Novice amp builders looking for guidance on first amp build

[scollier28]

My friend and I are high school students currently in the process of making a tube amp from scratch, as we thought it could be a cool project to undertake being into both electrical engineering and both being guitarists. Currently we are about to begin the building process, but want to be cautious and have our schematic be assessed by some more experienced builders first.

In terms of notable specs, it's a 25 watter, and plan to put in a single 12-inch, 8-ohm celestion greenback in for now. We have two 12AX7s, two JJ 6V6Ss, and one 5Y3 rectifier for our tubes – this build is sort of inspired by some of the more powerful tweed amps, like a deluxe. It includes the following pots: Volume, Drive, Bass, Treble, Reverb mix, and Dwell. Two seperate circuits are being powered by some op amps (TL072, NE5532): our drive circuit, and our reverb – a 600 ohm reverb tank (fender-style 2-spring). We got a 347AX Hammond Power Transformer, and some kind of chinese built 8K 20W Output Transformer. Feel free take a look at our schematics for a more detailed description of the signal path, and explanations of each circuit, each tube, the B+ design, and other components.

Here are some additional documents that you could dig into if you so choose:
List of all materials: https://docs.google.com/document/d/17Xo ... it?tab=t.0
Cabinet design on TinkerCad: https://www.tinkercad.com/things/epO7zY ... uoSDSAWDTQ


Sam and Fynn

[TUBEDUDE]

An ambitious undertaking for a first build. Some small things first:
Position your grid stoppers after your 1M grid bias resistors. As they are now, they form voltage dividers, dropping signal level.
Rewire your potentiometers to take signal from the wipers. This way the tubes see a more constant load to ground, instead of shorting the signal to ground with varied wiper positioning. They will perform more predictably and be a little less interactive.
You will be doing a great deal of troubleshooting with an unproven design. Just accept it and perservere. Star soldering now! Get good at it before you start. No sense getting frustrated by compounding your errors.
Get it working then find where to re-do things to eliminate noise and random oscillations.
I like your spirit of adventure, but believe a 5E3 as a stepping stone will help workout your novice skillset with a success. Then you'll have confidence and some experience to build upon before this more massive project.

[xtian]

Welcome!

No guitar amp has a Volume pot right after the input. It's redundant, because your guitar has a Volume pot. Better to move it following the tone stack.

[alnight]

Have you been here yet?

https://robrobinette.com/Amp_Stuff.htm

For what it's worth, I think you have a neat idea and you should totally experiment with it. Maybe a tad ambitious for a very first build. If I may be so bold, I would suggest you start with something simple (and fairly cheap) like one of RobRob's micro amps for the following reason. You will be able to quickly and cheaply rebuild the circuit with different parts (primarily resistors and capacitors) to learn how individual components shape the sound of your amplifier. That way when you build your original circuit and get it working fine, but it doesn't sound great, you'll have an idea of what to change to bring it more in line with what you want.

A bit of first hand experience with this can help mitigate a lot of frustration.

[dorrisant]

I'd make it possible to completely bypass the reverb. Like xtian said, move your volume pot. You need one somewhere else, but where you place it makes a difference.

Pulling up a chair...

[LOUDthud]

Pot in the overdrive circuit is drawn incorrectly. Opamp plus and minus inputs are wrong pin numbers. The minus inputs are next to the output so you can easily make a voltage follower

Do you know the gain equation for a non-inverting opamp ? That Plus One term means a replica of the input is added to what ever the feedback network does. The diodes will clip the input signal, but on the output the clipped signal will be riding on top of the input signal. is that what you want ?

Pots in the tone stack are drawn incorrectly. The Bass control will act more like a Volume control because nothing limits the high frequencies going to the pot.

Do you know what Common Mode input range is for an opamp ? (And what happens if you exceed it.)

[TUBEDUDE]

On the opamp "gain" circuit. It will sound like a cheap distorto pedal. Mitigate that by adding resistors in series with the clipper diodes to round off the square wave edges for a more tube sounding OD. Also an asymetric clip will sound better. Use different diodes/resistances in each half. Or a diode/ resistor pair in one half with an LED/ resistor in the other leg.
I emphasize the need to develop soldering skills now. Funky joints needlessly complicate troubleshooting.

[scollier28]

Welcome!

No guitar amp has a Volume pot right after the input. It's redundant, because your guitar has a Volume pot. Better to move it following the tone stack.

Hi xtian, makes total sense, thank you!

Sam.

