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Scott 99D Mono integrated: Restore or part-out

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Posted on December 23, 2020 at 16:05:59
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 I have a pair of these mono integrated amplifiers that have been damaged (cosmetics only) in shipment to me. I shall appreciate input on the question of trying to restore them, or part them out. I understand these amps sound very good when functioning properly, and I could reuse the transformers in a DIY stereo amp, but since there is a dwindling supply of Scott vintage gear I hesitate to pull them apart.

 

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RE: Scott 99D Mono integrated: Restore or part-out, posted on December 23, 2020 at 17:22:18
Uncle Mike
 They may not be pretty but will sure sound good. Restore them.

 

RE: Scott 99D Mono integrated: Restore or part-out, posted on December 23, 2020 at 19:11:10
bare
Audiophile

Posts: 1879
Joined: April 14, 2009
 Ohh just Bin it /them !
They are outdated /obsolete and apparently now Broken.. junk.
You took a chance and you've clearly lost.
Let it go.

 

RE: Scott 99D Mono integrated: Restore or part-out, posted on December 23, 2020 at 21:11:20
6bq5
Audiophile

Posts: 4385
Location: SF Bay
Joined: August 16, 2001
 If you have the time and patience, get them running in the best way - and don't worry about the cosmetics
You will enjoy the music-
Happy Listening

 

RE: Scott 99D Mono integrated: Restore or part-out, posted on December 24, 2020 at 07:20:30
Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008
 Maybe it's an opportunity to build something special with them. There is no way I would part them out. If the front panels are trashed, then be creative.

 

Volume adjust a pain on two mono amps, posted on December 24, 2020 at 09:26:34
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 Thanks for the nudge to be inventive with the front panels. The primary reason for considering to "harvest the transformers" is to build a stereo power amp with a single volume control. But I hesitate to trash two vintage Scotts.

 

RE: Volume adjust a pain on two mono amps, posted on December 24, 2020 at 09:56:04
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 The 99-D schematic suggests easily reversed field engineering changes could solve the volume control conundrum.

Take advantage of the already present tape monitor loop support. Connect a jack directly to the power section and disconnect the OEM 500 Kohm control. A fixed 470 Kohm part handles the grid to ground duty. Use the recording O/Ps of the pair to drive the I/Ps of an external stereo buffered volume control. Said buffered control drives the pair of power sections.


Eli D.

 

Here's What I Did, posted on December 24, 2020 at 13:18:14
tubav
Audiophile

Posts: 549
Joined: March 21, 2009



With the front panel of my Eico ST-40. Just veneer with contact cement. Covered up the unused holes after the rebuild. Just volume, 3 input selector, power switch. No phono, tone controls, etc.

 

Restore Them, posted on December 24, 2020 at 20:32:38
Ross
Audiophile

Posts: 1814
Joined: January 24, 2000
 Connect the tape outputs to a passive preamp with a stereo volume control. However I have used preamps with separate R/L volume controls and once set at a comfortable volume, you don't really need to adjust the controls. Separate volume controls facilitate channel balance, with out the addition of another pot.

 

RE: Here's What I Did, posted on December 25, 2020 at 21:14:35
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 Your wood veneer looks very attractive! Well done! How did you cover up the unused holes?

I own a couple of Eico St70 and St40 and I will keep your suggested "trick" in mind when I rework them.

Unfortunately this veneer application will not work for the Scott 99D, because the front panels, made from ~14 gauge aluminum, were badly bent at the corners.

 

I'm leaning towards harvesting the transformers for DIY monoblocks, posted on December 25, 2020 at 21:29:41
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 Thanks for your input and discussion. Most folks recommend restoring them. However, I have not heard persuasive arguments for keeping the phono, preamp and tone controls in two separate integrated amps.

Perhaps I'm wrong, but I'm getting the impression that the nostalgic motivation for restoring two obsolete mono integrated amps is not justified by any demand for, or usefulness in this configuration. Why should I not harvest the transformers and build to nice small monoblock amplifiers with them?

 

RE: Here's What I Did, posted on December 25, 2020 at 21:34:08
rivervalley817
 automotive bondo + grinder ? shave 'em and save 'em!

Merry Xmas!

 

RE: I'm leaning towards harvesting the transformers for DIY monoblocks, posted on December 26, 2020 at 03:58:24
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 If you follow the harvesting path, there are (IMO) 2 obvious candidates for O/P tubes, the 7591 and the 5881 equivalent Russian 6Π3C-E (6p3s-e).

The very easy to drive 7591 allows for a 2 stage "El Cheapo Grande". The 5881 equivalent 6Π3C-E could be used in a scaled back Harman-Kardon Citation V "knockoff".

You must choose between full pentode and triode modes for the "finals". Definitely regulate g2 B+, if full pentode mode is selected.

SS rectifying the B+ would free the 5 VAC winding up for feeding a 1/2 wave parallel multiplier that yields any/all necessary negative voltages. Given reduced forward drop in SS diodes, CLC B+ filtration where the 1st cap. is only large enough to keep the rail voltage up would reduce I2R heating in the rectifier winding.






Eli D.

