EL34 question...

[jgcable]

I have a Marshall 3203 Artist. It takes 2 EL34 power tubes.
The tubes that are in it are marked Dynaco EL34 and they are running at 33.1ma and 33.4ma. These tubes look very old. The amp sounds great although it is a tiny bid muddy sounding. BTW.. these Dynaco tubes are pretty thin in size.
I threw in a set of fairly new Sovtek EL34's and checked the bias. The tubes are running at 29.1 each. The amp now has a little less gain then it did but its more open, throaty with a better bass response that is tighter then before. It also seems louder for some reason.
I was told that I should bias the amp at approx 33.4ma per tube. With the old tubes in it, the bias was set perfectly.
With the new tubes its cold (I guess that is the term).
Did I do anything wrong? Is this bias ok if it sounds good to me? Should I bias it to 33.4 for the new tubes? If I do, what value should I change the 56K resistor to?
Thanks guys.
BTW.. my 3203 is a 1989 and has the original Drakes in it.
I REALLY like this amp. It has a certain something about it.

[markD]

Re: EL34 question...

well, i am not an expert, but if the tubes are biased cold it seems that would be better than having them biased hot. i think if you like the sound then keep it that way. but that's not expert advice.

i use winged C EL34 tubes in my riveras and i prefer them running a little cold.

[kmanick]

Re: EL34 question...

tubes will throw out different readings.
that's why when you retube power tubes
1. they should be matched so they are both running at the same level.
2. you need to re bias to the new tubes because it's most
likely the levels are different than the tubes you pulled out.
(sometimes if you stay with the same brand they stay pretty
close and you can get away without re-biasing).

29.1 is a bit cold but to 33.4 ..that's not huge, but it
will make a bit of a difference. bringing it up to 33.4
should open up and warm up the sound a bit.

[Fragle]

Re: EL34 question...

" you need to re bias to the new tubes "
i have yet to have an amp rebiased, without any problems at all, so technically speaking i don't think it's necessary. however, changing the bias definitely makes some difference.
i guess if you like the tone as it is right now, you don't have to change anything. remember the golden windows rule: "DON'T TOUCH RUNNING SYSTEMS" [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img] [img]/images/graemlins/laugh.gif[/img]

[Soloist1]

Re: EL34 question...

What were you getting for the plate voltage of each of the new tubes?

[jgcable]

Re: EL34 question...

[ QUOTE ]
What were you getting for the plate voltage of each of the new tubes?

[/ QUOTE ]

How do I check that? I have a bias probe that has 2 banana plugs on it that I plug directly into my Fluke Multimeter.
Thanks!

[Soloist1]

Re: EL34 question...

Not sure which pin it is. But just adjusting the current won't really be accurate unless you know what the plate voltage is. I use one of the Weber bias rite meters that lets you switch between current and plate voltage. You will want the EL34 at about 17.5 watts max plate dissapation for an AB type amp. So if your voltage on the plates is around 500 volt you'd want to run the current at .035 amps. Volts times amps will give you the plate dissapation in watts. I have no idea what the plate voltage is on your amp though.

[Soloist1]

Re: EL34 question...

If I'm reading this schematic correctly it looks like the plate voltage operates somewhere around 435 volts. Pin 3 is the plate.

http://www.drtube.com/schematics/marshall/3203pwr.gif

[jgcable]

Re: EL34 question...

So how do I measure the plate voltage? Do I remove the tube and stick my multimeter probe in pin 3? What about the other lead?
So, assuming my plate voltage is 435v and my current replacement tubes are running around 29ma each how do I determine the value of the resistor I need to bump them up to 35ma? It currently has a 56K resistor in it. I could yank it and replace it with another value but how do I know which one to use?

[DonP]

Re: EL34 question...

Why did you change the tubes? Was the amp not working correctly?

Does the amp sound good or bad with the new tubes?

With the new tubes running cold, they will last longer. If your voltage is 435v, you could run the tubes as hot as 40mA without exceeding the 17.5watts limitation. Changing the bias changes the voltages and the current. You'll need to re-check and re-calculate your wattage as you make changes.

[Soloist1]

Re: EL34 question...

John black on chassis and red to pin 3 for the plate voltage reading. Not sure on the resistor value. Personally I would go to a music shop that biases amps and see if they had variable pot type ones that can be used in your Marshall. And also I'd highly recommend getting the bias rite meter. You won't have to dick around with all the measuring. Just mount the probe between the tube and the socket and use the current/voltage switch to check the readings. You have to make sure you get the voltage reading option though. It's still under 100 bucks for the meter I believe. Well worth the dough.

[jgcable]

Re: EL34 question...

[ QUOTE ]
Why did you change the tubes? Was the amp not working correctly?

Does the amp sound good or bad with the new tubes?

With the new tubes running cold, they will last longer. If your voltage is 435v, you could run the tubes as hot as 40mA without exceeding the 17.5watts limitation. Changing the bias changes the voltages and the current. You'll need to re-check and re-calculate your wattage as you make changes.

[/ QUOTE ]

The amp sounded fine with the old tubes in it. I just like messing around with stuff. [img]/images/graemlins/grin.gif[/img]

[Twisteramps]

Re: EL34 question...

Do you like how it sounds? As long as the plates on the EL34 aren't cherry red, you're ok on a 'is this bad for the amp' standpoint. My Mesa ractifier was biased around *15* ma w/stock EL34s, so yours isn't that far off. Personally, I like amps a little (like 10-15%) colder than hotter - tubes last longer, and it still sounds ok. Just my opinion.

Pete

[jgcable]

Re: EL34 question...

[ QUOTE ]
John black on chassis and red to pin 3 for the plate voltage reading. Not sure on the resistor value. Personally I would go to a music shop that biases amps and see if they had variable pot type ones that can be used in your Marshall. And also I'd highly recommend getting the bias rite meter. You won't have to dick around with all the measuring. Just mount the probe between the tube and the socket and use the current/voltage switch to check the readings. You have to make sure you get the voltage reading option though. It's still under 100 bucks for the meter I believe. Well worth the dough.

[/ QUOTE ]

So I guess I have to remove the tube to check the plate voltage. Do I check it on standby or on?

Maybe I should get a book on this. Does anybody have one they could recommend me?

[DonP]

Re: EL34 question...

You can check it with the tubes in there, but it might be tricky supporting the chassis (pull the chassis out of the head).

The voltages will change if you pull the tubes because the tubes put a load on the power supply which drops the voltage.

If it ain't broke...