Ideas for a guitar case amp?

[Jennings]

A friend of mine who does a lot of busking and travelling asked whether I'd have a go at building a quality, lightweight amp into a guitar case for him.  That's totally new territory for me, and if I were trying it for myself I'd probably opt for a basic champ circuit and valves, maybe with a little alnico or neo speaker.  That would suit my blues/garage/rock tastes...but sadly my friend's tastes are different.  He likes reverb, and plays mostly folk/jazz predominantly clean and finger picked.  So I'm thinking maybe solid state is a good bet...which would help with weight too.

Anyone got any suggestions or examples of any builds they've done or seen which might fit the basic brief?  I'd like to build something, but part of me feels it would be simpler to get hold of a small solid state combo with reverb, and just ditch the cab and build that into the guitar case.

[kagliostro]

The only things I remember



http://el34world.com/Forum/index.php?topic=20720.msg218753#msg218753

http://el34world.com/Forum/index.php?topic=21473.msg228193#msg228193

http://www.filedropper.com/silvertone1449-sluckey

Franco

[Jennings]

Thanks Franco...much appreciated.  I quite fancy one of these myself, so even if I end up doing something different for this project I might also build something for myself later in the year.  Is there any practical way to do anything with the option of battery power...or does that pretty much rule out valves since the demise of high voltage wireless set batteries?

[kagliostro]

Don't know if this can be of your interest

http://el34world.com/Forum/index.php?topic=11342.msg104657#msg104657

Franco

[SILVERGUN]

I would build a case with a spot for one of these and a speaker.
A bonus would be that it will actually be strong enough to act as a standalone backup amp, or even his main amp, if needed. At 2 lbs. and 3" high this might be as good as it gets.
 https://www.quilterlabs.com/index.php/product/101-reverb/

[1blueheron]

Lithium batteries opens up a world of possibilities that weren't available back in the days of tube based car radios and silvertone guitar case amps.   I think a full on tube amp is fully doable. I would pick up a rechargable drill at a pawn shop or flea market that has a good rechargeable battery and charger. A laptop battery would be great but you would need to keep the laptop intact for charge control or build a proper charger.

You can scrap the PT/rectifier because you already have  isolated DC power from the battery so that eliminates a lot of weight and expense.  A DC/DC transformer can boost your voltage to neccesary levels. Subminiature tubes will help conserve space if that is an issue. Look at 6021, 6205 and 5902 tubes. Maybe something like this could be used.   https://jjs.at/electronic/class_a_subminiature.html or this https://hackaday.com/2018/02/19/tiny-guitar-amp-rebuilt-with-tiny-tubes/



I built several LM386 based amps like the ruby amp prior to getting my first tube amp to play with. For what they are, they are very impressive in tone. They also have some really nice breakup if you play with it a little and get the right capacitors in place to suit your style.  I built one little amp that used the LM386 as a preamp section a drove  a TDA based power section.  It sounded excellent.  The key to these amps sounding good is not to skimp on the power supply and buy the LM386N-4 version that will handle up to 18V. Running them at less than 12V they will get farty.

Then there is the Hybrid option.  Something like the tuby ruby with a very efficient speaker would be a nice option as well. 

possibly a firefly

&index=2

Cool project thanks for posting.

[Jennings]

Thanks for the pointers so far guys...some really inspiring ideas there proving the battery tube amp concept isn't dead in the water after all!  I hadn't thought of that route.  Keep the ideas coming by all means.  I'm actually getting the idea of trying one of these suggestions myself...I think I'd enjoy it, whether it's my friend's kind of thing or not!  I wonder whether he'll go for something like the Quilter suggestion...or perhaps even just buying something like the Vox Adio Air GT, which he could probably fit in his bag of leads and bits anyway without needing to convert or build and in-case amp.  From checking out some of these tube ideas on You Tube they have more grit and breakup than his clean loving jazz tates might allow...but they're certainly my thing!

