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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Mon, Oct 30th, 2006 @ 20:24 ( . )

Check it out:

[link]

It's a flea-power rock & roll amp with a parallel SE output stage using two 6AK6 power pentodes. The input stage is also a pentode with a 6AU6A. In between is a cathode follower - Marshall tone stack - gain stage using a 12AU7A. Sorry, no sound clips yet. It sounds good, though -- very rock & roll.

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Miles O'Neal (registered user: 13998 posts )
Date: Mon, Oct 30th, 2006 @ 23:57 ( . )

Sweet!

I love that hammertone (or whatever flavor it is). You planning to paint the top? or is it painted already?

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Merlin (registered user: 4774 posts )
Date: Tue, Oct 31st, 2006 @ 03:42 ( . )

Absolute beauty! Nice to see the 12AU7 used sensibly for once!

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Balijukka (guest: search)
Date: Tue, Oct 31st, 2006 @ 05:17 ( . )

It's so ugly it looks good.
jukka

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Tue, Oct 31st, 2006 @ 14:14 ( . )

On 10/31/2006 @ 05:17, Balijukka wrote :
It's so ugly it looks good.
: jukka
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I'm all *sniff* choked up. That's the nicest thing anybody's ever said to me. :-)

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Tue, Oct 31st, 2006 @ 14:13 ( . )

On 10/31/2006 @ 03:42, Merlin wrote :
Absolute beauty! Nice to see the 12AU7 used sensibly for once!
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Thanks, Merlin. At one point, I was going to use another 12AU7A in place of the 6AU6A, to get many low-gain stages in a row. But that kind of overwhelmed me when I tried to design it, so I tried to simplify matters by using the 6AU6A.

I think the 12AU7A would make a good bottom half in a cascode stage. It has very high transconductance, and that's what matters for cascode. I hope to try that sometime. It wouldn't have worked in this amp, because the supply voltage is just too low.

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Tue, Oct 31st, 2006 @ 14:07 ( . )

On 10/30/2006 @ 23:57, Harrison Ford Prefect wrote :
Sweet!
:
: I love that hammertone (or whatever flavor it is). You planning to paint the top? or is it painted already?
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Yep, it's that hammertone spray paint that most hardware stores sell. Personally, I think all amps should be blue. It's simply the best color. :-)

The top comes from Hammond already painted gray. I'm not that crazy about the color, and I'm thinking of painting it either blue or black. What do you think?

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Balijukka (guest: search)
Date: Tue, Oct 31st, 2006 @ 17:29 ( . )


Don't you dare change anything. Noe it is perfect sample of early sixties professional functionalistic design P.A. Amplifier.
Only thing missing is the steel horn speakers and 70V output.
jukka

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Miles O'Neal (registered user: 13998 posts )
Date: Sat, Nov 04th, 2006 @ 02:39 ( . )

Black was the first color that jumped into my mind, with blue following.

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:JuanChanKane (registered user: 184 posts )
Date: Tue, Oct 31st, 2006 @ 06:02 ( . )

Looks really good John. Very interesting tubes choice.

One obs: R18 might be too close to the filter cap.

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Tue, Oct 31st, 2006 @ 14:16 ( . )

On 10/31/2006 @ 06:02, JuanChanKane wrote :
Looks really good John. Very interesting tubes choice.
:
: One obs: R18 might be too close to the filter cap.
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Thanks, JCK. That's a great point about R18, and I'm going to move it. Luckily, it only dissipates about 1W, so it doesn't get very hot.

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Doug H (registered user: 5397 posts )
Date: Tue, Oct 31st, 2006 @ 15:36 ( . )

John, it looks very good! I love the blue too.;-)

I've been curious about using 6ak6's for a "mini-amp" ever since I designed my pentode driver years ago. Glad to hear you have had good success with them. I've still have many of them lying around.;-)

Another one for the mile-long "to-do" list...;-)


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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Tue, Oct 31st, 2006 @ 17:19 ( . )

On 10/31/2006 @ 15:36, Doug H wrote :
John, it looks very good! I love the blue too.;-)
:
: I've been curious about using 6ak6's for a "mini-amp" ever since I designed my pentode driver years ago. Glad to hear you have had good success with them. I've still have many of them lying around.;-)
:
: Another one for the mile-long "to-do" list...;-)
:
:
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Thanks, Doug. I think a push-pull amp using two 6AK6 tubes would be pretty cool ...

