New 6L6 amp project

Hi All, Well, thanks for Redboy for a Conn amp and Kegger for a good schematic, I am going to start building a 6L6 amp. You can see in the pic of the top plate I am using, there is a meter up top. I'd like to replace that with a meter for reading bias and a switch to switch over from one side to the other. Any ideas there? Also, on the pic of the top plate, it says 6V6 but I am using 6L6. I will be punching holes where all the knobs are now and mounting tube bases in the holes as indicated on the pic. The Conn amp has a boat load of good parts so I don't expect to need much more than what is there with the exception of another output tranny. I may end up upgrading to the Edcors or some other. That plate will go on a bigger base with all the iron off to the left. The base will be a piece of art so that will take a couple weeks, but I am really looking forward to this project to keep my mind off the winter. Cheers.

Damn, that's about as simple as it gets, I like it.:scratch2:

I wonder how that would do with a 6SL7 in place of the AT7:scratch2:

Any chance of seeing the complete circuit (with power supply)?

I'd also like to see the power supply. Seperate filament supply would be mighty nice here.

Does the AT7 have less gain than a 6SL7? :scratch2:

Please be patient with my limited knowledge. The question I am asking: the arrow of the bias is connected to what or where?

Looks good Mr. Nagle
Hammond 1650P (or PP for the potted version) would also work well in that circuit IMO (about $100 each new) and you'd be good up to 60wpc
But ~300V plate seems a little low to me, is that all that Conn power transformer can muster? Or are you planning on using vintage 5881 or 6L6 metals and not newer 6L6GCs? (If staying to low voltage (in Class A?) and expecting no more than 15-20wpc you could likely get away with Hammond 1650H actually - a bit less expensive)
Whatever class and OPTs you decde on, it will be interesting to see the 1K/10K square wave performance as there's zero negative feedback on this circuit.

Plus, you may want to add a DPDT triode > ultra-linear switch, just for playing around. It doesnt take much, but you would need to have adjustable bias.

Reggaeaut, those arrows would probably become the wiper of a 10K pot strung from the negative bias generator that's tapped off the power transformer. For that circuit at that plate voltage it probably wouldnt need to be much more than -20V to -25 max, then adjusted down (less negative) to the desired setting by the pot. If you upped the plate voltage to 400V though, you would likely need to create at least -36V from your negative bias, so I guess a lot depends on your PTX.

WopOnTour

PS> That's quite a panel for your top plate! But this is a power amp correct? So you wont need/want a volume pot. Besides, I think you're gonna need a lot real estate for mounting the Conn PTX, choke, and whatever OPTs you go with. You may even need to put all the 6L6s to one side

One of the things I have to do is figure out how much that power transformer is putting out. I assume that it puts out more than 312v. We will see, then we can adjust the schematic from there. Don't have a schematic for the power supply yet. I am NOT knowlable about the details of a curcuit, that is all Kegger. I am a builder and need A LOT OF HELP when it comes to the details. The only reason I thought about the Edcor OTX is because they are only 70 bucks vs. 100.

I am going to the woodworkers shop today to pick up scraps on any exotic woods I can find, then off to West Marine to pick up some West System epoxy. What I have planned for the base will be kind of trippy and should light up like a Christmas tree. Cheers.

I will be sitting in; I just hope the circuit does not get too complex.

reggaenaut said:

I will be sitting in; I just hope the circuit does not get too complex.

Click to expand...

Well, one good thing about Kegger being involved, is he usually always takes the time to explain his circuits in a way that even "slingers" like me can understand.:thmbsp:

Well I'm not trying to trounce Kegger's schematic in any way shape or form, but here is a couple of suggestions for the circuit. I add this only because it is almost identical to the 6L6GC circuit from the H/K A250/A260 integrated amps that I have altered in a near identical fashion (adjustable bias, triode switching) except the H/K OPTs are not UL.(and the phase inverter is a 7247/12DW7)

The ? resistor can be determined to give your 10K bias pots a nice adjustability range and mitigate the possibility of accidentally dropping the -bias voltage too much when adjusting (ie. turning them too far or all-the-way the WRONG direction- multiturn pots could also reduce this risk) and over-cooking the plates.I would guess ~500 to 5K ohm so you might start with something in between or just use a test pot to determine the best resistance. You may also need to add a small cap (50-200pf?) on the wiper of each bias pot to stabilize.

For the triode>UL switch you use a PAIR of DPDT switches (I've only shown channel A with a single DPDT switch in my quick schematic below, but of course you need to do the other channel as well) It's best to be using 250VAC rated switches but I've used 125VAC without issue.Then just wire the 100 ohm resistor off of each G2 screen (pin 4) to the center pair pins on the switch. Then connect a wire from the plate (pin 3) to one side, and the UL tap from the OPT to the other. When you switch make sure your power is OFF, swing you bias pots out all the way to 0K (max neg volts), flip both switches, power ON, re-bias and enjoy! On non ultra-linear amps (like the H/K integrateds) pin 1 goes to the 1st dropped voltage at the PSU (or whatever WAS on pin 4) and you're switching between triode and pentode modes of course.

