2nd Gen OS and #124 Mashup Questions

[dvgreen]

EDIT 30 Nov 2022: BTW: the schematic in this post was put together by me (fairly new to amp building and tweaking) and this journey could go anywhere - if you happen to be looking for a proven 2nd gen hybrid a bit like this, your best bet might be here:
https://ampgarage.com/forum/viewtopic.php?f=4&t=29845
2nd Gen hybrid
Post by erwin_ve » Sat Jan 28, 2017 5:22 am

EDIT 27 Aug 2023: I finally finished this build and couldn't be happier with the way it turned out. Thanks very much for the comments above that helped finalise the circuit for this build. Here s a video showing the overall build and is filled with sounds from the amp:



I'm planning a 50W Dumble OS style amp build based on some layouts and schematics from this forum. Something along these lines:

• a power section from a 2nd Gen OS style - as per https://ampgarage.com/forum/viewtopic.php?f=5&t=12870 - 2nd Generation Final v2.jpeg - Layout drawing by Tony Albany

• the preamp section, and passive send and return from a #124 OS style - as per https://ampgarage.com/forum/viewtopic.php?f=5&t=5719 - 124_Dumble Overdrive special Complete Layout3.jpg - Layout drawing by Martin Manning & Tony Albany

• adding a presence circuit in the style of the #124 layout drawing that can be switched in or out of the signal chain,

• adding a 'clean blend' control into the overdriven signal in the OD channel. I sometimes like using a y-cable to blend distortion and clean in pedal chains, and if I'm not mistaken the Klon pedal does something along those lines internally, so thinking to try something like it in this build

I'm not an electronics tech or engineer, and have only a basic understanding of circuit design, but have built and modified valve amps and many other guitar related bits and pieces. I've found a lot of answers and information in this forum and others, but have some specific questions in relation to this design and the 'mashup' schematic I've put together and attached, that I was hoping this forum might be able to help with:

2nd gen and 124 mashup schematic v1-1.png

• 1). When the presence circuit is switched on (dark blue DPDT switch on the schematic) the current design has a 4.7k NFB resistor (taken from the #124 layout) switched in instead of the 820R NFB resistor (taken from the 2nd Gen layout). Is it necessary for the NFB resistor to be set to 4.7k for this presence circuit to work? Is anything about this design an obvious bad idea, or have the potential to damage anything? My understanding is that there would be less negative feedback (or maybe even none in the milliseconds while the switch is used and changes position), so higher output and load on other components, but I'm pretty keen to just give it a try, but not if it might damage something

• 2). The 'clean blend' idea might be flawed in any number of ways but I'm keen to try it (dark blue items in the OD channel on the schematic). The signal is split prior to the OD trim, and the trim value changed from the recommended 250K to 500K, and there is a new 'clean blend' pot in parallel with the trim (also 500k). The thinking behind that is it hopefully gives the proceeding stage the same load to ground as it would otherwise have had (I have no idea if that is necessary or even actually achieves that though). The clean signal is mixed back into the overdriven signal via a 150k resistor to match the 150k resistor at the end of the OD stage (to act as summing resistors - again I have no idea if that will actually work properly). The question about this is much the same - is there anything about this idea that is obviously wrong or something important that hasn't been considered?

Any answers or tips would be greatly appreciated, but I'll just go nuts and build it anyway and likely post a video of this build with samples to youtube afterwards if anyone is curious to see how it all ends, and what this amp sounds like (whenever that might be).

[martin manning]

The 'clean blend' idea might be flawed in any number of ways... ...is there anything about this idea that is obviously wrong or something important that hasn't been considered?

Yes. You've created a positive feedback loop around the OD stages.

[dvgreen]

OK - Thanks for the feedback! I'll research what that means in detail (beginner) and see if I can make it work as hoped.

[erwin_ve]

You're a beginner. Why don't stick to a proven and well sounding existing specimen?
Frankenstein amps need tweaking.

[dvgreen]

I'm thinking worse case scenario is I'll need to remove my mods. I can tweak it.

[dvgreen]

It's a good question though. I started by wanting to build from the 2nd gen layout but then also wanted one or two features of the #124 layout then ended up with this design and thought I'd try a few mods along the way. I've built amps before but definitely a beginner when it comes to design.

