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Express with a 2x6AK6 power stage

 
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harryk



Joined: 29 Jul 2007
Posts: 148

PostPosted: Sat May 08, 2010 2:43 pm    Post subject: Express with a 2x6AK6 power stage Reply with quote

I have been thinking a low wattage Express style tubeamp for home playing purposes (1-4 W). It came to mind that maybe a 2x6AK6 power amp could be a solution for that. The idea is to use Express preamp in it. I already have one Lil Devil by Dana Hall, but I would like now to build an amp with tubes that are easier to find and p-to-p basis. Has anybody done anything like that? Harry
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passfan



Joined: 10 Jan 2010
Posts: 1188
Location: Central Florida

PostPosted: Sat May 08, 2010 4:18 pm    Post subject: Re: Express with a 2x6AK6 power stage Reply with quote

harryk wrote:
I have been thinking a low wattage Express style tubeamp for home playing purposes (1-4 W). It came to mind that maybe a 2x6AK6 power amp could be a solution for that. The idea is to use Express preamp in it. I already have one Lil Devil by Dana Hall, but I would like now to build an amp with tubes that are easier to find and p-to-p basis. Has anybody done anything like that? Harry


Here is one with a 6sn7 output.
http://www.ax84.com/4-4-0.html

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harryk



Joined: 29 Jul 2007
Posts: 148

PostPosted: Sat May 08, 2010 7:38 pm    Post subject: Reply with quote

I have tried those tubes, but I am now seeking a real pentode tube sound with reasonable sound pressure. I made also one amp with 12AT7 powerstage, but I did not like the sound. I have built almost sixty tube amps by now but never found a killer sounding small one. Harry
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Cliff Schecht



Joined: 30 Dec 2009
Posts: 2585
Location: Seattle

PostPosted: Sat May 08, 2010 8:41 pm    Post subject: Reply with quote

I like the 6AK6 for PA situations. I've used them before with great results and they are surprisingly loud for such a tiny little 7-pin tube. They are no EL84 but I like the idea.
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bancika



Joined: 22 Aug 2006
Posts: 60

PostPosted: Sat May 08, 2010 10:59 pm    Post subject: Reply with quote

I built a SLO with el91 which are similar. Use 12ay7 for PI or some other way of reducing PI output to avoid driving the heck out of them.

http://diy-fever.com/amps/5-watt-slo/

I also built a 6sn7 version of express, you can hear some clips here

http://diy-fever.com/amps/ax84-4-4-0/

Got a few of 6ak6 but haven't tried them yet
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harryk



Joined: 29 Jul 2007
Posts: 148

PostPosted: Sun May 09, 2010 6:19 am    Post subject: Reply with quote

I think that a PPIMV can prevent driving power tubes too hard when using a normal 12AX7 PI. My aim is to get as much as possible of the original Express sound where the biggest part of it comes from PI PA. I know that those tiny tubes are not a pair of EL34, but I think that I will try it anyhow. Harry
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RJ Guitars



Joined: 14 Nov 2006
Posts: 1922
Location: White Rock, New Mexico

PostPosted: Mon May 10, 2010 12:31 am    Post subject: Reply with quote

Harry - Do it, you will like the results.

Sometimes I think I am really innovative but usually somebody has already done almost everything before. I made a couple push pull 6AK6 amps before I did the Express build and one major surprise and maybe even a disappointment was how loud they can be.

If you go back and look at old Epiphone and Gibson schematics you can see these tubes (7 pin pentodes) were in the hunt then sorta disappeared when the 9 pin boom hit.

Since then I have discovered several 7 pin output tube options that I think are preferable to the 6AK6, but honestly there is a boat load of them that sound pretty decent.

In this amp I decided to really explore some and went with all 7 pin preamp triodes as well.

Currently I have a 7 pin Liverpool in the works. I'll sort out the output tubes based upon performance (how they sound) although I am going to use 9 pin preamp tubes in the first iteration. I know it's sacrilege but I am considering a PCB board for the 7 pin Liverpool build. It is all based on some of the work I did with the sub-mini tubes in the "V-2 -Dwarf Tube Rocket". It makes assembling the amp complete an easy task. You can build it all on the board then just put the fully populated board into a chassis and your off to the races.

Hope you'll post up your results, it will be a cool amp.

rj

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Bear



Joined: 29 Mar 2005
Posts: 316

PostPosted: Mon May 10, 2010 1:04 am    Post subject: Reply with quote

RJ Guitars wrote:
I know it's sacrilege but I am considering a PCB board for the 7 pin Liverpool build.


It's really hard to think of a modern high-gain production amp that isn't PCB, and the SLO cloners have embraced PCB, too. There's a reason -- once you get it stable once, almost any build you do from the same PCB design should be stable.

