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

[Paul Birkeland]

Support is maybe a little vague.  We can make suggestions and assist with ideas, but for us to be able to sort out hum from a modification we haven't tried, we would have to build a Crack and attempt to modify it to exactly your specifications. 

As far as hum from switching R1's, the only experience I have was finding noise when long wires were used to try different resistor values. 

[Loon]

Totally understand Paul. 

You often go above and beyond the call of duty.  Thanks.

Have tested the problem further changing cathode follower.

6080 - hum on startup, but goes away after approx 20 seconds
6AS7G - hum on startup, but goes away after approx 20 seconds
7236 - hum on startup, increasing in volume after approx 20 secs
5998 - Hum on startup, increasing in volume after about 20 secs
7802 - Hum on startup, increasing in volume after about 20 secs

All the tubes types with persistent hum have a higher mutual conductance than those that do not.  In another thread Paul mentioned that mutual conductance is a "tendency to oscillate". 

What would the sonic symptoms of oscillation be?  My hum would be around 120 Hz (speaking as a musician, not an electrician).

Do you think my hum could be a symptom of oscillation?

I have read about grid stoppers in this context?  Might a grid stopper help?

Thanks again.

[Paul Birkeland]

For the 10 cents that a pair of resistors will cost you, go ahead and give the stoppers a shot.

[Loon]

MJE 350

OK.  I have just read a thread re hum problem where the problem was a faulty transistor in the speedball. 

My Crack is a new build.  I installed switch in Speedball prior to installing Speedball in Crack.  The Crack worked fine with all the mentioned tube types prior to installation of speedball.  Perhaps, therefore, the problem is with the Speedball and not the switch as I ordered my amp about six weeks ago and received it about two weeks ago.

Will replace MJE 350 and see what happens.

[kscwuzhere]

Ok well I find myself liking the two 12bh7's that arrived in the post this morning I have been listening to them on the 12au7 setting but am mindful of how well the e80cc sonics improved with the resistor mod once better matched so once again interested in following progress here.   

I just finished modding the crack to run the 5687 and the improvement is absolutely incredible. The soundstage and the imagining on my 650s has enhanced tenfold! The 5687 is an even bigger improvement than from the e80cc to the 12bh7! I'll post pictures soon

[Loon]

Just had a look at the data sheet.  Crickey!

Looks like it will draw about 20 mA with a bias of -1.5 V and a plate voltage of 75 V.  I assume you have made a number of mods to make this work?

Looking forward to seeing what you have done.

[kscwuzhere]

Just had a look at the data sheet.  Crickey!

Looks like it will draw about 20 mA with a bias of -1.5 V and a plate voltage of 75 V.  I assume you have made a number of mods to make this work?

Looking forward to seeing what you have done.

Yeah, I upgraded the transistors on the small speedball boards and had to bias some LEDs. I installed everything on switches so I can switch between 12au7/e80cc/12bh7/5687

edit: Oh and the pin out on the 5687 is a little different too so some mods had to be made there too! The more and more I listen to this tube the more I'm being blown away because every aspect in the sound has improved, most noticeable difference is in the soundstage(incredibly whole-bodied) and imaging(instrument separation).

[kscwuzhere]

Ok well I find myself liking the two 12bh7's that arrived in the post this morning I have been listening to them on the 12au7 setting but am mindful of how well the e80cc sonics improved with the resistor mod once better matched so once again interested in following progress here.   

I ended up changing the original SPST set up to a SPDT set up with an On(235ohm)-Off(470ohm)-On(150ohm) to run the 12au7, e80cc, and 12bh7a, respectively. I'd recommend modding it with the SPDT design for ease of use For the resistance of the 5687, I'm in the process of installing a spst in parallel to one of the ON positions on my SPDT toggle to reach the correct resistance; not the most elegant, but itll do.

[onelivewire]

I have my crack running the Raytheon 5687 with my T1s right now, and I can say that the imaging has definitely improved. Bass response may be a bit reduced by comparison to the Sylvania 12bh7a, but the detail retrieval seems to be improved. The biggest difference is, as Paul suggested, the 3-dimensionality of the soundstage. Where other tubes would generally place all of the musicians relatively equidistant from me in different sized rooms, the 5687 is able to place some instruments right by my ears and others far away in the room. I don't know if anyone else uses binaural recordings, but the percussion-jazz in "Explorations in Space and Time" really came to life through this tube.

Edit: I should add that I am running R1 at 117ish ohms to get 77V on Terminals 1 and 5.

