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Old 19th April 2010, 12:56 AM   #1
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Default Kofi Annan in: "Pushed and Pulled Back!"

Hey!

So, it's been a while, I know. Things got a bit sticky here at the Annan household over the last year or so and I was not able to keep a presence at DIYAudio for some time. Everything is coolsville now though, so it's time for another project.

This time, I'm looking to build a push-pull amp for a friend. He's currently got a small set of Polk Audio bookshelf speakers (meh) and a 12 x 16 room so I'm thinking a modest push-pull amp would do the trick.

I'll need to come in under $500 for the project and I would like it to push about 15 - 20 watts, but more would be great. Here are the designs I'm considering:

1. The new Pete Millett Engineer's Amplifier (I love Pete's stuff and I always wanted to one of his designs)

2. SY's Red Light District

3. Gingertube's Baby Huey

4. Another Morgan Jones Bevois Valley

5. Some other monster KT88 PP or somesuch

Any recommendations among these listed or others I should consider?

Thanks in advance for any advice you can give. Dumb Kofi questions to follow...

Just like old times.

Kofi
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Old 19th April 2010, 01:08 AM   #2
Sheldon is offline Sheldon  United States
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Quote:
Originally Posted by Kofi Annan View Post
Hey!

So, it's been a while, I know. Things got a bit sticky here at the Annan household over the last year or so and I was not able to keep a presence at DIYAudio for some time. Everything is coolsville now though, so it's time for another project.

This time, I'm looking to build a push-pull amp for a friend. He's currently got a small set of Polk Audio bookshelf speakers (meh) and a 12 x 16 room so I'm thinking a modest push-pull amp would do the trick.

I'll need to come in under $500 for the project and I would like it to push about 15 - 20 watts, but more would be great. Here are the designs I'm considering:

1. The new Pete Millett Engineer's Amplifier (I love Pete's stuff and I always wanted to one of his designs)

2. SY's Red Light District

3. Gingertube's Baby Huey

4. Another Morgan Jones Bevois Valley

5. Some other monster KT88 PP or somesuch

Any recommendations among these listed or others I should consider?

Thanks in advance for any advice you can give. Dumb Kofi questions to follow...

Just like old times.

Kofi
Could add Eli's El Cheapo to the list.

Sheldon
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Old 19th April 2010, 03:04 AM   #3
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Hi Kofi! The UN isn't the same without you.

Your biggest cost item is probably going to be the OP transformers but you might find some good bargains on eBay. Stick to economical tubes that suit the OPTs you select (i.e. avoid DHTs, KT88s and other costly bottles).
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Old 19th April 2010, 03:57 PM   #4
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Quote:
5. Some other monster KT88 PP or somesuch
Don't think you'll be able to build something within your given budget with 4 * KT88.
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Old 19th April 2010, 04:20 PM   #5
rknize is offline rknize  United States
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If you are building it for a friend, you might want to stick with something that uses tubes that they will not have trouble sourcing in the future. That's my only "thing" with Pete's amp. Otherwise it is awesome and has a lot of potential to go further.

What about:

4.5) Some other medium-sized EL34 PP or somesuch

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Old 19th April 2010, 04:40 PM   #6
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Originally Posted by rknize View Post
If you are building it for a friend, you might want to stick with something that uses tubes that they will not have trouble sourcing in the future. That's my only "thing" with Pete's amp. Otherwise it is awesome and has a lot of potential to go further. .....
Initially, I was worried about the "future" tube availability issue as well.
So before I built Pete's amp, I checked my sources for the required tubes.
I now have more than a "lifetime" supply.
Also, the design is conservatively rated. The tubes are not working hard, so should last a long time.

I'm very happy with the Millett "Engineer's Amplifier". It's the best tube project I have ever built. Highly recommended by this hobbyist.
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Last edited by neonthecat; 19th April 2010 at 04:41 PM. Reason: text
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Old 19th April 2010, 06:05 PM   #7
rknize is offline rknize  United States
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Indeed...but stocking up his friend for life would cut into the budget some.
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Old 19th April 2010, 07:22 PM   #8
hkoetz is offline hkoetz  Netherlands
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Default Kofi Annan in: "Pushed and Pulled Back!"

You may want to consider this one. The Simple EL84: auto inverting with CCS. It is super simple which leaves you a lot of money for good OPTs
Updating the Simple EL84
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Old 19th April 2010, 08:43 PM   #9
rmyauck is offline rmyauck  Canada
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You could add George from Tubelab's new Simple PP board to the list too. Great support as I've seen. He has new section now in diyaudio under Vendor Forms.

#1. If you need more power still at low cost/ but great sound.

Under the thread on Pete's amp it has been mentioned that it could be converted to use present production tubes. Maybe one could look at that. I don't think it's too much of a problem to even run wires to sockets like ST-70 amps do. As George (Tubelab) says values of caps would have to be changed to a higher Voltage as he did with bigger sweep tubes when he increased power. He also mentions lifting resistors to change screen voltage supply along with some rewiring to board.


Plate voltage may be okay for a number of lower cost OT tubes 6L6GC 6CA7 & EL 34. I've seen there are new production 807 tubes out there for low cost. Maybe they could be run a bit harder? EH6CA7 @ $80 look like a great value and sound for $. KT-88's could be added later with bigger PS Trans or could be run at lower points for longer life.

Using Edcor OT as Pete used, but uprated to 100W rating say 4.2K Primary @ about $93 ea. Bigger PS Trans. shouldn't cost much more if needed.
OT tubes $60-80. Might be able to make it work for $500 or a bit more plus have more reserve power with at least 30W/CH depending on which tubes and how they are run. Cabinet or chassis would be extra.

If that's too much money maybe 6V6GT or EL84 @ $40-60 would work saving tube and OT trans. cost. A 8-10K Primary OT would be needed so existing one in BOM would work. 10K would be better for 6V6GT. Should get 12-18W depending how hard tubes are pushed. 6V6GT's worth a watt or two less per CH. PS changes again need to. Maybe simpler and less costly.
I've read Genelex New production EL84 & Tungsol 6V6GT's are amongst the best sounding today. Equal or close to NOS.

Russian NOS OT's could be run for all of the above for less $ though I read 6V6 types aren't very tough or good sounding. 6N3C-E for 6L6 types are liked too.

I don't know what present production tubes would work for input?

Seems like a great design with distortion cancellation loops like Cit II, so should sound great. Maybe this would be enhanced with regular audio tubes!

As Pete mentioned you could also use higher end OT for less distortion and increased frequency response range.

I have this board so would like set it up for production tubes too!

What mods are needed?

Cheers!

Randy

Last edited by rmyauck; 19th April 2010 at 09:01 PM.
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Old 19th April 2010, 09:29 PM   #10
rmyauck is offline rmyauck  Canada
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Ran out of time to make corrections.

I should say Pete first mentioned increasing PS Voltage is possible if Cap value is increased in voltage appropriately.

EH6CA7 should be EH 6CA7.

5881, 7591, 7868, KT-66, & KT-77 also in new production should work too.

Maybe NEW Tung Sol KT-120 coming in May would work too!

Randy

Last edited by rmyauck; 19th April 2010 at 09:40 PM.
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