20 watt single speaker bassman?

[Frederick59]

Hi folks,
I've been toying with the idea of building a 59 Bassman on the basis of the 5F6A schematics as my first DIY amp. I found a kit online on Onlineshop Startseite Tube Amp Doctor Onlineshop. Tweed Bass Four-Ten Amp-Kit 5F6A - AMP-Kits TWEED Tubeampdoctor Store From what I know they get their cabinets from Mojo Amps.

However, the amp kit is designed as a four speaker 40 watt amplifier. But four speakers and 40 watts are overkill for me. I would prefer a single speaker 20 watt version to keep it a bit more portable.

So, what do you think?
1. Would this work?
2. What would I have to change to downsize the 5F6A schematics to 20 watts?
3. Could I run into trouble with the 20 single speaker combination?
4. Should I just get the single speaker 16 watt kit they have for sale just to get my first build under my belt? Tweed One-Twelve-16, 5E3 Style Amp-Kit - AMP-Kits TWEED Tube Amp

Also, some info on me: I went to a technical college for electrical engineering, which was focused more on automation and programming. I do have general knowledge of circuits and electronics though.

I would appreciate any pointers or suggestions.

Best wishes
Frederick

[bob p]

If what you really want is a Tweed Bassman circuit then build a Tweed Bassman circuit, and adapt it to suit your needs. I wouldn't build a different kit if the Tweed Bassman kit is what you really want. If 40W and 4x10 is too much, then it makes sense to scale down to 20W and a 1x12 if you think that will be better for you.

There are tweed bassman builds that have been posted here (in the archvies) that have substituted a 4x6V6 power section for the 2x6L6 power section of the 5F6-A. There's no reason that you couldn't scale farther down to a 2x6V6 setup to bring the power down to lower levels, and there's no reason that you couldn't get by with a different speaker compliment if that will work better for you.

The most important issue to watch is the B+ voltage. Ask TAD how high the B+ voltage is on their finished kits. I looked for that information on the site that you linked but I could not find it. The key consideration would be whether the B+ ends up being too high for modern 6V6 tubes. Assuming that their B+ is reasonable and hasn't crept up compared to the vintage levels, you should be OK. That is to say, modern 6V6 like the JJ can tolerate voltages in the range of 430VDC as you'd find on a vintage 5F6-A. But you wouldn't want to drop them into a higher voltage 6L6 application, so it pays to ask TAD what kind of voltages their kit provides.

Assuming that their kit works with reasonable B+ that would be safe for a 6V6, you'll have to rework the bias circuit. You'll also have to re-scale the phase inverter to provide appropriate drive voltages for a 6V6. Those are inexpensive, easy tasks that will only amount to the cost of a few resistors.

The good news is that you won't have to change the OT. By moving from 6L6 (which typically require Raa to be about 4200R) to 6V6 (which prefer about double that), you'll also end up increasing the desired secondary load on the stock OT from 2R to 4R. A 4R speaker would work fine if you've got one, and you could probably get by OK with a mismatch upwards to an 8R speaker without having to buy expensive parts like new iron.

The good news is that you could do this type of conversion if you want to, without having to replace any of the expensive parts (iron). You should be able to make things work by just changing a few cheap passive components... provided that the TAD kit doesn't run the B+ voltages too high. If the voltages do run high then you might have to resort to some trickery to bring the B+ down, like back-biasing or the zener trick, both of which are well documented on the web.

What you're asking about isn't all that hard to do -- It's all been done before. If you should need help, you should have no problems getting it here.

I'd be calling TAD to ask for more details about that kit.

[g1]

And if the B+ is too high, they may be willing to swap in a different power transformer with lower high voltage winding.

[bob p]

yes, that would be a great solution if they'll accommodate you.

[bob p]

if 1x12 with a slight Z mismatch doesn't suit you then another option would be to go to a 2x10 configuration. with the change to 6V6 that would give you a proper 4R secondary OT match if you used two 8R speakers in parallel. and it would still be more portable than a big 4x10 bassman.

[Enzo]

And your amplifier has no idea how many speakers are connected to it. All it sees is the total impedance there. SO four 16 ohm speakers in parallel making a 4 amp total looks exactly the same to the amp as a single 4 ohm speaker. You can find speakers of any size you want, and in any number you want, then pick the impedances to combine for what the amp wants.

I would urge you to consider a smaller amp if the Bassman is more than you want. A Deluxe or something.

[bob p]

> I would urge you to consider a smaller amp if the Bassman is more than you want. A Deluxe or something.

that could work. a lot of people do just that. but there are quite a few people out there who like the bassman tone but don't like it's volume, and aren't interested in settling for a deluxe because the bassman sounds very different than a deluxe. to solve that problem there have been quite a few people who have been interested in shrinking down a bassman to deluxe levels, which is my impression of what the OP is trying to accomplish.

a bassman is a popular kit. so is a deluxe. but it's not so easy to find a kit that marries a bassman preamp to a deluxe output section. that's too bad, because it's an absolutely KILLER combination. IMO that's where the effort of building your own custom amp allows you to reap a unique reward. it's very much worth the effort, IMO, and the TAD kit that's local for the OP seems like a good starting point for a custom build like he's described.

