohm secondary tap is connected. So you can use 1x 4 ohm cab (the internal speakers), 2x 4 ohm cabinets, or 1x 8 ohm cabinet.

They put a lot of faith in those little fiber washers.

There seems to be an issue wth the external speaker jack as you have described. I found that the quality of sound and hum in the amp changed greatly when poking the connections on the ext speaker jack, with the speaker plugged into the regular jack.

So, I removed the external speaker jack from the chassis but left it connected electrically, just hanging in the air. If I took my chopstick and moved the jack so that it's sleeve touched the chassis, the sound would degrade/crackle and/or drop away almost to nothing.

I returned the jack to the chassis, making sure that the fiber washers were arranged appropriately on the threaded sleeve and I also used a fiber washer before I put the nut on the sleeve so the nut wouldn't short on the chassis.

The amp's sound improved a lot but it seems that the hum as controlled by the "Output Tubes Matching" pot needs to be tweaked every time I turn off/turn on the amp. There is still a fair amount of crackling which comes and goes.

Is it possible that the external speaker jack affects the bias of the power tubes?

Has anybody else had and solved this or a similar external speaker jack problem with this style of amp?

Is there a better replacement part or external speaker jack arrangement?