Just purchased a 68 Fender Bassman

[flemingmras]

And of course I have since changed it to this.

Jon

[Bluesbishop]

Thanks guys you have been very, very, helpful I'm gonna rewire my greenback loaded 4x12 toi 4 Ohms tonight and crank it!!!

I still think I'm gonna purchase two Avatar 8 Ohm cabs...( I'm 40 years old...Too old for BIG cabinets!) I will give you feedback when I gert them.

How do you bias this amp???...Don't se any bias adjustments, as I would really like to run it with KT-66..( I know I'm getting Old on the KT-66...but they are my Fav's!!

Ken

[Flames1950]

Well....the AB165 has that funky bias balance control....... (the pot to the left of the board, with the tube side of the amp facing you -- one tube's draw is kind of "set" and you balance the other tube to it for minimum hum -- assuming tight, original tube specs of course!!)
I have some 6L6WGB's that I've been meaning to try in my '66, but I keep putting it off 'cause I don't wanna have to strangle that bias balance circuit!!
So you're dead set on those KT66's.....hopefully the bias supply range is correct, considering the similarity between KT66's and 6L6's (an EL34 bias circuit would usually need modification for KT66's....) KT66 heater currrent looks to be more than a 6L6, almost an amp more filament draw for two tubes. I don't know what spec Fender's trannies were rated for; some guys seem to think you shouldn't run EL34's off them for the added filament load, while other guys do it and don't seem to have a problem.
If that bias balance circuit gives you fits trying the 66's, I'd just convert it to the earlier, more "acceptable" bias circuit per the AA864 Bassman.

[Flames1950]

I thought I'd mention something from hard-learned experience too.....
If you happen to get a wild hair and try to wire the entire power amp like the earlier AA864, Fender engineers had a nasty little surprise in store for you......when they designed the AB165 output section's feedback circuit, they changed the spot the feedback came back into the phase inverter at, and thus the output tranny's secondary wires (going to the speaker jacks) are REVERSED. If you rewire to AA864 and don't swap these, the sound that hits you when you hit the Standby switch is, well, UGLY to say the least.
But if you leave the output section as is, you're fine of course, and you can change to the earlier bias circuit without changing anything else in the output section.

[Bluesbishop]

Flames, The amp is loaded with pretty decent tubes, Ruby 6L6GCSTR in the power and EH in the Preamp and phase inverter. I'm not "dead set" on the Kt-66....I'll just have to wait and pray for George to get them JTM-45 Kits done soon!


I tried it out tonight with both the Les Paul, and my SRV Strat.. WOW... sounds incredible! Nice break up, and when slammed with my TS-9, I'm in Tone heaven! I'm gonna play it for a bit before I decide to do anything, but I really appreciate all the input you guys have been giving me!! You are all TONE GODS!!!!


Thanks

Ken

[Flames1950]

I really like the crunch these beasts put out, thicker but less "intense" than EL34 Marshalls. They're a great blues amp for sure.
Does yours "ghost note" too badly? Both of my Bassmans got some extra power supply filtering, they both ghosted so bad it sounded like I was running a ring modulator! If it's a problem on yours I can put up a pic of where I added my 100uF 450V Sprague inside the chassis.

[Bluesbishop]

Yup...A little bit of Ghost, so details would be nice I "colored" the tone a bit tonight though... was jamming some trower, and using my Nobels ODR-1, and Sweet sounds MOjo Vibe....I love univibes!!!

Ken

[Flames1950]

Yup......3:30 in the AM and I'm wide awake. Don't know what the problem is, I blame the horribly dissonant piano crap the radio had on, sounded like a cat jumping on the keyboard, after twenty minutes (that I was awake for) of it I had to get up. Must be time to pull an amp chassis.....
Here's the filter I add inside the chassis to my Bassman heads, Bluesbishop. I'm using a 100uF 450V Sprague Atom right now because it fits a little cleaner in there; originally I had two 40uF Spragues for 80uF total (you can, of course, just buy 80uF Spragues instead of two 40uF's.) This is the blackface '66 AB165 (which is so clean I won't even replace those "chocolate turd" caps on the right hand side!!)