For your meter you can send the 4 test point circuits to a 5-pole rotary dial switch (one dead/off on one end or in middle of rotary travel) that's connected to your voltmeter. On at least one of those H/K amps I used a rotary switch but just wired it to a single probe test point that I mounted to the top-side of the chassis.

See what Kegger thinks
WopOnTour

PS> I am envious of your woodworking skills. Your chassis will look amazing I'm sure. I cant even pound a straight nail!

That's a nice looking circuit too. I've been meaning to put a switch like that in my ST35. What changes (besides obvious pinout and socket) would you make to run the 6SL7 instead of the AT&?:scratch2:

http://www.nj7p.org/Tube4.php?tube=6SL7gt

and the AT7 for comparison.

http://www.nj7p.org/Tube4.php?tube=12at7

Thanks Wop, that switching with be simple to wire up. Now I just need to find the right meter. Cheers.

First off I didn't realize there was any action on this thread, somehow missed it..

Second the original Circuit I drew up came from the mods to the ASL wave 20's I did
and some of the choices made were based on what the amp had and available room.
http://www.audiokarma.org/forums/showthread.php?t=194959

Dennis has those amps now in service at his place doing bass duty, they do perform
rather well if I say so myself with I doubt the greatest iron, presently they are using
6829 Black Plate Ratheons as the driver/splitter tube in each mono amp.

When Dennis was to build his we were going to tweak the circuit and PS accordingly,
adding in an Triode/UL switch if the outputs he ends up with have UL taps otherwise
triode only as Dennis doesn't need more then 12-25wpc for his setup.

I was keeping the voltage low at the time as that's what the ASL was capable of, but
yes I also felt leaving it somewhat low would make the use of old (5881, 6l6G, 6BG6G)
tubes optional and could be cool as well.

I feel the circuit is very simple and performs well but also makes an easy first time DIY
build for Dennis that should yeild quite good performance with little to no tweaking.

---------------

Alright with that out of the way..

I have no problem someone making suggestions or alterations to the basic schematic.
It was kept simple and loop feedback free so anyone could build it an have work with
no issues or real test equipment for any specific output transformer being used.

(Thanks for the Triode/UL switch layout WOP, now I don't have to do it) Appreciated..:thmbsp:

Pentodes running in UL with a driver stage that doesn't have excess drive and has the
relatively low plate resistance of say a 12AT7 will not "need" loop feedback to perform
well, Triode definatly will not need it, if running Pentode you'll probably need it or some
other form of output tube based feedback design.
(this not a knock on anyone feeling the need or wanting to use loop feedback it's just
what I've found works easily with these type circuits and provides predictable results)

I recently updated and posted on Jay's (Urizen) thread what I would basically do when
not being tied to what the ASL amps had originally. (Posted that a few days ago here)
http://www.audiokarma.org/forums/showpost.php?p=2450781&postcount=76

At the moment there's no power supply here until we see what Dennis has in the power
tranny being used from the organ amp.

I'm attaching a supply that should work using the Triode Electronics ST-70 replacement
power tranny with solid state rectification for ease of build if someone wanted to use it.
(this would be for a stereo amp)

Also what I believe should work for a 6SL7 as the driver, someone would need to build it
and do some quick measurements then possibly tweaking of the drivers plate and or it's
cathode resistors to dial it in, but I bet it's pretty darn close..
(I try to keep the cathode of phase splitter 99v or less and the drivers plate to as close
as I can to 100v without making the splitter higher then the 99v I just said, SO the driver
will be at somewhere around 97.5 when the circuit is tweaked for the highest gain tubes)

The original circuit for the 12AT7 is set as that being the (Highest Gain) tube, BUT the
6829, 5965, E180CC can all be used with no adjustments needed. (others as well)

The heater/cathode voltage is 100v on quite a few tubes and in this case the tubes are
under that BUT if a heater voltage reference is provided would certainly be a good thing.

Hey Keg, that new schem looks great. When it says 6.6 or up on the OTs, what would be the advantage of say a 10K? I am going to go for the Habours and can order any size I need.

Also, you know me, I am into the looks of things and I've been reading a lot about the 807 tube as a drop in replacement. Is it really a drop in replacement? I have always like the looks of the top cap.

807's should work fine..(I believe Terry and Doug went 10K and 350v on his build)

Also a larger Impedence output I actually like better, not much of a difference in power
going to say 8-10K, but less distortion and even lessar need of feedback to keep things
linear as well, all good in my book!

OK, thanks to Bauhausler, I have all the wood I need to build the base for my amp. I think we have covered all the exotic wood available. The only thing that is not in the picture is the progressive sized dowl rods, five colors of food coloring, and 1/2 inch schedule 80 PVC tubing. Can't wait to get started. Cheers.

Well, I guess I am committed to the 807.....I just bought four of them from the Ukraine. I am also in a group buy for 10k OTs from Hayboer. I think I will start the base this weekend. Where is a good place for sockets for these tubes? Cheers.

I've looked at those a few times, the price sure is right.

Let me see about sockets, hey you can't get a hold of the sellar before the
tubes ship can you? I ask because you have no spares in case any problems.
I'd like you to have at least 6 of them total.