Greg's new tube amp build (video included).

[Phoenix]

Hi everyone, I thought I'd share with you my most recent tube amp build.

I'd been going through all my junk and came across an old Philips EL3541D reel-to-reel/radio that I'd picked up years ago and forgotten about. It was in very poor condition, the plastic case was turning to dust and was cracked everywhere, and there had literally been a wasps nest in there at one point, so this was definitely a salvage job, not a restoration.

So I pulled it apart, salvaged the power and output transformers, the voltage selector switch and the tubes - ECC83 dual triode, EF86 signal pentode, ECL82 dual triode/power pentode, EZ80 rectifier and EM84 magic indicator tube, then set to drawing up a schematic. I came up with a bom and ordered parts about half an hour later.

When all the parts arrived, I decided to try an experiment, and rather than planning a layout as I normally do, I decided to follow my intuition and wing it, and see how it turned out - I could always start again. About 4 hours later it was making sound, but I decided to try some tweaks, the first was an LND150 source follower to bootstrap the input gain stage, which I liked, and then I tried another LND150 source follower to replace the cathode follower and repurpose the triode as another gain stage, but after playing with different bias points and the interstage attenuation, I decided I wasn't a fan and returned the circuit to the earlier iteration.
The layout came out ugly, but quite functional - the amp is dead silent despite being quite high gain. And I'm very pleased with the sound, which is the most important thing of course.

It's single-ended, fixed bias, puts out around 4-1/2W. The EM84 is hooked up as a VU meter. It has a "squish" control - variable screen bypassing on the EF86, and a "shift" control - variable slope resistance in the tone stack to move the center frequency up or down. With the shift at 12 o'clock the tone stack matches a JCM800 2203/4 tone stack. Everything else is normal - gain, bass, mids, treble and master. There's an inbuilt dummy load, and line output, so I can crank it and run it into a bigger amp for more volume (or to add reverb or something on top of power amp distortion, that sounds great, like post-processing but live!), or equally, to run it into a smaller amp/a big amp turned down to get the cranked tone at bedroom/nighttime levels.

Here's a video demoing it. Sorry for the poor lighting and picture quality, it's all my potatophone can manage. Also note the audio recording came out darker than in person due to mic placement.
Please go easy on my playing, I'm very out of practice, I had DVT in my right leg a couple of years ago which I wouldn't have survived if it had happened a couple of months earlier before they'd introduced new treatments, and I damn near lost my leg anyway, so I've spent most of my time convalescing. And I'm only 31, so pretty traumatising I'm sure you'll understand.

Note that I've left the schematic as built, there are some redundant parts in there (the cathode bypass caps on the input gain stage).

So thanks for reading, hope you enjoyed!

[PRR]

Nice build.

That B+ rail is un-commercially extravagant. You can put two sequential stages on each node. Four B+ nodes, not eight. Or were you trying to work-down an overstock of caps?

[Phoenix]

Thanks Paul!
Haha, yes, the filter caps are over the top, I do often decouple each stage in my designs, if for no other reason than it entirely irrationally makes me feel better, albeit usually with smaller values like 10uF.
This time however, I wanted the 47u reservoir (just below the 50u max for the EZ80 without adding series resistance) but didn't have any in stock. Local supplier sells in packs of 10, was originally only planning to use 47u for the first two filters, and use 10u for the rest, but got carried away. Won't hurt anything, and hardly expensive in the scheme of things.

[rankot]

Beautiful build!

[anotherjim]

It's a beauty.
Coincidence - last night I got first sound from an ECL86 in a basic cathode biased tri-pent circuit. I have a couple of ex-equipment ones. A simple small power amp is in the future. Also in good health, a Dubilier can multi capacitor that reformed nicely.
When I switched the HT on, I thought it was dead, but it's completely silent without signal input, but then I haven't any cathode bypass caps or tone stack in there yet.

[antonis]

Excellent work, Greg..!!!









but






MosFet in a tube amp...??   
   

[Phoenix]

Thanks Antonis!
Mosfet? Yeah, but it's only a source follower (analogous to cathode follower), so it's not providing any gain, and it can't be overdriven. Plus, if it makes you feel any better, its a depletion mode (normally on), just like a tube, not like the usual enhancement mode (normally off) fets that are ubiquitous.

[pinkjimiphoton]

that is one of the cleanest "wingits" i have ever seen man. that thing is so shiny and sexy i think i need to change my shorts!!! well done! sounds great, too!!

[Max999]

Nice build. You made me want to experiment with a variable tone stack slope resistor in my own amp. Thank you for the idea!