Modifications to run e80cc/12bh7a/5687 for Crack+Speedball

[NightFlight]

Oh yes.

Thanks CB and Paul for your input in this thread. Its appreciated.

[kscwuzhere]

Oh yes.

Thanks CB and Paul for your input in this thread. Its appreciated.

Why not install a SPDT ON-OFF-ON so that you can roll between the 12au7, 12bh7a and the e80cc? That's what I did.
After rolling all these tubes at the right resistance I decided to retrofit for the 5687 and 7119 and I must say, I'm never going back to the 12au7, 12bh7a, and e80cc; the 5687 destroys these tubes. It has way better imagining and the soundstage is massive. Details and data retrieval are as good as the e80cc but it has the lower end warmth that the 12b7a had. After installing the 7119/5687 I found myself questioning why the Crack wasn't designed to run this tube in the first place. The way i retrofitted it also allows me to switch back to the 12au7 but I havent found myself ever wanting to.

[NightFlight]

What are the potential (haha) drawbacks of trimpots in this application?  Cycle life is one I have seen so far.  But I'm looking for a fit with effectively infinite turn cycles. Say 100ohms in series with a 500ohm trimpot.

http://www.mouser.com/ds/2/428/accutrim1240-7897.pdf

Edit: OY! Price could be one issue.... An the Power Derating Curve...

[Paul Birkeland]

Oh yes.

Thanks CB and Paul for your input in this thread. Its appreciated.

150 Ohms is about 6mA.  The voltage across the C4S is roughly 80V, and with 6mA of current, you'll dissipate 1/2 a Watt through the MJE-350.  A tiny heatsink probably wouldn't hurt here, and the MJE5731A will handle this with aplomb. 

[Paul Birkeland]

What are the potential (haha) drawbacks of trimpots in this application?  Cycle life is one I have seen so far.  But I'm looking for a fit with effectively infinite turn cycles. Say 100ohms in series with a 500ohm trimpot.

A trim pot would function here, though you'll end up with a fair amount of useless adjustment if you parallel the two when the trimmer is set to a low resistance.  A 100 Ohm resistor in series with a 500 Ohm/10 turn trimmer may work a bit better.

These are indeed expensive little buggers, and they would have to be glued down so that adjustments could be made reliably. 

[NightFlight]

I think I can just use some of the free space on the C4S board to get a 100 Ohm resistor in series with a trim pot easily enough.

[NightFlight]

Why not install a SPDT ON-OFF-ON so that you can roll between the 12au7, 12bh7a and the e80cc? That's what I did.
After rolling all these tubes at the right resistance I decided to retrofit for the 5687 and 7119 and I must say, I'm never going back to the 12au7, 12bh7a, and e80cc; the 5687 destroys these tubes. It has way better imagining and the soundstage is massive. Details and data retrieval are as good as the e80cc but it has the lower end warmth that the 12b7a had. After installing the 7119/5687 I found myself questioning why the Crack wasn't designed to run this tube in the first place. The way i retrofitted it also allows me to switch back to the 12au7 but I havent found myself ever wanting to.

The MJE5731A and some cheaper cerment trim pots are on their way in the mail. You've definitely peeked my interest in the 5687 and they seem to be readily available. I have a about 5 flea-bay 12BH7's on their way as well.  It was hard to turn down 4 for $10 and a NOS Tung-Sol '59 milspec for $20. I'll give the 12BH7 a bit of a listen and then likely hard wire for the 5687 once I get the urge again.

I'd also like to re-cable my HD800, but I think I'll wait for the mainline. Then this crack stuff might become moot.
This photo initially lead me to believe an HD800 cable kit might surface from BH, but has yet to materialize:

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fbottleheadnginx.bottleheadcorp.netdna-cdn.com%2Fwp-content%2Fuploads%2F2014%2F01%2Fmainlinehd800.jpg&hash=2bdd49d68e1d87de04d2f0a1ed7bbceba04181bd)

[Doc B.]

It's too difficult to hand build a reliable connection with those terrible plugs. My cable breaks each time we take it to a Head Fi meet. They really need a molded plastic strain relief.

[Jimb0]

So I've been lurking on this thread and willing to try modding my crack for the 12BH7a and 5687's tubes but would still allow me to switch to the 12au7's.

From my understanding, I would need to get the following parts:

2 x DPDT On-Off-On
1 x DPDT On-Off-On
2 x MJE5731A
2 x HLMP-CB1A-XY0DD LED's

I'll also need to test out different resistors to see what matches the tube for my 12BH7's.

Could you guys tell me if I have any missing or incorrect parts?

Thanks!

Jim

[kscwuzhere]

So I've been lurking on this thread and willing to try modding my crack for the 12BH7a and 5687's tubes but would still allow me to switch to the 12au7's.

From my understanding, I would need to get the following parts:

2 x DPDT On-Off-On
1 x DPDT On-Off-On
2 x MJE5731A
2 x HLMP-CB1A-XY0DD LED's

I'll also need to test out different resistors to see what matches the tube for my 12BH7's.

Could you guys tell me if I have any missing or incorrect parts?

Thanks!

Jim

Everything seems right to me. I recommend getting more RED leds and blue leds as they tend to break easily during construction. A simple resistor pack may be a good idea, as well as alligator clips to test them out.

[Jimb0]

Thanks kscwuzhere,

Do you know where I can find the voltages for 12BH7's. What range is good to run them at?

[kscwuzhere]

Thanks kscwuzhere,

Do you know where I can find the voltages for 12BH7's. What range is good to run them at?

My 12bh7a ran perfectly at 70v with 140ohms

[Jimb0]

My 12bh7a ran perfectly at 70v with 140ohms

Should all 12BH7a's run around that voltage?

[Paul Birkeland]

Should all 12BH7a's run around that voltage?

I would be satisfied if ten 12BH7's that tested good hit that voltage (or thereabouts).  If only one 12BH7 was used to dial in the R1 value, then a worn or even an overly fresh specimen could throw things off a bit for the average tube.

[NightFlight]

My first 12BH7A (unknown test value) hit 70v/75v @ 150ohm on R1. I dropped in MJE5731A to replace the MJE350 and they seem to work well enough as drop in replacements with no change to the output voltage.  Experimentally putting in cermet trim pots tonight. The trim pots I grabbed are 500ohm with 20 turns and seem to be stable to around .1 ohm at room temperature. We'll see how that goes once in the amp. I'm thinking in series with a 75ohm effectively in R1 to have more room to play with. 

It definitely be nice to be able to dial in each side of the triode!;-)