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

[Loon]

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[serpent68]

Specifically what improvement do you guys hear when moving from 237 ohm to 470 ohm resistor for the E80CC + 5998 combo?

I'll probably go get myself the 470 ohm resistors tomorrow to try out.  Is it worthwhile to get some higher quality resistors since I'm at it? 

[JamieMcC]

I used just some ordinary 470 Ohm 0.25w Metal Film resistors £1.19 free delivery for 10 at that sort price its a throw away mod if you don't like it just stick the old ones back in. My original ones came out quiet easily just heated up one lead at a time and pulled it out with some forceps once the solder melted.

http://www.ebay.co.uk/itm/120531828406?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649

[kscwuzhere]

Specifically what improvement do you guys hear when moving from 237 ohm to 470 ohm resistor for the E80CC + 5998 combo?

I'll probably go get myself the 470 ohm resistors tomorrow to try out.  Is it worthwhile to get some higher quality resistors since I'm at it?

For my HD650s, it really tightened up the bass (after a/bing it with against the 237ohm, I found the old resistance to make the bass noticeably muddier) ,slightly improved the imagining/instrument separation and soundstage definitely widened.

[onelivewire]

Can anyone just confirm the 200ohm net resistance needed for the 12bh7? I haven't seen anybody put this on a switch yet and just wanted to make sure.

[Paul Birkeland]

Can anyone just confirm the 200ohm net resistance needed for the 12bh7? I haven't seen anybody put this on a switch yet and just wanted to make sure.

You'll have to smoke it and see...  The 12BH7 curves aren't well enough resolved to really make a good estimate of what the current should be at 1.5V of bias to get 75V on the plate. 

As R1 drops, the plate voltage will increase. I might try something around 100 Ohms first.  You can run the amp with a 12BH7 plugged in and no 6080, then adjust the resistance of R1 till you see ~75V.  If you have a few different 12BH7's, you can then swap them all in the socket to confirm that they are all in the appropriate range.

Depending on how much current you end up running, a small heatsink or a beefier transistor may be a good idea.

[kscwuzhere]

Can anyone just confirm the 200ohm net resistance needed for the 12bh7? I haven't seen anybody put this on a switch yet and just wanted to make sure.

You'll have to smoke it and see...  The 12BH7 curves aren't well enough resolved to really make a good estimate of what the current should be at 1.5V of bias to get 75V on the plate. 

As R1 drops, the plate voltage will increase. I might try something around 100 Ohms first.  You can run the amp with a 12BH7 plugged in and no 6080, then adjust the resistance of R1 till you see ~75V.  If you have a few different 12BH7's, you can then swap them all in the socket to confirm that they are all in the appropriate range.

Depending on how much current you end up running, a small heatsink or a beefier transistor may be a good idea.

How should I go about installing a small heatsink, (where on the board would i do this)? Which transistor should I replace with a beefier one to run the 12BH7A with the proper resistance?

[Paul Birkeland]

Start by determining the R1 that gives the correct plate voltage.

[onelivewire]

Alright, time to order a pack of resistors and start trying! Would testing these using some 21" alligator leads be alright just for multimeter purposes? I realize the leads are far too long for listening purposes, but would they be alright to keep the solder/unsoldering time down in testing?

[NightFlight]

I would just use a pot in the right range to dial in what you want. Then measure it when your done.

[onelivewire]

Haha well, I went ahead and threw the 470ohm resistors we had in parallel to each other to get about 160ohms with 3, and about 117 ohms with 4.
@160ohm
T1: 64.7V
T5: 69.5V

@117ohm
T1: 79V
T5: 75V

I'm about to give this a listen. I did add some ~1.5" leads in order to tack on resistors, so hopefully that doesn't make itself too obvious of a bottleneck.

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?

[Paul Birkeland]

Looks like 147 Ohms would be a sweet spot.

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.  The MJE-350 has a maximum heat rating of 150C, but will eat itself in your case if that temperature goes beyond ~130-ish.

IIRC, the temperature measurements that we have seen have been between 30 and 40C, so adding 75C to that still leaves you in safe territory.  Replacing the MJE-350's with MJE-5731A's (they face in the opposite direction) would decrease the temperature rise to ~47C, which gives a much higher safety margin (and longer life most likely).

[onelivewire]

Alright, I think I'll put some MJE-5731ag's on order, as it seems that the "a" variants aren't very available. 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?

I plan to test these numbers with a Sylvania 12bh7 that I have coming in tomorrow, and then committing on the proper resistors to get a 3-way switch running properly for the e80cc, 12bh7, and 12au7

[Hornet900]

So guys  if I wanted to try the  E80cc  tube I should "our its best" to replace my  237 ohm resistors with the 147 ohm ones for the lower voltage?  I don't intend to install a switch just yet ..I'll try the tubes out for sound 1st.
I received some E80cc tubes today but haven't tried them yet.

[onelivewire]

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.