Due to limits on manual dexterity and funds ( sound familar? ) this project took a LOT longer that I had anticipated. I will factor these into future estimates!!
I also chose to build the unit in two modules.
The power supply ( cage removed ) is Dr. van Waarde's basic configuration with a few mods:
1. I wired the low and high voltage sections as separate circuits allowing the unit to run in "standby" mode, or for "burn-in" heating of new tubes before high voltage is supplied. Each has independent, switching, fusing, and indicators.
2. I chose HAMMOND transformers: 166P10 @ 10V/5A and 261G6 @ 250VCT/45VA units. This allows me to run the B+ at 200VDC, adjustable downward, if desired. ( I prefer an "at plate" reading of about 170 - 175VDC on the 6AS7G ) Hi/Lo/Gr test jacks on back side.
3. All resistors are at least double the design power rating to insure lower temperature operation. Low voltage rectification was through a Motorola MDA 48VAC/25A chassis mounted Metal Case Bridge for the same reason.
4. I found the LT1083 regulator ( Rated at 7.5A ) to run much cooler than the 1084 unit specified, hopefully leading to a longer lifespan. The ambient internal chassis temperature never exceeds 113F. even in prolonged use and no fan cooling is required.
5. I added an octal 6VDC/60 second time delay tube ( contacts in series with the HV primary AC ) to allow the amplifier tubes to heat and stabilize before high voltage "kicks in". ( This was standard in all our military equipment )
The amplifier is as you see a rather unusual interpretation:
1. The glass chimney is very functional and uses the abundant heat radiating from the 6AS7G to supply convection cooling for itself and the components in the closed die cast chassis. The ceramic octal socket is suspended in a 2 1/2" Dia. opening ( the inside dia. of the glass chimney ) and cool air is drawn upward through screened/filtered vents in the bottom plate. Also keeps little fingers away from a VERY HOT tube!
2. The input RCA jacks are on front as a "tip of the hat" to DYNACO and matches my DYNACO ST70 PA. I also added two output RCA jacks on back; powered when the headphone plug is pulled from the "shorting" headphone jack to supply "line preamp" functionality.
3. Internally only one really important change. The 3.3Kohm cathode resistor, R6, is very underrated at ten watts, and generated excessive heat in seconds! I changed to CADDOCK MP820's, TO-220 style chassis mounted, film power resistors. Rated at 20 Watts/3.3K/1% they barely get warm to the touch. ( Strongly recommended mod. )
The two modules are connected by a fully shielded power cord with a NEUTRIC 4pin plug/bulkhead assembly at the amplifier. Both circuit's grounds "float" and both chassis shield/ground through a three prong AC power cord. All parts are premium grade; my reasoning being that given the amount of time invested in this project, "cutting corners" in parts quality is not cost justified.
If you have ANY questions at all, please feel free to contact me at:
VAKLOV@PEOPLEPC.COM
and I will try to answer them as best I can.
James A.Deaver
