Adding low impedance out to B-15 Pre circuit?

[tdvt]

As a small side project, I would like to build a B-15 pre-amp but add a low impedance out, to be able to use it as a DI as well.

The original circuit is pretty well self-contained with a pre-out as part of the original design (that I assume would drive a SS power amp).

I would also like  a line-level out, which would have it's own level control, to go to a mixer & could be used at the same time as the standard pre-out.

From my Google searches I am finding 10K/600Ohm transformers specified for tube pre use, but I am unsure of transformer placement. Not sure if these are even the right trans values for this circuit.

Should it go parallel with the stock pre-out? 

[PRR]

> a pre-out as part of the original design (that I assume would drive a SS power amp).

No. It will drive a tube power amp in the same box, or a few feet away with reduced treble.

The Ext Amp comes out of a James tonestack at at-least 140K impedance (300K in bass) plus a 270K resistor. A 10K SS PA input would suck it WAY down. 3 feet of cable (100pFd) would high-cut at 4KHz.

You won't find transformers with >410K impedance on one side; for useful low Z on the other side the step-down would be huge and output voltage tiny.

You want a cathode follower or other HIGH-Z to low-Z buffer. To drive multiple modern loads, a fairly substantial cathode follower; or a chip (opamp, MOSFET). Ideally this will eat a lower voltage and higher current supply; this is awkward.

So do you want another tube, or a small added PT and a chip?

[tdvt]

 Thank you  very much for your reply.


 I wondered about the James tonestack impedance & the fact that it was at the end. I also recall mention of a cathode follower in the search, but didn’t save that info. I wouldn’t know where to start calculating any of that.


After changing my search terms some, I found nearly the same project proposed on another forum where you weighed in (8 years ago!), so I guess you’ve been down this road before.


In that post, you proposed tacking the VT-40 CF (exact circuit) to the end of the B-15 pre & possibly paralleling the sections of the second tube.  Does the same suggestion apply to this version of the B-15, as the other had the tone-stack between the first two stages? All tube sound like a plan.


In a couple of things that I read, people mentioned that they thought that the “Ext Amp” out on their B-15s were malfunctioning, as they were getting a degraded signal.  I am guessing they were just experiencing the huge mis-match you described.
 
Thanks, TD
 

[2deaf]

Pre-out, Line Out, Effects Send, and a bunch of other names are all the same thing.  For an unbalanced output like the ones coming out of guitar amps,  the average voltage is (or should be) about 0.5V_p with a low impedance.  Not only do you need to convert your high impedance to low, you also need to lower the voltage that comes out of those tonestacks.  Attached is what I think you mean by tacking the VT-40 CF to the end of the B-15 pre and paralleling the sections.  Included is a voltage divider in front of the CF's that can be tweaked for whatever you want.  The way it is drawn, you should be able to turn the pre-amp down a lot and still get enough voltage at the line out.     

[tdvt]

Pre-out, Line Out, Effects Send, and a bunch of other names are all the same thing.  For an unbalanced output like the ones coming out of guitar amps,  the average voltage is (or should be) about 0.5V_p with a low impedance.  Not only do you need to convert your high impedance to low, you also need to lower the voltage that comes out of those tonestacks.  Attached is what I think you mean by tacking the VT-40 CF to the end of the B-15 pre and paralleling the sections.  Included is a voltage divider in front of the CF's that can be tweaked for whatever you want.  The way it is drawn, you should be able to turn the pre-amp down a lot and still get enough voltage at the line out.     

Wow, thanks for taking the time to draw that up!

That is what I understood PRR to suggest in the other thread I came across.

What would be the preferred location for an isolation transformer on one of those line-outs?  From the cap to ground?

[DummyLoad]

Pre-out, Line Out, Effects Send, and a bunch of other names are all the same thing.  For an unbalanced output like the ones coming out of guitar amps,  the average voltage is (or should be) about 0.5V_p with a low impedance.  Not only do you need to convert your high impedance to low, you also need to lower the voltage that comes out of those tonestacks.  Attached is what I think you mean by tacking the VT-40 CF to the end of the B-15 pre and paralleling the sections.  Included is a voltage divider in front of the CF's that can be tweaked for whatever you want.  The way it is drawn, you should be able to turn the pre-amp down a lot and still get enough voltage at the line out.     

What would be the preferred location for an isolation transformer on one of those line-outs?  From the cap to ground?

yes.


i love these


--pete

[tdvt]

Thanks for the confirmation.

I'm old, but I don't remember those Altec transformers. Looks like they're pretty popular with the hi-fi guys too.

15K:600 is about the right value?

 

[DummyLoad]

Thanks for the confirmation.

I'm old, but I don't remember those Altec transformers. Looks like they're pretty popular with the hi-fi guys too.

15K:600 is about the right value?

yes. we use them in a bass preamp for balanced lo-z out. we drive them cap coupled off a plate amp. they drive as easily with a CF or source follower.

if you're pinching pennies, the edcor work well too...

if you want current state of the art, then consider cinemag or jensen ouch! my wallet hurts. the cinemag are less expensive. i paind 65 each a few years back, probably near the $100 mark now.

for a guitar amp, i'd put up with the build wait and run with the edcor. the cinemag and jensen are usually stocked shelf items. OTOH, vintage iron is hard to beat, and i'm fond of altec iron.

