Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: markmalin on April 04, 2020, 02:38:24 pm
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Hey guys. Please excuse the hand drawn schematic, and my lack of design experience...
I've laid out this Marshall style tube preamp pedal. It's based on a standard Marshall preamp (JCM800 or JTM), with Rob's switch mod on V1B's cathode resistor. I've built a few of these, have custom enclosures all set up, it sounds amazing and I was ready to start building and selling them, but I have a question I need some help with. In it's current incarnation, if I turn everything up to 10 and play into it with a guitar fitted with humbuckers, it's capable of putting out a signal in the rage of like 6 to 10 VAC. The extra 1M Audio "Output Level" pot is used to balance this out - this way I can dial in my favorite Volume and Master settings for the tone I want, then dial back the Output Level to give a lower level signal into the front end of my amp.
My question is - Is there a simple way I can reduce the max AC output to something more reasonable, like 1.25VAC? Granted this IS a preamp, not an effects pedal, and my original intent was to consider this something you could either run into the front of any amp to get that Marshall sound (which it does well), or run into the Poweramp In or Return on an amp. I've used it both ways, it works great, but I worry about someone doing something stupid like cranking it all to 11 and risk damaging their amp.
Humbly
Mark
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The 1Meg is Bass, the 25k is Mid.
> reduce the max AC output to something more reasonable, like 1.25VAC?
Reducing signal is the easiest thing in audio. So what is the output now? Figure your desired loss, design a semi-fixed pad to reduce it.
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The 1Meg is Bass, the 25k is Mid.
> reduce the max AC output to something more reasonable, like 1.25VAC?
Reducing signal is the easiest thing in audio. So what is the output now? Figure your desired loss, design a semi-fixed pad to reduce it.
Yah, that was a label mistake. Right, 1M Audio is Bass, the 25K is mid
I'm new to the term "pad", but I've heard it before. Basically a passive attenuator, right? I see there are a few types "T" "pi" and "L". Seems simple enough, I guess. If I'm currently seeing up to 10 VAC and want a max of something more in the 1.25 range, I should be able to figure that out.
When you say semi-fixed, you mean use a variable resistor (trim pot maybe) as one of the resistors so I can dial it in?
Mark
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The two cascaded pots at the end ARE pads, and redundant. Change one for heavy semi-fixed loss.
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Just use a simple 10:1 voltage divider, maybe 100K:10K
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The two cascaded pots at the end ARE pads, and redundant. Change one for heavy semi-fixed loss.
Thanks PRR. I love the picture of the glob of resistors - I think you made my day.
I totally get the voltage divider. Thanks. I guess the second pot is redundant.
Mark
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Just use a simple 10:1 voltage divider, maybe 100K:10K
Thanks for breaking it down slucky. Sometimes my head is a little thick...
Mark