adding a tone stack and possibly a tube preamp section

[estch71]

Hi,

I have a Univox U50 PA head that has a 6L6 power section with a 12AX7 phase inverter.  It's a hybrid amp from the 70's.  I'm trying to see if I can alter the tone section in order to add a tone stack.  It has three knobs (four channels) - Volume, Treble, and Bass.  I'm also trying to see if I can add a tube preamp to it, taking out the solid state section.  Here's the schematic with markings - the part on the left is the tone section:

[Roly]

Hi @estch71, welcome.

I'm trying to see if I can alter the tone section in order to add a tone stack.

The preamp as drawn already has what I would call a "tonestack" or tone control network.  What did you have in mind?

You could replace the preamp section with a couple of valve stages, say around a 12AX7, but you need to be a bit cautious because it might be possible then to feed excessive signal voltage into the s.s. amp that follows the Master volume control, and possibly the reverb driver too.  The tonestack will also need to be changed from the current low impedance type to a high impedance type to suit the valve.

If you are intending to modify all four channels you should make sure that you have the heater power available.  One channel won't make a lot of difference, but four will require more than an extra amp from the heater winding.

Far be it from me to discourage experimentalism (and I am a bit of a valve buff), but this is not a mod I would undertake myself because I don't think any tonal gain will be worth the effort and degraded noise performance of the front end.

You may find it more profitable to just modify the tone controls of one channel to suit guitar.

[estch71]

Thanks, Roly.

I'm looking to add a midrange control and deepen the bass, possibly with a different cap on the bass pot?  It doesn't have a mid control.  I've seen tone stacks for Fender and Marshall amps but, they lacked the use of any transistors - this one has BC169s in it. 

[phatt]

You could just gut it and have one decent Valve preamp,, as roly says you may not have enough heater grunt to run more.

The other option is to have an insert point at PI (see pic) and build a separate dedicated preamp (which would be my personal choice) leaving the Amp in its original state.

possibilities are endless just don't ask to keep the reverb as that will make life very hard. xP
Phil.

[estch71]

Ha!  Thanks.  I've already removed the reverb circuit.  It's got a couple of BC169s that I can salvage for other projects.  I don't have the tank anyway. 

[estch71]

The P1 point.  Is that both input and output?  I'm imagining that I can use a few preamp builds I've seen around the net to insert into the schematic.  My only issue is the connecting points on the board itself.  I admit that I'm sort of intermediate with electronics - I build stomp boxes and have done a few mods to amps before. 

[phatt]

PI is the input to power amp.

Ok then maybe just add a tone pedal in front and leave the amp alone till you become more confident.

If you are itching to build something You can always have a crack at my PhAbbTone circuit it has a big midrange control.

This will just enhance what you already have as you will have two tone circuits to tweak.

I've built 14 of these units now for local players and loosing count.
Phil

[Roly]

The other option is to have an insert point at PI (see pic) and build a separate dedicated preamp (which would be my personal choice) leaving the Amp in its original state.

I like this idea.  I was kinda nibbling in that direction myself.

Don't get hung up on the BC169 - it's more than likely just a different packaging of the BC109 family, nothing too exotic.

You can alternatively take any tonestack intended for use with valves and scale the values to lower the impedance to something more suitable with solid state, e.g. increase all the caps x10 and lower all the resistances to 1/10th.

Or you could build a PhabbTone.   

[estch71]

I like the PhAbbTone idea.  I'm assuming this sits in its own enclosure as a pedal of sorts, right?  I have to try and convert it to stripboard.

[phatt]

Yes you can build it as a floor stomp box but I built quite a few as small top boxes as you will likely have it on all the time so no real need for a stomp box.
I used old printer splitter boxes as they throw them out nowadays, pick em up few $2 at junk sales. :tu:

I use a very basic hand drawn pcb as I'm no way gonna fork out big bucks for a hobby.
You can very easy empty your wallet with fancy gear only to find you are no better off that a cheap simple build.

I did all these with Eagle (free version) mainly to get all the pot pins and opamp pins to line up then print a track layout and lightly centre pop all the holes then just join the dots with a light pencil ,, check and double check then indelible ink the pencil lines.

Dip it in enchant for 10~20 minutes,, drill the already poped holes and you are done.

Of course I don't recommend doing it this way if you want to build a whole Amp but it's doable  for such a small unit like this.

Strip boards are ok also for small stuff with low parts count but you still have to run air wires out to 4 pots which I find tedious but for a one off,, go for it.