 

RE: Here's What I Did, posted on December 26, 2020 at 07:26:21
tubav
Audiophile

Posts: 549
Joined: March 21, 2009



I just veneered over the holes. I guess I could have done the old masking tape on front and pour epoxy from the back trick but didn't. The middle black strip is thin basswood from the hobby store. Thin styrene would have worked too. Gary Dodd (sadly deceased) did the rebuild. He took it back to the chassis, transformers, and tube sockets. He kept the original circuit but beefed up the power supply. He eliminated tone controls, phono stage, the goofy speaker wiring scheme, etc. Sounds great. Attached is a pic of the underside wiring after he was done.

 

RE: I'm leaning towards harvesting the transformers for DIY monoblocks, posted on December 26, 2020 at 08:27:27
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 Eli,

Thanks; all very good suggestions.

If I go the "transformer harvest" route I will probably stay close to the original 99D power amp circuit, in honor of the Scott origin. (You mentioned DC for the first tube(s); Scott did some interesting things with the first two 12AX7s, lighting them with the cathode current of the output tubes, at starved voltages.)

For my revived Scott 99Ds, in whatever incarnation, I plan to use a batch of very nice 1960's Russian 6P3S (without the "-E"), which appears to be a close equivalent of 6L6 GB/5881. The 6P3S-E version is probably overkill in the Scott 99D with merely 365V on the output plates and 280V on the screens. I see folks are reporting that the -E version begins to redplate at >40 watt dissipation. On the hint of member Michael Sampra, several years, back I have been using 6P3S-E in my Eico HF-87 with very good results.

 

Beautiful work!, posted on December 26, 2020 at 08:33:20
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 Beautiful work! A joy to look at it. Sorry we lost him.

What output tubes are you using? I'm curious if the modern versions of the 7591 "deliver the goods"

 

RE: Beautiful work!, posted on December 26, 2020 at 14:02:01
tubav
Audiophile

Posts: 549
Joined: March 21, 2009
 I'm using vintage 7591 in the amp. On Jim McShane's site he warns that the new ones may not work. I also have a vintage backup quad just in case. I also have a Dynaco SCA-35 that he did. No tone controls, phono stage, etc.
Sounds great. He did amazing work. If you ever decide to mess with your ST-40, let me know and I'll send the schematic of the power supply mods he did.

 

RE: I'm leaning towards harvesting the transformers for DIY monoblocks, posted on December 26, 2020 at 15:15:48
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Quite a few manufacturers used the "Cheap Charlie" method for DC heaters. FWIW, I loathe it. The thought that the signal might modulate the temperature of preamp tube cathodes makes me cringe.

Look closely at New Sensor's offerings. The tube sold as the Sovtek 5881 is 6Π3C-E. The variant is definitely stout, but it's not a 6L6GC equivalent.


Eli D.

 

Would the real 6P3S please stand up, posted on December 28, 2020 at 03:52:05
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 Indeed, the 6P3S-E is no 6L6GC and plate dissipation of > 40 watts is unproven (to me). Although they are more robust than their conservatively rated 20.5 watt dissipation would indicate I had some initial issues with 6P3S-Es red-plating in the Eico HF-87 mentioned above. However, that amp routinely runs it's EL34s at above 30 watt dissipation, so the 6P3S-E is not a drop-in. In this amp, after increasing the cathode bias (don't remember how much) they have been very satisfactory and good-sounding tubes to my ears.

See: https://www.audioasylum.com/audio/tubes/messages/22/226635.html

6P3S tubes are less robust still, but as best as I can tell they are rated for at least 19.5 watt dissipation, and plate to cathode voltages of around 375 V, so I'm going to give them a try in my revived Scott 99Ds, that originally had 6L6GB tubes.

This link is the best source of tube data I have seen for the 6P3S; please let us know if you have a better source:


 

RE: Would the real 6P3S please stand up, posted on December 28, 2020 at 06:11:04
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Perhaps a clue lies in the TungSol 5881 datasheet. That document shows the conservative design center value for max. plate dissipation = 23 W. 25 W. would be reasonable for the design maximum number.


Eli D.

 

Scott 99D iron: What to build now?, posted on January 16, 2021 at 09:10:31
Spantou
Audiophile

Posts: 115
Location: Central Illinois
Joined: October 5, 2016
 Thanks for the feedback so far.

It seems there is little reason to try to salvage the 99Ds in their original, integrated amp configuration, so the decision is to harvest the iron. The question then becomes what to build. I'm leaning towards a pair of small monoblock power amplifiers.

Eli suggested the 7591 El Cheapo Grande, or a "scaled-down" 5881 Citation V knock-off (not sure how to scale it down, though). I could also consider a 7591-based Baby Huey. Or is there something special about the original Scott 99D power amp design that would justify a rebuild of the original circuit? Or something else?

Further suggestions and comments shall be appreciated.

 

RE: Scott 99D iron: What to build now?, posted on January 17, 2021 at 19:40:10
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 I suggest you start a new thread for the monoblock pair project on the Tube DIY "board" and link back to this thread.

As for scaling back the H/K Cit. 5, that's pretty straight forward. The Cit. 5 used a big "mamu", the 7581, of the 6L6 family as its O/P tube. You will use the Russian 6Π3C-E (6p3s-e), a 5881 equivalent, as your O/P tube. The 5881 is a 6L6 family member that is (IIRC) a tad "beefier" than the 6L6GB used by Scott in the 99D and should serve you very well.

FYI, the tube New Sensor sells as the Sovtek 5881 is, in fact, 6Π3C-E.


Eli D.

 

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