6SJ7 Pentode 18 watt Lite...Formerly the ABSOLUTE Minimum

StarGeezers · Post by **StarGeezers** » Tue 10/21/08 5:57 pm

KY, gotta' get one of those shorting jacks...scratchiness on the guitar pot indicates DC somewhere...maybe time for ZPs cap on the input... It does have really great tones ,and is VRERY LOUD!!!! slight noise while not playing

kyoungsteadt · Post by **kyoungsteadt** » Tue 10/21/08 9:15 pm

Put a .1uF cap on the input and let'er rip...

GUITARmole · Post by **GUITARmole** » Wed 10/22/08 12:14 am

StarGeezers wrote: B+ 356 from SS recto
B+1 338 to OT
B+2 322 to screens
B+3 281 to PI
B+4 268 to 6SJ7

StarGeezers, are those numbers what you're getting from a 215-0-215 PT like in the jpg you posted? I wanted to build a similar circuit but w/ EL84s and using the MPS 275-0-275 PT...now I'm concerned my B+ will be too high w/ the SS rectifier ??

StarGeezers · Post by **StarGeezers** » Wed 10/22/08 7:57 am

Mole , using an Edcor XPWR106 PT...with SS recto gives me 364V at B+..
Cathodes of the 6V6s have settled out but still a difference of some 20 V..

The amp sounds great and really LOUD!!!!!! The hum was caused mostly by me playing/testing with my guitar right in front of the amp ...Duh!!!

Waaaaaaaay more gain than I ever expected ...on 1 its clean and loud ...after that just Insane ...The amp is on the bright side,which will work great with my 4/12 Celestion cab... Couldn't be Happier...

KY , I'll add the input cap and switching jack ...Thanks !!!
Big , could you explain that V divider a bit more...Please???

A BIG THANKS to all for the help and your patience!!!!!!!!!!!!!!!!!!!!!!!!!!!

bigbeck · Post by **bigbeck** » Wed 10/22/08 8:33 am

StarGeezers wrote: Big , could you explain that V divider a bit more...Please???

Just put a 220K after the coupling cap,in series with it and another 220K to ground. A perfect example would be R6 and R7 in the VJ. This should cut the brightness a little too.

Post by **zaphod_phil** » Wed 10/22/08 8:59 am

StarGeezers wrote:Mole , using an Edcor XPWR106 PT...with SS recto gives me 364V at B+..

For EL84s I'd feel inclined to put in something like a 120 ohm 5W resistor after the rectifier, to drop some volts.

StarGeezers wrote:Cathodes of the 6V6s have settled out but still a difference of some 20 V.

Have you split the cathodes? If not, there must be a flaky joint somewhere in there.

StarGeezers wrote:The amp sounds great and really LOUD!!!!!!

That's an 18W for you.

StarGeezers wrote:Waaaaaaaay more gain than I ever expected ...on 1 its clean and loud ...after that just Insane ...The amp is on the bright side....

Sounds like the EF86 - or were you using a 6SJ7...? Anyway, those pentodes have a lot of gain. If you lower the B+ voltage some of the brightness may go away.

andrekp · Post by **andrekp** » Wed 10/22/08 9:00 am

SG,

It's really hard to see what you have going one with that beaver dam of components on each tube socket.

Do you still have different cathode voltages on the 6V6's? Did you use separate components for each tube, or do the two cathods run through a common resistor and bypass cap? If separate, I'd start looking for a wrong value, either on the Cathode or plate resistors. (if common, then just the plates).

Gain-wise, a "normal" 18watt is not all that gainy in a modern sense. Don't know how much your choice of tubes causes so much gain, but I suspect it should still act normally. If you are hitting distortion just about 1 on the dial, chances are something is wrong. Either you need to dump some more signal in the preamp, or maybe the wild imbalance in the PP pair is causing it.

What I would do, is pull all the tubes are start measuring resistance and continuity between each and every point on the amp - even opposite ends of the same wire. There's just about no way NOT to find your problem that way. Use the schematic and "layout." Don't eyeball it, and don't think that because it looks like you expect, that it is. Use your DMM between every point on the amp. (you may need to disconnect your trannies for certain points).

Andre

bigbeck · Post by **bigbeck** » Wed 10/22/08 9:35 am

zaphod_phil wrote: Have you split the cathodes? If not, there must be a flaky joint somewhere in there.

