i've built a simple SE amp using hammond 125ESE OT and have parallel cathode biased output tubes each with their own MV for blending different tubes at different volumes.
i would like to use 1 6V6 and 1 6L6 but am not sure how to determine which output tap to use for the correct speaker load. i assume 1x6V6 as approx 5k load, 2x6V6 as 2k5 load, and 1x6L6 as 2k5 load.
can someone point me in the right direction to calculate the mix of 1 6L6 and 1 6V6?
thanks
mixing output tubes in parallel se amp
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gingertube
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Re: mixing output tubes in parallel se amp
Just looking at datsheet values for SE operation.
single 6L6 wants about 4200 Ohms (300 or 330 Ohms cathode bias resistor)
single 6V6 wants about 8K (390 Ohms cathode bias resistor).
Simplest thinking would just say that you need about 4200 Ohms parallel 8000 Ohms which is 2754 Ohms which would suggest using the 2K5 tap however I would first strap the tranny for 5K and see how that works out.
The thing here is that those "recommended" load impedances on the datasheets are for best HIFI performance. A lower impedance will give a bit more power and a bit more distortion - up to a point.
As you drop the load impedance the tubes will try to deliver a bit more current for the same voltage swing. You go too low with the impedance and the tubes run out of current swing capability. Conversly if you go too high with the impedance the tubes run out of voltage swing capabilty instead (set by the rail voltage).
Those typical values were for a rail voltage of around 300V (6v6) and 350V (6L6).
I think 5K will give the best result if using a rail of >300V. If you are running a B+ rail of <300V then you could try the 2K5 strap instead.
Why? Basically that 80mA max idle current spec for the ESE suggest that the tranny is designed for a current swing down to 0 and up to 160mA. THat suggests to me that you are better off with a higher impedance tap so as to not go too far over the 160mA peak. Eventually the core will saturate (say at maybe 180mA peak). A bit of that is good for a guitar amp but if you start trying to put peak currents of say 200mA then I think you will find it looses to much detail and your touch reponsivity will suffer.
Don't be too hung up about the 80mA idle current limit quoted for the 125ESE. If you run a bit higher then primary inductance drops and you loose bass response - mostly irrelevant for a guitar amp where you not only do not need HiFi'ish bass response but indeed you do not want it (else it sounds "muddy") - try not to exceed it by more than say 10 to 15% though.
The 15W power rating is also not a problem, it will handle 20W in a git amp. In fact I have a "Super Champ" project half built where I'm running a 125ESE Output Tranny driven by an 845 Filamentary Triode, rail of around 800V 70mA idle, Tranny on the 5K tap - this should give me about 20 Watts. I'm driving the 845 with a 6V6. I am actually a biot worried about this since the very low internal impedance of the 845 will drive the low primary inductance of the tranny to much lower frequency and I will need to limit bass response earlier in the amp chain.
Some "random" ravings but I hope there is something useful in there.
Cheers,
Ian
single 6L6 wants about 4200 Ohms (300 or 330 Ohms cathode bias resistor)
single 6V6 wants about 8K (390 Ohms cathode bias resistor).
Simplest thinking would just say that you need about 4200 Ohms parallel 8000 Ohms which is 2754 Ohms which would suggest using the 2K5 tap however I would first strap the tranny for 5K and see how that works out.
The thing here is that those "recommended" load impedances on the datasheets are for best HIFI performance. A lower impedance will give a bit more power and a bit more distortion - up to a point.
As you drop the load impedance the tubes will try to deliver a bit more current for the same voltage swing. You go too low with the impedance and the tubes run out of current swing capability. Conversly if you go too high with the impedance the tubes run out of voltage swing capabilty instead (set by the rail voltage).
Those typical values were for a rail voltage of around 300V (6v6) and 350V (6L6).
I think 5K will give the best result if using a rail of >300V. If you are running a B+ rail of <300V then you could try the 2K5 strap instead.
Why? Basically that 80mA max idle current spec for the ESE suggest that the tranny is designed for a current swing down to 0 and up to 160mA. THat suggests to me that you are better off with a higher impedance tap so as to not go too far over the 160mA peak. Eventually the core will saturate (say at maybe 180mA peak). A bit of that is good for a guitar amp but if you start trying to put peak currents of say 200mA then I think you will find it looses to much detail and your touch reponsivity will suffer.
Don't be too hung up about the 80mA idle current limit quoted for the 125ESE. If you run a bit higher then primary inductance drops and you loose bass response - mostly irrelevant for a guitar amp where you not only do not need HiFi'ish bass response but indeed you do not want it (else it sounds "muddy") - try not to exceed it by more than say 10 to 15% though.
The 15W power rating is also not a problem, it will handle 20W in a git amp. In fact I have a "Super Champ" project half built where I'm running a 125ESE Output Tranny driven by an 845 Filamentary Triode, rail of around 800V 70mA idle, Tranny on the 5K tap - this should give me about 20 Watts. I'm driving the 845 with a 6V6. I am actually a biot worried about this since the very low internal impedance of the 845 will drive the low primary inductance of the tranny to much lower frequency and I will need to limit bass response earlier in the amp chain.
Some "random" ravings but I hope there is something useful in there.
Cheers,
Ian