First post, my “new” 18W

Skydog6653 · Post by **Skydog6653** » 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. 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.

Post by **JMPGuitars** » Wed 04/17/19 8:48 pm

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

geoff 1965 · Post by **geoff 1965** » Wed 04/17/19 9:38 pm

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

Skydog6653 · Post by **Skydog6653** » Thu 04/18/19 10:41 am

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/

Skydog6653 · Post by **Skydog6653** » Mon 04/29/19 4:36 pm

@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.

geoff 1965 · Post by **geoff 1965** » Mon 04/29/19 5:25 pm

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

Skydog6653 · Post by **Skydog6653** » Mon 04/29/19 5:40 pm

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

Skydog6653 · Post by **Skydog6653** » Mon 04/29/19 5:54 pm

Here are my pin readout values.

Skydog6653 · Post by **Skydog6653** » Mon 04/29/19 5:58 pm

781E3A53-26CF-439F-AF8C-BCD1DBF4DB97.jpeg

geoff 1965 · Post by **geoff 1965** » Mon 04/29/19 7:46 pm

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

Skydog6653 · Post by **Skydog6653** » Mon 04/29/19 8:03 pm

1-4, respectively: 47.7, 479, 0.81, 480. What does B+ refer to?

geoff 1965 · Post by **geoff 1965** » Mon 04/29/19 9:49 pm

B+ is your rectified dc voltage from the ez81.
how is the amp sounding,has the squeal stopped now?

Skydog6653 · Post by **Skydog6653** » Mon 04/29/19 10:17 pm

The squeal hasn't reappeared. The amp is a little hissy at startup, but plays fine. After 20-30 minutes it develops a pop/crackle noise.

geoff 1965 · Post by **geoff 1965** » Tue 04/30/19 7:36 am

skydog,
can you tell me the value of the coupling capacitors i've arrowed on the pic?

B5B600DD-547F-4103-B0B1-3804F9EB5AAC.jpeg

i'm looking into the phase inverter,particulary the plate & tail resistor values,just an idea but it might explain why he's played with the attenuation of the marwatt channel.

Skydog6653 · Post by **Skydog6653** » Tue 04/30/19 9:31 am

The yellow one reads: 150M, .022 uf, +/-10%, 630V, 0701

The blue one reads: STK 2594, 600VDC, +/-5%, MP85S

geoff 1965 · Post by **geoff 1965** » Tue 04/30/19 9:45 am

does your multimeter measure capcitance? the blue one is polypropylene and should measure close to .01uf,the other is mallory m150 polyester.

Skydog6653 · Post by **Skydog6653** » Tue 04/30/19 11:04 am

I don't believe so… I was recently given the Simpson analog meter. The GDT-11 I've had awhile. The Simpson was unused and still had plastic film on the screen!

896F0465-1EC9-4AC3-A039-467D08BFCE83.jpeg

geoff 1965 · Post by **geoff 1965** » Tue 04/30/19 11:11 am

which one have you used to check your voltages?

Skydog6653 · Post by **Skydog6653** » Tue 04/30/19 12:51 pm

The digital one; I'm still learning how to read the Simpson.

geoff 1965 · Post by **geoff 1965** » Tue 04/30/19 1:26 pm

okay,with the red lead plugged into the middle terminal not the 10A DC as in the pic,just checking!
i've been looking into the 82K & 100K plate resistors for the phase inverter and it's used to balance the signal on amps like the old plexi's and the one below which have signal going into one half of the inverter.

AMP-schem.png

however! your amp is TMB/marwatt type which has signal going into both halves of the inverter and this is what the aiken amplification site tells you;
Making the first tube's plate resistor (R1) 10-20% smaller than the second tube's plate resistor (R2) will compensate the gain difference between the two amplifier sections, and should be done before manipulating the tail resistor. Note that this should done only if one input is used as a signal input, and the second used for a feedback input. If both inputs are used as signal inputs, for channel 1 and channel 2, for instance, the plate resistors should be identical, because compensating the balance of one channel will make the balance of the second channel even worse;
so if that is true that may be the reason he's attenuated the marwatt volume pot? stock TMB/marwatt both channels go into the inverter from the pots with no attenuation,also the stock tail resistor is 56K.
doe's anyone have an opinion on this?

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