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Let's Mod my Allen 75's! Please help make them sing.
- Thread starter Fasterdamnit
- Start date
Fasterdamnit
Super Member
Nekkid!
OK-
Here is the current state of build and they sound VERY good. I am excited to see how far they can go with further tweeks.
power supply caps went from:
140UF and 140uf in series to 470uf-470uf + 0.47uf poly bypass
Multicap 40-20-20 to 160-140 (lost a leg due to removing the 12AY7) + 0.1uf poly bypass.
Bias supply 30-30 to 47-47-100 plus 0.1 poly bypass
Diodes replaced with hexfreds.
All mylar wax covered coupling caps replaced with Panasonic polypropylene.
The 0.1uf coupling caps feeding the output tubes were upped to 1uf.
250pf replaced with silver mica.
Bias circuit resistors replaced with 1% wire wound.
Bias measurement resistors changed from 5r to 10r.
Since the rest of the resistors are well thought of IRC metal film, I now have quite a few new spares.
Input bypasses the gain pot straight to the EF86.
Two carbons left in the bias balance circuit so I need to find some 510k's.
More to come!
Currently spinning Beatles- Love..
Tubes:
CCCP EF86
One GE and one RCA 6SN7 per amp.
Little ol' KT120 outputs biased at .65
OK-
Here is the current state of build and they sound VERY good. I am excited to see how far they can go with further tweeks.
power supply caps went from:
140UF and 140uf in series to 470uf-470uf + 0.47uf poly bypass
Multicap 40-20-20 to 160-140 (lost a leg due to removing the 12AY7) + 0.1uf poly bypass.
Bias supply 30-30 to 47-47-100 plus 0.1 poly bypass
Diodes replaced with hexfreds.
All mylar wax covered coupling caps replaced with Panasonic polypropylene.
The 0.1uf coupling caps feeding the output tubes were upped to 1uf.
250pf replaced with silver mica.
Bias circuit resistors replaced with 1% wire wound.
Bias measurement resistors changed from 5r to 10r.
Since the rest of the resistors are well thought of IRC metal film, I now have quite a few new spares.
Input bypasses the gain pot straight to the EF86.
Two carbons left in the bias balance circuit so I need to find some 510k's.
More to come!
Currently spinning Beatles- Love..
Tubes:
CCCP EF86
One GE and one RCA 6SN7 per amp.
Little ol' KT120 outputs biased at .65
Attachments
holmesuser01
Active Member
A couple of years later, I just aquired one of these fine amplifiers. I've got some bias issues between the outputs, but it's good condition,
I'll be actually using this amplifier in an otherwise solid state unit to drive a pair of horns.
I'll be actually using this amplifier in an otherwise solid state unit to drive a pair of horns.
nyone?
I now see the mod schematics on page one. Torch browser was not allowing their viewing... iPad’s browser does. On the AO75...keep noticing that the ceramic disc cap on the power line that is attached to the chassis by a screw, is not being removed by all. I unbolted it from the chassis, wrapped it well in electric tape and left it dangling. I don’t know what the designers of the AO75 were thinking of when they did this..
If I may, these caps can filter noise on the AC main to some degree, but mainly are used to create a virtual ground for the amplifier. That is because with a power transformer, chassis ground will normally float completely free relative to earth (or true) ground (defined by the neutral side of the AC line back in the day), other than for leakage between the primary and secondary windings in the power transformer. Since there is a disconnect between these two ground systems, it leave the chassis as actually being able to pick up its own noise signals, which can be amplified and reproduced as noise in the speakers in some applications. The purpose of the cap from one side of the AC line to the chassis then is to provide enough of a low impedance path to short out any noise that might be developed between the two ground systems, without being so large in value as to pass any significant current to be harmful to anyone should be AC plug be inserted so that the cap is connected to the hot side of the AC line. A value of .005 - .01 uF is common. As such, the cap is technically called an AC Bypass Cap, but has gained the nickname death cap should it ever short and the plug be inserted so as to make the cap reference the hot side of the AC line.
For those manufacturers that chose to use ceramic caps in this application, there are virtually never any problems because such caps normally have a near indefinite life span. For manufacturers that used mylar caps in that application however, they should be immediately replaced (if retained) without question. Special "fail open" caps are available to fulfill these applications to eliminate any concern for safety should they fail.
Personally, for equipment that use these caps and employ non-polarized power cords, I have always recommended the power cord be replaced with a two-prong polarized power cord set, connected so that the AC Bypass Cap is connected to the neutral side of the AC line. This has proven to be a good compromise in achieving low noise, while providing an improved safety standard over that provided by the original design.
Dave
For those manufacturers that chose to use ceramic caps in this application, there are virtually never any problems because such caps normally have a near indefinite life span. For manufacturers that used mylar caps in that application however, they should be immediately replaced (if retained) without question. Special "fail open" caps are available to fulfill these applications to eliminate any concern for safety should they fail.
Personally, for equipment that use these caps and employ non-polarized power cords, I have always recommended the power cord be replaced with a two-prong polarized power cord set, connected so that the AC Bypass Cap is connected to the neutral side of the AC line. This has proven to be a good compromise in achieving low noise, while providing an improved safety standard over that provided by the original design.
Dave
