Swapping output transformer for more volume

[TacosSupreme]

So last year I built the infamous Bassman Micro based on Rob Robinette's schematics. Since this was my first amp build I did the one with cathodyne phase inverter towards the bottom of this page (only 3 tubes using a solid state rectifier):
https://robrobinette.com/Bassman_Micro.htm

I freaking love it and it sounds fantastic, I just wish it had a tiny bit more juice (because I'm using a 16ohm speaker).
Hence the question: can I make it slightly more powerful buy just changing the output transformer?

On his page, Rob mentions that one can achieve this but using the Hammond 125C instead of the 125B, but doesn't give any details.
Can this work without changing anything else to the amp?
According to the Hammond data sheet, this would bump my amp from 5 to 8 Watts, is that likely to be significant (it's still for home use mind you, I don't need the thing to be crazy loud).
May this alter the sound in any way?
May I achieve a similar bump in volume by just using a 8 Ohm cab (or even 4 Ohm) instead? (rewiring the output transformer accordingly, obviously).

https://www.mouser.ca/datasheet/2/177/125a_125e-1389927.pdf

I'm looking for the guidance of the community because this amp works fantastic otherwise and it would be a shame to damage it at this point.

[tubeswell]

Increasing the iron size may help increase the OT's ability to handle bass frequencies - if these lower frequencies are already produced in the rest of the amplifier (but are somehow at the moment not getting through the OT to the speaker) - and if that all 'works', it could add to a perceived effect of loudness if the speakers can handle the additional low frequencies etc. However, unless it's a huge increase in iron size, probably not really noticeable. You really need to increase the capacity of the power supply (higher B+ voltage, more PT iron etc), upgrade the output tubes, use a more efficient speaker etc to increase loudness.

[danhei]

Your Bassman Micro is primarily limited by the 12AU7 output tube, not the output transformer. And Hammond’s ratings are a suggested max. A 12AU7 won’t put out anything close to 5 watts, much less 8.

[TacosSupreme]

Your Bassman Micro is primarily limited by the 12AU7 output tube, not the output transformer. And Hammond’s ratings are a suggested max. A 12AU7 won’t put out anything close to 5 watts, much less 8.

now THAT makes sense. And now that you mention it, I already swapped the 12AU7 (ECC82) with a 12BH7 and it did increase volume a bit.

I'm reading that changing the output impedence of the amp of the amp from 16 to 8 Ohm may not significantly alter volume.
Would the best route for me be to go with a more efficient speaker then?
I currently have a WGS12L in a thiele ported cab. I can't find any info on the speaker efficiency though. I was thinking of getting an open back cab anyway. Seems appropriate with a bassman type amp...

[tubeswell]

Would the best route for me be to go with a more efficient speaker then?
I currently have a WGS12L in a thiele ported cab.

Ported cabs are pretty efficient if they're tuned properly. However, 2 speakers will increase loudness by 3dB (all other things being equal)

[Willabe]

Are you sure that speaker is a WGS12L?

That's a 200w ceramic speaker, spl; 99.67db. Weighs 18 pounds! 

Specs are on the WGS web site, scroll down the page, click on specs;

12" WGS12L - 200 watts - Warehouse Guitar Speakers

[glass54]

Hi Taco
Have you considered using 2x12AU7 Tubes? ie 2 Triodes per side. 
I have seen it done before and looking at RobRob's info on 125B Tx, you could probably get your impedance's close with 2 Triode per side of the PP Tx. This would possibly give you the increase in Power that you want. (Add extra Cathode R's on pin 3, 8. I would split ie one resistor 820R/3W per phase).
Regards
Mirek

[TacosSupreme]

Are you sure that speaker is a WGS12L?

That's a 200w ceramic speaker, spl; 99.67db. Weighs 18 pounds! 

Specs are on the WGS web site, scroll down the page, click on specs;

12" WGS12L - 200 watts - Warehouse Guitar Speakers

Yes I'm sure 
It's a heavy piece of hardware for sure. I bought it years ago to put in a ported cab (similar to a Mesa). Of course it's waaaaay overkill for this low volume amp.
Is the SPL of 99.67 dB the same as what people call "efficiency" or "sensitivity"?
If so, that's not super efficient is it? Might I get a significant difference by plugging this amp into a cab with, let's say, an Eminence Wizard? It's rated at 102.8 dB.
What kind of cab/speaker would you use with a micro bassman amp?

