Possible to mod a Bassman Micro to use two power tubes (quad triode)?

joulupukki

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Random thought...

Out of the four amps I've got, 95% of the time I flip on my Bassman Micro (JTM45 Micro). It's just such a great versatile amp, takes pedals well, etc. It definitely gets loud for bedroom/home application solo, but let's say I wanted to use it with a band outdoors. I'm curious to know if it would be possible to double up the power output tubes so there'd be 2x 12AU7/12BH7? I'm guessing I would need to change the output transformer as well to do that to something maybe around 10 watts instead of 5w?
 

joulupukki

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Hmm, maybe, though it means I have to sell one of them to make room. Not sure I'm ready for that ... yet. Was just curious how it'd turn out or if anyone had experimented with such a thing.
 

robrob

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The Bassman Micro with 12AU7 power tube puts out around 1 watt. As printer2 said, doubling the power to 2 watts will add 3 db of volume, more than likely not nearly enough to use with a band outdoors. Switching to EF80 power tubes would get you to around 4 or 5 watts (still not enough) and would use your 12AU7 output transformer. See the Bassman Micro EF80 layout on the Bassman Micro webpage for more info.
 

SerpentRuss

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You probably don't have 5 watts of power now. Did you do a line out? How about treating your micro as a pre-amp and build a power amp, just a PI with master volume and power tubes.
 

RetiredUnit1

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You would probably get more volume by switching to a more efficient speaker, or driving more speakers. My 5e3 is WAY louder on my 4x12 than it is through a 1x10...
 

joulupukki

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You probably don't have 5 watts of power now. Did you do a line out? How about treating your micro as a pre-amp and build a power amp, just a PI with master volume and power tubes.
I didn't do a line out. If I'm gonna build a power amp, I might as well rebuilt an entire amp. :)
 

joulupukki

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The Bassman Micro with 12AU7 power tube puts out around 1 watt. As printer2 said, doubling the power to 2 watts will add 3 db of volume, more than likely not nearly enough to use with a band outdoors. Switching to EF80 power tubes would get you to around 4 or 5 watts (still not enough) and would use your 12AU7 output transformer. See the Bassman Micro EF80 layout on the Bassman Micro webpage for more info.
Ah my mistake. This is why I ask before doing. :)

On your Bassman Micro site near the beginning you state:
The 12AU7 with both triodes in push-pull will develop around 3 to 4 watts of output power. Swap in a 12BH7 and the output increases to 4 to 5 watts.
Later on, when describing the EF80 option:
This version of the Bassman Micro LTP uses two EF80 novel pentodes in push-pull for true pentode power tube overdrive with about 2 watts of output power. Pentodes overdrive differently than triodes due to their screen grid so this is a way to get pentode overdrive in a 2 watt power amp.

This is why I decided to go with the 12**7 build instead of the EF80. I figured I'd get more volume out of the 12BH7 than the EF80 power tubes, but it sounds like maybe I didn't quite understand it all...and obviously still a little confused.

No worries. I was just curious. It's a great amp regardless. If I could rebuild it (or make another one) I would shoot for something in the 12 - 15 watt range with a built-in attenuator (or Type II MV). That seems like it'd be the sweet spot for any gigs I would do (smaller indoor/outdoor shows) plus still be great for indoor use.
 
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robrob

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I'm sorry, joulupukki, those 12AU7 and EF80 numbers were from my pre-build estimates. I edited and corrected the page.
 
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joulupukki

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I'm sorry, joulupukki, those 12AU7 and EF80 numbers were from my pre-build estimates. I edited and corrected the page.
No problem, it’s still a great amp. Out of curiosity, if I added another noval tube socket for two EF80 power tubes (and replaced a few resistors to the right values), would the Hammond 270AX PT still be fine to use?

With the 12BH7 as the power tube these are my B+ readings:

B+1: 310.6 V
B+2: 308.6 V
B+3: 264.6 V
B+4: 183 V

I had the fine pleasure of meeting @King Fan in person earlier this week and we played through each other’s newly-built Bassman/JTM45 Micro amps. Man does he build a super clean amp! Very smooth, very quiet, etc. One thing we didn’t try out exactly was to crank them all the way and do a quick A/B comparison. But, they did seem both plenty loud for in-the-house use. One main difference between the two amps is that he’s got a 12AY7 in V1 and in mine a 12AX7 so mine seemed a bit more growly, as expected.
 

