seti Posted June 21, 2023 Share Posted June 21, 2023 I bought these monoblocks many years ago from a Little Rock Central High School teacher. I sent them to hot glass audio for a rebuild. Anyone who remembers that con knows it was a miracle I even got them back. He broke all NOS tubes in shipping. I sold them to my brother because I was frustrated but he just sold them back to me. The nightmare continues. I completely gutted them and rebuilt them from scratch but I did have professional help. I added bias adjustments removed a 12AT7 preamp section and upgraded power supply with a beefier choke and caps. The power tubes switched from, 6550 to KT90's which are from the collection I inherited from Daddy Dee. Thanks again to Daddy Dee. The tube compliment is 2 x KT90, 2 X 5AR4, 2 x OD3, and 2 X 6SN7. They sound really good when turned up but still a little transformer hum to work out. I need to tighten them down. Not sure I need freaking 90 watt amps. 7 Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted June 21, 2023 Share Posted June 21, 2023 @seti Awesome Work..!!! miketn Quote Link to comment Share on other sites More sharing options...
seti Posted June 21, 2023 Author Share Posted June 21, 2023 2 hours ago, mikebse2a3 said: @seti Awesome Work..!!! miketn Thanks I also forgot to mention they are dated June 1958. 65yo.... Quote Link to comment Share on other sites More sharing options...
henry4841 Posted June 21, 2023 Share Posted June 21, 2023 As I am sure you know transformer hum is usually associated with being pushed to hard. Unless they are unreasonbly hot I would probably just turn the music up until you can no longer hear the hum. Being older than the amps I hum a little myself. Excellent looking rebuild. Quote Link to comment Share on other sites More sharing options...
seti Posted June 21, 2023 Author Share Posted June 21, 2023 2 hours ago, henry4841 said: As I am sure you know transformer hum is usually associated with being pushed to hard. Unless they are unreasonbly hot I would probably just turn the music up until you can no longer hear the hum. Being older than the amps I hum a little myself. Excellent looking rebuild. I don't think it is being pushed too hard as it is running lower voltage and current than spec. It was over 600v now it is 550v. The 0d3 cuts the voltage to the driver stages to around 300v. We even removed a preamp stage then installed a larger potted choke. All the caps are cool after an hour of running. All the transformers and chokes not even warm. We are going to do some distortion and power testing and then try to find the hum. I can't tell if it is 60hz or 120hz right now. I've never had a tube amp this powerful. My bench speakers are 84db1w1m if I hear hum on this I'm scared of 107db 1w1m..... This was something I had to get out of the way. If I can get it to a point where I am happy with it I might ebay or audiogon it so I can buy parts for other projects. Thanks! Quote Link to comment Share on other sites More sharing options...
henry4841 Posted June 21, 2023 Share Posted June 21, 2023 I misunderstood the kind of hum you are having. I was under the impression you were talking about mechanical hum coming from the transformer itself but what you are saying is you are hearing hum from your speakers. Sometime it can frustrating finding hum coming from the circuit itself. I like the software program called Arta which will show which you have 60hz or 120hz with ease. Distinguishing 60hz from 120hz by ear is not something I am good at but is easily seen with Arta or any distortion analyzer. If it is noticeable with 84db speakers you definitely do not want to hook it up to Klipsch speakers. Quote Link to comment Share on other sites More sharing options...
mark1101 Posted June 22, 2023 Share Posted June 22, 2023 Nice job Seti. Those are cool as heck. 1958 and back to life. 1 Quote Link to comment Share on other sites More sharing options...
seti Posted June 22, 2023 Author Share Posted June 22, 2023 7 hours ago, henry4841 said: I misunderstood the kind of hum you are having. I was under the impression you were talking about mechanical hum coming from the transformer itself but what you are saying is you are hearing hum from your speakers. Sometime it can frustrating finding hum coming from the circuit itself. I like the software program called Arta which will show which you have 60hz or 120hz with ease. Distinguishing 60hz from 120hz by ear is not something I am good at but is easily seen with Arta or any distortion analyzer. If it is noticeable with 84db speakers you definitely do not want to hook it up to Klipsch speakers. The hum could be circuit or transformer laminations or maybe both. Yeah we still got some work to do. I'm optimistic we can knock the hum down. You know how it is when you just finish a big build and your just amazed any sound comes out at all. That is how I am anyway. I'm curious about the actual power rating and distortion measurements. That should follow. Quote Link to comment Share on other sites More sharing options...
rigma Posted June 22, 2023 Share Posted June 22, 2023 Great work as always! 1 Quote Link to comment Share on other sites More sharing options...
seti Posted June 22, 2023 Author Share Posted June 22, 2023 2 hours ago, rigma said: Great work as always! You were the first to push me towards DIY. THANKS! Quote Link to comment Share on other sites More sharing options...
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