Sansui AU-70 DIY

[MrIgotNomoney]

Guys
Going thru my stuff I have a set of output transformers for a Sansui AU70.
found a 250 mill 385-0-385 power transformer also.
This amp used 7189 tubes as outputs and 6AN8 as pre/driver, Im figuring I can use Russian output tubes, 7189 are pricey and scarce.
Fixed bias on each output tube.
Separate screen supply
Really neat cathode coupled output stage, Im thinking maybe a 5AR4 rectifier feeding a choke input power supply.
Any comments ??????


bob

 

[LinuxGuru]

I'm not sure how you will use CFB (cathode feedback) with cathode BIAS. You can simply take AU70 schmeatic and assemble it as is on your own chassis.

 

[PakProtector]

hey-Hey!!!,
Those CFB outputs should be fine. IIRC they've got the cathode part split so an individual tube cathode bias can be used( allow me to suggest a Blumlein Garter arrangement; it can balance nearly anything ). Fixed bias would be my first choice though. A long-tail-pair inverter/front end stage would also work better than the 6AN8 though it has a different set of things to get just 'right'.

I had one of those amps, and regret sending it to another home.
cheers,
Douglas

 

[jaymanaa]

Let me second the garter circuit that Pak introduced to me a couple years ago. To test it, they actually doubled up tubes on one side and used a single on the other. It came out amazingly close to perfectly balanced. There's some rumors that he was the father of modern stereo as well.

 

[MrIgotNomoney]

Hello
Im looking at the original circuit from Sansui.They used fixed bias and also cathode coupling using the secondary windings of the output transformer to earth the cathodes of the output tubes.

bob

 

[radiotubes]

Hey Mr.! The Sansui AU-111 integrated 6L6 amp uses the same arrangement on the output transformer as your little AU-70 does with surprisingly pleasing results. The only issue I have with that particular design was in adjusting the bias circuit. The cathode winding's DC resistance was different for each tube. I ended up adding a 1 Ohm precision resistor in the cathode winding to give a measurement point for setting the bias more accurately. I would personally stay away from a pentode front end and stick with all triodes. Gain stage ~>direct coupled to a split load phase inverter ~> gain stage ~> output is a tried tested and true design. I'll be interested to see and hear your results!

 

[PakProtector]

I would personally stay away from a pentode front end and stick with all triodes. Gain stage ~>direct coupled to a split load phase inverter ~> gain stage ~> output is a tried tested and true design. I'll be interested to see and hear your results!

hey-Hey!!!,
That topology gives less than great results; the pentode input stage allows very low plate voltages in an increasing grid signal and this a suitable mu-ed triode cannot hope to match. Try a pentode LTP and make it a two-stage amp.
cheers,
Douglas

 

[MrIgotNomoney]

Guys,
I was also thinking a diff amp feeding another diff amp, Possibly 6DJ8 to 5687 both running constant current.
I saw the circuit in a book called Tu-be or not Tu-be published by Audio Dimensions back in the late 70's. I actually had the book, but cant find it for the life of me..
I always wanted to try their circuit. Another thought has been the driver circuit from a Dyna ST35 or possibly a Leak Stereo 20, both all triode.

bob

 

[MrIgotNomoney]

Guys Im thinking about this circuit.
I like the input layout of this, It is Mcintosh MA230.
bob

 

[henniekuyper]

HI Mr....

Have you constructed that circuit?

Hennie

 

[sachind4u]

Hello,

I have Sansui AU70 Tube amplifier, one of the channel output transformer gone bad. When I gave it for rewinding the technician made mistake with count and now the re-winded transformer doesn't sound good (very low on volume).

I need help for winding data of output Transformer if any one has, don't want to risk on 2nd channel working transformer opening.

Please help me restore this old Sansui beauty.

Thanks,

Sach

 

[primosounds]

Have you tried contacting Hashimoto transformers in Japan? If i am not mistaken they were the original transformer producer for Sansui. They are still making extremely high quality transformers, but they ain't cheap.