[66Strat]

Rather than a guitar case amp, I would look towards making a backpack configured amp and speaker. Maybe something along the lines of a mini flip top cabinet with shoulder straps. One of the Hotone amps paired with a 6 inch speaker would be a good starting point. Get your mojo ho tone workin.

https://www.hotoneaudio.com/products/nano-legacy

[Jennings]

That’s a good idea...I don’t have any experience with the Hot Tone amp heads, but their little cab is ace. I got one last year to pair with a Wangs VT-1H mini valve head. I rolled valves a bit to tune the breakup and love it...It’s a great little rig! I do play with more crunch though than my friend.

[Jennings]

My friend has decided to think on it all a while...however I’m now planning something for me inspired by this thread. I’ve got a couple of old Selmer lap steels from the 40s/50s, and the case amp discussion reminded me of the early RSA Truevoice Selmer case amps and combos that accompanied these. Anyone tried to build one, or know a good source of schematics for these early Selmer amps? Did they use the Universal 8 circuit?

[sluckey]

Plenty of Selmer schematics here...

     https://el34world.com/charts/Schematics1.php

[Ritchie200]

To follow up on 66Strat's recommendation: Just for reference, I have a Hotone Nano Heart Attack amp.  It is awesome going direct or into a cabinet and will easily drive a 4x12 to uncomfortable levels.  It does require 18v.  That is the high gain model based on the dual rectumfrier - and it sounds perfect, but others like the AC30 and Fender knockoffs sound really good as well, are true to their namesake, and are also 5watt out.  I have several of their products and they all sound really good and are affordably priced (meaning I can sneak them in and my wife doesn't know...).  In fact I am about to order one of their Ampero modelers.  I would not hesitate to use one of their products.

MHO Jim

[sluckey]

What is the Universal 8 circuit?

[kagliostro]

I think he refers to this

https://el34world.com/charts/Schematics/files/Selmer/Selmer_truvoice_universal_8u.pdf





Franco

[tubenit]

Just for reference, I have a Hotone Nano Heart Attack amp.  It is awesome

What!!!  This is from a man who owns a Marshall Major???   

[Ritchie200]

I come down from heaven and dabble with the common folk now and then...

Jim

[Jennings]

Just catching up with things!  Apologies for the delay...and thanks for the replies folks.

Franco's correct...that's the Selmer schematic I'd come across, but not managed to confirm whether it was the kind of circuit that the company put in their early guitar case amps. 

This potential project has evolved a bit...my friend's doing his own thing now, but it's ignited my interest in doing something for myself.  I've recently got hold of a circa 1940s Selmer lap steel missing its case, so I'm considering making one as part of the refurb.  I'm happy with it running from mains power too, so batteries aren't required.  As these lap steels often used to come with case style amps, I'm thinking I'll maybe make a case amp to build in.  I thought it might be fun to base the build a bit on the kind of circuit that might have been originally paired with the guitar.  I'm happy to tweak or modify it to suit my needs though.  From reading that schematic the circuit uses a U31 rectifier, KTZ63 preamp pentode (similar to a 6J7G apparently), and a KT33 output tetrode.

I've got no experience with playing through this kind of amp, but I'm guessing it was designed for low power clean sounds, in keeping with Hawaiian music of the day...so maybe I'd need some kind of additional gain stage adding for my more retro blues/rock slide type tastes?  I'm not a high gain kind of guy, but I like a bit of grit.  I'd also probably want a single tone knob.  And maybe a Vox-style cut control.

[kagliostro]

May be you'll like to give a look here

http://www.vintagehofner.co.uk/gallery/solids2/sol52.html

http://www.vintagehofner.co.uk/selmer/early/sel7.html





Franco

[Jennings]

Thanks for the links Franco...those are the exact bits of kit I'm talking about, in fact the lap steel at the top of the page on the second link you sent is the exact model of guitar I'm refurbishing - a sort of stepped deco shape, but without the wings on the earlier model.  The second pic on that page (with the combo and curvy lap steel) is of a model I refurbished last year in fact.  Anyway, it looks like the first link also shows a picture of the original blue manual/info for the case amp...and lists the valves used.  They're almost the same as the Universal 8 schematic, except the preamp triode appears to be an L63.  It also seems they built it as a separate combo too in the form of the TV8, which true to the schematic has a volume and tone control as opposed to the lack thereof (so far as I can see in the pictures) on the case amp.  So that's good enough confirmation that I'm on the right lines basing any build off the U8 schematic...which appears to have a basic volume and tone control already.  True to the name, Selmer appear to have used varients of the circuit for a series of applications. 

