Ack!!!

Allterr · 09-13-2009, 10:28 AM

I just checked the bias on my DSL and it was set at 117 mv for left and 122 for right. I rebiased it out to 84 mv for both. Does this sound good? And why would Marshall bias it so hot? Or does it drift from shipping, etc?

T-Bird · 09-13-2009, 01:38 PM

Hi.

Can't comment about the values as I haven't worked on that amp, but bias shifts quite a bit over the lifetime of a tube.

Mesa uses that in their bussiness-model of selling rebranded tubes for twice the price to replace perfectly good ones.

The best way by far is to have individual bas pots on every tube and to monitor the bias current (voltage

) closely for the first 20 or so hours of operation. Once a month after that. A definitive bonus is that the constantly shifting bias current on one tube is nearly foolproof sign that the tube is failing. Easy and cheap to replace before the meltdown.

Of course Marshalls usually don't have individual bias pots, but there's amps out there that have

.

Regards
Sam

MajorNut1967 · 09-13-2009, 02:13 PM

Quote:

Originally Posted by Allterr

I just checked the bias on my DSL and it was set at 117 mv for left and 122 for right. I rebiased it out to 84 mv for both. Does this sound good? And why would Marshall bias it so hot? Or does it drift from shipping, etc?

What was your plate voltage (Pin3)? Your amp has EL34's in it right? Lets say for discussion you have 450vdc on the plates and since an EL34 is 25watt tube and the so called rule of thumb is 70% of the total plate dissipation? So @ 450vdc each tube should be drawing about 39mA? So approx. 78mA per side. by the way 84 sounds a lot safer.

MartyStrat54 · 09-13-2009, 03:18 PM

Quote:

Originally Posted by Allterr

And why would Marshall bias it so hot? Or does it drift from shipping, etc?

Because Marshall uses the cheapest tubes they can get so that they can make more profit on the amp. I have said this a hundred times on the forum. When new current production tubes are made, a great percentage of them fail. The one's that don't are put into amps, or they are sold on the open market.

I have talked to the employee's at GC and MF and they told me that it is a crap shoot every time they turn an amp on, because usually a tube will go.

There is a whole lot more to this story, but all you have to know is that a current production tube can fail at any time and it could possibly damage your amp. Take the junk out and put in some better current production tubes. Your amp will thank you for it. Better yet...NOS tubes. Yeah they cost more, but they are reliable and they sound better.

Allterr · 09-13-2009, 03:18 PM

How do I check the plate voltage? (That may be a dumb question....)

Allterr · 09-13-2009, 03:20 PM

Quote:

Originally Posted by MartyStrat54

Because Marshall uses the cheapest tubes they can get so that they can make more profit on the amp. I have said this a hundred times on the forum. When new current production tubes are made, a great percentage of them fail. The one's that don't are put into amps, or they are sold on the open market.

I have talked to the employee's at GC and MF and they told me that it is a crap shoot every time they turn an amp on, because usually a tube will go.

There is a whole lot more to this story, but all you have to know is that a current production tube can fail at any time and it could possibly damage your amp. Take the junk out and put in some better current production tubes. Your amp will thank you for it. Better yet...NOS tubes. Yeah they cost more, but they are reliable and they sound better.

I do plan on getting some NOS preamp tubes sometime next week. There is a fairly large tube supplier in town.

MajorNut1967 · 09-13-2009, 09:10 PM

Quote:

Originally Posted by Allterr

How do I check the plate voltage? (That may be a dumb question....)

OK you need to stop right here! If you don't have a lot of experience with High-Voltage tube amps DO NOT TRY TO DO THIS YOURSELF! Get a tech to do it! Because if you do it wrong the plate voltage can kill you or at the least you will blow up the amp!

AND THERE IS NO DUMB QUESTIONS ON THIS FORUM, ONLY THE DUMB PEOPLE WHO DIDN'T ASK AND GOT DEAD OR BLEW UP THEIR AMP!

barryg · 09-15-2009, 01:32 PM

I couldn't have said that any better MajorNut !!!! Allterr take note, Doom awaits the Dumb ! .......barryg

thrawn86 · 09-15-2009, 01:37 PM

'got dead'.......awesome name for a song!

