Problems? Maybe? Measuring bias

Discussion in 'Amp Tech Center' started by zachlovescoffee, May 4, 2021.

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  1. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    In post #85 I mentioned you could swap tube position. Usually the amp makes them somewhat uneven and the tubes are somewhat uneven. If the stronger tube is in the weaker side it is as even as it can be with that set of tubes.

    Usually one does not want them perfectly balanced in a guitar amp.

    There are balance pots that can added if the tubes are not matched.
     
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  2. zachlovescoffee

    zachlovescoffee TDPRI Member

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    LLC, good reminder. I actually did swap the weaker tube (v5) into the V6 position and it brought the balance different from 7ma to 5ma.

    The Genalex tubes are around a 5ma different by first test. I’ll swap them to see if that evens them out a bit.

    I just recalled that I bet I need to replace the grid leak resistors (220k pair).

    Looking at the robrob bias balance deal it looks like I’ll have to turn the now 10k bias pot into a balance pot and use a 50k trimmer pot to become the bias pot, plus using lugs, adding more caps into the situation and what not. I can probably figure it out but I wonder if it’s really necessary at this point? Is a 5-7ma imbalance that big of a problem?

    mad long I can bias them cool enough (reduce my 27k now 18k to 15k to give me more swipe) and the sound is good, then I’m good?
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    There is still that saying... "If it sounds good it is good".

    Some of the shock brothers would chase this down and end up with a balance pot. Ime it is not necessary with a guitar amp. I do understand striving for perfection so if you want to continue down the path that is OK. It may give you a better education. If you follow along with oscilloscope threads you will see the wave form as larger on top than the bottom. The larger one has clipping and the smaller may not. Your amp would show a similar pattern due to the mismatch. *Asymmetric clipping.* Chances are you want some of that.

    If you do change the 220K resistors, note their position and meter their resistances. Perhaps some of the difference could attribute to their actual resistance.
     
  4. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Both of those 220k resistors measured in at 230k or 231k. Within or right on the edge of their 5% tolerance. That's pretty freaking amazing for 52 years, assuming they were original. I still think it might make sense to replace them since I'm already in there.
     
  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Sounds like they are still good. A little less resistance there will help slow the onset of blocking distortion so I suppose you could make an argument for changing them. If you only play at clean volumes it wouldn't make a difference.
     
  6. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Howdy! I play clean/boosted with only mild overdrive engaged. I have a few different O/D pedals in my chain but use them only for light lead work. In general my chain is:

    Geetar > Compression (low comp) > EHX Soul Food (Klon Centaur clone) > Caline Crazy Cacti (Fulltone Full Drive 2 clone) > Caline Pure Sky (Timmy clone) > Delay > Loop > Amp.

    They are never all on at the same time. So it's only for fattening up the clean tone with very low gain added whenever I'm using any.
     
    Last edited: May 17, 2021
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  7. zachlovescoffee

    zachlovescoffee TDPRI Member

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    I popped in a pair of NOS JAN Philips 6V6GT and the TAD GZ34 and those tubes are biased 19.6 (v5) and 20.1 (v6). They sound amazing. Glassy highs, crystal clear, transparent, touch response unfelt before. Smooth as hell. Great bass response.

    it’s like the best of those way off RCAs and the new Genalex combined.

    Seems to be biases just a hair over 60% MPD (464 plate voltage). I’ll back the negative bias resistor down to 16 or 17k so I can roll off a few more milliamperes if I need to in the future.

    At this point I’m very very happy with the sound.

    my last changes in the amp before I button it back up is that in replacing the crappy PVC coated heater wires with some nice 18awg waxed green cloth solid core wire I have from some previous builds.

    I still want to unhook the AC outlet and 3-way switch and I think I’ve deciphered what @Wally was saying.

    Directly connect (by soldering together) the AC cord neutral/white to the black wire coming out of the PT that is currently going through the 3 way switch to the neutral on the outlet. Then connect the hot/black from the other side of the AC outlet to the fuse.
     

