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Old 7th July 2012, 03:51 AM   #1
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Default Hello, looking for advice on a tube project...

This is my first post here in the forum so I would like to say "Hello" to everyone. I've arrived here because I offered to take on a project for a friend and build a tube amplifier for him to use in his stereo system, so I'm looking for some advice and guidance...

First some background, I have plenty of experience repairing electronics, and have built a few tube guitar amplifiers in the years past, so no need to go over the basics. Plenty of experience repairing CRT monitors has taught me to respect electricity and how to work on it safely. Building high gain tube guitar amps has taught me a few of the finer points, like star grounding, biasing circuit schemes, different methods to power heaters etc. However, I have never built a tube amp for Hi Fidelity audio reproduction, my work with guitar amps was quite the opposite in fact...

So let me lay out what I am trying to achieve here and I hope some of you can offer me some advice and perhaps point me in the right direction.
My friend is into high end audio, or at least to the extent he can afford. He has some nice stuff, Threshold, Hafler, McIntosh, Carver etc, kind of a bit old school but nice quality. Lately he's been eying high end tube amps but they are way out of his price range, so he called me asking about these cheap Chinese tube amps on Ebay. Quick research told me they are a crap shoot at best and I advised him to stay away, at which point I offered to try and build him something if I can find a good design that we can afford to build.

He has a component system with an active crossover, the low end is covered by a 15" JBL Pro bottom end cabinet driven by a big SS amp, he would like to build a tube amp to cover the top end. He's running a pair of Bose 901's on the top end, so he doesn't need a ton of power, 35-50 WPC would be good I'm guessing. When he moved to Hawaii he had to sacrifice some of the larger/heavier audio equipment, electricity in Hawaii is also very expensive, so I was thinking a more efficient Push-Pull design would be better for him than a Class A design. It doesn't matter much to me if we go with a stereo design or two mono-block amps.

With that in mind, I'm wondering if anyone has a suggestion for something I could build that would suit his needs? I think perhaps a P-P pair of EL34's or KT88's on each channel would do? I would want to use solid state power supply, I don't see a need to bother with a rectifier tube on a hi-fi amp? The amp will not be reproducing bass at all, so were looking for sweet balanced mids and highs with ultra low distortion and noise levels.

As long as I can source the iron I can work from just a schematic and fabricate the rest from there, but I wouldn't be opposed to following a completed and tested design, or even building a kit. I just want it to be nice quality. If anyone would be so kind as to suggest what they might do, or point me in the direction where I might find something that works for me I would appreciate it...

Sorry for being long winded...
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Old 7th July 2012, 10:27 AM   #2
Fenris is offline Fenris  United States
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Tubelab has some nice kits as starting points. Edcor iron is has a great quality/price ratio.
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Old 7th July 2012, 11:29 AM   #3
cjkpkg is offline cjkpkg  United States
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+1

Plus there are some others that come to mind...
Peter Millett has a few boards out there, the "red" board has been discussed here quite a bit and he has a single ended board too.
There are some boards available at Shannon Parks site with the ST35 being a good starting point as well as some SE boards.
Another good resource just for research and inspiration is Tom McNally's Amps

Remember, tube "power output" is not apples to apples with modern solid state or chip gear. My Tubelab SSE only puts out like 7 or 8 watts but it makes plenty of music.

John Broskie's blog has a lot of data as well and I have built 2 power amps inspired by what I saw there. A PPEL34 which I did a DIY PCB for and a PPEL84 which I wired P2P.

I really like Edcor iron but you must understand that there is a typical 5 week leadtime.
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Old 7th July 2012, 06:41 PM   #4
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Quote:
Originally Posted by cjkpkg View Post
+1

Plus there are some others that come to mind...
Peter Millett has a few boards out there, the "red" board has been discussed here quite a bit and he has a single ended board too.
There are some boards available at Shannon Parks site with the ST35 being a good starting point as well as some SE boards.
Another good resource just for research and inspiration is Tom McNally's Amps

Remember, tube "power output" is not apples to apples with modern solid state or chip gear. My Tubelab SSE only puts out like 7 or 8 watts but it makes plenty of music.

