More Amp Construction


Active Member
I got these wrapped up and off the bench today so I can resume work on the T-20 stereo amp.

These have cobalt/M4 Magnequest TFA-2004JR parallel feed output transformers, M4 EXO-003 plate chokes, Magnequest filament chokes, Mundorf and Auricap coupling caps, Cardas posts and jacks, and meters for good measure. These parts were recycled from a very nice amp that got cosmetically damaged beyond repair, and ultimately they leant themselves most convincingly to a 2A3-40/300B-2.5 output stage. I used a 5965 up front, mostly because I have tons and tons of them sitting around.


Active Member
I wrapped up the T-20 amp tonight.
This amp uses Magnequest 40mA Dowdy plate chokes which have a beastly amount of inductance (over 100H IIRC), and I paired them with the little Bottlehead OT-2 output transformer that we use in our SEX and Stereomour amplifiers. The driver tubes are EL84s driving 6BX7s. I used Solen SM film and foil coupling caps and some Jantzen parallel feed coupling caps. The brown colored caps visible through the window have been brought up a few times by those who have seen the amp, but they are just screen bypass caps and are good quality metalized polypropylene caps. I actually prefer them to some of the lower end boutique caps I have used in the past and the alternative would've been using an electrolytic capacitor instead.
Most of the parts live between the metal sub plate and the top plate, so making adjustments is tough. I needed to change one resistor value to get things working properly and it wasn't the most fun thing in the world, but not completely impossible.

I took some quick measurements:

Output impedance is just under 3 ohms.

Output power at 1kHz 10% THD is 10W.
Output power at 1kHz 5% THD is 5.6W.
Output power at 35Hz 10% THD is 8W.

The amp is less than -0.5dB at 22Hz and 20kHz.
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I've used Meanwell supplies for filaments; first directly and then feeding VCCS regulators. What made you choose the RSP range over any of the others?


Active Member
I finished these which I mentioned here: 807 Thread

I used the 6K7VG power transformer and found that a 5Y3WGTB helped shed enough voltage to keep things happy. I have some 0C3W tubes on the way so all the tubes in these brown amps can have brown bases. As drawn in that thread, this circuit runs the 807 right at or slightly above the maximums, but I say let them cook!
The build inside is built around a couple of 12 lug terminal strips with a window (I have a piece of paper towel in there for this photo to keep debris and flux off the glass). Caps are Miflex/Mundorf and I'm so thankful that 500V electrolytic caps are readily available these days.