Building a Bassman - Should I design for 6V6s?

Discussion in 'Shock Brother's DIY Amps' started by gabasa, Oct 23, 2021.

  1. gabasa

    gabasa Tele-Meister

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    Hey all, I'm getting the itch to build again. This time it's going to be a tweed Bassman in a 2x12 cabinet, paired with Jupiter speakers.

    There's a part of me that thinks it'll be a cooler amp if I use smaller transformers and design it around a pair of fixed-bias 6V6 resistors. I could probably make some clean looking adapters from a thin plate of aluminum to fit the smaller transformers into a standard Bassman chassis. These transformers might work:
    • Hammond 290BX power transformer.
    • Hammond 1750EP output transformer
    • Hammond 194A choke
    I could probably borrow the power amp circuit from the 5F10 and wire everything up on one eyelet board, with no cap doghouse.

    I feel like 6V6 output tubes would have more chime, jangle and nuts than 6L6s at any reasonable volume, but still be plenty loud to play at band volumes. Just wanted to know your thoughts and opinions of this idea.
     
  2. gabasa

    gabasa Tele-Meister

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    So basically an 80W tweed Twin that's 20W.
     
  3. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So do you want the lower voltage of the 5F10?

    You could keep the voltage higher with the 6V6 like the Deluxe Reverb to maintain the Bassman voltages.
     
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  4. gabasa

    gabasa Tele-Meister

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    Thanks for coming in.

    I don't want the voltages as low as in a 5F10, I'm just thinking it would be easy to borrow that part of the schematic, especially because I built a Harvard earlier this year and I can copy the bias circuit and board layout that I used for that part of it, just up the filter cap values a bit.
    • Hammond's 290BX PT (for 5E3) is rated at 660Vct @138mA
    • Hammond's 290CX PT (for BFDR) is rated at [email protected]
    • Both have 50V bias taps.
    I'm wondering which might be a better choice.
     
  5. gabasa

    gabasa Tele-Meister

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    For what it's worth, I'm going to risk embarrassment and share a bit of history that explains why I want to build this amp.

    Somewhere around 1990, before I ever had internet, I decided to modify a vintage Traynor Bass Mate head. They were cheap and plentiful back then and I wasn't crazy about how they sounded stock. I punched the chassis for a third 12AX7 and a tube rectifier, then I rewired it to sound more Bassman-like. It had cathode-biased 6V6s, and the midrange & presence controls were set internally with resistors.

    I loved that amp. It was Marshally in flavor when cranked, but it felt like an extension of my central nervous system. I loved that amp and miss it, so I thought that a 6V6 Bassman build might be the ticket for me this time. I am considering cathode bias this time too, just not sure. The embarrassing part? Here's the schematic I drew up back then, it looks like it was drawn by a kid in elementary school ..... and I honestly think I'd do worse if I tried to recreate it today.

    Modded YBA-2A Schem.jpg
     
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  6. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Hey nothing to be embarrassed about. I kinda prefer the hand drawn schematic anyway.

    Either PT will work. I might consider the 180mA version if you think you might decide to use 5881 tubes at a later date. I think Traynor used heavy metal so maybe to stay with that thought the 180mA PT, but cost may drive your decision.
     
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  7. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Briefly looking at your schematic, it looks like you've copied off the original schematic, which is not how the majority of 5F6A Bassmans were actually built. Look at the reissue schematic for the the correct tone stack values and PI changes.

    Also, 6V6GT(A)s have a higher amplification factor than 6L6GCs, so you don't need as much signal to drive them. There's a few different ways to deal with that, each with their own pros and cons.
     
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  8. gabasa

    gabasa Tele-Meister

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    Thanks. I have a 2x10 6L6 Bassman in the house which had all the correct component values, then I modded the tone stack to resemble the original drawing, which I prefer. I might be wrong about this, but I felt like the correct tone stack with 0.1uf bass cap and 100k slope resistor sucked mids a little more, which made the bright channel less usable. With the schematic's tone stack, I feel like both channels have their uses now. Might be hearing things though....
     
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  9. JamesAM

    JamesAM Tele-Holic

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    The 290bx I used for my 5e3 actually measured 342-0-342 with my wall voltage- not off by much, but high enough over spec to red plate tung sol 6v6es with a 250r bias resistor. If you’re going to use the 50v bias tap on that PT, you may want to use a trim pot instead of fixed bias to make rolling tubes easier. Ymmv of course!
     
