First post, my “new” 18W

Skydog6653 · Post by **Skydog6653** » Wed 05/01/19 1:50 pm

Before, or after the “software update”?!

Post by **JMPGuitars** » Wed 05/01/19 2:06 pm

Skydog6653 wrote: ↑
Wed 05/01/19 12:57 pm
One guy on youtube recommends using “j” hooks on lead ends to solder in new components to avoid disturbing other joints sharing the same turret. Is this an accepted practice?

The correct method is the j hooks face opposite (or alternating) direction.

This is a great (and ancient) video on the subject: https://www.youtube.com/watch?v=Mrhg5A1a1mU

Skydog6653 · Post by **Skydog6653** » Thu 05/02/19 4:21 pm

I don't know if this will help or not, but here is the map I drew up, along with a photo.

geoff 1965 · Post by **geoff 1965** » Thu 05/02/19 5:44 pm

can you read the value printed on the capacitors i've circled?

DD60424D-F334-4510-8663-B933B185B92C.jpeg

Skydog6653 · Post by **Skydog6653** » Thu 05/02/19 6:23 pm

Top-bottom: 50V 470uf, 450V 20uf, 450V 20uf, 50V 1uf, the last one I can't see a value, it's marked Nichicon N9750.

Skydog6653 · Post by **Skydog6653** » Thu 05/02/19 6:32 pm

I replaced the two Colber 1k resistors today. When removing the one that read 0, I saw that it had a broken lead.

146CF090-E678-4EA0-AB16-FF9379C4A728.jpeg

when I fired the amp up, it again squealed loudly after 20 minutes or so. on the advice of a guy on tgp, I pulled V1 and although there was a hum present, no squeal. I replaced V1 and pulled V2. It squealed almost immediately.

geoff 1965 · Post by **geoff 1965** » Thu 05/02/19 7:25 pm

the replacement 1K's are a bit overkill! cement one's would have done but don't worry they will do the job. your capacitance value for the el84 bias is quite a bit out,but if the amp was built in 2003 like the cabinet then they will be 15 years old and i would replace that and the 2 20uf450's.
the 82 from the hiwatt vol pot is that 82K or 82 ohms also the measurent of the red one across the marshall vol & tone pots is that nf?
ive arrowed a couple of resistors if you can check their values and tell us if in ohms or K ohms.

A9A61785-8D7E-4A53-AC83-E8FC9D0EC18A.jpeg

generally the resistors in the amp will be 1 watt carbon film,but double check with us before ordering any as there is also metal film and wirewound.
note; the 470uf can be 100uf this will cover both marshall/hiwatt,what is your plan for the amp,do you want classic marshall or marwatt values for channel 1?

Skydog6653 · Post by **Skydog6653** » Fri 05/03/19 12:14 pm

I have prepared a better map of the amp. I hope this helps. The arrowed resistor on top is 1M ohm, the lower one 46.7 K

9135FC6F-5B7D-4890-98E7-52F7F9E975A3.jpeg

geoff 1965 · Post by **geoff 1965** » Fri 05/03/19 12:52 pm

okay i've added the values to the pic and numbered the turrets,this will help trouble shooting and can be used as a reference for things to check.there is a couple of issues that stand out to me but first look at the pic and check that the wires from turrets 23 & 24 go to pins 3 & 8 of V1 also wires from turrets 32 & 34 go to pins 7 & 2 of V3.

A9A61785-8D7E-4A53-AC83-E8FC9D0EC18A.jpeg

Skydog6653 · Post by **Skydog6653** » Fri 05/03/19 1:38 pm

Yes to both with the exception that a turret was hidden under the shielded wire so if I shift the numbering 1 to the right, turrets 33 & 35 go to 7 & 2 respectively.

geoff 1965 · Post by **geoff 1965** » Fri 05/03/19 2:14 pm

forget the hidden turret and stick to the numbers on the pic to save confusion! the first issue is the wiring to the normal channel vol & tone pots,i don't know what he's done here but it's not marshall,marwatt or "trem mod".this is how it should be;

18watt-layout.gif

the resistor is 100K and the coupling caps are 4.7n & 10n marshall or both 10n for marwatt.
as mentioned earlier you have signal into both halves of the phase inverter so i would balance the plate resistors i.e.both 100K.
it's hard to see from the pic but does the wire from turret 48 go to ground at the transformer nut? it's already grounded to the bus wire and i wonder if this will cause a ground loop?
note; you can have the 100K going from lug1 to lug1 and C1 going to lug 3 tone pot.

