Ok: all 12Ax7's or slip a 12AT7 in V1?

[PraiseCaster]

Ok, here's my amp schematic:



All my modifications are in red.

Its a Pignose G40V that has been modified into Plexi territory, albiet with 6L6 bottles.

Now, my amp has a Master Volume, and some recent talk I have heard/read about MV amps:

In discussions, I have heard that an amp with a Master Volume, does not necessarily need say a 12AT7 in the V1 position to, "Smooth it out and give it more headroom", because that set-up was primarily for non master volume amps.

This point of view, begs me to ask these questions:

1) is this statement true, partially true, or false?
2) If I were to go with all 12AX7's, would I loose out any headroom, or would it remain the same, just with more gain?

Thanks!

[JohnnyCrash]

Its not entirely true. Its a matter of fine tuning and/or taste - even in MV amps.

Headroom is primarily the function of the power section, but overall gain will be affected for sure.

I'd say try what the amp was designed for first... then figure out what you like and what you don't like. At this point you can more easily determine if a preamp gain "downgrade", speaker swap, tonestack mod, or bias adjustment would serve you best.

[JohnnyCrash]

BTW, if you're referring to a recent thread where I suggested avoiding a preamp tube swap... that was for different reasons :)

It seemed to ruffle some feathers too.

The user was new to tube amps and was complaining of not enough headroom in a 60w amp that can get plenty of good cleans at high volumes... I suspected amp trouble, but I may not be understanding everything he was going for or describing.

For a tube experienced dude like you with a "plexi" ish amp, I figure gain/Rock would be the name of the game - but I'd suggest trying both tubes :)

[nonvintage]

While your at it try a 5751. It's basically a lower gain 12ax7. and Mullard 12at7's are nice in V1. Of those 3 you should find something that floats your boat. Whether you'll perceive more headroom is the question?

[jhundt]

that's a cute little circuit - looks kinda like an old Bassman or Marshall 50. I never saw a Pignose like that, only that horrible little shoe-box thing. How's it sound?

[PraiseCaster]

Originally Posted by jhundt

that's a cute little circuit - looks kinda like an old Bassman or Marshall 50. I never saw a Pignose like that, only that horrible little shoe-box thing. How's it sound?

Yeah, I have been told that Dennis Kager (Ampeg fame), designed this little piggy on the old Bassman circuit. This is what it originally looked like:


This is what it looks like now:


How does it sound? Awesome! Absolutely awesome! The mods that a buddy of mine came up with really brought a lot out of the amp. Just the improvement in the presence control alone seems like easily a hundred-fold improvement.

[PraiseCaster]

Originally Posted by JohnnyCrash

BTW, if you're referring to a recent thread where I suggested avoiding a preamp tube swap... that was for different reasons :)

Partially. I have had other discussions where this same train of thought has come up, so combined, I felt it was a good question to ask.

I should also add, I like to set my amp up to run clean, and I push the tubes with a TS808 Tube Screamer for my overdrive. Thats why I ask, does it effect the clean headroom.

I like the dynamics of going from clean to od in my single channel amp. Though, when I push the volume up, and then dial in the level with the mv, we are talking some really sweet tones! But my ts really does get a great od as well.

[mad dog]

In my limited experience, it really depends on the amp. Some builders voice amps in a certain ways, have a V1 tube in mind for that sound. By all means experiment, and see for yourself. I tried it recently with a SR and VR. Pulled V1 (to subtly boost gain in the reverb channel), then tried a 5751. Me, I preferred them that way, a little less volume, a little more headroom and clarity in the middle volume ranges. But the guy who owns the amps liked the 12ax7 in there much better.

[11 Gauge]

I swapped just about every component in that circuit, so I can give you a few pointers that will push it in one direction or another.

I believe it's R3 (first series resistor after V1A), which is 510 K - try swapping for a 1 meg. Also, lose the little 47 pF cap that bypasses the whole affair and ties back onto the volume (or gain) pot's wiper.

Now run a 1 meg resistor from that wiper terminal to ground. Alternatively, bypass it with a 220 pF to 470 pF cap.

The 68 K resistor that was added after the 470 K resistor to ground (R15) is way too subtle - you won't really hear a difference. Even the stock 470 K value on Marshalls is subtle. Either bump it up to 1 meg, or just eliminate it. You can get much more tonal variation/gain levels by fiddling with R15 itself. Some folks drop it as low as 150K (the lower it is, the more the gain is dropped).

Replace the cap to ground after the treble pot wiper, but not with the stock value (way too large). Try 220 pF to 470 pF. Alternatively, move it after the MV, to it's wiper, instead of the treble pot's wiper.

Completely bypass the cap that falls right before the master volume (roughly 500 K).

Most of these changes will "balance" the signal a little more through the preamp circuit.

The problem with it in stock form is that it's too hot on the front end, and too loaded down on the back end. My suggested changes above will shift that around a bit.

