More amp surgery...iron transplant in a Dumble Clone

[212Mavguy]

Marsh amplification builds some really nice sounding amps.  I ordered one of the first 50 watt Overlord models as a build to order on Fleabay, got it at a discount because it was a new model.  It used some really nice Ceriatone boards inside, but was built in the USA instead of Malaysia.  The amp came with a Mercury Magnetics power tranny, I don't know who made the choke, and remember it advertised as having a Heyboer output tranny.  It is a copy of an 80's Overdrive Supreme by dumble, #124.  I have enjoyed playing through it, I had a couple additional features installed as part of the build, one is a lift negative feedback switch using the same switch formerly used and called the ground switch.  It still has that original label, a secret tone weapon.  The other was a half power switch that sits on the back of the chassis, it works by operating the power tubes in triode mode instead of pentode.  Using half power results in a slightly quieter, less punchy, darker tone set.  

with only five tubes in the whole amp one would think that the inside would be relatively simple, in fact it has way more parts inside that chassis than my '68 Fender Super Reverb, and it has nine tubes.  I really enjoyed the tones, the OD has a full, fat sound with a lot of lower mids providing the girth.  However in the bass frequencies that tranny was lacking.

I first listened to a Classic Tone output transformer that came with a custom build of a 50 watt Marshall Jubilee.  I noted that the tones had a robust bass response.  However, the clipping diode type of distortion found in that circuit was not my cup of tea.   But I could tell that I liked the way that iron sounded.  So I remembered the part number...

Last week I saw an auction of that model, 40-18025 for 25 bucks plus shipping for what was advertised as new.  When it arrived I noticed that one of the leads had been extended by a foot, and all the other leads were full length, however, each lead had a female spade fitting crimped onto the end.  Other than that it looked like it had never been used.  This is a 50 watt replacement/upgrade for the Drake transformer found in 50 watt  Marshall amps.  I thought that it would be a great science project to transplant it into the Overlord.

Only seven wires to cut and solder, right? Should be easy one would think...Haaah!  Not in a this case.  I didn't build the amp and I did not have a schematic to help.  So The only option was to leave the circuit intact.  Only problem was that two of the four bolts mounting it to the chassis were underneath the eyelet boards.  So both of those boards were removed from the chassis standoffs in order to have room to work underneath, it ended up being nearly impossible to do, and definitely impossible to remove wires in order to make more clearance to work under the Garolite eyelet boards because of no schematic and because I did not build the amp.  The sheer number of parts made getting to the the tranny primary and secondary wires a tight places proposition, so I used my 25 watt pencil instead of the beefier 40 wat one that I favor.  The wimpy iron had a longer tip that I previously bent down to improve reach into tight spots.  Using that iron was good, but each solder joint was a much slower  process to heat up the work properly..  

When I finished, I was tired, but the anticipation of first fire up caused some nervousness.  Getting the primary wires reversed would have resulted in a really loud high pitch whine that would make the amp unplayable.  However, I wrote notes as to the color of each primary and secondary wire and what colors went where on the power tubes.  So after diligently following the example from the Heyboer, I was ready to fire it up.  At first blush, I noticed a bit of low volume hum that the amp did not have before.  I was perplexed because I left the lead dress as similar to what it was originally.  Found the source of the problem, the half power switch was on, triode operation is a little noisier than pentode.  Then I plugged in a guitar...

I was floored! The amp was a good third louder at similar master settings and the preamp gains in the clean and dirty channels were also similarly affected.  Gain was as much at a setting of 2 as before at 3 1/2 in the clean channel.  The bottom end now was tight and huge, and the dirty channel definitely had that plexi rip yer face off  vibe going.  The new OT had about the same lamination stack thickness, but the end bells were a good half an inch wider across than the Heyboer, I think that there was more copper inside that Classic Tone.  Add in the factor of the new tranny being designed for el34 rather than 6l6, the primary impedance in the Heyboer was likely at or over 4,000 ohms, while the new one had around 3700.  That also had a lot of influence over the louder volume I was getting.  So now this beast goes from a fendery clean to a Marshall scream.  It is still possible to set that clean channel for the smooth jazz vibe as well.  

