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Full Version: Two Chassis Amp - Wiring between them.
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Hi Kevin and fellow TUT followers,

I am building a large combo for a local guitarist; Push-Pull 2 x 6CA7, 3 x 10" speakers.
Due to the large cab and wanting to keep the pre valves out of the speaker box, I have built it with two aluminium chassis (like some old amps).
One, in a separate compartment above the speakers, housing the Controls, Preamp and the PI. 
The other, on the floor of the cab, housing the Power Supply and Power Amp (the valves are shielded from the speakers).
This two chassis build is new to me and not something that I have done before now.

This build requires approximately 700mm of wiring/cabling between the two chassis for the;
HT for the PI & Pre (PT: 290HX = 356VAC at 420mA), Heaters, Chassis Ground, Speaker Ground, Feedback wire (circuitry is with the PI) and Signal from PI to PA.
I am going to use a relay, via a control panel switch, to switch the wall supply to the PS.

I have some ideas about which way to do this, but I thought I might ask what you blokes recommend, so I can get the best result, with no oscillation or noise and to ensure it is safe.
All suggestions gratefully received.

Cheers, Noel
Hi Noel

There are obvious wire pairs required between the two chassis:
1 - Twisted pair for heaters (6V)
2 - Twisted pair for the mains switch - no need for a relay as that requires an auxiliary PT.
3 - 2-conductor shielded or 2x coax for the output stage grid lines
4 - twisted pair for B+ and ground
5 - coax for the feedback connection
6 - a single bonding wire from the bottom chassis to the upper chassis

Note that in the bottom chassis, there is the main supply up to the screen node feeding the output stage. The #4 pair takes the ground and B+ from this node up to the upper chassis.

Similarly, the FB coax provides the ground reference for the isolated speaker jacks in the bottom chassis, ground referencing occurring at the splitter, as usual.

The elevation for the heaters is done in the lower chassis off the screen node.

The grid-lines could be a twisted pair or the 2-conductor shielded or two pieces of coax. With either shielded cable option, tie the shield to ground in the upper chassis and float it in the lower chassis.

We assume that you are using proper Galactic grounding and that the chassis are for shielding only. In the bottom chassis, the safety ground has its own tie point. The signal ground is made from the Vs-node via a ground-link-isolator. All jacks are isolated in both chassis. The wiring above maintains this principle between the chassis.

Have fun
Thanks for the reply Kevin.

I wasn't expecting to get any replies yet, due to the late hour there.

#1 makes sense. Would a shielded pair be a viable alternative to twisted wires?
#2 The relay idea is probably too much trouble; I'll dump that.
#3 That's what I have planned - coax. Tie the shield to the PI ground node? For the signal Ground; what would you suggest for the Ground-Link-Isolator?
#4 Makes sense and the PS board is set up to take B+ and Ground from the Vs node.
#5 That confirms my thought that the FB wire needs shielding.
#6 No problem there.

What wire gauges would you suggest for the various functions, over this length?

Yes, I always use Galactic Grounding with the chassis ground at the Input Stage node, the jacks are always isolated and I have the heater elevation on the PS board.
The power cord will plug into the bottom chassis and the safety earth will be there.
I may put the power switch in the bottom chassis also. The client isn't bothered with not having it on the panel and there will be a Mute switch, for his convenience. I haven't used Standby Switches before and see no reason to have one here.

This amp is a single Clean channel design to be used as a pedal platform and is a HIWATT design from the Tone Stack to the Power Amp. I am aiming for 60W+ with plenty of headroom.

Thanks for your help Kevin.

Cheers, Noel.
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.