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:ZoeNIain (registered user: 4522 posts )
Date: Tue, Oct 31st, 2006 @ 17:41 ( . )

Cool! Similar to the one we're working on at the moment. Though ours uses a 6BR7, a 12AU7 cathode follower, then another 12AU7 cathode follower, Big Muff Pi/Supro Thunderbolt tonestack, ECL86 triode floating paraphase PI, ECL86 output.

Incidently, the cascaded cathode followers are for compression as much as impedance matching. If you use a boost pedal to get the input level up, the pentode compresses, the first CF compresses it some more and the second one even more. With 1v input level, we are getting 50% compression on the bottom of the signal.

Hopefully it will sound as nice as it simulates. ;-}

How do you find the sound with a pentode driving into a 12AU7?

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Wed, Nov 01st, 2006 @ 14:59 ( . )

On 10/31/2006 @ 17:41, Zoe N Iain wrote :
Cool! Similar to the one we're working on at the moment. Though ours uses a 6BR7, a 12AU7 cathode follower, then another 12AU7 cathode follower, Big Muff Pi/Supro Thunderbolt tonestack, ECL86 triode floating paraphase PI, ECL86 output.
:
: Incidently, the cascaded cathode followers are for compression as much as impedance matching. If you use a boost pedal to get the input level up, the pentode compresses, the first CF compresses it some more and the second one even more. With 1v input level, we are getting 50% compression on the bottom of the signal.
:
: Hopefully it will sound as nice as it simulates. ;-}
:
: How do you find the sound with a pentode driving into a 12AU7?
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Wow, the 6BR7 is a new one to me. I hope it works out well in your amp.

About the sound of a pentode driving a 12AU7 ... I'm no good at describing that kind of thing. My amp as it stands now sounds good to me for rock music, but has a little too much bass for my tastes. I don't think I can say much beyond that. In any case, the preamp is very clean, so I doubt that my preamp tube choices had a lot of influence on the sound. Well, that's not quite what I mean. I mean, the tube choices affect the gain and the impedances, that that interacts with the tone stack, etc., to affect the sound; but I don't think the preamp tubes themselves are contributing a lot of their character to this amp.

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Jaap (guest: search)
Date: Wed, Nov 01st, 2006 @ 03:00 ( . )

Great John! And interesting to read your experience while building this amp. And yes, one or two watt is still quite loud at home, also IMO. I like the way you want to switch between one and two outputtubes; never thought of that solution. I'll keep it in mind.
Jaap

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Joe Henrich (guest: search)
Date: Wed, Nov 01st, 2006 @ 21:34 ( . )

John
Looks great, just the kind of project I was looking for to use up "on hand supplies"
Have any further advice?
Regards
Joe

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Fri, Nov 03rd, 2006 @ 12:48 ( . )

On 11/01/2006 @ 21:34, Joe Henrich wrote :
John
: Looks great, just the kind of project I was looking for to use up "on hand supplies"
: Have any further advice?
: Regards
: Joe
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Well, I think the amp is too bassy -- maybe even what you'd call flabby. I haven't had a chance to work on fixing that much yet. I tried reducing C8, and that helped. But I had to reduce it all the way down to 0.001uF to get the bass where I wanted it, and that cut out too much midrange and made it sound thin.

I also tried adding various amounts of negative feedback, by adding a 5K - 10K resistor from the 8 ohm speaker tap to the cathode of V2B. The scope showed that the NFB was working properly, but I didn't feel that it improved the sound.

I think the final solution may involve doing several things: (a) reducing C1, though I'm worried I'll have some hum if I do that; (b) adding a bypass capacitor of just the right size across R9; and (c) adding a resistor in series with the top leg of the gain control, with a capacitor in parallel with it to pass all except the low frequencies.

