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. ;)

Altec 9476A Monitor Amplifier

I've got this pair of Altec 9476A from ebay without amplifier boards and metal covers.

But other components are valuable to rebuild.

See link below for complete operating instruction.

http://www.greatplainsaudio.com/vintage_altec/9476A_monitoramp_oper_inst.pdf

This is 8 watt monitor amplifier @ 4 ohm unbalanced or 150/600 ohm balanced load.

Use external module power supply (Altec 9550A) for two 9476A mounting on the tray.

Original schematic use 2N3055 power transistors operate in push-pull

and coupling with a huge value capacitor (5,000 uF) for 4 ohm load.

Really worth for money are input and output transformers,

Peerless 4814 (10k : 60k) input transformer

and Peerless 16688 (4 : 150/600) output transformer.

Output transformer can reverse to 600 : 4 ohm, easier to find coupling capacitor.

Pair of Altec 9476A amplifiers.

Coupling capacitor and filter capacitor are same value. ;)

Missing amplifier board at the slot.

RCA 2SK3027 replace 2N3055 ?

Bottom side of amplifier.

Peerless 4814 input transformer with complete audio frequency response.

Peerless 16688 output transformer, larger than spec !

I think build tube amplifier with these transformers in original casing will more interesting

than find an amplifier board or make a new solid state amplifier.

Thyratron tube with "Birdcage" grid

Just heard the song from my new single ended amplifier.

http://mellowgroovy.blogspot.com/2015/08/WEamplifierPart1.html

And blue glow from mercury vapor so stunning me every time to see.

Will share detail soon after finish drawing schematic.

Well-built German Style Signal Transformers

I  have got these transformers for a long time and already test them with alligator clips.

Their sound present dynamic, detail and sweet too.

Many of German transformers have well-built and use mu-metal shielding 

for reduce noise and hum from environment.

So worth to put them into permanent enclosures for easier using with the system.

First is VEB Tonmechanik Berlin isolation transformer for isolate 

between CD/DAC and preamplifier.

Impedance 600 : 600 with no gain but get more dynamic and larger scale of mid-range.

Too heavy with thick metal shield cause lower noise floor.

Second is Telefunken input transformer that brought out from my Telefunken EL156 amplifier.

This transformer contain 2 channels inside, easy to get from single amplifier.

Application for amplify microphone, so can use with low output mc cartridge too.

Step up ratio is 1 : 30. If use with Denon 103 family and others in same output range,

it will be louder for some systems.

But if tune load resistor at input of mm phono preamplifier, 

you will hear a lot more fun with the dynamic as I do.

Assembly with Western Electric cloth wires.

Using wires as short as possible.

Finished and ready to join the system.