Fender Pre and Bassman Volume questions

[Subjecttochange]

Anyone have a link to or know of a killer fender-ish preamp?

I'm not happy with the pre I'm using based on the 5F4 super.
I've got an amp that I am working on with rocket and fender type preamps into tweed Bassman power section. The problem is that the rocket pre sounds so much better than clean fendery channel... it's just more dynamic and lively even when they're both running clean.

Second- the amp has never felt loud. I modeled it after (just like everyone does a one point) an old bassman- then changed things here and there. The thing is... it's never seemed horribly loud. I can't even crank my 18W or TW Express clone past 11 o'clock in my living room for fear of permanent hearing loss- but this thing can be cranked all the way up. Though it is loud- it's not THAT loud. Is there something wrong or is that how bassmans are? I understand that the TW and 18W are really heavy on the mid range which makes them perceived as though they are louder- but this thing just isn't quite there.

[Structo]

No a Bassman should be very loud.
If you are using 6L6 tubes, very, very loud!

Can you show a schematic of your build?

If the Rocket pre sounds good why not use it?

Do you own an oscilloscope?
Maybe the signal being fed into the phase inverter is weak.

Many feel that the AA864 is the best Bassman circuit.
Maybe pick up some pointers from that one.

http://www.schematicheaven.com/fenderam ... _schem.pdf

[M Fowler]

You need to go through that second preamp and find the problem as my 59 bassman is loud and my Rockets are loud. Maybe blocking distortion, bad coupling cap or ground issue.

[Subjecttochange]

Ok Thanks.
Fowler- I'll go through the whole amp and make sure. When I say the amp isn't loud I do mean the whole thing. Not just the fender pre. I'm not sure if I was clear on that or not. My guess is that it's a grounding problem out of those- but I'll check out the coupling caps too.

Structo- Unfortunately I don't own an O-scope. I had suspected possible a weak signal to the phase inverter before. What would be the best way to figure this out sans the scope?

I attached a schem of the amp using cannibalized versions found off of Prowess Amps Schematic/Layout Library.

The amp has a 5F6A poweramp with SS rectifiers in the power supply. It has an AA864 phase inverter section and preamp (bright cap engaged permanently) and then a Rocket preamp without the tone controls as you know. Unfortunately I don't have any 250K pots left so the Bassman pre is stuck with 500Ks- which may make it sound a little worse.

Here it is- there may be problems with it as it's small and I just threw it together.

What drawing software do you guys use? MS Paint is just not cutting it anymore.[/img]

[Structo]

OK, I'm trying to understand what you are doing.
Are you saying that you have two different preamps? One Rocket and one Super feeding into one Super power amp?

If so, then I think you need to work on how they each enter into the phase inverter.
I am no brainiac with this stuff but You probably need some kind of mixing resistor/ coupling thing to have them interface with the power amp.
One side might be loading down the amp.

Perhaps study how Fender joins the vibrato channels to the power amp with the normal channel.

The 6G4 for example shows a .05uF and 1M coupling to the PI.

I would look at some of the other amps Fender made for ideas how to incorporate the two different preamps.

[M Fowler]

The 1 & 2 preamp w/tone stack take up one 12ax7 itself each channel has its own 12ax7 then you need a mixer stage another 1/2 of a 12ax7 then onto the PI I don't see where you have a second stage amplifier after the preamp.

[Subjecttochange]

Yes two completely separate preamps (used one at a time) into the PI->Poweramp.

From what I have seen- the two channels (as in the vibrato/normal) feed into the PI via their own 220K resistor into the right side of the pi (I'm using a .0047uF cap but is shown as a 500pF cap on the aa864- different models vary this cap quite a bit). They're not jump-able as is the case with a plexi- from my understanding- since they're not really sharing any of the same circuitry up to the PI. As where the plexi's (and other fenders) have two distinct first triodes (bass/normal) but then both feed into the rest of the shared gain stages/tone stack together starting at the V2 position.

My goal was two distinct tone stacks- two distinct preamps into a single poweramp.

Maybe I should be looking at this as- "whats the best way to accomplish my goal?" instead of "how can I adapt my current amp".

[Subjecttochange]

So you're saying I need an additional triode to mix the two signals before feeding it into the PI?

I am a little confused. A second stage after the preamp and before the PI?

I do have one free triode. How would your recommend using it? Do you have a schem or layout of what you're talking about?

[M Fowler]

You would build your amp the way the Bassman is on the attached schematic.

clean fendery channel and rocket channel each channel is using a 12ax7 but a third 12ax7 combines the signals prior to the PI

[Subjecttochange]

I'm not sure why that wasn't making sense before... I guess I needed someone to guide me through it. Ok- I see whats up. I actually have the rocket pre wired up through what would be a mixing stage... the other channel is not.

Ok- let me go wire it up. I'm sure I'll be back with more questions.

You guys are awesome thank you so much. I don't think I would be getting through it without your help.

Let me know if you're ever in San Diego. I owe you at least a beer.

[M Fowler]

I was just in San Diego during the playoffs, Go Chargers.

Your amp is going to suddenly get louder I can feel it from Minnesota

[Subjecttochange]

Oh the Chargers... they pull our poor heart-stings every year... and now the whole thing with LT and Merriman possibly leaving. Someone needs to fire AJ.

You a Vikings fan then? Had to love having #4 this year.

The amp- not sure what the deal is... but that didn't work.

Rocket sounds great still- but I just played it at full volume for about 20min and I can still hear.

Fender doesn't make any noise. Hmm.

It seems to me as though only that the bass channel is the only one going into the last triode before the PI... is that the case? Isn't that the last gain stage for that channel? I directly copied the aa864 as my orig. preamp going into the last triode of the bass channel into the PI- but have no sound... weird. Both still run into 220K resistors into the PI

[Structo]

The 500pf coupler into the PI sure seems small. As in it would block most all bass frequencies.

Do you have two preamp tubes, then the PI into the power amp?

With one 12ax7 as the Fender preamp and the other a Rocket preamp?

Do you want to switch between the two preamps with a relay or switch?
Or just plug into the preamp of choice?

Also how are your voltages to the preamp tubes? 200v or less?

I'm having a hard time picturing all this in my head.

[Subjecttochange]

Yeah the 500pF seemed too small to me too. I used a 0.005uF instead.

The preamp arrangement is:
3 Preamp tubes, one PI tube into the 2 Powertubes.

Currently I have it set up as
V1 and V2a are dedicated to the rocket (as it takes three triodes before the tone stack if I'm not mistaken)
V3 dedicated to the aa864 preamp.
V4 PI

I'm planning on using a relay/FS.

I'll convert it back to what I had- then check the voltages.

[M Fowler]

Well I am also having trouble because we have Rocket preamp on one channel. A fender preamp on another chanel.

The PI is AA864 phase inverter section into a tweed Bassman power section 5F6A with SS rectifiers.

So bouncing back and forth to my schematics not seeing how your wired is not an easy task.

I am a Viking Fan yes.