tube pre schematic explanation to a nublet

I've been looking at a schematic for the pre section of a marshall amplifier for a long time now, trying to muster the courage to remove it from the overall schematic (which has KIND of already been done) and make a standalone pre from it.  Im sure to the seasoned pro the schematic is a no brainer, but im far from a pro and the only "tube" project I've done so far is a royer 2001 mod, with point to point power supply.  (tubes and their voltages scare the crap outta me!)

Id be happy to compensate someone for their time to help me through this so I dont kill myself.  (if someones willing to just be kind and answer my questions pro-bono even better!) Biggest questions are regarding power supply and necessary plate voltage(s).  Theres also a mysterious 3rd "pre" ECC83 tube that either IS or ISNT in the preamp depending on who you ask. (apparently its a phase-splitter which feeds the output stage, which i wouldn't be building). 

Im looking to build this more for a "re-amping" purpose than for jamming (i play bass anyway ) and plan to integrate the jensen re-amp circuit/trafo into it as well for studio use only.

http://www.drtube.com/schematics/marshall/1959prem.gif - pre schematic
http://www.drtube.com/schematics/marshall/1959pwrm.gif - power schematic

Interesting project.

3rd tube is the 'phase inverter' or 'phase splitter' - it creates 2 output waveforms from the input, 180 degrees out of phase.

These out of phase signals are also amplified to a sufficient voltage and current to drive the 'push-pull' 'finals' (power tubes) to full power.

You don't strictly need this for a preamp, although a tube in this position may be used to provide a 'line amp' for you preamp's output so as to drive a studio style low impedance output. You can get away without it, but it's nice to have a transformer balanced studio output.

If you do use a transformer driven output, you can retain the 'phase splitter' tube as line amp driving a push-pull transformer, or you could replace it with a 'single ended' tube line amp driving a single ended or parallel fed transformer.


For your 'reamping' application, you could also lose the first tube and pair of inputs. They are mixed with a second tube and pair of inputs and fed to the rest of the preamp so as to provide a pair of tube mixable hi/lo inputs.

I would say, lose tube 1, keep tubes 2,3 and lose the finals. Replace output transformer with a suitable push-pull  10K:600 Carnhill and your done.

The power supply could use a tube rectifier or a solid state. Given you are already going the tube route, keep it closer to the original and use a tube rectifier + choke. About $50 extra over the diode route.

The transformer can a smallish 250Vac centre tapped at 50mA or so (3mA tube 1, 5mA tube 2, 27mA tube 3) to provide 325Vdc or so into the R-C 'pi filter' for unregulated HV rails.

Heater could be 6.3Vac unregulated at 2A or so for the 3 tubes. Additional 5V heater for a tube rectifier at 2A or so depending on which rectifier tube you use. You could use a 6.3Vac heater type as this unit has smallish HV current requirements.

Throw in a JJ dual-cap can and choke and there it is.

Oh, and point-point wiring is the go - just invest in some eyelets, an eyelet punch and some blank board. You can make this quite small as it is pretty simple stuff. And keep the layout close to the original for easy implementation - just find the 'wiring diagram' for the Marshall and retain the basic layout.

The enclosure style has a few options - you could do as I like and mount the tubes on top of a rack enclosure or you could have them out the back or make some L-brackets and mount inside.

A relatively straight forward one like this looks very nice on a moderate size hammond grey steel/alu box with tubes/traffos on top.

Good luck with it 

wow! ;D ;D  Thanks so much for the reply.  Found a place that sells replacement transformers for old marshall amps so should be pretty easy to do, they have the choke as well.  The carnhill... gapped or ungapped?

For a push-pull style line amp, you need ungapped and make sure you have a centre tap available to apply B+ voltage.

The Carnhill 9600:600 has 2 seperate coils on the primary, so connect them in series and use the join for the centre tap. You could also use an Edcor xsm but I much prefer Carnhill. Also, something like a UTC A15 will work if you are after top-mounted aesthetics.

For the power traffo, try to find the smallest one for the job - excessively large will mean excessive potential for EM field induced noise. I would say something for a fender champ size or so would be the kind to get. The choke also need only be small - 50mA 5H would be plenty.

Cheers