Thanks Raymond.  After getting mine up and running, it is very quiet, so the filter cap on pin 7 is definitely not required in my case.  I will hopefully get the 50x gain boost connected to a switch soon enough to see what that is like.

The amp is built, and it works!  Powered by a 17vac salvaged transformer, through a diy full-bridge rectifier, into a cheapo buck convertor set at 16vdc, and housed in a salvaged VCR.  Played it through a 1×12 Celestion Gold.  It's gritty, but sounds pretty great for blues and rock.  It's also very quiet at idle.  It was a fun build!  Took some extra time due to moving components off-board.

I haven't finished my amp yet, but the option of having a gain boost sounds fun.  I discovered that the gain can be adjusted by adding a resistor in series with the 10uF capacitor.  In this case, a 1.2K resistor in series, will result is approximately 50x gain (as opposed to the 200x with no resistor).  I am confused however about what to do with pin 7?  In this schematic, it shows that pin 7 has a capacitor labelled as "bypass", and the lines are broken.  Can anyone please shed light on this?  I think I will do this mod, and make it switchable to have the option of a gain-boost.You cannot view this attachment.

I have a question regarding grounding of my setup.  As some of you know, I scavenged a stepdown transformer from an old Dell powered computer speaker (120 to 17), and added a full-bridge rectifier to change that to DC.  Finally, that will be regulated down to 16vdc using a buck converter.  This will then feed the amp (once I assemble it), and feed an external speaker.  The current transformer does not have a grounding plug, a fuse, or a power switch. The enclosure that I am using is plastic.  I am planning to add a power switch and an inline fuse, but am unsure about a ground.  I know from working with tube amps, that the voltages in there are lethal, and therefore grounding protects people from high voltages.  In the case of this amp, after the first stage of the transformer, the voltages are around 17VAC.  Is grounding necessary here?  Is the lack of a metal enclosure going to introduce a bunch of noise?  Will the lack of an earth-ground make this amp noisy?

Thanks for the help with this.  I bought a DIY full-bridge rectifier kit, and powered up the transformer.  My output voltage is 25.3 VDC, so I will definitely need to step that down.  I order up an inexpensive buck converter that is adjustable down to 3.2v, so I will easily be able to step this voltage down to the 16vdc that I am going to need.

I was under the impression that a full-bridge rectifier would drop the voltage?  I guess too much voltage is better than too little as it should be easier to drop it than to increase it.

Hi all, I got my kit last week, and was looking at options for powering the amp, and discovered I have a power transformer from an old Dell powered-speaker set.  It outputs 17v AC and 2.5A.  I have a full-bridge DIY rectifier kit on its way to convert it to DC.  I am not sure what the final DC voltage will be, but I am hoping for 16 VDC.  I cut open some shrink wrap on the power supply, and found a 0.1uf 275VAC capacitor in there.  Any ideas what this is for?  It appears to be wired in parallel?  Any thoughts on my proposed power supply?  I figure it's free, and it's great for learning more.

Hopefully this isn't a zombie post.  You said you burnt up components using a laptop charger.  Was it too much voltage, or was it an issue with current?

Regarding the excess gain from the 1-8 jumper, what cap did you use?  I might dabble with this option once I get my kit, and put it together.

Quote from: g1 on April 11, 2024, 12:24:42 PM

Quote from: RookieRecurve on April 11, 2024, 10:09:19 AMMy main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues? 

It can cause issues with ground loops or worse.  Some amps require their output (-) terminal to be isolated from ground for things like current feedback schemes.
If you want it to be safe as possible, use a Cliff S2 style jack, which will be insulated from chassis, and also will allow switching of both the positive and negative wires.

Thank you!  This is what I had suspected, but your answer confirms this.  Much appreciated.

Quote from: Tassieviking on April 10, 2024, 03:33:55 PMThe safes way to do this is to add a socket behind the speaker that the speaker goes to directly.
Also a speaker out socket on the back of the amp chassis, then you need a short speaker jumper cable with a plug on both sides to attach the combo amp to the combo speaker.
Edit: I have done this to some of my combo amps as it turns them into a separate head and separate speaker cab.
Sometimes I want to play the Marshall through the Fender speaker or the Sunn speaker, I just un-plug and plug into another combo for the sound I want.

Thanks for the reply!  Yes, I have considered doing what you have done, and adding two jacks with a short jumper.  To me, this makes things more simple.  The reason I am considering an alternative approach is that it is even more idiot-proof if it works in the same way that a headphone jack does, where plugging in disconnects one, and connects another speaker.  My main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues?  I am comfortable that I would not have both heads on at the same time, but if I ever sell this, or someone else is using this amp, that it is safe for them to use it as well.  I am pretty sure the 13A has a common ground terminal.  I have seen other jacks referred to as 'cliff' jacks that I am unsure about if the ground is common or not?  I have also seen 12A 'tip shunt' style jacks.  I might just order up a few different style of jacks, and see if any of them fit my intent of switching the hot and the ground when a plug is inserted.  My other question still remains though of whether or not a ground can be shared between amps.

Quote from: Loudthud on April 10, 2024, 03:06:02 PMNot sure what you want to do exactly.

Is the combo amp like a Fender where the internal speaker(s) just plug into the back of the amp chassis ?

Are either of these amps tube amps ?

Franken amps are notorious for poorly done grounding. Do you get any hum when you just connect a wire between the chassis' ?