[Mordicai]

Changed out the MJE350's; installed the 470 switch-resistor mod, and big sound change like everyone else. Bass, detail, and hearing new things. My problem and surprised no one else mentioned it, is a big jump in gain. I can only just crack my volume pot. Would appreciate suggestions on the the best way to proceed. Potentiometer or a resistor in the volume pot leads? Is this normal. Have others had this gain jump? Ordered a Sylvania 12bh7  today so lots more to go. Beats building more headphone cables( although I do have some Litz wire coming}.

[Paul Birkeland]

The 12BH7 shouldn't be that noticeably different than the 12AU7.

There are, however, likely cases of other tubes that are close to a 12BH7 being relabeled as a 12BH7.  Some computer dual triodes that are kind of a cross between a 12AT7 and a 12BH7 come to mind.  I wouldn't adjust anything until you have a few 12BH7's to triple check.

-PB

[kscwuzhere]

So the mod is finally done and my crack now works at the correct resistances for the 12au7/e80cc/12bh7a/5687.

In order to wire this mod to run the 5687 you're going to need 2 dpdts and the following leds and transistors:

http://www.digikey.com/product-detail/en/HLMP-CB1A-XY0DD/516-2275-1-ND/2428165 - blue LEDS for 5687

MJE5731A - install these in place of the transistors on small speedball boards in reverse orientation

You're going to want to cut the connections on the following lugs of the 9 pin socket:
A6 - coming from terminal 1
A9 - coming from B7
RED LEDs

Connect terminal 1 to the one of the inputs of the dpdt. The two outputs for this position should be going to A6 (for 12au7) and A9 (for 5697), respectively.

Connect the wire from B7 to the other input of the dpdt. The two outputs for this position should be going to A9 (for 12au7) and A8(for 5687), respectively.

Cut a piece of wire and connect it from the center lug of the 9 pin socket to the two inputs of the second dpdt.
Connect the RED leds to the two outputs of one of the ON positions on the dpdt going to A3 and A8 (make sure to orient correctly).
Connect the BLUE leds to the two outputs of the other ON position on the dpdt going to A3 and A6 (make sure to orient correctly).

Once you've installed this all thats left is to make sure the 5687 runs at the correct resistance so adding another resistor with a switch in parallel may be necessary. Luckily for me, my Raytheon 5687 runs at 70V at 140ohms which is the exact resistance that my RCA black 12bh7a needs in order to run at 76V.
So I have a 470 ohm resistor at R1 with a ON-OFF-ON DPDT in parallel to it. One of the ON positions has another 470ohm resistor to make R1 run at 235ohm. The other ON position has a 200ohm resistor in parallel to the 470ohm, making R1 140ohms, which runs both my 12bh7a and the 5687.

I'm sorry if the instructions were a little unclear so please let me know if anyone has any questions. I've attached some pictures but they wont be of most help since its getting very crammed up under the chassis. My Crack now has a cool blue glow when the 5687 is running.

Sonically, the 5687 is my favorite tube out of the lot. Its sound signature makes the hd650s sound very balanced with a quite incredible soundstage (the biggest difference with the 5687 tube). My second favorite is a tie between my RCA 12bh7a and my Telefunken e80cc which has a more lush bass than the 5687 but has noticeably worse imagining and soundstage.

[kscwuzhere]

Just to clarify before I order a bunch of tubes, with the mod working with the 5687 mean that I'd be able to roll both the 7119s and e182cc's with my crack now? Or would further modifications be needed?

[onelivewire]

Here's my build. I still prefer my Yellow label Sylvania 12bh7a (long grey plates, o-getter) to the Raytheon 5687 for the T1s, for its warm sound signature, despite slightly less articulate imaging.

Album: http://imgur.com/a/TTaME

Underside
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FroJHG0V.jpg&hash=07e9ccdb3200f913275d07a2fe0495f0bf27ab05)

Pinout Switches
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2Fu0Uuovs.jpg&hash=e33fc5b49e69ef454f7d40260d2101cf9b26b443)

Variable Resistance Switches
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2Fb1YB5f2.jpg&hash=eb4023c15dfc37f9d42dcf4451b3c448ae926679)

Running 12BH7A
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FPX5VyNV.jpg&hash=d9f5aaa70b331492152dc8c5a15b8537ab3e28c7)

Running 5687
(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi.imgur.com%2FRK0ek6i.jpg&hash=a8d3a2d12f62fda1ea91ac0a6ce4865e3687799f)

[kscwuzhere]

Thank you onelivewire for pictures that are actually helpful!