6au6 pentodes in push pull clean low watt practice amp. uses 1363 smps and 9v

[zambo]

It looks like I didnt correct the cathode R on the schematic. I doubled the 47R so its really 94R and its also bypassed with a 220uf 50v cap. The pi tap should also be between the 1k5 and 47k resistors as stated by amptramp. Thanks! OT is a hammond 125c with pins 5 and 6 on the secondary for 22k at 4 ohms so a 8 ohm speaker reflects 44k. I did this based on someone elses research and it seems to work well. All the heaters are running on 6v dc, even the 12au7. I used a 1spot 9v adaptor cause it can pump out 1.7 amps. I just ran 6v to pin 4 an 5 and grounded pin 9 on the 12au7. I didnt like overdriving the PI so much on this design as its a concertina type and can get ugly pretty fast. I am working up a mini amp using  this PA and a long tail pair PI with a simple but effective high gain preamp. I have been doin trainwrecks so much that I want to do something a little different but with good solid face melting distortion. I like this one because its clean and if you put a decent tube screamer or whatever pedal in front of it it sounds really good at low volume.    Keep the corrections coming and sorry for the mistakes. Glad you all like it though. I owe my diy career to the members of this sight so thanks again for all the help! I will post another one sooner rather than later I hope. Cheers, Greg

[iccaros]

Ok,updated drawing, check for correctness then we can make the layout cleaner..

just refresh page one...

[iccaros]

Yeah, but I've got a stash of 12V - 15V wall warts for 12.6 volt regulated output...besides, I like the lower heater current (even though I realize power is the same).  It just seems easier to find beefy 12V wall-warts compared to 9V warts with enough power output.

you can also run that 12V with the 6au6 and the 12au7 in series with each other, they have the same draw so you would use the same current as a single 12au7 at 6V. This would drop 6v across each tube

each are 21ohms so 21 * 2 = 42 ohms in series. drop 12V  (12/42 = .285) over this and you get  .285 amps

so to wire you would go

6AU6 pin  3 ---> + 12
6AUG pin 4  ---->  pin 4 12AU7
12AU7 pin 5 ----> ground

[Zipslack]

Any measurement on your voltage out of the SMPS or the B voltages?  I'm guessing 250, but I'd rather be sure.  My nixie supply puts out 250-300V under load, but is most comfortable at 250-260.

[iccaros]

the 6AU6 has a max voltage of 300 volts on plates.. With inductance from the transformer as close to 200 as you can get, so I would go 250  in your case.. Should not really change the sound, just a little louder (little)

Zambo listed 200V on the video..

[zambo]

yeah its putting out 200v at the first node. I cant measure the others as the amp is gone but I have another smps I can make something with. I only used the 100R dropping resistors because I was truing to keep as much voltage on the preamp as possible. When I build the next one I will use a 269ex (money permitting) and a 125d or e or whatever is still adjustable secondary in that style and around 10 watts or so. I would like to run 4 6au6's and two or three ax7s. I am thinking 5 or 6 watts is not out of the question. Would be a sweet bar amp! this one wouldnt be bad for a blues gig if miced.

[zambo]

here is the topic over at ax84 that i started from if it helps anyone.  Va0 for 6au6 >> 

[iccaros]

http://www.ax84.com/bbs/dm.php?thread=447825

[blackcorvo]

Yeah, but I've got a stash of 12V - 15V wall warts for 12.6 volt regulated output...besides, I like the lower heater current (even though I realize power is the same).  It just seems easier to find beefy 12V wall-warts compared to 9V warts with enough power output.

you can also run that 12V with the 6au6 and the 12au7 in series with each other, they have the same draw so you would use the same current as a single 12au7 at 6V. This would drop 6v across each tube

each are 21ohms so 21 * 2 = 42 ohms in series. drop 12V  (12/42 = .285) over this and you get  .285 amps

so to wire you would go

6AU6 pin  3 ---> + 12
6AUG pin 4  ---->  pin 4 12AU7
12AU7 pin 5 ----> ground

Only problem with this is: there's 2 6AU6's in this circuit. It could be a good solution for a SE design, but in this case, there's a better way to wire the heaters for 12v:
Instead of wiring the 12AU7 and 6AU6 together, wire the 2 6AU6's heaters in series and connect the 12AU7 for 12v.

[Zipslack]

That's what I was going for - 12au7 @ 12.6V (150 mA) with either: 2 6au6 in series (300 mA total @ 12.6V) or 2 12au6 in parallel (300 mA @ 12.6V).  Either way, still 450 mA minimum.  It's just a matter of what you have available...I have lots of 12V supplies.

[zambo]

just noticed as well that the cap i used from plate to plate in the output is a .001uf not .00005 like i had at first. this helped tame the nasty sounds quite a bit.

