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Scott 299A Rebuild
Scott 299A Rebuild
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Old 6th August 2018, 03:35 PM   #21
sony6060 is offline sony6060  United States
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Eli is no doubt a serious audio builder. But, forget the recommended Orange Drops, just use the PIO Russian K40Y-9. These caps sound much better.
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Old 6th August 2018, 03:39 PM   #22
sony6060 is offline sony6060  United States
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Quote:
Originally Posted by DAK808 View Post
I implemented this scheme in a vintage amp to use with currently made 5ar4 . Could you explain how the 2 diodes protect the inferior 5ar4 sold nowadays.
The diodes take the high voltage spikes by increasing the PIV (peak inverse voltage) 1000 volts higher. No internal tube arcs now. The UF4007 does not effect the sound at all. This is very smart- thanks ELI.
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Old 6th August 2018, 08:48 PM   #23
DAK808 is offline DAK808  United States
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Scott 299A Rebuild
Quote:
Originally Posted by Eli Duttman View Post
The Sovtek 5AR4 is fine, with 1 key exception. The variant gets into trouble at the top of the documented voltage range. The SS diodes provide some critical extra PIV headroom. No "fireworks" with the "sand" in series with the plates.

BTW, New Sensor is offering some ("surprise" more costly) 5AR4s that are supposedly flaw free.
So, during the reverse voltage the SSR will be able to conduct and take the load off the tube?
The reason i asked is i did this mod and put in a supposedly new sovtek 5ar4 which promptly flashed and died! I tried a Mullard and there was no problem. Me thinks i got took and bought "new" 5ar4 that were defective.
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Old 7th August 2018, 08:03 AM   #24
Eli Duttman is offline Eli Duttman  United States
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The series SS diode trick, along with the graphic, are not original to me. The tweak works and deserves publicity.
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Old 7th August 2018, 01:05 PM   #25
sony6060 is offline sony6060  United States
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I like Sovtek EL84M. Your 299A model uses a 6BL8 phase inverter. A Philips 7643 can sub for the 6BL8 phase inverter and is a better sounding tube. The Westinghouse 12AX7 short plate with narrow center bump is a very good phono tube. It sounds same as $125 Brimar 12AX7 box plate and will way outperform Sovtek 12AX7LPS for $30 in NOS. The Westinghouse is perfectly clear and what you want in a phono section. Have fun.
BTW- PM me for seller of NOS Westinghouse 12AX7 for $30 each.
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Old 7th August 2018, 02:33 PM   #26
Alan4411 is offline Alan4411  Wales
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Quote:
Originally Posted by thunderchicken View Post
August 2018 Update:

I ended up sending the amp out to AEA for a deep clean and some upgrades I wasn't comfortable with. That was June of 2017. I still don't have it back and Ken Leonard hasn't responded to an email since November and hasn't picked up the phone since April.
Sorry to hear you have had no communication, I thought it might be helpful to repeat your post.

Alan
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Old 7th August 2018, 03:02 PM   #27
sony6060 is offline sony6060  United States
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Some of these service persons are ridiculous. I used to rebuild tube amps & receivers and had a two week turn around. Played with rebuilds on weekends.
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Old 12th October 2018, 09:48 PM   #28
thunderchicken is offline thunderchicken  United States
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Some good news and some bad:

First, I got the amp back and it powered up and made sound.

Second, that was short-lived. The front four tubes have an intermittent power issue where they just don't come on. The amp also has a 25% or so channel imbalance where I have to move the balance pot to the Channel A 2 position.

I'm 90% sure the imbalance problem is a bias issue- it was fine with the NOS tubes AEA sent me, but the amp developed a steady pop through the speakers every two seconds or so after two weeks of daily use.

I had a dispute with AEA over billing and they requested their tubes back. I removed those and installed the Sovteks I purchased for the amp. To my surprise, the popping went away, but now Channel B (Right) is much louder than Channel A. The Sovteks were a "matched quad" checked on a curve tracer.

With my limited knowledge and experience, I could only reason through one explanation. Could it be because AEA adjusted the bias to match levels for non-matching NOS tubes? That would mean that my matched set is now biased as if they're not matched (on top of being biased for a completely different tube) and throwing the balance off.

I still can't explain the regularly timed popping.

I know that I'm supposed to set bias when I change power tubes, but I'm having trouble finding a guide to do it safely and efficiently. Do any of you know of a guide? The amp has bias pots, but I'm not sure of the testing procedure. From what I can tell, I have to measure resistance between two points when the amp is off, then use the values in some math to find the bias in mV between those points when the amp is on and then use the pots to find some mythical ideal value.

(How do I adjust my Scott 299C Bias and DC Balance pots? - Talkin' Tubes - The Klipsch Audio Community)

If I'm ripping into this thing again for any reason, I'd like to have an actual plan first. I thought biasing was done to keep distortion low and component longevity high, which suggests I'd need a distortion analyzer (which I don't have) to find the lowest distortion and a lab-grade voltmeter (also don't have) for balancing levels accurately.

What are my best options?
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Old 13th October 2018, 08:58 AM   #29
huggygood is offline huggygood  France
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for the DC balance adjustment, it's simple, you take your multimeter, you set it on VDC then you just put each touch point on the balance potentiometer and you adjust to be the closest to 0 volt dc on your multimeter (+/- 5-8mv is acceptable). but before, you have to adjust the bias, for that, you have to turn the amp on, let it warm up for a few minutes, take your multimeter set on Vdc, put the black key on the mass, and the red key at the junction of r43 and r48 (1K and 330K) and you set to have -18,5V) if someone else can translate into better English, you should understand easily
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Old 16th October 2018, 11:36 PM   #30
thunderchicken is offline thunderchicken  United States
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Okay, thanks!
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