Convert 5F6A Bassman into 5F8A Twin

[Aldo]

A theoretical question:
Could a RI Bassman 59 LTD be turned into a high power tweed twin? I'm not talking about changing the RI circuit to original specs, only about adding two power tubes and transformers if needed to the stock RI circuit.

I know it sounds crazy, everybody wants to lower this amp's power, attenuate, soften it... but what about the opposite, getting twin headroom? I see circuits 5F6A and 5F8A are the same except for 2 vs 4 power tubes and different transformers codes. Will the Bassman LTD transformers be up to the task? I've heard they're more than enough for the Bassman. I feel the most difficult thing would be retrofitting different sized transformers and making space and rearrange/drill new holes for the extra tubes

[Chuck H]

I know it's pretty tight fit in the stock Bassman. Some research into that would be paramount. The stock baffle board on the original Bassman amps is pretty thin too and might have problems handling twice the power. I don't know how the stock reissue speakers are rated, but that could also be a consideration. Higher rated speakers may take up even more space. Transformer replacement isn't likely to be compatible with the OEM chassis holes. Not a deal breaker since you'd need to punch some new tube holes anyway. If you're tube rectified you may need to add a second rectifier tube too. All this is making things even tighter and adding to the planning, labor and expense. The added weight, with bigger iron and speakers, probably wouldn't be a welcome side effect either. If you've ever had to lug a Twin around, a 4x10 version would weigh a little more than that and require lifting higher for clearance when carried.

If you're really into a project or just like the idea of having something cool and exclusive, that's valid. It could end up an expensive and cumbersome project though.

If you just want more headroom you might consider circuit adjustments and high efficiency speakers. If you just need the extra volume at gigs there's the option of using a mic or a line out circuit into the PA to just bump the volume a bit. Or... Sell the Bassman and get a bigger amp. Or... Since you'd probably need to get new iron, speakers and tubes as well as cabinet modification and chassis work anyway, just build a bigger amp, leave the Bassman stock and have two amps. OR... Since the project expense is likely large, get another Bassman. This has the benefit of being able to separate them for more stage coverage or stereo pedals.

Just thinking out loud.

[Aldo]

Hello Chuck, thanks your thoughts and for instilling some sense here!

It's just an idea rounding my head that I don't think I'll be brave enough to put into practice. I've already made adjustments for headroom, it's diode rectified, 7851A tubes biased cool, 12ay7 in V1, no V1 cathode resitor, stock slope resitor (100k), two ceramic speakers on the bottom and Emis alnico on top, I won't get the headroom I look for with different speakers. Love how the amp sounds, I just wish it'd stay cleaner and tighter at higher levels. The amp now is light, the weight increase would come from bigger transformers mostly, and I wouldn't mind the extra weight, still I don't think it would reach twin reverb weights. The most dificult thing seems to me accomodating the bigger transformers... maybe not even possible given the space, and making room for two extra tubes will be complicated as well. The most sensible thing seems building a Tweed twin clone, but that would be something I can't afford now!

[Aldo]

I have one doubt. If I were to beef up the amp with the 2 extra power tubes would I need to change both output and power transformers?

[pdf64]

two ceramic speakers on the bottom and Emis alnico on top, I won't get the headroom I look for with different speakers. Love how the amp sounds, I just wish it'd stay cleaner and tighter at higher levels

I think there's a strong likelihood that more efficient speakers would meet your goal more effectively and be way easier to implement than trying to beef up the amp.
My guess is that the alnicos are just squishing out on high power bass fundamentals, and even with more input power from a 5F8A, the SPL from them at low frequencies would barely increase; for loud tight bottom end, there's no substitute for heavy duty magnets.
Venting through the magnet for the voice coil helps to keep it cool and reduce the resistance increase with temperature (avoid the crappy weak tone towards the end of gig syndrome).

[Chuck H]

I have one doubt. If I were to beef up the amp with the 2 extra power tubes would I need to change both output and power transformers?

Pretty certain. Adding two power tubes would add 1.8A requirement to the filament circuit for the PT. That's a lot. I doubt there wouldn't be consequences. Even if the OT is up to the extra power (I wouldn't think so), the impedance match is wrong and you'd need to change all your speakers to 4 ohm units (I don't think there's many guitar speaker options in 4 ohm 10's) for a bizarre 1 ohm load.

[pdf64]

On the subject of heavy duty magnets, consider getting a Delta or two in there Speaker Detail | Eminence Speaker

[loudthud]

On the original Fender models, wasn't there so little clearance between the top speakers and the chassis that they had to take the magnet covers off of the top speakers?

[Aldo]

Chuck, about the impedance there's a Mercury Magnetics 5F8A OT with 2 & 4 ohms taps. Anyway... the two transformers alone amout to $400, add shipping & overseas taxes and it would likely go near $600

Pdf64, talking about speakers Deltas surely are great but for this amp I think they're too much at 6,65kg each. I was considering Emis Ragin Cajun, Copperhead, Lil'Buddy (all about 99 dbs) or Jensen Neo/Tornado. I would only need one, I would keep a Legend 105 (ceramic) that's already mounted. I know alnicos may add some compression but what I hear mostly is amp saturation when turned up... I want to keep two of them, right know they may be receiving about 12 watts each, doubling the power each would get about 20 watts

I plan to change the cap before the phase inverter of the Bassman from 0,022uf to some value between 0,001 and 0,01, I understand this may reduce some mids and clean things up some more. Which value you think would be a good compromise?

[Aldo]

On the original Fender models, wasn't there so little clearance between the top speakers and the chassis that they had to take the magnet covers off of the top speakers?

Yes, pretty tight up there, IIRC one of the speaker's alnico magnets is almost touching some part of the chassis, so I believe there's no space for even a ceramic speakers there, at least on that side?

[Aldo]

I experimented with the phase inverter cap and in this case I think it's not worth it, couldn't notice much headroom increase and values between 0.001 and 0,015uf gave saturation a shrill feel, I'll leave it stock at 0,022uf...