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  10. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Another thing is that using a 500pF with 1M grid leaks on the LTP can cause hum, and potentially intermodulation distortion with anything less than 10nF (for preamps that can actually push the PI into clipping).
     
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  11. gabasa

    gabasa Tele-Meister

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    Thanks. If I go fixed bias, I'll definitely have a pot in there.
    If I go cathode bias, I'll use a higher resistor value.
    If my memory serves me well, I think I've got 300r or 330r in my 5E3 for this reason. Now I need to check....
     
    Last edited: Oct 23, 2021
  12. gabasa

    gabasa Tele-Meister

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    Yeah, that was a mistake in my drawing. I definitely had 0.02uf in there. Thanks.
     
  13. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    bandmaster 005.jpg I can't get too technical nor can I attest to the Tweed Bassman, but I do like the results when you change a 6L6 over to a 6V6. The tone is superior.

    I present my '63 Bandmaster....
     
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  14. Gris

    Gris Tele-Meister

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    I’m a 6V6 lover. And I have a modded gen I Bassmate w 6V6. And I have a low voltage 5F6A running 5881. BUT…, if I was going to build another amp, a 5F6 type, I’d totally build a 4 x 6V6 one. Cause I have an old RCA PA head that runs 4 x 6V6 and the Marshally growl on that thing sounds better’n any Marshall I've ever heard.
     
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  15. gabasa

    gabasa Tele-Meister

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    Wow, did you punch a couple of holes in the chassis for the extra power tube sockets?
    I can't say I disagree about 6L6 vs 6V6.
     
  16. gabasa

    gabasa Tele-Meister

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    I'd love to use 8 Ohm speakers so that I can run the amp at 4 Ohm with a pair.

    Hammond's OT is 8 Ohm output, that's it.
    Triode has this OT:

    http://store.triodestore.com/tf65wscta48opaper.html

    I figure I can run the speakers with the 4 Ohm secondary but use the 8 Ohm secondary in the NFB circuit.
     
    Last edited: Oct 24, 2021
  17. NTC

    NTC Tele-Holic

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    I have a 290BX in a 5F11 circuit and end up with 425 V on 6V6's biased at about 66% dissipation - and about 420V on a pair of 6L6GC's in the same amp - and they com out at bout 70% dissipation with the same bias voltage setting. The CX will give you more headroom. I think the BX would be sufficient for your purposes if you want less headroom.

    I like 6L6's better myself - I think they sound cleaner. But when clean isn't the goal, perhaps 6V6's are a better choice. 6L6GB's or 5881's in a bassman circuit sound very sweet as well, but they will have more power than a 6V6...

    Traynor was known for big iron. Could you find an even bigger OT? Part of that sound is the increased low end response. An actual Bassman OT would be about 4K:2ohms, which would come out to 8K:4 ohms - might actually work if you stay with 6V6's. My 5F11 has the OT for a BF Pro, which is 4K:4 or 8K:8 (as I use it).
     
  18. Milspec

    Milspec Poster Extraordinaire Silver Supporter

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    The holes are already there from the factory (just capped) as they shared the chassis with the Showman amp. A couple of sockets and some minor tweaks and there you go....4x6V6.
     
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  19. rdjones

    rdjones Tele-Meister

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    6V6 Bassman = Bassman 20 from the early '80's "Rivera" era.
    But the original Bassman philosophy was clean with big iron, so you might want something bigger than the 20's Princeton size OT, with a lower (6.6 K) primary.
    Definitely fixed bias, cathode bias will make it sound more like a 5E3 or MusicMaster Bass.

    https://el34world.com/charts/Schematics/files/Fender/Fender_bassman20.pdf
     
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  20. gabasa

    gabasa Tele-Meister

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    The hardest part of building an amp is deciding which one to build; this part of the process literally makes my head spin.
    Now I'm thinking of how nicely a proper 6L6 Bassman can fill a room with beautiful guitar sounds.

    The monkey wrench in all this for me right now is that @Phrygian77 talks here and there about how great his 1x12 blackface Bassman combo is and I daydream about it.

    Me like..... Me want......
     
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