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Discussion in 'Amp Tech Center' started by DennisM, Sep 28, 2020.
...Like the ones Rob Robinette does. Couldn't find the 6g2 on his site. Thanks!
Our smart friend @82LesPaul had a thread about his version (and I added a version of mine there). IIRC, mine had a few errors, some mods, and wandered further from stock. I'll stick an updated PNG and PDF of mine below, but his may be more useful.
More basically, you can build from the Fender layout (and more importantly, use the Fender *schematic* to understand what you're building in terms of actual function). You just update a very few things: The AC power wiring, heaters, and grounds should be done the 'modern' way (Rob basically shows these on his layouts -- you just fit to number of tubes and grounds on a 6G2). The dropping and bias resistors are upgraded to 2 or 3W. Stuff like that.
Thank you very much! The original layout on the fender field guide site are harder to read...I can't make out some of the penciled in values.
I've got an old HiFi amp that has a lot of these components already that I could maybe repurpose into an amp head, and save a bunch on purchasing new components...hmmm...
Hmm, see if this is easier to read...
My copy of the schematic isn't perfect, but FWIW...
Yeah, that one is considerably better. I saved it to my PC. Thanks @King Fan !!
Thanks! Looks good, but why the high wattage values on the resistors?
Yikes, I'm not talking all the resistors, just the two at the left end of the board (the dropping resistors) and the first one (especially) on the bias board. Higher wattage ratings 'cuz they, um, handle high voltages? Fender could get away with 1W there, but we can easily afford to upgrade; also many folks want heat / flame resistant there (and maybe for the 1.5K grid stoppers on the 6V6 sockets, also a 'hot spot') and metal oxide resistors are most commonly available in 2-3W sizes.
This is not a layout, but it's a pretty high-res photo of my completed build. Maybe that would be helpful, too. You'll notice a few things, like increased power filtering, a negative feedback switch, the bias pot such as that one added in @King Fan 's layout, and so forth.
No particular reason. Just because. Although I have heard that higher wattage resistors have less noise. I just like the xicon 1 watt carbon film resistors. They have long strong leads and they are not very expensive and they just work.
Wow. Nice Job! Very clean wiring. May I impose and ask what you are getting at B+ 1, 2 and 3? How many mA on the bias current? Just curious, are those 25/50's on the bypass caps in the preamp? And one more, what's the old filter values fender used on the originals? They don't show it on their layout?
Mojo is now carrying the cabs, chassis', faceplates etc. for the 6g2. They said they would cut me a tag board, but I would have to use the cap can because it's true vintage replica. Nice job on your tag board!
I haven't measured lately, but here's where I was at some point when I was really liking the sound:
309 plate volts, with a pair of 6V6's at about 23 mA of current. I _think_ I used the lower-voltage taps on my power transformer (Classictone 40-18027) to get there. I'd have to pull the chassis again and do some measurements to get all those numbers. You'll notice that I used more filtering in the power section than on the traditional schematic. I think I've got two 40uF filter caps, followed by a 22uF for the preamp section. I don't know what the original values were on that cap can used in the real ones, but it certainly doesn't have two 40uF sections. And yes, I imagine those are 25uF/50V cathode bypass caps. That sounds right.
A few thoughts on a 6g2 in general, and one from Mojotone in particular. My opinions, of course:
I highly recommend more filtering for this amp, such as what I did with 40-40-22, or what the layout above from @82LesPaul shows, which is 30-30-30. I toyed around with several filtering schemes, and my choice is based on those comparisons.
And I really like those low plate voltages I mentioned above (ca. 309 DCV)
This amp really, really benefits from having an option of no negative feedback. I almost never turn on the NFB.
It's cool that Mojo is producing chassis and faceplates and such again. That will give you an authentic 6g2 look, with the knob spacing and the like. However, there are also advantages to building in a Princeton Reverb cabinet, like I did. You can easily get a 12" baffle in a PR cab, if you want, and a PR cab is pretty ubiquitous, whereas a 6g2 cab is a specialty item. However, if you do use a PR cab, you'll need to get a faceplate designed to fit it.
And while I'm at it, I've tried lots of speakers with the 6g2. If you're going with a 10" speaker, I highly recommend the Alessandro GA10-SC64.
If you're interested in putting the filter caps on a board instead of using a separate can capacitor the traditional way, I'm happy to share my board design that can be ordered from Doug Hoffman.
Related to the Mojotone kit idea: I wouldn't use Mojotone transformers unless I was prepared to bring the voltages down somehow. And that might be harder in an adjustable fixed-bias amp like this. The Mojotone transformers are well made, but they voltages are always too darn high, at least in my experience.
Thanks much. Tons of knowledge as always!!
Sure thing! Just passing on some knowledge and experience I got here at TDPRI!
I agree with @theprofessor -- he really knows this amp. Lots of discussions about 6G2 builds and choices on TDPRI; search '6G2' in the title for threads by the Prof, mjcyates, and @keithb7 , among others. Heck, you want TMI, search for mine -- I always overthink my builds, and my 6G2 planning was more confused / confusing than usual.
This is where the schematic is your friend. It shows 30-30-30 (above) -- which is probably fine. *But* there is evidence Fender actually used 20-20-20 in some real 6G2s, and OTOH more filtering is also possible. For a really smart discussion of 'if and how it matters' you can't beat this post from our smart friend @JD0x0 -- or read the whole thread for other folks' good ideas. Given that, I went with 30-22-22 (the same as 30-20-20) and it's great. BTW, you get to decide if you'll use a cap can -- simple, original, but then which can and what values?
A much more important choice is the PT, as the Prof says. (I like your question about B+ -- another area where the schematic is very helpful but *may* not agree entirely with measured values on vintage amps). FWIW mine, with the 40-18027 on the lower-HT taps, came in around 320 B+ -- and sounds perfect there.
I looked at the schematic after I asked the question. I was quite surprised to see 30 30 30. My 5e3 has 16uF's. My PRRI has 22uF. Mojo uses 5e3 PT with "reduced" voltage....330VDC. They also use the 5e3 OT. My PT (from AES) in the 5e3 I built is 330VDC without all the foreign primary choices. I'll have to check it again, but if memory serves my plate V is around 325/330. I don't like the sound of my 5e3 so I was thinking of taking it apart as using the PT and OT, sockets, tubes, switch, pilot etc. on the 6g2 build.
Have fun! The 6G2's are great amps!
Your 330-0-330 calculates to B+ north of 410VDC in a 6G2. The schematic B+ (315VDC) can be exceeded, but 410??? As @theprofessor says, maybe not a good idea. That's why you see many of us using something like the CT 40-18027 on it's lower, 275-0-275 tap. Hammond no doubt has something similar.