My First Amp build

[thelastleroy]

Hi guys. I'd like to build up a single-channel Marshall style amp with a medium-high gain capability. I have some transformers and parts from my old Mesa Nomad 45 amp, which runs 4xEL84 tubes.

I was originally thinking of a JCM 800 preamp into the EL84 power section, but anyone on the net who's tried this (that I've found) has run into problems with overloading the power section. Any attempt to pad down the signal results in lower gain (defeating the purpose of the high-gain preamp).

So I guess my options are to build a Modified TMB-type amp with 4xEL84 or try to run 2xEL34 and the 800 circuit on my transformers and hope they don't fuse.

I'm looking for some advice on this, I'm dying to get started.

Pat the Cat

[gldtp99]

I built an EL84 x 2 (cathode biased) amp with a JCM 800 2204 preamp and it works fine-----i also did a somewhat similar fixed bias 6V6 x 2 build----- i didn't run into any problems on either build---i later changed both of them to SIR/#34 Mod specs and added a Lar/Mar PPIMV to the 25 watt 6V6 amp---- and an adjustable NFB pot and a Pentode/Triode (halfpower) switch on the EL84 amp.
The EL84 amp was used for the lead guitar track (with wha pedal) in this old clip: http://soundclick.com/share.cfm?id=9924119
It's in the white smallbox head cab in the small pic on the linked page----- more pics in pre-#34 Mod specs: http://s46.photobucket.com/albums/f112/ ... 04%2014-7/

with your transformers you could easily run an EL34 x 2 or 6L6GC x 2 output section----your PT heaters have to supply at least 3A just to power EL84 x 4 (.76A x 4= 3.04A)---i built a #36 Mod experiment with 6L6GC x 2 and a Magnetic Components 40-18001 OT http://www.classictone.net/40-18001.html --- a guy came over to talk about a possible amp build and he bought that one on the spot after playing it---it didn't even have a faceplate, just Sharpie labeling the controls on the bare chassis frontedge---so don't think that you have to run EL34's to get a good sounding build....................gldtp99

[thelastleroy]

Thanks for the quick reply. I'm thrilled to hear I can run the EL34's with my iron. Transformers were going to be the budget breaker on this build, as I can fab up the chassis myself and salvage some parts from the old amp. I'm going to go with a JCM800 circuit.

I'm going to update this thread with build pics as I go along. Wish me luck!!

[thelastleroy]

Sorry, it's been awhile since my last post regarding this amp build. I've stripped all of the components from the Nomad 45 head and I've ordered a full compliment of parts from various sources. Im confident that my PT will keep up, and I've got a big beefy Hammond choke on the way, but that OT is starting to look mighty small. Theoretically, if I don't max out the master volume, my 40w OT should be okay, if a little hot, correct?

Just trying to go over the "ifs" while I wait for the parts to arrive.

Any preferred methods for using 1 input jack with a switch for the hi/low input modes?

[thelastleroy]

Moving right along with the project:

I've got most of the circuit board built up. There was a mix up with resistor values from one of my suppliers, so I"m still waiting for the 100k resistors and the diodes to arrive. In the meantime I'm working out the wiring for the power supplies, tube sockets etc.





I believe it's time to wire up the heaters. When I checked the schematic for my old amp, I noticed that my PT has a 50v auxilliary tap for things like channel switching, record out circuit etc. Check out the page that says power supplies:

http://www.freeinfosociety.com/electron ... omad45.pdf

Any good ideas for what to do with this 50v tap? I've thougtht about a DC filament supply for one or all of the preamp tubes, but wasn't sure if this would pull too much current out of that tap. I don't have the specs for this transformer.

little help?

[thelastleroy]

So... never mind about the heaters. I got my finger out this weekend, and as of very early this morning, my amp is complete. The amp is very quiet noise-wise, and sounds really great to my ears. I'm actually going to have to become a better guitar player to keep up with this tone. I'm toying with the idea of adding a "hair" switch with a Zener/1N4007 combo (gasps from the audience), for those self-indulgent times or when I want some more gain to hide my sloppy chops. Very cool amp, I'm glad I did this conversion. That Nomad I had was a pain; way too much going on and something was always broken.

Here's the pics, sorry about the blur/size, its from my IPOD camera.















Can't wait to get this thing out to Jam on friday for some real testing. I live in a townhouse, so I'm limited to testing at or slightly above bedroom volume. This particular master volume setup seems like it changes the sound of everything at different volumes, am I right? Maybe a master volume mod for me in the future. Everyone seems to rave about the LAR/MAR Master. I'll read up on it.

