New Build Crack-a-two-a

[SakoAmp]

Finished my first Bottlehead build - Crack-a-two a - the last electronic build was in the late 60's - a HeathKit ShortWave radio.

Finished build, no problems in functioning.  Using with Sennheiser HD650's. Great performance.

Did however have a question regarding voltage measurements (uh-oh !).

The tested unit functions well.  The question is about what seems like a "lower than spec" voltage in both:

1) The Low Current C4S (measured OB 42; range 60-90)
and
2) The High Current C4S (measured OB 72, 88; range 90-110). 

All other resistance and voltage measurements are OK.

This degree of voltage difference in each from the measured and anticipated range seems lower than acceptable/expected.  Re-rechecked wiring connections and solder integrity - everything looks OK.  Is this a cause for concern?

Thanks in advance for any information  / insight !

[Paul Birkeland]

It is interesting that your OB results are so low but your OA results are not.  What do the Kreg voltages look like on the center board?

Low voltage at OB would be the result of lower than anticipated allowed current by that center board's B side C4S, or a miswire.  It could simply by a misbehaving solder joint, likely on the board, that isn't quite letting things fall into line.  With the lights in your room nearly off, is the LED intensity identical for both pairs of LEDs on that center C4S board?

How about the LEDs on the 9 pin socket itself?

[Karl5150]

That is a good looking build. The black (painted?) hardware works well with the panel color.

[Raymond P.]

I like that top plate color. Reminds me of the color on my Crack (Sage). Good luck with your voltages.

[SakoAmp]

Envisioned this build as a WWII/mid century radio operators kit, left in the jungle and resurrected.  Would like a more distressed wooden/metal base appearance however.  Studied WWII radio operators kit - have some ideas - maybe for the future.  Will post.

TESTING AMPLIFIER:

All LED's throughout glow equally and bright.

Remeasured voltages:

LOW CURRENT
IA         148  (150)
OA        67    (60-70)
KregA   4.6    (3-6)
bRegA  149   (150)

IB          148  (150)
OB          60   (60-90)
KregB       5    (3-6)
bRegB    148  (150)


HIGH CURRENT
IA           196/197  (190)
OA          148/149  (150)
bA               0/0      (0)

IB                0/0      (0)
OB            72/88     (90-110)
bB             148/150 (150)

Everything works well / sounds great !
++++++

COSMETIC DETAILS
1) Wooden Base - 400 then 600 grit. Then hand rubbed 4x Tung Oil
2) Top (metal) -  400 then 600 grit. Krylon Primer x2 (sanded 600 grit between coats). Then Krylon Satin Italian Olive x2 coats with 600 between coats.
3) Black Plastic Knobs - as supplied by Bottlehead
4) Transformer Cap and screws - sanded, acetone, Birchwood Casey Perma Blue - achieved rust patina on transformer cap and black patina on all screws.

WISH LIST
3) would like if the RCA input jacks were deep red plastic instead of red-orange.
4) some sort of label for the volume, balance, selector switches (I may have the metal plate laser etched or apply hard plastic plate over control knobs).

++++++
Added Black painted headphone input (see image)

What's my next build ??
Glad places like this still exist !!

[Paul Birkeland]

Those are fine voltages.  You have a very, very strong 12AU7 that is presenting low-ish bias voltage to the 6080, but with the constant current sources none of this really matters. 

[SakoAmp]

Thanks for your input !

[oguinn]

Would you care to contribute to the gallery site? You’d be the first Crack-a-two-a. Shoot me a private message (the little email icon under my name to the left) if you’d like to and we can chat.

[ALL212]

"4) some sort of label for the volume, balance, selector switches (I may have the metal plate laser etched or apply hard plastic plate over control knobs)."

Ya - anyone have anything or found anything for this?  I'd like to label the tube positions.  I'm not going to be around forever and someday these might be handed down.  It'd be nice if the future generation would know for sure what tubes went where!

Nice build!  I would never have thought about using that color but I like it.

[oguinn]

I’m acid etching some tags to add to my amps this week. You could potentially do that for a chassis plate as well, although you’d have to etch deep to get the infill down enough to paint over and still see the etch, and you’d have to do it before the amp is built. I’ve been thinking about trying it to get a different color MourQuiet volume label plate.

[Paul Birkeland]

Ya - anyone have anything or found anything for this?  I'd like to label the tube positions.  I'm not going to be around forever and someday these might be handed down.  It'd be nice if the future generation would know for sure what tubes went where!

You can write this on the bottom side of the chassis plate with a Sharpie.

[ALL212]

awww...come on Paul...that's WAY too easy!!