18Watt.com

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!

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

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.

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!

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.

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.

Correction on pot values; when left on 2000k, the middle pot reads 023.

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?

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!

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. 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

I printed off the chart; is there one visible that has been completed, for reference?

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.

Got it. Do I just ground my tester to the chassis?

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!

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

The amp started acting up again today. I cleaned every pot, switch, and jack with deoxit. I then took a wood skewer and started probing around. When I got to the high input jack on the marwatt channel, it was quite noisy. When I hit it one last time, the amp went silent. My first thought was I messed something up, but when I plugged a guitar in, it played beautifully. Very, very quiet! I don't view this as a permanent fix, but I really feel it won't be an expensive/complicated one. Perhaps a ground issue. I did learn today that it is a Swanson cab, but not a DST chassis. In case the input jack is faulty, is there a link to the correct replacement part? Thanks again, guys.

Skydog6653 wrote: ↑

Wed 04/17/19 7:05 pm

The amp started acting up again today. I cleaned every pot, switch, and jack with deoxit. I then took a wood skewer and started probing around. When I got to the high input jack on the marwatt channel, it was quite noisy. When I hit it one last time, the amp went silent. My first thought was I messed something up, but when I plugged a guitar in, it played beautifully. Very, very quiet! I don't view this as a permanent fix, but I really feel it won't be an expensive/complicated one. Perhaps a ground issue.

Before you replace the jack, check the quality of the soldering around it. When you plug a guitar cable into the jack, it lifts the input signal off of ground. And of course, when you disconnect the cable, it reconnects the input to ground. This is primarily to keep the amp quiet when nothing is plugged in.

Skydog6653 wrote: ↑

Wed 04/17/19 7:05 pm

In case the input jack is faulty, is there a link to the correct replacement part?

This looks like the direct replacement: https://www.tubesandmore.com/products/j ... g-marshall

This is the version I usually use: https://www.tubesandmore.com/products/j ... t-marshall

Skydog6653 wrote: ↑

Wed 04/17/19 7:05 pm

I did learn today that it is a Swanson cab, but not a DST chassis.

This is what I suspected, but I didn't want to be a downer. If it was DST + Swanson, it most likely would have had the DST logo on the cab. Another thing is the lead dress and cleanliness of the components. Take a look in the gallery of some other amp builds, and you can do a little work to clean it up and make it look better/more pro level. Especially take a look at how straight the leads are on resistors/caps on the cleaner builds.

Thanks,
Josh

good advice and don't be disheartened about the chassis you've got the components for a great amp plus the bonus of learning and doing the work yourself,who needs DST!
Josh mentions the lead dress of components etc and there is one area that is driving me nuts every time i look at it! your plate resistors for the phase inverter are different wattage and also look to be different values? i don't know if this is an intentional "guru's trick" or what! could you safely measure their resistance and post them? make sure the amp is unplugged & capacitors are discharged.
good luck i've noticed the speaker is different to the original pics,is it 8 ohms to match your output transformer tap?

Same speaker! Hellatone was an outfit that took Celestion G-12 speakers and “exercised” them for ~15 hrs. They then slapped their sticker over the top. As an example of how brand snobby some of us can be (I'm guilty as charged!), the Celestions routinely get ~20% more than the Hellatones when resold! https://avatarspeakers.com/shop/speakers/hellatone-30/

@geoff 1965 @JMPGuitars I did the voltage checks today. How do I add the data? Some of mine, mainly on the preamp tubes don't really jive with the other guys values. With regard to the resistors, the white one on the left measured 150 exactly. The COLBER on the left measured 1062, the one on the right, 0. Thanks, guys.