Using Chassis as Ground?

[ioplex]

Hi,

I'm about to start my first tube amp kit from turretboards.com:

  

I know these guys are just trying to provide a faithful reproduction of the Supro 24 but I have to question using the chassis as ground.

Will using the chassis as ground not pick up hum?

Mike

[stratele52]

IMO it is only for save ground wires we use chassis as a "wire" . And it make easier and faster to built the amp.

[HotBluePlates]

... Will using the chassis as ground not pick up hum?

The concern is not "picking up hum" but whether hum may result from large power supply ground currents modulating small preamp ground currents.

See where all the filter caps share a single ground point to the chassis over by the power supply? Some would advocate distributing those filter caps among the amp circuitry, so that each filter cap is close to the circuitry it feeds, and the grounds for those preamp circuits connect straight to their associated filter cap.

So would be "best practice"; but many amps have been built with other than best practice, and with quite good results. You might ask the company if they've personally made that particular amp, and whether hum levels were acceptable. Otherwise, you may have a bit of head-scratching coming up with an improved layout that you'd prefer to use.

[terminalgs]

I know these guys are just trying to provide a faithful reproduction of the Supro 24 but I have to question using the chassis as ground.

Will using the chassis as ground not pick up hum?

His layout and grounding it isn't really a faithful reproduction,  its more of a turret or eyelet board approach (given the company's URL, seems fitting).  I think Valco builds were all point to point on tag strips and tube sockets.

If a lot of folks have built that kit, and if the reviews don't indicate otherwise, doing what the illustration says will probably sound good hum-wise. (hopefully, that's what a good kit provides you -- vs. a bad kit).

If you want to tweak it, I recommend reading valve wizard's grounding chapter.  Afterwards, examine the layout.   Once you visualize the current flows through the circuit, you'll probably see a few small improvements that can be made running an extra wire or two, or changing a common ground path for a few sub-circuits.

[ioplex]

I know these guys are just trying to provide a faithful reproduction of the Supro 24 but I have to question using the chassis as ground.

Will using the chassis as ground not pick up hum?

If you want to tweak it, I recommend reading valve wizard's grounding chapter.  Afterwards, examine the layout.   Once you visualize the current flows through the circuit, you'll probably see a few small improvements that can be made running an extra wire or two, or changing a common ground path for a few sub-circuits.

Yes, I am very familiar with Blencowe's awesome books (Strangely it seems the Power Supply book is being discontinued? I thought that book was quite good. Power supplies are so important!).

I fear now I am about to move things around enough that I might need a custom turret board.

This is my TODO list at this point:

1) Plastic isolated Marshall style input jacks.
2) Metal film resistors between all inputs and grids / ground.
3) Twisted ground wires from inputs to board closer to tube and possibly on to grid stopper soldered direct to grid (but terminated with only a little conductive tape under heat shrink so the ground wire is just acting as a shield).
4) Move last filter cap and possibly it's associated resistor into middle of board between preamp and PI with preamp an PI grounds converging there before running to star ground.
5) Create separate grounds for preamp / PI and output tubes to star ground.
6) Buss wire soldered to all potentiometer casings and then to chassis (or to star ground)
7) Verify chassis is isolated by testing non-continuity with chassis ground wire disconnected.

Note that I have not heard anything bad about this kit regarding hum. I should probably just build it according to the kit layout. But that would be too easy!

Mike

[sluckey]

Note that I have not heard anything bad about this kit regarding hum. I should probably just build it according to the kit layout.

I agree.

[phsyconoodler]

Don't waste your time with star grounding and isolated jacks.There isn't enough circuitry or gain stages to warrant anything exotic in this amp.
  Listen to Sluckey and build the amp as is.You will likely have zero issues unless you try and over-think the simplicity of the circuit.

[super&plexi]

two thumbs up.... one for advice to build as is, and one for phsyconoodler seconding that advice. so maybe 4 thumbs up total...Ha!. But seriously, especially if this is 1st build, keep it simple.

