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Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com
Asher Guitars WD Music Products Amplified Parts Mod Kits DIY Nordstarnd Pickups Warmoth.com

6v6 tweed bassman

Discussion in 'Glowing Bottle Tube Amp Forum' started by bluerolo, May 2, 2017.

  1. bluerolo

    bluerolo TDPRI Member

    22
    Jan 19, 2016
    NJ by the sea
    Hi all, hoping for some advise, I have a 5F6-a circuit bassman build that is true to the original circuit except for I had it built with:
    1. cathode/fixed bias switch
    2. Bias pot
    3. 2 x 8 ohm 10" speaker cab

    Great sounding amp as it is but for kicks I installed a 5Y3 with a set of NOS RCA 6v6's re-biased to 23 and really loved the sound even more as it did break up sooner I am just hoping to lower the power by using 6v6's. I am aware though that the OT that is in it with the 5881's is not correct for 6v6's.

    It has a Mercury Magnetics FTBP-59 power trans, ( a clone of the original)...and in speaking with MM I was told I could safely use a 5Y3 rectifier with this PT and then to match up with my 6v6's use a 8K primary OT. I would have to wire the 2x10's in series to get a 16 ohm load since the OT they offer with the 8K primary has only 8 or 16 taps. Their part number for this OT is FO50BM-8/16.

    So my question is does all this sound correct and will I be causing any reliability issues, I am aware that the 450v I have on the plates of those 6v6's is a bit high, any thoughts would be appreciated, thx.
     

  2. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    What's the primary impedance on your current OT? What taps?

    Example: If it's 4k, you can run your 16-ohm (series) cab off the 8-ohm tap, or 4 -ohm (parallel) off the 2-ohm tap--> and the tubes will see 8k reflected impedance. And that's just peachy for a pair of 6V6. I routinely do this with lesser iron than MM and have never had an issue.

    Also, if you are at 450V on the plates with your existing setup (5AR4/GZ34 rectifier?) running a 5Y3 will drop you to closer to 400V on your plates. Which also will be OK. And if your 5V tap has 3A you could also use a 5R4 or 5U4.

    Another also, if you don't want the sag of the 5Y3, but still need to drop a few more volts than the GZ34, get yourself a NOS (or used) 5V4.

    edit: I'm having trouble finding specs for MM stuff, like Primary impedance on OT and the voltages on the secondaries for PTs. Is this intentional on MM part? If so, that makes me peevish...almost as much as their prices do.
     
    Last edited: May 2, 2017
    Nickfl likes this.

  3. bluerolo

    bluerolo TDPRI Member

    22
    Jan 19, 2016
    NJ by the sea
    The 450v plate is what I come up with using a 5Y3. In my conversation with MM I did mention using my current parallel wired 4 ohm load with a 2 ohm tap and the response was in that scenario you are only using 50% of the OT and it would not be an optimum solution.
     

  4. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Just an observation, but that PT is not yielding proper voltages for a 5F6A circuit, which calls for 432vdc B+ with a GZ34 rectifier. Your voltages are high for a 5F6A even with the 5Y3. Vintage 5F6A's yield high voltages with modern wall AC, and they sound more like they should with a lower wall voltage.
    When doing a build, imho a voltage adjusted PT is preferable rather than a direct clone of an 8087 PT.

    https://schematicheaven.net/fenderamps/bassman_5f6a_schem.pdf

    I also don't quite understand the 50% thing when using that two ohm tap. ????? Anytime one is using the lower impedance tap one is using only a part of the secondary winding...unless the OT is built with separate winnings for each tap, which is less common than tapping points along a single winding for the dirrefent impedances.
     

  5. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yep, it's intentional. They will tell you anything you want to know but you have to call them. I like their stuff but that's weird.
     

  6. Commodore 64

    Commodore 64 Friend of Leo's Ad Free Member

    Age:
    42
    Mar 1, 2010
    Kent, OH
    The 5Y3 you are using, is it a current production? Many of those behave like a GZ34.
     
