Hi,
I am working on this amp at present. The complaint was that the fuse keeps blowing after a certain period. I checked all the components and realised that one 1uf 400V capacitor was dead. I replaced it. Plugged in a CD player and left in ON for more than 2 hrs. It seems to be OK. There is a problem here. According to the schematic, the bias should be set to 1.3V or 1.4V for an EL84. The bias preset is recommended to be turned fully anticlockwise. Turning clockwise would lower the bias. I have turned the preset fully anticlockwise, but the voltage reads 1.007V only. Thats the maximum it can go. Is it OK at this voltage. Is it not under biased?. Please can someone familiar with this amp advice on this.
Thanks
Carlos
I am working on this amp at present. The complaint was that the fuse keeps blowing after a certain period. I checked all the components and realised that one 1uf 400V capacitor was dead. I replaced it. Plugged in a CD player and left in ON for more than 2 hrs. It seems to be OK. There is a problem here. According to the schematic, the bias should be set to 1.3V or 1.4V for an EL84. The bias preset is recommended to be turned fully anticlockwise. Turning clockwise would lower the bias. I have turned the preset fully anticlockwise, but the voltage reads 1.007V only. Thats the maximum it can go. Is it OK at this voltage. Is it not under biased?. Please can someone familiar with this amp advice on this.
Thanks
Carlos
With the DSLs the current is normally sensed through a 10Ω resistor, meaning 1.0V sensed is 100mA in the output stage, or 50mA per valve.
50mA is enough in my opinion, and will help with the short valve life that many guitar amps suffer. You may well get contrary opinions though.
I would get the Player to try it. If the sounds pleases the owner at 50mA, all well and good.
The trouble with modern EL84 is that the sample-to-sample variation is huge, so it's unsurprising to find ones that come up out-of-range of the trimmer.
I'm assuming the EL84s are new though. If they are old, it could indicate the need for replacement valves. In that case yo may get the opposite problem of bias out-of-range at the HOT end.
best of luck.
50mA is enough in my opinion, and will help with the short valve life that many guitar amps suffer. You may well get contrary opinions though.
I would get the Player to try it. If the sounds pleases the owner at 50mA, all well and good.
The trouble with modern EL84 is that the sample-to-sample variation is huge, so it's unsurprising to find ones that come up out-of-range of the trimmer.
I'm assuming the EL84s are new though. If they are old, it could indicate the need for replacement valves. In that case yo may get the opposite problem of bias out-of-range at the HOT end.
best of luck.
Across R33 should be whatever you want to burn the valves at but 1.3V is rather high and if it cannot achieve that, try a new set of valves, they may be low emission. Personally I like the sound when set to 850mV and the valves run cooler plus last a lot longer.
An EL84 can stand 34mA maximum so 130mA is absolute maximum, for a quad, not leaving much room for error!
An EL84 can stand 34mA maximum so 130mA is absolute maximum, for a quad, not leaving much room for error!
MovedRod, just a note, this model has four power tubes, not a pair. The DSL201 has only one pair.
25ma per tube sounds OK to me, how does the amp sound set that way?
25ma per tube sounds OK to me, how does the amp sound set that way?
Yes, I was getting mixed up with the DSL201. 25mA per EL84 is a little lean with the trimmer set to HOT, I would say, and may indicate worn-out samples. But I agree: if it sounds okay, stand up and play.
I have that amp, and I love it. Those bias settings are from the first versions of it, up until 2003. It amounts to a very hot bias for those tubes, and led to some overheating problems. Then after 2003, they changed a couple of resistors in the bias circuit to allow a new recommended bias of 0.55V, which is way cold! I set mine to about 0.75V.
So you can turn it down as low as you can and it should be fine.
The classic issues with this amp are overheating, causing the bridge rectifier to melt its connections, resulting in loss of heater supply to the first three preamp tubes. also generally, the heat causes cracking for various solder joints, so while you have it open, its a good idea to reflow all the joints on the board.
Call into the Marshall forum for more info
So you can turn it down as low as you can and it should be fine.
The classic issues with this amp are overheating, causing the bridge rectifier to melt its connections, resulting in loss of heater supply to the first three preamp tubes. also generally, the heat causes cracking for various solder joints, so while you have it open, its a good idea to reflow all the joints on the board.
Call into the Marshall forum for more info
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