6V6 guitar amp design attempt

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Hi all,

I am having a crack at designing and building a VERY small guitar amplifier with a seperate PSU.

What do you think of my circuit? I know it will probably only put out a few watts at this HT, but my transformer dips horribly under load and i can only get around 150VDC at the current i will draw i estimate.
Any advice/ criticism is welcome ;)

Its my first attempt and is probably a bit odd and unconventional for a guitar amp but i just wanted to try it out.
i havent calculated the resistor values in the buffer stage yet, but i will do soon.

Here are some specs:
PSU = 0-220V@60mA, 6.3V@2A
8H choke, followed by 150uF.
0A2 and 0B2 stabiliser tubes if needed.

12AX7, 12AU7 and 6V6 tubes in amplifier.


edit: The schematic is wrong, the 0.022uF cap should go the the fist tubes anode.. not the grid.

Cheers
Craig
 

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Interesting design Craig!
When you say the cap is meant to go to the anode, don't you mean the cathode! It's an SRPP right?
(Or were you going for a cascode?)
You have no volume or tone controls?!
I would move the 1meg grid leak at the input to the other end of the 68k stopper, so you're not attenuating input signal.
Any particular reason for using a CCS cathode follower? In guitar amps, there's precious little advantage to using active loads on cathode followers. I would use that extra ECC82 triode as a gain stage or something.
 
Thanks Merlin.

Well i was going for cascode because i wanted lots of gain ( i want this amp to be dirty) but i guess i could change it to SRPP although id need to redo my resistor values.
Any sonic advantage to SRPP?

I forgot to put the volume control on the schematic but there is one right after the jack input. As for tone controls i may add some later but i just wanted to get the amp working first.

Good point on the resistor, ill move it.
WRT the active cathode follower load, i only put it there to use up the other half of the 12AU7. Ill give it another switchable gain stage instead :) maybe i can get some good compression with it at these low HT's?

Regards
Craig
 
Hey don't feel you're being press ganged into changing the cathode follower, it might make a nice change to have an active loaded one!

To be honest I would stick with your first plan of cascode for better input sensitivity, but it'll need a few extra parts! You need an anode load on the upper triode for a start! I guess about 47k.
Also, you need a large resistor from lower anode to upper grid since you're self-biasing it- something around 5Meg should do it, and decouple the grid with a capacitor.

Also, shouldn't the 1Meg grid leak brefore the CF go to the anode of the CCS triode?
 
Ok, im easy, the cathode follower load stays as is :D

I didnt realize you had to put a large resistance on the grid to use a tube as a CCS, i thought they were like JFETS.
I only have 3.3M, hope this will do. Also what is a suitable cap value for this decoupling?
I guess i have to do the same to the cathode follower load too.

I take it the grid leak resistor should go the the CF load anode then?
to be honest i dont know really, i thought it was just a reference to ground for the grid.
Could you enlighten me as to why it is good to do this? I would guess it is just degenerative feedback and raises the input impedance?


edit: doh ok i just realized why you need decoupling and large resistance on the CCS, to keep signals off the grid, so its not needed on the CF load.

Heres the sort of updated circuit diagram:
 

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Re the cathode follower, you need the grid to be only a couple of volts lower than the cathode- if the grid leak goes to ground the grid will be at 0V while the cathode is at 100V or something like that!

You CAN set the CCS using a large resistor like that (known as "zero bias") but it's very innacurate, and the max allowable grid leak for the ECC82 is only 1Meg which will only develop about 0.1V bias!. Really you should use a potential divider from the HT to set the grid voltage on the CCS (plus decoupling cap)

You can also use the same methode for the cascode, but the ECC83 can easily handle a 10Meg grid-leak so that's up to you. 3.3M will be ok, but expect the bottom of the output signal to be fairly clipped. If you decide to stick with zero bias, choose the decoupling cap using the 3.3M and you desired low F roll-off, say 20Hz, = about 2.2nF.

You don't need the 1k5 cathode resistor on the upper triode of the cascode. And take the signal off the anode!

It's getting there!
 
Thanks for the replies!

Ok i have been thinking about your potential divider suggestion and i think it is the best route to go for both tubes. I dont really know how to calculate the appropriate values for the potential divider to get the correct plate current.
I looked it up on Tubecad.com and came up with the values for the 12AX7 loaded tube.

The cathode follower load however, i am not so sure about.
Or how to go about calculating its resistor value, im guesing i just use the normal cathode follower equations for this resistor?

Heres the mutated circuit once again:

Thanks
Craig
 

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To work out the divider for the CF, simply decide what constant current you actually want, and what load the CCS has (you seem to have removed the cathode load resistor for your CCS!
Anyway, I guess a 10k load would do for an ECC82. From a load line I'd say abotu 3mA is the most you'll get at such a low HT. For this you'll need a bias of about -1V on the CCS with a quiescent cathode voltage of abotu 30V, so set the grid at 29V.
For maximumhead room, bias the CF at half HT, in ths case that's a bias of -3V, so:
3 / 0.003 = 1k bias resistor.

