Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TweeDLX on May 16, 2010, 09:20:52 pm
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I'm attempting to "design" an amp by assembling various parts from other amps. I have a schematic drawn up (I just used the Allen "Old Flame" and added/deleted what I needed) and would like someone to look at it and point out any obvious bloopers. I have a 1K/150K resistor combo after the volume that may be a problem. Anything else you can point out would be nice, too. Thanks! I seem to be having trouble posting my schematic. I'll try again.
http://i274.photobucket.com/albums/jj257/paintrdude/Amps/Super-Duper-amp-schem.gif
Mike
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The cap for your Brilliance control should probably be smaller. 0.0047uF is a typical value, as you want it to shunt only highs and not most of your signal.
I see you made one cathode bypass cap 10uF. Have you tested it and found that's a perfect value for you? I ask because as you start dropping the capacitance below 25uF, you're not guaranteed to hear a difference until, likely, 1uF, and at most 5uF. If you use 1uF, it seems to tighten up just the lowest octave without getting obviously bright.
And yeah... what's the idea on the 1k/150k divider?
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And yeah... what's the idea on the 1k/150k divider?
Well...I wasn't intending to put a voltage divider there. This is my first foray into amp "design". I got some of the ideas for this build from a Trainwreck schematic. I have recently modified a SF Princeton NR and my Mission 5E3 to be more "wreck-like", and liked what I came up with them. The 'wreck schematic had an 820 ohm (I used the 1K because it's what I had) series resistor from the volume pot. It also had a 56K-150K resistor to ground before the 3rd gain stage. Because I used the Matchless Clubman volume and tone layout, I wound up with the series resistor and the 150K on the same stage. Would it be best to ditch both of them?
As to the cathode bypass caps: I had great results using a graduated series of 25uF/10uF/4.7uf in my modded Princeton, so I tried it again.
The .01uf Brilliance cap value came from the Clubman schematic. I used the .0047uf on a previous build, and it sounded good.
Thanks for your response. Any other problems you see?
Mike
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Would it be best to ditch both of them?
Without researching the trainwreck schematic to figure out why those parts were use in the other amp, it appears best to ditch them. The 150k to ground is not needed as a grid reference, because the volume pot already performs that function. The 1k doesn't perform any useful function, although if it was right at the tube grid pin, it could be used as a grid stopper; that's probably not needed in your case, although you could use it if you wanted. The 150k, if used as drawn, would only serve to lower the input to the following tube stage.
I had great results using a graduated series of 25uF/10uF/4.7uf in my modded Princeton, so I tried it again.
Successful taste-test is the best design, so if it worked before, do it again. I'd argue there are good reasons to figure everything out with pencil and paper, but even the best designed frequency-shaping still needs to be heard to know if it's right. I'd keep your cathode bypass caps as you've drawn, until you hear that the completed amp doesn't sound right.
The .01uf Brilliance cap value came from the Clubman schematic. I used the .0047uf on a previous build, and it sounded good.
Since you know the 0.0047uF cap sounded good, do it again.
the frequency at which the control stops being effective (the "lowest highs" that it changes) depends on the cap value and all parallel resistance around the cap. You could calculate, but it's faster to use a smallish cap and see if the control affects the range of frequency you want it to. If you want more upper mids changed in the finished amp, or the control doesn't impact a low enough frequency, you simply incrementally make it bigger until it performs the way you want it to.
The Matchless may have had other design factor impact the size of the cap. I used to own a Clubman, and can tell you there are at least 2 different circuits for that amp, possibly more. The schematic floating around shows a 6SH7 in the preamp, and the majority of Clubman amps didn't use that tube (it's cheap enough, but I guess people didn't like it not being a stock item at Guitar Center). If I haven't thrown it away, I have a schemtic of the Clubman that uses an EF86 buried in storage somewhere. I couldn't tell you without a hard look at that if there are other circuit changes between the 2 versions of the amp.
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The .01uf Brilliance cap value came from the Clubman schematic.
Both Clubman schematics I have are with 0.0047µF and 0.005µF brilliance caps.
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Damn! I drew my schematic looking at a JPG I made of the Clubman schematic (so I could copy/paste). To my eyes (weak at best) it looked like a .015uf cap, so I settled on .01uf. Looking at the PDF file clearly shows .005uf. :embarrassed: Thanks for clearing that up!
Mike
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As to the cathode bypass caps: I had great results using a graduated series of 25uF/10uF/4.7uf in my modded Princeton, so I tried it again.
I've often used 10/2.2/.68 as cathode bypass caps with very pleasant results.
It is a common mod, simple and cheap. use a ON OFF ON spdt switch to move the 2.2 or 10uf in parallel with the .68
I find it makes a real difference when going from a single coil to humbucker
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I generally tweak everything once I have the amp built and a speaker picked out. Cathode bypass cap values are usually the first to change...
I have a couple other questions about this build. My PT puts out 330-0-330 @120ma. Is that going to be enough to run 2 x 6L6gc's? I have the 125C1A choke. Will that be a good fit, or should I go for the 125C3A? Sorry if the questions are lame...
:grin:
Mike