DIY all tube amp with $13 power transformer!

What’s it gonna take?

Can we really build a DIY all tube amp with $13 power transformers? Here are some amazing guitar amp heads that use the $13 power transformer linked below. Forget about 6.3V heaters, forget about full wave rectifiers, and say hello to the half wave rectified series heated tube amp.

https://www.mouser.com/ProductDetail/Hammond-Manufacturing/187D120?qs=%252BXxaIXUDbq3kPZhR0cJ2rQ%3D%3D

What tubes to use?

We can make about 2 watts with a 50L6GT (right side amp middle tube). The rectifier is a 35W4, and the preamp is a 12AX7. There is an insane amount of tube types out there that will work in a series heater set up, but the heater current draw of the tubes in your circuit need to match the draw of your rectifier.

What is a series heater amp?

The 35W4 rectifier draws 150mA. That means we can wire any tube that draws 150mA in series with a 35W4. The 12AX7 draws 300mA when wired for 6.3v; however, it only draws 150mA in 12.6v configuration, so 12AX7 is in for this design. The 50L6GT also draws 150mA what luck! 12AX7 tube here: http://12ax7b-shuguang-vacuum-tube-replaces-ecc83-7025-6n4-for-vacuum-tube-preamplifier-hifi-preamplifier-diy-amplifier-audio.

CAUTION!!!

BUILDING AN AMP WITHOUT A TRANSFORMER CAN CAUSE DEATH!!! The transformer in this case acts to isolate you from the mains of house electrical, and without that isolation you could be killed instantly from a shock.

Voltage consideration

Our 35W4 tube needs 35 volts for the heater, and 12AX7 needs 12.6 volts. If you guessed 50 volts for a 50L6GT you would be correct. Add that together 35+12.6+50 and we need 97.6 volts AC from our 120vac transformer (120v-97.6v=22.4v). We need a dropping resistor to eat up our extra 22.4v so we just divide that by our current draw of 150mA (22.4/0.150A=149.3). 150 Ohms is close enough and will drop 22.5 volts (150ohms x 0.150A=22.5), so we now know to use a 150hm 10 watt wire wound type resistor (22.5v x 0.150A=3.4watts). I like to double the watts from the calculation to account for fluctuations in the supply.

How do we get plate voltage?

The same transformer providing our heater current will also provide our plate voltages. Our $13 transformer has official taken the place of an $80 transformer, and we can be free to do more experimenting with our practice amps hurray!

What about an output transformer?

Don’t look past universal matching transformers because they are rated in volts 20v, 70v, 100, with leads taped in watts. Keep in mind that this will get an approximation. Use the picture below as a reference. Take the blue 5 watt lead and divide by the rated 70v. (5/70=0.071) and now divide 70v by the answer (70/0.071=986). The load of this transformer is 986z, but that’s only half the load required for the 50L6GT. Link below to see characteristics of our 50L6GT: https://frank.pocnet.net/sheets/127/5/50L6GT.pdf.

The transformer is good for 15 watts even though we used the 5 watt tap. The link below is a perfect match for our 50L6GT. The brown 2.5 watt tap gives us about 2000z at 70v which is what our 50L6GT requires, and its only $15!https://www.ebay.com/itm/125305555962?itmmeta=01J4Y91KCZA42CCSHT07BTNS8W&hash=item1d2ccb0bfa:g:kfkAAOSwmy9ieGse&itmprp=enc%3AAQAJAAAA0K3Ec9DlzRN0oomy2lLEb6azLuD731UXrEOVh8V0IoPjLAXpFLSn1dG%2BRUuu5FHpfH1LGUBgre4UkIlQkU8%2Bh3cQ9SB%2BnuDSLDjN6Lpx2tJRZ4rQ5g3mpmyBoeWwhQgFAXpMqPVDM4FccMSwR4N5tS4L6AyUaO6T8nl5F8bJloxFY6gFcMfvY4DQhwi96ux%2BDx5hCxlJbemp69xr2UFUFAAA1ek1DTvKRjeR24H49M6BnTe7QXDaSzUHDJ2KBPKhKZx6umWu0m5axnkT%2F8jXf%2Fg%3D%7Ctkp%3ABk9SR8q2hsmnZA.

Follow up

Yes we really can build a DIY all tube amp with $13 power transformers. I am building a schematic and layout of to post on Facebook. https://www.facebook.com/CrunchyTubeAmps/, and join my new group which has calculators you can use to make life way easier https://www.facebook.com/groups/485856047509004.