@Mikerodrig27 I sort of reverse engineered the plan, kinda knowing I'd eventually let Jon get his hands on it once I sourced the amp. We spoke a few years back. Kept an eye out for a super clean Threshold or Forte' as a core, and would decide once I got it. We talked some about pros/cons of heat and when he explained how close it would run in 1) op-temp, and 2) when he showed me the larger/taller power caps used with the conversion 3) the part about the change in sound stage was hook-line-sinker, in regards to Class-A operation. Would probably have kicked myself if I did not do it since it was already there, just did it. Yes, will do. Should get it back and settled with some burn-in, in a few weeks.
Check my thinking here, please. i.e. Class A/AB to Class A conversion on Forte 3 that is -
Jon indicated the Class A/AB amp bias can be around 47-50c on the heat sinks. That's when I touched the other threshold sitting there all warmed up. Did not feel overly hot to me. Not as warm as a few of my prior dual mono Class A/AB amps. And, when I pick up my Forte 3 he indicated it will be biased in the 47-50c range. Heat is the same, really, hmm. If so, I can see why others go for this.
My custom speakers are between 4-6ohm nominal imp and 92.5db @1w1m sensitivity, so I could not think of a reason to not have it upgraded and converted to full Class A while its there. He seemed to want to nudge me to do it, and I followed his lead on this decision. I think I also got hooked on seeing/comparing the larger power caps he uses in the Class A upgrade/change. And, reflecting on former comments here on DIY about Class A having more low end "grunt". Wishful thinking here - IF that somehow also translates into having more of that lower frequency midrange "bloom" I heard with my prior higher dual mono current SS amps, its something I listen for too. Since I have no other SS amps lying around any more to A/B, it will be a modded Tube Mono KT120/KT150 amp vs. upgraded Forte 3 amp comparison. Maybe unfair but that's what it will be. 🙂. This same comparison had me resale my former SS amps i did not feel came close to the non-fatiguing listening enjoyment of my current and former tube amps.
Preamp:
I will be using my Cary SLP-98 6SN7 tube preamp to drive this upgraded Forte 3 SS amp once I get it back.Same preamp I've used before on my prior SS amps and all of my prior and current tube mono amps. I had some light email conversation years back with Nelson Pass about this combo just before he came out with the Korg preamp. At that time he indicated having a number of customers who've used my same tube preamp with the Pass amp line. I'm a huge fan of this hybrid setup, just want to hear a better SS amp this time around.
For those who've made this change/upgrade will know a lot better on what to expect from converting the Forte 3 to full Class-A by Jon. If it does not match ~80% of what my tube amps offer, it will go up for sale to a lucky bidder and I'll be working on the next pair of tube mono amps in a different low-power configuration. Will this upgraded Forte 3 touch on "tube like sound" or some other great sound I've never heard before - maybe, wishful thinking... who knows, willing to try this. Hope to listen more in a few weeks. 🙂
Yes, temps will be the same. With the higher voltage power supply, you get more heat per class A watt but you get more class B. With lower voltage, you get more class A watts but less B ( generally still some class B). I believe there is a standard by which an amplifier is considered class A or class AB which depends on what % of total watt output is biased in class A.
The bias is set based on the heatsink temperature. Generally, 25 degrees celsius over ambient is the safe zone. Some people go higher, some go lower. Higher temperatures (higher bias) are reserved for people who have a good understanding of amplifiers etc.
I have had a number of amplifiers anywhere from 1-2 watts up to 1000 watts. 25-80 watt class A amps with a good clean power supply are my sweet spot. As a matter of fact, I don't even question the wattage. If the amp is a nice design, has a good output section and the power supply is good, I have generally had the experience of a powerful-sounding amplifier regardless of watts per se.
The bias is set based on the heatsink temperature. Generally, 25 degrees celsius over ambient is the safe zone. Some people go higher, some go lower. Higher temperatures (higher bias) are reserved for people who have a good understanding of amplifiers etc.