Thanks

[TacosSupreme]

Hi Taco
Have you considered using 2x12AU7 Tubes? ie 2 Triodes per side. 
I have seen it done before and looking at RobRob's info on 125B Tx, you could probably get your impedance's close with 2 Triode per side of the PP Tx. This would possibly give you the increase in Power that you want. (Add extra Cathode R's on pin 3, 8. I would split ie one resistor 820R/3W per phase).
Regards
Mirek

Interesting. Although I quite liked the idea of a minimal amp with just 3 tubes. I'm not sure I'm up to modifying the circuit again, drilling another hole in the chassis and so on...
but thanks, I'll consider it if all else fails.

[TacosSupreme]

Would the best route for me be to go with a more efficient speaker then?
I currently have a WGS12L in a thiele ported cab.

Ported cabs are pretty efficient if they're tuned properly. However, 2 speakers will increase loudness by 3dB (all other things being equal)

Oooh that's interesting. I was thinking about a 2x12 or even a 4x10. That would be fitting for a bassman (the original was bassman was 4x10 I believe).

[Willabe]

Is the SPL of 99.67 dB the same as what people call "efficiency" or "sensitivity"?

Yes.

Might I get a significant difference by plugging this amp into a cab with, let's say, an Eminence Wizard? It's rated at 102.8 dB.

Not sure how 'significant' a loudness bump it would be. But it will be louder. I think you would hear it.

But using 2 more efficient speakers instead of 1 lower efficient speaker should give you a nice little bump in volume. But now you have a nice small sized micro amp that just got fairly larger.

Might be better off with a 5w 6V6 Champ type amp with a Bassman preamp and a single 10" or 12" speaker. Still fairly small package. 

What kind of cab/speaker would you use with a micro bassman amp?

 

But if I built a micro amp I wouldn't want it louder. If I wanted it louder I'd build a more powerful amp.

[TacosSupreme]

Is the SPL of 99.67 dB the same as what people call "efficiency" or "sensitivity"?

Yes.

Might I get a significant difference by plugging this amp into a cab with, let's say, an Eminence Wizard? It's rated at 102.8 dB.

Not sure how 'significant' a loudness bump it would be. But it will be louder. I think you would hear it.

But using 2 more efficient speakers instead of 1 lower efficient speaker should give you a nice little bump in volume. But now you have a nice small sized micro amp that just got fairly larger.

Might be better off with a 5w 6V6 Champ type amp with a Bassman preamp and a single 10" or 12" speaker. Still fairly small package. 

What kind of cab/speaker would you use with a micro bassman amp?

 

But if I built a micro amp I wouldn't want it louder. If I wanted it louder I'd build a more powerful amp.

All good points. To be clear, I don't need it to be particularly loud, it's a home practice amp after all. I just noticed I'm closed to 10 on the volume when using it at normal volume and wondered if it wouldn't be best to have a bit more range on the volume knob.
Also I used to live in an apartment and now I'm in a house and can allow myself to push the volume a bit more.
Besides, I'm looking at my ported cab, it sounds good but feels a bit wrong to use it with a baseman.
Building a 1x12 or 2x12 open cab would be a fun project...

[Willabe]

Besides, I'm looking at my ported cab, it sounds good but feels a bit wrong to use it with a baseman.

If that cab sounds good with the amp, it's not wrong.

That's not a real clone of a Bassman, so it doesn't demand a 4 x 10" cab.

[kagliostro]

Of course the right way will be a more efficient speaker

that way you don't alter your circuit and preserve the particular sound of the amp

But there is one other way that will increase way more the power without modifying your amp and will maintain its sound

The way is to build a Super Scaler (a la London Power) and connect it between your amp and the speaker cab

The more, you can use it to increase power of other small amp if you have

Franco

[TacosSupreme]

Of course the right way will be a more efficient speaker

that way you don't alter your circuit and preserve the particular sound of the amp

But there is one other way that will increase way more the power without modifying your amp and will maintain its sound

The way is to build a Super Scaler (a la London Power) and connect it between your amp and the speaker cab

The more, you can use it to increase power of other small amp if you have

Franco

Interesting! I never ordered anything from London Power, but I see they have a lot of fun stuff for the tinkerers. Plus it's a Canadian company so 👍🍁

[kagliostro]

Unfortunately there is no a kit for a Super Scaler, KOC explain how to it works and explain how to build it on his books, for the components you can sure find what is necessary at Doug's online shop

Here you can find some info

https://www.tubecad.com/2008/12/blog0153.htm

Franco