Ampeg B15-N Bass Pre-Amp section ONLY! 6SL7's

I've been using my Ampeg B15-N as a Bass Pre-Amp by running the Line out (EXT AMP) into a direct box then into my Neve Mic Pre & I am really digging it...
BUT, it's a PITA because I have to load the output of the Ampeg with a GIANT Power Resistor when doing this.

Can I just pull the output tubes instead?

- OR -

What I'd like to do is rebuild the Pre-Amp section only - then build a new output stage (Line Driving Level) so I can make life easier & possibly better.

I'm not exactly sure how I should go about doing the output & could use some help.

The output section would be after the 270k mix resistors.

I was considering an LA2A type output or maybe NY Dave's Line Driver & using some EDCORE's Output Transformers. I have 10k:600's.

Since I'm going to rebuild this into a new box... I was wondering about making it more universal, like having a MIC Input TX... so I could also use it as a Mic-Pre.
I LOVE the sound of the 6SL7's.

ANY advise would be great. :green:

What about this compilation? :grin:

yes you can just pull out the two power tubes and therefore no need to have speaker/load connected with no adverse affect on the amp.

Alternatively you could wire it so there was one standby switch for the preamp & phase splitter and one for the power tubes.

Michael

your line out is the external amp isn`t?
you can pull the PI and output tubes without problem.

but is just one tube, easy to build in a more small box.
a friend of mine is building something similar I can ask him if he have a layout or something.

You also could move the hot terminal of the EXT AMP jack from the pot's wiper to the high side (where the cap attaches). Then turn the pot down to zero and you're there. Cost: $0.001. (A tiny bit of solder.)

Peace,
Paul

Thanks for the tips :thumb:

Cool... So I can just pull the output tubes when using it as a Pre. :green:
If I decide NOT to build from scratch, the switch sounds good... But I must say, I use it more as a Pre than a full amp so...

ANy more idea's for an output stage?

Xopek,
That looks good but is a little too much for my needs.
Thanks for sharing.

[quote author="pstamler"]You also could move the hot terminal of the EXT AMP jack from the pot's wiper to the high side (where the cap attaches). Then turn the pot down to zero and you're there. Cost: $0.001. (A tiny bit of solder.)

Peace,
Paul[/quote]

Really?
And this would give enough gain to drive an LA2A &/or my converters?

Also, what about volume control?

I tried driving my converters from the EXT Out & even at full gain it was NO were near enough.

Definitely worth moving the wire to the top side of the Vol pot, the level will come up a bit, not sure until you try. You don't have volume control now to the ext amp output, but you can control volume from your guitar.

Another option might be to come out of Channel 1 and jumper back into Channel 2, this requires a bit of modification, but will give you more gain and will probably saturate up nicely going into channel 2, you will want level control, going into channel 2 to control this.

Another mod, for lots of gain, (but no tone control) disconnect the wire at the junction of the 22K and the 47pF (in the tone stack) to ground and also the wire from the 270K attached to the wiper of the bass pot to ground and you might get 10dB or so increase. Try 1 discopnnect at a time for different results and then both disconnect for something different. Disconnect and use two alligator clip leads temporarily to switch on and off as you try it.

This is similar to mods I've done to a few amps, I put a switch in as the ground lift switch on the tone circuit. If you do something similar (partial lift) on a Fender Twin Reverb it can sound more like a Marshall ... one customer loved this mod on the normal channel, whilst the reverb channel remained standard - two very different sounding channels in one amp!

You can have lots of fun experimenting with these things, because you might just make that thing sing like it never has before and then switch it back to original when you want to. But be careful when modifying, discharge the big caps before touching anything or you might DIE.

Michael

Probably not going to be a big problem, but your plate supplies for the remaining tubes are going to run a little higher than design intent without the 6L6 loading.

yeah, make sure you aren't over any filter cap voltage tolerances when pulling power tubes, since you're removing around 3/4 of the current draw. You may well be. I see at least 3 of concern.

I am trying to do something similar, except with a B25 input stage. I posted on a few threads relating to this, but I can't seem to come up with a good solution. With the high impedence output of the preamp what sort of balanced output stage should we use?

as far as pulling tubes, so when I am testing an amp head, I can pull the power tubes to check the preamp sections? without a speaker load?