If you can find a TL062 I think they run very low current and hence battery lasts longer but I use 2x 9 volt delivering 18 VDC and the batteries last for around a year with LM833.

Just be aware these are HiZ circuits and are prone to picking up noise. so everything above R1 is super sensitive so keep those tracks short as possible.
Have fun, Phil.

[Roly]

You can very easy empty your wallet with fancy gear only to find you are no better off than a cheap simple build.

Now there is a statement that should go up in flashing lights.

these are HiZ circuits and are prone to picking up noise.

Which is where the all metal printer switch boxes help a lot with screening.  Failing that, ali die-cast.

[estch71]

Hey Phatt,

Do you happen to have a copy of the PCB layout?  I'm trying to convert it through PCB layout but, some of the symbols are different (phone jacks and such) while others, such as component values, are a bit difficult to locate.

Thanks

[phatt]

Hi estch,
Not sure I understand?
Do you mean locate actual part at the shop or locate on the software?
My layout is only going to work if you build it the same way. Besides I doubt it would load into your particular software.

Most of those cad programs have the basics but I've never used PCB layout so not sure how it works.
Not worth trying to mount everything on a PCB anyway unless you have exactly the same components that are in the library files. As to *Values* you usually edit them for the value you need and it's only for reference as the part is still the same package.

My biggest problem in the last few builds was getting hold of the Alpha 16mm pots with split shaft as one store here closed and the only other place, Jaycar is hopeless as management team have just never in ~30 years caught onto the simple (and kinda really obvious) point that * 16mm D shaft pots* are very limiting and if they had half a brain cell they would stock the much more versatile split splined shaft units.
To make matters even more confusing some of their D shaft pots are imperial shaft not metric. Arrrh tear your hair out if you're not up to speed on the details.
I guess one day I will just have to bite the bullet and source stuff from the net instead.
Rant over.

It can get a bit complicated when trying to home build stuff.
The idea of a sexy layout with all the components neatly sitting on a PCB may look impressive on a 2 dimension screen but make damned sure you have the actual components First And the case all drilled to match exactly. And I mean within about .5mm Exact otherwise you can break the epoxy base panel on the pots if you stress them sideways after they have been soldered.

Those pots have 3 pins which are pop riveted through the Epoxy panel, they are a press contact to the carbon film inside and not as tough as they may look. excess stress to the pins can render them intermittent even before you get to use the circuit.

If pots are mounted on a PCB then use an 8mm Drill for mounting holes which gives you .5mm to play with. (assuming a 7mm thread) (this also assumes you measured the spacing correctly)
centre pop all holes and pilot drill for better fit.

I do erk at the fact the pots are holding the whole pcb and it really is not the best situation some extra mounting bracing is really good idea but I can't source those fancy pots locally.

For one off jobs you are better off having sockets off board. Those 6.5mm sockets come in so many different versions that if you can't get the exact part the whole process comes to a grinding halt and you have to hack it up to make stuff fit which is why I don't go trying to make my circuits look factory perfect.

As to software pcb programs,, Take note most are aimed at a factory robot setup for etching,drilling and screen printing and assume everything must fit into an ipod sized gizmo,, yeah right. The default settings are very often narrow tracks, insanely small solder pads and for a OLDER home hobby chap like me who can hardly read normal parts let alone surface mounted crap,,,this is no friend.
Most allow for editing global parameters and you would be wise to make them larger.

Drills smaller than 1mm are not wise for hand drilling home builders and trying to etch a trace less than 1mm is just going to end up as land fill.
Phil.

[J M Fahey]

^^^^^^^^  Whay Phatt said  :dbtu:

Real Life experience vs. "wouldn't it be nice if ..... ?" which often is not

[estch71]

Wow.  That's a lot to take in.  I definitely see what you're talking about concerning pots.  I've used everything from Rat Shack pots with the long shaft that needs to be cut down to salvaged pots from old stereos and vintage electronics.  With the PCB aspect, I meant that the program didn't seem to allow for value editing, as in putting (for example) 1K on a resistor.  I think that some of the CAD programs take a bit more time and effort in order to understand how to use them.

Precision is absolutely key to placement.  In regards to soldering pots to the board, I prefer using short wires (solid 22 gauge) to connect.  I leave the board for the components.  Your build is absolutely amp style rather than the sometimes cramped stomp box.  I'm all about the sexy build.  It keeps wires from crossing over each other - power for example - while also giving it an amazingly professional look that most audiophiles are looking for.  It'll take me some time in order to understand how to read the schematics but, I'm definitely looking forward to the challenge.