Seems like every blackface/silverface Fender head needs some extra filtering when overdriven, at least with a Gibson's input; I haven't added any extra filtering in my Bandmaster, but it's usually a Strat-only amp for my tastes and I don't get it overdriven enough to have ghosting problems as bad. FWIW I originally tried just increasing the existing filter caps on my '68 -- instead of two 70uF's, three 20uF's and one 16uF I had two 100's and four 40's -- the cap cover wouldn't even fit over the 100's which wasn't very acceptable to me (don't wanna reach in there for something and forget the uncovered caps -- OUCH!!) and I still wasn't getting the ghost notes cleaned up. So I went back to the original values under the cover and added this cap, which is working very well, and surprisingly didn't "tighten" the sound up too badly, which I was afraid it might. Watch your plate voltage, I'm just barely exceeding the cap's rating on my Bassman heads (458VDC on the '66, 455 on the '68, on a 450V cap.)
As long as I was awake and had the chassis out, I popped in the JAN 6L6WGB's I had waiting for this head and gave the bias balance control a shot. If you look at the schematic on the AB165, you'll note that one tube gets most of its bias current before the adjustment pot; it has a tap to the other side of the bias pot that will only mildly affect its current draw, for the most part it draws what it draws. The other tube is completely adjusted by the bias balance, and you just have to get them as close together as you can without the amp humming (some guys claim that if you get the tubes perfectly matched with this circuit, it hums. ) Did my figures for the plate voltage this thing was putting out, and came up with a maximum target current draw of 39-point-something milliamps. And the bias balance system ran things right up to that point, I couldn't do anything with the first tube with this system, and it was drawing 39.7mA -- so I adjusted as best I could for the other tube (around 39mA, can't remember now, it's past 5:00 as I write) and plugged it into a cab to see if it was a hummer, but didn't hear any hum so I buttoned her up. Usually I'd only run around 34mA on a Fender head; I think that's where I set my AA864-wired '68 at. So this system is a little toastier than I would prefer.
No I didn't get to play it with a guitar, my neighbors are pretty tolerant of my loud amp antics but that would have been pushing my luck.
I guess you can expect these to run the tubes nice and hot. Can't wait to hear it with all-NOS tubes, I'm excited.

[flemingmras]

If you look at the schematic on the AB165, you'll note that one tube gets most of its bias current before the adjustment pot; it has a tap to the other side of the bias pot that will only mildly affect its current draw, for the most part it draws what it draws.

Gotta love those Silverface Fender bias circuits!

BTW Flames, whoever told you that load of crap about "If you bias them perfectly match they'll hum" is full of shit. Actually, since both tubes are running out of phase, the hum will get cancelled out so the closer you bias them the less hum you should have! I hate techs that mislead people.

BTW Flames, send me you phone number dude! My email is flemingmras@comcast.net. Would love to bullshit with ya about amps and shit!

Jon

[Flames1950]

I can't remember if it was someone off PlexiPalace, or the Fender Discussion Forums that came up with that "humming when matched" story. I got my 6L6's pretty damn close and didn't hear any hum problems, I'd have to suspect someone had other problems causing the hum.
And yes, you've gotta love CBS engineering.

[Flames1950]

I guess you can expect these to run the tubes nice and hot. Can't wait to hear it with all-NOS tubes, I'm excited.

Oh shit, I'm even more excited since I got to PLAY it with the 6L6WGB's today!! It chimes, it purrs, it howls, SPANK ME BABY.

SERIOUSLY good blues amp!!

[Ian]

I was writing about the 4 12 cab rewiring. What speaker ohm is it. I have my cab series/parallel and at 16 ohms due to the 16 ohm speakers. Is that the ohm rating you were referring to and do you have a wiring diagram for it. I know this may be obvious but I would like to make sure. And by the way running an amp with a lower ohm rating into a higher rated cab is not harmful to the amp or speakers. It just scrubbs off volume. Thanks

[Bluesbishop]

Ian,

I opted not to rewire my 16ohm cabinet, and purchased a Avatar G12H special with one G12H30 and One vintage 30. I love the sound of that cabinet!! With the Bassman it kicks all major forms of ass.

If you go to the first page of this thread, Jon ( flemingmras) tells us how to rewire a 4x12 cabinet loaded with 16ohm speakers into a 4 ohm cabinet. To quote Jon, "you can make it a 4 ohm cab by wiring all the speaker terminals in parallel with each other(i.e. all the positive terminals go togehter as do all the negatives). This will drop your load impedance down to 4 ohms. "

Ken

[Ian]

I saw his post but just read the thing about all in parallel. I was having a little trouble visualizing what it would look like. I orignially thought to y of the jack but I forgot about the negatives until about three seconds later and I just couldn't figure it out. Thanks a lot for the info.