Using a crappier output transformer would probably help, too. :-)

Before anybody recommends adding a bright cap across the gain control, it won't work because the gain is almost always cranked all the way up. Any other suggestions would be welcome, though.

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:Joe Henrich (guest: search)
Date: Sat, Nov 25th, 2006 @ 17:39 ( . )

Just finished up one of these, but with only one output tube as I don't want much volume. I didn't have a 6ak6 handy so I substituted a 6au6 in its place. Sounds good, gets dirty very fast. I'm guessing that the 6au6 is about at its limit as it starts to motorboat with the gain over 3/4. Will see what happens with the 6ak6.
Thanks John for a nice plan.
Joe

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Sat, Nov 25th, 2006 @ 21:12 ( . )

On 11/25/2006 @ 17:39, Joe Henrich wrote :
Just finished up one of these, but with only one output tube as I don't want much volume. I didn't have a 6ak6 handy so I substituted a 6au6 in its place. Sounds good, gets dirty very fast. I'm guessing that the 6au6 is about at its limit as it starts to motorboat with the gain over 3/4. Will see what happens with the 6ak6.
: Thanks John for a nice plan.
: Joe
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That's an interesting idea, substituting the 6AU6A in the output stage. I didn't realize it, but that tube is actually rated for higher plate dissipation than the 6AK6. Slick. You could build a whole amp using nothing but 6AU6A tubes.

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:AletheianAlex (registered user: 3532 posts )
Date: Sat, Nov 25th, 2006 @ 19:24 ( . )

Cool little amp John. I missed this thread the first time around. 6au6 is a versitile little tube.

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Sat, Nov 25th, 2006 @ 21:12 ( . )

On 11/25/2006 @ 19:24, AletheianAlex wrote :
Cool little amp John. I missed this thread the first time around. 6au6 is a versitile little tube.
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Thanks, Alex. I noticed you were AWOL for awhile there. Yeah, the 6AU6A worked out pretty well. I was worried it would add too much hum as an input stage, but all the specimens I've tried so far are surprisingly quiet. The amp is still bassier than I'd like, but it may be because I'm trying to play the wrong music through it. :-)

John

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:AletheianAlex (registered user: 3532 posts )
Date: Sat, Nov 25th, 2006 @ 22:25 ( . )

On 11/25/2006 @ 21:12, John Polstra wrote :
On 11/25/2006 @ 19:24, AletheianAlex wrote :
: Cool little amp John. I missed this thread the first time around. 6au6 is a versitile little tube.
: --
:
:

:
: Thanks, Alex. I noticed you were AWOL for awhile there. Yeah, the 6AU6A worked out pretty well. I was worried it would add too much hum as an input stage, but all the specimens I've tried so far are surprisingly quiet. The amp is still bassier than I'd like, but it may be because I'm trying to play the wrong music through it. :-)
:
: John
:
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I don't think that reducing C1 will cause much if any hum. If you have a handfull of 1u caps, you coulf parallel a bunch there and snip them out one by one until you have a value that works. You could also reduce c2 and see if that helps. I had apentode input stage that worked well with 4uf bypassing Rk and .3uF bypassing the screen. I think the roloff was sround 150Hz with no perceptable increase in hum.

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'New 6AU6A + 12AU7A + 2*6AK6 build'
Author:John Polstra (registered user: 654 posts )
Date: Sat, Nov 25th, 2006 @ 23:53 ( . )

On 11/25/2006 @ 22:25, AletheianAlex wrote :
I don't think that reducing C1 will cause much if any hum. If you have a handfull of 1u caps, you coulf parallel a bunch there and snip them out one by one until you have a value that works. You could also reduce c2 and see if that helps. I had apentode input stage that worked well with 4uf bypassing Rk and .3uF bypassing the screen. I think the roloff was sround 150Hz with no perceptable increase in hum.
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Reducing C1 was what I was planning to try next. I have some metal film capacitors in 1, 2.2, 3.3, and 4.7 uF. (I just haven't had time to try it yet because my wife and I are moving to Lopez Island, and we're trying to get our Seattle house ready to sell.) Interesting idea, to lower the screen bypass capacitor. I hadn't thought of that one. Thanks for the suggestion!

John

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