Marshall DSL40C No reading when checking the BIAS

[Zedi]

Hi I'm a newbie here I have just purchased a Marshall DSL40C 2015 Model Limited edition white made in Vietnam from ebay. It was sold as making a crackling noise when you move the vol knob. When I plugged it in to test it, it was making a loud humming/crackling sound so after some research I thought must be the EL34 Valves so did the old tap thing and nothing. The valves were both glowing so I thought I would check the bias on the old multi meter. So I connected the black to the middle on Con 5 and red to the left the the right side meter is set to 200mv but the reading on the meter stays at 0 and sometimes fluctuates to 1. It's a nice amp and I would love to keep it but I have until the 26th October to return it if I am not happy so I just want to explore every avenue before I decide on the former. My question is has anybody ever experienced this before where the bias reads zero?

Thanks for your help in advance.

 

[Leonard Neemoil]

Welcome to the forum.

Nice amps. I missed out on one of those a couple of years ago.

I have a 2016(or 2013?) DSL 40 C and it's been a couple of years since I biased it, but I'm pretty sure you're supposed to use the milliamp setting for the bias, not mV.

 

[Zedi]

Welcome to the forum.

Nice amps. I missed out on one of those a couple of years ago.

I have a 2016(or 2013?) DSL 40 C and it's been a couple of years since I biased it, but I'm pretty sure you're supposed to use the milliamp setting for the bias, not mV.

Sorry that's what I meant the milliamp is what I used

 

[Georgiatec]

Hi I'm a newbie here I have just purchased a Marshall DSL40C 2015 Model Limited edition white made in Vietnam from ebay. It was sold as making a crackling noise when you move the vol knob. When I plugged it in to test it, it was making a loud humming/crackling sound so after some research I thought must be the EL34 Valves so did the old tap thing and nothing. The valves were both glowing so I thought I would check the bias on the old multi meter. So I connected the black to the middle on Con 5 and red to the left the the right side meter is set to 200mv but the reading on the meter stays at 0 and sometimes fluctuates to 1. It's a nice amp and I would love to keep it but I have until the 26th October to return it if I am not happy so I just want to explore every avenue before I decide on the former. My question is has anybody ever experienced this before where the bias reads zero?

Thanks for your help in advance.

Nine year old amp off Ebay? First job change ALL the tubes for known good ones or new. You have to assume the seller put known junk in there. Set your multimeter to MA not MV to set bias.

 

[PelliX]

Nine year old amp off Ebay? First job change ALL the tubes for known good ones or new. You have to assume the seller put known junk in there. Set your multimeter to MA not MV to set bias.

^ This. In addition, crackling when turning the volume pot could indicate DC leaking to there. Any chance of at least a gutshot?

 

[neill]

First of all, you're trying to measure the milliamps through the valves BUT the way it's done is there's a 1ohm resistor inline with the cathode to ground connection and you measure the voltage drop across that resistor. As it's 1 ohm, 1ma through the valves will measure as 1mv on your meter so mv = ma. Don't set your meter to ma: it won't return any meaningful reading and you may damage the valves trying to adjust the bias without correctly measuring the effect of the adjustment. If there's sound coming from the amp, there must be current flowing through the valves. I'd double check your connection from the probes to con5: the pins might be oxidised. If the pins are close together, it's easy to short them out: won't cause any harm but your meter will read 0.

 

[malcrp]

"Set your multimeter to MA not MV to set bias."
NO NO NO! Sheesh.
Ohm's law, people. V=IR (Voltage=Current times Resistance).
In the DSL40C you check a VOLTAGE across a 1ohm resistor at those test points. Any CURRENT (in Amps) across a 1 ohm resistor produces exactly that amount of VOLTAGE..

If you set the meter to a milliamp range you're going to short out the resistor. Won't do any damage, but will give you no usable reading. Use the millivolts range and if you see 30mV you are reading 30mA.

In general, you should be getting something in the range of 28mV (cool bias) to 35mV (hotter). If one tube is showing 0 it's basically cut off. Swap the valves and see if the 0v moves to the other test point. If not, suspect the bias circuitry.

 

[Micky]

Use DC mv scale.
Bias should be 30-something millivolts DC, it is very important to balance the finals rather than set to some pre-determined arbitrary value. For example, set to 35.1 each side...

 

[Tederoni]

Hi I'm a newbie here I have just purchased a Marshall DSL40C 2015 Model Limited edition white made in Vietnam from ebay. It was sold as making a crackling noise when you move the vol knob. When I plugged it in to test it, it was making a loud humming/crackling sound so after some research I thought must be the EL34 Valves so did the old tap thing and nothing. The valves were both glowing so I thought I would check the bias on the old multi meter. So I connected the black to the middle on Con 5 and red to the left the the right side meter is set to 200mv but the reading on the meter stays at 0 and sometimes fluctuates to 1. It's a nice amp and I would love to keep it but I have until the 26th October to return it if I am not happy so I just want to explore every avenue before I decide on the former. My question is has anybody ever experienced this before where the bias reads zero?