 Maybe run twisted pair heaters, up away from circuitry, Fender style if looking for some tweaks...if you have to change something. I know it's almost impossible for me to stick to an old layout, without thinking I can improve it. Maybe jumper the channels,...Marshall style, (patch cord) or w/a switch wired inside, or switchable caps on input. those are simple tweaks, If you simply cannot or will not take their advice.............................................. But I'd still take their advice,.......................love to hear soundclips when finished. good luck!. s&p  

[stratele52]

If you look closely at the drawing, there is a very light wire hooked up, so this project is probably not chassis ground.  If you look, closely 

Sure it is a chassis ground.

[sluckey]

If you look closely at the drawing, there is a very light wire hooked up, so this project is probably not chassis ground.  If you look, closely 

Sure it is a chassis ground.

Make that a double sure it is a chassis ground.

[DummyLoad]

I'm about to start my first tube amp kit from turretboards.com:

i thot that mere mention of that site were to to be received with threats forum admin floggings?   

--pete

[sluckey]

There was a time that you could not post that domain name or it's other name. A script would automatically edit it out. How did you find out about the floggings??? Uh oh!   

[DummyLoad]

There was a time that you could not post that domain name or it's other name. A script would automatically edit it out. How did you find out about the floggings??? Uh oh!   

well i guess they mad(e) a mends? that's good. i was flogged for attempting to post said site in the not so distant past. i've learned my lesson. yes, i'm a forum felon.

 

--pete

[Willabe]

yes, i'm a forum felon.

 

[tubeswell]

From a safety aspect, your chassis should definitely be attached to the mains earth ground. The whys and wherefores of whereabouts on the chassis you run your signal (and other) ground(s) to varies. Here are some articles

http://www.valvewizard.co.uk/Grounding.pdf

[ioplex]

Maybe run twisted pair heaters, up away from circuitry, Fender style if looking for some tweaks...if you have to change something. I know it's almost impossible for me to stick to an old layout, without thinking I can improve it. Maybe jumper the channels,...Marshall style, (patch cord) or w/a switch wired inside, or switchable caps on input. those are simple tweaks, If you simply cannot or will not take their advice.............................................. But I'd still take their advice,.......................love to hear soundclips when finished. good luck!. s&p

I'm going to just do the layout as specified. I reason I won't achieve complete satisfaction unless I first find a problem and then fix it (instead of fixing a problem that I don't yet know exists).

Although for the heaters I used some stiff 18 AWG teflon wire and a cordless drill to twist. I think the wire provided is a little too flexible to maintain a good twist.

I'm making progress:



The board is pretty much done. I just need to add the transformers and glue all the leads to the pots and tubes ...

Mike

[stratele52]

Ioplex ,

Stif / solid wire are not bad , but easy to break if you do not remove insulation properly. Stranded wire is more flexible and tough .

[phsyconoodler]

Tightly twisted heater wires are totally un-needed.If you look at many boutique amp layouts you will see a great many have simply run the heater wire parallel to each other from tune to tune.No twisting in sight.I experimented a few years ago with an 18 watt build.I simply ran the wires inside a piece of shrink tubing.No hums at all.
  So wasting time twisting is simply for looks and has no real purpose.simply keep the wires running parallel and as far away from signal wires as possible.I still twist heater wires but only to make them stay put,not for hum issues.

[ajeffcote]

Don't have anything intelligent to add, but I am watching!
About insulated jacks and star grounding, I have read from time to time that it is a better way. I tried star grounding without insulting jacks and did not like the result. Went with Doug's grounding instructions and my amps became much quieter.

[HotBluePlates]

... About insulated jacks and star grounding, I have read from time to time that it is a better way. ...

I tried isolated switchcraft jacks once, but it didn't seem to offer a benefit in the amp, used by itself.

I recently used isolated jacks again, but only connected the chassis to the power cord ground wire, with a switch to connect the amp's circuit ground to the chassis, or not (a safe ground-lift). Used by itself, the amp is quieter with circuit ground connected to the chassis (and the power cord ground wire). The ground lift appears to be helpful only if I was playing through multiple amps and had ground loop issues.

[TIMBO]

Hi guys, This may or may not be relevant, but I put it out there anyway.

When I got this old amp up and running I was predicting that it would hum like a mother, but I was wrong  quiet as a mouse.

I marked the schem as all the preamp is connected to a ground buss and the only connection to the chassis is at the input jacks.
Power amp grounds are to the chassis.
And the node caps are also connected to the chassis

Again not a high gain amp so may be that style of grounding will give little or no hum.

