WineCaster
Tele-Holic
Here's what I'm working with. I have a bias pot as well.
Recommend an early breakup 12ax7 for V1 + Tremolo
- Thread starter WineCaster
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Here's what I'm working with. I have a bias pot as well.
Commodore 64
Friend of Leo's
Much: Unfortunately I had some Tung Sol (10,000 hr) 5881s break up in that way too! 
Ok, on a 6V6 amp that I wanted to compress and OD early and sweetly, I'd use cathode bias, cathodyne PI, and a prePI master volume control.
You got a schematic. I can't interpret layouts as easily as I should. I think you are cascading a 12AX7 into another, and out to cathodyne PI. With some bias wobble tremolo?
Your input stage feeds 3 different grid stopper resistances depending on which one you plug into. OK that's kinda cool.
Ok, on a 6V6 amp that I wanted to compress and OD early and sweetly, I'd use cathode bias, cathodyne PI, and a prePI master volume control.
You got a schematic. I can't interpret layouts as easily as I should. I think you are cascading a 12AX7 into another, and out to cathodyne PI. With some bias wobble tremolo?
Your input stage feeds 3 different grid stopper resistances depending on which one you plug into. OK that's kinda cool.
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muchxs
Doctor of Teleocity
Change that to a presence control at per the 5E5A Pro. If you eyeball the Pro schematic it has a 100k feedback resistor. You still have NFB with your 100k feedback resistor.
If you have NFB... make it earn its keep! A presence control is simply a 5k lin pot with a .1uf cap shunting some signal to the ground. You might actually want to increase your NFB to make your presence pot more effective. Use a 27k resistor instead of the stock 56k.
Just wire the pot inside the chassis. Set it so it knocks off some of the highs.
Another way to do it is a Matchless style "cut" control.
Opaltone
Tele-Holic
I agree.
And I understand how one might mis-interpret how I refer to tubes that consistently enable the circuit to break up earlier when they're installed as having "earlier-breakup" character - it may seem like I don't understand your point. My reference to early breakup is a linguistic convenience. And the term is only useful in a relative sense. For example: I know that nominally-new testing samples of '50s black-plate 12AX7 will break up in a particular circuit earlier than '60s short gray-plate 12AX7A every time. This is very predictable character, and I've always found it useful information to become aware of.
- Thom
muchxs
Doctor of Teleocity
You mean like a 5E3? Less the master volume of course.
Kinda makes me wonder why a tweed Deluxe overdrives instantly with 1/2 a 12AY7 on the front end while the Vibrolux stays clean. The Harvard does the same thing as the Vibrolux with its goofball 6AT6. It's essentially half a 5751 with extra stuff inside.
1/2 of V1 is the first preamp stage while the other half is the trem oscillator.
There may be a little gain to be had by swapping plate resistors.
theprofessor
Poster Extraordinaire
I neglected to say this at the outset. Winecaster, that's a really beautiful amp. I love the wooden cabinet and the stain color and the way the Vox grill cloth works with it. Very tasteful!
What ever terminology we use: As has been pointed out earlier in the thread, the V1 tube in this amp won't "break up" at all, unless you seriously boost the signal from the guitar before the input.
If you find that a certain tube has "earlier break up", put it in V2. Still, the 6V6's might be overdriven before the PI is.
If you don't want to add an extra pot, a cap over the NFB resistor could be tried to tame some highs.
WineCaster
Tele-Holic
Thank you for that! This is the third amp I've built. Each one is a venture into learning more about wood working and electronics. I clearly need more help with the latter but I think that's more likely for most folks.
Opaltone
Tele-Holic
You are welcome to believe what you choose to believe - it won't change the decades of experience I've had with tube swaps that I bring to my recommendations. Maybe try for yourself sometime?
In my previous post I seem to have confused you with the OP.
But I don't really understand what you mean, try what? Pushing the first triode in a Fender amp into overdrive with a normal guitar signal by swapping tubes?
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Opaltone
Tele-Holic
If you're really curious about this phenomenon, here's what I suggest:
Obtain a '50s long black-plate RCA 12AX7 and a '60s short gray-plate 12AX7A that both test as nominal new, and both with the same approximate transconductance values on the same tester. It helps if you have your own tester, for consistency.
Choose your BF Fender channel, and identify the first input tube for the appropriate channel (V1 for Normal, V2 for the Vibrato channel). Without changing any settings, A-B swap between those two vintage 12AX7 tubes. Play with a dynamic touch, try single-note runs vs. double-stops, and listen carefully to how clean the signal remains.
I chose RCA tubes for this example because they're the most commonly available samples to compare. There are others I can name (and have, previously), but they're often more rare.
Would like to hear your observations, if you feel like reporting back. If you lived nearby, I'd welcome you over for a demonstration. If you're ever in the San Francisco Bay Area, drop me a line.
- Thom
Commodore 64
Friend of Leo's
I don't think any of us are doubting you Opaltone. Some of us believe that we can modify the circuit to achieve the same results.
If I ever obtain those tubes I'd be happy to do that test, maybe even hook up an oscilloscope to the plate of the triode. And most likely, I'd be happy to just play that amp all day long
Opaltone
Tele-Holic
I'd like to hear what your scope tells you is going on.
Opaltone
Tele-Holic
I also firmly believe you can mod the circuit.
I'm happy being able to take less than 60 sec to swap a preamp tube.
- T
BobbyZ
Doctor of Teleocity
Just a though. You might try a pair of TS RI 6v6s. I've never actually compared them to JJ 6v6s but the internet says they break up sooner. . . .
Like them in my Reverberockets.
Also if you're running a real efficient speaker (like a Celestion) and playing were you can't get loud, try something else.
Like them in my Reverberockets.
Also if you're running a real efficient speaker (like a Celestion) and playing were you can't get loud, try something else.
I also firmly believe you can mod the circuit.
I'm happy being able to take less than 60 sec to swap a preamp tube.
- T
Maybe we could hear some slight "dirt" from V1 if we sit and listen carefully, rolling vintage tubes.
But it seems the OP is looking for something a bit less subtle. Plenty of things have already been suggested.
So call me a fool. But I still don't think finding a tube that somehow will have its headroom penetrated in V1 by a 200-600 mV peak to peak guitar signal would be top priority in this case. Chances are if he eventally finds it, it will not do much in solving his problem. JMHO.
Opaltone
Tele-Holic
Yeah - a clean boost can also be a beautiful thing!
I agree, JJ 6V6 would be last choice for that amp and what your wanting, being as when it starts to breakup, it will be the the power section that's gonna deliver the crunch. You've got the highest headroom, sounds more like a baby 6L6 tube in it there is.
I'd source some good old stock RCA or Tungsol, to build that amp (gorgeous cab by the way) and not get the best, saggiest 6V6 grind out of it you can seems counter productive. That amps breakup comes from the power section, not the preamp tube.
Sure find you a old RCA longplate so you drive those 6v6 as hard as you can, some of the RCA long grey plate pulls from Wurlitzer organs are some strong a** tubes.