If anyone's interested, I followed this Layout/schematic/BOM from dreamtone:

http://www.dreamtone.org/pdf/NEW2204Lay ... amTone.pdf

and this blog really helped me to put things together:

http://www.joepopp.net/jcm8002.htm

[thelastleroy]

Okay, so a week or so of playing this amp, and I absolutely love it. The simplicity in design of the 2204 circuit is a breath of fresh air to me, after owning an overcomplicated amp for years. I've installed a NFB defeat switch, and I would like to put a pot inline with the stock resistor to allow for custom tailoring of the the NFB signal, when It is engaged. Is 250k a good value for this pot? I'm going to use a trimmer for this, so that I'm not fiddling with it once it's set.

The other thing I'm noticing with this design is that it is VERY picky about the V1 tube, when it comes to noise. I've switched out 3 or 4 12ax7's so far, as they were too noisy in this position. I found a good way to test this is to turn up the master to maximum with the gain all the way down. I'm finding that the JJ preamp tubes are particularly prone to noise (2 out of 3 brand new tubes weren't usable), but that might just be the fact that I didn't opt for "extra testing" from my online supplier. I get the feeling that when these options are available you end up with the tubes that fail these same tests if you don't pay the extra bucks.

What are your suggestions for an Ideal high-gain, low noise preamp tube for V1 on a 2204?? Currently I'm using cheap, new-production tubes (JJ, Sovtek, EH etc) mostly so that I don't destroy anything expensive. Once I'm sure the amp is reliable after a good many hours of full-volume rock, I'll drop some dough on primo glass. What do you recommend? I've got the JJ E34L tubes in the power section.

[gldtp99]

Congrats on the build/conversion--- nice work---- as far as preamp tubes go, any tube of any brand can be microphonic/noisy in V1--- if you buy a certain brand of Old Production 12ax7 there's no way to tell if it's suitable for V1 in your amp until you try it.
When i first power up a new amp build, it will usually have Sovtek 12ax7wa's installed because that's the tube i put in the tube sockets while assembling the amp--- it keeps the tube socket solder tabs steady and can prevent solder from clogging the pin holes if I have a bad solder day. And i have loads of good, used Sovtek 12ax7wa's.
I do initial voltage/function tests with the Sovteks and they are almost always stable and non-microphonic--- If everything checks out then i start swapping preamp tubes to see what kind of tone i can get out of the amp----rarely do the "assembly tubes" (Sovteks) stay in an amp.
i'll try out many, many brands of New and Old Production preamp tubes to see what sounds best with that particular amp--- often several different examples of a brand that seems to sound particularly good (maybe trying 6 different RFT ecc83's in V1 to see which one has the best tone/stability-- or RCA 12ax7a's--- or several I63 Mullards, Tungsram, Phillips, GE, Sylvania, etc, etc).
Sometimes a Tung Sol 12ax7 RI will work as well as a big $$$ tube in some positions in some amps---or a Chinese, JJ, EH, and yes---even sometimes a Sovtek (WA, WB, WC, LPS).
People who try to tell you that a particular brand of tube will sound "best" in your amp usually don't know what they're talking about--- Or they don't have much experience with swapping tubes and they just recommend whatever they last bought at the guitar store---- there are general expectations for some brands of tubes (as far as what flavor of tone to expect) but i'm often suprised at what tubes sounds best in any amp.
you just have to plug them in--- play the amp---and listen----whatever sounds good, is good-- ...........................................gldtp99

[thelastleroy]

@gldtp99:

I agree with you entirely about trying as much variety as possible and using your ears. It's funny, because I have about a dozen mixed 12ax7 types, and the best sounding one for V1 (before it went noisy) was a throwaway Chinese non branded tube pulled out of a practice amp.

I'm going to wait until my local supplier has a "garage sale" on some of the NOS types, and pick up a few. Any insight into the NFB pot idea?

[gldtp99]

The 250k linear pot would be fine for NFB adjustment--- i've used NFB adj pots in some builds but i don't bother anymore, i just find what NFB value i like and go with it--- it does effect the overall tone/feel and a trim pot is not a bad idea--- but in the builds where i used one, it didn't seem to give enough "real world", easy to understand, range of adjustment to warrant keeping a NFB adj pot on the front panel.
I mean, i understood the function of the NFB pot and could use it as intended but others didn't seem to "get it" and looked at it as an extra knob that had little effect---- The effect of the control, through it's total pot sweep, wasn't as dramatic as a Gain, pre MV, or TMB pots. If you're the only one who's ever going to play the amp then this isn't a problem.
So most of my builds that had a NFB adj pot on the front panel were changed to have a Lar/Mar PPIMV in that position--- this has a dramatic effect and is quite useful--- and when turned up is not noticable in the circuit.
One exception is the 14/7 watt 2204 w/#34 Mods--- has cathode biased EL84's--- this one keeps getting borrowed by a local studio for client recording projects when the client's amp(s) aren't cutting it--- It's over there now and i need to go pick it up again--- been there for a couple of months--- and it still has a NFB adj pot on the front panel.
It's the leads and one rhythm track on this clip: http://soundclick.com/share.cfm?id=9924119