OT: Octal Fatness Update

[Doug H]

I did a minor tweak, dumped the superfluous input cap and 47k resistor and added a 33k grid resistor with the fender-style 1M on the jack routine. It's probably psycho-acoustic, but I think it may be a hair smoother when cranked now. Also added some verbiage about the MOSFET circuit on the schem and a blurb on my web page about the output tubes. I'll have the new layout up soon, kids.

http://home.cfl.rr.com/dbhammond/OF.html

Doug

[rocket]

Hi,

i am also planning to build an amp using an octal pentode (6sj7 or 6sk7)
do you have any comments about those tube?
are they compareable to th EF86?
Problems with noise or microphonie?

Ps it will be a 6v6 PP amp with a 6sl7 PI (maybe 6sn7)

[Paul Marossy]

Sometime I'm going to build this one...
I am curious about these 6SJ7s myself.
You might get more input on how these tubes compare to others over at the AX84 forum...

[Doug H]

Hi,

i am also planning to build an amp using an octal pentode (6sj7 or 6sk7)
do you have any comments about those tube?
are they compareable to th EF86?
Problems with noise or microphonie?

Ps it will be a 6v6 PP amp with a 6sl7 PI (maybe 6sn7)

I *love* the sound of this tube, very sweet and the compression is wonderful...

According to the datasheet it is not as gainy as an EF86. I have never played with an EF86 so I don't know how it compares.

No noise or microphonics. This amp is pretty quiet.

Doug

[RobB]

Doug,  the part number of that output transformer is P-T31.  Took me a while to find it.  Not too expensive at all.  (Power transformers are another story all together.)  

I don’t know much about valve amps but I understand there is a similar degree of black magic surrounding output transformers as there is with guitar pickups.  Would there be any tonal or reliability advantages in using a more expensive transformer?  

Been thinking of building your version of the Octal Fatness.  I would like to cut the chassis out one time only, I’d hate to have to have to upgrade this component later.  

What do you think?  Is it worth getting something in a higher price bracket, or would that be a waste of money?

[Doug H]

What do you think?  Is it worth getting something in a higher price bracket, or would that be a waste of money?

Rob, I really can't comment since I haven't tried it with a different output transformer.

Sorry,

Doug

[Paul Marossy]

Is that a Hammond part number? Some people seem to think the Hammond OTs sound "too hi-fi". I think they sound pretty good, myself.

Anyhow, output transformers do play a big role in how an amp will sound, but there a lot of other factors, too. I understand that West Labs makes really nice OTs, hand wound, etc., but be prepared to spend over $100 for an OT.  Heck, you could probably even go to www.hoffmanamps.com and get a replacement OT, if it had the right specs.

Hammond seems to offer quite a few different general purpose transformers. It's hard to beat them for selection and price.

[WGTP]

So Doug, are you going to model this with Jfets?