Crack;Bottlehead OTL

[Beefy]

a little remark on attenuation with the tube as well. hair trigger over the lowest volume gets it to regular volume. a tad more gets it comfortably loud.

This is good to know. I had heard some criticisms that the gain was fairly high.

My plan was to use a 50k attenuator with 50k resistors in front of it. Should give an additional 6dB attenuation for a bit more usable range.

[crappyjones123]

any suggestions for input tubes? i have a pair of hytron 5814A's, a mullard 12au7, and a pair of bugle boy 12au7s on the way. given that this amp has already sucked in close to $700 in tubes, wouldnt mind some medium priced options.

total cost so far ~$1030 with no casework done

[Nebby]

This is good to know. I had heard some criticisms that the gain was fairly high.

My plan was to use a 50k attenuator with 50k resistors in front of it. Should give an additional 6dB attenuation for a bit more usable range.

Wouldn't that bring the effective attenuation total to 100k which is the stock attenuator? I think I might swap my 100k for a 250k.

[Beefy]

Wouldn't that bring the effective attenuation total to 100k which is the stock attenuator? I think I might swap my 100k for a 250k.

No, that isn't how it works, because the attenuator is a voltage divider. You get the same attenuation range regardless of the size of the attenuator, it just depends how much voltage the amp sees and how much goes to ground.

Look at it like this...... a 100k attenuator has a maximum effective attenuation of -60dB, and you go from -60dB to 0dB in a full rotation. A 50k attenuator with a 50k resistor in front of it still has a maximum effective attenuation of -60dB, but at full rotation you can only get as high as -6dB; you can't get up to 0dB. It limits the overall range, but effectively gives you more precision per angle of rotation between -60 and -6dB.

[Nebby]

Thanks for mentioning the voltage divider, it makes a lot more sense now. I will have to give that a try myself.

[spritzer]

It's just two resistors per socket so go for it. You could also buy some of those audiophile attenuator for a fortune...

[Nebby]

Does a TKD attenuator count as audiophile grade?

[spritzer]

Yup, I heart TKD's. What I meant were those plugs you can add inline with the cables.

[Beefy]

Yup, I heart TKD's. What I meant were those plugs you can add inline with the cables.

I've got a pile of -6dB Goldenjacks that were pretty cheap. Many people poo-poo them, but they worked fantastically for me. Even so, a single 50k resistor right before a 50k pot will do almost the exact same thing, without the added cable bulk.

[Nebby]

Yup, I heart TKD's. What I meant were those plugs you can add inline with the cables.

Ah, those doodads! Bybee's are impressively expensive....though they're only 0.25ohm iirc.

[Nebby]

nevermind, they're only 0.025ohm

[spritzer]

This is more what I was thinking about, except the audiophile version.

Edit: They couldn't even spring for 1% resistors...

Edited September 16, 2010 by spritzer

[Nebby]

ahhh. The audiophile ones must have a fancy resistor stuck inside a rca adapter casing?

[spritzer]

Two of them actually, or the voltage divider doesn't work.

[Nebby]

Right

[Doc B.]

The 421A (and the 5998 ) has a higher mu than the 6080, so the hair trigger volume control thing that happens when you have a high level source or sensitive headphones will get worse. Ditto subbing a higher mu tube for the 12AU7. A lot of folks who are used to SS circuits mention hearing a little noise when the volume control is turned way up with no signal present, like to a setting way louder than one could ever really listen with music playing. That's just the nature of tubes with no feedback, a compromise necessary to get the good stuff that tubes offer. What we seek is to get a dead quiet background at what would be considered a very loud but not damaging listening level. Crack can definitely damage your ears (and your cans if they are reasonably sensitive), the source level is high, and you peg the level control.

One could try a shunt attenuator. It gives you the voltage divider for attenuation that you are looking for and allows you to set up the components so that the pot is just a variable resistor that is shunting signal to ground and you can use a high zoot resistor as the series element (that the signal passes through to get to the tube's grid).

[crappyjones123]

oh snap its doc! welcome to the boards and thanks for your input on the matter.

[dsavitsk]

The 421A (and the 5998 ) has a higher mu than the 6080, so the hair trigger volume control thing that happens when you have a high level source or sensitive headphones will get worse.

It's a cathode follower. Why would a tube with different mu matter?

[nikongod]

It's a cathode follower. Why would a tube with different mu matter?

Output impedance is lower, output voltage sags less due to the load.

[luvdunhill]

use a high zoot resistor

[dsavitsk]

Output impedance is lower, output voltage sags less due to the load.

The 75R Zout difference (1/Gm) between a 6080 and 5998 is probably swamped with all but low Z phones -- and note this difference has nothing to do with having a higher mu.

The "gain" (mu/mu+1) difference is pretty small, too compared to the high gain of the input stage. The mu of 2 6080 has a -3.7dB gain as a cathode follower, while the mu of 5.5 5998 has a -1.45dB gain.

[dsavitsk]

To put some numbers here, with the 6080, mu is 2 so gain of the cathode follower is ~2/3. Zout is ~150 ohms. With the 5998, mu is 5.5, so the cathode follower's gain is ~0.85 and Zout is ~75 ohms. Assuming that the gain stage is a grounded cathode with a CCS plate load, you should get a gain of ~20 from the 12AU7.

With a 1V input signal, the 6080's output is ~13.3V into an infinite load, and thus puts about 8.9V across a 300 ohm load. In the case of the 5998, it puts 16.9V into an infinite load, which is about 13.5V into 300 ohms. The difference is ~3.6dB.

[Doc B.]

The difference is ~3.6dB.

Thanks for crunching the numbers. It's not a huge difference, but enough to make a situation where one is experiencing a touchy volume control even a little more touchy if a 5998/421A is used. Don't get me wrong, I think they sound great and use a 5998 in my own setup.The 6080s and 6AS7s can vary a bit in actual performance so the measured numbers can vary a bit from the calcs. (5998 seems a little more consistent, but I only have a few samples)

[crappyjones123]

buncha tubes came in today. unfortunately one of the 6as7g's has a loose base (the top half isnt sealed to the base) but all other 11 tubes arrived safely (they pass the visual inspection and the rattle test). currently playing the 421A and an rca blackplate 12au7 which replaced the stock 12au7. the amp is full of surprises. MUCH cleaner on the top end. some of the bloomyness is gone. the bass is less flabby. will try swapping out the 421a for the 5998 later tonight and see how things change if they do at all.

for now, amp and i are very happy. thanks doc

[Doc B.]

Thanks for the kind words. I haven't scrounged up a 421A to try myself yet, but if it works like the 5998 (and probably better?) it should be the way to go. I happen to like Mullards (CV4003/M8136/12AU7) for the input tube and Tele ECC82 can be nice too. The Mullard adds a little more lower midrange flesh to the sound to my ear, and the Tele is a little more towards neutral. Both have a nice clean top end. If someone wants a brighter sound the rather rare Siemens nickel plate ECC82s are very clean and speedy on top.