[scollier28]

Pot in the overdrive circuit is drawn incorrectly. Opamp plus and minus inputs are wrong pin numbers. The minus inputs are next to the output so you can easily make a voltage follower

Do you know the gain equation for a non-inverting opamp ? That Plus One term means a replica of the input is added to what ever the feedback network does. The diodes will clip the input signal, but on the output the clipped signal will be riding on top of the input signal. is that what you want ?

Pots in the tone stack are drawn incorrectly. The Bass control will act more like a Volume control because nothing limits the high frequencies going to the pot.

Do you know what Common Mode input range is for an opamp ? (And what happens if you exceed it.)

Loudthud, I see what you mean about the pot in the overdrive circuit as well as the pinouts for the op amp. For the gain equation for a non-inverting op amp, I thought I reasonably calculated that the input resistor (10K ohms) and the feedback resistor (20K ohms) would create a gain of 3 (1 + 20K/10K), as not to overload the first tube with a heavily driven signal. I kind of just want the drive to be a cleaner boost, but do these calculations make sense at all?

I'm not super familiar with Common Mode input range for Op Amps, could you explain further?

[scollier28]

On the opamp "gain" circuit. It will sound like a cheap distorto pedal. Mitigate that by adding resistors in series with the clipper diodes to round off the square wave edges for a more tube sounding OD. Also an asymetric clip will sound better. Use different diodes/resistances in each half. Or a diode/ resistor pair in one half with an LED/ resistor in the other leg.
I emphasize the need to develop soldering skills now. Funky joints needlessly complicate troubleshooting.

Hi tubedude,
Ok, taking in the signal from the wipers seems like the better move here. As for the grid stoppers, wouldn't that configuration also create a voltage divider? For the OD, would placing 10K and 4.7K on each diode half resolve the issue? Or would you recommend changing the diode values too?
We'll make sure to develop our soldering skills, it seems like they can beget lots of unnecessary frustration. Thanks again!

[TUBEDUDE]

Screenshot_20260403_132734_Samsung Notes.jpg

In the first example the signal is split between the 2 resistors to ground, dividing the voltage. In the second the full signal is across the 1M to ground and the tap is from the top, so full voltage, instead of from a point in the middle of the combined resistance to ground. Remember, the grid draws no current, so the stopper drops no voltage.

Yes the resistors in the op amp feedback loop will round off the square waves.

[LOUDthud]

I'm not super familiar with Common Mode input range for Op Amps, could you explain further?

The biggest effect is that the output stops obeying the inputs. What you usually see is that the output suddenly slams the opposite rail instead of staying where it belongs according to one of the inputs being way more positive than the other. It can happen when you are just trying to make a Voltage Follower if the input gets too large.

[SoulFetish]

My friend and I are high school students currently in the process of making a tube amp from scratch, as we thought it could be a cool project to undertake being into both electrical engineering and both being guitarists. Currently we are about to begin the building process, but want to be cautious and have our schematic be assessed by some more experienced builders first.

Sam and Fynn

scollier28… is that you, the dude who we met at our shop when you were up in Boston looking at schools?
I’ve been waiting to see if you were gonna post about this project. I wish we were able to get into it a little more at the time, but it’s tough when you’re working.
You’re in good hands here, these folks are very well informed, experienced, and clever.
One of the things I wanted to show you if you made it here is a really great white paper written by RG Keen that contains foundational information on grounding, mains safety practices, and power supply operation, and importantly, practical construction concepts.
Apologize for being a little late to the party. But here is the most current version I know of. Maybe RG can correct it if there is a more current edit:
https://www.dropbox.com/scl/fi/vpijj3al ... zppfz&dl=0
Glad to see the project is advancing. I'm gonna get caught up on the progress.....

[scollier28]

My friend and I are high school students currently in the process of making a tube amp from scratch, as we thought it could be a cool project to undertake being into both electrical engineering and both being guitarists. Currently we are about to begin the building process, but want to be cautious and have our schematic be assessed by some more experienced builders first.

Sam and Fynn

scollier28… is that you, the dude who we met at our shop when you were up in Boston looking at schools?

Hey Corey! yes it is me! We have come a long way since that meeting! Hope everything at the shop is good. While you are here, could you take a quick peek at https://ampgarage.com/forum/viewtopic.php?f=6&t=38773? it is our most recent post titled: "The Amp Is Done". we are turning the amp on tonight and would love it if you could look at the final product!
Best,
Fynn & Sam