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Blonde Fender Bassman - Mismatched Impedance

Discussion in 'Glowing Bottle Tube Amp Forum' started by DannyStereo, Apr 10, 2018.

  1. Jakeboy

    Jakeboy Tele-Afflicted

    Jul 26, 2008
    Sedalia, MO
    I have a Lil Dawg Ol Yeller 6g6b clone...4 ohm OT.....I run it at 4 ohms and at 8 ohms all the time....and I CRANK it into an 8 ohm attenuator with no ill effects thus far since 2013.
    I like the difference in feel an 8 ohm load gives the killer amp. I keep reading both sides, but IME, I’ve never had an issue with an old Fender circuit, with vintage or a clone built to vintage specs, having an issue with a mismatch in either direction. Now Marshall, Vox, Mesa, etc, etc. no way!!! I have heard too many stories about them not handling the mismatch.

    Someone should invent a little device rated at 8 ohms to plug into the ext speaker jack so when an 8 ohm speaker is also plugged in the amp sees the correct 4 ohm load.
     

  2. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    I believe Weber makes a Z Matcher that will correctly match impedance.
     

  3. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    1523581052827.jpg 1523581059391.jpg 1523581064707.jpg

    Looks like there are in fact some wires taped off
     
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  4. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    It means on the 8 ohm tap if it's of equivalent spec to an original Schumacher it should tolerate 4 or 16 ohms. On the 4 ohm tap it should tolerate 2 or 4.

    The fact it has been replaced gives pause for thought - did a tube flameout and take it to, or did the original let go? I've seen several where the copper just coroded - my tranny winder calls it green spot disease.

    Ask Wally pointed out, fitting an extra jack for 4 and 8 ohm outlets is a no-brainer.

    Even if you can mismatch a load, that's not the same as recommended.
     

  5. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    I'm not sure why it was replaced. The gentleman I purchased it from did mention it had been but I'm not sure why? I dont have the setup to test what load it is looking for. Maybe I need a trip to a tech
     

  6. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    1523581927742.jpg it is for sure wired in parallel to the EXT. SPKR. Jack
     

  7. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    Also... What's this variable pot here? 1523582077831.jpg
     

  8. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    That looks like a variable resistor trim pot to adjust bias. That's where you'd fit it.
     
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  9. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington

  10. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    That's what I thought. Where would I measure with my meter from?
     

  11. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    That's good - one problem I can see, is the original shorting jack is in place, presumably the 4ohm connection. That may mean the tan wire is the 8 ohm.

    If that is the case, you can use the jacks as appropriate, but you should remove the shorting jack .Otherwise, if you have nothing plugged into that extension it's presenting a dead short, cooking your transformer and at least producing distorted output. It's meant to save the transformer in case you forget to plug something in, but if you have multiple outlets in parallel it should be removed.

    Fender has wiring on the Hotrod series where if you plug in an extension speaker it unshorts the jack if nothing is plugged in.
     

  12. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    That would enable you to fit 1ohm resistors on pin 8 and measure the cathode voltage drop - across them. P=IR so I=P/R 25mv = 25ma, for instance because 25mv/1ohm = 25ma. Measure plate Voltage at pin 3 (carefully!) and you can work out an approximate bias point.
     

  13. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    I only ever run one cab with the head. So this won't be an issue.
     

  14. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    If you run something into the 4 ohm jack no. But if you run something into the 8 ohm jack yes.

    The 4 ohm jack you'll notice has that extra spring loaded tang. With nothing in it, it's shorting the 4 ohm winding to the chassis. Not good.

    So if you run an 8 ohm cab in the jack to the left of your picture above, the other jack is a dead short.

    Edit: looking at that pic I can only see two wires out of the secondary through the grommet. So it's only got one speaker tap connection.
     
    Last edited: Apr 13, 2018
    DannyStereo likes this.

  15. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    Bump for other thoughts on an 8ohm load?
     

