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|24th June 2018, 06:16 PM||#1|
Join Date: Mar 2005
Location: Marietta/Moultrie GA
FS: Vintage 1960s homebuilt Dual-Mono Mullard 5-20 amp (EL34/Acrosound TO-300 OPTs)
This amp is pretty special.
It's a classic Mullard 5-20 design, built in the 1960s. It's done pretty much by the book, except for a few upgrades which allow more precise calibration, and it has some exceptionally excellent parts choices.
Here's the main features:
- Dual-mono construction (two of everything- transformers, chokes, etc)
- Everything built in mirror image, in both power supply chassis and output chassis
- Oil-filled power supply caps
- Output chassis is aluminum (hum and noise reduction compared to steel)
- Preamp/driver circuitry done on tag boards (very professional)
- All grounds on central bus (basically star grounded)
- Input gain pots
- AC balance pots on phase inverters
- DC balance pots on output tubes
- Adjustable bias resistor on output tubes per channel
- Heater hum pots
- Adjustable B+ voltage (tapped resistors in power supply)
- Teflon insulated wire
It's also built with all the RIGHT parts. Here's what's in it:
- Stancor (Chicago Standard) PC-8412 400-0-400V power transformers
- RCA 5V4G/GZ32 rectifier tubes
- Thordarson 20C56 8H 200ma chokes
- Acrosound TO-300 Ultralinear output transformers
- Mullard old-stock EL34 output tubes
- RCA 12AX7A phase inverter tubes
- Amperex mesh-plate EF86 tubes
- Sprague Atom 100uf 100v cathode bias caps
- Pyramid oil-filled coupling caps throughout
As you can see, this one was built to the nines, by a real craftsman. The layout is pretty much flawless from a technical standpoint- from quiet to noisy as you go from one end of the output chassis to the other, and the power supply is the same.
This amp has been checked out, and parts that were out-of-spec were changed, only as needed. This required replacing all the electrolytic caps (they had excessive DCR), and two of the output tubes, but everything else is as-built,. and in-spec.
The amp sounds wonderful (very smooth, detailed sound with great imaging and very powerful, controlled bass that goes deep), and it's virtually noise-free (no hum at all- and you literally have to stand with your ear in the tweeter of most speakers, to hear ANY hiss or other noise at all from this amp- and then only if you're in a completely quiet room with no surrounding noise AT ALL).
This is what I would call an "heirloom quality" build- something that would be worth passing down to future generations. An example of "Can-do" spirit, and an exceptionally performing piece of audio equipment as well.
Selling for $800.00 plus shipping. US shipping only (no international shipping).
Speaker Design, Restoration and Repair- since 1985.
Last edited by GordonW; 27th June 2018 at 12:52 AM.
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