Thanks for your help in advance.

The crackling is probably dirt in the pot. Get some Deoxit spray. It's very good at cleaning. It's super simple to clean pots if you can get to them straight away. Otherwise, take out the board. I'd clean all of them and all of the plugs while you're there with it. It saves you from taking it apart so soon in the future. Good luck.

 

[spacerocker]

So many replies giving the wrong advice! My general rule is that if I'm not 100% sure, I don't comment!

The best (and correct) replies are from "neill" and "micky" - To measure bias current, you set your meter on the DC mV scale (200mV setting) and measure the VOLTAGE in mV across the bias points. As neill has said this measures the voltage across a 1 Ohm resistor. 1mV measured is the equivalent of 1mA current flow, so 35mA DC bias current (for example) will give a meter reading of 35mVDC...

I have to say - if you don't understand the difference between mV and mA, and don't know how the bias circuit works, you probably shouldn't be poking around inside a valve amp! Not being patronising - but there are some very dangerous voltages present which need to be known about and avoided....

 

[Georgiatec]

"Set your multimeter to MA not MV to set bias."
NO NO NO! Sheesh.
Ohm's law, people. V=IR (Voltage=Current times Resistance).
In the DSL40C you check a VOLTAGE across a 1ohm resistor at those test points. Any CURRENT (in Amps) across a 1 ohm resistor produces exactly that amount of VOLTAGE..

If you set the meter to a milliamp range you're going to short out the resistor. Won't do any damage, but will give you no usable reading. Use the millivolts range and if you see 30mV you are reading 30mA.

In general, you should be getting something in the range of 28mV (cool bias) to 35mV (hotter). If one tube is showing 0 it's basically cut off. Swap the valves and see if the 0v moves to the other test point. If not, suspect the bias circuitry.

My DSL40hr has the external points for measuring the bias in MA. I take it the old 40c is different then. However any other amps I've set bias on with the 3 prong con port have all shown a normal reading when my dvm is set to MA and nothing when set to MV. Apologies to the OP if this amp is different.

 

[spacerocker]

My DSL40hr has the external points for measuring the bias in MA. I take it the old 40c is different then. However any other amps I've set bias on with the 3 prong con port have all shown a normal reading when my dvm is set to MA and nothing when set to MV. Apologies to the OP if this amp is different.

The external test points are the same as internal ones - they're just more accessible! They measure the VOLTAGE across a 1 Ohm resistor in mV, therefore the meter HAS to be set to DC mV! Because the resistor is 1 Ohm, the reading in mV on the meter is equal to the DC current flowing through the resistor in mA. I think you might be mistaken as to how your meter was set?

To directly measure current, it would be necessary to interrupt the circuit somehow, and connect the meter in SERIES with the cathode or anode. There is no easy way to do this using simple test points....

 

[Zedi]

My DSL40hr has the external points for measuring the bias in MA. I take it the old 40c is different then. However any other amps I've set bias on with the 3 prong con port have all shown a normal reading when my dvm is set to MA and nothing when set to MV. Apologies to the OP if this amp is different.

The point is when i connect the croc clips to con 5 it reads zero I have watched many videos so I know i'm doing it correctly I was just wondering has anyone come across this issue before?

 

[spacerocker]

The point is when i connect the croc clips to con 5 it reads zero I have watched many videos so I know i'm doing it correctly I was just wondering has anyone come across this issue before?

I have just re-read your initial post, and you said your meter was set to 200mV DC range - which is the correct range for measuring the bias at the test points.

You said the amp hummed and crackled, but you didn't actually say whether there was any guitar sound coming out of it when you plugged into it?....I'm assuming that there isn't - in which case I am puzzled why you checked the bias? If the bias was set too high or too low, the amp would still work and produce sound, which doesn't seem to be the case...

You say the valves were glowing - but that doesn't prove anything, except that the heater circuit appears to be working. But the valves will still glow, even with no High-Voltage supply (e.g like when the amp is on standby)

Reading this again, I think one of your power valves has failed, and blown the fuse for the high-voltage supply. Rather than poking around in the amp and changing valves and fitting a new fuse, I suggest you get a qualified tech to check this out for you....

 

[mark simpson]

If Measuring at the correct places and voltage is Zero, you have No bias and will soon be Releasing the Smoke from the amp. Remove the power tubes before troubleshooting further. With Respect to Ground, bias should be between Negative 20 to -50 volts...Not Millivolts! If bias returns after removing tubes, one or both of them are shorted. This can happen if amp is run with speaker disconnected or blown. Be careful of All Answers here or Anyplace on the Internet. If you Don't Know, Take it to a Tech. Good Luck