Almost forgot - cathode bypass caps. If you do nothing else, snip the one on V1B, and don't bypass it with anything. For V1A, IMO, that cap is way too big (220 uF). I would replace it with anything from 1 - 10 uF. Even 22 uF is overkill for that first gain stage. If you still need more of a low end boost, V2A can be bypassed with a cap in the 22 - 47 uF range, but bump the cathode resistor (820 ohms) up to 2.7 K, or better yet 4.7 K.

By shifting around the gain, you should be able to run 12AX7s in all 3 slots. The most crucial areas (i.e. too much unusable gain) with a 12AX7, are all within the V1A/V1B circuitry. If it's too tame, then you can add it back after that point.

That chassis is so easy to pull that I'd do it one cap or resistor at a time, 'til you got it just right.

Also - and very crucial - do not put any small value ceramic caps in that thing. You will get all kinds of awful noises. Silver micas, while pricey, are critical. It also will probably be tough to shoe horn them in the tiny board, but they will fit. If you are using larger value cathode bypass caps that will have to be electrolytics, I would personally go with Xicons, and no other manufaturers (unless you want to pay a fortune for Atoms). The voltage tolerance isn't critical (the 25 volt ones work fine), but most other brands will definitely snap, crackle, and pop on you.

You can also do a bunch of tweaks to the tone stack, but I'd start with the above stuff.

[PraiseCaster]

Hey, thanks for all the input guys!

11gauge, I'm gonna consult with my tech, and see about doin those mods, and yes I plan on a recap, and only good stuff is going in!

[11 Gauge]

I just took a 2nd look at the mods your tech did, and kudos to him for:

-adding the 33K input resistor (68K is cool, too).

-56K slope resistor. This value (or 68K) works great with single coils.

-220 pF treble cap. This works great in conjunction with the above slope resistor. Actually, anything other than the stock .001 uF cap is a huge improvement!

-27K neg. feedback loop resistor. The stock 100K, IMO, is way too bright.

As far as caps, I feel like in this circuit that less is more. People (even techs) tend to go crazy with the cathode bypass caps in particular in them. I think that only the first and/or third gain stages should get them. The second stage should NEVER be bypassed with anything. And while some techs will lower that 10K cathode resistor to 4.7K or lower, I personally never would. It sounds perfect just as it is.

Kager did a wonderful thing losing the caps on the 2 RC networks (and the resistor too on the 2nd one). I've always felt like the stock 2204's and 2203's were too bright with them. Some of his component value choices were a little odd (both cathode bypass resistors, 'post' load resistor after the MV, treble cap, and 68K plate resistor cap on the phase inverter), but a great starting point for anyone who likes to fiddle around with a somewhat classic circuit.

The other thing you may want to have your tech do is add a trimpot in place of one of the 2 bias resistors. With a good pair of 6L6's and a bias of 34-38 millivolts, the power section really comes to life.

The amp is like silly putty - you can endlessly twist it into any shape you want. Just do a little at a time, and know when to say 'when.' Or...get another one and voice it another way.

[11 Gauge]

Originally Posted by PraiseCaster

I should also add, I like to set my amp up to run clean, and I push the tubes with a TS808 Tube Screamer for my overdrive. Thats why I ask, does it effect the clean headroom.

I like the dynamics of going from clean to od in my single channel amp. Though, when I push the volume up, and then dial in the level with the mv, we are talking some really sweet tones! But my ts really does get a great od as well.

Sorry - I should have RE-read this first! If cleans are your game, it would be best to bypass an entire gain stage. Even with all the extra loading that I suggested in my first post, this circuit will still start breaking up at around 4, and I never venture above 6. This is with the MV either dimed or backed down no lower than 9, kinda using the amp as if it were a non-MV model.

Actually, if cleaner is your quest, the circuit could be simplified dramatically - by losing a gain stage and the cathode follower stage (plate load the tone stack), you can lose an entire preamp tube. In doing this, it would make sense to lose the MV as well.

Sorry to open a can of worms - just trying to help get this thing tweaked to your needs...

[PraiseCaster]

Wow! you got some great ideas there!

I'm gonna set down with my tech, and were gonna do some brain-trusting on all these great ideas!

Untill then, check out my next post here in amp central: all about those Winged =C= 6L6GC's.....................

[JohnnyCrash]

Originally Posted by PraiseCaster

Wow! you got some great ideas there!

I'm gonna set down with my tech, and were gonna do some brain-trusting on all these great ideas!

Untill then, check out my next post here in amp central: all about those Winged =C= 6L6GC's.....................

Those are my current favorite 6L6's.

[Silverface]

Quick note - lots of other threads on the subject, so I'll just add that th 12AT7 is usually NOT used as a "preamp" tube. Most are pretty harsh and "hard" sounding. I'd go with a 5751 or 12AY7 and see if that sounds right to you...but you can always try the 12AT7. who knows, you might like it.

[PraiseCaster]

Oh, I have the 12AT7 in there right now, and I love it. No harshness at all, unless of course the power tubes start to get tuckered out, then its harsh, brutal, ragged and ratty!

[PraiseCaster]

But I may just try the Y7 for kicks and grins!