Marshall VS100R hum problem

[g1]

Not sure why, it only seems to be with high cathode voltages. I had problems with them mostly in Marshall cathode follower circuits.

edit: http://music-electronics-forum.com/t29409/

[J M Fahey]

What components should I buy to convert the filament supply (around 14vAC) to an equivalent DC heater supply?

Letīs go step by step, I hate giving blanket answers, prefer to explain the method so it can be applied somewhere else if needed:

1) 14VAC will give you 1.41 times that peak, but in a bridge rectifier we have 2 diodes in series in the path, so :
14VAC*1.4142=17.8v .
each diode drops about 0.7V so we end with 17.8-1.4=18.4VDC .
We can use a 2200uFx25V electrolytic as first capacitor.

2) we need to drop from 18.4V to 12.6V , and filament current is 0.15A so we need a series resistor of:
(18.4-12.6)/0.15=5.8V/0.15A=38.7 ohms.

Power dissipation will be 5.8V*0.15A=0.87W

Nice numbers but we live in the Real World and suppliers can supply what they have on their shelves so, letīs pick "standard" values, close enough.

To begin with, raw voltage will drop a little respect to the ideal value, so I think a 33 ohm resistor will be fine ; IF available a 36 ohm one *might* be closer ... or not, as said above transformer winding might drop a little more than expected under load.

To begin with, I doubt Marshall wanted to cook te tube, what for? , so I bet they "expected" that filament winding to give around 12.6V ... under load that is, so .... I have a hunch that the 33 ohm one will be fine.

As of dissipation, a 1W one WILL work, no doubt, but for peace of mind specify 2W .

And after that resistor, add a second 2200*25 capacitor, this combination is called CRC filtering, because.... weeeeellll ..... you can imagine why

[nickb]

I lost track. What you suggest I can do it but it seems complicated; do you want me to run some tests again??? If we want to be sure just tell me what to do and I will

Have you abandoned this discussion? I was waiting for a response to this post 52

I don't think it's any more complicated than adding a DC supply - cut one track, add two resistors and one capacitor. On the other hand the DC supply needs wiring on the input and output, maybe add one resistor. It's more expensive but is guaranteed to work.

[avoverdu]

Letīs go step by step, I hate giving blanket answers, prefer to explain the method so it can be applied somewhere else if needed:

1) 14VAC will give you 1.41 times that peak, but in a bridge rectifier we have 2 diodes in series in the path, so :
14VAC*1.4142=17.8v .
each diode drops about 0.7V so we end with 17.8-1.4=18.4VDC .
We can use a 2200uFx25V electrolytic as first capacitor.

2) we need to drop from 18.4V to 12.6V , and filament current is 0.15A so we need a series resistor of:
(18.4-12.6)/0.15=5.8V/0.15A=38.7 ohms.

Power dissipation will be 5.8V*0.15A=0.87W

Nice numbers but we live in the Real World and suppliers can supply what they have on their shelves so, letīs pick "standard" values, close enough.

To begin with, raw voltage will drop a little respect to the ideal value, so I think a 33 ohm resistor will be fine ; IF available a 36 ohm one *might* be closer ... or not, as said above transformer winding might drop a little more than expected under load.

To begin with, I doubt Marshall wanted to cook te tube, what for? , so I bet they "expected" that filament winding to give around 12.6V ... under load that is, so .... I have a hunch that the 33 ohm one will be fine.

As of dissipation, a 1W one WILL work, no doubt, but for peace of mind specify 2W .

And after that resistor, add a second 2200*25 capacitor, this combination is called CRC filtering, because.... weeeeellll ..... you can imagine why

Ok, we are on holidays here and I canīt get the parts until Monday. But I have a question: I connected the filament heater to my 14VDC supply, turned the amp on and connected my guitar and there is still hum. The hum on the clean channel is loud!!! so I don't see the point on convert the AC to DC if the hum will stay after the conversion. Am I wrong?

[avoverdu]

Have you abandoned this discussion? I was waiting for a response to this post 52

I don't think it's any more complicated than adding a DC supply - cut one track, add two resistors and one capacitor. On the other hand the DC supply needs wiring on the input and output, maybe add one resistor. It's more expensive but is guaranteed to work.

Sorry mate I'm still here but we are on holidays and the stores are closed until Monday. Ill let you know when I do those changes. I appreciate it!!!

[nosaj]

Ok, we are on holidays here and I canīt get the parts until Monday. But I have a question: I connected the filament heater to my 14VDC supply, turned the amp on and connected my guitar and there is still hum. The hum on the clean channel is loud!!! so I don't see the point on convert the AC to DC if the hum will stay after the conversion. Am I wrong?