I have had a number of amplifiers anywhere from 1-2 watts up to 1000 watts. 25-80 watt class A amps with a good clean power supply are my sweet spot. As a matter of fact, I don't even question the wattage. If the amp is a nice design, has a good output section and the power supply is good, I have generally had the experience of a powerful-sounding amplifier regardless of watts per se.
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Update: Forte Model 3 has been refreshed and upgraded:
Local, 1 week turnaround
Converted to full Class A 50w operation
Voltage turned down, biased up a tad
4 main power caps replaced (100,000uf) each
Bias controls replaced w/upgraded ones
Heat sink grease, improved thermal transfer
IXYS rectifiers replaced, soldered direct
Cardas binding posts, speaker connectors
Spade connectors eliminated, soldered
Rebiased to 47-50c temp at heat sinks
---------------------------------------
Which one WINS for best sound, the upgraded Class A F3 underneath,
-or- the upgraded QS Tube Mono 120s next to it, hmmm?
Using my Cary SLP-98 6SN7 triode preamp for both amp setups.
Giving it 2 weeks of settling in before making early conclusions.
Warming up past 2 days, definitely hearing Class A 🙂
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I have owned two of these amps in the past and present (well, kind of. The present amp has been redesigned and gone through a couple of years ago). I would readily buy another if needed. As far as the difference between the 200W version and the 50W version is IMO the 200W version sounds more open. This surprised me, but that is my experience. Since you are using really efficient speakers to begin with, it would good to try it as is before jumping into class A. One thing that got me started in the remaking of this power amp was a failed input IC that caused a loud thump whenever it felt like it. Once identified, I found that replacing the original trim pot was good advice to go with since the originals have known to get out of whack now and then. Of course, going that far meant that I should replace the power supply capacitors and every other electrolytic capacitor since your in there and they all have a limited life, especially in that really warm environment.
Sometimes you can identify trouble just by touching the heatsinks and comparing the heat of each to the other. Mr. Pass designed and used pretty high bias in this amp and it shows. I remember that the heatsink temperature was normal at 50 C. or somewhere in that range.
FWIW, here is a shot of one of the model 3 that I own. I have another waiting in Denver, but have not made a decision as to what should be done to it yet.
As you can see from the shot, I had to 'widen' the chassis a bit to accept some of the new stuff. Power caps acme from a Nakamichi P7 and were a bigger diameter.
Sometimes you can identify trouble just by touching the heatsinks and comparing the heat of each to the other. Mr. Pass designed and used pretty high bias in this amp and it shows. I remember that the heatsink temperature was normal at 50 C. or somewhere in that range.
FWIW, here is a shot of one of the model 3 that I own. I have another waiting in Denver, but have not made a decision as to what should be done to it yet.
As you can see from the shot, I had to 'widen' the chassis a bit to accept some of the new stuff. Power caps acme from a Nakamichi P7 and were a bigger diameter.
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@Ixnay
Looks like a fun stretch project. Awesome. Like you, decided to upgrade this one b/c of the bones in it.
@Mikerodrig27
Well, can try, problem is - seems the sound is kinda changing from hour 1 to 12 to 24/48. New stuff settling in, hard to say. Hour 1 was kinda velvety and veiled at first. Notably different from hour 12 and hour 24/48 with heat in it seems to be a bit more open and more midrange forward now compared to hour 1-12. Okay, so I'm use to this type or early burn-in variance with new tube amps, new iron, new signal caps and tubes settling in. However in this case, using the same iron/retapped for Class A, same OPTs, however, different/larger power caps, different rectifiers, different (much better) Cardas speaker/binding posts, and some new wiring and direct solder joints. I'll try to keep track with these notes here. Maybe another 72hrs and it will settle in, not sure. Darn, some of Nelsons newer amps, all new parts, some reporting taking up to ~300-500hrs to find a set point. My tube amps settle with all new iron and bits, 150-200hrs max.
In this case of the Forte 1/3 amps, retapped transformer/path ?? with some new parts and connections, I dunno know, maybe 1-2 weeks at least - guessing, who knows?