Anyone actually played one though?  Or a similar circuit?  I'd be interested to hear whether there's a feeling that there's not enough gain, or it's too dark, in the as-drawn version.

Initially I'll try and read the BOM values and transcribe them out from the schematic.  If I build a version of this, what's the advice straight off on things that I might want to alter?  I'd probably replace the mains dropper resistors with a mains transformer.  I also don't need two jacks...and one of the ones in the schematic looks to have a weird transformer off it?!

[shooter]

a weird transformer

maybe for the 21st century 

guessing it's an impedance matching for use as MIC in

I'd probably replace the mains dropper resistors with a mains transformer

I'd consider that a MUST, but hey that tingly feeling you get from the guitar might be your thing 

[Jennings]

   Random electric shocks might inspire me to wig out more when playing!  Haha!!!  Yeah, let's go definite on the mains transformer.  Good call on the purpose of the input transformer too...I didn't think of that, but of course amps of that day often catered for mic and instrument.

I think I've almost transcribed the BOM as it's currently drawn, before mods...except I can't read the values of C6 or C7.  Does this look like I've read it right?

R1   470K      
R2   2.2M      
R3   5.3K      
R4   27K      
R5   3K      
R6   470K      
R7   47K      
R8   220r      
R9   450r   40W   
R10   80r   20W   
R11   40r   20W   
         
C1   0.01uF   450V   
C2   0.02uF   450V   
C3   50uF           25V   
C4   0.05uF   450V   
C5   0.02uF   450V   
C6              350V   
C7              450V   Across OT
C8   25uF           25V   
C9   16uF          450V   
C10   16uF          450V   
         
VR1   5M   Volume   
VR2   50K   Tone   
         
V1   KTZ63   Pre   Pent
V2   KT33C   Out   Tet
V3   U31           Rect   1/2 wave
         
L1   12Hen           75mA   Choke
T1   3000r - 15r   60mA   

[sluckey]

R3 = 3.3K
C6 = 4µF
C7 = .003µF
VR1 = .5M (500K)

[Jennings]

Thanks Steve, much appreciated! I think I can locate appropriate valves, OT and choke...what do folks recommend for a mains transformer spec?

[sluckey]

I would just use a 50 watt isolation transformer connected to the AC line input. 120V to 120V @ 50VA

[Jennings]

Thanks Steve 👍 I’ll try and get time this weekend to draw out the build schematic and think about sourcing parts. I’ve got the valves on the way.

[Jennings]

I've drafted an initial schematic (see attached).  Hopefully there aren't too many typos or mistakes.  Please do let me know if anyone spots anything silly!

One question I do have is about the heater requirements...I'm used to dealing with "standard" guitar amp iron, which accommodate the usual 5V rectifier tap and 6.3V tap shared by all the amp section valves.  Those are easy to get hold of at reasonable prices.  The data sheets for the valves here show the U31 and KT33C need 25V filament voltage, and the KTZ63 uses 6.3V.  How would people approach that scenario?  I have yet to look, but I'm not sure how easy it'll be for me to find an off-the-shelf transformer with secondaries for both of those.

[sluckey]

You don't need a transformer for the 6.3v tube. The filaments will be wired series just like the original schematic. The only difference between your redrawn schematic and the original schematic should be the added isolation transformer.

BTW, you need to replace C1 with a straight wire. It's no longer needed since the line is now isolated from chassis.

[PRR]

> 25V filament voltage

The original wired them in series for 56V, wastefully dropped from 220V line.

Are you UK or US? If US, how devoted are you to these non-US tube types?

In the US we would do it all on 110V, with 25L6 and maybe 6AU6. Or 50L6 and 12AU6.

