Scratch Build Tube Amplifier

The amp has been working great, doing 10-14 hours a day almost every day for nearly 4 months now.


I finally managed to get a decent picture showing all of the grids with decent depth of field.


The compacted MkIV version is still in progress. Time and the weather have not been on my side. I've done 90% of the wiring and I've got the front and back panels ready to fit. I just need to mount them and then wire up the mains input and switch. I also need to wire the input socket via the high pass filter board which will also need to be mounted. It sounds like a lot but it shouldn't take more than an afternoon to sort.
 
I've finished fitting the front and back along with the divider. The capacitors are all in place too. Not the prettiest as the wood is reused from previous builds. (the black on the back panel is where it got used to support something being painted)


Most of the wiring is now done. Hopefully I can have this finished tomorrow so I can test it.
 
I finished the Mk IV prototype and have now tested it. I made the usual mistake of reversing the phases leading to positive feedback, but I've managed to do it 3 times now so know how to quickly fix it. Takes 2 mins with a soldering iron to reverse the outputs from the phase splitter. Tested on the bench, the performance appears to be very similar to the other amp which is excellent.

1KHz square


5KHz square


10KHz square


It's a good looking amp, which would be even better with some decent wood panelling. (But I'm not wasting a good piece of oak or similar on a prototype)


Working together with the Mk III. First time having both speakers driven by my valve amps. Doesn't sound half bad. I've currently not got anywhere to put the second amp so it had to go on top of the speaker. (the Kef isn't in use)
 
Looks awesome mate, really nice frequency response judging by the CRO.

One thing that did jump out at me is that there is no dedicated earth for your top plate. You might have one and i didn't see it, but if there is not i recon you should add one.

Also what is the forward voltage on the panel mount switch LED? Iv'e been looking at a similar switch from TE and its 24VDC. If its also a 24V LED how are you powering it?
 
Looks awesome mate, really nice frequency response judging by the CRO.

One thing that did jump out at me is that there is no dedicated earth for your top plate. You might have one and i didn't see it, but if there is not i recon you should add one.

Also what is the forward voltage on the panel mount switch LED? I've been looking at a similar switch from TE and its 24VDC. If its also a 24V LED how are you powering it?
The frequency response is pretty good, though it could be better with a superior output transformer with higher bandwidth.

The earth from the IEC inlet is grounded to the aluminium plate via a single metal standoff with a star washer to ensure good contact. All the other standoffs are insulated in order to avoid ground loops. I'm not nuts enough to run an amp with 820VAC & near 500VDC in it without an earth and plenty of fusing.

The power switch has a 12V LED in it. I've not bothered to connect it up as I'm not bothered if it illuminates or not. (as the valves glow when it's on) It lights up on a diode test from my meter with 3V, so rectifying one of the heater feeds is an option as I'd get 6.3VDC that I could use. This is the one I used.
 
Last edited:
Bought some higher voltage rated capacitors with some matching capacitor mounts and more octal sockets. The F&T 500V rated capacitors may be reused to replace one of the 10uF capacitors. The additional belton octal sockets will replace the old finder sockets I've used on the rectifier as the pins are a bit loose. So far, I've swapped out the parts on one amp. Conveniently, the Mundorf tubecaps are practically the same length as the F&T's, only fatter so they fit in perfectly. Which is why I paid mundorf money instead of buying cheaper motor run capacitors or solen.
 
Finished off doing the second one this morning.


Doesn't look too bad considering the cobbled together nature of these prototype builds. Once I've figured out how I'm going to get the proper aluminium chassis made, I can get them built properly.


I've also invested in some DC blocking PCB's and all the necessary components to build them. I've always had some residual hum on all five of my amplifiers which has made me suspicious of there being some slight DC offset.


I've made a start on building one of them. I got 3 PCB's so I can do other parts of the house.
 
I've now built the box for the DC trap. I simply need to wire it up and solder in the diodes.
 
Finished fitting the diodes this evening. Just the box wiring left to do now.


Those large tracks absorb a heck of a lot of heat and are take a while to solder even with a 70W iron.
 
I finished the DC trap this evening. All the wiring seems to check out ok. Will test it properly tomorrow.

 
After someone mentioned trying to build a Mullard 5-20 using 807 output valves on DIYaudio, I got it into my head to look at whether it was possible using my spice simulation as the ideal output transformer for 6L6 based valves is 6.6K. (which is what the 5-20 happens to use) Turns out, based on my simulation, 6L6GC and it's 30W+ derivatives will work just fine in my amp. So on a whim, I bought some of the high dissipation (35W) 7851A's to try out.

With all of my output valves side by side, they look pretty similar to the EH 6CA7 on the far left, but the primary difference is that the cathode is quite a bit smaller. It does seem odd that it has a 35W dissipation limit whilst the rest are all 25W.


Fitted and running. Cathode biasing seems to work nicely. Plates sit at 428V and screens just below at 427V via the 43% ultra linear taps. Small bonus these have is the reduced heat output from the lower heater current (0.9A vs 1.5A) After a couple of hours listening, it sounds every bit as good as the other EL34's and their derivatives. I figure the ~20dB of negative feedback is minimizing any audible differences there may be between the various valves I've tried. A big added aesthetic bonus is that these have the blue glow on steroids compared to the rest.
​​

I will admit that the valve bug bit hard once this amp was functional. I've ended up 6 quads of output valves (x24), 5 pairs of phase splitters (x10), 10 pairs of input valves (x20) and 2 pairs of rectifiers + a spare. (x5)
 
Last edited:
Back
Top