Marshall 20 Watt on the bench...pics

[TubeGeek]

I have a 20 Watt Marshall head on the bench for servicing.  Nothing is wrong with the amp, the owner just wants me to go through it and do what I think needs to be done.  He asked for new tubes so I will at least do that.

I plugged it in to a few cabinets tonight and played it...it sounded really big and nice through a 4x12 of vintage 30's.  I also tried a detuned 1x12 cabinet loaded with a g12h30 and it sounded nice, a little brighter than the vintage 30's but very close. Lastly I tried a 1x12 avatar cabinet loaded with a weber greenback and it sounded very nice and solid too.

What I really like about this amp is it's not freakin loud.  It's perfect for home/studio.  I can crank it up and get a nice distortion without making my ears bleed.  It has the perfect classic rock tone, just like most Marshall's.  I do like 50 Watt Marshall's because they push a lot of air when used with 4x12 or 8x12's but I don't do that very often, only when I'm pissed off and want to vent.  

My initial thoughts when playing this amp were that I couldn't get the amount of highs that I like, it seems to be just a little shy of what I like.  I plugged into my 18 watt amp and played that, it definitely has more highs and mucho more gain on tap.  I almost don't like the amount of gain it has, when I crank up the gain on it there is a swirly phasey thing going on.  I've been meaning to address this but haven't made time to do so...yet.

Looking at the guts...there are a couple caps connected to the ac cord that I haven't seen before in an amp.  They look a little toasted where the leads connect to the cap.  Other than that, most everything looks common.

Is this an amp I shouldn't mess with at all, or should I go ahead and service it?  I'm considering replacing the filter & electro caps and removing those ac cord caps connecting to chassis ground.  Any thoughts appreciated.

[LooseChange]

Change the power tubes and then listen. Bet it's louder, bright and big after that. Want more bright and clarity? Replace the preamp tube with a Chinese 12AX7b.

[Leevi]

I would check the pots and change them if necessary.
Also a renewal of the critical coupling caps to better ones improve the sound.
I saw in the picture that mains ground has been soldered to the chassis.
At least in many European countries this should be done with screw.
/Leevi

[VMS]

If it ain't broken, don't fix it.

It seems that somebody has already biased it a bit colder by adding a resistor in series with the original Bias resistor. That would be one thing to check.

It looks like both channels are identical. Some schematics show a bright cap across the mixing resistor for the other channel, so if you want brighter sound you could try that.

How close is that amp to this schematic?

http://marstran.com/2022.gif

[Tone Junkie]

Thank You for the amp porn my friend I havnt seen one of those before. Bill
                                               

[jjasilli]

 

[jojokeo]

If it's like the schematic VMS posted you could still lower or remove the input resistors on one channel and then use shielded cable if you do. Another is remove that cap. Saw something else, lower the 680K resistors to 470k or even 220k and you'll hear a difference. There's three things to mix and match which will make high end AND gain differences.

[TubeGeek]

It will be a few days before I get to this amp...I'll report back when I work on it.

I just had to share the pics of this thing, I've never seen one either.

[navdave]

Wow thats a trippy phase inverter. Looks like a dc coupled PI but its ac coupled.
How does that work?

[The_Gaz]

Does that thing have screen resistors? Maybe that's why there was the mod for colder bias, because the thing was eating power tubes. I'd at at least 100R  for each tube like an AC30, or better yet 1K. Personally, in an amp like that, I'd mod for reliability only, or return to stock.

[tubeswell]

Wow thats a trippy phase inverter. Looks like a dc coupled PI but its ac coupled.
How does that work?

http://www.freewebs.com/valvewizard/dcltp.html

[navdave]

Well I've read Merlin's page before but this phase inverter is different than a normal DC coupled LTP.
This looks like a normal DC coupled LTP but it's not it's AC coupled..... I did not know you could do this so I'm stumped.
 

Wow thats a trippy phase inverter. Looks like a dc coupled PI but its ac coupled.
How does that work?

http://www.freewebs.com/valvewizard/dcltp.html

[Ritchie200]

Hey TG,
That (those?) cap on the line-in may be Marshall's death cap.  Sometimes noted in their schematics, sometimes not.  Is it .05 or .06?  It seems they used whatever was laying around.  I'd remove it (them?) and give it to your client in a baggy to save for "vintage" value.  

I've seen a few of the cascading gain mods/revoicing for this amp, but heck, build him a 18 watt if he wants that.  I would not mess with this one as they are unique in their own way and are rare enough to bring a few bucks if original.  Re-tube, check bias and remove bias mod (if there is one).  These things were reliable.  If they had to cold bias, they were compensating for another problem.

Let us know what you find!
Jim

[VMS]

   to navdaves question.

How does that work?

And what about this PI that is on the reissue model and also (looking at the gut shots) in some old 20Watts?

Is one better than the other?

[EL34]

I love the 20 watt heads

Those heads are where I came up with the Stout mod many years ago.

A guy brought me two of them and wanted more umpf..........

He loved the heads after the mods and I wanted to buy the heads from him, but he would not part with them.

[TubeGeek]

The amp is finished and turned out nicely.  I ended up replacing the v1 cathode bypass cap, bias resistor and capacitor, ac power cord and diodes as well as installing new tubes.

The brightness was restored. It sounds very nice indeed.  I think I just may have to build myself one of these someday.

[Ritchie200]

Cool!  A little bit of Marshall history there.  BTW that is a neat adjustable chassis cradle you've built!

Jim

[Ritchie200]

Hmmmm.  Just noticed something.  The first position on the transformer tag board is usually the 110v tap.  Did the voltages look ok throughout?  Did you replace a bias resistor that drifted or did you have to replace it with a different rating to get it in range?

Jim

[TubeGeek]

Yes I measured voltages and they seem normal.  I replaced the old bias resistor(s)...there was a 130 ohm in series with a 47 ohm...must have been a mod somewhere along the way.  I removed those and installed the 150 ohm 10 watter.


Plate voltages are 365 VDC which is close to the schematic where it states 370 VDC. 

[EL34]

Those heads really rock when you cascade V1A into V1B.

[TubeGeek]

Those heads really rock when you cascade V1A into V1B.

Even without modding this amp i found it had big balls.  It has just enough distortion when at 100% vol, i guess it is not really comparable to other high gain amps but for a classic rock tone, this can't be beat.  For me it is one of those amps that makes you play new things and inspires you to keep playing.  I really liked it through a 1x12 avatar cab loaded with a weber 1225.  It was ok through a 4x12 of vintage 30's.

If cascaded...it would sound more like your 18 watt stout with the hot switch engaged right?

[EL34]

If cascaded...it would sound more like your 18 watt stout with the hot switch engaged right?

Proabably, but it's a different amp with different trannies, etc

Throwing the switch is not just one sound
With switch on, you can vary preamp and master cobtrols to get everything from clean to mean.

The tone is just amped up because you are pushing more voltage through the tone stacks, etc