First post, my “new” 18W

18watt-specific Tech Talk - Building, Fixing, Parts, Mods...

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geoff 1965
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Re: First post, my “new” 18W

Postby 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!
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geoff 1965
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Re: First post, my “new” 18W

Postby 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.
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Re: First post, my “new” 18W

Postby 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.
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geoff 1965
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Re: First post, my “new” 18W

Postby 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!
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Re: First post, my “new” 18W

Postby 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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geoff 1965
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Re: First post, my “new” 18W

Postby geoff 1965 » Sun 04/14/19 8:14 pm

that's true in fact you are getting the squeal from the marwatt channel so you could just try a lower gain 5751 etc in V2 position and leave the 12ax7 in the phase inverter as per the schematic.
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Re: First post, my “new” 18W

Postby Skydog6653 » Sun 04/14/19 8:22 pm

Correction on pot values; when left on 2000k, the middle pot reads 023.
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geoff 1965
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Re: First post, my “new” 18W

Postby geoff 1965 » Sun 04/14/19 8:47 pm

yeah that's TMB pot values so try the easiest first and if a lower gain tube gets rid of the squeal you might have to attenuate the signal to use a 12ax7. there is also the C11 coupling cap into the phase inverter from the volume pot to consider.
a further thought is to check your lead dress to the tubes i.e plate wires are not touching the grid wires.
also when you ordered the JJ's did you get a balanced one for the phase inverter?
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Re: First post, my “new” 18W

Postby Skydog6653 » Mon 04/15/19 3:08 pm

I got a new set of tubes today. I didn't mess around doing them one by one, rather I pulled all the tubes and sprayed the sockets with CRC electronics cleaner. I then “lubed” the pins and replaced the three 12AX7’s and the rectifier with a EZ81. I kept the two Mullard EL84’s. I have a 5751 coming tomorrow for v2. Hopefully it will tame the amp a bit for better note separation/articulation. I powered the amp up and let it warm up for awhile. I only played at 1/2 volume and everything appears well! This was more than the volume that caused the previous issues. Once my guitar fund recovers :) I'm going to start replacing the preamp tubes with NOS stuff. Based on the observations made by you guys, I'm thinking I have a hobbyist hand wired amp in an authentic Jeff Swanson cabinet. Most importantly, it sounds fairly good even with inexpensive tubes. Thanks guys, for the support!
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geoff 1965
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Re: First post, my “new” 18W

Postby geoff 1965 » Mon 04/15/19 3:58 pm

that's good news!
yeah the cab is top quality and so are the GDS transformers,whoever built the amp did'nt follow the marwatt schematic or layout hence the TMB pots.another thing i've noticed is on the marwatt the signal goes to the phase inverter from the volume pot through the C11 capacitor with no resistance,if you look at your volume pot he has attenuated the signal with a couple of resistors,one going to ground from the wiper which i have'nt seen before.see pic.
7CD01F5F-BF73-4E5C-B121-6D8ED25E31A8.jpeg
it's a good idea if you could take some voltage readings and post them for future reference and tweaking,only do this if you are confident! keep one hand in your pocket and the other away from the turret board when probing,here's a chart you can use.
storage/18_watt_voltage_chart_template.doc
just a point of interest,Rob Dettorre made some alterations/tweaks from the original schematic to the production amps but they did'nt update the schematic,probably for copyright reasons.
good luck and keep us posted
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Re: First post, my “new” 18W

Postby Skydog6653 » Mon 04/15/19 4:52 pm

I printed off the chart; is there one visible that has been completed, for reference?
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geoff 1965
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Re: First post, my “new” 18W

Postby geoff 1965 » Mon 04/15/19 5:04 pm

not for the marwatt but if you click on downloads then technical info there is a standard 18W "tremelo" chart showing voltages,note that V2 on that chart is the phase inverter and yours is V3.
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Re: First post, my “new” 18W

Postby Skydog6653 » Mon 04/15/19 5:38 pm

Got it. Do I just ground my tester to the chassis?
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geoff 1965
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Re: First post, my “new” 18W

Postby geoff 1965 » Mon 04/15/19 6:37 pm

https://youtu.be/DkEc58-vWc4
yes,make sure it has a good connection, are you okay with the pinouts of the tubes? if you are unsure just ask,safety first!
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Re: First post, my “new” 18W

Postby Skydog6653 » Mon 04/15/19 9:36 pm

I have to make up a speaker dummy first. Pin 1 on v1 was 165.2
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