Scott 299B Restore

[Dandy]

Back your post #15, even though those two black tubular caps marked Pyramid see no voltage, I wonder about how well they age? Maybe replace? I think the Ceracaps that see no significant voltage are more reliable--but my experience is limited to the ones I took out of my Scott.

 

[Dave451]

Hey, Dandy. Well, I'm wondering too, because I've heard the Pyramid "black beauties" are worse for failure than the Ceracaps. These are 0.0012 uF 400 VDC caps. I have 0.0010 and 0.0015's handy. Can I use either of these if I decide to replace, or do I need to use exactly 0.0012? I hate to 2nd guess the engineers! No balance or sound issues at all with these Pyramids in place.

Here's another question for you; a bit of a mystery at least to me. I put the amp on the scope and was testing the old tubes after balancing and biasing them and one set had a bad one in it. I looked at the tubes and had 3 definite 6BQ5's and one "different" that might have been an EL84 or even a 7189 (markings gone). The odd tube was the bad one. So, I decided to put in a new matched quad of Sovtek EL84M's, along with two NOS 6BL8 PI/Driver tubes. Well, I couldn't get the Sovtek's to bias to full cathode current, even with the pots turned all the way CCW. The spec is 55 mA for each pair, and I was shooting for 51 mA. I could get one pair to 51 mA cathode current barely, and the other I couldn't get much past 40 mA. I had reset the DC balance for both pairs.

I tested output to clipping on the Sovtek's and still got 18.5 watts in each channel--pretty much full output for this amp. I decided to put in some known old tubes to see what happened and installed a pair of the 6BQ5's and a pair of tested good Sylvania black plate 7189's I had on hand. No problem biasing either pair at all! Went easily to 51 mA with plenty of room on the pots, then power tested at 18 WRMS for the 6BQ5's and 19 WRMS for the 7189's.

So what gives with the Sovteks? Do I need to let them burn in a while? The sound check on the Sovteks sounded fine to me. Can I run them this way, since I'm getting full output power? Appreciate any thoughts from you or anyone else watching. Dave

 

[Dandy]

Very puzzling. Sorry, I can't think of an explanation.

 

[Dave451]

Well, I figure it must be the tubes and these ones are pretty "cold." The vendor graciously agreed to exchange for a new quad, so that is in progress now. Work will proceed with the "veterans" for now.

 

[Dandy]

Good to hear of a helpful vendor.

 

[Dave451]

Speaking of tubes....noticed some Mullard action while working on the amp. One of the 'veteran' output tubes I put in the amp from my pulled tubes was a Realistic Lifetime 7189A. When I got it, I thought 'whaaaat...?' Looking at it closely, it appears to be a Mullard 'smoky' 7189A. It's indeed labelled 'Great Britain' and seems to have all the physical attributes of a Mullard 7189. The code markings are obscured at the bottom, so that's the best I can do identifying it. Thoughts?

The second tube is the GE 5AR4/GZ34 rectifier. It is marked with GE codes "FM" and "188-5," which some internet poking around indicates the GE Owensboro plant and a production date of March, 1978 ("FM"). The tube is also labeled "Great Britain" and a closer look shows tube codes 'f32' and 'B6C1'. It appears from this that this is a Mullard 'copper edge' GZ34 and, if I am reading correctly, the code indicates B Blackburn Plant, 6 is 1966, and C1 is month of March, first week in March. So, maybe GE had some NOS around at Owensboro and relabeled it for sale in 1978? 'f32' is apparently a change code, but I'm not sure what it signifiies for this tube. Anyone know?

 

[Dandy]

I bet the guys who came up with the 'Lifetime' slogan would be amazed to learn that their tubes really were doing well after all this time.

 

[Dave451]

I've read some funny stuff about guys who'll show up with one or more of these at the local RS and the responses they get from staff who've never even heard of tubes before. I don't think this one will be going back soon. Getting solid output and good sound from this paired with a Sylvania black plate (the other half of that pair seems to have died, unfortunately--no 'lifetime warranty' on that one, I guess!).

 

[Dave451]

So, I gathered up my courage and removed the power and output transformers for paint work. The bells have rusty spots and the laminates have gotten rusty. I labeled the wires and carefully de-soldered the fabric-covered wires. Now on to the 'paint shop' and cleaning up the chassis.

 

[Shadowdog]

Be careful with the laminates as you don't want to sand them ( can short them together) , but just remove loose rust with a bit of light scrapping and paint.

 

[Dave451]

Thanks for pointing this out. I was thinking about light sanding but now it's Plan B, light scraping and painting! Wouldn't want to ruin some nice iron. As you can see, a little rusty.

 

[Dandy]

Be careful with the laminates as you don't want to sand them ( can short them together) , but just remove loose rust with a bit of light scrapping and paint.

If sanding can cause shorting, wouldn't scraping risk the same? I recently read a thread where a guy asked Hammond if it was okay to sand the sides of a transformer lightly, and the answer was yes.

 

[Dave451]

Did a little reading and all seem to agree that you don't want to do anything too abrasive. Best solution I saw was to brush away loose rust with a toothbrush and then leave it or paint over it (I'll do the latter). Thanks all.

 

[drtool]

I use scotch brite, lightly. I have posted a few pics of tranies I have painted.

 

[dcgillespie]

Dave -- on the .0012 uF caps, consider just removing them and installing a shorting jumper in their place. They are part of a LF filter network that was relevant back in the day when changers, rumble, and record warp were all real possibilities. While record warp will always be around, for the vast majority of folks using these units today, the filter is an anachronism (particularly with enthusiast type installations), and also reduces LF performance with today's highly dynamic digital sources. Even without DC present on them, at least the Ceracaps can go internally intermittent just due to 50+ years of operating cycles, vibration, and lead stress. Shorting the caps out eliminates all these concerns, and gives the amp greater capability in dealing with today's wider range sources.

Good luck with your rebuild!

Dave

 

[Dave451]

Thanks, Dave. These two are Pyramids, which makes me even a little more gun shy (they were ceramic discs in the last 299B I did!). I'll be driving the amp almost exclusively with line sources, but I do want to preserve the ability to use the phono function. In any case, shorting them out is an intriguing idea. I'll give it a try and see how she sounds when I get things back together. I did finally locate a source of 0.0012 uF caps and they are on the way, but I am intrigued by your advice. It's getting harder, it seems, to find some of the old common values like 0.0012 or 0.0027 in audio-worthy caps. In the meantime, got the chassis clean and on to the 'paint shop' for the transformers!

 

[Dave451]

drtool--please send me a link or two of your work. Browsed your posts but not able to find from the titles. Thanks!

 

[Dandy]

That chassis looks immaculate! And great to have Dave G's advice on those small Pyramid caps. It would be good to have that LF network disabled.

 

[drtool]

Finished painting Eico HF 20 tranys.

 

[Dave451]

Those are awesome! Mine are much more humble. Going for a satin black finish like the originals with main objective to get rid of the rust. I do plan to put them in a cabinet, but I want everything to look nice anyway. The tube shields were very rusty, so I painted them too. Pictures tomorrow! Thanks for the link.