Nattawut Uamkruea, blogger and audio gear maker who passionate in vacuum tube, audio & music.
Producing electronics projects since high school. Seriously upgrade and modify vintage audio equipment for “Real Sound” concept.
He is a co-founder of Soundaries and Sentimental Studio, all audio solution provider.
Experienced and lively explore the listening rooms & audio shops in Japan as his most favorite to do.

Saturday, October 31, 2015

Bogen Sound Systems E-75 Power Amplifier

Last month, my friend told me that he just bought this integrated amplifier from ebay

and asked me for bring it back to working state.

So I've bought schematic from "Radiomuseum" website and surprised with design of this amplifier.

Bogen E-75 built under patents owned and controlled of Western Electric Company.

It was 2 channels amplifier but seem like dual mono in same enclosure,

1 main power transformer, 1 filter choke for main rectifier, 1 filter choke for negative voltage,

1 output transformer and 1 interstage transformer, total 10 for 2 channels !!!

Great for apply to use as stereo amplifier.

These are tubes that included with this amplifier from ebay seller, almost weak !

Layout look symmetry and right positioning.

Some electrolytic capacitors were changed but the rest weak and need to change too.

Coupling capacitors had cracked and still existing.

After analyze schematic, I've decided to cut microphone pre-amplifier stage from the circuit

and add RCA jack directly to grid of 6SL7 in first stage.

Replace all original electrolytic capacitors with new F&T from Germany.

And use Sprague "Black Beauty" in coupling position.

Carbon composition resistor and cloth wires still exist to make nice sound. 

New tubes for Bogen E-75,

GE 5691, RCA 6F6G, Sylvania 6L6G and Westinghouse 5X4G.

7Z4 tubes for negative voltage rectifier have good test result and continue using.

Original tubes like 6SF5 and 6SL7 are only used for fill sockets holes. ;)

Sound very nice with many kind of music in the first round, low hiss & hum.

Surprised me and my friend so much.

Worth to burn-in tubes and continue observe result after 100 hours again.

Interstage transformer seldom seen in commercial tube amplifiers of same age.

This might be cause different sound from others.

Got many ideas to improve mine. ;)

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