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Old 8th November 2012, 06:25 PM   #1
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Default My first tube power amp

Hello,

I'm about to start building my first tube power amplifier, and I still have some questions on my mind.

Any advice is welcome!

I already got a Audio Research LS8 tube pre-amp now driving a Rotel RB1080 2x200W/pc transistor amplifier but its time to get tubes in place.

What I want:
  • About 75/90 W/pc into 8Ohm.
  • Push Pull design, no single end Class A (2x KT88/120 per channel?)
  • E88CC/6922 input tubes.
  • None or minimal negative feedback.
  • No fancy tube like coloring sound, just amplifying the original signal as good as possible.

I want to build similar to the McIntosh 275, its a 60's design still very popular.

The filaments are powered directly from the transformer (AC) Isn't it better to have this DC? by placing some LM317 regulators instead?
LM317 is 12.6V powering 2 6.3V tubes in series.
I know running the filaments on DC wil decrease lifespan but its less prone to create hum or noises from the transformer perhaps.

Some other questions:
  1. Rectifier: Tube or Diode's? And what would be the best Tube or Diode.
  2. Transformer: Torodial vs laminated?
  3. Power tubes: KT-88 or go for a KT-120 design?

Please let me know if you have any recommended schematics, or any tips and ideas.


Schematics of the McIntosh
Mcintosh mc275 schematics.jpg
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Old 8th November 2012, 07:04 PM   #2
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A few answers follow in no particular order.

Use KT88s! If you look at the KT120 data sheet, you'll see FUGLY curves indicative of poor inherent linearity. Those tubes have to be bludgeoned with NFB.

Use high PIV Schottky diodes for B+ rectification. They are as quiet as vacuum rectifiers, while yielding bass performance all but impossible to obtain using "hollow state" for the B+ job.

A toroidal power trafo is fine, provided you install EMI/RFI filtration between it and the power mains.

AC heating is fine in a power amp. At most, DC heat the I/P stage

NFB is always necessary, when beam power tetrodes are employed as "finals".

"Unity coupled" O/P "iron", like that used by McIntosh, is VERY expensive. JMO, stick to good quality stuff with ultra-linear (UL) taps.

To employ 6922s at the circuitry's I/P, set up cascodes to do the voltage amplifying (regulated B+ needed here). One section of an ECC99 will do well as a "concertina" phase splitter. Use system 1 in channel "A" and system 2 in channel "B". When the triodes show signs of wear, exchange the bottles to get doubled service life. The topology suggested follows the lead of Scott, Fisher, and Dyna designs known to be quite satisfactory. Dig up a schematic of the Dyna MK3 amp, for a look at the generalities.
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Old 8th November 2012, 07:39 PM   #3
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You won't like this. If this is your first build, definitely stay away (far away) from McIntosh! It's a very complicated design that uses a very special transformer and a lot of feedback. At lease three feedback paths.

DC filaments are good for preamps but is not needed in a power amp because the signal levels are high enough that hum is seldem a problem. Perhaps DC on the first tube only makes sense.

Tube or solid state rectification is designers choice. Both will work well and each has their advantages and faults. SS will get you a tiny bit more bass if you are a bass freak. But tube rectification can be bolstered with large filter capacitors when done correctly.

I've never used KT120s because I'm old school and have plenty of vintage KT88s and 6550s. To get 90 watts from a pair, you will need 600 volts for the plates (and screens if UL). I would not put that much voltage on todays made tubes because they just can't take it like the old tubes could. So you will need push-pull parallel outputs with less then 600 volts.

Again, being old school, I like big iron transformers of which I have many. But I suppose tororids would work just as well if properly used.
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Old 8th November 2012, 08:29 PM   #4
tubemax is offline tubemax  Netherlands
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< No fancy tube like coloring sound, just amplifying the original signal as good as possible. >

Hi, maybe you should consider strait away building an OTL amp and get this kind of sound, for example Transcendent Sound > The Beast model...

OTL amps have this kind of sound as you describe in your wish list...
My experience with OTL amps are very positive ( in my case with 6AS7 triode tubes ).
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Old 8th November 2012, 08:37 PM   #5
TheGimp is offline TheGimp  United States
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75/90W ?

What are you powering? What speakers?

How loud do you want to play it?
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Old 8th November 2012, 09:12 PM   #6
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by tubemax View Post
..... for example Transcendent Sound > The Beast model...
I see two new smaller beastie sisters, nice
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Old 8th November 2012, 10:25 PM   #7
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Why build a copy, or near copy of the Mac??? It's been done already......the complexity & parts count will get you frustrated in very short order. BTW the Iron is going to sink this ship right off..........when you hear how much $$$ it's going to be.
If you can dial back your power requirements/target there are plenty of designs out there that will make you happy. The use of toroids can free you up on case design options as they are smaller & are of different dimensions than plain old EI Txs'.


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Old 8th November 2012, 10:48 PM   #8
tim614 is offline tim614  United States
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pp KT88, IT phase splitter
My first pp attempt, sound very nice and very simple to accomplish, should be enough wattage that you're looking for
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Old 8th November 2012, 10:57 PM   #9
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Quote:
Originally Posted by Osirison View Post
  • None or minimal negative feedback.
  • No fancy tube like coloring sound, just amplifying the original signal as good as possible.
The cheapest and easiest option for the Mcintosh idea is to find a Mcintosh and buy it!

You need negative feedback, but not around the transformer.
I'd say about 40W should be about right, and is a good ballpark for tubes.

You biggest challenge will get sourcing the iron, decent output tubes are the GU50 type, ECC88 is fine for input but my preference for the main gain stage is 6n2p in SRPP or similar.
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Old 8th November 2012, 10:58 PM   #10
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Let's see, 75-90W, using KT88's, without NFB.
That would mean triode mode since 'non-coloured sound' in pentode or UL would require NFB. To get the power you want, you would need 4 pairs of KT88 per channel, 16 KT88's in total...
Do you really need all that power to drive your speakers?
As a first project, I'd start with a PP KT88, around 420V to get 50W in UL, 25W in triode.
By the way: why did you choose these input valves? There are so many to choose from
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