Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mindcrazedbanjo on December 29, 2011, 10:18:01 am
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I bought this Marshall 2203 jcm 800, changed 4 x 50uf caps and all the valves and it works, before it kept blowing a fuse. Now to biasing, inside on the pcb there are 2 voltage regulators, VR1, VR2. I measured the plate voltage (new tubes in) and its was 465, by turning VR1 I adjusted the bias from 70, 56, 52, & 58, (4 el34's) down to around 37mA on all tubes, thats 25 watts divided by Plate Voltage and then taking 70% of that. my questions are.. Are these the right bias settings for this amp? and What does VR2 alter? because I turned this a little bit by mistake although I think I got it back to its original position.
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Without a schematic or model year on your amp,we don't know what VR2 is.I'm assuming it's dual bias?
37ma is rather hot for that amp as the EL34 is a true pentode and can often red plate if biased to hot.It depends on your plate voltage.
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The amp is 2006 model, a schematic for this amp head is here..
http://drtube.com/schematics/marshall/jcm800pw.gif (http://drtube.com/schematics/marshall/jcm800pw.gif)
thx for the reply
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The schematic you linked is for an old Marshall amp. There is only one adjustment shown on your schematic.
This is a more modern schematic (even though it may not be your exact amp). VR1 is the bias adjustment and VR2 is a filament hum balance.
http://www.drtube.com/schematics/marshall/1959-01-60-02.pdf (http://www.drtube.com/schematics/marshall/1959-01-60-02.pdf)
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thx for the info, as you can tell I'm new in this department and a filament hum balance doesn't mean alot to me, what does it do and what should it be set at? btw does anyone know the bias settings for this amp? trying to find them is like looking for the holy grail lol.
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sluckey are you sure that's the right schematic? he said the amp is a 2203. that schematic is no 2203
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sluckey are you sure that's the right schematic? he said the amp is a 2203. that schematic is no 2203
As I said above, I'm pretty sure it's not the exact schematic, but at least it shows VR1 and VR2. The older 2203s did not have a VR2. His amp is a modern 2006 vintage.
The hum balance is used to adjust 60Hz filament hum to a minimum level. Turn the amp on, stick your ear close to the speaker and crank VR2 for minimum hum.
I'd set the bias to 70% max plate dissipation at idle.
http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt (http://aga.rru.com/TechTips/disp.cgi?file=LV/BiasAdj.tt)
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thx sluckey .....where it says "REMOVE the ground wire from pin EIGHT on each tube, and REPLACE it with a 1-ohm resistor" does that mean solder one end of the resistor to pin 8 and the other end to the ground wire that has been unsoldered? and then I need to put the DMM probes either side of that resistor to take a reading, is that right?
A 1 Ohm resistor but what wattage? if I knew the color code I might be able to cannabalise some.
also, I built a couple of Octal probes that were shown on this site, couldn't the 1 ohm resistor have been incorporated in to the octal probes?
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does that mean solder one end of the resistor to pin 8 and the other end to the ground wire that has been unsoldered? and then I need to put the DMM probes either side of that resistor to take a reading, is that right?
Yes and yes.
You don't need to install any 1Ω resistors in the amp if you have a set of bias probes. But, do you have 4 octal sockets? If not, it would be easier to install the 1Ω resistors between pin 8 and ground for each of your power tube sockets. Then you don't need to turn the amp off to move the test sockets around. Doug has the 1Ω 1 watt resistors you need. The bias probes on this site don't use any resistors.
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Thats all alot clearer now thx. I could knock together a couple more octal probes thanks to Doug,
while I'm taking measurments from one valve do the leads from the octal probes on the other valves need to be connected together to complete a circuit, or can they just be left dangling?