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The Un-Official Sadowsky SA200 Thread...
- Thread starter jokerjkny
- Start date
The master was set approximately to where the output section hits the wall. Turning it up more resulted in the sound becoming fuller and more compressed, but it doesn't really seem louder. Where this transition occurs on the master is highly dependent on how the Gain and Volume controls are adjusted.
Feels like about 200W to me. It's a nice, strong 200W, but not quite as loud as my SVTs and CTM 300.
The amps were placed where I could easily access the input and speaker jacks. I plugged both amps in and turned them on in standby. For testing, I inserted a speaker chord and a cable from my wireless before setting the amp to operate. I set the amp back to standby before moving the input and output too test the other amp. I alternated between the amps several times.
Are both Amps from the same production run? I have one from the 1st run but just one LoL…Here's something you probably don't see every day.
View attachment 7004502
Finally got around to hooking up the amps in a way that allowed me to quickly switch between them. Both amps have factory preamp tubes. The top amp is running factory Koch EL34B. Pretty sure the EL34Bs are relabeled TADs, which in turn were made by Shuguang.
The bottom amp has a fresh set of JJ KT88s. I pulled out an old set of EL34Bs that had a mix of Koch and TAD labels. The old tubes still sounded great, but would become noisy and occasionally blow the HT fuse if I moved the amp. I am not certain if the problem is because the tubes are too old, or possibly because the pins in the tube sockets needed to be cleaned and tightened.
Even with both amps running EL34B, they had a slightly different sound and feel. This seems to hold with KT88 in one of the amps. The differences are so minor that I am not absolutely certain they are actually there. My impression is the amp running KT88s seems slightly more focused and clean/HiFi.
The amp running EL34B seems to have a slightly more old school sound and feel. It seems looser, bloomier, and woolier. I don't hear any obvious difference in tonal balance, but the mids seem warmer and to have more of a vocal quality. I also think it may be slightly louder and punchier.
IMHO both amps sound great, and as far as I can tell there is not enough of a difference to be concerned about it.
Low light so you can see the fairy dust a bit better:
View attachment 7004513
That's not a trick of the camera...the KT88s are noticeably brighter.
Edited to tag @gtirard to see the pretty pictures.
They are both 1st run amps. S/Ns are 2 apart.
You really only need one as far as I am concerned.
That is interesting! I can see slight tonal differences which could probably be attributed to the fact that it is a handmade amp.
They use PCB construction, so I don't believe there is a lot of variance in the layout or connections. However, I don't find it surprising that they don't sound exactly the same. It's not unusual for the tolerance of electronic components to be +/- 10% or more. Also, even though both amps are running factory preamp tubes, tubes are not precision components and there can be noticeable differences from the sound of one tube to the next.
Getting ready to upgrade the tubes on my SA200. My tech suggested Genalex KT88s.
What are you guys using in yours?
What are you guys using in yours?
I do t think the amp had stock KT88 tubes. It was EL34
If you go with KT88, it's still recommended to use the factory bias settings (25-35mA). With a higher capacity tube you bias to the capacity of the power supply. If you go with a smaller capacity tube, then you bias to the tube.
I have the factory tubes in one. I recently installed JJ KT88 in the second. The bias control needed about 1/2 turn to bring current into the proper range. Coldest tube is set to 27.3mA and hottest tube is set to 32.4mA
The amps did not sound identical running factory tubes. I preferred the sound of the second amp, but the first amp was louder. The sound may have changed slightly with the KT88s, but it's very subtle. The first amp is still louder.
I am practically a JJ KT88 fanboy. I have a lot of amps running JJ KT88 and no failures yet. I did have one amp that was popping tubes because the plate voltage was set to run about 800V. My tech changed a transformer tap to put the voltage in the proper range and I asked him to install a new octet of JJs. I believe he gave 6 of the original JJ tubes back to me and I have rolled them through several amps and all are working to this day.
I also have amps running Shuguangs and various New Sensors. The JJs have been very reliable for me. I have had quite a few failures with the Shuguangs and New Sensors. Most of the Shuguang failure were all very old tubes. The new set of Shuguang KT88s I bought has been very drifty despite being previum tubes. I believe the New Sensor failures were all relatively new.
There seems to be quite a bit of concern about running New Sensors in amp with high operating voltages. In this context, the SA200's plate voltage is relatively low at ~550V, but the screens run relatively high at ~525V. These voltages are within spec for both KT88 and EL34, but most high-powered amps have a separate supply for the screens that runs about 350V.
The relatively high screen voltage relates to why Sadowsky has recommended running KT88 over the factory tubes. KT88 has a higher screen dissipation rating than EL34. The hope is this will extend the life of the tubes.
Factory tube was Shuguang EL34B with Koch silk screen. AFAIK these were tested, matched, and supplied by TAD
I just dropped my SA200 off at the shop of a well-regarded tech ("The Tube Amp Guy" in Richmond VA) who has done work for me before, and I sent him some info culled from this thread regarding the use of KT88s. I'm super curious to hear what he'll have to say.
Edited to add his response: "Yeah I would agree after making some measurements. They are running the tubes at voltages well above the max listed in the datasheet. One of the EL34's is drawing a lot of current and that current varies. Even though it tests good on a tester, when exposed to continuous high voltage current load it was going as high as 400mA. Should be steady at 35mA (or whatever the bias is set for. ). I'll get you a price for a set of KT88's. I think the filament current draw in kt88 is similar to EL34 so that shouldn't be an issue."
Edited to add his response: "Yeah I would agree after making some measurements. They are running the tubes at voltages well above the max listed in the datasheet. One of the EL34's is drawing a lot of current and that current varies. Even though it tests good on a tester, when exposed to continuous high voltage current load it was going as high as 400mA. Should be steady at 35mA (or whatever the bias is set for. ). I'll get you a price for a set of KT88's. I think the filament current draw in kt88 is similar to EL34 so that shouldn't be an issue."
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Some of the EL34s in the amp that is now running KT88 were drifty and also making random noise. When the noise occurred I could see it in the current draw. This amp also occasionally blew the HT fuse when switched from standby to operate. I think it blew 2 fuses over about a 5 year time frame, so I wasn't in a huge hurry to replace the tubes.
Previously I was moving the amp around. Now many of my amps are stacked in dedicated positions on tables and wired for power, so they don't need to be moved. All I have to do is patch in my bass and a speaker and these amps are ready to play. With the dodgy SA200 immobile, all of the problems went away. The bias become stable, the random noise stopped, and the amp did not blow a fuse for an extended time.
It's possible the old EL34s I pulled out are mechanically degraded. I think I did a chopstick test in the amp and shook them after the were removed. Not of them produced excess rattle.
Another theory I have is perhaps the pins in the tube sockets needed to be tightened. Loose pins can cause all of the symptoms I described and more. I retensioned all of the tube sockets before installing the KT88s.
Followup post to say that the amp seems quite happy with the matched set of JJ KT88s.