Just asking the obvious. You did disconnect the heaters from the transformer when you clipped in the 14VDC?

nosaj

[nickb]

Ok, we are on holidays here and I canīt get the parts until Monday. But I have a question: I connected the filament heater to my 14VDC supply, turned the amp on and connected my guitar and there is still hum. The hum on the clean channel is loud!!! so I don't see the point on convert the AC to DC if the hum will stay after the conversion. Am I wrong?

In post 39 you stated there was no hum using the DC supply. All the tests and answers since that point have been based on that. You are now saying the opposite?

[avoverdu]

In post 39 you stated there was no hum using the DC supply. All the tests and answers since that point have been based on that. You are now saying the opposite?

The post 39 was without the guitar. Today I plugged the guitar with the DC supply hooked to the filament just to be sure that it worth to make the conversion and it hums.

ANYWAY I FOUND SOMETHING ELSE THAT MAYBE IS MESSING WITH ALL THE THEORIES: THE TIP OF THE JACK WAS SUPPOSED TO GROUND THE SIGNAL WHEN NOTHING IS PLUGGED IN AND IT WAS NOT MAKING CONNECTION. I GUESS THIS CHANGES EVERYTHING. PLEASE LET ME KNOW

[avoverdu]

Just asking the obvious. You did disconnect the heaters from the transformer when you clipped in the 14VDC?

nosaj

Yes I did

[nickb]

The post 39 was without the guitar. Today I plugged the guitar with the DC supply hooked to the filament just to be sure that it worth to make the conversion and it hums.

ANYWAY I FOUND SOMETHING ELSE THAT MAYBE IS MESSING WITH ALL THE THEORIES: THE TIP OF THE JACK WAS SUPPOSED TO GROUND THE SIGNAL WHEN NOTHING IS PLUGGED IN AND IT WAS NOT MAKING CONNECTION. I GUESS THIS CHANGES EVERYTHING. PLEASE LET ME KNOW

It should only make a difference if the second shorting contact were broken also as that one mutes the signal input to the tube. Earlier you said that it hummed with all the knobs at zero and no guitar plugged in. All the tests were supposed to done that way. If you changed them at any time it invalidates everything.

Right now I have zero confidence in any of the results - there are too many contradictions. I suggest you fix the jack and carefully check the operation of both switching contacts and we'll re-evaluate the situation.

[avoverdu]

It should only make a difference if the second shorting contact were broken also as that one mutes the signal input to the tube. Earlier you said that it hummed with all the knobs at zero and no guitar plugged in. All the tests were supposed to done that way. If you changed them at any time it invalidates everything.

Right now I have zero confidence in any of the results - there are too many contradictions. I suggest you fix the jack and carefully check the operation of both switching contacts and we'll re-evaluate the situation.

Hey Iīm back and I fixed the jack. These are the new tests:

Everything on zero, no guitar plugged in, all the values are in AC volts from measuring across the speaker:

1) Nothing, just turning on the amp: CLEAN CHANNEL=0.24 OVERDRIVE CHANNEL=0.184

2) R48 TEST: I placed one end of the jumper cable on the main ground and the other end on the side of R48 that connects with R49 (just to make sure im doing it right)
CLEAN CHANNEL=0.001 (no hum) OVERDRIVE CHANNEL=0.152

3) R77 TEST: I placed one end of the jumper cable on the main ground and the other end on the side of R77 that connects with R78 (just to make sure im doing it right)
CLEAN CHANNEL=0.145 OVERDRIVE CHANNEL=0.002 (no hum)

[nickb]

OK Thanks. Can you try the DC heater supply once more please and measure the ACV out across the speaker?

[avoverdu]

OK Thanks. Can you try the DC heater supply once more please and measure the ACV out across the speaker?

Everything on zero, no guitar plugged in, all the values are in AC volts from measuring across the speaker:

DC Supply: CLEAN CHANNEL=0.070 (approximately, it fluctuates up to 0.040) OVERDRIVE CHANNEL=0.012

Thanks

[nickb]

Everything on zero, no guitar plugged in, all the values are in AC volts from measuring across the speaker:

DC Supply: CLEAN CHANNEL=0.070 (approximately, it fluctuates up to 0.040) OVERDRIVE CHANNEL=0.012

Thanks

So the DC supply helps quite a lot. You can
(a) Fit a DC supply
(b) Try elevating the heater DC voltage [cheap to do] or
(c) keep trying different tubes until you find one you are happy with
(d) Live with it.

What's it to be?

[avoverdu]

So the DC supply helps quite a lot. You can
(a) Fit a DC supply
(b) Try elevating the heater DC voltage [cheap to do] or
(c) keep trying different tubes until you find one you are happy with
(d) Live with it.

What's it to be?

I didnt give up!!!! Iīve been busy at work this week but today im gonna get some parts. Im going to try all your suggestions until I find the cure LOL. By the way I did some tests that I will ppost later in detail.

Thanks guys!!