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Old 21st Dec 2011, 7:45 am   #1
mrmagnetophon
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Default 6v6 with 12ax7 valve amp

I am trying to put together a Single ended 6v6 with a 12ax7 valve amplifier, need only to be mono, just need a little amp to go with a valve reel to reel recorder that i have, that uses a 7591 valve. So any thoughts, all I can find is not very good guitar amp schematics, which I don't need.

Thank you,
-Chris
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Old 21st Dec 2011, 6:48 pm   #2
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Default Re: 6v6 with 12ax7 valve amp

I'd just start with the back end circuit for any old valve radio which uses a triode followed by a pentode for its audio. Just use half the 12AX7. If you need more gain, you can always just repeat the triode stage again at the front end.
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Old 22nd Dec 2011, 3:57 am   #3
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Default Re: 6v6 with 12ax7 valve amp

hmm any schematics you have? Thats exactly what i need, with solid state rectification.
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Old 22nd Dec 2011, 2:10 pm   #4
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Default Re: 6v6 with 12ax7 valve amp

How about this, dc coupled, adjustable bias / feedback. It uses the 12ax7 in parallel so no wasted section.
I haven't built it myself so I can't comment on its performance. I may have a go but use a EL34 in place of the 6v6 as I have a spare.
Purist might suggest there is a bit too much silicone in there!

Vic
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Old 22nd Dec 2011, 6:02 pm   #5
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Default Re: 6v6 with 12ax7 valve amp

The 12AX7 seems to be operating close to cut off and will be not particularly linear at such a low anode current, which should really be a couple of mA's per anode.
The cathode biasing arrangement is very wasteful of power and I don't understand why g3 is connected to g1 and not the cathode.
It looks like dc coupling for the sake of it, with no real benefit.
I'll see what circuits I can dig out tonight.
Rob.
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Old 22nd Dec 2011, 7:15 pm   #6
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Default Re: 6v6 with 12ax7 valve amp

Hi.
It looks as if someone is trying to re-create a Mullard 3-3 amp, no doubt it will work but I doubt it would have much ooomph!
A ECC83 and 6V6 amp is really simple especially with just half of the ECC83 being used. Just follow the convention ECC83 G1 strapped the the slider of a 1 meg log pot (volume) bottom to chassis top via a 0.01uf cap as input, anode of ECC83 220k to HT cathode via 1k & 10uf cap to chassis, output from the ECC83 anode fed via a 0.01uf and 10k grid stopper to G1 of the 6V6, G1 of the 6V6 has a 680k to chassis as grid leak, cathode of the 6V6 has a 270 ohm decoupled by a 47uf going to chassis. The 6V6 screen grid is fed from HT via a 1k ohm decoupled to chassis with a 0.1uf cap, the Anode is connected to HT via the primary of the output transformer, a 0.0047uf cap should be connected across the primary of the transformer.
There is no reason why you cannot use solid state devices for HT and aim for 200-250v HT, Its very basic the above and can be improved with NFB, better HT decoupling for the ECC83 and even to use the other half of the ECC83 for more gain that can be used with advantage if you have a lot of NFB.
Good luck
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Old 22nd Dec 2011, 10:00 pm   #7
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Default Re: 6v6 with 12ax7 valve amp

Hi,
Found this circuit published c.1957 in "Quality Amplifiers for A.C. Mains". Although 6SL7 is shown, ECC83 (12AX7) is very similar.

The first circuit is complete with tone controls. A potential problem is the fact tone correction is being implemented in the forward path of the feedback loop.

A second modified circuit illustrates removal of the tone correction but retains the high gain needed to implement NFB as mentioned in previous post.

If NFB is not required then a one triode stage will suffice.

