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I have a 1994 Sovtek Mig60 head. I really dig the sound of it. It was modded for more gain, Mercury PT/OT installed and an effects loop added. This is the schematic: http://web.aoct.org/marshall/Mig60.jpg. My question is about the bias settings.
The amp uses EL34 power tubes. I use new production Tung Sol EL34B power tubes and they sound great. B+ is 640VDC. The screens are at 350VDC. There are separate B+ and screen supplies (it's a stacked supply). There are 4 ohm (3.9 ohm, actually) bias sense resistors between cathode and ground. The schematic document instructs you to adjust the bias so that you read 80mV across the bias sense resistors at idle. I have modded my amp for individual bias controls so that I can utilize mismatched tubes more effectively.
If we do the math 80mV across 4 ohms yields 20mA of current through each tube. 640VDC x .020A gives us an idle dissipation of 12.8 watts per tube which is well below the 70% mark that is often cited when biasing tubes. The EL34 is rated at 25W so 70% of that would be about 17.5W. The amp sounds really good. These are a popular model in the Sovtek line even without modding. If you follow the instructions on the schematic you are biasing the amp somewhat cold by definition.
Here is my question. B+ of 640VDC is quite high for a guitar amp. My Marshall JCM 800 clone is about 475VDC. Is there some reason that an amp with much higher than usual B+ should be biased on the cold side like this or was Sovtek just erring on the side of caution? One day when I was rolling tubes I did actually adjust the bias for a more conventional 70% setting. It came to about 110mV across the sense resistor for a current of 27mA and an idle dissipation of about 17.3 watts. I saw no red plating. I really didn't notice a significant difference in tone so I set it back to factory spec.
I know some folks like the older Marshall amps from the 70s because some of those ran the EL34 tubes at a higher voltage and they felt it gave the amp more "grunt". This amp has grunt in spades. I'm just trying to wrap my head around the bias settings on this amp.
The amp uses EL34 power tubes. I use new production Tung Sol EL34B power tubes and they sound great. B+ is 640VDC. The screens are at 350VDC. There are separate B+ and screen supplies (it's a stacked supply). There are 4 ohm (3.9 ohm, actually) bias sense resistors between cathode and ground. The schematic document instructs you to adjust the bias so that you read 80mV across the bias sense resistors at idle. I have modded my amp for individual bias controls so that I can utilize mismatched tubes more effectively.
If we do the math 80mV across 4 ohms yields 20mA of current through each tube. 640VDC x .020A gives us an idle dissipation of 12.8 watts per tube which is well below the 70% mark that is often cited when biasing tubes. The EL34 is rated at 25W so 70% of that would be about 17.5W. The amp sounds really good. These are a popular model in the Sovtek line even without modding. If you follow the instructions on the schematic you are biasing the amp somewhat cold by definition.
Here is my question. B+ of 640VDC is quite high for a guitar amp. My Marshall JCM 800 clone is about 475VDC. Is there some reason that an amp with much higher than usual B+ should be biased on the cold side like this or was Sovtek just erring on the side of caution? One day when I was rolling tubes I did actually adjust the bias for a more conventional 70% setting. It came to about 110mV across the sense resistor for a current of 27mA and an idle dissipation of about 17.3 watts. I saw no red plating. I really didn't notice a significant difference in tone so I set it back to factory spec.
I know some folks like the older Marshall amps from the 70s because some of those ran the EL34 tubes at a higher voltage and they felt it gave the amp more "grunt". This amp has grunt in spades. I'm just trying to wrap my head around the bias settings on this amp.
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