The time was worth the major PITA factor, and I felt fortunate that everything worked as it should at first fireup with no freakups!  Yaaaaaay!  And the resale value won't be hugely affected, those amps hold their value well.

 

[myoldfriend]

Way to go!  Absolutely loved the Marsh Overlord Supreme that I owned...  Couldn't get a sour note out of that head despite my best efforts.

[TalismanRich]

Thanks for the story.   One thing that I keep looking at are the Marsh Overlords and Ceriatone OTS.   Having never played a D type amp,   I'm really curious about them and almost pulled the wallet out a couple of times.   

 

[Gitfiddler]

HOC's own 'mad scientist' is at it again...with wonderful results!  Well done, sir.

[212Mavguy]

Would you believe the damn thing took six hours to get done?  Aaaaauuuugh!  The two tough bolts...an hour to get them out.  2 1/2 hours to get back in.  They were a tight fit through the chassis slots cut out for them.  Lots of bad words said very loudly every now and then.  At least it sounds uniquely awesome.  But sheeeesh...glad I don't work on 'em to pay the bills.  Am my own laughing stock right now.  Very humbling.  At least it worked the way it should right off the bat.  Getting in and out of the amp as a project  reminds me of the limerick about "The girl from Peru"...

There once was a girl from Peru,

Who filled up her insides with glue...

She said with a grin, 

They paid to get in,

Now they'll pay to get out too...

 

 

[Spectrum13]

Was this a taste thing, a miss match of the transformer or a cost saving decision by the builder?  What do you think?

[212Mavguy]

Great questions!

Am not second guessing the builder who knows waaaay more than I do, rather will explain why I did it in the first place.    Marsh amps tend to be fantastic sounding!  I did have  previous experience from other mad scientist experiments, that is what prompted the amp surgery in the first place.

The foundation amp for Dumbles is the Fender Bassman, as is the earliest Marshalls.    That Bassman amp in 50 or 100 watt versions came with a large output transformer in order to put out a strong bass response.   Smaller transformers fail to provide that level of breadth in their frequency response.  I could be wrong, but it appears that my Marsh Overlord did not come with a clone of a Bassman output transformer.  The Ceriatone builds of the same circuit type feature a locally sourced clone of the Bassman OT.  Nik is pretty fastidious about his parts selection.

I learned a lot from writing back and forth the Chris Siegmund, his amps are known for being special in a very amazing way.  I have a Midnight Blues Breaker head of his, it has a slightly oversized Radiospares type output transformer from Mercury Magnetics, as well as a real pair of British KT66's socketed.   He has a term called the "power trio."  It consists of the power tube(s), the output transformer, and the speaker(s) that dominate the amp's resulting tone.   According to Siegmund, the most important is the output transformer, it is the "Heart" of the amp's tone.  This led me to test the theory using a 70's silver face Fender Champ as the guinea pig.  I ordered a Poshan brand singe ended output transformer from Taiwan that was double the mass of the stock Champ iron.  Was easy to do.  The difference in overall tone of the amp in a "good" way validated his words.  Then I noticed a 30 watt unit on eBay that weighed somewhere near 6-7 pounds, I got that and mounted it in the bottom of the cab because it was so oversized.  Again, a huge difference in volume, frequency bandwidth and harmonic content for the better...waaaay better!  The observations from the two swaps in same amp (changing nothing else) were that swapping out a less massive output transformer for a beefier one led to a louder, beefier, more detailed tone content.

