Marshall 18 watt. One tube red plating

[Phil_S]

I don't have the patience to decipher your build topology to check this. If you have reduced the grid leak resistors from 470K to 200K you have, to a large extent, neutered that Marshall vibe. Revert to the spec if you are not finding the tone to your liking. I do not believe this has any relevance to the buzz problem you are discussing. It only affects clean headroom.

[evirob]

I don't have the patience to decipher your build topology to check this.

Honestly, I'm not surprised and I don't blame you. I did say it was my own layout, and I only posted it because Richie asked.

Thanks for your advice though, any help is appreciated.

[Littlewyan]

I built an 18W using that Output Transformer from Ampmaker.com and had buzz issues. What fixed it for me was the Ruby Mod and a conjunctive filter on the anodes of the EL84s. Have to say though I was never happy with that amp and ended up selling it.

[Richie]

I don't have the patience to decipher your build topology to check this.

Honestly, I'm not surprised and I don't blame you. I did say it was my own layout, and I only posted it because Richie asked.

Thanks for your advice though, any help is appreciated.

Yes, because sometimes you can see things that may be overlooked or causing the problem. One thing I see, you have the red and blue as the wires going to the tube plates, and the CT being the brown wire.
According to the Hammond data on the 1760e the red wire is the center tap.
I'd look into making sure the OT is hooked up correctly.
Also as was mentioned an oscillation, you have the OT mounted to where the wires run close to the preamp. Have you tried moving some wires around while playing to see if it effects anything.

There are many you tube clips that show what an 18 should sound like.

I never was a fan of how they ran the tubes at the higher voltages.

[xtian]

One thing I see, you have the red and blue as the wires going to the tube plates, and the CT being the brown wire.

Well, THAT would cause a wee bit of trouble. Gripping my seat waiting to hear if this is the case!

[pdf64]

Yes, well spotted.
But even when that's amended and the amp likely sounding a heap better, the layout is asking for trouble - the speaker socket looks to be a couple of inches away from the 1st grid.
Suggest to try putting a screening can on V1, moving the speaker socket over to the other side of the EL84, and run the OT wires outside the chassis.

[Richie]

One thing I see, you have the red and blue as the wires going to the tube plates, and the CT being the brown wire.

Well, THAT would cause a wee bit of trouble. Gripping my seat waiting to hear if this is the case!

That's why I like pics, we could guess all day, and chase your tail trying to fix or make things right that is caused by something simple as a miswire. Hopefully correct that,then can go from there.

[Phil_S]

To be clear, pictures are always a good idea. In my experience, someone who knows things will look at it and tell you something. I was pressed to move on to work-related matters. Honestly, I simply didn't have the time to do it and I'm not particularly good at it. I didn't mean to come off like a curmudgeon. Hindsight tells me I sounded a bit crusty about it.

[evirob]

Yes, because sometimes you can see things that may be overlooked or causing the problem. One thing I see, you have the red and blue as the wires going to the tube plates, and the CT being the brown wire.
According to the Hammond data on the 1760e the red wire is the center tap.
I'd look into making sure the OT is hooked up correctly.[/quote]

Aaaaah! By gum you're right! Goodness knows how I got that wrong, or how the amp is even functioning at all. Thank you for taking the time to track that down, I don't know if I would have ever spotted it. I'll change it over and report back

[evirob]

Well, that was a major breakthrough. Since wiring up the output transformer correctly, the tubes are now seeing the same voltage. It's a little higher than before - both seeing 355v and dissipating about 12.5 watts each with individual 300 ohm bias resistors. I will tweak that to bring it down below 12 watts.

As for the sound, it remains to be seen if the layout or lead dress is causing further issues. I've plugged in and played at the lowest possible volume, which sounds fine - but then it always did sound alright at low volume. Once again it's late into the night here so turning it up is out of the question, but hopefully I'll get a moment tomorrow to bother the neighbours

[Richie]

This might go along with some things that were mentioned. A while back I had an amp here that needed some bugs worked out. And thought it was a good amp to video, and show how lead dress can effect an amp, in how it works,or keeps it from working,or causes noise problems. Or how shielded cable may not always fix things.

But this amp had the output plate wires close to the input, then run or routed down through the amp.
And was a bit of a wiring mess. As you can see or hear in the video, how a slight movement from the plate wires, the amp picks up noise or just cuts out completely.

So I recorded this video so it might be of help to someone else. As they can see how little of a movement can make or break the amp working or not working.

https://www.youtube.com/watch?v=8ryIXTL3mRU

[evirob]

Well, the mixed up primary leads on the OT were the culprit alright. I had a brief play with the amp wound up to full and I would say it's like night and day but I actually don't think that does it justice - the amp sounds incredible now! I finally get the attraction of the 18 watt circuit, it's just a joy to play. Beautiful creamy overdrive til the cows come home, not a hint of buzz in sight.

Regarding the lead dress and so on, noise and interference haven't actually presented themselves too badly. I wiggled various cable runs with no perceived change so I think I might be in the clear there.

Thanks everyone for helping, especially Richie for spotting my rookie error. The amp that was not of my dreams is finally of my dreams.

[Richie]

Glad that was an easy fix. Anyone can make a mistake, rookie or experienced.

[tubeswell]

Congrats on your 1st build success! If you survive them, mistakes are learning opportunities.

[evirob]

Congrats on your 1st build success! If you survive them, mistakes are learning opportunities.

Thanks! I was actually thinking the same. I learned a ton of stuff that I wouldn't have if I wired it up right first time. I'm just glad ididn't damage anything in the process!