Looking for Amp Building Mentor

[juansolo]

Most excellent cabbage there sir! 

[raulduke]

 

That is one sexy amp.

I love the look of it (like an old 40's valve radio).

Plain vanilla ½" Baltic Birch. 11×6×6"

dave

[jkokura]

Hey, so I'm still trying to track down that perfect Chassis. I'm thinking I'm going to go with a chassis that's mounted to the lower side of a head box, so just like that pretty all wood one that's pictured above this post.

Anyone have any advice about loading the transformers 'under' where the PCB also sits inside the Chassis? In other words, I want to mount the power, choke and output transformers in a line outside the chassis, and then internally, the PCB board will mounted 'above' where the wires come in. Then, in the space between where the trannies and PCB sits, the tube sockets will be taking up that open space. The transformers/PCB will be close to the knob side, the tubes will be on the output/power inputs side. Make sense?

Next question, looking at all the Lightning schems, and comparing them to the ceriatone project. There's some differences. I can post what they are, but if anyone is familiar with the project/schem, they'll know what I'm talking about (rectifier tube, missing cap, extra resistor from + to -). Can someone walk through them for me?

Also, I'm having a hard time figuring out which type of resistors to use. I'm getting 3 watt Metal Film for power stuff, and a 5or10 watt cement for one spot, but the preamp stuff I'm not sure about. I know 1/2 watt Carbon Comp is traditional, but I've read the noise isn't worth the tone. Then there's another camp that says the quiet Metal film is too sterile and... well - what type should I be getting?

Lastly, I'm trying to figure out if I want to go Tolex or bare wood. If I go bare wood, I'd like to use Maple for the look, but if I go tolex, should I just go with that 1/2 or 3/4" birch stuff? Also, Combo or head? I'm leaning towards Head, because I want to get/make a cab that I'll be able to partner with other potential projects...

Thanks, Jacob

[cooder]

Hi Jacob,
I do think you have to be very careful with the positioning of transformers in relation to each other (and also to the input section of amp) as the magnetic field of PT can induce hum into PT and choke (as well as circuitry). In a well designed amp the positioning of trannies should be well placed for that reason and not just somehow thrown for best looks of how it just happens to fit.
Same goes if you use the given spacing of amp but use different transformers, they might have a different magentic field under power and will likely influence each other (this is in my experience more important and a more likely noise source than what type of resistor you use).

The way to test positioning, well at least how I do it (and I've read this somewhere on a clever site, might have been valvewizard):
HIGH VOLTAGE warning: you need to do this with PT powered up, sitting on workbench, making sure no outgoing or bare cable touch each other or so, tape them of securely.
Connect line voltage onto PT (a fuse doesn't hurt in that setup!), nothing else connected to PT.
Take OT and connect a set of headphones on 16ohm (or whjatever highest) and ground on secondary side of OT. Make sure all other connections are taped of and can't connect anything.
Now move the OT beside the PT in a few likely positions and distance you would mount them on chassis, you will hear an induced hum in the headphones in MANY positions and you want to look for the position with least/no hum that practically possible in your design. Half an inch up or down can make a very noticable difference! Turning the OT or PT 90 degrees etc can help.
As there's obviously no connection (well there better be not!) between PT and OT all the hum is only magnetically induced. And this would like be the source of a hum in amp (there's of course other possible sources of hum in an amp too).
Same procedure with choke, hook it up to headphones and move it around powered PT to hear best spot and you actually have to have OT sitting in your best position there too as the mass of iron influences each other to some degree.

I know this sounds like magnetic voodoo but in my experience it's the way to determine the positioning.

What resistors to use? I'm going now with 1 watt carbon film as a happy medium in this neverending discussion. Probably most of all because Dr.Z has apparently settled for that too and Bob Reinhardt said he's gone away from carbon comp for the noise reason and not enough benefit. I think tohse dudes know more than me.

I like bare (oiled) wood look personally, I'm using mainly Macrocarpa and Rimu (native NZ wood).

[davent]

Great looking build Cooder!

Hi Jacob, I'll second what Cooder pointed out, use the headphone trick to determine where the transformers go, first saw it over at ax84. Made up a little box to take care of all the transformers' and power  connections plus headphone jack. I stow the PT secondary leads in a terminal block.



Final position required mounting OT horizontal.



I've always brought the transformer leads through under the eyelet boards, no problems. The boards are mounted on  3/4" or one inch tall standoffs so lots of room for the leads to come through (rubber grommet protected holes).

For resistors usually see wirewounds and metal oxides (saves a bit on the cost of films) in the power supply. The preamp usually see lots using carbon films. Carbon comps i've read may yield some interesting harmonic content if used on the plates of the PI. Tried it in my 18watt and really prefer the metal films that were in there before.

Some interesting reading on resisitors.  http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm
http://www.aikenamps.com/ResistorNoise.htm

Jacob are you thinking along the lines of a Trainwreck-style wooden cab?

And thanks for the kind words guys!
dave

[jkokura]

Hmm. I don't think I'm going to escape the leads entering under the board, so I'm glad that hasn't usually caused issues. I wonder if the West Labs guy knows about the best positioning thing. I emailed him, and he called me, straight up, and we spent about 20 minutes chatting. Nice guy. Sold me on buying his expensive metal.

