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10-16-2018, 12:54 PM
Hi Noel
If you think about the current loads of the preamp chassis, you'll find that #22 is ample for everything except the ground bond.
Say there are 4x 12AX7. The heater load is 4 x 300mA or 1200mA. #22 is good for 2A. Because of the length, you could go to #20 for less wire resistance, although <1m of wire has little DCR.
Signal wires carry essentially zero current.
B+ and ground carry a few milliamps for the plates, so #22 is ample and #20 is over-kill.
The bond wire should be #18 minimum.
Tying the signal ground to chassis at the input or the input stage is a mistake - damn that figure 2-11 - it was just a drawing convenience that everyone takes verbatim. Better to take the ground link from mid-circuit, as all the TUT3 and TUT5 projects show.
The ground-link-isolator (GLI) is a pair of anti-parallel diodes in parallel with 100nF and 100R. In the lower chassis, add the GLI from the Vs node to chassis. It might be good to have one in the upper chassis, too, even though this has the potential to create a ground loop. The fact is that whichever path presents the lowest resistance will dominate and make the reference for the whole circuit.
If you are Power Scaling the amp, the Power Scale pot is obviously in the upper chassis but the rest of the circuitry is in the lower chassis, meaning only the pot wiring is extended. Use a piece of coax for this since one side of the pot is grounded. This works fine even with the SB-style Power Scale circuitry, as shown in TUT6.
"60W+ with plenty of headroom" suggests building it as 80W with two tubes, or as 100W with either two tubes or four?
The 80W version would be like the Bassmaster, with about 560Va and 4k3-aa OT. You can do 100W with a pair of EL-34s but then need 800Va + 425Vs and 11k-aa. If you switch to 6550, a single pair gives 100W with 600Va+ - actually up to 150W with 700Va+ and a lower Vs. Use KT-88s in the same app for more of a KT-77 tone as the Hiwatts had.
You mention 6CA7 in the first post. True fat-bottle 6CA7s have a tone like 6L6 and 6550 - open and neutral, or dull and round if you don't like it. EL-34, all the KTs and EL-84 have a brighter tone due to their higher distortion. This is a factor in the Hiwatt and Vox tones. Actually, I find the KTs to have a muddy tone.
Your player would achieve more acoustic headroom with the cluster of drivers split between two or more cabinets spaced apart as TUTs 1&2 and SPKR demonstrate. With a lot less power, the sound field is as enormous as anyone can stand - actually, way louder than the Human Scale of loudness permits. You only need lots of electrical power if the acoustic design is compromised.
If you think about the current loads of the preamp chassis, you'll find that #22 is ample for everything except the ground bond.
Say there are 4x 12AX7. The heater load is 4 x 300mA or 1200mA. #22 is good for 2A. Because of the length, you could go to #20 for less wire resistance, although <1m of wire has little DCR.
Signal wires carry essentially zero current.
B+ and ground carry a few milliamps for the plates, so #22 is ample and #20 is over-kill.
The bond wire should be #18 minimum.
Tying the signal ground to chassis at the input or the input stage is a mistake - damn that figure 2-11 - it was just a drawing convenience that everyone takes verbatim. Better to take the ground link from mid-circuit, as all the TUT3 and TUT5 projects show.
The ground-link-isolator (GLI) is a pair of anti-parallel diodes in parallel with 100nF and 100R. In the lower chassis, add the GLI from the Vs node to chassis. It might be good to have one in the upper chassis, too, even though this has the potential to create a ground loop. The fact is that whichever path presents the lowest resistance will dominate and make the reference for the whole circuit.
If you are Power Scaling the amp, the Power Scale pot is obviously in the upper chassis but the rest of the circuitry is in the lower chassis, meaning only the pot wiring is extended. Use a piece of coax for this since one side of the pot is grounded. This works fine even with the SB-style Power Scale circuitry, as shown in TUT6.
"60W+ with plenty of headroom" suggests building it as 80W with two tubes, or as 100W with either two tubes or four?
The 80W version would be like the Bassmaster, with about 560Va and 4k3-aa OT. You can do 100W with a pair of EL-34s but then need 800Va + 425Vs and 11k-aa. If you switch to 6550, a single pair gives 100W with 600Va+ - actually up to 150W with 700Va+ and a lower Vs. Use KT-88s in the same app for more of a KT-77 tone as the Hiwatts had.
You mention 6CA7 in the first post. True fat-bottle 6CA7s have a tone like 6L6 and 6550 - open and neutral, or dull and round if you don't like it. EL-34, all the KTs and EL-84 have a brighter tone due to their higher distortion. This is a factor in the Hiwatt and Vox tones. Actually, I find the KTs to have a muddy tone.
Your player would achieve more acoustic headroom with the cluster of drivers split between two or more cabinets spaced apart as TUTs 1&2 and SPKR demonstrate. With a lot less power, the sound field is as enormous as anyone can stand - actually, way louder than the Human Scale of loudness permits. You only need lots of electrical power if the acoustic design is compromised.