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  7. bluerolo

    bluerolo TDPRI Member

    22
    Jan 19, 2016
    NJ by the sea
    Thx for all the replies fellas, It appears to me that what I'm trying to accomplish maybe cannot be done elegantly, that is have this 5f6 that is using MM 59 Bassman clone PT and an OT that is yet to be purchased, be set up so I can reliably use either 6l6 or 6v6s. Here are the 2 scenarios presented by Mercury:
    1. use a 5Y3 ( Commodore, it's an NOS), wire the speaks in series for 16ohm use their 8K primary FO50BM-8/16 OT using 16ohm tap and all is good and matched for 6v6, but then when I need to use 6l6's I use the 8 ohm tap so the reflected impedance is correct for the 6l6s.
    2. second is use a original design bassman OT with 2 and 4ohm taps, speaks wired to 4ohm, all good for the 6l6's then when using 6v6 use the 2ohm tap to the same 4ohm load so the reflected impedance is correct or close.
    ** use 5Y3 with 6v6 and I use a 5V4 with my 6L6.
    I don't understand the reflected theory, if one of you guys has a minute to give me an idea of how this works that would be great.
    thx again for help
     

  8. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Four 8 ohm speakers in parallel/parallel = 2 ohms (standard 5F6A Bassman)
    Four 8 ohm speakers in parallel/series = 8 ohms.
    Four 8 ohm speakers in series = 32 ohms.

    Original design 5F6A output transformer is 2 ohms only.

    You need a new output transformer with 2, 4 and 8 ohms selectable (MM sells one).

    Wire the speakers in parallel/series for 8 ohms.

    For 6L6 select 8 ohms.

    For 6V6 select 4 ohms.

    You're golden.
     

  9. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    From my website:

    Running 6V6 power tubes in a big bottle amp will give you small bottle tone but with lots more power tube overdrive because the preamp is tuned to hit the phase inverter and power tubes harder than in a small bottle amp.

    To compensate for the different power tube impedance we need to adjust the speaker impedance. Since the little 6V6s put out much less current they need a higher impedance load. For a big bottle amp with a 4 ohm output transformer secondary we need to connect an 8 ohm speaker. With a 2 ohm secondary we need a 4 ohm speaker or with an 8 ohm secondary we need a 16 ohm speaker.

    JJ 6V6S power tubes are good for 500 plate volts so they can handle 6L6 voltage but for normal 6V6 tubes you must do something to lower the plate voltage to keep from frying them. For amps with tube rectifiers you can plug in a lower rated rectifier like the 5R4 (less sag) or 5Y3 (more sag) to bring down the voltage. Another option is to plug the amp into a variac and turn down the amp's input voltage. A bucking transformer at its lowest setting may also do the job.

    You may also need to adjust the bias after installing the 6V6 tubes. See this to measure and adjust the bias.
     
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  10. bluerolo

    bluerolo TDPRI Member

    22
    Jan 19, 2016
    NJ by the sea
    My bassman is a 2x10 in parallel, so 4 ohms, if I understand you correctly I will use the 2 ohm tap on ot when running 6v6....so with mercury or classic tones 2/4 ohm tapped ot if I add a 2nd speaker jack to chassis will I be able to wire one jack to 4 ohm tap and one to 2 ohm tap so when I want to switch tubes I will simply switch the speakers to appropriate jack? ... And of course swap out rectifier to 5y3 for 6v6's...and adjust bias, I do have a bias pot in it...
     
    Last edited: May 6, 2017

  11. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, that will work.
     

  12. tombob

    tombob Tele-Meister

    Age:
    44
    379
    Jun 25, 2010
    Sonora, TX
    I'm looking to start a 5e3 build and I've been thinking about doing a build using Russian 6p3p output tubes (not the -E version but the 20w approx ones) and a gz34. The kit I plan on starting with has large iron for a 5e3 including a Deluxe Reverb 6.6k, 25w OT which makes it seem like a good candidate for that.
    Of course that got me to thinking about doing a 5f6a with 6V6s and a 5y3 as another way to get about the same output from a tweed amp. If you use a cathode/fixed bias switch and a NFB switch on either build they're going to have some similar sounds. The bigger iron on the 5f6a is going to give it a deeper bottom and bigger tone. The 5e3 with 6L6s vs 5f6a with 6v6s both with bias and NFB loop options is interesting to me. Maybe I'll just have to eventually build both so I can a/b them and see the difference first hand!
     

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