First post, my “new” 18W

Skydog6653 · Post by **Skydog6653** » Fri 04/12/19 8:23 am

My only other tube amp is a Siegmund Midnight Special that I really like, but I wanted something with less headroom, more crunch. So, I picked up this DST “MarWatt”. It has the classic tube lineup, 3/12XA7’s, 2/EL84’s, and an EZ81 rectifier. This has one Marshall channel, and one Hiwatt channel vs. normal/tremolo. Currently, the preamp tubes are 2/Mesas and one TungSol in V1. 2/Russian Mullards in the power tubes, and a Weber “Copper Cap” WCA4 rectifier. I'm getting a loud squeal onthe Hiwatt side that I believe is being caused by a faulty 12X. Anyway, here's my new amp.

geoff 1965 · Post by **geoff 1965** » Fri 04/12/19 10:25 am

hello,
i've seen good reviews for this amplifier,top quality cabs and GDS transformers etc. the rectifier is diodes & resistors which simulate an ez81 tube,the wiring and grounding look questionable though! can you add some more pics showing the wiring to the pots and tube sockets? there is a lot of variation with the preamp tubes with this amp so it might be an area to look at,some are all 12ax7's like yours and some use a 12au7 for the phase inverter and a 5751 for the marwatt channel which is basically a marshall TMB with a HiWatt tonestack.
keep safe when working around the amp.

Skydog6653 · Post by **Skydog6653** » Fri 04/12/19 3:08 pm

Here are some “gut shots”. Thank you for the input.

geoff 1965 · Post by **geoff 1965** » Fri 04/12/19 4:01 pm

thank's for uploading the pics,it helps a lot when trouble shooting as someone might spot something that's obvious.
first, have you tried a lower gain tube like a 12at7 or 12au7 in the V2 position? whoever built the amp might have had one in this position and using a higher gain 12ax7 might be the cause of the squeal.
second is safety, i don't know your skill level with electronics and don't want to advise you to do something and put you at risk! are you okay working with the high voltages and testing component values etc?
there is a good schematic and layout in the downloads section of the forum but we will need to confirm if it's the same as yours.

Skydog6653 · Post by **Skydog6653** » Fri 04/12/19 6:04 pm

No “death cap for cuties” for me! I have some tubes coming (inexpensive JJ’s) to use for diagnosing and then relegated to backups. If it needs to be taken in, I'll do that. Unfortunately DST is out of business, but I would like to learn the factory tube complement.

geoff 1965 · Post by **geoff 1965** » Fri 04/12/19 6:57 pm

same here! i was trying to research this before but with DST out of business i could'nt find anything.
the schematic in the downloads is by Jeff Swanson "DST" and dated 2002 but there is an oddity with the phase inverter tube! V1 is a 12ax7, V2 says 12au7 or 12ax7 and V3 shows 1 triode as 12ax7 and the other triode 5751!
the layout is Barry Maries and dated 2004 and the tube layout is V1 12ax7, V2 5751 and V3 12au7.
so taking into account a 12au7 has about 20% of the gain of a 12ax7 i would imagine there's quite a difference in character.
i'm going to delve into the old posts on the forum and see what i can find.

Skydog6653 · Post by **Skydog6653** » Fri 04/12/19 10:19 pm

I was under the impression that Jeff was the cab builder and Rob Dettorre was the chassis builder.

geoff 1965 · Post by **geoff 1965** » Sat 04/13/19 10:54 am

struggled a bit here because any of the links related to DST come up as "file cannot be found" and Jeff Swanson & Rob Dettorre did most of their responses through e-mail.
Jeff Swanson designed the original production amps with a 12au7 in the marwatt preamp "but say's a 12ax7 can also be used" and the tonestack pots were Hiwatt, 220K Hi,100K Mid,500K Lo.
Rob Dettorre was the engineer and did the clean up & layout and the production tonestack pots were changed to 500K linear treble,100K linear middle,500K log bass, and the tube lineup was V1 12ax7,V2 5751,V3 12au7. "there is a bit of variation here with quite a few people favouring a 12au7 in V2 position"
you can easily check your pot values to see if yours is the early or later model and if you're checking component values be aware that the 2002 layout does'nt match the 2002 schematic! also the tone pots on both layouts have log pots with the treble value at 250K!
Ceriatone amongst others did a kit version of this amp which i think yours might be looking at the wiring.
i found one post relating to squeal on the marwatt channel and there is a 1 meg resistor going to pin 2 of V2 which needs to be soldered to the tube socket not the board.
good luck with your trouble shooting!

