1959rr hum and hiss

1987XPW, 1992LEM, 1959RR, 2203KK

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munkyver
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1959rr hum and hiss

Postby munkyver » 02 Dec 2015, 03:31

Hi

I have the same problem as this:

https://www.youtube.com/watch?v=_gd85juTGeU

I pulled the PI and the hum disappeared. I tried 8 different preamp tubes but to no avail...

Any ideas guys?

Thanks

Anitoli
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Re: 1959rr hum and hiss

Postby Anitoli » 02 Dec 2015, 13:30

I noticed in the video that he pulled the guitar jack out before recording the noise. Does your amp make this noise with the guitar plugged in?

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Re: 1959rr hum and hiss

Postby munkyver » 02 Dec 2015, 19:28

Yes, with or without the guitar plugged in it is the same.

Last night I was able to manage to remove some hum by biasing it and moving the potentiometer for the hum.

Now, i am not sure if this is normal: I did not remove the biasing meter and played a bit... then i notice when i put the volume higher, the bias also goes up... from 30mA then when i move the volume the bias became as high as 90mA. is this normal? meaning is it pushing the tubes at that level? thanks.

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Re: 1959rr hum and hiss

Postby Anitoli » 04 Dec 2015, 14:38

Ok, first off all your bias meter is doing is breaking the cathode connection and inserting a 1 ohm resistor in between. Bias measurements are taken with NO SIGNAL applied. Once an AC signal is applied the bias reading will no longer be accurate as the current through the tube is not the idle current but the current used to to produce power to the secondary side of the OT.

Always measure bias with no signal applied.

How long has it been since the power tubes have been in the amp/replaced?

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Re: 1959rr hum and hiss

Postby munkyver » 04 Dec 2015, 16:48

Thanks for the reply.

The tubes have been there for a long time but I barely used this amp. the amp is 8 years old but playing time is very low.. but I guess the tubes can break anytime.

I tried the same thing with my self biasing AFD100: placed the bias probes on one power tube then checked the bias at 35mA then I played with it on and applied the volume up to full.. the bias did not change ... still at 35mA even if the volume was at 10!

very different with the 1959rr jmp because the bias changed as soon as I turned up the volume.. i wonder if it is normal for such an amp.

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Re: 1959rr hum and hiss

Postby Anitoli » 05 Dec 2015, 03:24

Not sure why there would be a difference between the two heads in that regard. Idle current is drawn through the cathode the same as the current used for amplification. Idle current should remain steady while the ac current ( or more precisely the half wave ) will fluctuate with the signal. Tubes are placed in pairs in 100 watt heads. If the pairs become too far apart in Ma draw there will be an increase in noise. Usually it starts to become more noticeable after the pairs get more than 6-8 Ma apart.

What is the idle bias setting on each tube going in a straight line from one side to the other?

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Re: 1959rr hum and hiss

Postby munkyver » 05 Dec 2015, 21:01

the bias are as follows:

tube 1 = 35.7mA
tube 2 = 37.7mA
tube 3 = 39.5mA
tube 4 = 41mA

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Re: 1959rr hum and hiss

Postby Anitoli » 06 Dec 2015, 06:05

Just for shits and giggles, swap tube 1 and 4 and see, (just to rule out the tubes ) if the noise goes away. This will bring the tubes in closer in ma.

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Re: 1959rr hum and hiss

Postby munkyver » 08 Dec 2015, 23:31

Saw another post from another forum with the same issue. People are saying that it is just a normal hum/buzz for the loud 1959rr:

http://www.marshallforum.com/showthread.php?t=42496

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Re: 1959rr hum and hiss

Postby munkyver » 09 Dec 2015, 23:09

Thanks for all the answers... I played it for the first time with the both volumes at 7 then both volumes at 10, problem solved! it was ear shattering good at that volume. The tube amp distortion was amazing! I can post a video soon.

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