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Old 20th July 2006, 10:15 AM   #1
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Default 6V6 guitar amp design attempt

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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Old 20th July 2006, 10:33 AM   #2
Merlinb is offline Merlinb  United Kingdom
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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.
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Old 20th July 2006, 10:45 AM   #3
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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
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Old 20th July 2006, 11:03 AM   #4
Merlinb is offline Merlinb  United Kingdom
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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?
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Old 20th July 2006, 11:25 AM   #5
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Ok, im easy, the cathode follower load stays as is

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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Old 20th July 2006, 11:43 AM   #6
Merlinb is offline Merlinb  United Kingdom
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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!
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Old 20th July 2006, 12:28 PM   #7
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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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Old 20th July 2006, 12:41 PM   #8
Giaime is offline Giaime  Italy
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The input stage seems to be very Aikido

http://www.tubecad.com/2004/blog0011.htm
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Old 20th July 2006, 01:02 PM   #9
Merlinb is offline Merlinb  United Kingdom
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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.
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Old 20th July 2006, 01:21 PM   #10
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Quote:
i want this amp to be dirty
Then you are going to need a capacitor across the cathode resistor in the output stage.

You also have the grids swapped on the 6V6.

Since you are using a relatively low B+ voltage, try some of the low voltage output tubes after the amp is done. 6Y6 and 6W6 come to mind.
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