DIY Valve Guitar Amp Head

[DrNomis_44]

Just read elsewhere that this amp build is to be based on a SLO-100... Is this true?... Do you have a circuit ?... Are you going to include clean/crunch to OD switching via a foot pedal ? ...

Yep, I've got the complete SLO 100 circuit, which includes the full two-channel preamp, power amp, power supply and footswitching circuitry, I got it as a pdf from the Sloclone.com website, and I'm going to be implementing the full footswitching system in my amp build too.


The only thing I'm going to be doing different is switching to EL34 power tubes, the circuit specifies 6L6 power tubes, so I'll probably get more of a British high-gain sound out of the amp.

And rather than use a set of four power tubes, I'm only going to be using two power tubes, there's a Triode/Pentode switch on the back panel of the amp which was something I added to the amp in a previous build, the Triode/Pentode switch basically re-configures the power tubes so that they can work either as Triodes, or Pentodes, the output-power drops to around half-power in Triode mode but that also changes the feel of the amp too, I thought it was cool enough to keep.


Interestingly enough, the SLO 100 footswitching circuitry included in the pdf is pretty simple, based on LDR/Led opto-isolators.


If you're interested in a copy of the pdf, here's where I downloaded it from:

http://diy-fever.com/wordpress/wp-co...CIAL-SCHEM.pdf

[Marcel]

Thanks Doc... the circuit is very useful as the circuits I have are difficult to read hand drawn versions and don't include the wiring in the actual foot switch.

Initially I was only interested in how the switching was done so I could incorporate something similar in my AC18W build to swap from a 12AX7 input tube to the EF86 tube, but over the course of the afternoon I've been increasingly tempted to gut my 2204 build and step it up to a build not unlike yours as a 50W SLO-100 type build or more probably a 50W hot rod. As my 2204 build originally in a former life had 6L6's and the output transformer is impedance matched for 6L6's I'll probably pull the EL34's it's currently fitted with mostly as due to Z issues there have been moments where my 2204 is not happy with the load it get presented with. I have a Marshall with EL34's so some new tones will become available if I do make the swap in this build from EL34's to 6L6's...

Keeping the T/P switch is a good idea, If I go ahead then I'll probably keep the PPIMV control that is already in my 2204 build.

[DrNomis_44]

Thanks Doc... the circuit is very useful as the circuits I have are difficult to read hand drawn versions and don't include the wiring in the actual foot switch.

Initially I was only interested in how the switching was done so I could incorporate something similar in my AC18W build to swap from a 12AX7 input tube to the EF86 tube, but over the course of the afternoon I've been increasingly tempted to gut my 2204 build and step it up to a build not unlike yours as a 50W SLO-100 type build or more probably a 50W hot rod. As my 2204 build originally in a former life had 6L6's and the output transformer is impedance matched for 6L6's I'll probably pull the EL34's it's currently fitted with mostly as due to Z issues there have been moments where my 2204 is not happy with the load it get presented with. I have a Marshall with EL34's so some new tones will become available if I do make the swap in this build from EL34's to 6L6's...

Keeping the T/P switch is a good idea, If I go ahead then I'll probably keep the PPIMV control that is already in my 2204 build.

Before I stripped all the original circuitry out of my amp, it was basically a Mesa/Boogie Mk I preamp circuit tacked onto a Marshall power amp circuit, what motivated me to strip all the circuitry out and re-build it as a Soldano SLO 50 was that, with the original circuitry, I was getting a reasonably good clean tone but the overdrive tone was very lackluster, no matter what I did with the tone controls, the tone of the amp didn't change much from a bassy tone that had a tendency to fart-out a bit when the gain was turned up too much, so, with the Soldano SLO circuitry I'm hoping that'll give me a good clean, crunch, and overdrive sounds, been checking out lots of SLO100 demos and I liked the sounds I was hearing.


It's interesting how in the Sloclone schematic, V2b is set up with a 39k cathode resistor, and a 100k anode resistor that's in parallel with a 1nF/630V DC cap, the 39k cathode resistor would set the biasing of that gain stage so it self-biases very close to cut-off if my valve theory is correct, I'm guessing that the 1nF cap in parallel with the 100k resistor helps to attenuate all the really high harmonics when V2b is overdriven, I'm guessing that V2b would be operating at very high gain due to the really low cathode to anode current so it would go into saturation and cut-off relatively easily.

[Marcel]

There is a lot going on around both sides of V2. I like how your circuit includes notes whereas mine is mostly the same yet has no notes and quite a few nasty errors. I'm pretty sure yours has an error around the power transformer as I can't see one winding with lots of taps doing the needed job correctly, most likely it should be 3 windings...

Much of the Soldano circuit carries many similarities to both Fender and Marshall designs, but with a few modern and very useful tweaks. I just love how a change in value here or there can make all that difference....