Cheers

Rich
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Old 23rd Dec 2011, 5:50 am   #8
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Default Re: 6v6 with 12ax7 valve amp

Hi thanks for the schematics, how many volts do i need for the HT supply?
The 6sl7 is quite good for my use, it will be fine.
Do i need to keep the choke?
-Chris
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Old 23rd Dec 2011, 10:27 am   #9
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Default Re: 6v6 with 12ax7 valve amp

Hi.
As with the simple amp I described the HT should be around 250v, a choke on the HT is the best route as a resistor generates heat and causes a bigger drop of HT than a suitable choke also it is less effective as a smoothing medium than a choke.
The 47 k on the anode load of the second half of the triode could be increased somewhat to give a larger voltage swing to drive the 6V6, one advantage of using a triode with a high Ra.
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Old 26th Mar 2012, 7:15 am   #10
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Default Re: 6v6 with 12ax7 valve amp

How much increase and will a 15H choke work?
-Chris
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Old 26th Mar 2012, 8:42 pm   #11
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Default Re: 6v6 with 12ax7 valve amp

Have a look at the various circuit suggestions in the datasheet below for the ECC83. There's not a lot to be gained from increasing the anode load really.

A 15H choke should be fine as long as its dc current rating is adequate for the total HT current. If its resistance is less than about 200ohms, the filter may be a bit "bouncy" but for a class A amplifier this probably won't matter much. You could probably reduce the smoothing capacitors to 10uF without much if any noticeable effect on hum.
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Old 28th Mar 2012, 11:21 pm   #12
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Default Re: 6v6 with 12ax7 valve amp

Quote:
Originally Posted by mrmagnetophon View Post
... and will a 15H choke work?
-Chris
Hi Chris

According to the parts list, the choke is 8H @ 70mA so a 15H will be fine. Power transformer HT secondary is 250-0-250V @ 70mA

Cheers

Rich
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Old 1st Apr 2012, 8:11 pm   #13
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Default Re: 6v6 with 12ax7 valve amp

Quote:
Originally Posted by Vicboduk View Post
How about this, dc coupled, adjustable bias / feedback. It uses the 12ax7 in parallel so no wasted section.
I haven't built it myself so I can't comment on its performance. I may have a go but use a EL34 in place of the 6v6 as I have a spare.
Purist might suggest there is a bit too much silicone in there!

Vic
I actually own the book where that circuit came out of.
If you'd like to read the full article I'll scan it for you.
However, I find that this Loftin-White style of amplifier is a terrible waste of supply-voltage.
Instead set the 6V6 up with a plate-voltage of something like 250-300V and apply a fixed bias at -12~15V and drive the thing from one section of a 12AX7.
That should set things off.
G2 can be Ultra Linear coupled or just supplied through 470R~1000R.
OPT 5K~6K:4/8Ohm
Pretty simple and pretty effective, should bring you around 5W.

rgds,

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Old 5th Apr 2012, 9:01 pm   #14
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Default Re: 6v6 with 12ax7 valve amp

Hi,
I know that this circuit is pretty old and has no tone controls (although a passive high-cut would be easy to add), but in its day, the Fender Champ was reckoned to be pretty good, if low in power. People still record with these things!
The Fender Princeton circuit does have a simple tone-control and is almost as ancient.
It is quite possible that Leo Fender had a big say in the design of these circuits, as he was a rdio-repairman. They are almost certainly based on circuits that the valve (or should I say "tube," since they were in the USA?) manufacturers suggested.
Regards, Colin.
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Old 6th Apr 2012, 7:39 am   #15
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Default Re: 6v6 with 12ax7 valve amp

Tri comp you should scan it, it would be interesting.
And Colin, that schematic is dead simple i have to try that!!!
Thanks guys!
-Chris
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Old 6th Apr 2012, 6:06 pm   #16
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Default Re: 6v6 with 12ax7 valve amp

Hi Chris,
If you want to build one of these Fender circuits, remember that they were built originally for guitar, etc amplification. There was generally some instructions in the user manual suggesting that if distortion was evident, turn the volume down! Of course, guitarists rather liked the distortion, eventually. Anecdotally, for those who are fans of Spinal Tap, the volume control on the little Fender Champ went up to 12. My point is that they weren't intended to be hi-fi. Having said that, it is possible to source re-manufactured original parts (and some improved ones) from various places. Watford Valves and Vyse Ltd in the UK and The Amp Doctor in Germany are two. In the USA Vibroworld, Weber Vintage Sound Technology, Hoffman Amplifiers and Antique Electronic Supply are good.
Have fun.
Regards, Colin.
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Old 7th Apr 2012, 9:52 am   #17
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Default Re: 6v6 with 12ax7 valve amp

