Back to the tube world !
Here’s a tube amp that I made almost one year ago. I owned a blackheart 5W litle something from Crate amplification. Although this amp was good and not as crappy as many other tube amp in this price range I wasn’t really happy with the sound . Something harsh and irratating in the high ends. I tried to modifiy it a lot but the PCB wasn’t helping at all so I decided to use the chassis to make a new amp.
I wanted something different (I thought so ) and absolutely not make a clone of something ! it’s boring to see all these marshall clones and I’m always suprised to see that guitarists want all the same gear ! do they have any personnalities ?
Therefore the idea was to use a pentode followed by a 12XA7. But in making researches I realised that wasn’t new , few amps also uses to kind of signal path like the 65 amp soho for instance. Nevertheless I kept the idea .
So the amp was composed of a 6J32P (russian equivalent of the EF86) followed by a 12ax7 gain stage , a Cathode follower , a marshall tonestack. The output tube is a 6P6S (a russian 6V6 ) .Regarding of the bigness and the datas of the ouput transformer I can actually put every octal output tubes I want, even a KT88. The ouput transformer is rated at 15W/160mA. The only change is to modify the bias and the current . The transformers were made by ESO , a small french manufacter located in South France. as you can see they are huge and heavy. The amp suffers of this weight as well ! need big arms to carry the amp around.
Why Russian tube you may ask , because they’re cheap and sold in bulk of 4,6 or 10. I hate buying pricey components ! (or call me cheap…)
The proper bias of the ouput tube is made of a transistor in the cathode path similiar to a current source . The current flowing into the cathode ( and the emmeter/collecter path ) is set by the base current of the transistor (with the help of a little trimmer). A 1R resistor is here to control and visualise the current.
I wasn’t satisfied with the marshall tonestack, it didn’t have real efficiency ! I spent more time tweaking the pots than enjoying this amp so it got out. The specific type of sound associated with fender or marshall amp have a lot to do with the tonestack . The tonestack really gives its footprint to the sound. If you take a look on a graphic representation you’ll see bumps and notches in the spectrum .
So I wanted something with less effect on sound . I tried the baxendall or James tone control but it ate too much gain to be really interesting even if it worked really good. Tired of wirring up I just put a 500k pot/10n cap as a tone control and ….Works good ! the amps still has this arrangement. Simpler the better as always !
here’s the original first schematic to give you an idea:
the fat switch was here to give a kin of dirt mode . I didn’t keep it in the final version.
and the power supply line :
this amp is more a raw and a kind of agressive amp. It distorts quick with humbucker and have a “fuzzy” sound with the drive at max (a put a master volume because the preamp generates lot of drive and it’s more useable at home with a master volume). I tried to create a “clean” mode in testing different kind of things (triode/pentode mode , no bypass cap on the screen grid ect…) and even connecting the pentode directly into the 6P6s wasn’t working good .Certainly because of its high output impedance (again ! ) . But this non-working circuit gave me idea that I’ll expose later.
The amp could make a cool two channels amp with clean and dirt modes. I wrote down this idea and see if what I can make with it in the futur.
Sorry for the crappy pics and the crappy look of the amp as well. I must say that the amp will be better looking with a faceplate and retolexed case but I get lazy with those sort of things.
thanks for reading