What I'd like to do is rebuild the Pre-Amp section only - then build a new output stage (Line Driving Level) so I can make life easier & possibly better.

I'm not exactly sure how I should go about doing the output & could use some help.

The output section would be after the 270k mix resistors.

I was considering an LA2A type output or maybe NY Dave's Line Driver & using some EDCORE's Output Transformers. I have 10k:600's.

Click to expand...

Still not sure how to do this.

If I use an LA2A (or NYD's) output stage... can I just hook the 270k's to the input of the out stage or do other changes need to be made?

I haven't tried it yet but I can see where grabbing the signal at the top of the VOL Pot may give me the gain I need... 270k is probably dropping a lot of gain, huh?

That brings me to my next question:
What if I grab the signal before the 270k's & had separate ETX OUT's?
I'd still have volume control but would the gain be enough?


Anyway, it doesn't have to be an LA2A (or NYD's) output stage.
Anything simple that could drive my converters, LA2A or 1176 inputs would be cool.

BTW, there's a few different preamp setups that AMPEG used but I prefer the one I posted... with the volume control LAST.

I'm working on my B15N now... putting in a new 3 prong Power cord.
The original 2 prong was shocking the shit out of me when using the EXT out. :shock:

While I'm doing repairs:
Do you think I should get rid of the "HUM BALANCE" & just use the 2x 100 ohms to GND method on the filaments :?:

[quote author="amorris"]as far as pulling tubes, so when I am testing an amp head, I can pull the power tubes to check the preamp sections? without a speaker load?[/quote]

Probably depends on your voltage & how close it is to your CAPs voltage rating.

Luckily for me, I'm not using the EXACT Power TX in my Ampeg.
The one I'm using is slightly lower voltage.

Can someone explain the drawbacks in using the hum balance? I know it is an old technique, but even if the resistors were exactly matched, isn't there still a possibility that there will be hum from the filament?

khstudio, have you tried the LA2A output stage? I don't have all the components now to build one, but could someone try it?

[quote author="mattnj"]khstudio, have you tried the LA2A output stage? I don't have all the components now to build one, but could someone try it?[/quote]

Uhh... not yet.
I can't just whip it up in a few minutes... it will take some time to figure out the power supply+voltage divider, etc...

I was hoping someone with more knowledge of this stuff could steer me in the right direction so I don't waist too much time doing the wrong thing.

I DID try pulling the EXT. OUT signal from before the 270k & the volume was "Almost" enough... actually not bad, then ran that into my LA2A (with 10K:10K Jensen input TX) for makeup gain... worked out pretty good.

I also realized that I like the "Basic" Single input (Channel 2) better... so this simplifies things even more.
So now I'm thinking about building ONE channel (& can probably drop the 270k) & come right out of the 1Meg VOLUME Wiper into the output stage:idea:

I have the feeling that the LA2A output will be TOO much, as in gain.

Maybe Jacobs SRPP output stage from the PULTEC? :?

I tried the SRPP stage on mine and I was having problems getting a solid signal out of it. I think that maybe I fried the OEP transformer though.
It had a very low level and tinny sound. that circuit has about 26 db right?
Any suggestions?

Try jumpering Channel 1 input into Channel 2 as well ... the circuit shows that Ch 1 has two input sockets, so plug your guitar into one of them, and then into the other input on Ch1 use a short guitar cable to parallel into Channel 2 input, this should give you another 3dB gain and might be enough to achieve what u are trying to do. .... but you will then need to maintain some mixing resistors, so you might lose your 3dB again., worth trying anyway

Michael

[quote author="Junction"]Try jumpering Channel 1 input into Channel 2 as well ... the circuit shows that Ch 1 has two input sockets, so plug your guitar into one of them, and then into the other input on Ch1 use a short guitar cable to parallel into Channel 2 input, this should give you another 3dB gain and might be enough to achieve what u are trying to do. .... but you will then need to maintain some mixing resistors, so you might lose your 3dB again., worth trying anyway

Michael[/quote]

It's not enough gain & really isn't the "Sound" I'm looking for. :sad: