First post, my “new” 18W

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

Post by geoff 1965 »

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

Post by Skydog6653 »

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

Post by geoff 1965 »

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

Post by Skydog6653 »

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

Post by geoff 1965 »

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

Post by Skydog6653 »

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

Post by geoff 1965 »

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

Post by Skydog6653 »

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

Post by geoff 1965 »

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

Post by Skydog6653 »

I have to make up a speaker dummy first. Pin 1 on v1 was 165.2
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Re: First post, my “new” 18W

Post by Skydog6653 »

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.
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Re: First post, my “new” 18W

Post by JMPGuitars »

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

Post by geoff 1965 »

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
E09959EE-C27B-433A-9E7A-C54240A44B74.jpeg
i've noticed the speaker is different to the original pics,is it 8 ohms to match your output transformer tap?
thumbnail_18wattOT.jpg
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Re: First post, my “new” 18W

Post by Skydog6653 »

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/
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Re: First post, my “new” 18W

Post by Skydog6653 »

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

Post by geoff 1965 »

the white one on the left measuring 150 ohms is your el84 bias resistor and sounds okay,the 2 colber resistors are your el84 grid resistors and both should measure near 1K = 1000 ohms so that needs to be looked at!
they need to be 5watt wirewound if you are going to replace them with new ones.
did you measure the resistance of the phase inverter plate resistors "the ones arrowed on the last pic"?
as a temporary fix you could remove the colber measuring "0" and use a jumper between both pin 9's of the el84's until you get new ones and see if that cures the squeal.
if you filled the voltage chart in you should be able to post it using the "attachments" then "add files" then "place inline"
good luck
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Re: First post, my “new” 18W

Post by Skydog6653 »

Thanks, geoff, I'm afraid I'm showing my ignorance! Of the two arrowed resistors, the larger one measures 83.0, the smaller one 102.4
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Re: First post, my “new” 18W

Post by Skydog6653 »

Here are my pin readout values.
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Re: First post, my “new” 18W

Post by Skydog6653 »

781E3A53-26CF-439F-AF8C-BCD1DBF4DB97.jpeg
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geoff 1965
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Re: First post, my “new” 18W

Post by geoff 1965 »

i take it those resistor values were 83K and 102K so the swanson and TMB circuits use 100K for both but we have to establish which circuit we are dealing with here i.e. a lot of the hiwatt circuits have unequal plate resistors for the phase inverter and the marshall plexi has 82K & 100K!
your B+ and el84 voltages look good,if you could measure the resistors for the inverter i've numbered on the pic it will help us work things out.
B5B600DD-547F-4103-B0B1-3804F9EB5AAC.jpeg
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