I have joined the Dark Side (well not really)

[fremenblue]

Ok, I've got a Sylvania JAN-CHS 12au7 on order. What to do with the 5751? Hmmm. . .

 

[barroso]

that's a great thread!!

 

[Psycho Bass Guy]

I'll prepare myself for disappointment then. I guess when I read this thread I keyed in on mgmadian's comment that he'd used a 5751 in a Kern and had experienced a small loss of gain but had cleaned up the sound as well, and also that you thought that a 5751 wouldn't mess up the tone controls the way a 12au7 did.

My conclusion from that was that a 5751 might be a better choice than a 12au7. Ah well. . . .

You need to read the thread again. You've mixed a whole bunch of stuff together in a way that it was not presented.

The 12AU7 only makes the tone controls act weird when it is used in the preamp position, not the driver. The reason that a 5751 cleaned up the Kern was that it was being subbed in a preamp gain stage.

 

[fremenblue]

You need to read the thread again. You've mixed a whole bunch of stuff together in a way that it was not presented.

The 12AU7 only makes the tone controls act weird when it is used in the preamp position, not the driver. The reason that a 5751 cleaned up the Kern was that it was being subbed in a preamp gain stage.

Thanks for straightening that out for me. Much clearer now, cap'n! So I could go ahead and drop the 5751 in the preamp gain stage of my Mesa amp then, if I chose to.

 

[quickervicar]

I had a GT 12AT7 on the shelf so I tried that. It cleaned it up nicely. Thanks for the tip. I am now finally loving the front end of my Mesa (and I've even bypassed my Alembic & SWR preamps)! Guess it's time to change my sig.

 

[Vanni]

I know the real test will be with my band.

Anderbass, did you have a chance to test the 400+ with your band?
how does it work? ... and comparing it with your SVT?

Thanks
Vanni

 

[anderbass]

Whoops, your right I never did follow up on my report.
It worked out really well with my band, here's my quote from another thread for ya:

My SVT-II has always had much fuller low-mids and seemed to sound great in the mix with my bands on most any stage with my normal (near-centered) tone knob settings.

I always liked my 400+ (w/full JJ's) but it had me constantly tweakin knobs trying to get comfortable with its sound. Shortly after getting mine, I modded it with power-tube lift switches because It seemed to work so much better at very loud volume levels, But this mod didn't really accomplish my goal.

Mine has always seemed to sound somewhat grainy and lacked the strong low-mids I craved. I also had trouble taming it's somewhat harsh upper register sounds without its overall tone becoming to dull.

Psychos 400+ mods ended my knob tweaking and the graininess. Its transformed mine into much more of a plug and play amp that just flat works way better in every way. It still doesn't sound or work like my SVT-II, but it does have its own voice and vibe that I really like now. Its now got a much fuller sound and a very tight punchy response thats simply amazing.
My current favorite leans towards the SVT, but it actually comes down to which ever one I'm playing at the moment.

 

[bigbajo60]

Just got through reading through the posts in this thread and was wondering if any of the older vets might be able to say whether this AX-to-AU swap brings the 400+ more in-line with being, say, an "old Fender Bassman on steroids"? That IMO, would be 'my cup 'o tea'!!!

 

[Vanni]

Whoops, your right I never did follow up on my report.
It worked out really well with my band, here's my quote from another thread for ya:

anderbass, thanks!!!

I'll report back my thoughts when I receive my 400+ (hopefully saturday)

Vanni

 

[anderbass]

Just got through reading through the posts in this thread and was wondering if any of the older vets might be able to say whether this AX-to-AU swap brings the 400+ more in-line with being, say, an "old Fender Bassman on steroids"? That IMO, would be 'my cup 'o tea'!!!

Yep, I think thats a pretty good description of mine. But make sure you have Chinese 12AX7's too.
(plus have the adjustable bias mod performed for best results)

I don't think the problem is necessarily just the JJ's. I think it's a combination of the amp's voicing and the tubes' inherent tone. I remember theorizing a long time ago that Mesa voiced its amps around the cheap Chinese 12AX7's they sold. I have bunch of Ruby 12AX7s from 10-15 years ago, the same ones that Mesa used to sell, so I put a full set of them in the amp. I was right The amp's preamp was nothing like it was with the JJ's. It was a little noisier, but the tone knobs became much more responsive, and even with a 12AX7 as a driver, the amp was much cleaner than it had been with the JJ preamp tubes. What's really funny is that those same Chinese 12AX7's are icepick treble machines in every other amp I've tried them in, but in the Mesa, they sound like RCA's in a Fender, just noisier. It's funny that JJ's are dark in other amps but too bright in the Mesa and that the Chinese 12AX7's that are too bright in other amps sound smooth in it. A plus is that those Chinese tubes are very cheap and abundant.

 

[AdmiralB]

FWIW, the driver tube is NOT the leftmost preamp in the 400+. It's the next-to-leftmost; the 400s have an unusual PI circuit that has a buffer amp after the actual driver.

I suspect that changing them BOTH to 12AU7s would be a decent experiment.

 

[Psycho Bass Guy]

FWIW, the driver tube is NOT the leftmost preamp in the 400+. It's the next-to-leftmost; the 400s have an unusual PI circuit that has a buffer amp after the actual driver.

I suspect that changing them BOTH to 12AU7s would be a decent experiment.

That's not true. The actual driver tube, NOT the inverter, is the last preamp tube on the left. It is a cathode-following gain stage meant to generate power to drive the output section, which is why a 12AU7 works better there. Using a cathode follower for drive is very common. The other, less common way, is to use a transformer. If you change the inverter (the tube you recommend changing), all it will do is lower the gain there, which will increase noise and lower headroom.

