Documenting a 30 watt build

[hesamadman]

I've never shown my step by step on any of my builds since I have been on here. I definitely should have already. I owe a lot to the people on this website for all of their help over the last couple years. Im finally getting around to doing it.


A build that I have been very fond of and have worked very hard on are 2 separate models of a 30 watt AC30 style power amp using a preamp similar to a 2204 that I have been working on for the last year now. The power amps in my two builds are obviously identical. The preamps are similar though one was set up to have a much heavier gain and the other set up with more headroom.


The Heavier gain version utilizes a couple of red LED's after the first gain stage. This gives the lead tone a more hi gain tone and brings out harmonics and pick squeals really well. Each gain stage is pushed pretty far as far as gain goes. Low cathode values, 100k plates, etc.


The cleaner version does not have the LED's. A few little fun things here and there. PPIMV. Reverb utilizing a solid state ICTL072 and a MOD reverb pan. I am using a 12AY7 in the first preamp tube. I know its less gain (why not just reduce the gain elsewhere) but I like the "flavor" of gain it produces.


This is technically the 5th one of these that I have built. However the first two were test prototypes so that I could get a good layout. They ended up getting scrapped and the XFMRs were used in the rebuild. The rebuilds turned out fantastic as I was able to use a smaller chassis and move some components around to go with the style of lead dress that I have researched and grown fond of. The last two were very successful builds. Very quiet. Very clean wiring. The heavier gain model was a little noisy at high gain settings though it was expected. I did implement a very slight noise gate in that one which took care of the issue. It doesnt seem to choke the tone at all at lower gain settings though I did put it on a switch just in case.


I have picked up a lot of tricks here. Sluckey has been a great influence. I bought some isolated turret lugs for the grid resistors of the EL84 on my last two builds. This build I implemented the isolated lugs on the LED resistor as well.


There are some things that I have done differently already with this build than the last two (one thing not purposely). The tube sockets I got are oriented 90 degrees counter clockwise than the ones I meant to use. I made a drilling template for these builds and already punched the chassis to accept the other sockets. I had not realized until I went to install the sockets. The main thing is, on my last build, the heater wires came from the bottom. This one they come from the right side. I am REALLY hoping that the grid resistors on my EL84 are not affected by this. I think it will be OK but we will see. I will be using shielded cable from here to the board. My board layout is pretty convenient. My preamp section is laid out in a way so that none of the wires cross each other at all. They leave the socket and go straight to the board. I will be using Merlins method here and twisting plate, grid, and cathode wires of the associated stage together. Im mainly doing this because of my tube socket orientation. Pin 6,7,8 are on the bottom rather than the left side life I wanted them to be. Ill have some nice 90 degree bends going on and will keep them as far away from the heater wires as possible.


I think that pretty well covers it all for now. I did make a mistake and punched an extra tube socket. I also punched a small hole for my cap can. I am out of 16/16 cap cans so I am adding in a small board over these holes and using some 16 uf caps I have here. This board will be high enough to clear my isolated turret lugs directly under the PT.


This was a blank chassis. Being an electrician I have a knockout set, but the only size I have that is useful is a 3/4" KO punch for my 9-pin sockets. I have a friend who owns a machine shop and im working on getting a punch for my IEC sockets. The IEC and PT I used a jigsaw. Since I have a chassis drilling template it makes this pretty quick.


I post a lot of questions on here and sometimes I dont get around to responding to my replies (as I work 12+ hours a day mostly wiring and repairing wiring in homes) but I do read them all and appreciate everyones time and help they give. Hope you enjoy this thread. Hopefully I will be finished with the amp this week. And hopefully I will have the enclosure build by next week.

[Willabe]

Looks really good! 

And if you like the sound of a 12AY7 your not alone, many guys like them.

Each gain stage is padded out pretty good as far as gain goes. Low cathode values, 100k plates, etc.

Maybe I'm not understanding you? When I read 'padded' I think of less gain. That would need a higher value cathode (K) R.   

[hesamadman]

Looks really good! 

And if you like the sound of a 12AY7 your not alone, many guys like them.

Each gain stage is padded out pretty good as far as gain goes. Low cathode values, 100k plates, etc.

Maybe I'm not understanding you? When I read 'padded' I think of less gain. That would need a higher value cathode (K) R.   

Thanks Willabe. Thats a good point. It should be worded differently.

[Paul1453]

Looks really good! 

And if you like the sound of a 12AY7 your not alone, many guys like them.

Those 12AY7s seem to me to be somewhat of a rare tube.
I run across plenty of 12AU7s and 12AX7s, less often 12AT7s, but I've only found 1 12AY7 in all the gear I've purchased.
I haven't tried it out to hear how it sounds yet.

So gain from these 12A_7 tubes from most to least is X,Y,T,U?
Is that correct?
If I want to keep my only 12AY7 tube, should I swap in an X or a T or the 7247 as a sub in a working amp that is up to be sold?   

