Non-Traditional Express... another take (updated presence)

Express, Liverpool, Rocket, Dirty Little Monster, etc.

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martin manning
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Non-Traditional Express... another take (updated presence)

Post by martin manning »

Here's the result of my recent forray into the TW. Inspiration came from rj and his PCB Express project, and he also supplied the excellent chassis and transformers (many thanks, rj!). This is a similar overall concept, distriubuting the power supply along the board, and accessing the centrally located tube sockets through large holes. I went a little farther afield by distributing the components around the sockets, and did not pay much attention to the original Express parts placement.

The ground scheme is also different, with locally closed loops for each node (except the first two share), and then separate wires to three points on the chassis. Preamp and PI ground to the chassis at the input jack, power tube cathodes, screen node cap and bias supply all ground at the bias test points, and the last point near the transformers is used for the reservoir cap and the PT center taps. Details are shown in the layout drawing.

The circuit has an additional trimmer to adjust the drive into the third stage, with a 10dB reduction available. This might be useful to add some clean headroom, but no harm done if it stays full-on. It also has a presonance control, a treble cut, and a PPIMV (KF type-2 with Lar-Mar mod). As it sits, I can get the amp to whistle with the presonance turned to the resonance side and volumes turned up, but cracking the treble cut kills it. There is a little work to do there. Without the presonance, I don't think there would be any stability problems, so the layout seems to be fine as far as that goes. The layout shows an alternate version without the PPIMV, and the presonance can be converted back to a presence by removing the 0.047uF cap going to ground, replacing the other with the orignal 0.1uF value, and installing a 25kB pot in place of the 10K, leaving the CCW terminal unconnected.

This layout style is pretty nice, and it was a fun and interesting excercise doing it. It looks pretty clean on top, but there are a number of jumpers on the underside of the board, and of course the filament wiring and a few other runs are hidden. I was kind of shocked and happy that I didn't have to go looking for errors, and that everything worked right out of the box. I was a bit concerned that the under-board wiring isn't very accessible once all the board-socket and etc. connections are made. This was a risk that seems to have been worth taking, and any later circuit mods should be do-able from the top side.

With only a little playing time it sounds very good- it's highly sensitive with a lot of sparkle, and it has the "clean-to-mean" thing going on where digging into the strings causes it to distort more. I find the PPIMV muffles the sound a bit when it's set below noon, but it is very useful to reduce the sensitivity of the input volume control. The pics here are "newborn" shots with a few fingerprints and flux spatters, and it still needs a power indicator.
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Last edited by martin manning on Mon May 28, 2012 11:31 am, edited 7 times in total.
Bob S
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Re: Non-Traditional Express... another take (updated presence)

Post by Bob S »

Wow Martin - lots to digest there.
Well thought out.
Maybe we will find a way to make TW clones without needing the magic touch that Ken had.
No disrespect to Ken intended - his unique amps inspired our quest.
Very well done.
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martin manning
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Re: Non-Traditional Express... another take (updated presence)

Post by martin manning »

Thanks, Bob! Like somebody said elsewhere, KF probably would applaud these kinds of efforts. I don't know much about him, but I've read about his support of DIY, so I think I'd have to agree.
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Re: Non-Traditional Express... another take (updated presence)

Post by jjman »

I have cathode bias, Larmar and attenuation (but coming out of) the 3rd stage on mine. I take it's output from the middle of 2- 50k series plate resistors. The PI was clipping soon so the channel volume knob now has a more meaningful range as a drive control. 3rd stage still clips later than the PI.

I'm not noticing change in the eq when turning the MV down. Maybe some low end loss. I use it way down much of the time. Maybe try separating the "in" from the "out" twisted pairs on it?

Very cool layout. 8)
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M Fowler
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Re: Non-Traditional Express... another take (updated presence)

Post by M Fowler »

Very nice Martin she looks great.

Mark
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Reeltarded
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Re: Non-Traditional Express... another take (updated presence)

Post by Reeltarded »

Martin, that scaling that happens with the PPIMV is the only reason I have PPIMV now. I never attenuate it there more than 15% off the top, so I retain at least a scaled amount of NFB. Above the needed 50%_ish. Removing my NFB control also left room for another control. :)

You guys are about to make me start chopping more amps. I really dig the tube holes in the boards with the components flying like that. Much more PTP amp with tags all around. It makes too much sense not to consider.

Neato!
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martin manning
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Re: Non-Traditional Express... another take (updated presence)

Post by martin manning »

Thanks, Mark and JJ. I'll have to spend more time figuring out how to set the controls on this thing. Luckily I have a very good in-house player that can wring it out for me as I tweak. Comparing it to the other amps he's pronounced it "a different animal."

I like the tubes-in-a-hole too. Interesting that the recently posted Dumble SSS PCB shots also show that arrangement.

I think I'll try a little larger resistance between the presonance pot and the NFB. I think that might eliminate the oscillation... hell, it worked before, right?

Here's the power indicator. I like the chrome 5mm holders available at Radio Shack, and high brightness LED's running low current, and didn't want to use the filament supply- too much wire. So here it is running off the AC line. A 15K resistor limits the peak current to 10-12mA, and a 1N4007 backs up the LED in reverse bias.
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Colossal
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Re: Non-Traditional Express... another take (updated presence)

Post by Colossal »

That looks killer Martin. Beautiful layout. Extremely well thought out!
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Re: Non-Traditional Express... another take (updated presence)

Post by passfan »

martin manning wrote: Here's the power indicator. I like the chrome 5mm holders available at Radio Shack, and high brightness LED's running low current, and didn't want to use the filament supply- too much wire. So here it is running off the AC line. A 15K resistor limits the peak current to 10-12mA, and a 1N4007 backs up the LED in reverse bias.
I like this implementation. I've been using a small tagstrip for this style indicator (also from radio shack). I think I'll steal this and eliminate the tag strip, thanks.
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xtian
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Re: Non-Traditional Express... another take (updated presence)

Post by xtian »

I love this power indicator LED idea! The black and white wires in your photo are the house 120vac? Will a 1/4 watt resistor work here?
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martin manning
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Re: Non-Traditional Express... another take (updated presence)

Post by martin manning »

Yes the indicator assembly is wired across the PT primary. I like to run 5ma or so through a high-brightness water clear LED. That means choosing a resistor that will pass 10 mA at 120V (it's only conducting on half of the cycles, and the LED usually can take 20mA continuously). So 120 / 0.010=12k. Power is I^2*R so 0.01^2*12000= 1.2W, but again you only need half of that because of the half-wave rectification, so 0.6W minimum. I found a 15k 2W laying on the bench and used that.
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xtian
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Re: Non-Traditional Express... another take (updated presence)

Post by xtian »

I appreciate the maths. Thanks.
Bruciep07
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Re: Non-Traditional Express... another take (updated presence)

Post by Bruciep07 »

That's a great build Martin! Super clean layout, easy to work on, easy to trace leads, I really want to try something like this now! Huge fan of the tube cut outs by the way, great job!
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Re: Non-Traditional Express... another take (updated presence)

Post by surfsup »

Martin, shouldn't the diode go across the LED, not in series with it, to protect the LED against reverse voltage?

Like this:
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martin manning
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Re: Non-Traditional Express... another take (updated presence)

Post by martin manning »

surfsup wrote:Martin, shouldn't the diode go across the LED, not in series with it, to protect the LED against reverse voltage?
That would work, but current would flow over 360 degrees and double the power consumption, the wattage rating for the resistor, and heat generated.
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