You might risk damage to one or both amps unless they are both OFF when you plug the cable between them. Can you do that or do you need to make this idiot proof ?

Thanks for the reply.  This is a basic SS amp (Samick 10R) that is connected to a pair of vintage 6.5" alnico speakers.  The amp is picking up a ton of noise from the reverb tank, which is likely my fault for not using shielded wire after relocating it to the bottom of the enclosure (I will be trying to solve that eventually).  I want to leave the amp connected to the speakers in the enclosure, as well as being able to connect some of my heads to the vintage speakers inside this combo.  I want to 'idiot proof' it as I have nephews and sons that might also use this amp at times when I am not around.  I am confident that using the 13A jack will allow me to have one plug that will allow me to switch the hot (+) connection from the internal amp to the external 1/4" speaker connector; what I am unsure about is the ground.  I believe that these jacks have a common ground.  I do not know enough about electronics to know if sharing a ground with the internal speaker, internal amp, and the external speaker input is ok or not?  I am also unsure if the 'cliff' style of 1/4" inputs switch grounds, or if the ground is common.

Hey all, I am looking to add a speaker jack to a fraken-amp that I assembled, but not in the traditional configuration.  I am looking to use the combo-amp as an external speaker cabinet, as opposed to having the amp switching to an external speaker cabinet.  I am looking at a Switchcraft 13A (transfer circuit) jack to achieve this.  Is this the right part for the job?  Will sharing the ground from the external amp, the internal amp, and the speakers work?  Do I need something to completely switch circuits (hot and ground)?  I have seen other setups that use two standard jacks, and then use a jumper cable to mitigate, but I prefer to have one input jack to keep things clean and simple.

Thanks

Quote from: JonnyDeth on March 31, 2024, 09:14:42 AM

Quote from: RookieRecurve on March 31, 2024, 08:39:55 AMI see these pop up often around me, but not for that price!  There's one I saw for $100 with only one of the two channels working.  I thought about buying it to try and fix it, then flip it.  It would be too loud for me, but they are pretty amazing amps.  The ease of access to incredible schematics makes these a great buy.

I saw a guy on youtube that flips gear for a living and he nabbed one for $40! He said it was his very first amp so he made a video with it then sent it on it's way to someone who had bought it.

I am a shred and sweep player so it's a little harsh in the bass due to the very high, even order harmonics added by semiconductors, but in combining the dirty channel with my modeler and an overdrive pedal I actually designed around this amplifier's dirty channel used to overdrive the modeler, I get perfectly useable lead tones. I also have a Zoom G5 on the way to replace the one I have that some crazy b*t** smashed, and those with the 12AX7 in them turn damn near any amp into a thoroughbred.
That coupled with this and I have a brutal metal amp but really, an every genre amp. It's so friggin loud I could never actually use this much volume and in a live setting, I would definitely be using earplugs.

It's all about gain stage cascades and teaming the right devices. Solid-States have some universal problems shared by 99% of them, but Crate is highly underrated and players with really high standards would rather spend 3 grand on known gear combinations than experiment and get something built for $500 that is every bit as good as the 3 G's arrangement. The Zoom G5 is an example within itself because when you run the processor without the tube engaged, it's very digital and quite truly sounds cheap, but when you switch the tube in and crank it's gain, it makes every single patch you design for modeling whatever amp, pedal and speaker combination sound amazing. The harshness, "swooshing" etc. to your bass is completely filtered out of the signal by the tube's limited bandwidth. The 12AX7 is said to have a maximum of 20Khz for audio, but typical is actually 10 Khz. The semiconductors will add ridiculous harmonic bandwidth like 40 Khz to a 100 Hz bass note!
This trait is why solid-states continue to be the whipping boy of the industry and because digital circuits are still using solid-state semiconductors, 90% of those units aren't designed to compensate so the war wages on! I shutter at the sight of digital modelers and FX I see selling for $1500 and I can hear all those extremely high, even order harmonics still coming out of them and trashing the signal, but not every player is sweeping and shredding for it to matter.
The technology guarantees people will always spend big money on tube amps, and even I'm still guilty of that.

Great explanation of why tubes are still so popular.  Its also a great reminder that great tones from inexpensive SS stuff can be had with the right piece(s) of equipment in front of it.  Sometimes I like to just plug into my fairly stock Valve Jr. and appreciate its simplicity, but other times I love playing around with different tools.  Your talk about having a tube in the signal path makes a ton of sense.  Thanks!

Quote from: Tassieviking on December 19, 2023, 12:06:40 PMIf you want to mess with the amp then you should leave the power amp section alone.
The pre-amp is where the tone shaping happens.
The closest you should go to the power amp is the 100nF cap going into the power amp, that 100nF cap with the 100k to earth is a HPF filter that drops any frequencies below 15.9Hz. Change to a 10nF cap and the frequency is 159Hz, experiment if you like.
I would concentrate on the tone stack, changes there will affect your tone more.

Are you sure it is not the speaker itself that is dark sounding ?

This is great advice here.  With the amount of variables in an entire setup, mucking with a SS can be difficult. Small electrical mods to your guitar, pedals, and adding an extension to play with cabinets and speakers will likely produce better results.

Old post, I know, but it's really solid information for anyone considering modding a SS.

I see these pop up often around me, but not for that price!  There's one I saw for $100 with only one of the two channels working.  I thought about buying it to try and fix it, then flip it.  It would be too loud for me, but they are pretty amazing amps.  The ease of access to incredible schematics makes these a great buy.