I have a couple questions about an amp schematic?

[vintagelove]

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

 

[Adolph Ernst]

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.

 

[abbey road d enfer]

One of the reasons the OP needs explanation is that there are some mistakes in this schemo...

 

[gyraf]

basically, it just switches between two different gain/master settings. Not two channels, but two settings of gain/master

 

[vintagelove]

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

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. Also, I don't mind paying a couple bucks as I had a couple other questions. Anyway...


1 - I get that it's really a "single channel" or really, a single signal path, and that the fets are changing the feedback loops and controlling the gain pushing through the tube clipping circuit. I just don't see the exact signal path. I would really appreciate (and by that I mean pay for) if someone could download the schematic above, and trace the feedback loop in green for the clean, and red for the distortion.

I'm confused by what looks like a feedback loop between the first two gain stages? I've not seen that before.

The tube master fet is just controlling whether that pot is in the feedback loop or not, and that controls the gain. Correct?



2 - My other important question... I would ideally like a little more bass, but I understand why they are rolling it off with both "channels" going through the clipping circuit. 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... it would be perfect here as the lows could be added post distortion.


3 - I take it the .047 cap in between tube stages is most responsible for cutting the lows? ... Once you get used to big old fenders, you can't help but miss that big full low end.



@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.



Thanks again for your time and responses. Take care.

 

[abbey road d enfer]

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.

A lot of members here are inveterate guitar players.

I'm confused by what looks like a feedback loop between the first two gain stages? I've not seen that before.

I don't see that. What makes you think so?

The tube master fet is just controlling whether that pot is in the feedback loop or not, and that controls the gain. Correct?

Actually, when the FET is off, the nominal pot value is what defines the stage gain.

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...

For good reason. Actually some SS amps had that but it makes things more complicated and there's the risks of instability.

it would be perfect here as the lows could be added post distortion.

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

@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.

The opamp that drives the output transistors has its inputs crossed, and the tube cathodes have no connection to ground.

 

[vintagelove]

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.

Many thanks.


So the "distortion channel" is when the fet is off, which puts the tube master pot in the feedback loop.


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...

Ok, so the first opamp has unity gain correct?

I'll skip wasting the space typing what is inevitably wrong, suffice to say, I just don't understand the circuit around the second opamp beyond the basic function of changing the level of gain depending on if the clean or distortion is selected.


Thanks again for the rest of the advice. Have a great day.

 

[abbey road d enfer]

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?

Correct.

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...

I suggest you read an opamp textbook. Figuring out gain is not more difficult than th erule of three.

 

[vintagelove]

Correct.

Correct.

I suggest you read an opamp textbook. Figuring out gain is not more difficult than th erule of three.

Much appreciated.

I will need to read more about opamps for sure.

I think my issue is more to do with the circuit/signal flow. For example, I get how the clean gain pot works in a feedback loop, I just don’t see how that feedback makes its way back through the feedback loop without the fet, but isn’t passing through the tube drive pot (which it doesn’t in clean mode).

Shocker, I need more experience, looking at more varied circuits, as well as more reading.

Thanks again

 

[abbey road d enfer]

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.


Drive:


Analyzing the circuit is not super simple, but here are teh results of simulation with LTspice:

 

[Newmarket]

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

SS ? Meaning ? I assume Solid State except I can see "Valve Stuff" there. ("Tube" if you must )

 

[abbey road d enfer]

SS ? Meaning ? I assume Solid State except I can see "Valve Stuff" there. ("Tube" if you must )

There's only one double triode, the rest is all opamps and transistors.

 

[vintagelove]

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

Hello, just wanted to say I really appreciate you taking the time to answer my questions. If you have a PayPal, please pm it to me, I'd like to buy you lunch.

Your post reminded me to dig out the old circuit simulator, which will answer most of the remaining questions I have. Thanks again.

SS ? Meaning ? I assume Solid State except I can see "Valve Stuff" there. ("Tube" if you must )

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.

Good sounding amp though, especially with a Strat/tele. LOUD too.

 

[Voyager10]

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.

 

[Cqwet Dbdfte]

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.

 

[vintagelove]

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.

Thanks for the advice.

The thing is, the amp isn't lacking bass like some other high gain amps. So I'm not sure how much more low end I could put through the clipping section before it gets flubby... could be good on a switch though.

That's why I was looking for solutions in the power amp (which I will definitely try what Abby posted). Of course, the easiest mod is to just throw an eq in the fx loop... but I amp hoping for something that requires less velcro.


Thanks again

 

[Voyager10]

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.

Pretty sure they should be grounded, the grid/cathode voltage won't change otherwise & I can't see how it would amplify anything.

The op-amp driving the output transistors is correct, I think, because they're both common-emitter connected, which inverts the signal.

 

[Cqwet Dbdfte]

If not a schematic error, the tubes may be used as signal rectifiers? There is enough gain available in those TL074's...

 

[Admiral-dk]

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

 

[vintagelove]

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

Hi, I ran the tone stack through the online calculator, and this was the result. If you see something I entered in error, please let me know.

Seems to be good in the low end here, but obviously there is rolloff elsewhere.