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Favorite pre-amp question

Illinest

Member
Messages
41
Aloha.

I'm building a few amps for myself (and maybe later for others). Now I need to pick preamps. My experience level is low, so maybe I can get informed opinions from a few of ya?

I'm leaning toward simple builds that have fewer parts, but I'm not sure if (for example) the difference between blackface Champ and blackface Deluxe Reverb is significant enough to say that 2 tubes is better than 1.

Now I know enough to understand that the additional gain stage is a LTP phase inverter, and therefore integral to the design of the DR, but the fact remains that there is additional gain from that PI.

I'm wondering what you gain (aside from - uh - gain) with the additional gain stage. If it only adds tube distortion then I think I know the answer.
I'm inclined to use the least possible parts as I said before, but I worry that I'm overlooking something. I know that some amps use many more stages than what you find in the DR, but I don't know why. A single 12ax7 should be able to drive a second 12ax7 well beyond the point where you could hope for clean signal, and the second 12ax7 ought to be able to blast the hell out of the grids of the phase inverter, and so on.

What's going on with all those extra stages?
 

pdf64

Member
Messages
7,073
With the Fender reverb amps, an additional gain stage was necessary in the reverb channel to recover the losses due to the mixing arrangement of the wet and dry signals.
For most models, the arrangement used increased the overall gain of the dry signal path.

For pure cleans and very flexible tone controls, I think that the regular '2 stage' BF normal channel is hard to beat.
 

axpro

Member
Messages
628
I think you are are over-simplifying the whole gain stage idea. If you look at a champ vs a Blackface deluxe, they are pretty much the same thing preamp wise. leave out the Phase inverter/power amp from the mix, and they are incredibly similar. One utility gain stage, driving a tone stack with a volume control following. Then a second stage before feeding the power section. the first stage is the same, the stack is pretty much the same and the recovery post tone stack is similar, but witha difference in value.... sharing a cathode resistor with another stage to save on parts cost with the deluxe...

The "extra stages" you speak of would be the second channel, the vibrato circuit and the reverb circuit if you are talking deluxe reverb.

The phase inverter is to drive the power tubes, and is part of that system. Yes it adds gain, but if we are discussing PREAMPS then wash it out of your mind.

The 2 amps you list use the same preamp. 2 stages, with a tone control in the center.

Other amps use different setups to get dirtier or cleaner. Personally i like Matchless' use of parallel first gain stages, a little more push, but nice and quiet.

Also it depends on how much gain you are trying to wring out of each stage. Technically, you could cascade 2 stages into each other and get a TONNE of gain out of them, but it might not sound great. I sometimes bias gain stages a little weaker, cause i like the sound it gets. A Marshall Plexi type preamp (3 gain stages, one of which being a cathode follower) and even a JCM800 (4 stages) don't just have multiple stages in a row maxing out the gain, they increase gain to specific parts of the frequency band, use filters that tailor the sound and lower overall gain that needs to be built back up again, all to get a specific tone.

Doctor Z's amps are pretty minimal gain stage and component wise, which i find sound pretty good, though getting an accurate schematic is pretty tough to examine.
 

TweeDLX

Member
Messages
3,756
The main difference between a Champ and a DR is the amount of clean headroom. Champs break up pretty quickly, and if you plan to gig with the amp you build, isn't going to cut it with a drummer. The extra gain stage in a DR, AND the push/pull dual tube output section make it an obvious choice for a stage worthy amp. I used 6L6GC tubes for this build. With the correct transformers, you could easily use 6V6GT tubes.
 

Illinest

Member
Messages
41
Thanks fellas.

I love simple, but I also worry that I'm missing some hypothetical advantage. I'm guessing it's safe to say I'm not.

I think that the explanation that axpro gave was particularly helpful. It never occurred to me before that you could reduce the signal and reamplify it to emphasize particular frequencies. Now it's starting to become a little clearer.

Is there any chance I could have a follow-up about tone stack and a bonus about cathode followers?
Why have a cathode follower at all? Is it about phase inversion? I don't understand the benefit.

And if I put a full tmb tone stack in a Champ, would that introduce too much loss between the stages?
I'm not interested in a conventional Champ sound anyways. I want a little bit of headroom.
 

axpro

Member
Messages
628
I am far from an expert, but there are lots of guys who can explain the ins and out of your questions far better than I can.

Just to touch on your "Champ with a full TMB tone stack", blackface champs (AA764 models) DO have a full tone stack, it's just that Fender subbed in a fixed resistor instead of a pot for the midrange control. As mods go, adding a control pot is pretty common for people who like to fiddle with the middle.

You can mess with "Tone Stack Calculator" (Is that from Duncan's amp pages? I forget where it lives) a program that lets you sub in values and see it's effect on frequency sweep. If you want headroom, then the champ is usually not the ideal amp, unless you like playing very quietly.

As for Cathode Followers, It was explained to me that it doesn't add gain but does add CURRENT to drive your signal through the tone stack (which does eat up a lot of your signal) I have tried marshall type circuits with and without the CF, and with seems to be my preferred
 

Illinest

Member
Messages
41
I am far from an expert, but there are lots of guys who can explain the ins and out of your questions far better than I can.

Just to touch on your "Champ with a full TMB tone stack", blackface champs (AA764 models) DO have a full tone stack, it's just that Fender subbed in a fixed resistor instead of a pot for the midrange control. As mods go, adding a control pot is pretty common for people who like to fiddle with the middle.

You can mess with "Tone Stack Calculator" (Is that from Duncan's amp pages? I forget where it lives) a program that lets you sub in values and see it's effect on frequency sweep. If you want headroom, then the champ is usually not the ideal amp, unless you like playing very quietly.

As for Cathode Followers, It was explained to me that it doesn't add gain but does add CURRENT to drive your signal through the tone stack (which does eat up a lot of your signal) I have tried marshall type circuits with and without the CF, and with seems to be my preferred
I think that the Red Bear amp uses a cathode follower as well. I always liked that one.

I am in fact planning to make it a very quiet amp. Gigging is not at all a concern.

I was also looking into the Matchless Clubman style of preamp. It seems interesting.

Well thank you.
 




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