[jeffreybehr]

Yesterday we replaced the main-PS rectifier diodes, from 25-Amp HEXFREDS to 30-Amp Fairchild Stealths.  I removed the heatsinks as they were never even warm to the touch, and the new diodes, too, are hardly warm.  I suppose if I were driving hundreds of Watts thru these things, the diodes would indeed require 'sinking, but I'm not, and they don't.  I think I'm finished.   

[bummrush]

One of my first amps a few years was a Sonograph 250 I bet you could have done a few things with that one.very nice amp .

[ptmconsulting]

Just out of curiosity, can you estimate the out of pocket $ you have now put into these changes (not including your time/labor)?

What would also be cool is a ranked ordering of the changes and the sonic benefit each made to the amp. I understand that may not be totally possible since a lot of these were made at the same time, or with little time between them. Just curious is all.

[Levi]

Excellent work and pictures Jeff.  This is a great project. I am curious you choose the Hexfreds vs the Schottcky diodes. I found them to improve the treble but I guess they are personal preference.  You are done but then again, you can still fine tune it when everything settles.

Yesterday we replaced the main-PS rectifier diodes, from 25-Amp HEXFREDS to 30-Amp Fairchild Stealths.  I removed the heatsinks as they were never even warm to the touch, and the new diodes, too, are hardly warm.  I suppose if I were driving hundreds of Watts thru these thing, the diodes would indeed require 'sinking, but I'm not, and they don't.  I think I'm finished.   

[sts9fan]

I am curious why you would remove heatsinks. They can't hurt.

[jeffreybehr]

Excellent work and pictures Jeff.  TYVM. 

This is a great project. I am curious you choose the Hexfreds vs the Schottcky diodes.  I didn't. I chose the Fairchild Stealths; c-j chose the HEXFREDS.

I found them to improve the treble but I guess they are personal preference.  You are done but then again, you can still fine tune it when everything settles.  Indeed.

[jeffreybehr]

I am curious why you would remove heatsinks. They can't hurt.

sts, you're correct, but (1) I had to removel the 'sinks with diodes attached to get the diodes out, (2) the Stealths are smaller than the original HEXFREDs, and (3) since the 'sinks were out, I simply chose not to reinstall them.

[jeffreybehr]

Just out of curiosity, can you estimate the out of pocket $ you have now put into these changes (not including your time/labor)?

What would also be cool is a ranked ordering of the changes and the sonic benefit each made to the amp. I understand that may not be totally possible since a lot of these were made at the same time, or with little time between them. Just curious is all.

ptm, I guess you haven't read my disclaimers about not being a Golden-Eared Audiofile.  I'm not, at least in being able to characterize subtle changes resulting from single changes.  I have to work hard at hearing some differences among cables, etc.  I'm much better at eventually recognizing the cumulative results of several changes, so I can't rank these many different things I've done to these amps.  Overall, the sounds have gotten both smoother--less edgy--and more detailed...more transparent.  The slight edginess that must have been there in the upper frequencies seems completely gone now, to the point that sometimes I think the amps sound a little dark.  This weekend I had to listen to one of my long-time reference recordings, Holst's The Planets*, one of EMI's great Christopher/Christopher recordings but also one that had a couple mics too close to the trumpets and trombones.  When blown hard, these instruments have always sounded a bit too loud, too edgy...too hard, unattractive.  Now they still sound a little too loud but never edgy or unattractive.  Other long-time reference recordings also sound bess bright.  But let me make one thing very clear--my system has NEVER sounded better, overall.  It's very transparent, musical, easy to listen to, rich and warm when it should be but NEVER thick sounding.  Only one other amp combination I've ever had--the quad of much-improved Marantz MA-24s--bettered it in any regard, that being transparency and only slightly, but those amps had little of the ease and richness that the '750s have.

I've ranked these general improvements in another thread, but if I had to start over on another pair of these, I'd do them in this order.
1. Replace the 8-per-amp 'DNA' caps with at least 16 (and as many as 24) BlackGate-N 680s, and add four 1uF SoniCap Platinum bypasses.  My amps have 20-each BG-Ns.  These DNA caps are in parallel with the main-PS caps but are after the 4 DC-rail fuses; the output-stage PS does not have 2 poles (other than the positive and negative halves).
2. Replace the main-PS caps with the Nichicon KG Super Throughs.  Add more BG-N 680s as bypasses depending on how hard you want to work at it; my amps have 8 each.
3. Replace the frontend-PS 2nd-stage caps with BGs and add SoniCap Platinum bypasses.
4. All the other cap and other passive-parts (speaker-lead cable, input RCAs, bindingposts, etc.) replacements.
Of course some of these steps include removing the OrangeDrop caps in those positions.

I've spent somewhere around $2K for all these parts.

* Adrian Boult and the LPO; LP #ASD-3649; CD #7243 5 67749 2 6 or #CDM 7 64748 2.

[jeffreybehr]

WOW was I ever wrong for thinking they sounded short of hi-frequency energy.  I listened to a couple different Rachmaninov Symphonic Dances SACDs today--Johanos's with Dallas* on an Analogue Productions 2-ch., #CAPC 34145 SA, and Jansons' with the Concertgebouw, RCO Live #RCO 05004.  On both recordings, the trumpets, trombones, violins, flutes, percussion, etc., had plenty of HF energy but never sounded too bright or harsh.  Both recordings sound excellent, overall, but I hadn't heard the Johanos recording for decades, and I agree with others' comments about the 'dry'-sounding hall and recording--there's just not much sound of the space around the orchestra.  The system, however, sounded EXCELLENT and not lacking in anything that I noticed.

Lucky me.   

* the old 4-ribbon-microphone recording released decades ago on a Vox Turnabout LP, all of which were rather noisy, IIRC.  This at least was on Harry Pearson's SuperDiscs list.

[jeffreybehr]

2017Nov13:  Oh my...7-1/2 years later I may be buying these very amps back from the friend to whom I sold them.  Would be to drive a pair of Revel Ultima Salon2s.  More soon.
-----------------------------------------------

2017Nov14:  I believe we'll soon have a deal and I'll end up with 'my' amps.  I plan to remove ALL the old 680uF BG-Fs and end up with, in addition to the Nichicon Super Throughs initial PS caps, only the original 8-count of DNA caps.  Those will be Blackgate (std.) 470/160s plus a total of 4uF of SoniCap Platinum bypasses per amp.  That would leave 87,520uF of capacitance instead of the original 99,760uF.  I've read this thread completely a couple times recently and am amazed at all the time and effort I and my goodbuddy Pat put in.  I also can't see anything else I need to redo.   

Because of the (temporary) low price, I ordered the same type of 2-outlet, plug-in-the-wall-outlet power conditioner, an Audience aR2p...
https://www.thecableco.com/adept-response-ar2p.html
...the standard version ($200 under RR!), for these amps.  That way I won't have to replace the still-shorted-on power switches AND I'll have some noise filtering and surge protection on them.