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octal front end worth it?

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  • octal front end worth it?


    I need a low powerded tweed am for a studio project that gives me that early 50s crunch at low volume. A 5C3 seems to be perfect. Since it'S not a presige project but rather a workhorse I am going the Mojo kit way (5E3) and modify , get me a Mercury 5C3 OT and all the other parts necessary for the conversion.

    here's the question: I heard that the 6SC7 preamp tubes are more or less identical to a modern 12AT7 apart from the pinout. Is that true? If so, will it be worth to drill out the chassis for octals and get the more expensive NOS tubes or would you go with novals, have more options for different tube combinations (AT, AY, AX, ...) and enjoy the easier tube replacement in case of a tube failure?

    any opinions are appreciated ... thanks!!!

  • #2
    Looking at the data sheets, it looks as though it's closer to a 5751 with Gm of 1325 vs.1200, both have a u of 70 and plate resistance 53k vs. 58k. Also bias voltages are close as well. Heater current draw isn't an issue either with them being very close to each other. I personally like the different 12A_7s because of the variety and not having to replace the socket. Others may disagree.
    Turn it up so that everything is louder than everything else.


    • #3
      Originally posted by Bluefinger View Post
      I heard that the 6SC7 preamp tubes are more or less identical to a modern 12AT7 apart from the pinout. Is that true?
      What DRH1958 wrote ^^^. Plus 6SC7 tend to be rattly & microphonic. If you have a big box full of 'em you may find a few that behave themselves. It was a big step up in audio quality when Fender moved to 12AY7 and 12AX7 early 50's.

      You might want to consider building with 6SL7 but the same rattle & micro observation applies. NOS are getting spendy & cost is no guarantee of trouble free operation. New Sensor sells a couple varieties of Russian made SL's, also costly. I haven't spent the $$$ to try 'em. When I run across an amp that needs 6SL7 these days, I make up adapters to fit 12AX7 or 12AY7 to their octal sockets. This has kept more than a couple B-15 owners happy.

      Should you opt for the 12A-7 types, note many guitarists find the 12AT7 as a preamp to have dry sterile tone. But you have to experiment, try a couple varieties, pick the one that makes your ears happy.
      Enjoy. Every. Sandwich.


      • #4
        Does the eye candy of the NOS 6s outweigh the utility of the modern tubes? Whether the modern tubes are 'equivalent' or not doesn't play into it, as I see it. You will be able to get the sound you want with whatever components. You may have to adjust for gain or other tube properties, but the early fender designs were nowhere near the edge of the envelope. Plenty of tweakability. Plus what Leo_G said; tube swapping as part of the design process makes tweaking easier!
        If it still won't get loud enough, it's probably broken. - Steve Conner
        If the thing works, stop fixing it. - Enzo
        We need more chaos in music, in art... I'm here to make it. - Justin Thomas
        MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey


        • #5
          I have built a couple of 5B3's using both 6SC7 and 6SL7's. Found it hard to find either that were not microphonic. Pretty sweet amp when you finally can get it right. By the way a Rickenbacker M11 is just about the same circuit. Sovtek is making both of these tubes. I think the SL is about $12 and the SC is about $20. Or maybe it is the other away around. Any way Sovtek has new production tubes.


          • #6
            I'm still on the fence about whether octal front ends are worth all the trouble that comes with them. I have these reservations because I've never seen an amp design that I think provides a fair, one chassis, gain-corrected comparison designed to minimize the differences between the tube types. I'm thinking that if someone actually did that, then maybe people wouldn't be so crazy for octals.

            Regarding the other topic -- based on a lot of experience, I'm convinced that MM iron is highly overrated and represents a waste of money for the most part. MM iron used to be a reasonably priced substitute for Hammond iron. There was a time when it was actually cheaper to source application-specific MM iron than it was to source equivalent Hammond iron. Those days are long forgotten though... MM has become a glorified iron pimp.

            I have a dealer's account with MM that I haven't bothered to use in almost 20 years. To me their products are just over-hyped and way too expensive for what they are. There are plenty of builders who have shown us that cheap iron -- like Fender used in the tweed era -- sounds great in old Tweed circuits and that it's pointless to pay through the nose for OT MM reproductions. I'm very reluctant to say that MM is worth the money that you have to pay to get it. And I say this having about 20 units that have been sitting in inventory for more years than I can accurately remember, because I ended up preferring other iron for projects. I need to get off of my ass and unload them.

            Are the octals worth it? I'm on the fence. There are plenty of people who assert that the large plate area of the octal tubes grants them a smoothness that you don't get with the minis, but I'm not sure how much that matters if you're looking for "sweetness" and you're not intentionally driving them into distortion. I think that they definitely have less graininess when they are "abused", but I'm not sure if that isn't an artifact of the difference in gm / amplification factor. The highest of the high amplification factor octals don't get all that close to a 12AX7, so there is definitely an apples to oranges component there. The 12AX7 is just a more gainy tube, which makes comparisons to any octal fairly meaningless. A more apples to apples comparison could be a 12A?7, but people who grew up on the gain of a 12AX7 view the others as "lifeless." I still don't have a really good grasp on how much of that is due to amplification factor differences, and how much is actually due to the sound of the tubes themselves, when other factors are made equal.

