by EWBrown » Thu Jan 11, 2007 8:10 am
The circuit for the monoblock version is exactly the same, except one channel only.
11MS8s can be replaced directly with 10GV8s, plug 'n' play, no mods required!
If you build the amp on the pine board, it will be hummy and buzzy, this can be reduced by wiring all the trannie frames together, then connecting to the ground trace next to one of the input jacks. My reccomendation, keep the wood for another crafts project, and use a metal plate or chassis.
If you use a chassis, mount all the components, except for the smaller resistors and diode bridge, on the etch side of the board. The smaller resistors can be mounted on the socket side. If you have the plastic sockets and O-rings, toss 'em and use the same-sized ceramic sockets (available at Triode, etc). Newer kits won't have the plasticrap ones)
And ditch the lamp cord switch, it is really "bush league", use a normal chassis mounted power switch, and fuse holder. (and a good time to connect a CL90 in series with the switch/fuse side of the primary circuit, see below).
I'll recap the basic mods:
Before anything else: connect a CL90 inrush current limiter (same as used on the DIY35) on the primary of the power trannie. This will make life a LOT easier for the tubes' filaments, and other PSU components.
If you use a metal chassis, ditch the crummy little board-mounted pot and use one of the Radio Shark 100K "Alps" pots for about $3, they're a bargain! Watch the pot wiring, it's easy to mess up Yellow_Light_Colorz_PDT_09
1: Replace all the 0.22 uF coupling caps with WIMA 0,15 uF "red brick" MKP caps - they fit the PCB holes perfectly!
2: Bypass the 300 ohm, 3W cathode resistors with 100 to 470 uF / 35V eiectrolytics. Improves LF response
3: Connect a 470 uF / 350-450 VDC electrolytic directly across the 220 uF PSU cap (don't jumper to any other part of the ground trace, this will create a huge hummmm ground loop situation). Gives better LF and transient response.
4: Connect a 2 - 4 uF MKP cap directly across the 100 uF PSU cap (this lowers ESR, and improves HF response).
5: Replace the BR1 bridge with four uF4007 rectifiers - this takes a bit of creativity to do. Reduces 120HZ buzz.
6: The OPTs can be replaces with Hammond 1609s, they closely match the primary impedance of the original UTK brand OPTs) 1608s will also work just fine.
Note: 11MS8s and 10GV8s aren't as UL-friendly as 11BM8s or 16A8s, try it if you want, but the results are rather disappointing.
For 11BM8s / 16A8s (note, the tubes have a different pinout) , cut the traces to the screen grid for each pentode section, then connect to the appropriate UL tap on the 1609s (1608s will also work, but 10K is a better match for the tubes.
I found that these amps will still have a residual 60HZ AC hum, that seems to be incurable. THis is caused by the poor PCB layout, the filament power traces cun between the pins of the four interstage coupling caps. This injects a little Ac signal right onto the pentode's input grids.
Two solutions, neither is really simple.
1: Cut the etches leading to all pins 4 and 5, on each tube. connect one end of a twisted pair to each tube's pins 4 and 5, and then connect the other end of the TPs directly to the filament trannie (this can be done where they connect to the PCB, or use a terminal strip.
2: DC filament voltage, though this will require a separate filament trannie, rectifiers and electrolytic cap, or use a 12VDC, 3A brick.
The power trannie's filament winding will melt down if you try rectifing and filtering it, the current rating isn't sufficient for this.
Best yet, combine the two methods!
3: Really going out on a limb: 11BM8s can be replaced with 16A8s, this requires a separate 16VAC, 2A (or greater) power trannie)
Similarly, 11MS8s (and 10GV8s) can be replaced with 6GV8, 9GV8, and any other available filament voltages). Also requires a separate filament trannie of the appropriate voltage.
What I did in my 11BM8 amp, was to add a 16VAC "junk box" filament trannie under the chassis, and then wire in a DPDT slide switch (mounted under the chassis) so that either 11BM8s or 16A8s could be used.
I like 16A8s better, because they are more easily available and dirt cheap for NOS.
6BM8s could be used, I didn't bother, just because they have a "premium" price...
50BM8s? Can be done, (use a 48VAC trannie, don't play with direct AC line power) :o Yellow_Light_Colorz_PDT_09 I haven't tried this, so you can be a "pioneer" if you dare Yellow_Light_Colorz_PDT_08
/ed B in NH
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