Separating tone stack from amp chassis?

[rokker]

So I know some amplifier designs use a separate preamp and power amp chassis. For example Standel does this with the 25l15 correct?

What I would like to do is just move the controls ( vol, tone stack, input etc ) to a separate small chassis and mount that on the top inside of the amp cabinet and leave the iron and the guts and all tubes mounted in another chassis in the bottom of the amp cab. If I were to do this, do I need to take any special care with shielding with these long wires? Will just using shielded cables be enough? I have an old 60s Cordovox amp that did this, vol, treble, bass on a small face plate on the top of the cab and the faceplate is wired to this huge chassis inside the cab via long wires. Will this work? Suggestions please?

[dude]

What's the purpose of this...?

[jjasilli]

Some older -- mostly pre-60's I think, including Cordovox, Silvertone, etc. -- guitar combo amps had the preamp up top, power amp at the bottom, connected by an "umbilical cord" containing B+, signal & filament supply.  Don't know why.  Seems to pose unnecessary issues of signal noise not to mention electrocution.  Sluckey I think has a website describing his restoration of such a vintage amp. 

[rokker]

What's the purpose of this...?

I want to keep all the weight at the bottom and still have the controls up top. Just something I want to do for a combo amp.

[rokker]

Some older -- mostly pre-60's I think, including Cordovox, Silvertone, etc. -- guitar combo amps had the preamp up top, power amp at the bottom, connected by an "umbilical cord" containing B+, signal & filament supply.  Don't know why.  Seems to pose unnecessary issues of signal noise not to mention electrocution.  Sluckey I think has a website describing his restoration of such a vintage amp. 

I think valco/gretsch did it too. I realize its unnecessary but that's what I wanna do. I was thinking of not even running the heaters to the smaller chassis. Just move the controls off the main chassis.

As I mentioned above the old Cordovox accordion amp I have did this. I have a ton of shielded wire that I got from the Cordovox amp when I gutted it.

[HotBluePlates]

So I know some amplifier designs use a separate preamp and power amp chassis. For example Standel does this with the 25l15 correct?

Yes, the 25L15 has a power amp chassis in the bottom of the cabinet and a preamp chassis at the top of the cabinet. The difference between this and what I think you're proposing is the preamp tubes are located in that preamp chassis, so the run of wire between tube and tone circuit is relatively short.

What's the purpose of this...?

I want to keep all the weight at the bottom and still have the controls up top. Just something I want to do for a combo amp.

That's fair. It seems the earliest TV Front Bassman had the input jacks and Volume/Tone controls in a separate chassis at the top of the cabinet. The schematic also shows the separate plate for these and the umbilical connection using octal plug/socket. The "tube socket at the bottom edge of the parts layout is the amplifier chassis end of this umbilical.

What I would like to do is just move the controls ( vol, tone stack, input etc ) to a separate small chassis and mount that on the top inside of the amp cabinet and leave the iron and the guts and all tubes mounted in another chassis in the bottom of the amp cab. If I were to do this, do I need to take any special care with shielding with these long wires? Will just using shielded cables be enough?

I think Fender must have run into some kind of problem with this arrangement and abandoned it (unless using an all-in-one chassis was a cost-driven decision), as they only appear to have used it in the earliest Bassman amps and nowhere else.

But let's say you absolutely must do it: my approach in my version of the 25L15 was to use 3-conductor XLR jacks/sockets for such a signal connection. That gives you 2 conductors (hot and ground) plus a shield, rather than using the shield as a ground signal connection.

Assuming you have no ground-lift switching, the shield will be connected to chassis ground and circuit ground in the amplifier chassis. The ground wire inside the cable will connect to ground at the point where the tone circuit has its ground in the amplifier chassis. At the control end, the shield connects to the control chassis, but does not connect to circuit ground anywhere. Instead, the ground wire in the cable gets connected to any point that must be grounded as part of the controls. I would personally not use any form of pot buss to form the grounds in the control section, but instead run insulated wires to each point needing to be at ground. All this is an attempt to avoid ground loops in the signal ground/chassis ground system.

[HotBluePlates]

Now, obviously you may need more than 2 wires to handle your signal connections between amplifier and control chassis. You may need one for a volume control and another for tone circuit output, with a third for ground. You can find XLR-type connectors and jacks pretty easily with up to 6 contacts plus shield. You may have to shop around for a supplier that has the matching shielded cable with the same number of inner conductors.

I would personally try not to have 2 signal points in that cable that have gain stages between them; doing so would seem to invite oscillation by coupling later, bigger signals back to earlier stages of the amplifier.

I would also look to have as low an output impedance from the source end as possible, say, in the form of a cathode-follower-driven tone stack. The cabling will present some amount of capacitance that's higher than the circuit would be dealing with if wired conventionally. Depending on how big that capacitance gets, it will take an appreciable amount of current to "charge" that capacitance and maintain your output voltage at all frequencies. A low-impedance signal source help drive cable capacitance. The penalty if you have a high source impedance may be unwanted treble roll-off, at a minimum.