Allterr · 09-15-2009, 10:29 PM

Yeah, I was just curious is all...I rechecked the bias and no drift at all (And i'm using a VERY cheap multimeter....) I just hear alot about plate voltage, and was wondering about how you find that value.

solarburnDSL50 · 09-19-2009, 07:05 AM

Quote:

Originally Posted by Allterr

Yeah, I was just curious is all...I rechecked the bias and no drift at all (And i'm using a VERY cheap multimeter....) I just hear alot about plate voltage, and was wondering about how you find that value.

I use one of these so I don't have to fool around with dangerous voltages that may kill you. I do not have a steady enough hand to hit pin 3 due to fear.

Weber BR-2 Octal Dual Bias Probe:

The Weber BR-2 is a self contained unit with dual 8 pin octal probes. You can measure both your plate current and plate voltge at the flip of a switch!

If you want to keep things in general and at safe settings then you can safely bias the amp. May not be an exact or best bias point due to not knowing the PV but it will do. A safe bet would be running the numbers with a PV of 460v. So at 70% plate dissipation with a PV of 460v your bias point would be 76mv.

I'd bring it down to 80mv from your 84mv. 80mv with a PV of 460 means you are running 75% dissipation which in my book is fine when I've done it.

09-13-2009, 10:28 AM	#1 (permalink)
Allterr Member Join Date: Mar 2009 Location: Apopka, FL Posts: 76	Ack!!! I just checked the bias on my DSL and it was set at 117 mv for left and 122 for right. I rebiased it out to 84 mv for both. Does this sound good? And why would Marshall bias it so hot? Or does it drift from shipping, etc? __________________ Amps 95 Valvestate 8100 100V 05 MG15CDR 09 JCM2000 DSL 100 Guitars 92 Jackson Stealth Pro 90 Washburn Mercury 88 Steinburger headless

09-13-2009, 01:38 PM	#2 (permalink)
T-Bird Senior Member Join Date: Jul 2009 Location: Finland (Northern Europe) Posts: 214	Re: Ack!!! Hi. Can't comment about the values as I haven't worked on that amp, but bias shifts quite a bit over the lifetime of a tube. Mesa uses that in their bussiness-model of selling rebranded tubes for twice the price to replace perfectly good ones. The best way by far is to have individual bas pots on every tube and to monitor the bias current (voltage ) closely for the first 20 or so hours of operation. Once a month after that. A definitive bonus is that the constantly shifting bias current on one tube is nearly foolproof sign that the tube is failing. Easy and cheap to replace before the meltdown. Of course Marshalls usually don't have individual bias pots, but there's amps out there that have . Regards Sam

09-13-2009, 03:18 PM	#5 (permalink)
Allterr Member Join Date: Mar 2009 Location: Apopka, FL Posts: 76	Re: Ack!!! How do I check the plate voltage? (That may be a dumb question....) __________________ Amps 95 Valvestate 8100 100V 05 MG15CDR 09 JCM2000 DSL 100 Guitars 92 Jackson Stealth Pro 90 Washburn Mercury 88 Steinburger headless

09-15-2009, 01:32 PM	#8 (permalink)
barryg Junior Member Join Date: Sep 2009 Posts: 15	Re: Ack!!! I couldn't have said that any better MajorNut !!!! Allterr take note, Doom awaits the Dumb ! .......barryg

09-15-2009, 10:29 PM	#10 (permalink)
Allterr Member Join Date: Mar 2009 Location: Apopka, FL Posts: 76	Re: Ack!!! Yeah, I was just curious is all...I rechecked the bias and no drift at all (And i'm using a VERY cheap multimeter....) I just hear alot about plate voltage, and was wondering about how you find that value. __________________ Amps 95 Valvestate 8100 100V 05 MG15CDR 09 JCM2000 DSL 100 Guitars 92 Jackson Stealth Pro 90 Washburn Mercury 88 Steinburger headless

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