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Old 29th April 2011, 03:08 PM   #1
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Default Building my 1st tube amp

I own a pair of Thiel CS1.2 speakers which have a sensitivity of 87dB. I would like to build 2 tube mono blocks to drive them, and am looking for advice on the following:

1) How much power is recommended to adequately run those speakers? From what I've read, I will need a minimum of 50W per channel. Does this sound right?
2) Which amp should I build? Are schematics and parts easily available? What do you recommend?
3) I have good soldering skills, a basic understanding of electronics, plenty of very good tools. Is this enough, or am I biting more than I can chew for a 1st build of this nature?
4) My budget is about $300/amp, is this reasonable, or am I way out of line thinking I can do that on this "limited" budget?
5) What am I missing here?

Thanks in advance.
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Old 29th April 2011, 04:59 PM   #2
ryuji is offline ryuji  United States
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50 watts would certainly be push-pull, unless you want to parallel tubes in SE

two GU-50's in pentode mode single ended could handle it i suppose.
if you dont know about the GU-50, heres a datasheet for 99% identical german tube
there inexpensive on ebay. your going to wait a while for them to arrive from russia tho, takes about a month for shipments from russia for me.
i got a real russian datasheet/curves floating around my hard drive somewhere if you want it.

plenty of projects here for push-pull and single ended KT88 and 6L6 you can use the designs for.

for your budget, your iron is going to be your most expensive item. see what you can afford/locate for power and output transformers. Otherwise, more information is needed on what you want. pentodes or triodes, SE or PP

Last edited by ryuji; 29th April 2011 at 05:02 PM.
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Old 29th April 2011, 05:48 PM   #3
jrenkin is offline jrenkin  United States
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I really don't think you need quite all that power. I suppose it depends on your listening volumes and how much headroom you think you need.

25W to 40W will be nice.

I think you would be fine with PP EL34 or KT-88
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Old 29th April 2011, 06:22 PM   #4
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300$ isn't that much for one mono block. You're going the best way with some standard EL34 PP circuit with common available 3,4K output transformer. Good thing is you need no choke. EL34, ECC's are cheap and always available.
Just do some calc:
Pair EL34 - 30$
Two ECC8X - 20$
2x9pin + 2x8pin sockets: 12$
soldier points, wire, components and you we reach easily 100$ !
And still no transformers and chassis !
I'm not familiar with american transformer prices. But lets say Power: 70$ ; and OPT: 80$
Means 50$ left for the chassis which is very optimistic.
Just my opinion.
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Old 29th April 2011, 06:25 PM   #5
ryuji is offline ryuji  United States
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edcor and antek are usa suppliers. my 400 VA transformers from antek were $50 for me, and my 10w output transformers for SE from edcor were $26 each. they get expensive fast tho as you go up in power $80 is probably about right
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Old 30th April 2011, 02:40 AM   #6
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Perhaps biting off a bit more, but maybe not. $300 per monoblock is a little optimistic, especially if you are considering KT88's, which is a good way to get 60w/channel out of a pair of tubes.

EL34's in UL PP are good for 30W or so; your power needs really depends on your listening preferences and room size. It can be surprising how little power you need.

It appears that your speakers are a relatively flat 6 ohms or so impedance....

Take a look at the soloist, UL KT88 PP based on the classic Mullard 5-20 design with a few modern improvements. (a long thread....)

"Mullard 5-20 KT88 PP blocks!

For a 30W version of the classic mullard circuit, check out Claude Byrith's El34 UL PP:

Edcor transformers are a good idea and a great value.

Last edited by boywonder; 30th April 2011 at 02:46 AM.
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Old 30th April 2011, 02:48 AM   #7
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What am I missing here?
The impedance curve of the speakers! Thiel speakers have large dips in the impedance curve. The big models dip down to 1 Ω. That behavior makes Thiel speakers POOR candidates as mates for tubed power amplification. It's a matter of damping factor. A "McShaned" Harman/Kardon Citation 2 stereoblock will work with your "small" Thiels. However, that is a $2000+ project and your budget is $600.

Build a nice tubed line stage, say 12B4 based, and mate it with the best SS power amplification your bank balance will tolerate.
Eli D.
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Old 30th April 2011, 03:16 AM   #8
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Originally Posted by Eli Duttman View Post
The impedance curve of the speakers!
Eli: Is this too lumpy? It was posted by a (ahem...) popular magazine. Looks like 6ohms more or less. I don't see any 2 ohm dips....
Attached Images
File Type: jpg Thiel CS1.2.jpg (26.8 KB, 805 views)
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Old 30th April 2011, 05:06 AM   #9
TheGimp is offline TheGimp  United States
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87dB/W-M is a bit low but not really bad. My 96dB/W-M speakers will play at reasonable level with 32mW average and 1W peak.

The difference between 87 and 96 is 9.

9/3=3, 2^3=8, so 256mW average and 8W peak for normal listening levels isn't all that bad.

Figuring that the meter was c weighted you will need a bit more so a 30W amp is probably sufficient.
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Old 30th April 2011, 07:28 AM   #10
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I'm with Eli.

@ $300 a bloc, I'd do it this way - tube front end as a buffer and then LM 3xxx or other chip amp for output.

viola - tube looks (and sound-ish), 50w output, oddball speaker impedance tolerant, on budget.
"It may not be easy for some to not hear differences, even if they are not there." - Vacuphile,
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