Skydog6653 · Post by **Skydog6653** » Sat 04/13/19 12:57 pm

If it's a kit, someone went to a lot of trouble to hide it. My Hiwatt treble pot reads 232.

geoff 1965 · Post by **geoff 1965** » Sat 04/13/19 1:09 pm

that's suprising because the wiring especially the grounding does'nt look like a production built amp. just my opinion!

Post by **JMPGuitars** » Sat 04/13/19 1:39 pm

Geoff cited the correct schematic, and I agree with him that it doesn't appear to be a production model, or even made by Jeff or Bob. It's more likely somebody else built it from the schematic on this site.

Swanson and DST both used logos on their cabinets, and it doesn't appear as there ever was one on the front of that cab?

You can see some of Jeff's wiring here: gallery/picture.php/2009/category/349

And read more about DST here: https://www.vintageguitar.com/3616/dst- ... mplifiers/

All that said, it doesn't matter too much. With maybe a little work you could clean that up and make it a great amp. GDS/Heyboer iron is great, and that's got it.

Thanks,
Josh

geoff 1965 · Post by **geoff 1965** » Sat 04/13/19 3:10 pm

MarWatt_version21.gif

can somebody explain what the diodes from the output transformer do?
skydog, can you tell if the pots are linear or log and do you have the 1meg resistor going to pin2 of V2? it's not on the schematic but it shows it on both layouts.

Post by **JMPGuitars** » Sat 04/13/19 4:40 pm

geoff 1965 wrote: ↑
Sat 04/13/19 3:10 pm
can somebody explain what the diodes from the output transformer do?

viewtopic.php?f=4&t=3090&p=23812&hilit= ... des#p23812

Skydog6653 · Post by **Skydog6653** » Sat 04/13/19 5:02 pm

Pic=1k words!

Skydog6653 · Post by **Skydog6653** » Sat 04/13/19 9:13 pm

Josh & Geoff, thanks for all your input. I'm afraid I may have come across a little gruff. A classic case of “shooting the messenger”. I'm just bummed at the possibility of this amp not being what it was purported to be. Mainly, I think I might have overpaid for it. I'm in it for around $775 including the new JJ’s. The bitch is, the amp sounds amazing until the trouble starts. If the new tubes eliminates the issue, I'll feel better about it. If it turns out to truly be a DST, I’ll be ecstatic! I sent an email with pics to both Jeff & Rob. Hopefully, they’ll have good news for me. Thanks again guys!

geoff 1965 · Post by **geoff 1965** » Sat 04/13/19 11:00 pm

no worries!
you've got a good deal,those amps were nearly $2000 new and like Josh said you have the main components for a very good amp.don't bother too much about it being built by DST and lets focus on sorting the gremlins!

geoff 1965 · Post by **geoff 1965** » Sun 04/14/19 1:59 pm

just measure the resistance of the bass and mid pots,looking at the wiring i think this is a TMB tonestack.

Skydog6653 · Post by **Skydog6653** » Sun 04/14/19 2:42 pm

The bass measures 1044, the middle and treble both measure 232. I feel a little better as when measuring I saw the attached signature on the baffle board.

geoff 1965 · Post by **geoff 1965** » Sun 04/14/19 6:36 pm

that's good,looks like you've got a genuine swanson cabinet,early days as well 2003! the 1 meg to ground from pin2 V2 is "marwatt" just trying to figure out the pots,1meg bass,250K mid & 250K treble? i wonder if they changed from the Hiwatt linear to a more TMB type tonestack?
this could be an easy fix by just using the lower gain tubes in V2 & V3 position. the layouts from 2002 and 2004 both have V2 5751 & V3 12au7 your cabinet date falls between these and if you check the link Josh added it says 3 12ax7's in the preamp and that was 2007.
interesting!

Skydog6653 · Post by **Skydog6653** » Sun 04/14/19 7:53 pm

A 12au7 only has a gain factor of 17. If the circuit spec's out a 12xa7 (gf=100) will the lower gain tube cause an issue? What about the reverse scenario? Correction on pot values; when left on 2000k setting, the middle pot reads 023.

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