Add a 3rd stage to a preamp, tie it all together with a DC cathode follower, and get ready for some serous overdrive with a tone to die for. This project is not for the faint of heart, so lets build a 3 stage and cathode follower amp:)
Most 2 stage amps have plenty of gain to deliver that fun crunchy overdrive we expect from our tubes, but what if we want more? While there’s an endless world of possibilities, I recently answered that question with a 3 stage preamp. Why 3 stages? Because 3 is more than two:) I am going to walk you thru what I’ve done, so here is a 3 stage preamp with a DC cathode follower. I will post options for transformers and tubes along with the usual Layout and schematic.
I have read, discussed, and heard about 3 and 4 gain stages with a DC cathode follower providing amazing tight breakups and smooth gain at every volume level. Sounds great, but where are the examples? There really isn’t a basic schematic or model I can point to with the exception of some Marshalls amps using extra gain stages to compensate loss of gain from tone circuits. That is why I decided to experiment with a very basic 3 stage and cathode follower amp.
The purpose of this amp is to deliver hard rocking overdriven gain to the speaker. With that in mind I decided to omit the 1st and 2nd stage volume pot and instead opted for a pot before the 3rd stage. Added another pot in front of the output tube, and went with a switch that could remove the 2nd stage for a clean option. No idea if this was going to work, so opted for a lot of used parts (which came back to bight me in the end).
If you’re interested in designing you’re own preamp please refer to the link below:
https://crunchytubeamps.com/use-the-preamp-tube-of-your-choice/
I have decided to use my new favorite low noise tube the Shuguang 7025. This tube is a noticeable improvement over the 12AX7B. I chose to use a lower voltage for the preamp because my output tube only uses 150VDC on grid#2. I like to use lower grid resistance when using lower plate voltages as a way to keep out unwanted parasitic oscillations. Instead of the usual 1MegΩ grid resistor, I went with a 470KΩ at the input of the 1st stage and the input of the 2nd stage. I also decided to do away with cathode bias bypass capacitors, but you could experiment with those if you would like. Personally, I love the low volt low grid resistor tone that you get from a preamp like this because its very similar to early Silverstone practice amps. The difference here is that we’re slapping on an extra stage to really punish that output tube.
Here’s where things get tricky. I really had no idea how a direct coupled cathode follower (DC CF) worked. I only know that when a DC CF is directly coupled to the 3rd stage triode we will have above unity output for that stage. Compare that to an AC CF that is intended to be less than unity output like used in our stand alone reverb. You can review our CF load design here: https://crunchytubeamps.com/budget-stand-alone-spring-reverb-3-of-3/.
The first triode section with the input signal uses the same load as the 1st and 2nd stages. No coupling capacitor needed for the DC CF. Since our signal is going to be above unity, we need have a cathode load that will allow for a tiny swing. A normal CF uses a 10KΩ to 33KΩ load to a very near vertical load line. I decided to stay between 47KΩ and 82KΩ for the load based on that information, and I settled in the middle with 56KΩ. You can play with numbers if you want. A higher resistance will give more distortion, but 56KΩ will run at 7.8mA measured across the resistor at idle. The best example to explain the difference between a 2 stage and a 3 stage with DC CF would be like if Chuck Berry plugged into Randy Rhoads amp by mistake.
A while back I wrote about using a 6146B output tube to design a simple SE amplifier. I decided that this would be perfect to accept a 3 stage preamp because of how robust this tube is. It also fit well with my design parameters. Read about the output design here: https://crunchytubeamps.com/uncommon-power-output-tubes/.
Having an old 275V transformer on hand with 5v and 6.3v leads, I decided to make a 10W single ended. 295VDC on the plate @70mA and 150VDC @ 4mA on the grid gives a bias of 22.5V. I wanted something robust with a fairly high bias voltage for this amp and the 6146B really fit that description, but I could just as easily used a 6CU7 or 6L6. I also choose to power this with 5Z2P drop in replacement for the 5Y3GT, and I was not disappointed. Very glad I decided to test out this rectifier.
Adding a 3rd stage to the preamp with a DC cathode follower yielded some over the top results. I was surprised how much of the tone quality survives the explosive amount of overdrive thrown at this circuit. Is a 3 stage amp better than a 2 stage amp? Maybe, but you could easily ad a switch to go between a 2 or 3 stage amp. Never underestimate the power of options. Below are some links to the tubes you can use to build this amp along with the transformers. Please enjoy this 3 stage and cathode follower. I hope this adds to a growing potential for your future projects.
FU-46 6146B Vacuum Tube New – Crunchy Tube Amps
2024 NEW Shuguang 7025(ECC83,12AX7B,12AX7-T) Amplifier HIFI Audio Vacuum Tubes – Crunchy Tube Amps
I used the 2.5 watt tap on this output transformer from mouser.
https://www.mouser.com/ProductDetail/Visaton/1803?qs=bT6MOr62zwIPrMVzUGvvxw%3D%3D
Alternatively you could play it safe and use the 2.5 watt tap on this slightly beefier transformer
Multi Tap Universal SE Audio Output Transformer – 15W Chassis Mount – Crunchy Tube Amps
I do not have a suggested power transformer that has a 5V tap; however, the below transformer will work fine with an EZ81 tube
Crunchy Tube Ampshttps://crunchytubeamps.com/product/270v-0-270v-3-15v-0-3-15v-0-6-3v-0-12v/
Would it be possible to purchase these plans? I think this’ll be my next project. I wanted something brute simple, single-ended, and with great overdrive.
These are my shared projects and there’s no cost. I can put a parts list together. Let me know what else you need to get started.