I did the promised scan.
Be carefull with this circuit. In the book (Audio Valve Amplifiers by Rainer zur Linde) are a lot of errors. Schematic and dimensioning errors. I would say running a paralleled ECC83 at an Ia total of less than 0,5mA, as in this case, isn't 'normal'. If you can indeed count on the shown voltages, which I doubt. Also running an ECC83 at Va around 84V is a bit on the low side; there are tubes better suited for a low Va.

rgds,

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Old 9th Apr 2012, 1:01 pm   #18
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Default Re: 6v6 with 12ax7 valve amp

I agree with tri-comp in respect of the book by Rainer zur Linde. I too have a copy and it is full of errors. The book was published by Elektor Electronics (remember the mag?) in 1995. I am prepared to give the author the benefit of the doubt as to his knowledge; I think the editing and translation (from Dutch) are likely to be the worst offenders. If you notice in the text, it suggests that the heater voltage is stabilised by diodes D1-D5. I don't think so!! There is also no mention of the function of P2. It is associated with the negative feedback loop, so I would guess that it would limit distortion, similar to a "presence" control on a guitar amplifier. The problem is that with a stereo pair, these two controls would have to be set in a matching fashion, as they would have an influence on the frequency response and gain of each of the stereo pair.
When I first got this book, I thought of documenting the errors and informing the publishers, then I got further and decided that it would be like emptying Loch Ness with a mustard spoon.
Regards, Colin.
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Old 9th Apr 2012, 5:48 pm   #19
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Default Re: 6v6 with 12ax7 valve amp

Actually I did just that (NO, not try to empty Loch Ness...)
I took as an example a circuit that when built as shown will produce some serious smoke until whatever gives.
The answer is shown here:

Dear Sir

Thank you for your comment. The book referred to was produced many years ago beyond the control of the editors of Elektor magazine.

Having examined the PSU circuit diagram on page 156 I can concur with your findings and advise that the cathode of the EZ81 should not be connected to the filament.

Kind regards,

Jan Buiting

Editor, Elektor UK & US editions


...obviously nobody cares about this publication any longer.
I'm not saying interesting knowledge can't be harvested from this book.
Just keep a healthy critical view on what you read.

rgds,

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Old 10th Apr 2012, 3:13 pm   #20
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Default Re: 6v6 with 12ax7 valve amp

Quote:
Originally Posted by tri-comp View Post
Actually I did just that (NO, not try to empty Loch Ness...)
I took as an example a circuit that when built as shown will produce some serious smoke until whatever gives.
The answer is shown here:

Dear Sir

Thank you for your comment. The book referred to was produced many years ago beyond the control of the editors of Elektor magazine.

Having examined the PSU circuit diagram on page 156 I can concur with your findings and advise that the cathode of the EZ81 should not be connected to the filament.

Kind regards,

Jan Buiting

Editor, Elektor UK & US editions


...obviously nobody cares about this publication any longer.
I'm not saying interesting knowledge can't be harvested from this book.
Just keep a healthy critical view on what you read.

rgds,

/tri-comp
Ho, Wow!
EZ80 & EZ81 rectifiers were made for low-current PSUs, such as in portable record-players. They had a good cathode-to-heater insulation and a heater voltage of 6.3 V, so could be put in parallel with the other couple of valves in the device. I think they got used in the original (non-solid-state) Watkins Copicat tape-echo units, too. Then this circuit, which clearly takes great advantage of the heater-to-cathode insulation of the EZ81. Obviously a drawing error, but a corker.
The initial maths pages can be informative, sure, but one has to be careful as some subscripts and superscripts have been forgotten, which makes interpretation tricky. There are also some rather huge leaps in the mathematical reductions, too.
Perhaps we should stop kicking poor old Rainer and get him a mustard-spoon.
Colin.
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