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Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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Old 9th January 2018, 09:23 PM   #1
wanders is offline wanders  United States
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Default Where to start

Over the past five years I've built a number of projects, mostly solid state. I made all the usual mistakes, but have gained skill and patience. A year ago I completed a build on a 6SN7 preamp. It was fun, I learned a lot, and I'm very happy with the outcome. I want to get started reading and thinking about building a tube amp - somewhere in the 75 - 125 wpc range. I'd like to start from a proven design/schematic and go from there. Any suggestions as to a starting/research/reading point would be welcome.
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Old 9th January 2018, 10:01 PM   #2
Alllensoncanon is offline Alllensoncanon
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Originally Posted by wanders View Post
somewhere in the 75 - 125 wpc range. I'd like to start from a proven design/schematic and go from there.
IMHO, 75-125 watts per channel is a lot of amp using tube. You properly don't want to start there until you gain some experience working with high voltage. They also typically require either massive tubes (power) and/or using multiple large tubes. It is best to deal with these designs after some experience with burning a few small things up first.

An example of a good stable quality circuit with good documentation is the Mullard 5-20. A simple search will result tons of good info for this classic beauty,
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Old 9th January 2018, 10:28 PM   #3
Eli Duttman is offline Eli Duttman  United States
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I strongly 2nd the Mullard style circuitry idea, as it's "idiot resistant". You can get into the desired power range with PP KT88s by employing mild Class "AB2" (positive control grid current) conditions. MOSFETs are DC coupled to the O/P tube control grids. Tubelab and Pete Millett show the way.

As was previously stated, this is a highly ambitious 1st power amp project and a mistake with the nearly 500 V. B+ rail could be lethal. Do you have access to mid 90s sensitive speakers? If yes, an "El Cheapo" could let you "get your feet wet", at modest cost.
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Old 9th January 2018, 11:23 PM   #4
flysig is offline flysig  United States
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A good resource to get you going is the Valve Wizard's site. How to design valve guitar amplifiers On the left side of the page is a list of links to his tutorials. He is very practical in his approach. There is some math but it is all relevant, like how to calculate the value of a capacitor or resistor to get the result you want. He stays away from the intense mathematics, which you really don't need at first.

When it comes to the practical aspects of physically building the amp, Morgan Jone's book is a good one. Online I like Aikenamps.
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Old 9th January 2018, 11:25 PM   #5
rayma is online now rayma  United States
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Quote:
Originally Posted by wanders View Post
I want to get started reading and thinking about building a tube amp - somewhere
in the 75 - 125 wpc range.
Please choose a 35-40W per channel design instead. Unless you have very inefficient speakers
with large woofers, this is the sweet spot for tubes.
http://www.tubebooks.org/Books/GEC_approach.pdf

Last edited by rayma; 9th January 2018 at 11:29 PM.
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Old 10th January 2018, 05:54 PM   #6
wanders is offline wanders  United States
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Thanks for the leads and the admonitions as well.

I have Vandersteen 2Ce sigs which are somewhat inefficient. I'm currently using a McIntosh mc 2105. The Mac is adequate, but not over powered, IMO.

As discussed, I'd like to do several things: 1) Take on a challenging project; 2) Get more involved in tube components; 3) Build a "keeper" tube amp that fits my system. I don't necessarily have to kill all three birds with the same project.

Last year I completed the Tubes4HiFi, SP14, 6SN7 preamp. I purchased only the pcb's and documentation from the designer. It took me three or four months to complete. In the process I learned quite a bit but never considered the work to be dangerous. I'm very happy with the final product and expect to keep it around for the foreseeableI future. I considered the Bob Latino 125 wpc mono block kits, but I really don't want to purchase a kit. Looking at the sales materials, I feel I could knock out both amps in a month or so. If he would sell the boards and docs separately, I'd be more interested.

I recognize that building a high powered tube amp from scratch, even from a proven design, could be problematic and perhaps dangerous. Point to point wiring can be much more challenging than working with pcbs.

I've looked a little at the Poindexter Musical Machine. It would certainly be fun, and instructive, to build, but I don't know what I would do with an 8-10 wpc amp when I finished. If I could build it for a couple of hundred bucks, it might be worthwhile. It's possible I could find something between 40-60 wpc that would be beneficial to the Vandersteens, so I'll read up and look around.

Anyway, sincre thanks for the advice.
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Old 10th January 2018, 06:09 PM   #7
aardvarkash10 is offline aardvarkash10  New Zealand
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hmmm. Spec on the speakers indicate an unhappy match to tube amps. Claimed 7 ohm nominal and 4 ohm minimum is almost certainly under ideal (almost imaginary) conditions, and its interesting theat no upper value is given. 86db sensitivity is similarly dubious and likely overstates things by 3db.

None of this would matter with a set of beefy MOSFETs providing the stick, but with a tube these things matter.

Raw power is only a part of your design criteria if the speakers are keepers - damping and impedance range will also need to be seriously considered.
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Old 10th January 2018, 07:37 PM   #8
Eli Duttman is offline Eli Duttman  United States
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Right on about the derate. You have to treat the Vandersteen speakers as 4 Ω, when they are mated to tubed circuitry and 2.83 V. into 4 Ω is 2 W., not 1 W. Applying Paul Joppa's 102 dB. rule to the "true" 83 dB. sensitivity yields a power requirement that's upwards of 64 WPC.

Hopefully, Vandersteen's complex crossover includes 1 or more Zobel networks to stabilize the impedance presented to the driving circuitry.

Quote:
I've looked a little at the Poindexter Musical Machine. It would certainly be fun, and instructive, to build, but I don't know what I would do with an 8-10 wpc amp when I finished. If I could build it for a couple of hundred bucks, it might be worthwhile.
Poinz's setup is an assault on state of the art (SOTA) and is quite costly to construct. Look closely and you will find a strong family resemblance between "Musical Machine" and "El Cheapo". A few $100 does build an "El Cheapo" that plays quite a bit better than its cost suggests. Wire the "finals" ultra-linear (UL) and 12 WPC is a reasonable expectation.
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Old 10th January 2018, 08:01 PM   #9
wanders is offline wanders  United States
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Quote:
Originally Posted by Eli Duttman View Post

Poinz's setup is an assault on state of the art (SOTA) and is quite costly to construct. Look closely and you will find a strong family resemblance between "Musical Machine" and "El Cheapo". A few $100 does build an "El Cheapo" that plays quite a bit better than its cost suggests. Wire the "finals" ultra-linear (UL) and 12 WPC is a reasonable expectation.
Thanks, Eli. I'm digging around on the El Cheapo.
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Old 11th January 2018, 05:32 AM   #10
aardvarkash10 is offline aardvarkash10  New Zealand
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The Engineer Amp on steroids could fit the bill. Search for Tubelab's many postings on torturing over 100w out of this setup from Pete Millett
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