[iccaros]

Yeah, but I've got a stash of 12V - 15V wall warts for 12.6 volt regulated output...besides, I like the lower heater current (even though I realize power is the same).  It just seems easier to find beefy 12V wall-warts compared to 9V warts with enough power output.

you can also run that 12V with the 6au6 and the 12au7 in series with each other, they have the same draw so you would use the same current as a single 12au7 at 6V. This would drop 6v across each tube

each are 21ohms so 21 * 2 = 42 ohms in series. drop 12V  (12/42 = .285) over this and you get  .285 amps

so to wire you would go

6AU6 pin  3 ---> + 12
6AUG pin 4  ---->  pin 4 12AU7
12AU7 pin 5 ----> ground

Only problem with this is: there's 2 6AU6's in this circuit. It could be a good solution for a SE design, but in this case, there's a better way to wire the heaters for 12v:
Instead of wiring the 12AU7 and 6AU6 together, wire the 2 6AU6's heaters in series and connect the 12AU7 for 12v.

good point, I was thinking in my head that this was SE.. clearly not..

[zambo]

ok here is the new idea.

parts are ordered and on the way. 4 6au6wa with a LTP PI and a simple 2 stage pre with eq and bright switch. Anyone see any major flaws in this plan? PT is 269es and OT is 125c. Feed back resistor is 100k. 2 gain pots and a post PI master volume.

[zambo]

Here is a revised schematic


you could still run this on a 1363 smps if you only use 2 power tubes

[amptramp]

I am not sure why you would need the dual ganged volume control at the output tube grids when you already have one at the input to the phase inverter and another one going into V1B.  You might want small value (>1K) resistors in series with the grids of each output tube to avoid certain types of oscillation or instability.  Turning the feedback control to minimum resistance (the 25K connected to the 0.1 µF cap) gives you a feedback lag that almost guarantees oscillation.  You will get better stability with a cap across the 100K feedback resistor going from the transformer secondary to the phase inverter.  The inverter has a 100K plate resistor for the lower side and 82K for the upper side.  You are correcting for a µ vs. (µ+1) gain, so 101K vs. 100K may be a better idea.  The plate-to-plate capacitor in the inverter stage reduces the high-frequency gain but also adds a phase lag.  Not a bad thing in an amp with no feedback, but may upset the stability with feedback applied.

[defaced]

I am not sure why you would need the dual ganged volume control at the output tube grids when you already have one at the input to the phase inverter and another one going into V1B

I agree, but I know the reason.  Its there to allow overdrive of the PI without clipping the power tubes. 

The only change I'd make is lowering the input coupling cap of the phase inverter to a more standard value.  I know there are reasons why the input and ground caps are different, and it has to do wtih the internal negative feedback and the different caps will give the same frequency response, but since I never use LTPIs, I don't remember the exact whys and hows.

Also, watch the B+ dropping resistors in the preamp stage.  Since the B+ is sufficiently filtered by that point in the amp, the caps are there more to decouple the tubes from the rail than anything.  Usually a -3dB cutoff below 1Hz is used in these positions.  Not that other combos won't work, just that if you run into issues with the preamp, that could be a place to look.  Usually 22u and 10k is a good starting place. 

[zambo]

Yeah the post PI master is to adjust the power tube drive as stated. Since I am in weird power tube situation I figured it was easier to just have gain 1 gain 2 and master volume so I could control clipping in various parts of the amp. I am not an electronics guy buy any stretch of the imagination, I just like to stick parts together and make noise and I am addicted to tubes  I am now going to decipher your responses for the next 3 hours lol.I saw the different PI caps on a few things and thought about the smaller .01 cap on top. I may do that. The neg feedback / presence knob is just a lift from the liverpool drawings. I may not use it at all as I am not going for a super gain amp after all. It is nice in the Trainwreck amps though. any idea how I can calculate the plate to plate impedence for 4 tubes. Its 44k for 2.  Thanks for the comments! I cant wait to build this thing.

[iccaros]

That volume pot after the PI can also act like an attenuator, I have one setup in my Carvin for this reason, I get my tone all the way through and then turn down the powertubes.. Just a slight loss on the Bass side of things.  I like this new design.. I was sondering about the feedback on only one side of the PI, would this not come to the imput to the PI?  but I could be way off as I only looked for a min..

now to ebay to get more 6au6's.. I have 4 but they are going into pedals..

[zambo]

It only goes into the one side because on the other side it would be a positive feedback loop i believe. I guess i never really questioned it but I am pretty sure thats the answer.

[iccaros]

sorry, that is a long tailed pair right, I did not notice the flipping of the PI tubes and thought you had a weird cathodine with two tubes..