E04 - Blue-line 1973 (or so) Ampeg SVT: Quick Preamp Repair; Set the Bias, Balance, and Hum.
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- Опубліковано 28 лют 2020
- NOTE: Press "SHOW MORE" for the linked Table of Contents!
This early SVT, owned by a back-line company, came in with channel one cutting out intermittently. They need the beast back ASAP. We quickly diagnose and repair the causes, and then bias the output, balance the 'push' wit' da 'pull,' set the hum balance, and ship it!
2:17 - Open the back
5:44 - Power-on and diagnostics
7:55 - Tear down
9:38 - PreAmp inspection
12:25 - Cleaning tube sockets and circuit board
13:20 to 14:30 - Messed up fade - kind of spacey though...
14:55 - Identifying the fault
15:25 - Discovery and repair
17:36 - Repair validation
19:56 - High temperature validation
21:11 - Tap testing and reassembly
24:40 - Bias adjustment
27:40 - Driver balance adjustment
30:20 - Hum balance adjustment
31:45 - Final testing
32:45 - Power output validation
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Thanks! I just got a late 70s SVT and it was definitely nice to watch somebody bias 'er up. Would definitely be interested in seeing deeper dives into a beast such as this; mine has a bit of hum that doesn't go away by just adjusting the Hum Balance pot.
The split chassis SVTs have more hum than most single chassis amps. Make sure all of your grounds are clean.
Very interesting video sir 🙏🏻thank you for this!
Very nice to see you service this amp ,I have a 1970 VT22 V4 head and its super loud but very clean so clean it dosnt want or like pedals and never get to get it to breakup ,the only way that could happen is being outside and a 20 foot cord
14:37 - Q-Tips soaked in Acetone did fine on my V-4B. No damage to board, rips right through decades-old flux.
Chrome power chassis says 1969 to me as that was the only year for the chrome... Someone changed it over from the original 6146 power tubes to 6550’s... pretty nice amp...
that is a beast of an amp! the music man amps run extremely high voltages as well!
They sure do!
Learnd some new stuff! Thanx
Happy to hear this! I have not done anything here since April of 2020, but am going to start again.
LOL !!! Get out the good old mallet. I like your style .
nice
. . . so did Keef!
I enjoyed the video. You mentioned that you may make a complete SVT service video in the future. Did this ever happen?
Thank you. What happened with the bad power tube/socket you diagnosed at the beginning of the video?
Hey there Wally I’m trying to buy an oscilloscope to do balance in my blue line SVT and I’ve found an RCA WO-91B and was wondering if you could tell me if that would do the trick? It’s got really nice tubes inside and it’s a good price here locally in Canada like $100 so about $60US and was wondering what you thought?
Recently did one with 6146b tubes. Nightmare.
WALLYS WATTAGE COTTAGE, what is the balance and hum trim pots adjusting? the AC or DC voltages to the preamp and power tubes heat voltages?
The drive balance pot is adjusting the signal level balance between the "push" and the "pull" sides of the power amp. This is kind of like a front/rear control for speakers. The idea is to get the output signal positive and negative wave peaks the same voltage (height).
The "ends" of the hum balance potentiometer (or pot) are connected between the 12.6V AC heater supply; and, the center wiper of the pot is connected to ground. The idea is to set the control until the preamp tubes are picking up the least amount of AC hum from the heater circuit. I hope this helps.
@@wallyswattagecottage1275 thanks for the help
1.) I'm confused how the Hum Balance pot can cancel out the noise/hum from adding more or less resistance from +12.6vac to ground because that is what the hum balance is doing is just adding more or less resistance from +12.6vac to ground or it is changing the heater vac voltage up and down?
2.) The Balance pot is adjusting the plate voltages or the cathode voltages of the power amp tubes to adjust the positive and negative peaks of the voltage height?
@@billwilliams6338 He answered you. If you still don't understand, let it go. He has other things to do.
is the heat gun trick a conventional method? ;) Was it used to simulate the output tubes heating the preamp section?
THE BIG B O Y E that thing'll kill ya
Great video except using that squealing signal generator that parts of you talking we could barely hear what you were saying otherwise make some more videos on these amps thank you
Can you work on my 73 svt needs roadworthyness answer comment if possible arrange transport if close to northeastern us I'm in quebec southeast steve
Thats a 1969 blueline
How can you tell? Curious because I’m having a hard time identifying one I found.
The chrome power amp chassis was only used in 1969 Blueline SVT’s… This is chrome… Great amp!
That’s not a 73
It’s a 69 ❤️
Wally !!!!!!!!!!!!!!!!! I just learned HOW NOT TO FIX MY OLD SVT !!!!!!!!!!!!!!!!!!! Go to LEE JACKSON UA-cam page, Much better way to do things !!!!!!!!!!!
@@JohnLysonmusic I'm so sorry I rattled your emotions. I can't even imagine what you'd say if I said, this guy was a FUCKING HACK !!!!!!!!!!!!!!!
@@JohnLysonmusic tell me when and where. I'll meet you half the way. Let's make it fun. Bring an amp. I'll bring mine. The one NOT RATTLED gets an amp !!!!
You couldn't fix a mains plug junior