I think Geezer made a typo here. In another post he stated that his cathodes were 21.5V. His big voltage difference was between the two 6v6 control grids,pin 5.

StarGeezers · Post by **StarGeezers** » Wed 10/22/08 9:52 am

Big, Thanks for the correction...you know how us Geezers are...

What I meant to say was the 6V6 input control grid Vs were different...single components in the cathode produced identical Vs there... sorry, my losin' it!!
Now might have that been due to the different Rs in the PI???
Big , Thanks on the Vdiv..Got it !!! need as Andre says to dump some gain , just a little ...hahahahaha
Andre , I'm quite used to working on these Rats Nests ..hahaha But if I had it to do again I think I'd put the PI on a little board just for convenience..The 12AX was a bit "crowded"...

I think using insulation on the parts leads also makes it look more complicated , the usual bare leads hardly being seen...
ZP, got a 100R after the diodes for the 364V B+1 to the OT...Using National 5693, the RED one ..

bigbeck · Post by **bigbeck** » Wed 10/22/08 10:03 am

Did you try a lower gain tube in the PI yet? Is the PI cathode resistor 820R?

GUITARmole · Post by **GUITARmole** » Wed 10/22/08 11:08 am

zaphod_phil wrote: For EL84s I'd feel inclined to put in something like a 120 ohm 5W resistor after the rectifier, to drop some volts.

You mean 220? The StarGeezers schematic has a 200 ohm resistor there now...

bigbeck · Post by **bigbeck** » Wed 10/22/08 11:37 am

Geezer currently has a 100R in that position,which is not even needed as he has 6v6's and not EL84's. The 6v6's can take a lot more voltage than the el 84's.

The schematic should be updated to reflect his build,or there will be constant confusion

StarGeezers · Post by **StarGeezers** » Wed 10/22/08 11:50 am

Big, not yet ... got a 12AU7 (old) I can plug that in and see the difference...got a 5751 too for somewhere in the middle ... we'll try them all...
PI Cathode R is 820R...Gray red brown ...confirmed by visual inspection...
I think I'm getting all the extra gain in the PI,cause' the 6SJ7(5693) doesn't have anything like that in the "other" amps..you know ..
Sorry, I'll make the changes to the schematic, didn't know anybody else was interested...

StarGeezers · Post by **StarGeezers** » Wed 10/22/08 12:19 pm

Ok, here's the latest with what I have in it Now!!!

If anybody has a layout on a small board for the PI, this would be a real EASY amp to build...the rest P2P...not so expensive either...

StarGeezers · Post by **StarGeezers** » Wed 10/22/08 12:38 pm

Tried the 12AU7...staticy' (probably bad tube) ...the 5751 sounds really nice and not so crazy...I can open it up to 9:00 o'clock before I get dirty.. Sounds GREAT!!! Beautiful Tones!!!

and LOUD!!! Tweed TC is just perfect!!!
Seems like all the extra gain's coming from the PI...eh?

bigbeck · Post by **bigbeck** » Wed 10/22/08 12:41 pm

You might not notice much of a difference with the 5751. It's not that far down on the gain chain

Try 12at7,12ay7 and 12au7. With at being highest and au being lowest.

StarGeezers · Post by **StarGeezers** » Wed 10/22/08 1:14 pm

Big, I'd try em' if I had em...I'll have to dig around in my friends tube collection (antique radio collector) He's got boxes of tubes...
The 5751 made a noticeable difference and sounds good too...
Think restoring the tail R back to 56K would tame that wild PI??? 22K now as ZP suggested...

kyoungsteadt · Post by **kyoungsteadt** » Wed 10/22/08 1:22 pm

SG,

You have to understand that ZP wants your amp to sound wild...

StarGeezers · Post by **StarGeezers** » Wed 10/22/08 1:26 pm

It is Wild and Hairy!!!!!

...Have to call it Bigfoot...hahahahahaaha

bigbeck · Post by **bigbeck** » Wed 10/22/08 2:19 pm

StarGeezers wrote: Think restoring the tail R back to 56K would tame that wild PI??? 22K now as ZP suggested...

I think I remember Phil saying reduce the tail to increase drive to the 6v6's. Try it. you can't hurt anything.

Edit: a 12at7 might be perfect for you,but might as well try raising the tail resistor to see what happens.

Clean up to 9:00 sounds good as long as it's loud enough at that setting.

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6SJ7 Pentode 18 watt Lite...Formerly the ABSOLUTE Minimum