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  8. Wally

    Wally Telefied Ad Free Member

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    Interesting. I would have to know what those power tubes would do with a 5U4 and then a 5R4...out of curiosity. One would wonder what those JAN Phillips would do at lower voltages. You might find more results with these Phillips tubes and various rectifiers with to compare to what they do with the TAD GZ34...whatever that tube is. They might act very differently from the other tubes you were using. Obviously they cause different...lower...plate voltage than did the other power tubes even when those tubes were running on rectifiers that should be yielding lower voltages than that ‘GZ34’. It is interesting how different power tubes will draw different currents at different plate voltages even with the same rectifier. I have put various power tubes in the same circuit and seen very different plate voltages and current draws.
    As for this TAD GZ34, this is when it would be of interest to have a real GZ34/5AR4...NOS or UOS...to discern what type of tube this TAD GZ34 is...does it act like a GZ34???? One wants to know.
    Imho, it would be of interest to hear those tubes at a lower voltage and biased to that 60%. I like when preamp voltages come down closer to schematic numbers. Ommv. Ime, one very valid use of a Variac is to move the voltage up and down, rebias, and compare Sonic changes.
     
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  9. zachlovescoffee

    zachlovescoffee TDPRI Member

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    I’ll pop the 5U4GB in tomorrow and bias and see how they sound.
     
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  10. Wally

    Wally Telefied Ad Free Member

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    A complete voltage chart for the whole circuit with the GZ34 compared to the voltages in the circuit with different rectifiers and with the same plate dissipation in the power tubes would be of interest.
     
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  11. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Probably a stupid question but is there a chart that lays out which readings (ac vs dc) and which pins to measure and record on?

    This resource seems to have some useful measurements: https://chuck.cranor.org/mpr/testin/index.html
     
    Last edited: May 18, 2021
  12. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    upload_2021-2-17_8-14-1.png
    @King Fan posted this one on another thread. Something similar will do.
     
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  13. zachlovescoffee

    zachlovescoffee TDPRI Member

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  14. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Interesting development. When I soldered in the 22k resistor to the bias pot the entire unit choked.

    No output and no bias voltage. I pulled the resistor out and clipped in a different 22k and voila it’s working again.

    Has anyone experienced anything like this? I checked all my solders were perfect. I did have to run a little wire off of the resistor to reach the pot lug. Again, I wrapped the wire around the lead, soldered them together and heat shrink tubed it.

    Thanks all. Will report back once I solder this one in. Of course, this happens right when I have some free time from the family to take my voltage chart measurements!

    cc: @Lowerleftcoast @Wally
     
  15. Wally

    Wally Telefied Ad Free Member

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    Did you measure the resistor that did not work...all,of it from one end to the other end of the splice?
     
  16. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Yeah it measured out at 22k. That’s what I cannot figure out. I measured it in circuit and pulling it again.

    makes me think there is some kind of short inside the pot?
     
  17. Wally

    Wally Telefied Ad Free Member

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    Or a bad solder joint??? Curious it is.
     
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  18. zachlovescoffee

    zachlovescoffee TDPRI Member

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    Weird as heck. I pulled it out of the circuit and tested it and it was fine. I went ahead and put in a brand new 22k 1w I had that had nice long leads. No extra joins to deal with.


    Okay! The long awaited voltage chart as best as I could put together.

    I did as many measurements as I could pull together. Please take a look and let me know if anything looks askew or dangerous.

    Tube config:

    V1: 12ax7
    V2: 12au7
    V3-4: 12ax7
    V5: JAN Phillips 6v6gt bias at 18.1ma
    V6: JAN Phillips 6v6gt bias at 18.6ma
    V7: Tube Amp Doctor GZ34

    The amp sounds pretty amazing. The only thing I will say is that the tremolo doesn’t sound as lush as I would like. Plate voltage on pin 3 for those output tubes is 450vdc and I biased just a hair above 60% mpd. So I dunno. Also, it breaks up much earlier now around 4-5. Perfect at 2-3 for bedroom and if I really dig in I can break it up. 4-5 it’s lush just barely broken up for every pick. Past that it’s roaring. I’d like to get some more clean headroom out of it so I can use a few pedals on it. It’s pretty darn quiet now too, which is good. No weird hum or hisses or anything.

    take a peek at the chart and let me know if anything pops out! Thank you so much for this re-learning opportunity!

    cc: @Lowerleftcoast @Wally @Paul G.
     

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  19. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    You might try different 12AX7 in V4 and/or reduce the bias a little more to try to get more out of the vibrato.

    If the V1 cathode resistor is 3.3K, it can be 1.5K. It might help for a tad more clean. Reducing the 18K dropping resistor on the last filter node might get more clean. (You now know to clip a parallel a resistor across that 18K to try a lower value.) You might shoot for about 300VDC on V1 pin1.
     
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