King Fan

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No problem, it’s still a great amp. Out of curiosity, if I added another noval tube socket for two EF80 power tubes (and replaced a few resistors to the right values), would the Hammond 270AX PT still be fine to use?

With the 12BH7 as the power tube these are my B+ readings:

B+1: 310.6 V
B+2: 308.6 V
B+3: 264.6 V
B+4: 183 V

I had the fine pleasure of meeting @King Fan in person earlier this week and we played through each other’s newly-built Bassman/JTM45 Micro amps. Man does he build a super clean amp! Very smooth, very quiet, etc. One thing we didn’t try out exactly was to crank them all the way and do a quick A/B comparison. But, they did seem both plenty loud for in-the-house use. One main difference between the two amps is that he’s got a 12AY7 in V1 and in mine a 12AX7 so mine seemed a bit more growly, as expected.

Hey, @joulupukki builds a super clean amp too -- and plays some gorgeous guitar riffs. It was a fun comparison. His 12AX7 version with a dual triode and my 12AY7 with dual EF80s sounded pleasantly similar given the V1 differences. Maybe non-identical twins? We did use the same speaker and cab (a 12A125A).

Re loudness both amps easily played nice loud household volumes; as noted above, if there's a wattage difference, it's apparently small enough that the dB (and SPL) difference isn't that noticeable.

We didn't swap V1s to run the same tube in both, but when I've run a 12AX7 in the past, mine sounded even more like his; a more throaty, more gutty OD.
 

robrob

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310v B+1 is a little high for the EF80 but I would try it and if you burn up a power tube too quickly then a B+ dropping resistor may be in order to get it down about 15 volts.
 

King Fan

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A smallish dropping resistor would be easy in this amp; might sound good too. Hmm. Could the B+ change a little with the EF80s? Don't know.

FWIW, @joulupukki , I found my favorite B+ calculator page nicely predicted my actual B+ -- and current draw. If I had a 200V Antek, for example, this shows how I woulda filled in the 'custom' choices for EF80s and my 22.5K OT. The results include a correction factor if you have a choke, and a pretty good estimate of the current draw across the B+ -- which allowed me to calculate ahead for possible dropping resistor sizes.

1660325171571.jpeg
 
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joulupukki

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@King Fan thanks for that. This probably won't be the last time I admit confusion over power transformers, but I'm not quite sure what value to enter for the Power Transformer Voltage for the Hammond 270AX, which in their documentation shows 480V C.T. @ 58ma. in the "A.C. High Voltage Secondary R.M.S." column of their spec sheet.
 

King Fan

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So 480 C.T. means 240-0-240; you'd just put 240 for custom transformer voltage.

Side note though. Based on our nice A:B comparison, I'm wondering what you want to gain by going to EF80s. Your goal mentioned above, an amp that's meaningfully louder, doesn't seem likely based on what our smart friends point out -- doubling wattage increases volume by just 3dB, and as I'm always reminded, "A change of 3 dB is accepted as the smallest difference in level that is easily heard by most listeners."
 

joulupukki

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Ah, gotcha. 240-0-240 makes more sense. That was my suspicion but I just wasn’t positive. Thanks for setting me straight on that.

So yeah, the goal would be to give the amp a little more umph. But, as pointed out, that’s not really gonna do much. I actually plugged it into both 1x12 speaker cabs I’ve got and it was a bit louder. For now, I’m not gonna worry and I was mostly curious about it all … and now I’m set straight on the actual power capabilities of the 12AU7. I do wish I could get a bit more clean headroom so it could be a tiny bit louder before breakup, but it’s fine – I’ve got an 18 watter right next to it that has the exact opposite characteristic – SOOO loud. ;)
 
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joulupukki

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Ok, the wheels are still turning. I was reading more about the differences between the AB763 circuit on Rob’s site and he says that with a higher plate voltate on the preamp tubes you can get more gain and clean headroom. I’m curious if that same idea would apply on the micro amp — if I lower the value of the voltage dropping resistor between V2 and V3 on the micro amp could it have the same outcome? It would be nice to eek a little more volume out of it before it starts to break up.
 
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