[DrNomis_44]

There is a lot going on around both sides of V2. I like how your circuit includes notes whereas mine is mostly the same yet has no notes and quite a few nasty errors. I'm pretty sure yours has an error around the power transformer as I can't see one winding with lots of taps doing the needed job correctly, most likely it should be 3 windings...

Much of the Soldano circuit carries many similarities to both Fender and Marshall designs, but with a few modern and very useful tweaks. I just love how a change in value here or there can make all that difference....

Yep, the circuit pdf I downloaded does include some very useful information, like for example the voltages on the cathodes and anodes of the preamp valves, definitely going to be useful in the event that you need to do any troubleshooting, the circuits are easy to follow and understand too, I think that the errors in the power supply may have been due to the limitations of the software used to create the circuit diagram more than anything else, I've also got a set of Turret Board layouts in pdf form for the Soldano circuits, but I may need to make some modifications to the layouts to enable them to work with the chassis i made for my amp, and in particular the power amp Turret Board layout, it was designed for 220uF/350V RB style Electrolytics, but I can only find some 220uF/350V RT style Electrolytics amongst Evatco's stock of caps, I've got two big Electrolytic caps installed on the chassis, they are dual 50uF Electrolytics rated at 500V DC, i want to include them in the power supply circuitry since removing them will leave a couple of big holes in the chassis.


The SLO 100 circuit looks like it borrows a few ideas from Mesa/Boogie and Peavey, I think Mesa/Boogie were one of the first companies to come up with the concept of gain-stage cascading, but Mesa/Boogie basically took a Fender circuit and added gain-stages to it and ended up with the Mk I circuit, it was around the early 80's when gutarists were taking their Marshall amps to amp repair techs to get them modded for extra gain in order to get overdrive sounds suitable for playing music in the then new genre of Thrash Metal, other amp manufacturers, like Peavey, followed suit, if you compare the preamp circuit of a Peavey 5150 with that of an SLO 100 and Mesa/Boogie Dual Rectifier, you'll notice some similarities, I reckon they borrow from each other.

[Marcel]

Can't but love those drawings done on a CAD PC, so nice and clear and easy to read... it is just so sad that they often contain as many mistakes, errors or omissions as drawings done in pencil on a paper beer coaster ... suffering the use of either or both is part of the trade I guess ...

You're right Doc, they do all borrow from each other and we are just as guilty of it. One thing you don't see much of is inventive new ways to implement tubes... everything is common cathode bar for the two other and often identically copied by anyone and everyone versions of common anode (cathode follower) and common grid (2nd 1/2 of a PI) circuits. There is a lot of fiddling with bias but it is rare to see to see a "WTF!!" implementation doing something Wow! ... Maybe things were tried eons ago but have fallen into infamy due to lack of that 'Wow' thing becoming popular or most likely because 'transistors can do that' took its place...
But I lament, and I must shoot that melancholy in the foot.!!

In my hunt for more SLO-100 info I've stumbled on some old Peavy and Dumbel circuits... I'm still keen on a 6L6 finals amp, but now I'm hugely at odds as to which front end I should feed it with. Something my JVM210 doesn't do overly well is cleans, the AC18W is better yet it too has its own failings and for reasons I won't give here I want to avoid the typical Fender circuits so the search is on for a pedal friendly clean option that I can dirty or crunch up to taste.

Lol... Thrash Metal...lol .... My first, in I think it was '82 I had one guy in Melbourne ask me to 'hot modify' his 240VAC US version Fender Super Twin...The dude wanted "buckets of gain" and the amp also had to feed two Marshall Green back loaded quad boxes (essentially a full stack)... I was in the RAAF at the time so I did the mod at home, out with all the AT7's and in with a full fresh set of AX7's plus a few tweaks, and Damn that rig made some noise, so much that while I was testing it my landlord (and mentor in tube electronics) got complaints from neighbours 2 streets away but that could also have just been because of my mediocre playing into something so wicked in the OD department. Actually, thinking back, even when using my cheap Japanese plywood Ason strat of the time this Super Twin ended up at gain/vol setting of less than 4's for cleans and at 9's sounding very similar to a wound up OD2 in a modern JVM410.

[Marcel]

A small step back for the topic... Dummy loads.

A shot of my 8ohm 100W + 8ohm 100W load, also has two 600ohm terminators for other audio stuff I play around with. If I feed it with 1kHz @ 100W you can just hear the coils singing as they use our planet Earth's magnetic field to push against. If I dunk the load in a bucket of clean (distilled) water the load will comfortably handle two 500W RMS channels (as in a big PA system or a big bass amp) although for a test of a few seconds at 500W RMS (to check for clipping or max power out) I usually don't bother with the drama of messing with water.

[DrNomis_44]

I might see if my local Bunnings store has any of those electric kettle heater elements in stock, and if they do have them at a reasonable price I'll buy some so I can make a decent dummy load, in the meantime I'll have to make do with a 50 watt 8 Ohm dummy load that I scored from my local Uni some years ago.

