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I killed my '66 Bassman - any ideas?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by smurfco, Feb 11, 2019.

  1. smurfco

    smurfco Member

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    Atlanta GA
    Sooo here's a sad story. I got a 1966 Fender Bassman head and cab on CL for a decent price. I've wanted to start working on amps for a while, having built pedals, rewired guitars, etc. I am not an electrician but I know some basics, and I make certain all caps are drained / check with a voltmeter / etc.

    All that is to say, when I am working on a mod, I am following instructions very closely, but I don't always know why I am doing what I am doing. So when something doesn't work I am at a loss for where to start troubleshooting.

    Back to the Bassman. A few months ago I replaced the electrolytic and filter caps, and the resistors that are prone to getting noisy. Everything was great. Amp sounded nice and some crackling / weird noises that had been present were gone. All seemed well. This week I decided to convert the balance bias to a more normal style bias so I could properly bias the amp. I followed the instructions here:

    http://el34world.com/charts/bias_conversions.htm

    I take out the resistors to the right of the bias pot, add my 220K resistors to the board, and get everything looking like this:
    [​IMG]

    Put it all back together, plug it in, and the volume is now way too low. I mean I had the amp on 10 and it was so quiet you could easily talk over it without raising your voice. I tested with multiple cabinets, speaker cables, guitar cables, guitars... I even tried new power tubes. No luck.

    I will say that the bias part of the mod at least seems to have worked - when I plug my EuroTubes bias probe thing in and adjust the bias pot, I can see the values I'd expect to see and they change for both tubes.

    Any ideas? I am at work now but can post board pictures later if that might be helpful. Like I said I'm learning this stuff and any help given would be much appreciated... Thanks!
     
  2. gldtp99

    gldtp99 Member

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    Are you sure you used 220k resistors in the bias splitter positions and not 220 ohm ?

    Also, is the speaker cable plugged into the Speaker jack and not the Extension Speaker jack on the back panel of the amp ?
     
  3. xtian

    xtian Member

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    You made the bias mods...but then, did you SET the bias? If so, how?
     
  4. smurfco

    smurfco Member

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    I am pretty sure they are 220k ohm but I can double check.

    And yes - Speaker jack not EXT jack (though I did try both).
     
  5. smurfco

    smurfco Member

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    Yes I did - I have the EuroTubes bias probe that gives you the current and the plate voltage. I used the calculator here: http://www.tedweber.com/webervst/tubes1/calcbias.htm
    ...to get the correct mA, then I turned the bias pot so that the mA reading was where it should be (around 48mA if I recall correctly). I then checked the other tube and was getting the same 48 mA or so.
     
  6. smurfco

    smurfco Member

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    Oh my God. I owe you a beer.

    I could have sworn I ordered the 220K ohm resistors from Amplified Parts, but I just checked my receipt and they are 220 Ohm.

    So my next question is - did I damage any other parts by doing that? Or if I take them out and put the correct resistors in will it work?
     
  7. gght

    gght Member

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    The knowledge here is amazing!!!
     
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  8. xtian

    xtian Member

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    No problem. By installing 220R resistors, you just dumped most of the volume to ground. And most of your bias voltage, too--so I'm glad you didn't red plate your tubes!
     
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  9. gldtp99

    gldtp99 Member

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    No damage should have occurred.

    It was just dumping more signal to ground (actually to the Bias circuit and then to ground).

    Remove the 220 ohm resistors and install the proper 220k's in their place---- bias up the output tubes and you should be in business.

    Here's my '66 Bassman:

    [​IMG]
     
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  10. smurfco

    smurfco Member

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    Awesome, thank you everybody. I feel foolish but also relieved that I didn't do any real damage.
     
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  11. gldtp99

    gldtp99 Member

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    You're very welcome--:)

    You're also not exactly the first one to make an error like this.

    Another common one is on a Marshall clone build is that the builder will install a 470k resistor on the cathode of the PI when the right value is a 470 ohm.

