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

[Paul Birkeland]

When installing these, will the back still butt up to the heatsink in the same way, even given that "they face in the opposite direction" or will some orientation changes need to be made?

Butt up to which heatsink?  These transistors go on the smaller PC boards, they do not go in place of the TIP50's on the bigger PC board. 

[Hornet900]

The E80CC's need something along the lines of 470ohms across R1, which is why many of us have been replacing R1 with a 470ohm and putting a second 470ohm on a switch in parallel, so when switched on, the resistance totals 235ish, and both resistances are usable. The 12BH7's are the dual-triode tubes that we're finding require a resistance of ~147ohms.

Thanks 

[Paul Birkeland]

Edit: Plugged in, this thing sounds crazy! Bass extension, detail retrieval, soundstage are all the best I've heard on the t1, even though the treble can be a bit more cutting than the Mullard ECC82. I must say though, the tubes are getting extremely hot with this setup, even after just 10 minutes or so of play. What can be done heat sink / transistor wise to fix this while still allowing for stock resistance use?

Yeah, this is the difference between just plugging any old tube in and actually sitting down and figuring out what needs to be done to properly operate said tube.

I don't understand the question about heat and the stock resistors.

[Paul Joppa]

... This is about 6mA, which in an MJE-350 being cooled by air will give a temperature rise of 75C beyond whatever temperature is under the hood of the Crack.  ...

For what it's worth, ever since we had problems with the early Foreplay III upgrade and the Paramount, I have designed for a maximum chip temp of 100C and assumed 60C under the hood - so a 40C maximum temperature rise.

[Paul Birkeland]

That's a good reference, thanks for the reminder.  It seems like an MJE-5731 should be OK on most line voltages.  (I calculated for a drop from 200V to 75V, which is worst case scenario)

[Stanshall]

I don't know if anyone else has mentioned but this resistor mod has also completely removed the zero volume hum.

Also, really rather surprised but quite delighted to hear the audible improvements. As with most upgrades to the Crack, it's 'just the same but more'. Nothing I've done has removed or fundamentally altered the qualities of the amp, it just keeps opening out and stretching a little further in every direction.

[onelivewire]

I don't understand the question about heat and the stock resistors.

I just wanted to make sure these new transistors on the small speedball boards will work just fine with everything else in "stock" configuration, which seems to be the case.

I wanted to throw this out there to anyone thinking of playing with the 12bh7's too- I just grabbed a vintage Sylvania 12bh7 to check the operational differences against a Tung-Sol 12bh7, and got the following readings:

@117ohm, w/SYLVANIA
T1: 64V
T5: 69V

So it seems the proper resistance definitely changes between these 12bh7 tubes.
That all said, the soundstage on my t1s has never been so massive and three-dimensional with any other tube.

[Paul Birkeland]

That all said, the soundstage on my t1s has never been so massive and three-dimensional with any other tube.

Try retrofitting for the 5687.

[onelivewire]

Just threw in a couple more resistors,

@93ohm, w/SYLVANIA
T1: 74.8V
T5: 70.8V

Try retrofitting for the 5687/

I'm intrigued - any recommended branding? I'll try and scoop one of these up for play. Would resistance be the only mod needed for this dual-triode as well?

[Paul Birkeland]

I think you could run 10mA on the 1.5V LED, but you'd want the MJE5731A for that.  Do also note that the 5687 has an annoyingly different pinout, so you'll have to do some work there as well.

[serpent68]

I finally installed the 470 ohm mod and am very pleased with the result.  Bass is more controlled and the mids are more detailed and dimensional.

I wonder if anyone with this mod will actually use the 235 ohm switch position.  With this mod, is there any 12AU7 tube that is superior to the E80CC?

When using a 6AS7G (with E80CC), should the switch remain at 470 ohm or changed to 235 ohm?

[Paul Birkeland]

When using a 6AS7G (with E80CC), should the switch remain at 470 ohm or changed to 235 ohm?

The switch is to optimize the plate voltage of the E80CC, so if the E80CC is plugged into the amp, the 470 Ohm resistor should be there.

[Loon]

Just had a crazy idea.

I wonder if it would be possible to put a voltage regulator between the anode of the input tube and the grid of the cathode follower, setting the output voltage at 75V.  This would provide a steady bias to the 6080 at the level required by the circuit. 

As I understand it, the voltage into the regulator would need to be greater than the sum of the output voltage and the voltage drop across the regulator.  This in turn would require revision of the operating point of the input tube.  I wonder if a plate load could be chosen that would

1. Result in a linear operating point for 12AU7 (and variants), 12BH7 and E80CC
2. Result in a plate voltage that met the requirements for running the said regulator

If this could be done, you could swap input tubes between these three families without having to turn off the amp and flip switches: Tube rollers heaven.  Wonder how much it would cost to implement.

Does anyone have any thoughts to add.  Do you think it is possible?

[Paul Birkeland]

I wonder if it would be possible to put a voltage regulator between the anode of the input tube and the grid of the cathode follower, setting the output voltage at 75V.  This would provide a steady bias to the 6080 at the level required by the circuit. 

Voltage regulators have low impedance and require capacitors at the input and output. Therefore, you would be shorting all the signal to ground at the plate of the 12AU7, and the grid of the 6080.  On top of that, the regulator will work its butt off to null out any signal present at the input from appearing at the output. 

The plate voltage of the driver tube can move around without a whole ton of consequences, it's when it goes down into the 50's or up past about 90V that things may get out of whack.  Being obsessed with getting exactly 75V is neither particularly healthy for one's psyche nor a concomitant to getting better sound out of the Crack.

[Loon]

Told you it was a crazy idea! 

Thanks for the learning experience.