Skydog6653 · Post by **Skydog6653** » Fri 05/03/19 2:34 pm

Yes, it appears to be “double grounded”

33B114F5-C61F-44E5-9E19-0BAA6D6F8E77.jpeg

geoff 1965 · Post by **geoff 1965** » Fri 05/03/19 2:51 pm

it's not double grounded but part of the bus rail,the ground from the bias goes to a seperate ground point from the bus rail if you look at the star grounding layouts,it might be an area to address when you do your grounding.

Skydog6653 · Post by **Skydog6653** » Fri 05/03/19 6:12 pm

If possible I would like to have channel 1 as Marshall and channel 2 as Hiwatt. I know it's a bad plan to just swap out components until I find the culprit, but as you noted, the caps are old anyway. What caps should I replace the “big 3” with? Other than those, and the two plate resistors on the PI, is there any other components that scream for replacement?

geoff 1965 · Post by **geoff 1965** » Fri 05/03/19 7:42 pm

if you are in the U.S. the link Josh posted for the input jack's have all the components,you can look them up and buy elsewhere that's up to you.regarding the squeal,it could be numerous reasons but if we start with the grounding,B+ filtering,and channel 1.
channel 1;
your cathodes are seperate and the marshall has 50uf/820R with a jumper across pins 3 & 8 so you could do that or use 25uf/1K5 on each cathode "i tried this and it was superb!"
2 X sprague atom 25uf-50V axial
2 X carbon film 1 watt 1K5 resistors
1 x carbon film 1 watt 100K resistor
1 x 4n7 ".0047uf" polyester film axial
1 x 10n ".01uf" polyester film axial
note; regarding the polyester's,mallory 150's,mojo,sozo nexgen are all good depends on what you want to pay?
B+ filtering;
2 x F&T 22uf-500V axial
phase inverter;
2 x carbon film 1 watt 100K resistors
1 x carbon film 1 watt 56K "this is stock value you might need it"
el84 bias cap;
sprague or F&T 100uf-100V axial
grounding;
12awg tinned bare for bus wire
22awg 600V tinned stranded wire for hook up "black"
these are recommendation's but i have done a lot of testing on my 18W and the sprague atoms were the best for tone on the V1 cathodes.
extra note! your dual can capacitor does'nt have a bleed resistor,so as a safety precaution get a metal film 2 watt 220K as well.

geoff 1965 · Post by **geoff 1965** » Sat 05/04/19 8:31 am

Skydog,
i forgot the 10n polyester film going into the phase inverter for the marshall channel,whilst we are on the subject i'm not sure about those old polypropylene couplers.we know polyester films are tried,tested and good in this amp and for what they cost i would fit new ones.has anyone else used polypropylene's or have an opinion on them?
here's a tip,get 4 small crocodile clips and solder them together,you can clip them to the turrets and swap in & out components when testing.

002.JPG

geoff 1965 · Post by **geoff 1965** » Sun 05/05/19 10:52 am

how about this for grounding?

77AA2B69-8A41-4CDB-AC50-54CAB0B16692.jpeg

cut the link and leave the el84 bias ground where it is.
still going through the preamp,this is definately not marwatt, more TMB/plexi-ish!

Post by **JMPGuitars** » Sun 05/05/19 1:48 pm

geoff 1965 wrote: ↑
Sun 05/05/19 10:52 am
how about this for grounding?
77AA2B69-8A41-4CDB-AC50-54CAB0B16692.jpeg
cut the link and leave the el84 bias ground where it is.
still going through the preamp,this is definately not marwatt, more TMB/plexi-ish!

The pre amp and power amp sections shouldn't be connected to the same ground or bus. That should be split, and the preamp side should be connected through the input jack ground.

geoff 1965 · Post by **geoff 1965** » Sun 05/05/19 2:08 pm

true Josh, i have my input jack & cathodes grounded seperately from the phase & power section,an oversight on my behalf!

Skydog6653 · Post by **Skydog6653** » Sat 05/11/19 8:53 pm

I'm having a hard time finding exact matches for the electrolytic caps. How “crucial” is the volt rating? The uf rating? As said, I'm not finding the exact volt/uf pairings. Ican find some with the same volts or farad but not both. Initially, I'd like to find the 50v/470uf, the 50v/1uf, and the 2 450v/20uf. This week has been crazy work-wise and I had to let the amp go a bit.

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