--pete

[DummyLoad]

almost forgot about these guys, they are the spin-off of peerles, (altec back in the day) and are reasonably priced. you want the  bee seven series model B7-10K or the B7-15K.

--pete

[vibrolax]

I have used Edcor WSM15K:600 to provide balanced line output in a few projects.

Here it is in my dual Pultec MB-1 microphone preamp
http://www.frontiernet.net/~jff/pultec_mb1/DualTubeMicrophonePreamp.html
It is driven by a cap coupled 12AU7 parallel plate cathode follower

I used an Altec/Peerless 15095A in another single channel Pultec MB-1 I made.  I happened to have one that I acquired in a $10 box of miscellaneous transformers I bought at a hamfest.  Way too expensive at ebay prices.

Here is the Edcor in my dual Fender Blackface preamp, driven by a cap-coupled optimal White Cathode Follower
http://www.frontiernet.net/~jff/SonOfAlembic/SonOfAlembicF2B.html
This puts out 16V P-P into 600 ohms before clipping.

I intended to use the Edcor in my Ampeg SVT preamp, driven by a wimpy 12AX7 cathode follower. But I abandoned it due to hum I couldn't get rid of.  I ended up using an OEP A262AE3 ($16) (hooked up backwards), inserted in the separately available Mu-metal shel ($10).
http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifier.html
http://www.newark.com/oep-oxford-electrical-products/a262a3e/audio-frequency-transformer/dp/24M0686
The maximum output level is lower than the Edcor, but it's perfectly quiet, and a good level for input to recording/PA console.  I am happy with the OEP.

Before I received the OEP, I was paranoid about it working, so I also ordered a pair of Cinemag CM-2810 9.6k:600  http://cinemag.biz/output/PDF/CM-2810.pdf  About $42 apiece.

I haven't actually used the Cinemag's yet, but I have no doubt they will rock.

I designed a B15-N preamp with a DC coupled cathode follower + Edcor for another builder.  He was very happy with it.  The transformer isn't in the LTSpice model, I simulated it as a 15K resistor.  The attachment is a print of the LTSpice schematic.

Jon

[tdvt]

Thanks for all the matching transformer info. The Edcor sounds like my price range., hope I won't run into the hum issue.

Those are great pages you have, really well documented!

Did you ever add the output level control?

Best, TD

[vibrolax]

No, I didn't add a level control.  I designed a switched balanced pad with -6 db and -12 db cuts on a dpdt center off toggle for another builder.  I'll post it if I can find it.  Of course i can find it!  It's in my sent emails. There are lots of online calculators for pad design.

Another variation I designed for builders wanting a master volume used an ac coupled cathode follower, so a level pot could be placed between the gain stage and the cf.

Jon

[vibrolax]

Ok, I found what I designed.  It is a single level -10 dB H pad @ 600 ohms in and out on a DPDT switch.

The attached PDF shows it.

An H pad calculator http://www.nu9n.com/tpad-calculator.html makes it easy to obtain the resistor values for other attenuations.

Jon

[tdvt]

Decided to move ahead with this one. Have parts & chassis material coming as well as the PT.

I have drawn up the circuit (no power supply) which is one channel of the B-15 pre-amp grafted to a VT-40 cathode follower at PRR's suggestion & so generously drawn up by 2deaf.

The pre-amp circuit in the drawing is lifted from the recent re-issue B-15 "Heritage" model, 1964 channel (which has a few component values different from the original), as opposed to a later '66 model I included in the original post. The tone stack is moved between the tube stages in the '64.

I have also added a isolation transformer as suggested by Dummyload & Vibrolax on one side. I would use the 10K side to drive a power amp & the 600R side for a line level out or for recording.

There were 270K mixing resistors in the original Ampeg design (shown on 2deafs sketch) that I deleted since I have only one channel & it now goes straight to the voltage divider. Not sure if these values are still correct.

I was hoping for the more savvy to look it over & point out what I have missed.

Thanks, TD

[tdvt]

Any takers for schematic review?

Layout is done & I plan to make the board this week.

Thanks,

TD

[sluckey]

Looks workable to me.

[vibrolax]

I reviewed it somewhat casually, and didn't see any immediate issues.  I don't know that the 820K/200k voltage divider after the volume control is strictly necessary.  I guess it depends on whether the input to the CF's will be overdriven too much at high volume settings.  Probably won't hurt to leave it in.  You can adjust the divider values if the levels aren't quite right.

[tdvt]

Great!

Thank you both for checking it out.

Making the turret board is the beginning of where the rubber meets the road & things aren't so easy to change if there's an issue.

Onward!

TD

[vibrolax]

Just in case you're running short on turret board space:

In my Ampeg SVT preamp, which includes the same James tonestack as the B-15, I found it convenient to wire most of the bass/treble circuit on the back of the pots: http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifierWiring.html

[tdvt]

Just in case you're running short on turret board space:

In my Ampeg SVT preamp, which includes the same James tonestack as the B-15, I found it convenient to wire most of the bass/treble circuit on the back of the pots: http://www.frontiernet.net/~jff/SonOfSVPCL/DIYSVTBassPreamplifierWiring.html

Thanks for that.

Some how I hadn't seen your SVT project, quite an undertaking!

I actually started with the tone components hung on the pots but have since moved them on to the board. I can fab the board any size so that's not a problem.

With everything mounted on the board, except filter caps (using a new but orphaned cap can) & iso transformer, I am at about 3.5" X 6.25" w/50 or so turrets 

Thanks, TD