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Thread: Strange results with new 6550 tubes

  1. #1
    ppp Pianississmo jonhut's Avatar
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    Strange results with new 6550 tubes

    Hi,
    First, I should say that I am a beginner in all aspects of this.

    I have always loved the Hammond organ sound and have thought that some day I might have one and learn to play it. Last summer I was unexpectedly given a 1947 BV with a 122-RV. Both had issues. I committed to learning how to fix them. I recapped the TWG and the pre-amp in the organ which made it work and sound much better, but with any volume it distorted so I started looking into the Leslie.

    Fast forward to now- I recapped the amp which did make it sound even better, but did not resolve the distortion issue. Today I received a new set of tubes. I started by replacing the two 6550S with new Sovtek ones. Here's what happened:

    First thing I noticed was that it was way louder, and way clearer. It sounded great, better than I thought possible. Then I switched the Leslie to tremolo and the volume dropped. It still sounded good though and pushing the peddle brought the volume up, and it still sounded good. Then the volume jumped back up again......still sounded good so I played it some more for several minutes, switching the Leslie speed back and forth to see if it had any affect on the volume again- It didn't. Then, after about 5 minutes the low notes started making a very bad distortion sound, I thought I must have blown the woofer (i was cranking it a bit to see where it would start to distort), it sounded awful. I turned it off. -Here's where it gets weird- Just to see if it could be the new tubes causing the problem, I took them out and put the old ones back in. I was expecting it to be bad, I was pretty sure I had fried the woofer and expected it to go back to it's previous less clear and and more distorted (anything beyond low volume) sound.............But.....it didn't. Not only was the new terrible low end sound gone, but it still sounded fantastic! Remember, this is with the OLD tubes put back in; it never sounded like this before. It is clear, and bright, and clean, and loud; no volume jumping it sounds great! And for the first time after hearing about it for months, I know what key click is, I never heard it before.

    So what the heck is going on?; what do I do now? It sounds great now but something must be wrong ..right?

    Again, I am learning all of this as I go, I greatly appreciate any advice you can give me.

  2. #2
    ff Fortissimo Sweet Pete's Avatar
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    Welcome to the Forum!
    Did you also change the volume control pot on the 122? Does the 122 kit have a fresh cap and resistor? How many V DC do you have on pin 5 powering the B+ to the preamp?
    Sounds to me like cathode bypass on the 6550's woke up when the pin finally made contact. A dirty tube pin can do this. Also a dirty broken socket .
    The relay dragging down the switching is most likely a cap/resistor inside the box....
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  3. #3
    fff Fortississimo David Anderson's Avatar
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    Quote Originally Posted by jonhut View Post
    Again, I am learning all of this as I go, I greatly appreciate any advice you can give me.
    The problem with this case is that you have too many parameters in play here to draw any definite conclusions.

    You don't know the source of the distortion. What you know is that you did something that affected it, but what? When you replaced the tubes, did you disconnect and reconnect something that improved a connection? Did simply removing and replacing the tubes a couple of times clean a dirty tube socket contact? We can't know.

    For example, in Leslies, the 12AU7 pins are a frequent source of intermittent connection problems. I've seen cases where just tapping the side of the amp would cause a pin contact problem for the 12AU7 to come and go.
    I'm David. 'Dave' is someone else's name.

  4. #4
    ppp Pianississmo jonhut's Avatar
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    Thanks, both you guys for the quick responses.

    I did not change the volume control pot, as it seemed to be working; I was always adjusting it between various stages of repair -i did things in small stages and put everything back together between stages and tested it each time- and it seemed ok. It never did the volume jumping and dropping before today (only right after the new tubes). The volume is now set between 4 and 5. To clarify, when the volume jumped back up, it was not when I changed the speed. When it first dropped it was when I switched to tremolo, but it popped back up when I was just playing along in coral... After putting the old tubes back in it has not done it at all. I played it for about a half hour after putting the old ones back and it just sounded great with no change.

    I replaced all the caps and resisters except the two .05 "death caps?"; the replacement kit I got did not include them and there was a note that said if they were there to remove them. There was no further explanation and i was not sure if I should just remove them, as in just cut them out, or if they need to be replaced with something (they tested good). In the process of all this I had the amp in and out of the cabinet multiple times. I cleaned the volume pot, the tube pins and sockets with deoxit. Each time I took the amp out and worked on it, I removed all the tubes and then replaced them each time i put it back in to try it. Aside from various general sound improvement, with the stages of part replacement, it never displayed any of this behavior. Even though the sound quality improved with each stage, it still distorted with any volume past med/low and the general sound was still a bit muddy? I have been playing it for about two weeks since I finished the recap and it has been consistent in its sound and behavior.

