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Beer of the Bass

6SL7 valve preamp

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After having played around with building a B15n style preamp into my old Dynacord hybrid head;
[url="http://basschat.co.uk/topic/277312-new-old-amp-day-dynacord-eminent-ii/page__view__findpost__p__3252758"]http://basschat.co.uk/topic/277312-new-old-amp-day-dynacord-eminent-ii/page__view__findpost__p__3252758[/url]
I have found that I really like the sound of that particular preamp circuit and I've started thinking it would be a handy thing to have a similar sounding preamp I can use as a standalone unit and plug into other power amps. A good DI output would be useful too. I also have a good few of the parts I'd need hanging around already; I have some valves in the stash, a neat little toroidal power transformer and a couple of salvaged audio transformers I can try for a DI output.
After mulling it over for I while, I've worked out what I want to build. The B15n preamp circuit sounds great, but with only bass and treble controls the EQ is not very flexible. I've been reading up on the inductor-based active midrange used in the SVT, which seems attractive apart from the high parts count. But there is a more economical version of the Ampeg midrange circuit used in other amps like the 90s V4BH which uses only one triode for the midrange, with the other half of the valve used for a cathode follower for a low impedance output. The multi-tapped inductor used by Ampeg is a proprietary part which is hard to find, but I have found a useful set of instructions on how to wind my own.
So the rough plan is to have a 6sl7 valve for the initial gain stages (with a pre volume and bright switch in between), a bass/treble James tone stack, then an ECC82 for the active midrange and cathode follower output stage. That should be able to feed the jack output directly, with a transformer for the DI out. If I take some care over the layout, I think I can get it into a small project box of about 250 x 170 x 50mm, which should stack quite nicely with most micro heads. I thought about using a 1U rack case, but I have never got into the habit of using rack gear.
The first thing I'm going to attempt will be winding the inductor for the midrange, as the rest of it depends on whether I can do that successfully...

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I've most of the parts for a similar build just waiting for me to free up enough time to start building.

I digested quite a bit over on the other forum, particularly from username JGR, who builds under the name Reiner Amplification. His bass preamp has an inductor midrange, based on the Ampeg V9 topology, probably similar to the V4 going by your description. His earliest designs had lower and upper midrange controls using wah inductors!

In the end up I decided to start with the B15n pre, simple as it may be, but add a sweep control to the tonestack, a la Arkham Zephyr http://arkhamsound.com/zephyr.html

I can always revisit later and add an inductor midrange. I have a multi-tap carnhill inductor at the ready!

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[quote name='Bigwan' timestamp='1493105252' post='3285549']
I've most of the parts for a similar build just waiting for me to free up enough time to start building.

I digested quite a bit over on the other forum, particularly from username JGR, who builds under the name Reiner Amplification. His bass preamp has an inductor midrange, based on the Ampeg V9 topology, probably similar to the V4 going by your description. His earliest designs had lower and upper midrange controls using wah inductors!

In the end up I decided to start with the B15n pre, simple as it may be, but add a sweep control to the tonestack, a la Arkham Zephyr [url="http://arkhamsound.com/zephyr.html"]http://arkhamsound.com/zephyr.html[/url]

I can always revisit later and add an inductor midrange. I have a multi-tap carnhill inductor at the ready!
[/quote]

Yes, the V9 looks like the same midrange circuit I'm thinking about. I wasn't aware of the Carnhill inductors - having just googled them, that could be an option if I can't get a hand-wound inductor to work. I found this page which describes making a copy of the Ampeg multi-tapped inductor;
[url="http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampInductor.html"]http://www.frontiern...mpInductor.html[/url]
I have found a ferrite core of the same size with an AL value close to the one he used, I've calculated how many turns I should need, and now I just need to borrow an LCR meter to check that I'm in the ballpark of the right values.
My build will essentially be a B15 preamp, followed by the midrange stage and an AC cathode follower on the output.

Edited by Beer of the Bass

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[quote name='Beer of the Bass' timestamp='1493110614' post='3285605']
My build will essentially be a B15 preamp, followed by the midrange stage and an AC cathode follower on the output.
[/quote]

Pretty much what I'd arrived at and tried to model in LTSpice. My midrange was more like a steve bench eq section though. When I tried more than one midrange control LTSpice didn't like it, but it was probably user error!

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Interesting, will be following this!

I also have a cupboard full of tube amps and parts, been meaning to make a bass preamp for a while now so will be interested to see this thread develop :)

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There's an old Talkbass thread on a couple of my 6SL7/6SN7 builds: https://www.talkbass.com/threads/a-diy-tube-preamp-line-driver-by-passinwind.345415/#post-4388452

Some of the pics have gone missing in the thread due to the TB software changeover after I posted them but I can probably scare them up if anyone is interested. JGR is a good friend of mine and it's well worth gleaning what you can from his posts over there too. I opted for an active bass and mid format, closer to the Jule Monique's approach than the Ampeg thing.