Life gets easier if you have the preamp tubes along with the controls in that upper chassis, as it simplifies wiring and circuit issues inside the preamp chassis. The 25L15 uses a 12AU7 phase inverter, which also simplified the output impedance issue for the signal cable. The downside is you need a separate power cable. In my case, I used a 6-conductor XLR cable ( which I made from raw cable and ends) with wires for the two sides of the heater winding, B+ high voltage and ground, and 2 wires for either side of the 120vac power switch. I was worried the 120v may cause hum to show up in the B+ wiring, but that turned out to not happen.

When in the planning stage, I made sure my signal cable and power cable connected on widely-spaced points in the power amp chassis, although it turned out I could place them side-by-side in the preamp chassis. If you use this approach, it is advisable to have whatever filter caps the preamp uses physically in/on the preamp chassis to provide short ground return wiring between a circuit and its filter cap.

The drawback to all this is it may take a significant amount of planning to pull this off. No modern amps seem to do it, and you'll have to figure out your own layout and parts requirements. I myself spent many months working out the details of how I'd build my 25L15-style amp, as I didn't exactly copy every detail of the original. Even if I had copied the original, the approach and parts are not typical of guitar amps and are not available in many places.

[rokker]

HotBluePlates, thanks for the info and advice!

"I think Fender must have run into some kind of problem with this arrangement and abandoned it (unless using an all-in-one chassis was a cost-driven decision), as they only appear to have used it in the earliest Bassman amps and nowhere else."

Leo might have done this cuz he could have saved a couple bucks per amp right?? 

My build is going to be a hoffman style 6l6 ac30 ( have another thread on that but didnt want to get too many differerent subjects in it. ) If I was going to go the 'easier' route and do preamp one chassis / power amp another - I would want to keep the PI ( v3 ) on the power amp chassis correct? Looking at the hoffman ac30 layout, seems like it wouldnt be that difficult to separate right before v3?

[darryl]

The current Fender Pawn Shop Excelsior amplifier uses "Bottom-loaded primary chassis with top-loaded control chassis for low noise and operating convenience", to quote their advertising literature.   

[rokker]

The current Fender Pawn Shop Excelsior amplifier uses "Bottom-loaded primary chassis with top-loaded control chassis for low noise and operating convenience", to quote their advertising literature.   

I remember seeing these too. Looks like they have heaters, B+, signal and everything in the same umbilical chord of wires.

[Ed_Chambley]

I have a tonestack chassis I use when building.  Contains nothing but pots and input jacks.  I do not like to tweak the tonestack inside a new build.  I have never built it in an amp, but have ran it on my bench with about 3 or so foot of shielded cables with ground connections inside the tone control box. Shield grounded to the back of the pots as you might do in a high gainer.

There is a slight increase in resistance, which can be measured if you want, but it is not noticeable in practical use.  Never noticed any increase in noise.  The reason I do this is different than you, but the results are the same.  I do it so I can make changes in the tonestack and not have to be concerned with getting hit with high voltage or turning off the amp every time I make a change.

[rokker]

Ed, Thanks for the info!

The reason I do this is different than you, but the results are the same.  I do it so I can make changes in the tonestack and not have to be concerned with getting hit with high voltage or turning off the amp every time I make a change.

So when your on your workbench you can make the changes to the tonestack since you dont have B+ running to it, correct?

I have a tonestack chassis I use when building.  Contains nothing but pots and input jacks.  I do not like to tweak the tonestack inside a new build.  I have never built it in an amp, but have ran it on my bench with about 3 or so foot of shielded cables with ground connections inside the tone control box. Shield grounded to the back of the pots as you might do in a high gainer.

Im thinking if I went this route I would prob only need 2ft max of shielded cables.
When you say shielded cables are you talking about the braided steel type? As I mentioned above, when I gutted the Cordovox amp I got a bunch of braided steel shielded cable. I was thinking of using that stuff for this application. Im not familiar with high gain type amps, but are you suggesting to solder the braided steel to the back of the pots per each wire? Could you elaborate more please?

Thanks

[terminalgs]

I've done this several times.  The Why was usually due to chassis size constraints, and not wanting the controls on the floor. 

Here's the last two I did.  The one on the left is a 6G3 Deluxe with a 8" speaker, the one on the right is a 5E3 Deluxe with a 10" speaker.  the small cabinets provided the constraint...  or if you enjoy the industrial engineering aspects of amp-building, provided the challenge!! :-)





These are quiet amps, but of course, they aren't as quiet as you'd get if everything is in a single chassis.  All your standard rules-of-thumb and amp design practices you'd do to keep the amp noise-free apply here.   star-grounding,   filament wiring best-practices,  keeping the signal line isolated with shielded cable and away from the 120V and the 6.3V (on the 5E3, the power switch is on the bottom, so I didn't have to run 120V to the top).

