Need Help with Loud Hum in ToneKing Falcon Grande Amp

Discussion in 'Amp Tech Center' started by aucklajc, Jan 22, 2020.

  1. aucklajc

    aucklajc TDPRI Member

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    Last year I purchased an early, used, Falcon Grande amp through Reverb.com. It worked very well with only minimal use in my home since then. Yesterday I decided to try some good (Sencor tested) vintage 12AX7’s that I had used in other amps to see if I could make the clean/rhythm tones less boxy sounding. After trying several tube combinations the amp started to hum, loudly. I put the original TAD 12AX7’s back in, but the hum continues. It sounds like 60hz hum, not fizzy at all, sort of like when you bring a guitar too close to the amp. The hum is not affected by volume, tone or reverb controls. The attenuator does reduce the hum to almost nothing at its lowest setting. My guitars will play through the amp, but the hum is louder than the guitar. Unplugging the guitar cord from the input makes no difference, the hum continues. I moved the amp to another room and plugged into another circuit – no change in the loud hum. My other amps continue to work fine. I tried removing one tube at a time to see how that would affect the hum: V1 – no change, V2 – no change, one power tube – lower hum, both power tubes (all tubes removed) – no hum. There is no hum with the amp in standby. With the power tubes installed and turning the standby to operate before the tubes are fully warmed up, the hum goes from nothing to full noise as the tubes warm up. One other interesting thing is that prior to the hum, the vintage USA 5881 power tubes measured 41- 42ma at 400VDC (amp is cathode biased). After the hum started, one power tube measures 56ma and the other 38ma, both at 400VDC! I have tried a dozen different 5881 and 6L6 combinations and the results are similar. The higher current socket is always higher regardless of tube rotation. I have taken the chassis out of the cabinet to look for any obvious sign of bad component(s) or loose ground connections – nothing. I have looked at the solder joints through magnifying glasses and checked for continuity on all ground connections from the mains to the end of the ground conductor in the pcb. I have tap tested all component connections to the board. The voltages going to the power tube coupling caps are identical. Filament voltage is 6.7VAC.
    This amp is not covered by warranty.
    I am an amateur guitar and amp player and repairer (my retirement hobby). I have overhauled/fixed dozens of tube amps, but am not an expert as many of you are. This one has me stumped, so I am looking for any suggestions of what else I can try before I have a professional tech look at it.
     
  2. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    Do you have access to a schematic for the falcon grande?

    60hz hum or 120hz hum? Maybe filter caps, or power tube bias cap? just guessing.

    Tube sockets crack/dirt/solder joint?
     
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  3. EsquireBoy

    EsquireBoy Tele-Holic

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    Since it has started after swapping tubes several times, I’d vote for that too.
     
  4. tizianod

    tizianod Tele-Meister

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    I would suspect the caps (filter caps, bias cap) as well. You say it's starts as soon as the tubes warm up in standby mode? Maybe you should check out the filament circuit as well.
     
  5. aucklajc

    aucklajc TDPRI Member

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    Thank you all for your quick and thoughtful responses. I really appreciate your help with this amp!
    It is 60hz hum.
    The amp is dated June 2015.
    I don't have a schematic for it, nor can I find one anywhere on the internet. If anyone has one, please give us a link.
    I have checked the tube sockets and their solder joints - all look good and solid.
    I paralleled the first and only primary filter cap (47uf) with a new 47uf. The hum was reduced somewhat, very similar to when I remove one power tube, but still loud.
    The hum is not present in standby, only when I put the switch in operate.
    It sounds like a bad ground somewhere, but I have checked all of the ground points and everything seems solid. It is also strange that the current draw on the power tube plates became so mismatched at the same time. It is cathode biased, there is no bias that I can adjust - similar to an old Fender Deluxe. If I leave one power tube in, and then replace the second one with a cold tube and let it warm up in the operating mode, I can hear the hum start as the plate current rises to about 30ma, then the hum disappears until the plate current reaches 40ma, then the hum starts again as the current increases to above 50ma and stays loud. This is consistent no matter what power tubes are installed in either socket (I have tried about a dozen different 5881s and 6L6s).
    Thanks again for your help. Please keep asking questions/making suggestions!
     
  6. Tenderfoot

    Tenderfoot Tele-Holic

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    Mark Bartel <bartelamps@gmail.com> might be able to help you as he is (IIRC) the designer of the amp. I know he designed and supported the Tone King Imperial (which he designed) until he sold his interest to Friedman. He was very helpful helping me correct a low level hum in my Tone King Imperial. He now designs and sells high end Bartel Amps.