Looks like a fun stretch project. Awesome. Like you, decided to upgrade this one b/c of the bones in it.
@Mikerodrig27
Well, can try, problem is - seems the sound is kinda changing from hour 1 to 12 to 24/48. New stuff settling in, hard to say. Hour 1 was kinda velvety and veiled at first. Notably different from hour 12 and hour 24/48 with heat in it seems to be a bit more open and more midrange forward now compared to hour 1-12. Okay, so I'm use to this type or early burn-in variance with new tube amps, new iron, new signal caps and tubes settling in. However in this case, using the same iron/retapped for Class A, same OPTs, however, different/larger power caps, different rectifiers, different (much better) Cardas speaker/binding posts, and some new wiring and direct solder joints. I'll try to keep track with these notes here. Maybe another 72hrs and it will settle in, not sure. Darn, some of Nelsons newer amps, all new parts, some reporting taking up to ~300-500hrs to find a set point. My tube amps settle with all new iron and bits, 150-200hrs max.
In this case of the Forte 1/3 amps, retapped transformer/path ?? with some new parts and connections, I dunno know, maybe 1-2 weeks at least - guessing, who knows?
@Mikerodrig27 you asked I keep you posted. Some changes again at 72hrs, opened up some more, a little more detail, tone, texture coming through now. With the upgrades and new parts, Jon is telling me at least 72hrs to 1week of actual play time to settle in some more. Since I use a 6sn7 tube preamp, have not been leaving on continuously. So, at least another week to go+. With my dacs and other SS gear, seems like everything starts to level out at 14-21 days, we will see. This upgraded Forte 3/1 Class A amp conversion seems like its going to be a very nice alternative to my mono tube amps.
Very cool! I am burning in a recent build now too. I perceive the sound changing which could just be my ears familiarizing myself with the new equipment, or the new equipment burning in. Either way, I like it. Glad to hear that the sound is heading in the right direction for you.
For me, I am just happy if a new amp sounds different. It is kind of nice when you get too used to a sound, that you have the option to shake things up again. I switch between amps about every couple of weeks or so. Sometimes for a day, sometimes for a month.
So you have been leaving the tube amp on less?
For me, I am just happy if a new amp sounds different. It is kind of nice when you get too used to a sound, that you have the option to shake things up again. I switch between amps about every couple of weeks or so. Sometimes for a day, sometimes for a month.
So you have been leaving the tube amp on less?
@Mikerodrig27 I'm not playing the mono tube amps at all right now. Completely disconnected. I'm not much of a fast-flip-flopper whenever I swap prior gear out and new gear in. Reason is, I really like to get familiar with the sound, have it etched in my memory for some times up to 30 full days. Then, I switch back. This really helps me to pick up on changes and the differences, fwiw. After I learn the signature of a new piece, then timing of rotation more similar to your timetable. Im kind of an ocd case about not disturbing interconnects/cables too much though.
@4days burn-in post upgrade
Ok, now its beginning to show itself.
Opened up, plenty open now.
Nice tone & texture surfaced.
Midrange is present, just there.
Smoothing some, touch of organic.
Comparing: I can’t conclude if my tube amps (KT120, KT150) tubes are simply more linear sounding -or- if this Forté Model 3 now upgraded and changed to Model 1a Class A 50w configuration simply offers more Midrange presence than what I was anticipating It could provide. Pleased with the upgrade results so far. More next week.
Ok, now its beginning to show itself.
Opened up, plenty open now.
Nice tone & texture surfaced.
Midrange is present, just there.
Smoothing some, touch of organic.
Comparing: I can’t conclude if my tube amps (KT120, KT150) tubes are simply more linear sounding -or- if this Forté Model 3 now upgraded and changed to Model 1a Class A 50w configuration simply offers more Midrange presence than what I was anticipating It could provide. Pleased with the upgrade results so far. More next week.