[kagliostro]

On the schematic Jennings draw a 240V line, then a 120V/120V transformer (seems 120V and 120V are two separed secondary windings)

I suppose he is in this side of the pond or, may be, Australia / New Zeland

I'm curios

--

However if he has a 120V winding he can apply PRR council

Franco

[PRR]

> Jennings draw a 240V

Ah, yes.

Then I would use a second 240V:24V (standard furnace transformer) for the heaters. The KT33 direct. The small tube with about 18V of dropper resistor. And for KT33 fed from a 120VAC supply, and a miniature amp, _I_ would skip the bottle rectifier so this fairly big tube can have 160VDC.

[DummyLoad]

just a suggestion. i used a hammond 80VA iso x-former for the B+ and heater supplies. preamp pentode is US 6J7 equivalent.

--pete

[DummyLoad]

another approach to dealing with the odd filament voltages; take advantage of the KT33C CT filament. wire as shown in attachment  for 13V @ 600mA.

antek PT has 2 x 6.3V @ 2A windings. strap them is series and tap V1 filament for V1. R16 balances the load across the windings. heater artificial CT can also be a 250Ω pot (omit R14,15) with the wiper grounded, or just simply ground the green + blue wires that are essentially a CT.

--pete

[sluckey]

There are plenty of cheap used 240/480v to 120v control transformers on eBay. That's what I would use...

https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2060353.m570.l1311.R1.TR6.TRC2.A0.H2.X240+to+120.TRS0&_nkw=240+to+120+transformer&_sacat=0

[Jennings]

Thanks for the replys, corrections and options folks...much appreciated, and some good advice to consider.  I'm in the UK, so the mains is around 240V here (though "officially" it's 230V).  I've got a number of control transformers (for 110V, 220V etc) as I'm used to running equipment from various countries of origin anyway.  Gives me plenty of flexibility for using up whatever spares I have too.  Also, my main source of mains transformers recently supplies units with dual primaries for 120V and 240V use, so I'd probably given mixed messages in my drawing.  I'll clarify the schematic and upversion.  I'll be looking to work off 240V mains as it's a scratch build and have the choice to build it that way.  I'd originally stuck with the tube choices just as an hommage to the original circuit...though I will say the KTZ63 are the hardest to get hold of here at the mo, so I could always substitute.  I've already got the odd U31 around from some old valve radios, so I can leave that in the design or take it out without too much fuss. 

Pete - Thanks for the schematics etc...what did you make of your build performance/sound wise?  Anything you'd go back and build differently after playing the amp?

Re the Vh supply: 
When simply run in series as per the original Selmer schematic, what are the pros and cons?  Aside from the obvious component cost savings that is.

If I go the route of a second transformer for Vh supply, I can cheaply locate a 240/24V unit, and use a 60 Ohm / 10W resistor to give the circa 18V drop to around the 6.3V, so that's not too much fuss to practically implement.  And the Hammond 185E230 is also similarly cost effective and freely available here, so implementing that is also no bother.  The Antek unit I'd have to ship over, but I might be able to find something similar here and try making use of that KT33C heater CT.

[kagliostro]

+ 1 for

..... _I_ would skip the bottle rectifier so this fairly big tube can have 160VDC.

I agree with PRR

the rectifier didn't matter on an SE amp

Franco

[DummyLoad]

just to be clear: i have not built any of the plans that i presented. they are simply variations of what i would try based on my limited knowledge of the original circuit.

KT33C are near non-existent on this side of the pond, so if i were to build something similar with more common tubes found in the US, i'd probably use a 6Y6GT for the power pentode with the antek toroidal PT, and some modifications for the 6Y6GT screen supply.

respectfully,

--pete

[PRR]

> probably use a 6Y6GT

It is hard to look at the KT33C and think anything other than "better-built 25L6/6Y6".

Series heaters and resistor is cheap, dangerous, and hot. (Wasting 75% of heater input as resistor heat.) Yes, we use that much heat inside a piano in summer damp, but not a guitar case.

Going with transformer for isolation, we can avoid the need to waste-off 160+ volts. But now a LV and HV winding.

[DummyLoad]

the stateside model...improved(?) screen grid supply to compensate for the shortsighted design of the 6Y6 screen grid.