You'll want to decouple the cathode of your lower cascode triode by the way, 1uF should do it. Or you could use a diode instead or a resistor since you'll only require abotu 0.5V bias.
 
Thanks to everyone for the replies.

Yes it does look a bit like the Aikido! i probably have the topology on the brain as i finished an Aikido amp a few months ago.

I think i have managed to make all the corrections that have been suggested, this has turned from a very simple circuit into quite a hefty one!

Thanks for the calculations Merlinb, i have got the divider ratio now and have used you suggested resistor values in my CF.

So what do you guys think now.. is it ready to go?

Regards
Craig

new schematic:
 

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Hi,

I built the first stage last night, the 12AX7 stage. I cant get any gain out of it, in fact it attenuates?
The problem seems to lie with the tube bias, the cathode of the lower tube is only at 0.44v accross 1.65k which gives 0.27mA. When i looked at the data sheet for these tubes at 150V HT i found they needed 1V accross 1.65K.

To correct this lack of current should i decrease the plate resistor?

(HT is slightly higher than 150V at the moment because the 6V6 is not connected.)


Thanks
Craig
 

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Well you want to drop about a third HT across the bottom triode which you have done. However, at such a low HT you're never going to get more than about 1mA through it and thats pushing it! You could lower the anode resistor to 33k, but not much lower.
To get the most out of it, you really want Vgk of the upper triode to be about -0.5V, at the moment it's too high- hence the really low current! To be honest, you need such a low Vgk I would simply go back to using that 3M3 resistor from grid to cathode, that should get you in the ballpark.

Looking at the mutual characteristics of the ECC83, at 50V HT you want Vgk lower triode to be about -0.4V. If we assume 0.8mA current that's a 470 ohm cathode resistor on the lower triode, see if that helps.

When i looked at the data sheet for these tubes at 150V HT i found they needed 1V accross 1.65K.
Don't forget you are having to share the HT between two triodes, so it isn't as high as 150V, it's only 100V upper, 50V lower triode.
 
Thanks Merlinb, i added the 470R, the anode resistor is now 33K, and i went back to the 3.3M//2.2nf bias scheme.
Now i do get amplification! although it is only about 12x.. and the output is clipped at about 20V peak to peak.

Is this enough swing to drive the 6v6 with?

I have another tube on my chassis i could easily wire in another regular anode follower gain stage or two.

Thanks
Craig
 

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Hi Craig,

and the output is clipped at about 20V peak to peak.

Is this enough swing to drive the 6v6 with?

This 20Vpp swing, did you measure it while the driver was working into a load, like the output stage presents (grid leak, and so on)? Probably not.

Depending on operation point chosen for the power stage, 6V6 usually is biased somewhere between -7V and -14V.

Theoretically, you could get away with the 20Vpp swing for 6V6 operation points with Vg1 at -7V to -10V, but you won´t have any reserves. This might be intentional for a guitar amp, but also remember that your driver has to swing the driving voltage into the actual load the output stage presents.

Tom
 
Tom, you are right this was with no load connected i can expect a bit of a drop in output swing when i connect the next stage. The next stage is actually a cathode follower so the current for driving the output tube will hopefully be sufficient coming from this.

I will certainly need to add another gain stage with a gain of about 25 to reach full swing from a guitar input.

Thanks
Craig
 
Luckily your cathode follower has extremely high input impedance so your measurement of 20V is probably perfectly accurate.

Unfortunatly your low HT of only 150V is restricting your cascode to operation in the very crappy corner of the grid curve graph. The gain of the lower triode is multiplied by the gain of the upper triode, but unfortunatly both values are extremely low. Since you at least have it working you can try increasing the anode resistor again and see if you can improve on things, but placing another gain stage between the cascode and CF would certainly be preferable since you really want to hit the power valve hard. Personally I would remove that active load on the CF and make it into the extra gain stage, maybe even DC couple it to the CF?

Incidentally, you could try replacing the cathode resistor and cap on the cascode with an ordinary silcon diode That would maintain a bias of about -0.7V while you play with the anode resistor. Ok so 0.7V is a touch higher than optimum, but it might allow you to tweak other values more easily.
 
Good suggestions, thanks.
I have another 12AX7 tube wired after the cascode stage and now i get all the gain i need which is great. So i think ill leave the CF load tube alone.
I know my supply voltage is letting this circuit down a lot in the performance department and probably in the hifi sound department too, but i am hoping it will sound ok for guitar use.

I have wired and tested all but the 6v6 section so far and its working ok.
Although i am having some supply ripple issues that shouldnt be there at all, i think i might have leaky supply caps because large ripple is getting amplified in my signal path.

Regards
Craig
 
I am regretting the use of the cascode input stage now, it has very poor PSRR and unfortunatly i have to use use configuration where the bias comes from the active loads own cathode.

1V peak to peak ripple on the supply is exactly what i get at the output of my cascode.
This 1v is then amplified to 20V by the next stage, basically i wont be able to hear my instrument with this much hum!

Any suggestions for a cure? i have read John Broskies pages on improving cascode PSRR but he uses the potential divider bias in his cascodes.

Or would it be best to just cut my losses and go for a boring anode follower input stage? :xeye:

Regards
Craig
 
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