Crack modified for 6SN7 voltage amplifier

[Natural Sound]

Hi guys,

I'm not sure if I'm the first to do this but... Back in July I asked a few questions about using a 6SN7 instead of the 12AU7. Why you ask? Because I wanted to do something a little different. This was a simple modification. Since the 6SN7 is very similar to the 12AU7 all I needed to do was punch out the miniature socket hole to accommodate another 8 pin octal like the power tube. Then it was just a matter of having 2 data sheets to refer to when making the necessary wiring changes. Piece of cake!

How does it sound? It sounds great driving my beloved Sennheiser HD600's equipped with bottleheadphone cable. For testing purposes I plugged in an old run of the mill GE 6SN7 that is used but still tests strong on my B&K 707 tube tester. If I had made any wiring mistakes I didn't want to take out any of my more exotic 6SN7's. Anyway, the resistance and voltage measurements were spot on perfect. I'm going to let things burn in for a while and then I'll start rolling through some of my stash and see if there is a magical combination in there somewhere. Actually its pretty magical already and thats stock (no speedball ordered yet.)

I do have one minor concern. The two 3K 10W wirewound resistors get pretty toasty. To a point where there is a slight burning smell. Is this normal? I do have adequate ventilation underneath and made it a point not to block the slots on the top plate with any wiring. I think it will be OK but a little reassurance from the designers or fellow bottleheads would be comforting.

All thats left is a little woodworking and finishing of the base and I'll be set.

Oh yeah, some time this winter I'm going to try a side by side comparison to my Enhanced S.E.X. amp. Stay tuned.
     

 

[JC]

That is very cool!

So, did you run it with a 12AU7 in it first?  Are there audible improvements with the 6SN7?  Did you have to make any modifications to allow for the increased heater current used by the 6SN7?

I'm sure there will be many other questions!

 

[ironbut]

Looks excellent already.
I really like the way you routed the bottom of the chassis. Classic!

Regarding the wirewounds, they do get awful hot. I imagine any residue on them from the manufacturers would burn off and have an odor for a bit. I think they should be OK since I haven't heard of anyones top plate finish being messed up by the heat. Definitely too hot to touch after it's been on for a while.
Those resistors are pretty darn tough. I had one in another project hanging by some jumpers and sitting on my bed. It burned right through the comforter!

 

[JC]

One of my first lessons in "lab" at electronics school had to do with just how hot one Watt was when it was confined to an area the size of a 2 Watt resistor.  Suffice it to say, it was plenty hot enough to raise a blister!

Having said that, I don't know the Crack circuit or what those two 10 Watt resistors do in it.  But, I assume that since the manufacturer chose 10 Watt resistors, it meant that 5 Watt ones might not dissipate enough heat to be reliable.  So, the additional expense of the higher Wattage must be justified.  Same with the additional expense incurred having the vents lasered into the chassis plate.  Must have been a good reason.

So, the question is, then: Are his resistors running at the dissipation expected, or are they running hotter for some reason?  More specifically, could they be running hotter because of his modification, or are they even involved in a part of the circuit that would be so effected by the modification?

My completely wild guess is that, if the Voltage measurements were good, then all is well.  Is there a Voltage measurement that involves those resistors?

 

[Natural Sound]

That is very cool!

So, did you run it with a 12AU7 in it first?  Are there audible improvements with the 6SN7?  Did you have to make any modifications to allow for the increased heater current used by the 6SN7?

I'm sure there will be many other questions!

I didn't run it as a 12AU7 first. I may buy or build a 6SN7 to 12AU7 adapter so I can roll some miniature tubes to compare.

If you look at the data sheets for 6SN7 and 12AU7 the heater current is the same. The 12AU7 running in parallel mode (6V) and the 6SN7 both draw 600 milliamperes. 

 

[JC]

I've never seen a 12AU7 draw more than about 300 mA in six Volt operation.  For twelve Volts, they draw  about 150 mA.  IIRC, this has always been the case, and one of the first considerations in determining the possibility of using a 6SN7 in it's place in existing equipment.

 

[Natural Sound]

I've never seen a 12AU7 draw more than about 300 mA in six Volt operation.  For twelve Volts, they draw  about 150 mA.  IIRC, this has always been the case, and one of the first considerations in determining the possibility of using a 6SN7 in it's place in existing equipment.

You're probably right, I've never actually measured the current, I only went by the data sheets. I asked Paul B. who had had a hand in designing the crack. He said, "no problem using a 6SN7", that why I went for it. Additionally, the FPIII uses the same Bottlehead PT-3 power transformer as the crack. Several fellow bottleheads have used three 6SN7's in Extended Foreplays.

 

[JC]

Well, Paul would know!  No doubt, there was sufficient additional current available.

So, was there a Voltage measurement during your check-out that involved those two 10 Watt resistors?  It would be interesting to know what they are dissipating at "idle".