    This will bias the PI so cold that the amp will barely pass signal, if at all.

    Putting in the correct value resistor solves that one, too.
     
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  12. pdf64

    pdf64 Member

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    48mA - there’s no technical reason to idle the power tubes that high. Everything will run hot and wear out more quickly. 35mA should be plenty.
    If it sounds better then fine, but make sure you’re getting a tangible benefit.
     
  13. gldtp99

    gldtp99 Member

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    I agree with this.
     
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  14. BluntForceTrauma

    BluntForceTrauma Member

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  15. Plexiglass

    Plexiglass Member

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    Smurfco

    No wories, it happens, and it happened to me before. My mistake was 100 times too much resistance on the screen resistors with a similar result to you LOL. We are in the resistor hall of shame together.
     
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  16. HotBluePlates

    HotBluePlates Member

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    Yeah, I've seen this reported many time on another amp forum. It's an easy mistake to make, everything looks like it should, but the amp doesn't seem to function properly.

    I don't always do it, but many who have experienced making this mistake check their parts with a meter before installing in the amp. There are some brands/types of resistors whose color code is very hard to read (especially under yellow light from an incandescent bulb).

    Experienced guys don't always avoid making this same error. The real difference between a novice and an experienced tinkerer is the experienced guy knows when the amp doesn't work, he made some wiring or parts-value error, and goes looking for it immediately.

    Last time I did something like that, I mis-wired a negative feedback loop as positive feedback (and not the forgivable version of this mistake at the output transformer). As soon as I switched the amp on it emitted a full-power HOWL! Shut it off, knew right where the offending loop was in the preamp, and fixed the wiring error. Took all of 45 seconds to go from screaming banshee to purring tube amp!
     
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  17. Dan40

    Dan40 Supporting Member

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    I'm in the habit now of measuring every resistor with my meter before installing because I too have made this same mistake.
     
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  18. clcwarlock

    clcwarlock Member

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    Reading a resistor value wrong is easy to do. Always double check the resistor color code or just throw the meter on ohms to check. I’ve done this before reading the multiplier color band wrong.
     
  19. Tron Pesto

    Tron Pesto Member

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    Disregarding any extended discussion about the difficulties of reading color bands which has been done elsewhere in the forum, I agree it's best to do both - that is read the bands and test - particularly in those positions in the amp that a narrower tolerance is important like phase inverter plate load resistors.

    You could have 10% tolerance resistors labeled as 100K and 82K that have actual values of 110K and 74K or arguably worse; 90K and 90K - which would noticeably affect the performance of the amp compared to "dead-on schematic" actual values.
     
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  20. Tron Pesto

    Tron Pesto Member

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    I can't claim to be as experienced as some (there is always someone much more experienced out there) and I've made plenty of these types of mistakes, but here's a quick story of a novice (kudos to him) who recently decided to do a full overhaul of a true point-to-point two channel amp (it really was too big of a jump with his lack of experience, but hell, he learned A LOT). He basically replaced everything in the circuit!

    I got involved remotely - email and phone - helping him wire a three prong and actually remove the standby, then fix an anemic volume in all but one of the 4 input jacks. I kept coming back to the actual input wiring although we did find many things in other spots. It went on for weeks and weeks and I kept trying to get him to focus on the input wiring - it looked right in the pictures- at least from a connection point of view, but testing gave some odd results and at that point, we literally redid/tested everything else in the amp that could be the problem. And on one of the inputs (one of those "microphone" inputs without a grid stopper) provided full strong volume - so I knew the issue was up front.

    As discussed elsewhere in this forum, I'm color blind and particularly when looking at pictures, I can't even begin to take a shot at reading color bands... so I never focused on reading the nominal values of the resistors. Yeah, it ends up that he had installed 68 ohm resistors at the jacks instead of 68K ohms! And what made it tough before that was discovered is it was really tough remotely to convey to him how to properly read the tip to ground and tip to grid resistances with four inter-wired switched input jack wiring so I could never be certain of the values he was providing when we were testing it.
     
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