    But, it does seem that putting the new tubes in today changed something or, as you say, "woke up" something. Then the bad sound on the low end,... took the new tubes out, put the old ones back....sounds better than ever (I have not replaced the the 12AU7s or the 38205, just the 6550s.)

    I have to confess, I did not test any voltages (of anything). I will test the DC voltage on pin 5 (After I learn how). I tested the capacitors and resisters and several of them where out of tolerance so I replaced them all. When you say the "box", do you mean the volume box? I replaced the caps in that when I did the preamp, but not the resisters.

    I am wondering if I should put the new 6550s back in and see if it does the bad sound on the low notes again... I am not going to do anything without more research, and hopefully more advice from you guys. I am willing to learn and do whatever I have to, to get this thing working and sounding its best.

    Thanks again



  5. #5
    mf Mezzo-Forte Jaim's Avatar
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    Check out George Benton's site for Leslie switching info: http://bentonelectronics.com/servici...2-amplifier/#7

    The "death caps" should be removed. Again there is good info in the archives.

    Jim

  6. #6
    p Piano Jyvoipabo's Avatar
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    Quote Originally Posted by jonhut View Post

    I have to confess, I did not test any voltages (of anything). I will test the DC voltage on pin 5 (After I learn how).
    Hello,
    Voltage shall be Zero measured between chassis (Voltmeter -) an pin 5 (Voltmeter +)
    http://bentonelectronics.com/servici...2-amplifier/#1
    'The first step in servicing the Leslie Amp is to check the Voltages at various points to determine the amp’s operating condition.'

    JP

  7. #7
    ppp Pianississmo jonhut's Avatar
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    Thanks for all the help, I will do all of this. It will take a while (still have a day job).

    The Benton articles are helpful, thanks.

    I looked up more about the death caps and I am still not clear if they need to be replaced with something or just taken out.

    One thing Benton mentioned is that sometimes a tube can work fine when first turned on but once it warms up, it "over conducts" and starts causing distortion. I wonder if that's whats going on with the new Sovtek 6550s? Doesn't explain why the it worked great after putting the old tubes back though....but it certainly wouldnt be the first time two unrelated things happened at the same time...

    I guess I just need to get at all this voltage testing. I will report back with findings.

    Thanks

    P.S. After playing it (on and off) all day yesterday, I noticed the high end is starting to sound fuzzy....low and mid still good. This is still with the old tubes. I have not not tried the new tubes again yet.

    I know...check the voltages.

  8. #8
    fff Fortississimo enor's Avatar
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    Quote Originally Posted by jonhut View Post
    I looked up more about the death caps and I am still not clear if they need to be replaced with something or just taken out.
    Just take them out.
    Current organs: AV, BC, A-100
    Current Leslies: 22H, 142, 147, 760
    Organs in the past: L-100 (several), M-100 (x2), T-100, E-100, CV

  9. #9
    mf Mezzo-Forte Jaim's Avatar
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    The purpose of the caps is to shunt AC line RF interference to the chassis. If the caps fail by shorting, then you will have AC line voltage on the chassis looking for a path to ground which can be supplied by the user. Remove the caps. If you have an RF interference problem, safety caps (Y2 rated) can be installed. These caps are designed to open when they fail.

    You should also preform the stray voltage check which is in the front of the service manual.

    Jim

  10. #10
    fff Fortississimo David Anderson's Avatar
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    Quote Originally Posted by Jaim View Post
    The purpose of the caps is to shunt AC line RF interference to the chassis.
    It's a minor quibble, but the actual point of death caps is to shunt RF picked up by the amp chassis to AC neutral, which is supposed to be tied to local ground at the breaker panel. If were just to eliminate line noise, you'd only need one capacitor across the AC lines and wouldn't need a connection to the chassis at all. Leslie used two capacitors since, with a non-polarized two-wire setup, you don't know which is Hot and which is Neutral, the same reason that older Fender amps had a Ground switch that toggles the death cap from one AC input to the other.

    General practice these days is to remove them for safety reasons, or, as Jim notes, to replace them with special safety capacitors designed to fail open if you're in an area that suffers from high levels of RF interference, usually near a radio transmitter.
    I'm David. 'Dave' is someone else's name.

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