Oddball transition Tweed Deluxe---anyone seen this before?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by cheezit, Aug 14, 2008.

  1. cheezit

    cheezit Member

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    I've got a tweed Fender Deluxe from the 50's. I've had it since about '89, played it on and off, and just pulled it out and played with it some more...

    So here's the odd bit. The label inside the cab says it is a 5C3, and it lists the tubes. A normal 5C3 has two 6SC7's, and that's what is printed on the label, but those are crossed out, and instead 12AY7 is written in. The amp actually runs 12AX7s.

    Opening up the amp, the circuit is for sure a 5C3, I've compared it to the schematic. The sockets and all wiring appears original (though I had some preamp caps replaced at one point).

    So has anyone heard of this happening? It's like a strange transition model between a 5C3 and a...5E3??? The 5D3 had 6SC7's as well.

    The amp has always been a bit woofy and woolly for me---too much umph and not enough kerrang or chime. I'm wondering if this frankenstein tube/circuit combo might be why.
     
  2. phsyconoodler

    phsyconoodler Member

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    Try a speaker change first and see if it loses some of that wooly nature.
    Fender often had stuff laying around that had to get used up and some stuff slipped through in a different guise.That includes Labels.
    Sounds like you have a gem there though.
     
  3. cheezit

    cheezit Member

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    I just hunted through my copy of the Pittman book and it said the 5D3 took 12AY7s...then i found this:
    http://www.ampwares.com/ffg/schem/deluxe_5d3_schem.gif
    ...which shows 5D3s taking a 12AY7 and a 12AX7. However, my circuit is definitely a 5C3. So maybe this is a transitional amp after all. I'm just thrown by the fact that they dumped a new tube in without changing the preamp cathode r/c circuit or anything.

    I checked the date code and it was built in June '54. Cosmetically it looks like a ratty 5C3, but I can throw some pix up tomorrow.
     
  4. brad347

    brad347 Member

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    Fender amps from that era are very loose in terms of their model designations and what is going on inside them. There's lots of variation in what you see in practice.
     
  5. VikingAmps

    VikingAmps Member

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    I've seen at least one "5C3" model with 9 pin sockets that appeared to be factory. I don't think the 12AX7's are what's causing the woofyness tho. You could have a circuit issue or it could be the speaker like mentioned earlier. The Jensen bluebells work great in that amp.
     
  6. scottl

    scottl Member

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    I dunno.... The 12AX7 has a much higher internal plate resistance than the AY or AT. Therefore, not only does it have a lot more gain, but it has much more bass. It also has much less highs.....

     
  7. phsyconoodler

    phsyconoodler Member

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    Less highs?Hardly.More gain,yes.My 5E3 has lots of treble with a 12AX7 and sounds mellower with less treble bite with a 12AY7.
     
  8. cheezit

    cheezit Member

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    As far as the speaker goes, it's the original Jensen blue label---unfortunately, I stupidly blew the cone and the recone just has never had the same tone. It's close, just not quite as nice.

    I'm going to try some AY and AT tubes and see what happens. The part that throws me is that I don't think I have seen any "standard" circuits with an input stage from that tube family and the kind of cathode circuit (straight to ground) that the 5c3 has. Everything I can find has an r/c in parallel for the first stage. The 5D3, which is otherwise sort of close to the 5C3, has that r/c setup at the cathode of the input stage.

    Now that I look around I don't know if i have *ever* seen a 12AX7 with the cathode going straight to ground...
     
  9. brad347

    brad347 Member

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    unbypassed cathode resistors in any tube types are relatively common. The 5F1 champ has an unbypassed first stage on the schematic (sometimes the bypass cap is there in the amps though, underscoring what I said above about fender amps being loose in that time period).

    Most magnatone amps had unbypassed cathode resistors at some point or another. Even the 5F4 and 5E7 have unbypassed cathode resistors at various points in their circuits.

    You could always try sticking a bypass cap across it if you are curious. It won't hurt anything. An unbypassed cathode resistor is a form of negative feedback, and it is harder to clip a stage with an unbypassed cathode resistor because the audio will actually modulate the bias to a degree.
     
  10. cheezit

    cheezit Member

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    Sorry if I wasn't clear. The 5C3 has the cathode of the 6SC7 connected straight to ground, no resistor, no cap. The 5C1 Champ is set up like that using 6SJ7 tubes.