John Broskie's blog has a lot of data as well and I have built 2 power amps inspired by what I saw there. A PPEL34 which I did a DIY PCB for and a PPEL84 which I wired P2P.

I really like Edcor iron but you must understand that there is a typical 5 week leadtime.
Thanks a bunch for the feedback. I just got done looking through the Tom McNally amp page, lotta amps there, plenty of food for thought...

It seems he is mainly into SE amps, which have a power range from a couple watts up to about 7W, which is a bit light on power IMHO even considering how much more sound per watt you get from tubes. To keep things in perspective, he has a 15" JBL pro sub in a folded horn driven by a Hafler XL-600 which puts out 900W when bridged to mono. I know tube watts put out more sound but paired with that 900W sub, it's gonna need some power! LOL

Ironically, the amps I was most interested on Tom's page were the P-P kits from Triode that he built (the rest seemed to be all SE lower wattage amps), I have purchased quite a bit of stuff from Triode in the past when I was building guitar amps, so I guess I should familiarize myself with my old friends over there...

That said, I had been thinking about more of a P2P design using some terminal strips or perhaps a small board with eyelets or turrets rather than an actual circuit board. A P-P EL34 design seems to be what it may be boiling down to...

I'm going to try and search out John Broskie's blog and other builders that were reference but not directly linked above and see what kinda builds they got out there.... Still open for suggestions...

Thanks again for your input, I appreciate it very much.

Last edited by ToneMonkey2; 7th July 2012 at 07:02 PM.
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Old 7th July 2012, 07:36 PM   #5
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Its going to have to be PP to get the power you want here. Look at Allen Wrights Vacuum state PP1C for inspiration. This can be achieved on a modest budget if you don't go with the 300B version and will better almost any SE amp out there. Its mostly class A triode which in PP is a wonderful combination.

Start here Rundmaus at work - PP1C without sand

Shoog

Last edited by Shoog; 7th July 2012 at 07:39 PM.
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Old 7th July 2012, 07:51 PM   #6
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Quote:
Originally Posted by Shoog View Post
Its going to have to be PP to get the power you want here. Look at Allen Wrights Vacuum state PP1C for inspiration. This can be achieved on a modest budget if you don't go with the 300B version and will better almost any SE amp out there. Its mostly class A triode which in PP is a wonderful combination.

Shoog
Thanks, looking that one up now...

One question though... You said it's Class A AND PP, which I was under the impression were pretty much mutually exclusive? Maybe I am confused about the classifications. For Class A I was assuming that all tubes must amplify the complete audio signal (both positive and negative side of the wave), and a Class A amp is also drawing full current always, even when the sound output is low or mute. Are these bad assumptions?

Since this amp will reside in Hawaii, where it is hot and the weather is generally tough on electronics, and electricity is very expensive... I was thinking I should stay away from Class A because of its constant current consumption, which will cost a lot for electricity and also make the amp run hot all the time and therefore more likely to fail sooner. I figured a Push Pull amp would give more power output with less current draw at idle and would therefore run cheaper and cooler than a Class A amplifier? Then you said "Class A Push Pull" and got me all confused! LOL

If I am way off base here please correct me, its been a few years since I dug into tube amp design and operation theory...
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Old 7th July 2012, 07:56 PM   #7
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Hi, as suggested by others I think a good place to start would be millet's red board

DCPP Amp

I built one an it sounds terrific. Very balanced, detailed, plenty of power and since you know your way around a circuit you will be pleased to know Tubelab and others have modded the the h*** out of the board to produce in the excess of 125w of power.

The red board is very nice as basically every component is on the board itself (aside from transformers) so basically impervious to PTP induced hum etc.

Hope it helps.
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Old 7th July 2012, 08:27 PM   #8
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Quote:
Originally Posted by Alexontherocks View Post
Hi, as suggested by others I think a good place to start would be millet's red board

DCPP Amp

I built one an it sounds terrific. Very balanced, detailed, plenty of power and since you know your way around a circuit you will be pleased to know Tubelab and others have modded the the h*** out of the board to produce in the excess of 125w of power.

