Scratch build head(s)...

[Marcel]

Have been pondering the idea for the past few months on doing a scratch build amp...

On the current wish list is 1/ all tube audio path design with 6L6 finals, 2/ 40 to 50W output, 3/ an EF86 clean input channel, 4/ a Marshall'esk crunch or Soldano'ish OD channel, 5/ possible tremolo, 6/ bass, mid, treble and presence controls, 7/ master volume control, 8/ effects loop, 9/ maybe a built in spring reverb, 10/ 4 and 8 ohm outputs, 11/ footswitch control of channel, and if fitted tremolo/reverb,

The physical building isn't the problem. What is the problem is the style of tone that would be most universally useful, by which I mean 'is a Soldano style OD through a Fender 6L6 type final worth listening to'? My gut says the EF86/6L6 combination will produce very interesting cleans, as will the near universally used 12AX7/6L6 combination will produce very useful Fender type crunch tones ... And other things like what frequencies should the tone controls be set around.. Fender or Marshal frequencies, or something else?

'Universally useful' is ambitious, probably too ambitious... but guidance through wonderful examples would be immensely helpful. Even loves or hates about existing amps can have a important role in how this one turns out ....

As a start I'm looking at a Fender Hotrod Deluxe style amp as the starting point and adding the EF86, and then tinkering the design from there... If it all works out then I might make a few of them and offer them for sale...

Comments anyone???

[DrNomis_44]

I seem to remember reading somewhere that the Soldano SLO 100 came from the factory with 5881 power tubes installed, or I could be wrong, being an American made amp, it most likely also came fitted with 6L6 power tubes, and from studying the SLO clone schematic, the Soldano SLO 100 is basically a Hotrod-modded Marshall circuit, for the typical Fender/Marshall tonestack you could incorporate a toggle switch, or a rotary switch to switch in different capacitor and slope/resistor values to get either the Fender tonestack or the Marshall tonestack with the Marshall pot values, it should be possible to do so.

Fender Tonestack:

Treble Cap: 250pF

Bass Cap: 100nF (.1uF)

Mid Cap: 47nF (.047uF)

Slope Resistor: 100k

Treble Pot: 250k Log

Bass Pot: 250k Log

Mid Pot: 10k Log


Marshall Tonestack:

Treble Cap: 470pF

Bass Cap: 22nF (.022uF)

Mid Cap: 22nF (.022uF)

Slope Resistor: 33k

Treble Pot: 250k Lin

Bass Pot: 1M Log

Mid Pot: 25k Lin (or could be a Log).


Note that in a Marshall amp the tonestack is driven by a cathode follower, whereas in a Fender amp it is driven by a common cathode stage.

[dave.king1]

Without considering the technicalities what about considering one of Ken Fishers fabulous Trainwreck designs as a reference as well.

[Dedman]

Take a look at some of Egnators designs too.

[tiltbell]

You should look in to the Soldano Lucky 13 as a reference. This is a dual channel with Fender tonestack on the clean channel and a hot rodded Marshall type of OD channel. It runs 6L6's in the power section. There is a guy on eBay that sells PCB's for these out of the Ukraine if you are willing to wait about three months for it to arrive. I have one that I am about 80% completed, I have a blank chassis that needs to be drilled out sometime over the next few weekends....

[Marcel]

Ooooh.. Ahhh.... Soldano Lucky 13... Can you post a copy of the circuit here? A link to the circuit on the net?

[DrNomis_44]

I'm doing a search for the Soldano Lucky 13 schematics right now, so far haven't found anything yet.

[Marcel]

Yeah Doc, I can find references and photo's of the Soldano Lucky 13, but nil circuit which for me provided it's drawn correctly gives the critical info to do anything as a tech.

Repair or bias or copy, the circuit is the key. Fumbling around without a circuit is simply asking for trouble.... Maybe Soldano want this amp to only be returned to the factory for maintenance. Sadly that kind of attitude doesn't help sales, particularly outside the USA ...

Interestingly, Roy, the guy I'm working with at the moment has earned himself a bit of a reputation fixing Stanton equipment.... Even people in NZ are sending him gear. That's alongside all the LG electronics stuff that comes to him from across the country. So it's not unheard of to have gear shifted to get to the fewer and fewer repair guys.
Our throw away society has developed a downside which is in the future on faulty gear if you paid more than $1k for it then it might get fixed under warranty, less than $1k and it will simply get replaced, and if the warranty is over then good luck finding anyone that has the skill or can get the parts to fix your item...
An example - Panasonic HDMI recorder with netflix etc for $850 at uncle Hervey's.... Warranty 12 months, unit is 15 months old and HDD died. Nil spare HDD's in Australia, Only 15 spares held in country of manufacture being Taiwan for $450 ea. Model is superseded, HDD is specific to the recorder and nil more to be made... Too bad if you're the 16th person in the WORLD with one of these recorders to have a HDD fail.!!.. and we all know how often that happens...

[DrNomis_44]

I would have to agree with you there Marcel, trying to fault-find and fix a piece of equipment without a clearly read-able circuit diagram is like trying to solve a mystery blindfolded....been there and done it lots of times in the past, I think a lot of these manufacturers are worried that other competing companies will steal their intellectual property, and profits by copying their product designs, that's probably why they're being coy with circuit diagrams, but as you say, it leaves us poor service techs in a bit of a lurch when we have to repair things.


The rationale seems to be going along the lines of deliberate "Planned Obsolescence" in order to keep making money, meanwhile electronic equipment that's deemed un-economical to fix ends up as e-waste, something that I reckon is not good for the environment we live in, there are some quite dangerous chemicals used in the electronics industry, as I'm sure you know, which is where the ROHS standards come into play.

[Marcel]

The back door of any techs workshop has little regard for any well meaning ROHS standards...

Dead $8k plasmas stacked against equally dead $100 Kogan LCD's... it's all a little sad when we fill the tandem trailer with the un-repairable gear for the once a month trip to the dump. All too often there are 3yo TV's making the journey due to the failure of a single component that is not held in stock and no longer in production... and nobody wants to pay for or is setup to do the recycling that the community at large thinks is happening. It's no longer like the old days where a toolbox full of parts (most often tubes) could fix every electronic item in the house... The things in electronics that break are so vastly different from job to job. Over the last 6 months I can count on one hand the number of times where the same exact part was used to fix more the two different jobs. I mean that we can get 5 Samsung TV's in for repair, and all can have the exact same fault, yet due to changes during manufacture maybe only 2 will use the exact same item to be fixed.... Did you know there are at least 4 different versions of the Playstation IV and not including production/manufacturing changes all have totally incompatible electronics inside...

Tube amps make a tech's life so much simpler...