[erwin_ve]

It's a good question though. I started by wanting to build from the 2nd gen layout but then also wanted one or two features of the #124 layout then ended up with this design and thought I'd try a few mods along the way. I've built amps before but definitely a beginner when it comes to design.

If 2nd gen is what you like: search for 2nd gen hybrid. Is some of the features you listed, and is a great sounding amp.

[martin manning]

All of your changes to/from 4.7/5uF or 0.0047/0.005 are negligible. Modern cap values are consistent with their tolerances. Basically 47 is the new 50.
The resistor feeding the bias supply will probably need adjustment to get the bias range in line with the needs of the output tubes using the selected power transformer. The schematic value is simply a good starting point.
The 8k2 source resistor on the FET must be selected to suit the particular FET, and the amount of distortion you want it to introduce. See https://ampgarage.com/forum/viewtopic.p ... 91#p211891

[dvgreen]

Thanks for the replies - very much appreciated.

"If 2nd gen is what you like: search for 2nd gen hybrid. Is some of the features you listed, and is a great sounding amp."
I thought I did take a look a couple of these, but somehow missed the 2nd gen hybrid - yep, that is almost exactly what I'm after! I'll go though it and compare it to what I put together component by component and work out what to do next. As much as anything though, I'm enjoying digging into the circuit and using it as a learning experience to try to understand how these amps work better and how I might be able to tweak them.



"All of your changes to/from 4.7/5uF or 0.0047/0.005 are negligible. Modern cap values are consistent with their tolerances. Basically 47 is the new 50."
That's good to know, because that was all I could find available, so changed it on the diagram as well

"The resistor feeding the bias supply will probably need adjustment to get the bias range in line with the needs of the output tubes using the selected power transformer. The schematic value is simply a good starting point."
Thanks. I think I took that value from the layout diagram - if I'm unable to get the bias required, I'll take a look at changing that one.

"The 8k2 source resistor on the FET must be selected to suit the particular FET, and the amount of distortion you want it to introduce. See https://ampgarage.com/forum/viewtopic.p ... 91#p211891"
The FET input is going to be a bit experimental - I'm even considering using a 2n5457 JFET that I already have. I'm guessing there is a lot of tweaking in this section to be done but I'm not aiming for anything in particular either, and possibly won't use this input much.

[bepone]

first is necessary to build one good unit, which is very difficult for the start, and then thinking about tweaking, not opposite

[dvgreen]

agreed done that a few times - now I'm up to the tweaking stage

[dvgreen]

I finally finished this build and couldn't be happier with the way it turned out. Thanks very much for the comments above that helped finalise the circuit for this build. Here s a video showing the overall build and is filled with sounds from the amp:

[WhopperPlate]

Congratulations on the build!

Curious, what was your reasoning for going with the 2nd generation power amp over the stock 124?

Also , are those black ends on the signal caps marking the the outside foil ?

Cheers

[dvgreen]

Thank you!

It has been a while since making that decision, but from memory I started out wanting to build the 2nd gen amp as per the circuit/layout diagrams on this site (with the Bassman style iron and lower gain PI), but wanted the tone stack and gain stages from the 124. I don't know nearly enough to modify one of the circuits to arrive at what I wanted - so really just cut and pasted the 2 together.

Yeah, that's right - the black ends were coloured with a black sharpie indicating the outside foil.

[WhopperPlate]

Thank you!

It has been a while since making that decision, but from memory I started out wanting to build the 2nd gen amp as per the circuit/layout diagrams on this site (with the Bassman style iron and lower gain PI), but wanted the tone stack and gain stages from the 124. I don't know nearly enough to modify one of the circuits to arrive at what I wanted - so really just cut and pasted the 2 together.

Yeah, that's right - the black ends were coloured with a black sharpie indicating the outside foil.

Certainly a mashup choice I have personally made that’s not hard to implement. I like the stoutness it provides.

I was wondering about the black ends because the general practice is the only two caps with the outside foil oriented away from the tubes are the caps off the grids on v3. It’s certainly not the end of the world , and if you like the sound it really doesn’t matter , but I figure I would mention it fyi

If you get bored of the sound and get the urge to break out the soldering iron that might be something to have fun with

Cheers