vintagelove
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- Oct 26, 2013
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you could just ask about this in the regular forums. There's nothing special going on in the schematic and the swithing is nothing more than couple JFETs used as switches.Hello, I have a few very basic questions about the signal flow on a guitar amp I picked up (it's SS, and it has some channel switching I've not seen before). I'd happily buy someone lunch who could answer them for me. Thanks, schematic posted below.
PS, payment via PayPal
View attachment 132696
you could just ask about this in the regular forums. There's nothing special going on in the schematic and the swithing is nothing more than couple JFETs used as switches.
One of the reasons the OP needs explanation is that there are some mistakes in this schemo...
basically, it just switches between two different gain/master settings. Not two channels, but two settings of gain/master
A lot of members here are inveterate guitar players.I appreciate the responses. I would have posted it in the regular forms, but I wasn't sure which one it was really appropriate for, as there isn't too much discussion of guitar amps here.
I don't see that. What makes you think so?I'm confused by what looks like a feedback loop between the first two gain stages? I've not seen that before.
Actually, when the FET is off, the nominal pot value is what defines the stage gain.The tube master fet is just controlling whether that pot is in the feedback loop or not, and that controls the gain. Correct?
For good reason. Actually some SS amps had that but it makes things more complicated and there's the risks of instability.I was wondering if there was an easy way to implement a "resonance" circuit like you see in tube amps (feedback loop in the power amp that emphasizes lows). I never see that in SS amps...
You may want to try fiddling with NFB on the output stage, by sticking an RC (R in series with C) across the 220k resistor. Try 68k and 4.7nFit would be perfect here as the lows could be added post distortion.
The opamp that drives the output transistors has its inputs crossed, and the tube cathodes have no connection to ground.@abbey road d enfer .... I don't expect you to explain this in detail, but if you could just point to the sections where you see a mistake, that way if I ever need to do any service in the future, I'll know to be wary of those areas of the schematic.
Many thanks.A lot of members here are inveterate guitar players.
I don't see that. What makes you think so?
Actually, when the FET is off, the nominal pot value is what defines the stage gain.
For good reason. Actually some SS amps had that but it makes things more complicated and there's the risks of instability.
You may want to try fiddling with NFB on the output stage, by sticking an RC (R in series with C) across the 220k resistor. Try 68k and 4.7nF
The opamp that drives the output transistors has its inputs crossed, and the tube cathodes have no connection to ground.
Correct.So the "distortion channel" is when the fet is off, which puts the tube master pot in the feedback loop.
Correct.Ok, so the first opamp has unity gain correct?
I suggest you read an opamp textbook. Figuring out gain is not more difficult than th erule of three.With regard to the first two stages. The short version is I don't understand the circuit. The only SS things I've done were either simple circuits, or work that I did on my old MCI 428 console... which... was pretty simple circuits...
Correct.
Correct.
I suggest you read an opamp textbook. Figuring out gain is not more difficult than th erule of three.
Hello, I have a few very basic questions about the signal flow on a guitar amp I picked up (it's SS, and it has some channel switching I've not seen before). I'd happily buy someone lunch who could answer them for me. Thanks, schematic posted below.
PS, payment via PayPal
View attachment 132696
There's only one double triode, the rest is all opamps and transistors.SS ? Meaning ? I assume Solid State except I can see "Valve Stuff" there. ("Tube" if you must )
Here are the the equivalent schemos of input stage in both modes:
Clean: you see the drive pot is shorted, so only the other pot (Gain) is active.
View attachment 133349
Drive:
View attachment 133350
Analyzing the circuit is not super simple, but here are teh results of simulation with LTspice:
View attachment 133351
The amp is a Tube Driver amp (as in the famous tube driver guitar pedal). Ironically, despite the name, the emphasis on the word TUBE is ... a bit... well... let's call it good marketing.SS ? Meaning ? I assume Solid State except I can see "Valve Stuff" there. ("Tube" if you must )
Thanks for the advice.The way to analyse something like this on paper is to consider the network as a voltage divider, from the op-amp output to its negative feedback input.
With the 'Drive' mode engaged, there's a first stage of divider (R3-C3/R2-C2) followed by a second stage (potentiometer / R1-C1). Very roughly you get 2.5x from the first and (up to) 500x from the second, which is why max gain is (in theory) in the 60dB (1000x) range.
You can also see that the LF gain depends on the R1*C1 value. For 1K and 100nF this comes out as 1.5KHz (-3dB), as Abbey Road's plot demonstrates.
Increasing C1 will lower this frequency and increase the bass level.
Pretty sure they should be grounded, the grid/cathode voltage won't change otherwise & I can't see how it would amplify anything.Is that tube supposed to have a floating cathode?
10k on the grids to cathode is not going to be enough for gridleak bias.
Strange.
Beside the 'Missing Cathode resistor' in the schematic ....
I see a bigger than normal Treble Pot value and a tenth of normal size Bass Pot => increased Treble and a huge Bass cut in the Tone Control Stack .... (the rest of the values there do have influence too). But I can't say that it surprice me as a lot Guitar Pedals (and other related Gear) often are missing the bottom of sound - especially on older gear.
Per
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