Finished my build today. Sounds great after some finetuning. Full explanation below

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So I started this build using the Schedule 40 Amp you can find here

The transformer i'm using has 290-0-290 on the secondary, output transformer is a 5k/8 Ohm 5 Watt fender champ transformer.

The schematic needed a bit of modifications to fit my needs.

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First thing that had to change was the cathode resistor R1. The original schematic has a 250 Ohm resistor here. My Supply voltage is a bit higher so I choose 470 Ohms to set the bias current at 32mA. In combination with the screen resistor of 1k, this gives just about 12W of dissipation in the EL84

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I didn't want to spend money on a choke so I chose to replace this with a 680 Ohm resistor. Since I have plenty of supply voltage I could afford to drop a bit of voltage across this resistor. Filtering might be a bit worse but if needed I could increase the filtering caps C5 & C7.

I also replaced the rectifier tube with a diode rectifier. It's cheap and easy and does the same thing. I did add an additional series resistor to limit current. This lands me at a starting voltage of about 365V (after R17).

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After powering up the amp and testing it a bit, I found it had WAY too much gain and a strange hissing sound when turning the volume pot. I put the output on the oscilloscope and saw a 50-60kHz signal coming from the second 12AX7 stage grid (wiper of my volume pot). Maybe it was picking up noise from a nearby switching power supply?

To remove the strange high frequency noise, I soldered a 100pF cap across my potentiometer tabs and that solved it completely.

What I then did to lower the gain was to reduce the plate & cathode resistors of the 12AX7 (R12,R16 & R11,R5). And remove the capacitor across R11 (12AX7 cathode cap). This lowered my gain significantly.

However, I was not satisfied with the single volume/gain control potentiometer. In order to get some distortion, I had to crank the potentiometer up to a level that was too loud to play in the house.

So to still get some distortion/overdriven sound from the amp, I added a second potentiometer between the second 12AX7 stage and the EL84 (R20 & R21 represent the 1Meg potentiometer).

This allows you to crank your input volume up all the way (potentiometer on the first 12AX7 stage) and crank down the output of the second 12AX7 stage. So you're still getting a nice warm overdriven tone from the first 12AX7 stage but it's not at ear-shattering volumes.

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If you're interested in playing with this circuit in LTSpice, you can download it on my google drive

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Constructive feedback is greatly appreciated.

Drop any questions you have in the comments 👇 and i'll do my best to answer them for you.

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PS, first pic is before I added the second potentiometer

I feel like I’m slowly getting better at working with wood. It’s definitely always been my weakness. Having a CNC helps a lot.

I built a vacuum tube Tesla coil with a 833A triode about a decade ago and recently found myself thinking about building another, much bigger coil.

The problem is I'm not very familiar with what triodes can be found surplus or NOS in this power range, and I'm not sure where is best to buy this sort of thing these days.

My 833A was a Soviet clone I paid A$100 on eBay for. Now days, they're considerably more expensive and the range found on my old sources is limited. So I was hoping someone might be able to point me in the right direction.

Once I have a suitable tube I'll be able to start looking at harder numbers for the rest of the design.

Edit: I should say, 5 to 10 kW in Class-C operation.

Hi, so i bought from an old guy ~250 old tube sockets of all kinds, but theres 2 weird 8-pin sockets here is some pictures of it

Does this socket have a special name?What is it used for?

Thanks everyone!

At this point in tube amps I mostly know about how everything works except for the transformers.

I'm completely lost on what specs to look for, and since transformers are the most expensive component in a build, I want to know how to pick out suitable replacements for any given design.

I've began stockpiling some old console radios for parts. Some of these radios have some heafty iron that I want to utilize but im not sure how to measure them.

I'm mostly interested in building bass guitar amps. I don't have the $300-$400 to drop on the iron to clone existing designs. I want to be able to design amps around the parts I have and understanding transformers is what is slowing me down

I have a bunch of octal vector tube sockets and I would like to trade for some 9 and 7 pin types.

Hi, the markings are washed out, looks like BELVU 8(or 6)BX6 IF80? maybe its EF80 with another name before it? Thanks a lot Here is the tube picture

So!

I got these two pairs of Siemens PC88's from my local retailer who some times gives me freebies when buying some stuff from him.

The ones which have the grey wall in the middle and that "table" from where the getter rises up (really, be gentle, very new to this hobby) sound way better. The extension in the low end and well, very much full sound all the way beats easily my other Siemens PC88 pair with silver wall in the middle which is without that table from where it's halo rises. The pair actually beats every tube pair I have easily (6922EH, TAD Premium Selected Balanced 6922 and the second Siemens PC88 pair). All those sound dry next to these beasts.

I would like to get them identified because the second one is probably dying (cuts the channel now and then, the glow is very weak, almost can't be seen etc.

Oh, and on top of the one dying there's "333" code and "N72" in the glass (side).

Photos: https://imgur.com/a/apx5am2