Marshall Haze 40C Bias problem

Haze 40watt and 15 watt

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HAZE MHZ 15 HEAD- 15W, guitar head-unit, five-tube chassis, preamp: 3 X ECC83, power: 2 X 6V6, solid-state rectifier, two channels, reverb, echo, vibe, chorus, front gold control panel, single input, nine knobs (effects adjust, effects depth, r, b, m, t, OD v, OD g, Clean v), power switch, effects switch, channel switch, bright switch, rear panel: emulated line out, two speaker jacks (16 ohm, 8 ohm), effects loop with on/off switch, footswitch jack, two-button footswitch included, black covering, gray grille with white piping and white script Marshall logo, 20.5 lbs., mfg. 2009-2012.

HAZE MHZ 40C COMBO - 40W, 1-12 in. Celestion G12T-66 speaker, guitar combo, five-tube chassis, preamp: 3 X ECC83, power: 2 X EL34, solid-state rectifier, two channels, reverb, echo, vibe, chorus, front gold control panel, single input, 10 knobs (Normal Ch. v, OD Ch. v, OD Ch. g, t, m, b, p, r, effect depth, effects adjust), bright switch, boost switch (Normal Ch.), channel switch, boost switch (OD Ch.), effects switch, standby switch, power switch, rear panel: emulated line out, two speaker jacks (16 ohm, 8 ohm), effects loop with on/off switch, footswitch jack, two-button footswitch included, black covering, gray grille with white piping and white script Marshall logo, 44.7 lbs., mfg. 2009-2012.

HAZE MHZ112 SPEAKER CABINET - 1-12 in. Celestion G12-66 Marquee speaker, closed back, straight or angled front, designed for use with the Haze MHZ 15 Head, black covering, dark gray grille cloth with white piping and white script Marshall logo, mfg. 2009-2012.

JHaus
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Marshall Haze 40C Bias problem

Postby JHaus » 20 Mar 2013, 10:54

Hey all,
I've been a happy owner of a Haze 40 Combo since Oct. of 2009. Lately I've been having some issues, after smelling that telltale "plastic burning" smell at a gig last week, and I opened up my amp to check it out.... seemingly nothing wrong no pots/caps/resistors looked burned out. A little silicone drip from the input jack so i thought that it maybe heated up and dripped onto a hot tube or speaker housing. No residue anywhere and i check the jack/tighten it up, that's fine. Anyway since it's open i figure i need to change power tubes (it's been 3 1/2 yrs!!!) so i get a set and my trusty bias tool (asharp BiasPro and octal adapters).....

Here's the problem...
When i first plug in VR1 is around 35mA, but VR2 (tube farther away from the pre tubes) reads -0.9mA???? This is on my old tubes.
So i put the new tubes in and check the bias.
Sooo i'm getting a reading on VR2 that ranges from 0.1 to -2.3mA (from pot min to max), VR1 was 11.1-14.7mA (min to max) with VR2 at min. With VR2 at max VR1 reads 141-165mA(min to max). From what i've been looking for on forums online here, the power tubes are supposed to be bias-able independently and are suggested at around 39mA each.
Anyone have any clue what's going on here? And any suggestions? I am fairly new to biasing but not to repair and modding in general.
Any help would be appreciated!
:dunno

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Re: Marshall Haze 40C Bias problem

Postby Jamesm » 05 Apr 2013, 07:38

Hi,

Can I first check that you have found the fact there are two bias pots, one for each valve.

There is a three way connector that allows you to read the current going through each valve, which means you dont really need your BiasPro.

Take the EL34s out and measure the bias voltage on pin 5 of each octal base and adjust the voltage to maximum negative.

Replace the EL34s and power up. adjust one of the bias post and check which valve each one controls. On the connector, pin 2 is ground and 1 and 3 are the voltage to read. Watch out here, because it is possible to see some difference on the other valve as you make adjustments, as the HT voltage is loaded.

The best way to work is making small adjustments on each bias pot and then check the bias voltage on pin 1 and 3. The factory setting is 39mV (which equates to 39mA) on each valve. There is some leeway here, but I would consider 45mV to be the maximum and valve life will be shortened at this setting.

Once both valves are at about the right voltage, leave the amp for 1/2 an hour and re test.

All of this is with no signal applied by the way!

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Re: Marshall Haze 40C Bias problem

Postby Eddy Rigotti » 05 Apr 2014, 16:28

Hi everyone, I am new here.

I loove my Haze 40. But on my last gig, I had a very weird crakling sound.
Anyway it seemed that it was one of the tubes, I changed it to spme JJ but the sound really changed for me. After reading a lot about this I found out that the stock tubes were actually EL34B Shuguang Gold Logo or Ruby EL34-STR, and that I need to the bias.

I am trying to figured out to do the Bias on that Haze 40 but it is very hard to find something as simple as a photo showing the pots to turn to change the mva and where are the pins to connect to. I saw two screws with a red dot over it, is it this?

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Re: Marshall Haze 40C Bias problem

Postby Eddy Rigotti » 05 Apr 2014, 17:34

I read the schematic so it seems that it is those red screw VR1 and VR2 and set to 39mv, sorry to ask.
I bought a [air of EL34B Shuguang Gold Logo and a pair Ruby EL34-STR. I hope that I will be able to find the exact same sound that I had before.
I guess that it is a good idea to test how many mv I have with the old tubes first, or it doesn't matter?

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Re: Marshall Haze 40C Bias problem

Postby Eddy Rigotti » 06 Apr 2014, 18:27

So I tested my old amp it seems that it was 28mva and 25.4mva. I was surprised that it was that low.
I actually read that the tubes are too cold it is when you might have eventually a crackling distortion. SO I thought maybe I can try to make it hotter around 39mva like it is supposed to.

I tested my new JJ they were at 41mva and 42.9mva maybe a little bit to high.

But I wanted to retry with the old ones, and it was really 28mva and 25.4mva, and then I wanted to see how the tubes react when you play. I started to play, to finally realize that the crakling sound was gone. I couldn't beleive it. I played and played and no the noisy sound was gone. I put everything back in the amp again, try again, and the crackling sound was back. What is going on??? Can someone give me an explanation to this? I might receive some new EL34B Shuguang tomorrow, but still I would have to know if my tubes are really bad or not.

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Re: Marshall Haze 40C Bias problem

Postby Eddy Rigotti » 08 Apr 2014, 19:14

Yeah I received my new EL34B Shuguang, did the bias just like how it was 28 and 26, and Voila, my sound is back! I am so happy!

I am wondering if this forum is still working, but anyway, THE HAZE 40 ROCKS!

My sound is so good that I gave up to use another overdrive or fuzz with it. I don't need it, when the sound is good, the rest is all in your fingers. I know some people complained about a muddy sound. Doesn't bother me at all. It takes a while to set it up, but with the pick ups on my guitar and the setting on the amp I am really pleased. Right now I just use a wha and sometimes an Otava magus, but seriously just the amp, you have a beautifull clean, a crunchy channel that you can set with just a tiny bit drive like 6.3 that is really responsive to your finger, almost clean if play really softly, and a boost that sounds almost like a fuzz but again with more control with your fingers and harmonics. Yeah I love my Haze 40.

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