Marshall DSL50 questions on tubes and biasing.

[Mr 9finger]

This is what I love about my Mesa's. Just stick your new tubes in and have at it! Not so much for the Marshall.

I've never biased an amp....ever. After looking around the YouTubez, it's not that hard actually. After the DSL started cutting in and out, I figured it was a tube problem some where. So I've ordered new tubes for it and one of them little bias meters. All of which should be here some time this week. Now I just need to figure out what preamp tubes do what and double check with you guys on the bias thing.

Here's a pic of the back of my amp.


Right now it has Tun Sol 12ax7's in the preamp slots and EH EL34's in the power. I've got some new Tung Sol preamp tubes coming since I've grown fond of them. A couple EH 12AX7's and I think I ordered some of the Mullard reissue 12AX7's. For power tubes, I ordered a set of Tung Sol's and Mullard reissues...both EL34's.

What I need to know is, what preamp slots do what. I can't seem to find this info anywhere and it's not in the manual. Also on the bias, I'm pretty I know what's going, but I want to double check. Right under the power tubes, you'll see a little slot. It has a trim pot, then 3 pins, then another trim pot. The trim pot below it's corresponding tube is how you set the bias for that tube. I'm assuming the 3 pins are the meter points. left for the left tube, right for the right tube, and center is common ground for both. Correct?

I also need to figure out what mA the tubes to be biased to.

 

[Gearjoneser]

Re: Marshall DSL50 questions on tubes and biasing.

If I recall correctly, the bias points on the back are for voltage, not mA. The little bias tool you bought is probably for mA though.

I like biasing with a mA bias tool, as well as my ears. 6L6's sound best to me around 32mA, and EL-34's around 38mA. That's just a rough guide, so I know I'm in range. What you don't want is the plates to start turning orange by having the bias pot turned too high.

The main objective is getting the amp to sound big tight and bold......sorta like setting the dwell on a muscle car. They used to use a timing light, but you know when the engine is running healthiest.

 

[FuseG4]

Re: Marshall DSL50 questions on tubes and biasing.

from what I remember v1 was green channel, v2 was red channel, v3a was cathode follower and v3b was reverb recovery or something, and v4 is the PI

 

[hermetico]

Re: Marshall DSL50 questions on tubes and biasing.

This is what I love about my Mesa's. Just stick your new tubes in and have at it! Not so much for the Marshall.

I've never biased an amp....ever. After looking around the YouTubez, it's not that hard actually. After the DSL started cutting in and out, I figured it was a tube problem some where. So I've ordered new tubes for it and one of them little bias meters. All of which should be here some time this week. Now I just need to figure out what preamp tubes do what and double check with you guys on the bias thing.

Here's a pic of the back of my amp.


Right now it has Tun Sol 12ax7's in the preamp slots and EH EL34's in the power. I've got some new Tung Sol preamp tubes coming since I've grown fond of them. A couple EH 12AX7's and I think I ordered some of the Mullard reissue 12AX7's. For power tubes, I ordered a set of Tung Sol's and Mullard reissues...both EL34's.

What I need to know is, what preamp slots do what. I can't seem to find this info anywhere and it's not in the manual. Also on the bias, I'm pretty I know what's going, but I want to double check. Right under the power tubes, you'll see a little slot. It has a trim pot, then 3 pins, then another trim pot. The trim pot below it's corresponding tube is how you set the bias for that tube. I'm assuming the 3 pins are the meter points. left for the left tube, right for the right tube, and center is common ground for both. Correct?

I also need to figure out what mA the tubes to be biased to.

1)
I've spent days testing my DSL-50 with any kind of new production tube and, I don't recomend you anything else but a set of TESTED JJ ECC83S. That amp seems as if it was developed with such a tube in mind.
Just the PI tube can be a Sovtek 12AX7-LPS, for the rest, throw there JJ ECC83S.

V1 and V2 are voice tubes for both channels
V3 is a cathode-follower position (don't throw there an spiral filament tube) and gets rid of FX Loop, reverb and tone stack, if memory doesn't fail
V4 is the Phase Inverter.

My recommendation is:
V1 - V3 = TESTED QUALITY ECC83S (TAD 12AX7-Cz worked really nice)
V4 = Sovtek 12AX7-LPS

BTW, let me know how those Mullard EL34 reissue sound, the 12AX7 and EL84 are really good.

(2)
In the bias spot, yes, the center lug is for ground but, maybe left trim pot is for right tube and viceversa.
Anyway, take into account that, as soon as you increase the bias in a tube, the other tube goes down and, viceversa.
So, you will have to be patient tweaking both tubes at same time up and down until you reach a balanced situation.
Remember to completely roll off gain and volume controls, set your tone stack to noon and warm your amp in bypass mode for a while, before biasing.

The reading doesn't matter if you do it in mA or mV. Biasing resistor is 1 Ohm, that means that A = V.
What is important for you to know is that you are reading there Plate current + Grid current so, you should review your biasing value taking into account that.
My recommendation is to bias those tubes to 42.5 mA (around 61% of plate+grid disipation power) but you can go up to the 45 mA recommended by Marshall and, still be in a safe zone (around 65% of plate+grid disipation power).

Maybe you can learn about my own mistakes. This is my own biasing experience with such an amp:
http://hermeticoguitar.blogspot.com.es/2013/02/amps-retubing-marshall-1923c-85th_24.html

 

[Mr 9finger]

Re: Marshall DSL50 questions on tubes and biasing.

Awesome! I'll give it a read!