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PostPosted: 24 Apr 2020, 17:41 
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I just completed the build of my first tube amp. It is based verbatim on Matt's Marblewood design. Though aesthetically it is different. I refer to it as an "inside out" amplifier. My idea being that the stuff that is usually hidden out of site inside the chassis would in this case be visible, and celebrated. Wiring, resistors, capacitors etc, are meant to be "hanging out in the breeze" so to speak, so that someone looking at the amp could see what actually goes into making things happen. It mostly worked as I had hoped. I also tried to go for a vintage military vibe on the design. I like this look and my 1956 Ford F100 has the same thing going on.

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Being my first build, I was not expecting much when I fired it up. I followed the advice in the first power up sticky on the forum. When I plugged it in there was no smoke or fire. But then again, there was not much else either. No heater voltage, no B+, nothing. After checking all the resistances and cross-resistances on the power transformer I figured it was ok and started looking further. Turns out my military surplus on/off switch was intermittent. I had an identical NOS unit and swapped that in. Then bam, everything came up, all the voltages. Everything seemed stable. I let it cook for a bit then powered down and hooked up some speakers and an iPod. Voila, music!

Many thanks to Matt for his design and the explanations given on his website. That made this first project something a newbie like me could take on.

Here are some more pics of the inside of the amp and the build process...

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I cobbled up my own home-brew speaker binding posts. They seem to work great:

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This was prior to start of wiring:

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Construction of chassis:

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We are listening to the amp here in our office and it sounds great. The tubes are some pretty cheap NOS units for now. Not sure how long this stuff takes to break in. The amp did sound pretty sketchy at first, but noticeably improved over the first 15 minutes or so.


Last edited by Dr G on 24 Apr 2020, 20:13, edited 1 time in total.

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PostPosted: 24 Apr 2020, 18:31 
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Joined: 14 Feb 2010, 13:13
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Location: Lisbon, Portugal
WoW!!!

Amazing build and nice design!

Congratulations! :)


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PostPosted: 24 Apr 2020, 19:02 
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Joined: 06 Apr 2009, 10:08
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Location: US Pacific Northwest
That looks awesome! I love the gray on the transformers.

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PostPosted: 24 Apr 2020, 20:09 
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Thanks Matt and HTP. It was a lot of work but very satisfying.

One other thing I had an issue with. I wired the volume pot backwards. I conflated rising resistance with rising volume. Anyway, easy enough to flip around. But... the pot I got, although it has a logarithmic calibration, still feels very weird. The way in which the volume changes in relation to turning the knob does not seem natural at all. I could not bring myself to install a $2 pot on this amp, so I went with a PEC from Digikey. But it clearly not right for adjusting audio volume.

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What is a good volume potentiometer for a tube amp, and where can they be purchased? If anyone has input that would be great.

Gustave


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PostPosted: 24 Apr 2020, 21:16 
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That is really cool. I love amps that are also art.


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PostPosted: 25 Apr 2020, 03:29 
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Location: australia
Love it. Great layout!

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PostPosted: 25 Apr 2020, 09:26 
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Dr G wrote:
But it clearly not right for adjusting audio volume.
This statement confuses me. That is actually EXACTLY thew same part number pot I've used on many amps and projects. And I've never had any issue with it whatsoever.

I'm not sure I understand your statement "clearly not right". Could you elucidate?

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PostPosted: 25 Apr 2020, 12:37 
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Suncalc wrote:
Dr G wrote:
But it clearly not right for adjusting audio volume.
This statement confuses me. That is actually EXACTLY thew same part number pot I've used on many amps and projects. And I've never had any issue with it whatsoever.

I'm not sure I understand your statement "clearly not right". Could you elucidate?
OK, good that you use the same part. Because I think I might know what is going on. It's the fact that I accidentally wired up the pot backwards that is making its response feel strange. If it had a linear calibration then it would make no difference. But since it is logarithmic then it does.

I'll get the wiring sorted out and then that should show if my theory is correct. I'll report back with findings.


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PostPosted: 26 Apr 2020, 19:05 
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Suncalc wrote:
Dr G wrote:
But it clearly not right for adjusting audio volume.
This statement confuses me. That is actually EXACTLY thew same part number pot I've used on many amps and projects. And I've never had any issue with it whatsoever.

I'm not sure I understand your statement "clearly not right". Could you elucidate?

I got the wires flipped and now the potentiometer sounds perfectly normal.


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PostPosted: 27 Apr 2020, 09:55 
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Well done,

It looks great very retro styled.
I have a question, do you have a pic showing your earth connection to chassis?
Do you have an earth connection that links the top back and main chassis metalwork together?
Also although the mesh cage is fitted inside the base I would fit something like a spade connector with ground or something that gives a good earth to the mesh cage (I can't tell if its all bolted together).
However if you take the chassis apart for some reason and did some testing if you had something short to the chassis the fault should clear ie no metalwork should become live because it should be earthed and blow a fuse etc.

Just a couple of comments but it looks really interesting. :up:

One last thought do you have a discharge resistor on the B+?
Something to think about, If the live to the switch was to touch to the mesh for some reason.
Will the fuse blow?

Regards
M. Gregg

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