Sansui 1000A question

[akadrhouse]

I recently acquired a very nice looking Sansui 1000a.




It does look pretty much original, so I will be replacing all the Suzuki Oil Caps eventually and going through it. The guy I bought it from has used it as a daily driver for two years, so I did take the chance and turned it on to give it a listen. I cleaned out all the pots first to get rid of static.

My question is this:
The bass sounds fantastic when first turned on, and over the course of ~45 minutes it thins out. Is this likely some of the tubes?

This is my first piece of tube gear. The main output tubes are 7591A GE Westinghouse. I "think" that is stock but not sure.

Second question(s): Source for tubes? Looks like I may need to acquire a matched quad? Which tubes would be closest to the original sound?

Thanks in advance.

 

[akadrhouse]

Underside Pic

 

[tom3]

I'd take a close look at all tubes when good and warm, sounding a bit thin. Look for a reddish look on the metal shields around the glowing filaments. Red plating they call it. Could be a sign of impending problems that'll need fixing quick. Nice looking receiver and real quality unit. Maybe the last gasp of tube receivers.

 

[Mr Boochie]

Those electrolytic capacitor cans are prolly starting to or are completely dried-out. When the cans get warm, they lose their filtering and reserve properties. That maybe what is happening to your bass, as the unit gets warm.

You will most likely need to replace all the electrolytic caps. Particularly, in the power supply, bias supply (if the unit has fixed bias), then each bypass cap. Do you have access to a tube tester?

It's a worthy unit to restore properly. Classic amp of that era.

 

[AE Bob]

It could be that one or more output tube grid coupling capacitors are developing leakage, removing the required negative bias from the tube(s). It will cause a much higher steady load on the power supply, This can cause the symptom you describe. It will also damage the output tube(s) rather quickly if you continue operating it that way, as the current through the tube(s) goes up dramatically, even enough to make the plate of the tube begin to glow red.

 

[akadrhouse]

Thanks for the comments so far. No red-plating or flare from any tubes. Only a nice little glow as shown here in pic: This was after ~ 1 hour and the receiver was quite warm with regards to heat coming off.

 

[GregBilodeau]

No matter how good you think it's working, at the very least, replace the coupling caps to the output tubes before applying power again. As is you risk destroying a set of 7591 tubes as well as transformers. I can pretty much promise you they are leaky and causing harm to the unit. I have seen the damage many times and it's not worth the trouble a couple dollars worth of caps can save you. I have a Sausui AU70 that the original owner thought was working perfectly until both output transformers were destroyed. A few dollars worth of caps would have saved those original transformers as well as the set of output tubes.

Gregb

 

[akadrhouse]

Interestingly, it looks like 3 of those caps were replaced in the past, but not the 4th. 3 red nichicon .33/630

 

[andy]

The remaining leaky caps are probably getting more leaky as they warm up. Don't bother trying to make sense of symptoms until all the rest of the leaky coupling caps have been replaced.

 

[Catmanboo]

Stock 7591's were Toshiba, fwiw. Still have 3 out of the 4 for mine. Somehow one got broken just sitting in the bundle of 4 rubber-banded together.

 

[gadget73]

Those Suzuki caps were notorious for failing. I'd honestly give re-capping a serious consideration before buying tubes. Even if that doesn't fix it, good caps will ensure your new tubes don't get killed from bad caps.

 

[akadrhouse]

Yes I'm going to recap everything. I'm having a hard time figuring out what to replace the different kinds of caps with.

 

[gadget73]

keep reading, you'll be more confused

I'm a basic yellow film cap kinda guy. Illinois MPW. They have leads coming out the same ends as the old ones, they aren't big, not expensive, and they work fine. if its an electrolytic, it usually just depends what the form factor is. Axial caps you don't get that many options in.

@larryderouin has done a couple of these. He might be able to offer specific advice.

 

[egellings]

Does any know what those two black circular plugs near the tuning flywheel are for? My 1000A does not have that large white cylindrical resistor near the IF section of the receiver.

 

[BuzzK]

As others have said, plan on replacing all of the gray coupling caps. Every single one.

Replace the electrolytic cans, too.

Then check for out-of-spec resistors -- the high value ones may be VERY high.

I would install 10 ohm resistors on the 7591 cathodes and 100 ohm screen grid resistors too.

It's a very nice piece and worth the [considerable] time it takes to refurbish.

 

[akadrhouse]

Does any know what those two black circular plugs near the tuning flywheel are for? My 1000A does not have that large white cylindrical resistor near the IF section of the receiver.

I took off one. Looks like an octal plug facing down. But I don't see what would plug into it from below. I thought the big white thing was an antenna. But no idea tbh.

 

[gadget73]

If the owner's manual is to be believed, they are for optional step-up transformers for low output phono cartridges. See page 6

http://akdatabase.org/AKview/albums...ngular 4 Voltage Selector Manual# 36050M5.pdf


"When the cartridge output is less than 2mv, employ input transformers (we recommend type Sansui A-604) into the amplifier and connect to "phono 1" terminals on the back of the amplifier". It has a pic of what looks like trafo cans plugged into that spot.

 

[larryderouin]

+1 Gadget. LOMC cartridge step up transformer. I've had mine over 11 years now and have never seen a A-604 in the wild (or on E-PREY).

 

[egellings]

Thanks for the clarification on this.