Please review the site Rules, Terms of Service, and Privacy Policy at your convenience. Rules, TOS, Privacy
Get familiar with the reaction system: Introducing the Reaction System

Bravo Headamp > Borbely Preamp



  • The big blue Alps pots are pretty much the standard for diy audio projects. 50k or 100k are typical for tube circuits. You are replacing the grid resistor with a pot, correct?

  • This would be for the input side volume control, Tom.
    Looking around the net, it appears most dvd/cd/dacs/etc. have an output around 470∩ and preamps/amps an input around 47k∩. That leads me to believe 50k would be the best choice.
    Researching volume pots, it looks like the favorite Alps are the rk271 conductive plastic Japanese, not Taiwanese made units. Doesn't look like Mouser has that in stock. Bourns 91 series was another that came up often and another conductive plastic, but only the 10k is available. Thinking that's a little lower than optimum for a preamp line level input. Then found mention of the Vishay/Sfernice, also conductive plastic. Mouser those in 50k and in stock.
    Look ok to you guys? It's going in the cart unless somebody has an objection.

  • I like Alps pots. PE part number 023-208 is my go to.

  • edited January 2021

    @PWRRYD said:
    I like Alps pots. PE part number 023-208 is my go to.

    Though I might add some tubes to the order but PE's kinda slim on 12au7 stock and unfortunately, they don't have enough of the other stuff needed to get free shipping nor does Mouser have that in stock. For what it's worth, be leery of buying an Alps rk271 (I think the rk271 series is referred to as the blue velvet) or rk270 (is the kax2 you linked in the rk270 series?) off ebay as it appears fakes are more common than the real thing.

    Tom, that duncanamp design link went to a has-been page.

  • I agree on ebay being full of counterfeit parts. Just too risky, especially with electronic components.

    The Duncanamps page works for me -

    Looks like he has a user group too -

  • Those links worked.

    Everything but a pair of back-ordered diodes. For some odd reason, the tubes will not insert into the tube sockets. They look to align, and I pushed as hard as I dare, but they will not go in. There goes another week waiting in parts.

  • edited January 2021

    re the tubes; try a firm push in combo with a bit of circular rocking

  • edited January 2021

    Yeah, it's those PC board mounted sockets. I'm running into the same thing with my ST-70 rebuild. Find an old tube and wiggle & push and they'll finally go in. The first time is the hardest.

  • edited January 2021

    Wiggling and working it in sounds kind of kinky. braaaaaaaaaap

    Bent the pins on a new Tung-Sol trying to wiggle and push. Me thinks these are miniature pc sockets since the solder pins had to be bent outward to align with the board's solder holes and that 'regular' pc board sockets have a larger spread of the pins? Spreading the pins on the socket probably misaligned the female part of the pin inside the socket and now that it's soldered, they will never align correctly. Going to try and open the socket pins up with a small nail before going through the hassle of desoldering the sockets.

    Next time I'll insert an old tube into the socket before soldering it to the board to insure the pins stay aligned.

  • Upon powering up, what parts of tube need power first?

  • The heaters (filaments).

  • Where can I find those small couplers that can be used to connect the volume pot to a long shaft so the pot can be mounted in the rear near the rca jacks? What's it called?

  • I don't know what those are called but I have seen people use environmental heat shrink. The adhesive holds the shafts together.

  • Found em'. Shaft couplers, ordered some from ebay but going to try heat shrink till then. Thanks for the idea.

  • The top plate has only two tubes sticking out and venting. Too sparse. What about adding some transformer covers to fill the empty space? Could this be as simple as a couple electrical outlet boxes smoothed out and painted?

  • You can use hammond alluminum boxes like the ones used for guitar pedals.

  • Good call. Those look speciously like electrical outlet boxes. Hardware store tomorrow.

  • Slowly making progress. Will try and get some pics up today.

    How do I wire the ac input of the power supply for 110v? Not finding it in the pdf.

    HAA24-0.6-AG Here's the pdf

  • I think this stain will lighten up a bit with some age.

  • I think this is it - took me a while to find it.

  • edited February 2021

    Where did you find that?
    As shipped for 120v, terminal 4&2 are bridged, and 3&1 are bridged. Hot to 4/2, neutral to 1/3, and ground to 5?

  • I saw something in the Bel Tech Library that pointed me to that document. Yep - 4 & 1 for your AC line in - but don't put earth ground to 5. That's a tap off the 1st winding and it would probably trip your breaker and/or let the smoke out.

  • edited February 2021

    On the left is the ac power to/from the switches. Do they need to be twisted? The switch wiring/wiring is a bit more than 1/2" from the caps at its closest and ~1" from the boards, is this far enough away?

  • I would twist anything AC just to be safe. Hard to really know if it'll make any difference without trying it both ways. That is pretty close to that board. It looks like you raised those boards up, so can you route the AC wiring along the bottom?

  • Was running low on hookup wire and had to take the short way around but should have more wire in next week. I'll work on it then. Wish that stuff was more pliable.

  • Pin 1 goes to the shield of input/output, or ground, or both? Does it matter which #1 pin, or both (are they shorted internally?) goes where? Ground it at the same spot everything else is grounded at, this includes the input/output shield?

  • Ground the body of the pot itself if it's not mounted to a metal part of the chassis that already has a reference to ground. Tie it to your star ground point. If the shields are grounded at the input jacks, leave them floating at the pot. Grounding both ends could introduce a ground loop. The two channels are not tied together inside the pot. Is there a ground pad on the input of those boards? If so, that's where I would terminate pin 1(s), either through a shielded cable or a twisted pair.

  • edited February 2021

    Just thought of something - if your input jacks are mounted in wood or a metal panel that's bolted to a wooden chassis, you can tie the shields to pin 1 and then bring those back to the boards. That would be akin to running the RCA cables right up to the pot itself. Just make sure you aren't separately grounding any other jacks, like the outputs, on that same panel, unless they are all isolated/insulated jacks

  • edited February 2021

    I know you guys are either getting tired of me asking dumb questions or having a good laugh at my complete lack of things electrical. Why quit when I'm on a roll ...

    In reference to the first drawing ...
    +/- vcc _ +/- 24 volt input and com from board power supply.

    Out to amp _ audio to amp. Use this plus SGND/PGND for outputs to amp?

    6v and neutral from heater supply _ looking at the schem I assume the board is configured to allow 12v series or 6 volt parallel hookup of the heaters. Using 6v, both of these inputs will be +6v and since pin9 is center tap, the neutral side of the heater power supply is the hole marked -inp. Not sure of this but the trace does fall back to pin9.

    +inp _ +audio input from potentiometer.

    SGND _ (top right) ground or shield from potentiometer goes here. Bring input shield wire from potentiometer to this point?
    Trace from SGND also runs to pin out just below -inp, does this unlabeled pin also go to ground or is doing this asking for ground loops?

    Pin out just above -inp lettering goes to R9 which ties in with pin7 (grid) and R8/C2 (is this a filter to remove voltage from the grid for audio output?). What is this point for? What does it connect to, or should it?

Sign In or Register to comment.