            I keep telling myself that I'm going to build an amp that has an octal preamp on one side, and a noval preamp on the other side, equally scaled for gain, to get a better handle on this, but I've just never gotten around to doing it.

            I'm not convinced that the octal preamp difference is all that it's made out to be -- I think you can get the same "sweetness" out of the mini tubes while avoiding all of the headaches inherent in octals, by tailoring your load lines to provide similar operating conditions, and avoiding the overvoltage cascading distortion sort of thing.

            That said, there seems to be a strong contingent of people, especially in the swing guitar, jump blues and rockabilly/psychobilly world, who think that octal preamp tubes and MM transformers make all the difference. Those guys don't seem to be hesitant to shell out $2500 for boutique 15W amps with plain-jane simple circuits. Some are honest attempts to reproduce an EH-150 with 6SC7 and 6N7, while others are less genuine reproductions. I've seen at least one boutique builder who discontinued his Valco reproduction and relabelled it as an EH150 when the prices for Gibsons went up. And another is selling a Gibson repro that really looks like it's based on a 6SL7 Ampeg B-12. Caveat emptor.

            Some of these builders specifically pimp their wares as having authentic MM repro iron. Using 1930s reproduction PT iron has to be the worst approach that someone could take, as it's guaranteed to generate over-voltage problems. I'm not sure whether the builders who are sourcing the repro PT iron do this becuase they're just catering to the demands of buyers who have been drinking the MM Kool-Aid, or whether they're just taking a short-cut in designing the circuit because they don't know how to spec the correct modern transformer for the job. To me there seems to be a disconnect between the good design practices that these guys should be following, and the idea of just pimping a vintage reproduction that turns a blind eye to basic design problems that have followed the gradual increase in source voltage between 1930 and 2016.

            I know this post may be rambling a bit, but I've been looking at these octal amps lately, and scratching my head. Some of the things that people love to buy just make no sense, it's all about perceived mojo. If this bothers me long enough I might just have to build that comparison amp so that I can stop wondering about this. I know, this isn't the answer you were looking for. sorry about that. Maybe someone else has a better handle on this?
            Last edited by bob p; 08-10-2016, 08:03 PM.
            "Stand back, I'm holding a calculator." - chinrest

            "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H


            • #7
              Agree with Bob, if it's not a prestige amp, as you say, you could save a lot of money by using a non 'prestiged priced' OT.
              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey


              • #8
                Kind of went through this last year building a 5C3. They are microphonic somewhat but i used rubber grommets on the tube socket. If you can find the glass version 6sc7gt they are great. I got rid of the grid leak bias also. Overdrive with humbuckers was just too much.Click image for larger version

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                • #9
                  ooh -- grid leak bias -- I didn't even think about that. IME it's a recipe for AC coupled hum unless you go to the trouble of DC filament supplies.
                  "Stand back, I'm holding a calculator." - chinrest

                  "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H


                  • #10
                    Guess that I should admit that I scrapped the last 5B3, salvaged what I could and built an AB763 20 watt light to replace it...There is a reason that Leo used the AB763 pre amp circuit in so many amps.


                    • #11
                      I'd go with the 12--7s I have an old PA amp that I converted for guitar use. The circuit is similar to a 5C5 pro. When I got it it had 6sc7 preamp tubes which were very microphonic, so I rewired the sockets to use 6sl7s They were better initially but went microphonic too after some use (or maybe I just started noticing it) I think I tried another 8 pin tube also, but I don't remember which one. Finally I got fed up and made some converters so I could use 9 pin 12--7. Not only did that stop the microphinics, but sounded better too. Some think octals sound "warmer" to me they sounded dull in this amp.
                      Vote like your future depends on it.


                      • #12
                        Thanks a lot for all the responses. I'm a bit relictant to go with the octals straight away. So I'm not going to drill out the tube sockets, start with a regular 5E3 and modify it from there. It's a simple enough amp to try two or three different different boards.

                        I am just looking for that early 50s tone with low headroom and swwet breakup you hear on the old chess records, played by the Myers brothers, Muddy, Jimmy Rogers and so on. There is something happening tone wise that I don't hear in a regular 5E3. smaller amps sound too boxy but I need low volume for a recording situation in teh same room with everybody. Maybe I'll just try a lower voltage PT for a start or drop the preamp voltages a but below 5E3 specs ... it's supposed to be a reliable rather quiet amp too. Guess I'll try a couple of things.


                        • #13
                          Hi Guys

                          The guys who like octal preamp tubes like them a lot. There is a nuance of tone found with those large-structure tubes that will be lost doing direct stage-by-stage replacements with 9-pin tubes. But...

                          If you replace each octal gain stage with parallel 9-pin stages - a whole 9-pin bottle per octal stage - then you can mimic the smoothness of the octal and have 9-pin economy of replacement. When you parallel the sections of, say, a 12AX7, the mu stays the same, rp goes down, gm goes up but the important thing is that the tone is slightly mellower.

                          A good trick to know.

                          Have fun