help adding 'preamp in' to existing amp...

[HTH]

I've been getting a Marshall 9001 rack preamp and want to run it into one of my existing homebrews to use the output stage.

The amp I plan running the 9001 into is a simple LTP PI (no -'ve fb) into a pair of cathode biased 6V6s.

Is adding an input to feed the rack preamp into the 6V6 amp as simple as breaking the existing PI connection between the master volume and the PI input, then feeding the rack preamp straight into the PI ???

Or, is it more involved than that?

I can set the output on the rack preamp at -20dB, -10dB or +4dB - which should I use?

[daz]

yes, that'll work. I've done that myself, tho you'll want to either go before the master or break it after but put a 1 meg resistor to ground at that point.

By the way, one thing that may not be apparent right away, and i believe the reason rack seperates never are quite the same as a regular amp is that when the preamp and power amp are using different power supplies, this kills dynamics. for the most dynamic tone it's always much better to have an amp thats all in one. So you may get a great sound, but it also may have very little touch response. i used rack gear for a while and did what you're doing too using regular amps with a pre going into the loop return. i gave all that up when i realized i just could no get any good touch response from any of the gear i tried, and i tried some of the best preamps out there.

[chipaudette]

Wow, interesting reply. I'd never thought about the importance of (unintended?) coupling via the power supply. Especially at louder volumes (where the power tubes are drawing a lot of current from the power supply), I could imagine how the sound of the pre-amp would be affected by the sag in the power supply. Cool. It's not something that I had thought of.

Like many people, I had a rack system that I played for a long time. I eventually gave up, though. I decided that it just didn't sound right and that I was spending too much time fiddling with it to try to make it have a more natural sound. I had a Mesa Boogie Triaxis with a Mesa Boogie 2:90. So, it's not like I wasn't using serious equipment. But, despite all that tube heaven, it just didn't have the sound of a "real" amplifier.

I always assumed that the problem was with all the manipulation of signal levels between the separate units. In the Triaxis, right at the very end, they knock down the signal level to line level. They then push out out the cord where I plug it into the 2:90. In the 2:90, they then have to re-boost the signal just to get it to the right level to hit the phase inverter. I always assumed that the lack of mojo in the system was due to this knock-down-boost-up business.

I like your theory better. Coupling through the power supply. Interesting.

Chip

[daz]

Maybe at this point you may not remember it well enough to say, but i bet if you had thought about it back when you were using rack gear you'd have realized it wasn't the sound that wasn't like a real amp, it was the dynamics. I got some great sounds that way. But the dynamics were not there at all. Great tone but coupled with a feel that was like a cheap SS amp from the 70s ! The first gig i did with a real amp after a year or 2 with separates was like a huge sigh of relief. It was like, yes, this is what i was missing ! all of a sudden i could play again.

[HTH]

I've played around with the sag on the HT line before with a high-gain amp I built - used 1N4007s in line with the B+ supply between each node. it's supposed to stop each node dragging down the previous one and give a punchier response.

I'd imagine this is more of the rack 'feel' you describe, but it's actually what I'm aiming for with higher gain tones.

As it happens, I've had a play with the Marshall 9001 preamp tonight and must say it's as dynamic as any amp I have - you can pick soft and get a cleaner tone or dig in and the amp spits the notes back at you. Controlling the gain from the guitar works too. It actually sounds very much like a good JCM800 with some tastefull mods for a little more gain (it has that aggressive crunch in the mids).

Maybe I've just found a nice preamp (???). I will say however that it's got a trio of NOS Mullard ECC83s in it, so it's got a good chance of delivering the goods from the start.

btw, are you saying that I should take the signal from before the master volume so that I still have a 'master volume' control or for some other reason?

Oh, and thanks for the help, always appreciated.

[HTH]

the 'poweramp in' mod on my homebrew AC30 worked, but the volume with the Marshall 9001 rack preamp is much lower than when the amp (AC30) is fed by it's own preamp.

Any ideas? - I'm assuming that the line level signal from the Marshall preamp needs boosted up a bit before hitting the PI directly ???

Any/all help appreciated.

[HTH]

ok, so I've done some digging and come up with the partial schematic below. If I insert this after the existing 'power-amp in' on my amp, it should bring the signal upto the correct level ??? (currently sounds too quiet plugged straight into the power-amp in using a Marshall 9001 preamp)

[dai h.]

no difference even if you set the output at +4dB?

[pdf64]

Will the booster be in circuit even when the external power amp input is not used? If so the 100k input impedance might load the MV / tone stack excessively.
Alternative to the op amp booster, if you reduce the cathode tail resistor of the LTP, the stage gain will increase, at the expense of balance, which you could compensate for. Or try other types of phase splitters, paraphase or cathodyne, which would give you more gain from the 2 triodes which the LTP is using.
FWIW, I went through this when I used my digitech 2101 and built a stereo power amp for it. I eventually decided that I missed the extra something that 5F6A preamp bright channel adds to the tone, and so added it in. A flat response power amp into the speakers just sounded dull and lifeless in comparison, no matter how I tried to compensate on the pre amp output graphic eq. Peter.

[HTH]

no difference even if you set the output at +4dB?

yeah, it's louder but the tone is bass-heavy and doesn't sound 'right' somehow. anyway, I'd prefer this to work correctly - it should be fine with a -10dB line level signal coming in like most stand-alone power amps.

[HTH]

Will the booster be in circuit even when the external power amp input is not used? If so the 100k input impedance might load the MV / tone stack excessively.

I was planning on having a switch to 'true bypass' the TL072 circuit. However, I could just increase the value of the 100k resistor between pin5 and ground so that it doesn't load the tonestack down as much. OR, I could put another opamp before the booster circuit wired as a buffer - that would solve the low impedance/loading on the TS.

Alternative to the op amp booster, if you reduce the cathode tail resistor of the LTP, the stage gain will increase, at the expense of balance, which you could compensate for. Or try other types of phase splitters, paraphase or cathodyne, which would give you more gain from the 2 triodes which the LTP is using.

I played around with the LTP resistor values quite a bit when I initially built this amp and it sounds best as it is (470ohm and 10k with the two 1Megs I seem to remember, no NFB), so I don't want to change that aspect.

FWIW, I went through this when I used my digitech 2101 and built a stereo power amp for it. I eventually decided that I missed the extra something that 5F6A preamp bright channel adds to the tone, and so added it in. A flat response power amp into the speakers just sounded dull and lifeless in comparison, no matter how I tried to compensate on the pre amp output graphic eq. Peter.

I know what you mean, the tone is just tighter and punchier overall if I plug the Marshall 9001 direct into the front end of my amp. However, this is something I want to sort now on principle (don't like to be beat by these things).

[dai h.]

if you want to try boosting the gain with a tube stage I can send you an efx. lp. schematic if you want to try that. I guess you'd just use the return stage (single 6AV6 or 1/2 of a 12AX7). It sort of looks like it's meant to give voltage gain without distorting. Or how about trying an EQ after the preamp?

[HTH]

if you want to try boosting the gain with a tube stage I can send you an efx. lp. schematic if you want to try that. I guess you'd just use the return stage (single 6AV6 or 1/2 of a 12AX7). It sort of looks like it's meant to give voltage gain without distorting. Or how about trying an EQ after the preamp?

thanks Dai, I'll definately be interested in that fx loop schematic (media_ian(AT)yahoo(DOT)co(DOT)uk).