MT15 Bias

[SilverSurf9]

Anyone know how far the bias pot will allow you to adjust?

If the bias range for the 6L6 is 25-30mV , does the pot have enough range to dial it to 45-50mV for a different tube?

Thx

 

[Em7]

What you are trying to do? If you talking about the test points, instead of measuring the actual grid bias voltage, that is a lot of idle current. If the bias test points on the MT15 are anything like the other PRS amp, they actually measure the voltage drop across high precision 1 ohm resistors that are in series between the cathodes of the power tubes and ground. One milliamp of current drawn through the resistor equals one millivolt. You talking about an idle plate dissipation of near twice that at which the amp is designed to operate. The power supply more than likely will not source that much current without the B+ sagging.

 

[SilverSurf9]

I just want to see if it's possible to adjust the amp out of the factory cold bias setting especially with a different tube like a 6ca7, or kt77 - el34. The cold bias sounds great especially for metal but I was curious how flexible the amp is before buying. Having a normal bias in the center of hot and cold shouldnt be too much to ask for but with an amp that has a plate voltage of 300 or under 300, normal dissipation for a el34 would be like 50-55mv right? Not sure if the dail would turn up that high?

 

[McMahon67]

What you are trying to do? If you talking about the test points, instead of measuring the actual grid bias voltage, that is a lot of idle current. If the bias test points on the MT15 are anything like the other PRS amp, they actually measure the voltage drop across high precision 1 ohm resistors that are in series between the cathodes of the power tubes and ground. One milliamp of current draw through the resistor equals one millivolt. You talking about an idle plate dissipation of near twice that at which the amp is designed to operate. The power supply more than likely will not source that much current without the B+ sagging.

Dude… I’ve seen a lot of your responses to different queries here. Gotta say, your knowledge is like… Mensa level. Amazing!

 

[Em7]

I just want to see if it's possible to adjust the amp out of the factory cold bias setting especially with a different tube like a 6ca7, or kt77 - el34. The cold bias sounds great especially for metal but I was curious how flexible the amp is before buying. Having a normal bias in the center of hot and cold shouldnt be too much to ask for but with an amp that has a plate voltage of 300 or under 300, normal dissipation for a el34 would be like 50-55mv right? Not sure if the dail would turn up that high?

The power supply on that amp is more than likely not designed to source that much idle current. Have you measured the plate voltage? I bet that even if you manage to mod the amp to be able to set that much idle current, the B+ voltage is going to drop. It is not a 50W amp. I am going to have to take a look at one of those amps.

 

[SilverSurf9]

The power supply on that amp is more than likely not designed to source that much idle current. Have you measured the plate voltage? I bet that even if you manage to mod the amp to be able to set that much idle current, the B+ voltage is going to drop. It is not a 50W amp. I am going to have to take a look at one of those amps.

I did read that the plate voltage was around 300 or slightly less. I ended up going with the RM50 for this reason. Just wish it was scaled down to a 15 or 20 watt amp with the full sized tubes but oh well

 

[Deleted member 5962]

The power supply on that amp is more than likely not designed to source that much idle current. Have you measured the plate voltage? I bet that even if you manage to mod the amp to be able to set that much idle current, the B+ voltage is going to drop. It is not a 50W amp. I am going to have to take a look at one of those amps.

Actually, it is designed so that with only a few mods it can be a 50 watt amp and pull the current needed for EL34s. Look for Matt's thread on modding his, or for comments by Doug and PRSh in videos, that it could easily be modded to a 50 watt amp.

Matt didn't reply to the question about what the mods were, but it should be simple to figure out what is required.