The pre-amp tubes are in the top, so B+, ground, heater supply, (plus 120V for the switched one) all had to be run from bottom to top.

Maybe not required, but I made chassis ground a separate connection, and ran an isolated circuit ground from bottom to top to maintain a star-grounding type scheme.   I also put the preamp stage filter caps up top, as close to the plate resistors as possible.

I used an octal plug/socket and an umbilical wire bundle for everything but the signal, which I used an RCA jack.  I've used molex style connectors too (like a Magnatone M-series).

Here's a design decision I made for the 6G3 build, that if I did it again, I'd do differently:  the tremolo tube is up top, near the two control knobs.  space was tight on the bottom chassis, and I had space to spare up top.   (I should say, when you do this, you draw a line through a schematic and say "everything on this side goes in the top, everything else, goes in the bottom").  SO, I ended up putting the bias supply circuit in the top as well, and running some neg. "raw" DC voltage up to the top, and the controlled bias voltage to the bottom.  This means that the bottom chassis cannot function without the top chassis connected.   Not an optimal design.  But it works. 

I've got another one in the works right now, and probably 5 more cabinets in the garage that it might happen to someday....

What I would like to do is just move the controls ( vol, tone stack, input etc ) to a separate small chassis and mount that on the top inside of the amp cabinet and leave the iron and the guts and all tubes mounted in another chassis in the bottom of the amp cab. If I were to do this, do I need to take any special care with shielding with these long wires? Will just using shielded cables be enough?

Be aware of the capacitance of your shielded wire over distance, but for 2' runs, it is probably nothing to worry about. 

If  the input jacks are up top, then you'd run the shielded wire to the bottom pre-amp triode (you might stick grid-leak and grid-stopper in the bottom close to the gain stage,  which would rule out the nice fender style input wiring tricks...),  then after the coupling cap, you'd run shielded wire back up top to the tone/vol controls,, and again, signal back down...  (All the up and down is why I stuck the pre-amp tubes up top on mine)

[rokker]

Great info, thanks! Those are cool looking amps. What did the cabinets start out as?

[Ed_Chambley]

Ed, Thanks for the info!

The reason I do this is different than you, but the results are the same.  I do it so I can make changes in the tonestack and not have to be concerned with getting hit with high voltage or turning off the amp every time I make a change.

So when your on your workbench you can make the changes to the tonestack since you dont have B+ running to it, correct?

I have a tonestack chassis I use when building.  Contains nothing but pots and input jacks.  I do not like to tweak the tonestack inside a new build.  I have never built it in an amp, but have ran it on my bench with about 3 or so foot of shielded cables with ground connections inside the tone control box. Shield grounded to the back of the pots as you might do in a high gainer.

Im thinking if I went this route I would prob only need 2ft max of shielded cables.
When you say shielded cables are you talking about the braided steel type? As I mentioned above, when I gutted the Cordovox amp I got a bunch of braided steel shielded cable. I was thinking of using that stuff for this application. Im not familiar with high gain type amps, but are you suggesting to solder the braided steel to the back of the pots per each wire? Could you elaborate more please?

Thanks

I use the shielded cable Doug sells.  The braid is soldered to the back of the pots and to the ground on the input jack.  The I simply solder the inner wire where you would to the amp turrent or eyelet and shrink tube the shield going to the amp to insure no ground loop.  Yes, there is no high voltage.  When I make the arrangement I want to use I simply braid the wires and connect them inside the amp.

[terminalgs]

On the shielded cable, you only need to ground one end of the shield. do this to avoid ground loops (I think HPB mentioned this). it doesn't matter which end,  just chassis ground.  If I were you, in addition to the 3 shielded cables run top to bottom, I'd run a circuit ground wire (if you are using a star-ground topology),  and a separate chassis ground.  (you could ground both ends of one of the shielded cable's jacket,  but I'd run a dedicated chassis ground, so any of the shield cables could be disconnected, and you'd always have chassis ground)

also, I'd put the plugs/sockets on the lower amp chassis, as apposed to having the umbilical going upward to the upper chassis.

If you want to use a fender style two/three knob tone stack, you either need to run high voltage up to the upper upper chassis, or run a bunch more shielded cables.   

Great info, thanks! Those are cool looking amps. What did the cabinets start out as?

50's record players.  I started doing this because I absolutely hate doing the upholstery/tolex work,  plus I like the vintage look you get immediately with an old cab. the record player cabinets tend to be light-duty and require gussets, re-enforcements,  and a lot of interior wood work to carry the weight.

Also, these are somewhat lightweight pine plywood boxes, smaller than they should be for the size speakers I put in them.  The result of putting Fender Deluxe circuits and 8" and 10" speakers in these small boxes, is they sound more like Supros than they do Deluxes.

[HotBluePlates]

Also, these are somewhat lightweight pine plywood boxes ...

Better than the lightweight pressboard of the portable record player I fixed for my Mom to resell!