• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Workhorse Tube Power Amp

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Wulumay

Member
Joined 2010
Hi All,

Could someone please recommend a design for a workhorse power tube amp for me to build? My criteria:

- I have solid electronics skills, although have never worked with tubes.
- Power output not that important; will be driving medium sensitivity 8ohms.
- Ideally a tried & tested design that I can replicate without customisation.
- Relatively easy to source components.

Thanks in advance for any guidance.

Cheers, Jake
 
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Eli Duttman

Member
Joined 2004
The OP mentioned medium efficiency speakers. What is the specified sensitivity? What is the nominal impedance? What does the impedance curve look like?

The characteristics of the speakers dictate, in large part, what the associated amp "looks" like. A "flat" impedance curve will work well with a no NFB SET, provided sufficient power is available. Speakers with modest fluctuations in impedance will be OK with a design that employs NFB, as damping factor does matter. Then there are speakers, like big Thiels, whose impedance dips below nominal are so bad that they never (IMO) should be mated to tubed power amplification.

Detailed info. about the speakers the OP plans on using is essential to making good recommendations.
 
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Eli Duttman

Member
Joined 2004
FullRangeMan said:
Medium efficiency speakers is a vague term, to me is 93-96dB.
Click to expand...

Agreed. Applying Paul Joppa's 102 dB. rule to a 93 dB. sensitivity yields upwards of 8 WPC, as the requisite power. Unfortunately, manufacturer claims frequently have to be derated. Frequently, claims are made for 2.83 V. of drive. However, 2.83 V. into 4 Ω is 2 W., not 1 W. Derate the claimed sensitivity by 3 dB., for that situation. View speaker manufacturer claims with suspicion.
 
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Soldmyquads

Member
Joined 2017
Removing the feedback opens up the sound-stage tremendously and brings a warmth that is well known with the ST70. There is a sharp (very narrow) spike at 14 kHz if you look with test equipment but as far as I remember it was never reported as being heard. I noticed no distortion or degradation through a pair of vertically reconfigured Quad electrostatic speakers, Audio Research tube preamp, Oracle Delphi turntable, ET-1 tonearm, and Koetzu Onyx MC cartridge. I do not know if this modification works with other tube amps but I do know that removing the feedback cap in the ST35 sounded terrible and made a lot of weird noise.
 
pblix

pblix

Member
Joined 2011
For a first build I would focus on building something simple and to get it right, like Tubelab's SE. Even with a lot of general electronics experience there is plenty to mess up. Gradually take on more complicated builds if you catch the bug. My $.02
 
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Eli Duttman

Member
Joined 2004
Wulumay,

Please provide a link to the specifications for your speakers. As I previously stated, without an understanding of the speakers, a rational recommendation can't be made.

Also, I strongly suggest you cultivate a relationship with Ian, AKA Gingertube. He's a fellow "Aussie" and some variation on his Baby Huey theme rates to get the job done for you.
 
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