 

[AdmiralB]

It is a cathode-following gain stage

It's not cathode-following, it's plate-driven. Which is kind of weird, IMO. See: http://schematicheaven.com/boogieamps/boogie_bass400.pdf

I would hardly say CF drivers are 'common'. The ONLY ones I can think of offhand are the Fender Super Twin and Studio Bass.

I've also never seen a transformer driver. I've seen transformer inverters, though.

 

[Psycho Bass Guy]

It's not cathode-following, it's plate-driven. Which is kind of weird, IMO. See: http://schematicheaven.com/boogieamps/boogie_bass400.pdf

For starters, you've got the wrong schematic. You have the Bass 400, not the 400+, and second, what you are describing IS a cathode follower- where the plates of the inverter feed through to the plates of the driver for no increase in voltage gain, but an increase in power output. The driver section "follows" the cathode potential of the inverter, but at a higher power output. It's a pretty standard way of driving a large tube power section.

I would hardly say CF drivers are 'common'. The ONLY ones I can think of offhand are the Fender Super Twin and Studio Bass.

The SVT in all its versions (even the solid state ones!) has it, as do most 150+ watt tube amps.

I've also never seen a transformer driver. I've seen transformer inverters, though.

Lots of transmitters use them, as do the Fender 300PS and 400PS.

 

[AdmiralB]

what you are describing IS a cathode follower- where the plates of the inverter feed through to the plates of the driver for no increase in voltage gain, but an increase in power output.

It seems we have some confusion in terms. A cathode follower takes signal from the CATHODE of the tube in question - see the driver for a Marshall tone stack, for example. Hence, the term CATHODE follower.

The only CF in the Bass 400 is the triode that drives the effects loop.

There is very definitely voltage gain in both stages of the Bass 400 PI.

Further, if there's no voltage gain, there's no power gain. Tubes are voltage-triggered devices, and the only advantage to current availability is when the driven stage is of low input impedance (e.g. a tone stack). The power amp does not 'draw' current from the phase inverter.

I presume Mesa uses two triodes because the PI circuit isn't a typical LTP (it's based on the weird transistor-driven cathode setup in the D180), and they use two to get the voltage swing they want.

You're correct, the SVT does use a CF driver section. But the Fender PS amps use the transformer for phase inversion, not driving ability. The only reason low output impedance is desirable in a driver circuit is to minimize blocking distortion, not to offset loading.

 

[Psycho Bass Guy]

It seems we have some confusion in terms. A cathode follower takes signal from the CATHODE of the tube in question - see the driver for a Marshall tone stack, for example. Hence, the term CATHODE follower.

Link Removed

Pay close attention to figure 1.3

Further, if there's no voltage gain, there's no power gain.

I got ahead of myself; the power gain is in the output stage. A CF is current amplifier that has a net power LOSS.

Tubes are voltage-triggered devices, and the only advantage to current availability is when the driven stage is of low input impedance (e.g. a tone stack). The power amp does not 'draw' current from the phase inverter.

The larger the output section (number of tubes), the lower their plate impedance, which requires a lower impedance driver, hence the use of a CF.

But the Fender PS amps use the transformer for phase inversion, not driving ability. The only reason low output impedance is desirable in a driver circuit is to minimize blocking distortion, not to offset loading.

Take another look at the schematic for a 400PS; you'll find the driver transfromer AFTER the 6L6 but before the power section.

 

[AdmiralB]

Link Removed

Pay close attention to figure 1.3

I did. What I see is output from the cathode. Which is not what I see in the Bass 400, nor what you described above, when you said this:

a cathode follower- where the plates of the inverter feed through to the plates of the driver

Take another look at the schematic for a 400PS; you'll find the driver transfromer AFTER the 6L6 but before the power section.

I'm quite familiar with the PS series, I have a 160. Regardless of the terminology the schematic uses, the transformer IS the inverter, there is no tube inversion happening.


To get back to the original topic...the reason the 12AU7 cleans the amp up is because it's simply not hitting the power tubes with as much signal swing. Current capacity/impedances don't really apply, since the tubes are plate-driven.

 

[r379]

Admiral B

Not to argue with you; just curious. Why wouldn't you want signal swing?

 

[AdmiralB]

Why wouldn't you want signal swing?

The idea is to reduce overdrive; if you decrease the swing to the power tubes, you don't drive them as hard.

It's not that different from turning down the master, except that higher master settings can overdrive the PI/driver tubes, which might be desirable I guess. And a 12AU7 will have different characteristics as far as being overdriven by the 12AX7 PI tube in front of it.

If you have the amp cranked so far that you're getting blocking distortion (and I doubt that anyone using it as a bass amp would do so), then I can see a 12AU7 maybe helping out, but everything else would be so dirty by that time that I don't think it'd make a huge overall difference.

 

[r379]

The idea is to reduce overdrive; if you decrease the swing to the power tubes, you don't drive them as hard.

It's not that different from turning down the master, except that higher master settings can overdrive the PI/driver tubes, which might be desirable I guess. And a 12AU7 will have different characteristics as far as being overdriven by the 12AX7 PI tube in front of it.

If you have the amp cranked so far that you're getting blocking distortion (and I doubt that anyone using it as a bass amp would do so), then I can see a 12AU7 maybe helping out, but everything else would be so dirty by that time that I don't think it'd make a huge overall difference.

OK, I get where you're going. I was just remembering from my radio days that "swing was king" although that was output from the finals and not the gain. Wasn't thinking clearly when I asked my question. Thanks for straightening me out.