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[quote name='Passinwind' timestamp='1493142810' post='3286032']
There's an old Talkbass thread on a couple of my 6SL7/6SN7 builds: [url="https://www.talkbass.com/threads/a-diy-tube-preamp-line-driver-by-passinwind.345415/#post-4388452"]https://www.talkbass...5/#post-4388452[/url]

Some of the pics have gone missing in the thread due to the TB software changeover after I posted them but I can probably scare them up if anyone is interested. JGR is a good friend of mine and it's well worth gleaning what you can from his posts over there too. I opted for an active bass and mid format, closer to the Jule Monique's approach than the Ampeg thing.
[/quote]

That does look nice, and it looks like a deeper design process than I'm getting into since I'm essentially pilfering sections of existing amps in a building-block approach. I should probably look into SPICE modelling at some point...
I'll be using less premium parts than some of your builds - no PEC pots for me! I'm going to make up eyelet boards for the power supply and high-voltage bits of the audio circuitry, though I might put the resistors and caps for the James tone stack on a little perf board mounted on the back of the pots to save some space, since those are low-voltage parts.

Edited by Beer of the Bass

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[quote name='Passinwind' timestamp='1493142810' post='3286032']
There's an old Talkbass thread on a couple of my 6SL7/6SN7 builds: [url="https://www.talkbass.com/threads/a-diy-tube-preamp-line-driver-by-passinwind.345415/#post-4388452"]https://www.talkbass...5/#post-4388452[/url]

Some of the pics have gone missing in the thread due to the TB software changeover after I posted them but I can probably scare them up if anyone is interested. JGR is a good friend of mine and it's well worth gleaning what you can from his posts over there too. I opted for an active bass and mid format, closer to the Jule Monique's approach than the Ampeg thing.
[/quote]

I don't know much about Monique. Do you know what topology Jule uses for the midrange control? Similarly I'm clueless about his sensitivity control. I'd assumed it was just a simple gain, but reading between the lines there seems to be something else going on there.

What topology did you use for your midrange control? Is it similar to this: http://www.angelfire.com/electronic/funwithtubes/images/Amp-Tone-7-A-A.gif

I can't see any pics from early in that thread unfortunately, although I've read it at some point in the dim and distant!

Edited by Bigwan

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[quote name='Bigwan' timestamp='1493193012' post='3286297']
I don't know much about Monique. Do you know what topology Jule uses for the midrange control? Similarly I'm clueless about his sensitivity control. I'd assumed it was just a simple gain, but reading between the lines there seems to be something else going on there.

What topology did you use for your midrange control? Is it similar to this: [url="http://www.angelfire.com/electronic/funwithtubes/images/Amp-Tone-7-A-A.gif"]http://www.angelfire...-Tone-7-A-A.gif[/url]

I can't see any pics from early in that thread unfortunately, although I've read it at some point in the dim and distant!
[/quote]

The EQ in the Monique has been described as a Baxandall, and from what I've read about it I'd guess it's probably a true, active Baxandall circuit. Jule is a little cagey about how the Sensitivity control operates (quite understandably), though he hints that it also affects the input impedance. Without knowing what topology he has used for the input stage it's hard to be certain what's going on there, though it does look like he's come up with something fairly distinctive rather than borrowing from the same classic designs most other people do!

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[quote name='Bigwan' timestamp='1493193012' post='3286297']
I don't know much about Monique. Do you know what topology Jule uses for the midrange control? Similarly I'm clueless about his sensitivity control. I'd assumed it was just a simple gain, but reading between the lines there seems to be something else going on there.

What topology did you use for your midrange control? Is it similar to this: [url="http://www.angelfire.com/electronic/funwithtubes/images/Amp-Tone-7-A-A.gif"]http://www.angelfire...-Tone-7-A-A.gif[/url]

I can't see any pics from early in that thread unfortunately, although I've read it at some point in the dim and distant!
[/quote]

I'm planning on going to Jule's shop sometime this summer. He's never given away a whole lot about the nitty gritty of how his preamps work.

My mid control and active feedback schemes are a bit different than the one in that link, but the general idea is more or less the same. My experience is that there are too many compromises once you go beyond two bands using that topology. I do a passive treble control later in the signal chain, and in the SS builds it comes after the parametric EQ. The idea is that any extraneous noise doesn't get reamplified by a bunch of subsequent stages. It also gives a means of toning down down high end harshness if you drive the midrange or PEQ sections hard. Sorry about the pics, but the early ones are just of the Aikido line driver anyway. Here's a gut shot of the second build, which had new production 6SN7s fitted for testing (two left tubes, the one on the right is 6SL7) rather than the NOS ones I ended up using:

Edited by Passinwind

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After getting distracted with a few other things for several weeks, I've had a go at winding an inductor for the midrange circuit using these instructions.

[url="http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampInductor.html"]http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreampInductor.html[/url]

I couldn't find exactly the same core in the UK, but I found some with a very similar AL value which (according to my back-of-an-envelope calculations) need just a handful more turns of wire. My winding method was crude - I cut notches in both ends of an ice lolly stick and wrapped wire around that to use as a shuttle to pass through the centre of the core by hand. I used 0.4mm enamelled wire, which should be about the same as 26AWG. The resulting inductor looks like this. I still need to attach leads to the taps and I may dunk it in some varnish to stop the wire from moving around.


The winding is less tight and more scattered than the original Ampeg part, but I borrowed an inductance meter today and the values seem close enough. The target values are 800mH with taps at 100mH and 300mH. Mine measures 99, 290 and 767mH, which is within a 5% tolerance. It doesn't seem like an application where the values are especially critical, so I'm sure that will be fine.

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