Cabling HT between two chassis in a combo

[NoelW]

I am building a large Combo Guitar Amp for a local guitarist, to his requirements, using 2 x 6CA7 for ~60W, with 3 x 10” speakers.
Due to the size (and potential weight) of this amp, I decided to use two aluminium chassis: one for the Power Supply and Power Amp, mounted in the floor of the Cab and one to contain the Preamp, PI and Control Panel, mounted in the separate compartment above the speaker section.
This way, the weight of the Transformers and therefore, the centre of gravity are lower down. This should make it more stable and a little bit easier to carry.

This then presented the issue of cabling and connecting the HT, Heaters, Feedback and Ground to the Preamp and PI and sending the Signal from the PI to the PA.
The HT will be 500V+, with about 450V supplied to the PI.
I am thinking; one cable to connect HT, with a shield that will also provide Ground, a cable to connect the Heaters and shielded cables to contain the Signal and Feedback.

I have never done this type of layout before and if anyone here has worked with this type of layout, I would be grateful for any info and/or advice about suitable cabling (and maybe plugs) to connect between the two chassis?
I would rather have plugs, so the two chassis can be easily separated, for maintenance work, but will hard wire if this is not a recommended approach.

Thanks,
Noel

[Tom Phillips]

A couple of comments. Put the PI in the bottom chassis with the power amp. Use a multi conductor cable with 600 volt wire for the power suppy lines or make your own cable with individual wires in a sleeve to run between the two chassis. When I'm back in my shop I may be able to find pictures of older amps that use this methodology and I'll post them here.

[NoelW]

A couple of comments. Put the PI in the bottom chassis with the power amp. Use a multi conductor cable with 600 volt wire for the power suppy lines or make your own cable with individual wires in a sleeve to run between the two chassis. When I'm back in my shop I may be able to find pictures of older amps that use this methodology and I'll post them here.

Thanks for your input, Tom.
I was originally going to put the PI in the bottom chassis but decided that sending the higher level PI output along the ~700mm of cable may be better than the lower level Preamp output.
I will be using coupling caps at both ends of the PI to PA cable, to ensure there's no DC from either end.
It will be good to see the older amps cabling method.
What is your opinion (particularly from the safety aspect) of using plugs, at one end of the cables and hard wiring the other, to allow removal of the individual chassis for maintenance purposes?

Here are some pix of the layout.
There is a divider above the speakers, which isolates the preamp chassis from the speakers; not yet fitted in the photo.

[nevetslab]

I would suggest that within the shield, one of the conductors is the Ground, and the shield NOT be used as a current-carrying conductor. Ground the shield at one end only (power amp/power supply end). Braided shield would be my preference. Flat braid is available, and with some practice, you can open that up and pass conductors thru the center. Though there may be some industrial instrumentation cable available thru Belden, Alpha, other cable mfgrs of similar stature.

I agree with Tom on putting the PI in with the Power Amp. If you have NFB, then it's well contained as a stable gain block, and not with a flexible cable, with it's inductance and capacitance to make it conditionally stable.

For a suitable cable, one having at least 16AWG conductors for the HT and Ground. Signal would be contained in it's own shielded cable (2-cond + shield). If AC heater, then 2-16AWG conductors. Maybe separate shield around that, though this now sounds like making your own cable. Tech Flex works well to contain a harness bundle. Amp makes some nice large-dia pins/sockets for something like this. Male end at the power amp/power supply, female end mating with the preamp, so no exposed HT pins.

[Tom Phillips]

After a dinner break (and I see that nevitslab has also contributed to the discussion and made a really significant point about the feedback signal path)
I offer the following:

...sending the higher level PI output along the ~700mm of cable may be better than the lower level Preamp output.

Maybe. However, I say not significantly better if at all. Remember that the pre-amp output is a line level signal and, if it is routed through a shielded cable, then I would not expect any issues. This situation is pretty much identical to the signal connection between the preamp and power amp of a component stereo system. The signal level is high enough that it will not pick up significant noise.

...I will be using coupling caps at both ends of the PI to PA cable, to ensure there's no DC from either end....

That won’t hurt anything but it is overkill and redundant. Blocking the DC on one side is sufficient. I would just use the cap on the send side.

...
What is your opinion (particularly from the safety aspect) of using plugs, at one end of the cables and hard wiring the other, to allow removal of the individual chassis for maintenance purposes?...

That is how I would do it. The safety rule to follow is that the plug style be chosen such that the power/voltage is applied to female recessed contacts. In that case, when a plug is pulled that exposes male pins, there is no voltage on those pins. This is the same principle that applies to the power wall plugs that we all use.

Attached is an annotated photo of a 1965 Supro Royal Reverb amp that was built by the Valco company. The annotations were done for my personal records but some apply to this discussion. The power line cord attaches to the upper preamp chassis where the ON-OFF switch resides. When the amp is turned on line voltage travels down the large multicomputer gray cable to the power supply / power amp chassis and the B+ & heater voltage is fed back up to the pre-amp. (The cable used 18AWG conductors by the way) The pre-amp audio output signal to the power amp is connected via a separate coax cable.

[Enzo]

I agree, keep the PI with the rest of the power amp. After all it is part of the power amp.

In fact I might further suggest, with a few extra parts you could make these real stand alone preamp and power amp units, aside from common power supply. That would allow later mix and match with other power amps and preamps.

[pdf64]

In favour of putting the phase splitter in the preamp chassis is that the signal would be balanced, and so reject common mode noise, eg hum from ac heater wires, especially if they share the cable.
But yes, as the signal level is high, it may not be an issue either way.
For me, the deciding factor would if whether the power amp has a NFB loop; if yes, then it would be much more logical to site the phase splitter with the power amp, keeping the NFB loop within that chassis. If no, then putting it with the preamp may have the edge.
The cable and multi way XLR connector types used for tube mics could possibly be used if you want to keep it to a single cable but I think they may have a lowish voltage limit. If 2 cables, there's a special XLR type for mains that could be used for the HT.
Whatever, the connector mounted to the power amp chassis should be female, to avoid electrocution hazard if the cable is removed whilst it's energised; same for thre cable end feeding the preamp. Just follow the convention used for tube mic cables.

[olddawg]

I would be VERY aware of what the application of this amp will be. Over the years I’ve used and repaired a number of “split chassis” amps from various manufacturers. They are extremely prone to ground loops and ambient hum. The examples I have seen can work “ok” for low gain applications but usually become monsterous hum generators with a boost pedal. My question is why? 3x10” is only one speaker more than a 2x10” It can’t be that much heavier (or even taller in a triangle configuration) than a standard 2x10” combo. Personally, I would consider mounting the whole/complete chassis at the bottom and avoid the grief..... then POSSIBLY put the input, a volume control and maybe an SS active eq in an upper panel. Or.. just mount the chassis on top with the 2 speakers at the bottom. I doubt it will be horribly unbalanced or unwieldy if the cab isn’t made of MDF.

[nevetslab]

In looking thru my Alpha Cable catalog, I found some 600V Multicore Instrumentation cable, overall braid shield, their Series XM cable, in 18, 16, 14AWG sizes. The cable dia gets very large quickly as the conductor count goes up. In your application, the only wires that need the 600V insulation system is the HT wires. If AC and S/B switches are in the preamp, depending on how the S/B circuit is designed, that could be done using relay switching in the amp/supply chassis, reducing the cable count. In multicore cable, 300V insulation system is considerably smaller in dia than 600V. And, finding cable that would be suitable, then being able to buy it may be two different things altogether...min lengths imposed by vendors.

While I like having nice jacketing on cables, you're limited to PVC on all but custom built cables as a general rule. Tech Flex offers a good solution for building custom cables, and is relatively durable, depending on the connector ends, and cutting it with a hot knife so it doesn't unravel on you. Usually hot knife cutting and then shrink tubing applied in the end terminations takes care of that sort of thing. It does allow placing AC mains, HT wires, LV wires & shielded signal cables all within one jacket.

I see my 'blinders' have been on. I forgot to look at the photos, to see that the preamp section is just sitting at the top of the cabinet, with the power amp/power supply sitting down below it in the same cabinet. Forget my comments on custom cables and such....we're just dealing with a long harness, that could be cable-laced or tie-wrapped into a suitable form.

I just started looking at connectors. Now, at the power amp end of things, Heyco makes some very nice circular strain reliefs, rubber inserts with a collet-clamp type locking device, which would allow the amp and to live without a connector. Just with suitable service loop within the chassis.

The Preamp end is still where safe-and sane wire-to-panel connector(s) would be in order. Tech Flex could still be a solution to contain the wiring so it looks like one large flexible cable. My mind was somehow seeing a 1 to 1.5 meter cable. Sorry about that.

[NoelW]

My thanks to nevetslab, Tom, Enzo, pdf64 and olddawg for all your cable info and advice on my amp layout.
My apologies for not responding sooner. I got called away to tend to an issue.

The cable info is very helpful and having Techflex in Australia is handy.
I have 600V and 1kV PTFE wire and 600V PVC in suitable gauges, so I can make up a cable assembly to suit, or use some shielded 600V dual cable for the HT and ground wires.
My intent was to individually shield all the different function cables, between the two chassis, so there is no interference between any of them, or from any outside noise.
i.e., shielded heater wire pair, shielded HT & Ground wire pair, shielded signal wires and a shielded NFB wire that would be separated from the others.

To avoid having all the valves upside down beside the speakers, I decided on the split chassis layout, with the preamp chassis and valves horizontal and the cab divider to isolate them from the speakers. The PS/PA chassis allows the power valves to be upright and they will also have a shield on the speaker side. I’m not a big lover of, upside down valve, combos and I wanted to build this as a head and cab; but he wanted a combo, so here I am, hoping you blokes can help me out.

It seems that the main concerns, which you have raised, are the feedback, ground loops and hum (which are enough potential problems).
I am confident that I can avoid the ground loops and hum with careful layout and shielding, but the amp has NFB in the design and because of your comments about stability, I am now concerned about this.
I would rather not redesign (and rebuild what I have already done of this amp), if it is possible to implement some changes in design, to avoid the possible problems with the NFB stability and separate chassis layout.
I don’t really want to move the PI to the PS/PA chassis, unless absolutely necessary, as I have already made the boards and installed most of the components and if I re-position the PI valve, it will only be 20mm away from the power valves.

What can I do, to avoid potential instability in the NFB loop, with the long wire?
If it is going to be too much of a risk, maybe I will have to leave the NFB out.

I really appreciate the help that all of you have offered.

[Dave H]

What can I do, to avoid potential instability in the NFB loop, with the long wire?

I'd put the NFB resistor inside the preamp chassis so that the long wire between the two units is carrying the less sensitive speaker signal.

[NoelW]

I'd put the NFB resistor inside the preamp chassis so that the long wire between the two units is carrying the less sensitive speaker signal.

Thanks Dave, I agree and I have already done that.

[nevetslab]

I'm assuming your power connection will be IEC panel mount. With the rear speaker panel bottom cleat blocking most of the amp chassis, looks like it leaves you with the connector into the region by power tubes, though no real problem there. Fuse holder adjacent to it. I'd not suggest the IEC/Fuse holder assembly, since it would be recessed, and tool access to pry open the fuse cover would be a PITA.

I've attached some Heyco collet-style cord grips to consider for feeding the cabling to the preamp. I attempted to browse TE Connnectivity-AMP connector's website, but it was too tedious.

Is your power amp an adjustable bias format or cathode bias? If Cathode bias, your Standby Switch could just open the cathode connection as a mute function. Or, since you have the PI in the preamp, you could also short the two AC-coupled output leads together to mute the amp, saving extra cabling between the two chassis.

Heyco Cord Grips-1.pdf Heyco Series-35 Cordgrips.pdf

[NoelW]

Yes, I am using the IEC connector, with a separate fuse holder nearby

I like those cord grips and have used similar ones before; much better than the old style ones.

The power amp is adjustable fixed bias and I'm using a Mute Switch.
I might try the PI output short method, as you suggested. I haven't used that method before and it will be convenient.

Thanks again for the suggestions.