A lot of folks would like an easy to build of a stable Express or Liverpool without too much worry about getting the lead dress dead on. Something that sounds and behaves like a good clone amp if not quite like the best clone ever would probably be enough to make some builders very, very happy. Probably a butt-load of work to get the PCB design down, though, so unless the quantities are right, it might be easier to chopstick a handful of hand-wired amps.
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UR12



Joined: 28 Nov 2005
Posts: 1548

PostPosted: Mon May 10, 2010 1:23 am    Post subject: Reply with quote

You will probably get about 10 watts depending on plate voltage but the 7 pin 6AQ5 is a great tube. Sounds very much like a 6V6.
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harryk



Joined: 29 Jul 2007
Posts: 148

PostPosted: Mon May 10, 2010 5:30 am    Post subject: Reply with quote

Dana! I think that a pair of 6AK6 tubes will give some 4-5 W power, not 10 W like a pair of 6AQ5/EL90 tubes. I am interested in to hear if there is a better sounding alternative to 6AK6 tube giving maximum 5-6 W output power in push pull. Harry
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RJ Guitars



Joined: 14 Nov 2006
Posts: 1922
Location: White Rock, New Mexico

PostPosted: Mon May 10, 2010 6:21 am    Post subject: Reply with quote

Harry,

Here is the list that I tried for output tubes in my various 7 pin builds. There is one pin that you can jumper (i put it on a DPDT switch) or not and make any of the 7 pin pentodes work in the same amp.

*6AN5 - 0.45mA - 300V
6HG5 - 0.45mA - 275V
6HR5 - 0.45mA - 260V
*6AQ5 - 0.45mA - 250V
*6005 - 0.45mA - 250V
*6AS5 - 0.8mA - 150V
*6DL5 - 0.2mA - 300V
6DS5 - 0.8mA - 275V
*6CA5 - 1.2A - 130V
6CU5 - 1.2A - 150V
6EH5 - 1.2A - 150V
6BF5 - 1.2A - 250V

After the tube number is the heater current and the max plate voltage. I don't recall what the asterisk was for but I think it might indicate a Beam Power Tube or maybe some of my own personal favorites. The Beam power tubes tend to be the preferred choice for output pentodes.

I don't know if any major guitar amp builders used anything but a 6AQ5 as a 7 pin output tube. It's possible but they are pretty common and affordable and as Dana mentioned they are famous for sounding good.

The 6AK6 sounded very good in the amps I put together although it has a really high plate resistance and according to the spec sheets it shouldn't be a great guitar amp tube... The resulting amps were "too loud for church" when I got them cranked up enough to sound good. I went back to Octals and just started dropping the voltage and also played around with triode outputs. I was looking for the ultimate 1 to 2 watt amp for small venue work - churches and small clubs. Right now sub-mini tubes are tough to beat.


If you want lower output, you can always just drop the B+... or use one of Dana's VVR's and dial the wattage in to your preference.

rj

PS - here is a look at a good start on a low output Wreck - unfortunately the thread sorta dead ends. http://ampgarage.com/forum/viewtopic.php?t=3842

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harryk



Joined: 29 Jul 2007
Posts: 148

PostPosted: Mon May 10, 2010 7:02 am    Post subject: Reply with quote

Thanks RJ. I already have one Lil Devil by Dana with subminiature tubes, but as a tube amp builder (www.bluetone.fi) I am always seeking something new to my customers. I have installed several VVR systems to smaller and bigger tube amps with success and tried all combinations with power scaling them. VVR has it`s benefits, but as an "innovative" Finnish person, I am keen to try also something new. I have once used a pair of 6AQ5/EL90 tubes and I liked their sound like I like 6V6. Harry
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sst4270



Joined: 08 Jan 2007
Posts: 255
Location: Glen Burnie, MD

PostPosted: Tue May 11, 2010 9:24 pm    Post subject: Another Option Reply with quote

I've built a few AX84 4-4-0's

Just another option for low wattage express tone with easy to obtain tubes.

Look here: http://www.ax84.com/4-4-0.html

And FWIW one of my customers who has played through several original wrecks called me the day he received it and was blown away that I had "absolutely nailed" the tone with the 4-4-0.

Photos attached below.

Good Luck,

Steve

OOPS - Missed Passfan's link above. Oh well. Enjoy the shots anyways.
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harryk



Joined: 29 Jul 2007
Posts: 148

PostPosted: Wed May 12, 2010 7:04 pm    Post subject: Reply with quote

Smile I am happy to see a nice and clean amp inside. It must be same also soundwise. I am aware of AX84000 4-0-0 project. I made one Firefly with a 12AU7 power stage and was not so impressed of triode sound in power stage. That project went to recycling. I am now wondering if one 6SN7 is a better alternative than a pair of 6AK6 tubes. I bought today a 8 ohm L-Pad to make a poor man attenuator of it to this forecoming lowatt project instead of VVR this time. My idea is to push power tubes really hard to get distorted lead sound nirvana. Harry Bluetone
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Structo



Joined: 17 Oct 2007
Posts: 14041
Location: Oregon

PostPosted: Thu May 13, 2010 12:32 am    Post subject: Reply with quote

Nice clean looking build Steve.
It has to sound good looking that nice! Very Happy

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