    I'm looking at the 5F1 schematic (http://www.ampwares.com/ffg/schem/champ_5f1_schem.gif) and the input stage has a 1500 resistor to ground, but no cap bypass. Agreed, that's plenty common, esp. in phase inverters, etc. I just don't know what effect it will have on the 12A*7 family of tubes.

    I tried a 12AY7 in the first stage and it does degunk the sound somewhat. But it's still not where I'd like it, I'll keep tube swapping and maybe get a combo I like.
     
  11. brad347

    brad347 Member

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    Connected directly to ground huh? Now I gotcha!

    So what you were trying to say is that the tube is grid-leak biased. :)

    If I had one of those early grid-leak biased tweed amps and I wanted it to sound good, I would convert it to cathode bias straight away. In my opinion the grid leak bias will really hold it back, and it's unrealistic to use any sort of distortion or boost pedals with a grid-leak biased amp.
     
  12. Schwalbe

    Schwalbe Member

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  13. brad347

    brad347 Member

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    The main disadvantage you always hear about grid leak biased designs in guitar amps is that they cannot handle a very large input signal. This is a disadvantage in my mind if you want to use any sort of overdrive pedal with the amp.

    In my opinion it's no coincidence that the TV front and wide-panel Fender tweeds are less sought-after as instruments than the narrow panel tweeds.
     
  14. cheezit

    cheezit Member

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    Thanks to everyone who has taken the time to respond so far! I'm new to the forum and it's great to find so many folks who post useful and well-informed messages.

    This has cleared up the mystery somewhat and now I have some mods to try. The amp is far from mint, both outside (ratty tweed) and inside (preamp caps have already been changed, and I'm about to replace the power supply filter caps). So I'll probably go ahead and make a few targeted updates to get it more like the 5E3 designs and see what happens.
     
  15. cheezit

    cheezit Member

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    Wooo! I tried a targeted change on the first tube stage. I popped the cathode off ground and inserted a 2.2K resistor in parallel with a 15uf cap (the values aren't ideal but it's what I had on hand), making the first tube use a more conventional bias method.

    The change is very noticeable. The tonal balance has shifted, with more and crisper highs and upper midrange, and the bass is tighter. When I turn it up, I've got to go farther to get the equivalent level of grind as before, but the overdrive is much more articulate, with more snap and more "string sound". I've been a/b'ing the change and there's no going back.

    Amazing what a little tweak will do!
     
  16. brad347

    brad347 Member

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    Sounds like you're moving in the right direction!

    As you noted, I think 2.2k is a bit high for a cathode resistor and 15µf is a bit low for the bypass cap.

    If you get a 1k (or 820Ω) and 25µf in there, you'll be getting into the zone even more, I bet. :)

    You DID remove the stuff from the grids, right? Two 5 Meg resistors going from grid to ground and two .1µf caps in series between the grid and input jack?
     
  17. cheezit

    cheezit Member

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    Not yet, though that's in the plan--to sub a 1M for the 5M pulldowns. The plate resistor is also 250k which is a lot higher than the standard 100k, so I may try that too.

    I took voltage readings with the old cathode-to-ground setup and the plates for that tube were at 50v---I thought that was due to the 250K plate resistor (the supply side of that resistor is at 357v) but then realized it was mostly due to the cathode config.

    The input caps are not affecting things too much, I thought...I'll try bypassing them and see what happens.
     
  18. brad347

    brad347 Member

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    I would remove the caps completely.

    (edited to fix)
     
    Last edited: Aug 16, 2008
  19. Axekisser

    Axekisser Member

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    I'm no expert but I think you need a grid resistor to reference the grid to ground with cathode biasing. That way the voltage drop across the cathode resistor is going to be "seen" by the grid. It also sets the impedance of the tube stage and does some filtering when combined with the Miller capacitance of the tube. Aiken's amp site has some good info on this.
     
  20. Pedro58

    Pedro58 Member

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    A possibility is that someone modded it to fit/run the 12AX7 type tubes. Lots of folks did this when they couldn't find the 6SC7's that weren't noisy. Thus, you might look at going back the 'original' tube for that pre-amp, the 6SC7. I've got a 5C3 with the metal ones and it is a bit woofy, but cool.
     

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