    Good luck.
     
  7. aucklajc

    aucklajc TDPRI Member

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    I have contacted Mark and am hoping for a response. In fact this is one of his personal prototypes of the Falcon Grande.
     
  8. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    Filament centertap or artificial ct? Check elevating resistors and solder joints.
     
  9. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Or one of the tubes you tried had an issue and blew up something else.

    If you're not comfortable with cracking open an amp and playing with high voltage, it's tech time. Even without a schematic, that type of amp wouldn't be difficult to work with.
     
  10. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    Is the power tube bias circuit just like 5e3 with one resistor and cap for both tubes? Or, does each tube have a resistor and cap?
     
  11. aucklajc

    aucklajc TDPRI Member

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    I assume that the filament supply is artificial center tap because there is no green and yellow wire from the PT. However, I can't locate any 100 ohm resistors on the pcb, and the traces are very hard to follow due to the thickness of the board. The two green wires from the PT measure 6.6VAC between them with the switch in standby, but when I put the standby switch in the operating mode, and the hum starts, voltage between the green wires measures 0. However, the voltage on the filaments in the preamp tubes and power tubes measures 6.3VAC in both positions of the standby switch. Not sure I understand how it works on this amp, but the heater voltages seem right on.
    All of the tubes that I tried were previously tested on my Sencor (not the best, but hasn't failed me yet?), and worked in other amps - but things do happen!
    The power tube cathode bias is like the 5E3 except the one cathode resistor is 300 ohms, and it does have a 22uf bypass cap. All connections from both power tubes seem solid and the cathode connections to the 300 ohm resistor and from the resistor to ground measure correctly.
    Thank you again for your help and questions, keep them coming.
     
  12. Wally

    Wally Telefied Ad Free Member

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    You have an imbalance between those two power tubes. That is your hum issue. Since you have ascertained that one socket is drawing much more current than the other no matter what tubes are in the amp, I am going to think there is a problem in that socket....something the circuitry attaches to that socket has gone bad. That circuitry would include the bias circuit. It is tech time.....
    IF you are your tech, then start taking some voltage and continuity measurements in that circuit.
     
  13. Wally

    Wally Telefied Ad Free Member

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    And...ime...Mr.Bartel is a great fellow. He will probably be interested in how one of his prototypes is doing. Now I am wondering what I have done with my Imperial MKII schematic....or is it locked away in my dead PC????
     
  14. Wally

    Wally Telefied Ad Free Member

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    The coupling cap from the phase inverter can throw bias off if it goes bad. Look at the one feeding that HOT socket.
     
  15. Dacious

    Dacious Poster Extraordinaire

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    Also check that in juggling tubes you haven't managed to unseat something - I'm not familiar with that amp but something like a cocked reverb cable can cause this.
     
  16. Unionjack515

    Unionjack515 Tele-Holic

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    So you’ve got a prototype of the Falcon Grande? Did you buy it from Mark directly on Reverb?

    Interestingly, I bought his personal Falcon (1x10) directly from him on Reverb.

    Though when I asked him to sign an letter authenticating it...silence...ha
     
  17. aucklajc

    aucklajc TDPRI Member

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    Yes, this amp came from Mark through Reverb. And he has graciously offered to take a look at it and repair it at minimal cost plus shipping. I feel more comfortable having him work on it since it is his prototype. And, I suspect that it will not cost much more to have him fix it than it will to have a tech an hour or two away look at. This way I know it will be fixed right.
    BTW, the PI to power tube coupling caps and resistors all measured OK and within 10% of each other.
    Wally - if you find your Imperial MkII schemo, please let me know, it might be helpful.
    Thank you all again for your help with this. If I hear from Mark about what he found, I will let you know.
    Jim
     
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  18. Wally

    Wally Telefied Ad Free Member

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    I can understand that. All one has to is to look at one of these amp’s circuit to know that it is authentic.
     
  19. Unionjack515

    Unionjack515 Tele-Holic

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    Ah...no. Authenticating that it was his.
     
  20. Wally

    Wally Telefied Ad Free Member

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    Yes, I understand that, and I think that I understand why he might have not wanted to ‘authenticate’ an amp that he built as being anything other than an amp he built. But hey.....do you have a receipt or record of the transaction? There is your authentication, right?
     
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