  16. Wyatt

    Wyatt Tele-Holic

    998
    Nov 3, 2004
    You're there.

    If all the data you posted is correct,...it is already wired for 8-ohms in traditional Fender style.
    • Yellow wire is the 8-ohm tap
    • Black wire is the common
    • The other cloth wire connects the jacks in parallel
    • The shortening jack is NOT a liability — running for a duration with a shorted secondary will not kill an OT, but running for minutes, even seconds, with an open secondary will kill an OT — so that's why Leo's boys used a shortening jack in the first place, to save the OT in the case that you forget to plug a speaker in
    • The only caveat is, unless they also changed the negative feedback circuit, you have roughly twice the negative feedback voltage as stock because it was wired for a 4-ohm output. It that's true, you amp will run cleaner (and be a little less lively) than stock
    You have two additional options if you want,...
    1. Remove the Ext Speaker jack and add an impedance switch, allowing you to set the remaining jack to 4-8-16
    2. Wire the two jacks up like a Hot Rod Deluxe, where the 8-ohm tab is connected when the Ext jack is NOT use, and when you plug a second cab in, it switches the circuit to the 4-ohm tap
     
    Last edited: Apr 13, 2018

  17. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    I think I like the idea of an impedance switch.

    For all those who care, the gentleman I bought it from and I e-mailed a little today. He said he wasn't sure why the old tranny went or if it DID go as he bought it with the new one already installed by the previous owner's tech.

    He bought and rocked the Bassman in combo format (with the 8ohm Eminence above) for about 10 years, playing a Hamer Doublecut LP style guitar straight in for a blues trio and never had any issues.

    Seems to me, from what I can surmise based on the evidence, that at some point someone WANTED this head to live out its days in a 1x12, 8ohm combo. If only I could ask this amp the things she's seen...

    All in all, I DO notice a significant tonal difference when running at 8ohm vs 4ohm. It seems like the amp is more alive and responsive at 8ohm. That's part of why I've been scratching my head!!

    The inspiration for this whole tone quest into a Bassman is I've had some conversations with a fella named Price Stevens, current guitar player for John Mark McMillan. His advice for what I wanted from an amp was "get an old Bassman." Boy, was he right.
     
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  18. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    ‘Get an old Bassman’.....there are a fair number of ‘old Bassman’ circuits, and each one is different. The hottest Bassman circuit is the AB165 Bassman and its descendants.....through the Bassman 50in the SF era. These amps have three gain stages for the Normal(guitar) channel and therefor have hotter preamps....more agressive breakup and livelier sonics, ime.
    The 6G6B is a very different amp, ime, from that hot AB165 circuit. The 6G6 is cleaner and not as agressive when pushed...there are only two gain stages in the preamp compared to the 3 in the AB165 type of circuit.
     

  19. DannyStereo

    DannyStereo Poster Extraordinaire

    Aug 13, 2014
    Kelso, Washington
    As I'm doing more digging, this old girl is likely a late 1961/early 1962. The tube chart inside is actually marked 6G6-A, production run "22."

    Ampwares distinguishes the amps based on their covering as well - mine has rough blonde tolex, oxblood cloth. I was orginally thinking 63 because of the flat Fender logo, as evidence by my message to my friend below... But the research would indicate she's maybe just a wee bit older.

    My mom was born in November of 1961, and I kinda hope this amp was too .Kind of a neat coincidence. I may tell her it DEFINITELY was just so she can get a kick out of it. Lol

    Screenshot_2018-04-15-20-26-21.png IMG_20180415_202059_437.jpg
     

  20. Dacious

    Dacious Poster Extraordinaire

    Mar 16, 2003
    Godzone
    Well actually if he runs the shorting jack unoccupied i.e. shorted, even with a speaker plugged into the extension, I'd suggest at the least he's going to have distorted and low volume output.

    If you have a shorting jack and it's not occupied it needs something holding the poles open.

    Annecdotally Eddie Van Halen played a Brown Bandmaster IIRC that way for recording.
     

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