Dummy loads are a very useful bit of test equipment when it comes to building amplifiers, in that you can run the amp at full power for testing purposes without blowing out your eardrums, or annoying your neighbors.

Might be a good basis for a future DIY project.


Update:


Just had a quick browse through Evatco's range of power transformers that they've got in stock and I think I have found the perfect one to replace the existing power transformer installed on the chassis of my Valve amp head, the Sloclone Schematic I'm using for my amp re-build specifies a power transformer with the following specs for a 50 Watt Sloclone amp:


Primary:

240V AC / 50Hz.

Secondary :

360-0-50-360 @ 200mA DC (HT and Bias Supply).

6.3V CT AC @ 6A (Power Tube Heater supply).

6.3V CT AC @ 2A (Preamp Tube Heater Supply).

And here are the following specs for the Evatco Power Transformer I have selected:

Primary:

240V AC / 50 Hz.

Secondary:

375-0-50-375 @ 201mA DC (HT and Bias Supply).

5V CT AC @ 2A (Rectifier Heater Supply).

6.3V CT AC @ 6A (Power Tube Heater Supply).

6.3V CT AC @ 6A (Preamp Tube Heater Supply).


I think this transformer is going to serve the purpose perfectly, it is listed as Evatco's 374bx Power Transformer, and costs $200.00 all up, which is fine for me, I've decided that I can re-use the existing output transformer, so I don't need to order a new one.

Here's a link to the webpage:

https://www.evatco.com.au/http://eva...o.com.au/374ax


Since the amp uses four UF5408 Silicon Power Rectifier Diodes instead of a Rectifier Tube, I can use the spare 5V CT AC @ 2A Secondary to power the channel switching circuitry, I reckon it should work fine.


Evatco also seem to have a pretty good range of Metal Film resistors with power ratings from 1 Watt to 3 Watts, now since my Sloclone 50 amp is a high-gain amp, I'm going to elect to use Metal Film Resistors throughout the amp's circuitry, reason being is that Metal Film resistors don't generate anywhere near as much hiss-noise as traditional Carbon Composition resistors do, which is essential for low noise in a high-gain amp, I'm also going to try and over-rate the resistor for the job it does in the Sloclone amp's circuitry, by over-rate I mean ensure that the power dissipation capability is more than adequate, for example I would choose to use a 1 watt resistor when a 1/2 watt resistor is normally called for, this increases the reliability of the resistor and ensures that the amp is not going to break-down when I least want it to, incidentally, this is also why I elected to build the majority of the amp's circuitry on Turret Boards rather than some Etched PCBs, in the unlikely event that a resistor or other component fails, I can simply de-solder the faulty component and replace it with a new one without having to completely remove the board like you have to with an etched PCB, that's the main advantage with Turret Boards, ease of serviceability although it does mean more time is needed to build the amp and there's more likelihood of errors cropping up, but since this is a one-off build rather than multiple build, time taken to build the amp is a moot-point.


Each one of the 5 Preamp Tubes is a 12AX7/ECC83 type, the heaters of these can be powered two ways, either by 6.3V AC @ 300mA (Heater filaments in parallel), or 12.6V AC at 150mA (Heater filaments in series), I'm going to be powering them by 6.3V AC so the Heater filaments will be in parallel with each other, now since there are five 12AX7/ECC83 Tubes all up, the 6.3V AC Heater supply needs to supply 5 X 300mA, or a total of 1.5A if my maths is correct, this is about a quarter of the maximum supply current of one of the 6.3V AC @ 6A secondaries of the new power transformer so there's plenty in reserve.

Each of the two EL34 Power Tubes I'm going to be using requires a Heater supply of 6.3V AC @ 1.5A, which means a total supply current of 3A, this is half the maximum supply current that the other 6.3V AC @ 6A secondary is capable of, again plenty in reserve.


The only issue I can foresee is that the HT Voltage may end up a bit higher than noted in the Power Supply Schematic.


Note that the maximum supply current of a power transformer secondary winding is the maximum current that it can supply before the supply voltage starts to drop due to Ohm's Law, the power transformer will start to get warm but won't burn out unless a dead-short develops across the secondary winding for some reason, hopefully that won't happen provided it gets wired-up correctly.

[DrNomis_44]

Update:

Went to Jaycar electronics today and bought a few new bits for the Sloclone 50 amp head, a couple of beefier toggle-switches to replace the current thin ones mounted next to the inputs on the front panel, a Green 3mm Led and a Blue 3mm Led for the channel indicators, two 1/4 inch sockets for the inputs, and nine new pointer-style knobs.


Here's a couple new pics of my Sloclone 50 amp, it has some new pointer knobs and toggle switches installed on the front panel:

[DrNomis_44]

Update:

If anyone is interested, here's a link to the Sloclone Forum website, you may need to register as a member before checking out the various forum threads though, registration is free:

www.slocloneforums.com