[super&plexi]

Oh I get it, this is your 1st amp from DoNotLinkToThisWebSite -...Not your 1st amp!.... NICE!... is this the model sometimes referred to as ''Lightning Bolt"?, the smaller sibling to the Thunderbolt?  Whatever model it is,   your work looks Great-clean-tidy...nice joints,...not a lot of space to work with in that chassis.
 just wondering why the switch to glue, instead of solder?....

 Hope you post soundclips, we had a Vintage/Used, Instrument shop, with everything imaginable passing through, and out of all the amps, the one marked  'God Of Tone'... 'not for sale', was a little blue Lightning Bolt, with 12'' 8 ohm  Greenback, and same tube compliment. we wouldn't sell it. kept it for after hrs. jamming, and mic'd club gigs.

[ioplex]

I think it's done. I just need tubes. Should get those in the next day.






This last graphic is a quick and dirty birch ply head cabinet just to try it out in it's proper orientation.

[HotBluePlates]

Looks like an outstanding build! Everything looks very neat & tidy.

[ioplex]

Done. There is no hum or noise whatsoever in this amp. Using the chassis as a signal ground is absolutely a non-issue.

Here's a sample:

http://youtu.be/YBYWFZSRWgA

The dual inputs can create a nice variety of clean tones. If you study the schematic the front end is nicely designed. There's a lot of tone shaping going on. The Treble inputs make a 300 Hz HP filter. Channel two has 2 pole HP filter and both Volumes make HP shelving filters. The Tone controls make LP filters and are mixed together with big grid stoppers. It kinda looks like the front end of the 59 Plexi circuit but with more high passing.

Strangely the kit came with 250K pots instead of 500K which is going to shift things up slightly more. I asked Watts about the change but received no response. The kit also came with two extra caps (1n and 33n). In theory I suppose you could use the 1n in place of one of the 500p and then that would compensate for the change in pot value.

The output of this amp is unusual. It's not the usual PI. I can't say the 6973 tubes sound different from EL84s for example. Strangely this amp is not as loud as an equivalent EL84 circuit. My other tube amp is a DSL401. I'm not really into shredding anymore so I've been plugging germanium circuits into the return of the DSL401 so that I'm just overdriving the last tube, the tonestack and output. So that would be fairly similar in terms of gain. And yet the DSL401 is considerably louder. I have a home made T-attenuator circuit with a big CPU heat sink on it (you can see it in the video) and it gets super hot with the DSL401. But with this Supro 24 amp, it doesn't get that hot. But that's good AFAIC. I like to mic the amp and listen to it with filtering, compression, reverb and so on so I don't want too much bleeding through the headphones.

If you're looking for something to overdrive, this is not the amp for that. I put a Rangemaster circuit in front (which sounds really cool into the return jack of the DSL 401) but with the Supro, it's iron-y and shrill. Even without overdrive it doesn't sound good to really jam on the strings. OTOH if you are good at controlling pick attack, muting with your palm, and generally controlling pressures, this amp has great response. It feels good. If I had to pick an iconic tone to compare this to, I would say it's a lot like a good GD concert recording.

Regarding Led Zeppelin, if you want your amp to sound like Jimmy Page, you are going to have to have Jimmy Page come over and play it for you. However, if the rumors are true, and this is the circuit used to record Led Zeppelin I, I don't hear anything in this amp to dispute that. I think a lot of the heavy Page sounds are the Fuzz and maybe also the Wah. Note that even though I don't think this amp sounds good overloaded, the Fuzz (or Tonebender or whatever) actually does not overdrive the input as much as you might think. If you look a the output of a Fuzz circuit, the pot only taps a small portion of the total output swing. In fact, the Fuzz is actually limiting the output swing to a couple hundred millivolts whereas if you really jam on your guitar, it can put out a couple volts. I have to wonder if some iconic Fuzz sounds were actually recorded direct (Reellin' In The Years solo). A really good Fuzz (which are virtually impossible to be had these days BTW) can sound really good listing to it direct. So if the Supro is just used to provide "clean" character, it would all make sense ...

Mike

[JPK]

Nice sounding amp!

[Ghetto_Soundwave]

I've bought a few parts from Ken Watts in his shop in St. Pete, Fl. His prices are reasonable, Doug's are better.