Two weeks ago this was to be another experiment to try thinking I'd see how it sounds, then resale the F3>1A amp fully upgraded with what I have into it, and move on to some Threshold SA/2 monos to upgrade next and test next with my custom speakers. That's what I'm doing, trying different amps to see how my custom speakers respond with different tube and ss class A amps of different types. @Day4 of listening, sort of getting pulled in to how it sounds now, wondering if I'm going to have a hard time letting this one go. Seeing the bones inside and upgrading the right bits, can say it's worth doing. If you decide to let it go later someone is going to want to take it off of your hands. Almost seems more no-brainer to me now, after listening to the upgraded result, it is more engaging than I expected it to be, fwiw.
I completely agree with the resale of the 'additional amp' syndrome here, but like you, I have doubts that I would want to ever sell it. Idid that once with a Threshold T50 and still hand slapping my forehead! What a cool amp it was.
I have the same problem with first building speakers to die for and then wondering what amp will work best. In short, I have found that every stinking thing has to be in line for best results before the amp is worth the effort.
I have the same problem with first building speakers to die for and then wondering what amp will work best. In short, I have found that every stinking thing has to be in line for best results before the amp is worth the effort.
So now that you have it pretty much dialed in, how do you feel about those 50 watts? Are they keeping up? 🙂
The stasis amps are a real gem. If you set them up right, they really have a nice sound. I have had a lot of amps come and go and my rebuilt/modded stasis seems to always stick around. The way they image is really neat.
I am really surprised Nelson Pass's new stasis front-end thread doesn't get a lot of attention. Zen Mod even went through the trouble of doing a great job of drawing up boards and making really nice gerbers. The trick is a really really good power supply. Hence Jon's suggestion of the large caps.
Yes, more than keeping up. Plenty, I can barely turn it up 20% with my speakers too. Jon shares this 50w class a configuration has "more grunt" than its former self. Really depends on your speakers.
Totally UNEXPECTED benefit I'm discovering about this now upgraded F3 > F1a to Class A conversion, and cannot fully understand why?
This is NOT the case with any of my prior solid state A/AB amps -or- my current Tube amps which take at least 60-120m to start coming into their own. All of my my prior class a/ab solid state mosfet dual mono based amps would some times take about 2-4 hours to begin to shine, i.e saturated transformers.
ASK:
Is this remotely possible because it's running at full Class A as soon as I turn it on and it simply gets to the optimum temperature faster or ???
This is NOT the case with any of my prior solid state A/AB amps -or- my current Tube amps which take at least 60-120m to start coming into their own. All of my my prior class a/ab solid state mosfet dual mono based amps would some times take about 2-4 hours to begin to shine, i.e saturated transformers.
ASK:
Is this remotely possible because it's running at full Class A as soon as I turn it on and it simply gets to the optimum temperature faster or ???
It fooled me pretty good when it took my tube phono pre 1-2 Hrs. to even be listenable! I was worried.
@Mikerodrig27 @Zen Mod @Ixnay reading over some posts from the past here on this forum and others, I'm wondering if any of you guys have discovered any newer non-tube Class A preamps or simple LineStage units you prefer to pair up with these older Forte/Threshold/Pass amps?
Note: I've seen and peeked at the Forte F40,44 and some of the Threshold FET preamps, but kinda wondering if I can do just as well or better with a simple modern LineStage. I don't need a remote or tone controls. Don't need XLR/Balanced, SE okay. Clean and simple is better for me. Something to swap my 6SN7 tube preamp out for once in a while. Wound not mind hearing this amp with a preamp you've tried and felt was a little more musical than some of the vintage units.
Note: I've seen and peeked at the Forte F40,44 and some of the Threshold FET preamps, but kinda wondering if I can do just as well or better with a simple modern LineStage. I don't need a remote or tone controls. Don't need XLR/Balanced, SE okay. Clean and simple is better for me. Something to swap my 6SN7 tube preamp out for once in a while. Wound not mind hearing this amp with a preamp you've tried and felt was a little more musical than some of the vintage units.