--pete

[Jennings]

Thanks folks...with your comments and suggestions in mind would something like this be a reasonable idea?  It's not expensive and I could make use of the 24V secondary for the KT33C Vh, using a dropper resitor to the preamp pentode.  Or even use the 12V and a dropper and give the preamp its own separate Vh supply winding.

https://www.ebay.co.uk/itm/CHINT-NDK-CONTROL-CIRCUIT-PANEL-TRANSFORMER-20-415V-OUTPUT-12-24-48-110-230-VOLT/253457845500?var=552632020432

[PRR]

You surely don't want the 150VA job in a guitar case. There is a 50VA.

The single secondary makes it difficult unless you use the high-V heater tubes, series heater resistor, and half-wave rectification.

[DummyLoad]

https://www.toroidal-transformer.com/shop/high-end-audio/power-transformers/313/2n1357-powertransformer-for-valve-amplifiers--48va-.html


use FW with solid state rectifiers.



--pete

[Jennings]

Thanks for the input and help folks...much appreciated.  It's been an interesting few months working on other bits and bobs with this on the back burner, not least because I've found it a little hard to get hold of things lately, but I'm coming back to my little project now things are moving again.  I think we're all sorted on iron...I'm going with Pete's 48va Amplimo toroidal suggestion on the power transformer, and I'll go full wave solid state on the rectification.  So that's the biggest design decision out of the way! 

I've found a suitable choke here:
https://www.ebay.co.uk/itm/GUITAR-Amp-Output-transformer-15W-Single-Ended-for-EL34-EL84-EL84-OEP-V29A04F/163984018749?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

And what I hope to be a suitable OT here:
https://www.ebay.co.uk/itm/GUITAR-Amp-Output-transformer-15W-Single-Ended-for-EL34-EL84-EL84-OEP-V29A04F/163984018749?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

I'm going to build this into a small combo format, I think...so it's time to hunt a suitable chassis and cab, and decide what to go with speaker-wise.

Now, a funny thing happened yesterday...I got a good deal on a Selmer lap with original case!  Happens to be the same model as the one I gig with, so the guitar's a good backup and spare as I'm fond of the pickup in these.  But this one comes with case and amp intact!  Amp apparently isn't functional, but it looks in great nick, so it'll be interesting to look at first hand while I build my inspired by version.  I also need to weigh up whether I restore the original as-is, leave it as a piece of history, or modernise that too. 
https://www.ebay.co.uk/itm/Selmer-Truevoice-Lapsteel-Guitar-With-Amp/184322086450?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

[Jennings]

The new purchase arrived...and I think it's helping direct the project itself.  The amp's currently disabled, and a quick open up reveals that someone's already modified it at some point previously.  Pics attached.  Currently a number of wires arent connected, and the output transformer's clearly been moved.  I'll the new mains transformer and circuit mods discussed to rebuild this chassis rather than do a separate new build.  I'll leave the U31 in situ but not connected just for show.  That large wax and paper packet has a Jun 46 date stamp, interestingly.

[PRR]

Is there a power transformer??

Is it this plan?
https://el34world.com/charts/Schematics/files/Selmer/Selmer_truvoice_universal_8u.pdf

[shooter]

 
I like how R9 was schematically "drawn to scale" 

[Jennings]

   I'd have really laughed if they'd drawn that old box capacitor to scale too!  Haha!!!

I haven't dismantled it yet to really get a proper look, and can't see all the connections and printed values for definite, but it looks like the amp was a slimmed down or earlier version of that Selmer Universal 8 Drawing 1003 circuit.  Notable differences I can see already are:
1) There's no sign of the non-guitar (mic) input having had an input transformer, in keeping with some Selmer literature I've seen saying they used to sell these separately as some kind of after-market mic input adaptor.
2) There's no volume or tone adjustment...just preset component choices.