My Marsh came with a Mercury Magnetics power transformer, those are really expensive.  Both Heyboer and Classic Tone iron give a lot of bang for the buck.  Classic Tone's parent company, Magnetic Components, built transformers that are found as original equipment in lots of vintage tube amplifiers, including guitar amps.  It may be that in order to offset the cost of the Mercury iron that the choice for what was used happened.  I don't know for sure.  Heyboer iron has a great reputation as well.  I am not familiar with pricing difference between those two makers. I do know that Classic Tone builds great sounding transformers that sound like the original equipment in Fender amps, I did a swap in a '68 Super Reverb using model 40-18000 to get 2, 4, and 8 ohm secondaries for using a single 15 inch speaker a la Vibroverb.  Have never heard another Super that could compete with mine in the tone department either.  But that's another happy story...and that OT choice was only a small part of what was done to that secondhand, DOA amp with two blown speakers that I paid chump change for.

So the past results of earlier output tranny swapping did lead me to the current experiment.  I expected some difference, but not to the extent that I experienced.  I previously touted the Marsh Overlord as a great amp to cover a huge range of tones.  That capability became more enhanced as a result of installing the Classic Tone unit.  It covers the Dumble roots, and now gets more aggressive if desired. 

Hope this helps.

 

 

[bolero]

very interesting stuff!!

 

[212Mavguy]

Part 2...

While I was inside the Overlord I noticed a small, mouse turd sized bit of white stuff that had oozed out of the ground side of one of the filter caps.  They help the amp change alternating current to direct current.   There are several, some in diverse locations. This one was on the main power supply eyelet board.  There was another, identical one next to it that did not appear to have anything wrong with it.  

When the amp was new, within the first couple weeks one of the board standoffs came unscrewed, there was no glue holding the nut in place as the others had. So those  parts came undone as a result of loud playing and vibration from the speaker cabinet, the now loose parts floating around inside got into the wrong places and shorted out something so that the amp would not make sound.  Rather than have to pay to ship it both ways to the builder for a warranty repair, I took it to my super skilled tech, one Mike Christenson at House of Guitars in Salt Lake City.  He fixed what  kept it from working without the aid of a schematic.  It came back with only three corners of the main eyelet board supported, but no biggie since each board is made out of 1/8 inch thick sheet fiberglass, called Garolite, very rigid and no problem with one of the corner supports missing at all.  I think that when those parts came loose that the filter cap in question got shorted out.  Instead of it exploding, it just got hot enough to ooze a bit of the electrolyte material out.  I ordered a new pair of caps, a higher quality brand than was originally installed.  

It was a bit difficult to get the old leads out of the eyelet holes, tough enough that I just snipped those leads off the old part and grabbed hold of each one with a pair of pliers to yank it out when I put the soldering iron on to melt the old solder.  Eyelets are easy to stuff one wire or lead into, but the more wires that go into one, the more difficult to insert the new or remove the old and broken when going in for a repair.  For that reason, I prefer turret boards to eyelet boards for a boutique build, easier to repair.  You can wrap more wires and leads around a turret than stuff those same number of wires into a small hole.  So since the other cap looked good and there was going to be a tight fit getting the new lead to jam into one little eyelet hole that already had a wire plus leads from two other parts inserted,  I did not fix what appeared to be unbroken. I just replaced the one bad cap.  

The amp ran dead quiet before, and when I fired it up is was the same.  However...on the clean channel the volume/gain knob could not be turned up past  about 3 1/2 before a bit of breakup happened, right at pick attack for a very brief moment, my buddy and I refer to that as "splat".  After the repair, I could crank it clear on up to past 6 on a scale of 10 without any splat at all.  This changed how I could play the amp significantly... Made it even more of  a 50 watt Marshall eater!  Can still get the smooth milkshake tones out of the lead channel as well as the Kerraaaaaaaang!!!... tasty nasty!  Big win...little money!  Start to finish less than a half hour!  Yaaay!

A printed circuit board amp would have taken me several times as long to fix.  I was surprised to have a longtime friend come over to play some right after I finished. We played a bunch of different guitars through that amp 5 hours straight...

 

Boutique amps for boutique guitars!