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

[jboehle]

Here's a sneak peek at my solution for easy switching between 12AU7, 12BH7, and E80CC tubes. It fits right into the existing resistor holes on the Speedball boards.

With no jumper installed:

With jumper installed for 12AU7 operation:

The back:

I will post more pics once I get the 2nd one made and both installed.

 

[2wo]

how cool is that...John

 

[JamieMcC]

Creative solution!

 

[kscwuzhere]

Here's a sneak peek at my solution for easy switching between 12AU7, 12BH7, and E80CC tubes. It fits right into the existing resistor holes on the Speedball boards.

With no jumper installed:

With jumper installed for 12AU7 operation:

The back:

I will post more pics once I get the 2nd one made and both installed.

Any update with the mod? I'm curious to see the results!

 

[NightFlight]

Well, I've been sitting on this Rogers(Canadian?) D Getter 12BH7A that I got with a bunch of other stuff about a year ago now   I was digging through my electronics kit, and found some 150ohm resistors so I decided why not give it a go and put them together.

Glad I did.

In a stock SB crack the 12BH7A was sitting in the low 60V on T1 and T2. Dropping R1 to 150Ohm brought up T1 and T2 to ~70V. Acceptable given I just soldered the resistors them straight in without much thought to reverting. Honestly, re-reading this thread from the start tonight indicated to me that I was likely be happy with this choice anyhow.

I gotta say before modification the 12BH7A showed a ton of potential, but was a bit fat in the transients and flabby on the bottom end at ~60V. Bringing the plate up to 70V put this tube in the zone! Still using the stock MJE350s, so I'm hoping their not going to go into runaway with the extra load to dissipate... because I'm not reverting. NOPE. ;-) No sir. I won't like it.

Whole new texture appearing across the spectrum compared to the 12AU7s I've give a fair shake over the last year. I really don't roll tubes that much these days. I may just setup switching because I'm curios about the E80CC, but that's about it. I've read the E80CC sacrifices some bottom end for clarity, so I'm likely prefer this 12BH7A.

Strangely with the 12BH7A I find the RCA 6080WA stock tube totally new. My 5998 has been my staple since I got it - even selling the 412A in favour of this particular 5998. But the 6080 has a midrage texture reveal that is a bit lost on the 5998. Interesting.

 

[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:

 

[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!;-)