Hello guys, I don't think I posted this, but I built an octal preamp tube based Express. I was influenced by the Red Iron T-Rex to a certain degree, but I ended up changing a lot. The Tone stack is a James style and the values are exactly as a Garnet G250TR. I also added a PPIMV. The amp is a bit of a Frankenstein, but is sounds amazing to my ears....
Cool to see it again though. It sounded really good.
Thanks, I haven't been around and I forgot..... Doh! I will use the search engine next time....
But, the tone stack values changed drastically from the last thread. Also, the PI couplers went from .1's to .039's. Also added a cut control.
I also removed 50 uf of filtering on the screen grids. So now the filtering is:
50 uf, 50 uf, 50 uf, 22uf. 22uf, 22uf, 22uf. Yes, that is 7 B+ steps in this little beasty...
The schematic posted above is the latest. I think it is now where it should be. I went back to the .027 as the first coupler and .1's on the PI to power tubes. I separated the cathodes in the power section and added 1 Ohm resistors so I can check the current that way too. I was playing it again today and it flat out sounds amazing. I got a pair of ANOS TungSol 6550's and they sound amazing! The tubes are: V1-Modern Tungsol Reissue 6SL7, V2-NOS TungSol, V3-NOS GE, V4-5 ANOS Tungsol 6550's. BTW, ignore the 65' JTM45 in the background...
First, the last two B+ strings in the preamp all have 1.5k 3 watt resistors.
Second, the 120K on the grid of V2 is not on a switch, it is always connected to ground. There is so much gain available, if you took off that resistor, you would immediately go into grid overload.
Third, there is no 220pf between the cathode and plate on V2a.
Fourth, the coupler between V2a and the grid of V2b is a 2200pf or .0022uf.
Fifth, the cathodes of V3 are connected to a 75 ohm resistor.
Sixth, the PPIMV pot is wired backwards like a 5E3 so that the .1 couplers feed the wipers. This way there is always a 250k reference to ground through the pots.
Seventh, the cut capacitor is a 390pf, not a 500pf.
Eighth, the power tubes can be either 6L6gc's or 6550's with the 3.6k primary Classictone transformer. I would use a TW style 8k primary if I were to use KT66's. Currently, I am running a pair of ANOS TungSol 6550A's and they sound awesome!
Ninth, I don't have a 220k across the B1+ cap to ground. All the caps are rated for 500+
Tenth, the Hammond PT I used has a 5V tap that I use for the lamp and there is a center tap for the filaments. Also, you only need two diodes per ac feed from the HV.
Eleventh, the OT has 4, 8, and 16 Ohm outputs.
Grounding Scheme:
Point 1 is on one of the mounting screws of V1. I have the input jack, Cathodes, ground of the B6+ cap, and filament center tap grounded there.
Point 2 is on one of the mounting screws of V2. I have the cathodes, B5-B4 grounds of B+ caps grounded there.
Point 3 is on one of the mounting screws off V3. I have the OT, speaker jacks, B3, PI Cathode, PPIMV grounded there.
Point 4 is on one of the mounting screws of the second can cap. I have the V4-V5 cathodes, both can caps, and the HT center tap grounded there.
Point 5 Is near the input of the power cable and is the chassis ground.
I have learned so much here, I hope this is a helpful little grain of sand contribution to the rest of you. This is honestly the best OD amp I have ever built....
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