Ampeg B15 build -- question about lead dress

[hylaphone]

Hi good folks.
I am building a single channel B15 clone for a friend, using an old Hammond PT and a BF Bassman OT.
Just starting punching the chassis and it occurred to me - these amps have the output transformer way over near the preamp tubes, OT secondaries running to the output jack between v2 and and v3, the PI.
Might this be problematic? And is there any protocol I should follow, short of moving the OT to the middle of the chassis?
I'd like to keep the transformers on each end if possible, but would love to hear any tips!
Thanks ---

[loudthud]

Check this thread: http://music-electronics-forum.com/t33258/

It covers many of the issues. It took me a while to find it because B15 and B-15 are not equal as far as the search function is concerned.

[hylaphone]

Lots of great information in that thread, thanks for the reply.
The OT leads concern me. As this layout stands, they poke through the chassis right under the board around the tone stack. Similar to an original B15, from what I can tell from pictures...
I could run them above the chassis like you suggest, to enter between the output tubes, but that's a bit unsightly for this upright chassis..
Any other tricks? Wrap them in braided shield or something?
Just trying to plan carefully!

[Mick Bailey]

When building a clone an important point to remember is that it often isn't a clone! If the transformers are different this can change the magnetic field patterns and induction relationships, so that what may have worked fine in the original sometimes doesn't work out too well in the new build.

With an amp layout and build that I haven't done before, there's a simple way to optimise the transformer layout: Connect up the mains transformer with terminal block, and terminate or otherwise insulate the secondary leads. Position it on top of the chassis such that it can be safely powered and moved around. I now connect a crystal earpiece to the primary of the OPT. With the mains transformer energised you can listen to the induced hum in the OPT and move the transformers around to give the best relationship. I then work out the rest of the layout around the transformer locations.

Haven't tried, but standard MP3-type ear-buds or phones may work on the OPT secondary.

Apologies if this has recently been posted elsewhere.

[Steve Conner]

The OT primary wires will be the worst offenders. You can run them through a grounded tube of copper braid like the stuff on old pickup wires. Or tuck them into the corner of the chassis and stick copper tape over them.

In the past, I've replaced the OT primary leads with two coax cables, and used the shields as the CT connection. From a shielding point of view, B+ is almost as good as ground.

[hylaphone]

Thanks guys. Great tip, Mick! I'll definitely give it a shot.
I can orient the OT to run its leads in a corner -- can the secondaries run safely under the board, or should I bundle them under shielding with the primaries?

[ldervish]

Haven't tried, but standard MP3-type ear-buds or phones may work on the OPT secondary.

headphones worked fine for me

[loudthud]

I could run them above the chassis like you suggest, to enter between the output tubes, but that's a bit unsightly for this upright chassis..
Any other tricks? Wrap them in braided shield or something?
Just trying to plan carefully!

The danger in a bad layout is what you do if it causes problems. Do you have the equipment to find and correct an oscillation or objectional hum? It can be time consuming and frustrating without an oscilloscope. It may become necessary to scrap the chassis and start over. If you are ok with that, do whatever you want. Experience has taught me that layout matters and a bad layout is a waste of time and money. The impedances in the preamp and tone control network are pretty high and that makes them susceptible to noise and unwanted signal pickup.

What you can do is first get those plate leads as far from the input(s) and preamp as you can. Twist them together so that the signals cancel as much as possible. If you get an oscillation, try slipping a braided shield over the plate leads where they run close to the preamp. The right size braid may be somewhat hard to find. If that doesn't fix it, try running the plate leads across the top of the chassis. The secondary side of the OT radiates less, but the ground currents can cause problems so attention to ground becomes important.

[hylaphone]

Thanks for all the input! I will follow up as the build progresses.

[Randall]

I am the OP for the above B15 thread. It took me quite a while to decide on a layout, and then get a chassis custom made. I have a trade arraignment for amp work with a metal worker friend. In my build, I went with putting the trafos at opposite corners of my smallish chassis, and orienting the OT so the plate leads are pointed right at the output tubes, with the preamp tubes being at the front of the chassis, maybe 5" away. I hope this will be enough. I am just about ready to start building this. I am getting the chassis blue zinc plated next week.

[hylaphone]

Interesting, we have been in the exact same situation right down to the work trade for chassis fab!
This one is 17" x 8", I tried the headphone trick and ended up mounting the transformers diagonally in the corners rather than parallel to the face.
Also used a braided sleeve on the OT primaries for good measure. I am still tidying up the power section, but the amp plays beautifully and I am getting no bugs/noise in the preamp, and no oscillation from the OT.
May have to build one of these for myself!
Keep us posted on your build, it would be interesting to compare layouts. I'll post some pictures of mine next week.

[Randall]

What about pots? I am aiming for the B-15NC, the earlier cathode biased version with the gain>tone>gain>vol arrangement. All three pots will be 1M, but I don't see from Joe Piazza's drawing (all could I find) whether to go Linear or Log taper. My goal here is to build a bass amp that will fit an acoustic blues gig, so I am aiming for an upright sound on a Fender P. Tone is more import and than a lot of volume.

[loudthud]

You should use Audio 10% taper. With those, flat response will be at half rotation of the Bass and Treble. I've been having a hard time finding any pots with a 10% Audio (log) taper. The PE pots are about 15%.

[ldervish]

I am the OP for the above B15 thread. It took me quite a while to decide on a layout, and then get a chassis custom made. I have a trade arraignment for amp work with a metal worker friend. In my build, I went with putting the trafos at opposite corners of my smallish chassis, and orienting the OT so the plate leads are pointed right at the output tubes, with the preamp tubes being at the front of the chassis, maybe 5" away. I hope this will be enough. I am just about ready to start building this. I am getting the chassis blue zinc plated next week.

I recently finished the major work on a conversion to an SB-12 clone with a chassis just 5 5/8" x 10 5/8". The OT was placed on the corner diagonally opposite the PT using the headphone trick, parallel to chassis edges (rotation provided no advantage). The existing preamp tubes (shielded) were less than an inch away from the OT bells but I cannot detect any interference. The amp is very quiet. I did have an ultrasonic oscillation problem (solved with help from MEF members - THANKS!) but that was due to a senior moment and not xfmr location.

Regarding the pots, IIRC the tone stack is identical in many of the Ampeg B series (if that's the right terminology) and some others as well i.e caps of .001u, .01u, 47p and 470p, isolation resistor of 120k and 220k/22k resistors in the bass control divider. I just went through this, and the pots are log. One of the early early schems shows them as linear, but the later ones that specify it show log and the Duncan Tone Stack curves look better with log. It's just too sensitive with linear.

[hylaphone]

I used Plane-Jane Alpha pots and have no complaints, one thing to keep in mind if you use a 1m treble pot is to change the cap values there to those from the B-15-N version, which used a 1m.
47pF and 470pF become .00047uF and .0047uF respectively.