Greetings Friends! It's been a rough couple of years for the frugal DIY tube scene, tube prices are through the roof, and costs of everything else are up, too. So I've had an idea for a low-cost, simple to build amp that will use the cheap-yet-common EL84 power tubes for outputs, a choice of driver (6N1P, 6N23P, 6DJ8, ECC85) and the EZ81/6CA4 tube rectifier. Edcor XPWR013 power and GXSE15-5K output transformers are used, and a Triad C-14X 6H choke.
For the audio circuit, I have borrowed the "Suzuki" EL84 schematic from Single-Ended.com:
and for the power supply, I've borrowed from Matt's Marblewood Amplifier over at CascadeAudio.com:
with a different PT better suited for these power needs. I've retained the individual caps for R and L OPTs, but both channels will share a single Screen Supply, and Driver supply. (Unless that's not a good idea, only costs a couple more caps to make a full dual-rail supply.)
A simple 12x8x3" chassis can hold all components, and simple P2P wiring keeps costs down and allows for wiring flexibility down the road.
There's a BOM attatched, not sure how to include it in the post. Any thoughts, criticism, improvements welcome. thanks!
w
For the audio circuit, I have borrowed the "Suzuki" EL84 schematic from Single-Ended.com:
and for the power supply, I've borrowed from Matt's Marblewood Amplifier over at CascadeAudio.com:
with a different PT better suited for these power needs. I've retained the individual caps for R and L OPTs, but both channels will share a single Screen Supply, and Driver supply. (Unless that's not a good idea, only costs a couple more caps to make a full dual-rail supply.)
A simple 12x8x3" chassis can hold all components, and simple P2P wiring keeps costs down and allows for wiring flexibility down the road.
There's a BOM attatched, not sure how to include it in the post. Any thoughts, criticism, improvements welcome. thanks!
w
Also, a few questions about the audio circuit:
What's the purpose/role of the 1N4007 diodes?
What kind of switch should be used to select btw UL and Pentode modes? Assuming the switch is never changed when the amp is energized, would this Carling switch work?
https://www.tubesandmore.com/products/switch-carling-toggle-dpdt-solder-lugs-short-bat
It's rated for 3A @ 125v and here it would see < ~300v @ 15mA. Most switches rated for HVDC are quite expensive, so if the $10 switch won't do, might be best to just hard wire UL or Pentode.
What's the purpose/role of the 1N4007 diodes?
What kind of switch should be used to select btw UL and Pentode modes? Assuming the switch is never changed when the amp is energized, would this Carling switch work?
https://www.tubesandmore.com/products/switch-carling-toggle-dpdt-solder-lugs-short-bat
It's rated for 3A @ 125v and here it would see < ~300v @ 15mA. Most switches rated for HVDC are quite expensive, so if the $10 switch won't do, might be best to just hard wire UL or Pentode.
Using an ECC85/6AQ8 as a driver tube isn't exactly frugal. Do not forget the gridstopper on the EL84. E
sure, but you could go on Ebay right now and find an ECC85 for less than TubeDepot sells one of those new gold pin 12AU7s from the Exo-Pul factory...
Replace the 1N4007 in the screen circuit with a 100 Ohm resistor.
And as mentioned above, use a grid stopper on the EL84 control grid.
If the EL84 goes into Barkhausen oscillation, then replace the EL84.
One thing you do not want in an amplifier is a part or circuit that can cause a discontinuity.
There are lots of 250V rated toggle switches out there. The issue is not the amperage of the screen connection.
Never switch the UL/Pentode switch with the amplifier powered on. When the contacts are already closed, it is unusual for any arc to start when powering up, or powering down.
And the transient of switching from UL tap voltage, to Open, to Pentode mode is bad for the output tube, the output transformer, the tweeter, and your ears.
Open, you ask? Yes, the switch is Open from the time it leaves UL to the time it reaches Pentode connections (or the reverse).
Screen at high voltage, then zero volts, then back to high voltage creates Two terrible transients.
Just my opinions.
And as mentioned above, use a grid stopper on the EL84 control grid.
If the EL84 goes into Barkhausen oscillation, then replace the EL84.
One thing you do not want in an amplifier is a part or circuit that can cause a discontinuity.
There are lots of 250V rated toggle switches out there. The issue is not the amperage of the screen connection.
Never switch the UL/Pentode switch with the amplifier powered on. When the contacts are already closed, it is unusual for any arc to start when powering up, or powering down.
And the transient of switching from UL tap voltage, to Open, to Pentode mode is bad for the output tube, the output transformer, the tweeter, and your ears.
Open, you ask? Yes, the switch is Open from the time it leaves UL to the time it reaches Pentode connections (or the reverse).
Screen at high voltage, then zero volts, then back to high voltage creates Two terrible transients.
Just my opinions.
Certainly not interested in the ear-bleeding.
Is there a typical value for the grid stopper of an EL84?
Is there a typical value for the grid stopper of an EL84?
1k Ohm connected directly at g1 socket tab should stop; but will not affect the upper frequencies whether the tube is in Pentode, UL, or Triode mode.