[hesamadman]

If I want to keep my only 12AY7 tube, should I swap in an X or a T or the 7247 as a sub in a working amp that is up to be sold?   

I think the order of gain factor is X, T, Y, U. If you want to swap it. Go with a T. The U has a gain factor of about 40% of the Y.

The T has a bit more. The X obviously much higher. My numbers could be off but I think it's close.

[Paul1453]

Thank you!

So the T should give it about 50% more gain than the Y, which might be OK.

The X has greater than 100% more gain than the Y, and that might be too much.

The U would probably be to weak to be a good sub.

I'll have to see how this Y sounds to me, to really know if it is a keeper or not.

[hesamadman]

Yea I think the U would be too weak.

I think this is how it goes.

Gain factor:
12AX7= 100
5751  = 70
12AT7= 60
12AY7= 45
12AV7= 41
12AU7= 19

[hesamadman]

Had a little time before work to get some more wiring. Got the primary of the PT as well as the IEC/Fuse. Pots mounted. Input Jack. And my shielded cable for the grids. I have been buying Lava Cable Mini ELC for my shielded cable. It is a little bigger in diameter. Not as easy to bend and orient the runs like I would like, but its a GREAT cable and the soft copper shield is fantastic and easy to work with. I've been a fan of Lava Cables for years so now I implement them into my builds.

[hesamadman]

I have the amplifier all wired up. Still need to finish the head cabinet and speaker cabinet. One thing I did differently on this one is I did a traditional marshall style head cabinet mount layout. My last 30 watt was a top mount style. This touches a bit on one of the topics on the board recently about transferring from a marshall style to fender style mount but the reason I did a top mount chassis install before was it made it much easier to take the same amp and put it in combo amp form. Only thing I had to change was the tube sockets. Instead of on the top of the chassis I just put the on the rear. All controls remained the same and so did switches. Also made faceplate design simple by using the same template. Well, I decided to spend some time and make new templates due to the cathode bias EL84 being so hot. And with the tubes being underneath, all the heat rose to the top and my chassis got really hot. Switches got really hot. I cut a vent in the top of my last one. It worked great. Fixed the heat issue but now the circuitry is exposed due to vent and I'm not very comfortable with that. To change the way the chassis fit in the head cabinet (top to bottom) only required I rearrange my tone stack and volume controls. It actually lined up each pots much better with their associated components on the board. It eliminated any wires crossing on the tone stack but obviously bringing my PPIMV over required jumping across the board. I have it heavily shielded so Its nice and quiet. I still need to add the fail safe resistors to the pot of the PPIMV. I guess the reason I ever did a top mount install with my chassis was I thought it would be easier for the amp to be lined out left to right. The order I liked my controls laid out was the order in which they were in circuitry. (pre-gain, bass, mid, treble, MV) To keep that true here, I would have long leads running to my pre gain and it would just be messier than I want. So my goal here was cleanliness but still remain practical. My layout is now (Bass, Mid, Treble, MV, Pre) I think it would be better if Pre was before MV but this eliminated crossing any wires. It would be fine im sure, im just trying to keep it clean.  Nevermind my unused holes in the top of the chassis. Ill get some KO blanks for those. Hopefully I wont make that mistake again. I took a lot of notes during this build so that I can make some small changes to my templates and layouts.


Oh...and my switches are upside down 

[tubenit]

That is really a GREAT looking build!  Thanks for sharing your success.

With respect, Tubenit

[p2pAmps]

Very nice work, how does she sound?

[Paul1453]

Very nice work, how does she sound?

I like your layout, especially how your well is about as far away from your outhouse as possible.   
I expect she is nice and quiet as far as hum and crosstalk, and rather loud when you crank her up.   

[hesamadman]

Thanks everyone. It is sounding great. Ill try to get around to posting final photos of it put together and some clips.

[Willabe]

Looks really nice! 

[hesamadman]

Thanks again everyone.


You know, ive never built anything with a tube rectifier. I was thinking about filling one of these incorrect holes i drilled with a rectifier tube and having switchable between SS and tube. I know I've touched on this a tad in the past but what are some differences in tone with the two? I would like to do a true SS bypass if possible. I see some schematics for hollow state (with diodes in parallal with the tube) but I would like to hear the difference between the two.

[DummyLoad]

very nice work.


--pete

[hesamadman]

Can anyone confirm if this will work ok?


I would like to add this and some sort of mod (not sure what yet). I will have the rectifier switching on the back and I would like to add something on a switch on the front panel. I tried adding a cut knob and finding a particular setting on the pot that I liked and just replace the pot with a resistor of the value in which the pot was set, but I didnt really like it. Its why I never include the cut knob anyway I guess. May just do some switchable cathode values.

[sluckey]

That will work.

[hesamadman]

Thanks Sluckey