The rest of the circuit looks to probably be the same (though I suspect the unlabelled second input might be a later addition), and has the same valve compliment.  Power is the same U31 HW rectifier...except the bulky dropper is removed, missing and not replaced.  This also explains the unconnected wires.  I suspect it malfunctioned and was taken out but never renewed due to a lack of suitable replacement or easy/cheap transformer swap.  The choke and output transformer are intact, but look at have been moved slightly in the chassis when someone obviously did some rework before.  All capacitors are the wax/paper kind.  All resistors look to be the enamelled wire wound kind.  It's completely point to point wired...no boards anywhere.

My initial thoughts are to save the choke and OT (assuming I find they function...or swap with the replacements I already have), and build to the modernised SS rectified circuit we've developed from the Universal 8 circuit.  Using new components, wire etc.  Mount the new 120v/6.3v/6.3v torroidal mains transformer where the dropper used to be, and keep the choke below it inside the chassis, OT next to that (so in situ).  Plus use a turret board to mount the components, making it easier to work on.  I could add mounting posts either to the control panel side where the massive box capacitor once sat, or I could mount below (so effectively over) the underside of the valve sockets in the more normal orientation.  A good plan do you think?

I don't need the voltage selector switch, or the non-guitar input jack, so I have a couple of pre-existing chassis holes for mounting volume and tone.  Maybe even go with a concentric dual vol/tone pot and mount in the non-guitar jack hole to keep it away from the mains switch?

[Jennings]

I've been having fun with the next challenge on this project...the original chassis is so compact, space is a major issue.  I've had to:
- Rig a mounting bracket for the mains OT to sit vertically where the dropper resistor was. 
- Trim the mounting tabs on the OT to fit where the original unit was in the case.
- Squeeze a 16+16uF can where the U31 originally was to save in-chassis space. 

I'm not using the circa 60s Radiospares Standard replacement OT for this rebuild.  I've drawn a turret board layout for the circuit, but that won't fit in the remaining case space...so I've had to draw a more compact version (some items mounted together, rather than separate with linked turrets) which will.

I just have a couple of questions:

1) The original anode wire for the KTZ63 had a rubber insulation layer with a metal braided outer for screening.  Much like the standard Gibson hookup wire you see everywhere these days, except that has waxed cloth insulation or similar...However I can't find any that states what its voltage & current rating is.  I'm not sure this modern stuff is suitable for B+ supply use or not.  What would folks use?

2) Do folks think I should re-use the cool-looking old fiber wafer mains push/pull on/off switch, or replace with a modern unit for safety and reliability?

[sluckey]

1) The original anode wire for the KTZ63 had a rubber insulation layer with a metal braided outer for screening.  Much like the standard Gibson hookup wire you see everywhere these days, except that has waxed cloth insulation or similar...However I can't find any that states what its voltage & current rating is.  I'm not sure this modern stuff is suitable for B+ supply use or not.  What would folks use?

The top cap on the KTZ63 is the control grid. Voltage rating for wire is not a factor. Use any shielded wire you want.

2) Do folks think I should re-use the cool-looking old fiber wafer mains push/pull on/off switch, or replace with a modern unit for safety and reliability?

If it is functional and in decent shape I would use it.

[Jennings]

Ah yeah...I see, many thanks for helping clarify...much appreciated.  I popped the meter on the orignal mains switch and it appears to be functional, so I'll go ahead with keeping it.  I might add some shrink wrap insulation, as it's a bit close to the transformer frames if it comes loose and rotates.  I'm using the voltage selector chassis mount for a fuse holder.

[Jennings]

Apologies for the thread resurrection, but I’ve finally got back to this project and wanted to close the loop.


Particularly for real estate limitations, along with necessary safety/modernisation, I built using the toroidal mains transformer (located where the larger dropper resistor once was) and KBL04 SS rectifier. Concentric pot for volume and tone using a pre-existing chassis input jack hole. 16uF dual can in the space left behind by the rectifier tube. Turret board for ease with components…thought it was a squeeze round the internal choke (used the original) and OT.


Schematic as built below, and I can report that it’s a great sounding little amp! Superb full cleans, fair bit of volume and headroom for the size. Nice dirty crunch when you crank it…great for dirty blues lap steel. Thanks for everyone’s help and input…we’ve resurrected a little gem!