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help with amp building

7/20/1998 10:00 PM
help with amp building
Okay I'm a beginner at this amp building thing, and even though I already posted something, I still have a few questions.  
First a question about chassis building. Would it be possible to get a metal shop to make you one, and would a simple steel box be adequete. What is the best gauge of steel to use?  
I am going to buy a champ parts kit from STF electronics. For this amp, what would be a good speaker to use. I was thinking about a Celestion Vintage 30, or possibly a Greenback or something like that. Are these good speakers?  
Would it be possible to make a kind of "spectrum analyzer" for an amp. one that uses like 3 LED bar graphs, and shows the volume of a specific range of frequencies for each EQ control. Has anybody done this or heard of it before, and where would it be possible to obtain a schematic of this.  
As for component placement, I also need a little clarification. The capacitors and resistors are mounted on some type of board (different depending on different construction methods) and have leads that connect the appropriate areas to tube sockets, transformers and the like. The transformers are placed on the chassis, with the tube sockets, with leads running through (grommeted) holes to the places neccessary. The controls (pots, switches etc.) are mounted on one side of the chassis, so one can see them from outside the amp, and these should be insulated from the chassis ground, for whatever reason. Is this right?  
I need a good DMM. Has anybody tried the ones from New Sensor, or are these crappy.  
And last but not least, If I wanted to, after I have built my champ, could I add a couple of gain stages (one tube) to my amp, modify the wiring of the amp, and have a 4 gain stage (two tube) preamp. Also why do some marshalls have like 7 12AX7 tubes? Do they have 12 gain stages and a phase inverter, or is each tube used as a single gain stage. Is this common practice among large manufacturers, and for what reason.  
Sorry for the long post (Again) , but I need a couple of things clarified. Thank You Very Much for any help.
7/21/1998 1:23 AM

Hey Richard  
As for the 12" speaker in a champ circuit..  
I think that will be a little big for a 3.5 watt output. It might sound painfully bright. I would check on an 8" or so. Check WeberVST to see if they have a good alnico 8".  
good for smooth natural overdriven tone.  
For a small champ you could buy a hammond chassis that you could build a box around for probably 20-30$ Check Allied, Mouser and other electronics supply houses for this.  
You are correct about the trannies and tubes on the chassis, and the "little parts" on the inside on one of several types of boards.  
The New Sensor DMM sucks. Mine lasted 3 days... The rotary switch broke, and when I tried to fix it.....  
Marshalls that have 7 preamp tubes have more than one channel. I think 12 gain stages on one channel would feedback uncontrollably ALL of the time. They also have reverb driver tubes etc. You could probably gain up your champ, but I'm not the one to ask on that.  
I would try and drive the input a little for solos with a dist. pedal like a FF or TS.  
Spectrum Analyzer? you could probably do it, but I would buy a rack for that. It is such a small amp anyway....  
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7/21/1998 11:08 AM

A quality 12" guitar speaker sounds awesome with a small 3-7 watt amp!! Such a blast.....GREAT fun!  
Unless you are recording the amp in close proximity with an 8" speaker, you'll be BITTERLY disappointed with a wimpy 8" speaker after you've heard it in a real cabinet.  
I can't stress this enough!  
I recommend at a single 12" speaker with the little 5 watt SoulKicker jr.amp kits I sell but a 10" sounds much better then the 8". NO CONTEST.  
To hear one of these in a 4 x 12" bottom is unbelievable. Clean to mean!  
After you build a small power amp like this, you'll be looking for excuses not to use your 50 watt amps anymore.  
This will probably be the first time in your life you'll be able to hear real power tube distorion at full volume and not take your head off!  
Contact me by Email for more Blue Stone Amp info.  
7/21/1998 3:32 AM
Dave H

"The controls (pots, switches etc.) …. should be insulated from the chassis ground, for whatever reason. Is this right?"
No, the pots and switches can be grounded to chassis as long as you don’t solder components to the pot body. There’s no electrical connection between the pot element or switch contacts and mounting bush so you won’t make a ground loop by doing this. The exception is the guitar input socket. Use an insulated socket here and have a single point (star) ground system inside the amp.  
7/21/1998 7:12 AM
Jack Orman

"Also why do some marshalls have like 7 12AX7 tubes? Do they have 12 gain stages and a phase inverter, or is each tube used as a single gain stage. "
Some of the 12AX7 stages are used as cathode follower buffers. It raises the tube count but I don't know how much sonic advantage it offers - keeps the gain up by driving the eq stage with low impedance; I'm sure there are other reasons to do it this way...  
regards, Jack
7/21/1998 2:04 PM

Sounds like you need to send out and get alot of supply books,try antique can get alot of chassis from them and about every other parts,try new sensor,fair radio sales,mcm,digi key, and you can find alot more,as they say it pays to shop around! all these places will send you a catolog,there are alot more places,maybe some others will give you some info on some.As far as the multimeter,I agree that I have a bunch,I got the one from new sensor and was bad when I got it!the same problem the dial fell apart!invest on a good one because you will be using it alot and a good one will be more acurate and last longer,you can find them every where,flea markets, ham fests, etc.[Richie]
7/25/1998 9:00 AM
Steve F

Hi, Richard.  
For a quick and dirty chassis solution, I had  
a sheet metal shop bend me a fairly long U-shaped channel from .094 aluminum. I was using it for a specific group of projects, so I made the width 8", with 2" "legs". Then, when a certain length chassis is needed, I can  
just chop off that amount and get to work.  
This method leaves the ends open but I've not found this to be a problem since the aluminum is so thick - it's very rigid. I'm assuming you plan to mount the chassis in a cabinet.  
Aluminum is a lot easier to work with than steel and can be buffed to a nice finish without worrying about taking off the plating.  
Con's - can't solder to it. Not terribly important, could even be an advantage as you will be less tempted to create a Frankenstein  
grounding scheme.  
I'm not sure if there is any substantial difference in shielding properties. I doubt it - you see a lot of high quality snake cable with aluminum foil shielding.  
If you do buy a box off the shelf, you might consider aluminum just because it's easier to cut with the kinds of tools most folks have.  
Regards and good luck. Steve
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