Old Timey 6K6 build

[bmccowan]

I just finished, well except no cabinet yet, a simple PP 6K6 all octal build. Partially inspired by Leon C's Smoothie, but also my own vintage Gibson and Valco amps, I blended a Gibson type pentode preamp with a Valco paraphase PI (it was Pi day) but also going for lower volume with 6K6s running at <280v. I also dialed in some extra Valco mojo by using ceramic disk caps for anything under .02 and an undersized OT. The amp sounds great into a Greenback 12, but I will probably pair it with a Weber 10. I incorporated a James type tone stack with a raw control. But playing in raw mode, the treble is a bit much, and I'd like to tone it down a bit. Should I just change that 390pF cap? or other? BTW, I am amazed as to how loud those 6K6s are compared with 6V6s. Are there wimpier octal power tubes out there besides going all the way down to 6SN7s? I looked at 6W6s, but that did not seem like a logical choice. I know there are multiple 9 pin choices but my arthritic fingers like the octals.

[uki]

the treble is a bit much, and I'd like to tone it down a bit. Should I just change that 390pF cap? or other?

I'm no expert but if those 2 caps control the tone and its too treble, maybe experiment a bit with both, what makes the tone go towards lower frequency? maybe less pF like 200pF or maybe 100pF 

Have some pics ?!   

[tubeswell]

Are there wimpier octal power tubes out there besides going all the way down to 6SN7s? I looked at 6W6s, but that did not seem like a logical choice. I know there are multiple 9 pin choices but my arthritic fingers like the octals.

Pentode pre with paraphase inverter and cathode bias output tubes is a great choice for low-powered guitar amp.


Besides the 6K6s, there are 6F6 (6F6Gs are better than the metal canned variety) if you can find them. 11W tubes but with about 1/2 to 2/3rds of the gm of 6V6s - for about the same load resistance, which translates to a cleaner signal all other things being equal, which isn't necessary a bad thing with paraphase inverters YMMV. (I prefer the sounds of a 6F6 to a 6K6 as an output tube). 6F6 screens are rated for 3.75W (compared to 2.2W in a 6V6). Only thing is 6F6 heaters are 700mA each  - so check your PT heater winding.

[sluckey]

I think the caps on the bass pot are wrong. Try the values in this Ampeg tonestack. Nothing too trebly about this TS...

     http://sluckeyamps.com/phoenix/phoenix.pdf

[bmccowan]

Uki - thanks and I'll take and post a couple pics of the build.
Tubeswell - I think I have a few 6F6s and never used them. I always thought (apparently wrongly) that they were simply earlier versions of 6V6s.
Steve - I'll try those changes. I used no science or math in selecting my cap and resistor values - all guesses based on other circuits.

[AmberB]

Since you're already using octal sockets for a push pull pair, it might be interesting to use 2 of the 6SN7s with each tube's plates, grids, and cathodes in parallel, one tube on each side of the PP OT.  I have no idea what it would sound like, but you might get 3-4 watts out of it.

[bmccowan]

Steve - I tried that change. Like it and will keep it. Thanks.
Amber - thanks for the suggestion. At some point I will try 6SN7s for power tubes, but I will do so with a lower spec power transformer. How did you make out with your 6SN7 project? I recall that you were trying to get the voltage down.
 

[sluckey]

Steve - I tried that change. Like it and will keep it. Thanks.

That Ampeg tonestack is one of my favorites. I bought a Gemini II brand new back in '67. Wish I still had it.

Do the new values get along with your raw pot?

[bmccowan]

Yes - they do, I am able to dial in a good mix for the volume I am seeking, which is typically quite low. Before I added the raw pot I played the amp some to decide what I wanted to add to the extra control hole I had drilled. I started with a blank undrilled chassis. My bass to ground resistor was a 10k. The pot I used had about a range of about 10-240k resistance so I changed the bass to ground resister to 1.2K. I'm kind of new to adding a raw control to a tone stack but I have a few old amps with only a volume control - Gibson GA-5 and BR-6f and a couple of low wattage Valcos and have always liked how they sound. After all, there are tone controls on my guitars.

[bmccowan]

On that same note (no pun intended) one of my guitars has 2 p90s. I find that mixing the 2 pickups and modulating the vol and tone controls on the pickups provides quite a range of nice tones. Oddly I have never liked the tone of mixing 2 of the pickups on a strat.

[PRR]

... Special K 6K6 Baxandall v2.pdf ...

That's not any sort of Baxandall. Check where the output comes out.

And as drawn, there is no gridleak for the next tube?

[sluckey]

And as drawn, there is no gridleak for the next tube?

I see one...

[bmccowan]

I think PRR is right about the output. I drew it coming out of the wrong leg of the treble pot. I don't think I have ever drawn a schematic without at least one mistake! Fortunately I usually get lucky and wire it right.
I'll fix when I adjust those cap values.

[tubeswell]

FWIW, if you're trying some iterations on the tone stack values, here is one that I like and have built into a couple of amps, with a 12AX7 triode at each 'end'. It has a good flat response with treble and bass pots both at 50% rotation, but a slight mid-scoop at full rotation and a mid hump at full cut.

[shooter]

I see one...

that's the way to Lowes from home! 

[bmccowan]

I updated and corrected the schematic. I understand that James may be more accurate than Baxandall for the TS, but I like saying "Baxandall."
Tubeswell - thanks, that project looks good. I may try that on my next build that uses a dual triode preamp.

[thetragichero]

yes james is correct as baxandall has to be in a negative feedback loop, but of course just like leo's swapping of 'tremolo' and 'vibrato' some mistakes become part of our lexicon (strats have a vibrato, many fender amps have tremolo)

[bmccowan]

And when I read your posts, I have always read your handle as the Tragi Chero rather than the Tragic Hero. I just realized I had it wrong.