 

[Paul Joppa]

The 6080 draws 2.5 amps, more than FOUR 6SN7s, by itself. But the power transformer can handle 3.5 amps of AC heating.

 

[JC]

Thanks, Paul, it is good to know that there is still plenty "left over". 

 

[Grainger49]

I didn't run it as a 12AU7 first. I may buy or build a 6SN7 to 12AU7 adapter so I can roll some miniature tubes to compare.

If you look at the data sheets for 6SN7 and 12AU7 the heater current is the same. The 12AU7 running in parallel mode (6V) and the 6SN7 both draw 600 milliamperes. 

Poster Paully has a post with a link to a seller on ebay who makes adapters in one of the Eros threads.  He got an adapter to try a different pentode in the first stage of the Eros.

He also put 7N7s in his FP III because they are much cheaper (NOS) than NOS 6SN7s.

The FP 1 and 2 didn't have the heater current for a 6SN7.  The increased heater current and dual secondaries for the B+ were just two of the experimenter upgrades Doc and PJ made going to the FP III.

 

[Kingrat]

+1 for 6sn7 in the crack. I built mine with the 12au7, listened to it overnight and the next day installed the speedball, it sounded pretty good but there was just something missing from the sound that i was getting out of another amplifier i had built. what was it missing? detail and realism, could have been from the new 12au7 so i tried the rest of the 12au7's i have lying around (i dont have any of the fancy holland made tubes) but RCA, GE, black plate Westinghouse, CBS, and a couple others, and none of them really sounded right at least to me

so i decided, it needed some mods, so i added another octal socket for a 6sn7, fired it up and never looked back, even my new production 6sn7's sound better than any of the 12au7s i had for testing, i am sold on this mod and now i have a very nice Crack

i would highly suggest anyone give it a try with a socket adapter if they are interested, especially if you dont want to spend alot of money for decent 12au7's you may be quite pleased with the results

i think it would be cool if bottlehead offered a 6sn7 option on the kits as i think the selection of quality nos 6sn7s offer a better bang for the buck

edit: i think i may try the 12au7's in it again after it breaks in, just to see if that had anything to do with it

 

[Doc B]

Before you completely dismiss the 12AU7 I'd suggest trying something like a Mullard or a Telefunken, They have some qualities that I like better than other 12AU7s and 6SN7s.

 

[Kingrat]

It wasnt my intention to badmouth the 12au7, I will definately keep my eye out for a Mullard/Telefunken/Amperex and other types from what I have.

Just sharing my experience for those who asked if someone had tried it with the 12au7 first, and it's not entirely a fair comparison putting a lower quality 12au7 against Sylvania GTA/GTB and Kenrad JAN VT231 etc, but it just comes down to the limited selection I have on hand.

Happy Holidays guys (and any gals).

 

[Natural Sound]

Well I've had several months of listening to my modified crack. I think I've found my favorite tube combination (for now). For the power tube I'm using a Chatham Electronics 6AS7G manufactured in May of 1962. The driver (voltage amplifier) is a funny looking 6SN7 RCA coin base that reminds me of a 12AU7 RCA clear-top, only bigger. I say funny looking because of the long connecting wires that travel up the tall glass bottle. Its a great sounding tube. I picked up 4 of them very cheap. To me, in my current configuration it sounds better than some of my 6SN7's that cost me a lot more. Here is a picture of the coin base next to a standard base tube.

Note: this is not my photo. My camera is on the fritz. This is from tubeworld's site.

 

[levlhed]

That sounds like it sounds cool.  Looks neat-o too!
Maybe I'll try and get one of those adapters to try out a 6NS7 and see how it fairs against my Tele smooth plate...

 

[levlhed]

bah...gotta wait for the slow boat from China for the adapter!  Are there any reasonable USA sources for an appropriate adapter to stick a 6SN7 in the 12AU7 socket?

Also, since I did not build my own Crack (I actually own the factory prototype!) I'm not too familiar with the belly of the beast.  I know I saw a recommendation to install the octal socket before final assembly/Speedball/etc, but how difficult would it be to retro fit?

Thank you!

 

[Grainger49]

You can cobble one together till the slow boat arrives.  Attach wires to the octal tube socket and jam them into the existing socket.  It lacks any sophistication or grace but amazingly works.

It has been done before.  Just keep small children and pets away from the voltages on the wires. 

 

[levlhed]

yes, I suppose there's no reason that wouldn't work as a temp solution.  Maybe I'll try it...though I have to get an octal socket then hah
Is there a guide handy regarding the pin out/line up?  I'm sure you don't literally mean jam it in any old way

 

[Paul Joppa]

Tube data sheets are available from several sources on the web. I think the 1973 GE "Essential Characteristics" handbook is still available as a reprint, from Antique Electronics for instance - nice to have around.

I just measured the pins on a 9-pin tube. They are 1.0mm (40 mils) diameter, and AWG 18 gauge wire is very close to that size.