The red board is very nice as basically every component is on the board itself (aside from transformers) so basically impervious to PTP induced hum etc.

Hope it helps.
Well, I kinda skimmed over that one for two reasons.
First I was kinda thinking big bottle tubes in sockets that are soldered to printed circuit boards and run in the tropics is not the best idea? Seems that board is gonna get hot, and the tube sockets etc might need soldering touch up after running a while and changing out tubes... He's going through problems right now similar to this with other equipment. Unfortunately I will not be there in Hawaii to help him maintain and/or repair this amp , I was thinking an old school P2P layout would probably be more reliable for him?
Second, I didn't like the fact that it was designed to run "cheap TV tubes".
That said, after doing some reading I gather the red board only has the smaller input tube sockets mounted on board, and it can run a variety of power tubes, so my issues are pretty much non issues.... I guess I should take another look at the red board and reconsider going that route. However, I still kinda wonder if a pcb should be needed to build a mid sized mono-block amplifier?
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Old 8th July 2012, 12:14 AM   #9
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What's the budget? Do you have any particular output transformers in mind? Do you want global feedback or a no-feedback design?

I'd consider trioded KT88's; they'll do about 30W. In UL, they are good for a lot more than that.

Here are a few KT88 PP and a 6L6 PP design:

"Mullard 5-20 KT88 PP blocks!

OPUS 5.0 A Modern Mullard

6L6GC AB2 Amp

Do a google search for Triode Dick's Mono Bill (or regular Bill) as well.... http://translate.google.com/translate?hl=en&sl=nl&u=http://www.triodedick.com/&prev=/search%3Fq%3Dtriode%2Bdick%26hl%3Den%26client%3Dfi refox-a%26hs%3DMhL%26rls%3Dorg.mozilla:en-USfficial%26prmd%3Dimvnsfd&sa=X&ei=BdH4T5PcIKaC2 wW9pNXEBg&ved=0CFcQ7gEwAA

The Opus and ChrisH's amp both use mosfet followers (aka Tubelab's "Powerdrive") The Tubemack amp is an updated Mullard 5-20 circuit and the Opus is the 5-20 with mosfet followers. IIRC, these all use CCS in the tails of the LTPs. I would use SS rectification with any of these designs. Chrish's amp was developed with lots of input from Tubelab George and uses two stages of LTPs for very low noise.

If the budget allows, consider James, Hashimoto, Tango, Electraprint, etc OT's. The Edcor offerings are a great value if the budget is too tight for the above.

Last edited by boywonder; 8th July 2012 at 12:17 AM.
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Old 8th July 2012, 03:20 AM   #10
rmyauck is offline rmyauck  Canada
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Pete Millet actually has now about 3 different red boards for PP amps.

First was the transformer input one with the latest version on a red circuit board.

The second was the TV tube one with all tubes on it called the "Engineer's Amp"

The latest are mono driver boards with the output tubes outboard & you have to supply your own PS.

You can also pick a favorite out of the many Dynaco driver boards out there for 35-60 W/CH . Many of the aftermarket ST-70 ones will even drive KT-88 to KT-120's

Also look at Mark III & IV boards too.

Many of the LTP or Mullard style of Dynaco circuit boards out there now have CCS on the tail if you want that.

If you don't want any SS except the PS Diodes it looks like Triode's Boards still are all non SS
assistance.

If you don't want any SS and prefer P2P or even terminal boards, Eico's HF-50/60 look like a well liked design. There is even a mod by Dave Gillespie discussed on AK(Audiokarma) and on the Eico tube Form to change the input tube from EF86 to a 6CG7. Shannon Parks even has Mono boards for the same circuit Eico HF-87/89 which used different tubes. He calls them Eiclones. They have everything on the boards for easy foolproof wiring. I just thought I should mention the boards as they are a well liked design.

Another amp that used the Mullard was the Harman Kardon Citation V , and there are mods mentioned on the forms to sub the no longer in production 12BY7A.

Hope that helps!


Randy

Last edited by rmyauck; 8th July 2012 at 03:49 AM.
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