About the resistors - I was going to go with CC before I started hearing how many people were against it. Mojo means nothing in terms of these things, as compared to Pedals at least. I think 1 watt Carbon Film is what I'll aim for, with the Metal in the power section.

Any thoughts on the Ceriatone mods to the Lightning circuit?

As for the cab, no I'd still be going for a more traditional, grill cloth facing on the inside above the chassis. I'd like to go with a nice, box joint cab, but to be honest I'm pretty hard on gear. It doesn't take long for it to get banged up and damaged, and since this one will definitely be used in my regular church gig, it'll get moved lots. That's why I think I'll go tolex. I have a good supplier, and I just need to go through and look at the options and decide on one I like. Same with the Grill Cloth. I really like Basketweave style of the Egnater Tweaker I own...

Jacob

[cooder]

That's a really nice and well organised setup for trannie testing, Dave! I wish I was that organised.... LOL!
And your builds look fantastic, great stuff!

I find it amazing how much you can hear the induced hum (a 50 to 60 cycle hum depending on your line) with the earphone method... and how much a bit of difference in position can change that. Also it doesn't necessarily depend on distance between trannies, they can be rel. close together as long as the magnetic fields don't interact / cancel each other out.

[oldhousescott]

The diodes on the rectifier socket are insurance against a short taking out the first cap and are just a good idea for general practice. The cap on the rectifier is to quell any switching noise from the diodes.

The extra resistor is a bleeder resistor and again is just good insurance for working in the amp, ensuring that the PS will be drained at power off.

The missing cap and the different dropping resistor value feeding the second tube are significant changes from the original. The missing cap (and resistor feeding it) means the screen voltage on the output tubes will be higher and the amp will have less compression. This will make the amp louder and stiffer compared to the original. Likewise, the lower value dropping resistor will result in the second stage having more voltage and therefore more headroom, again making the amp cleaner and potentially louder before breakup. You'll have to decide if these are changes you want for your amp.

BTW, you picked a great first amp to build. The Lightning is a wonderful sounding amp that can do both clean and gritty with aplomb. The only part I'm not really crazy about is the cross-line master volume. I prefer a LarMar post-PI type MV in these low-watt, non-NFB amps.

[jkokura]

The diodes on the rectifier socket are insurance against a short taking out the first cap and are just a good idea for general practice. The cap on the rectifier is to quell any switching noise from the diodes.

Gotcha. Sounds like a good idea to include those, especially because it looks like they can just be added to the octal socket. Any particular value recommends on the diode and cap values/ratings?

The extra resistor is a bleeder resistor and again is just good insurance for working in the amp, ensuring that the PS will be drained at power off.

That sounds useful also. Is the Ceriatone value/rating a good one?

The missing cap and the different dropping resistor value feeding the second tube are significant changes from the original. The missing cap (and resistor feeding it) means the screen voltage on the output tubes will be higher and the amp will have less compression. This will make the amp louder and stiffer compared to the original. Likewise, the lower value dropping resistor will result in the second stage having more voltage and therefore more headroom, again making the amp cleaner and potentially louder before breakup. You'll have to decide if these are changes you want for your amp.

Yeah, my plan was to go with the actual Lightning schem on this one. I think I'm going with a dual 33uF cap though, to save some room. two 33uF caps in one body, one ground connection.

BTW, you picked a great first amp to build. The Lightning is a wonderful sounding amp that can do both clean and gritty with aplomb. The only part I'm not really crazy about is the cross-line master volume. I prefer a LarMar post-PI type MV in these low-watt, non-NFB amps.

Awesome. I was thinking about building a Blackface Princeton, but I can actually purchase one of those. The Lightning will come out way cheaper to build versus building one. Until I can figure out amp building particulars, I'm going to stay away from the inexpensive amps. I think I might try a 25w or 10w dumble style in the future as well, see what the fuss is about.

After that, you lost me talking about cross stitching and posting up.

jacob

[oldhousescott]

Those diodes need to be 1N4007s or UF4007s for the 1000v reverse breakdown voltage rating. You could also use 1N5408, but you don't need the 3A rating. The cap also needs to be rated for 1000v at least. I would stick with the 0.01uF 3kV as shown. Probably a ceramic disk, what is sometimes called a snubber.

The 220k value for the bleeder resistor is fine. You ideally want the value to be high enough to not draw too much current, but low enough that the B+ voltage is drained in a reasonable time, a minute or two at the most. 220k at a B+ of 400vdc is about 2 mA which is no drag on the PS. Ohms law says it needs to be at least a 1 watt part. You'll want that to be at least a 2 watt part for some thermal headroom, and I think the Ceriatone drawing shows 3w which is even better.

Yeah, the part about master volume types is just personal preference. Build it as shown. To me, it just doesn't sound great with the master turned way down, but you'll likely want to run it wide open anyway once you hear it scream.


Edit: the power law should correctly be attributed to James Prescott Joule, not Georg Simon Ohm.

[cooder]

Which one is actually the original or correct schematic/layout (as opposed to the modified ceriatone one) you guys are refering too? Is it the one on Trinity amp site or have you got a link?
Cheers

[jkokura]

I just googled it.

http://www.google.ca/search?q=Matchless+Lightning+Schematic&client=safari&rls=en&tbm=isch&tbo=u&source=univ&sa=X&ei=PbzwUf6pC8WFyQHK8YCYDQ&ved=0CC8QsAQ&biw=1440&bih=764

Jacob