[Tom Phillips]

Frederick59,
Several members have provided many good comments.

You said "I would prefer a single speaker 20 watt version to keep it a bit more portable.
So, what do you think?
1. Would this work?
2. What would I have to change to downsize the 5F6A schematics to 20 watts?
3. Could I run into trouble with the 20 single speaker combination?
4. Should I just get the single speaker 16 watt kit they have for sale just to get my first build under my belt? Tweed One-Twelve-16, 5E3 Style Amp-Kit - AMP-Kits TWEED Tube Amp"

However, you have not yet told us what you expect from the amp with regards to tone, overall playing response and how you plan to enjoy the amp. Is it home pleasure playing at low volumes or do you need enough volume to keep up with a drummer and other band members at a small venue gig? Tell us more about those things. Both the 5F6A Bassman and the 5E3 Deluxe are classic amps and each has their own unique qualities. There may well be other options that would be better for you so, once we know more about your goals, we can provide better guidance to help you make a decision.
Cheers,
Tom

[Justin Thomas]

I'd just plug JJ 6V6s in the Bassman, and bias accordingly... all that impedance stuff? Meh, "it's a guitar amp, not a NASA moon rocket." Close enough for R&R.

I'd also just build the 5F6-A in a blackface chassis, as 1x12" cabinets are available for a blackface Bassman chassis. Stick your ampguts in the chassis, stick your chassis & speaker in the cab, blammo. Might have to make a custom faceplate, and the layout would obviously have to change, but all that can be worked around.

Just me & how I would do it... though one day, I WILL invest in one of those combo cabs for my own blackface Bassman.

Justin

[bob p]

Just me & how I would do it... though one day, I WILL invest in one of those combo cabs for my own blackface Bassman.

Do you mean like one of these Mojo 1x12 Bassman cabs?



They're kind of on the large side as far as combo amps go, but that looks like a great option, and I've been giving some thought to this type of arrangement lately.

actually, now that I've got a Cream 90 12" on the way i've been thinking about hacking up my B-10 to convert it from a 4x10 CB to a 1x12 OB cab just like that one.

[Justin Thomas]

Yep, that's what I mean. A guitar instructor back home has his BF Bassman in one with a nice Celestion. Killer amp, perfect size. MUCH better than lugging my bigass 2x15" around for guitar...

(Okay, I get that some may brand me a hypocrite for that last bit. But, yeah - I <DO> know how to fit the amp to the , and there ARE times when I don't need to move ~354inē of air...)

Justin

[bob p]

Hypocrite? You must be referencing that thread from last week about willingness to cart about heavy gear for tone. You're not a hypocrite if you decide not to haul a 2x15 to your coffee house gig. It's all about matching the right gear to the task.

From a design standpoint, I really like concept behind that large open back 1x12 -- it's more along the lines of what's mathematically "correct" for a 12" infinite baffle design. The popular designs like the Fender Deluxes are smaller than they should be, from a speaker design standpoint. That Mojo 1x12 is almost as big as a Super Reverb cabinet. When loaded with the right speaker I'm thinking that it would absolutely kick ass for a 1x12 application. But there are a lot of people who would just turn their heads, not understanding why you would want a cabinet bigger than a Deluxe Reverb for a 1x12.

I'm giving serious thought to cutting up my B-10 to open the back and installing a switchable baffle in front. It's a bit taller than the SR platform. I much prefer the sound of a 4x10 open back to a 4x10 closed back cabinet, and if I go that route it's a short hop to being able to swap in a 1x12 or a 1x15 baffle. It's a big amp as far as combos go, but it gives a lot of different options and like you, I'm not bothered by the size or weight.

[Justin Thomas]

you got my reference... yeah, there's a big difference between fitting an amp to a room and just leaving it all behind because I'm lazy... ad before I get flamed for that, most of the people that <I> know gave up their amps for convenience, not because they legit can no longer lift and throw heavy amps. Medical reasons is one thing, laziness is another, ESPECIALLY when you then whine about the tone being "mostly okay for what it is..."

I wouldn't mind a Bassman 10 with a closed back... I'd use that as my main bass amp. Though, like my Concert, it'd drag in the dirt because I'm about 3" too short... I'd prefer a 1x12" with an option to open or close it, which means I'm actually more likely to go for an extension cab & leave my head as is. That way, I could use it for guitar or bass. Weber's Bass speakers sound fairly good for clean jazzy guitar, which is what I'd be using it for anyway, along with handling actual bass. Eminence makes a few speakers that I think might work well for both, also. Given that I play a Mustang Bass, anyway, and who needs below 80Hz? The SVT did a nice rolloff of fundamentals...

Anyway, enough derailing the thread, though I think we've thrown out some good ideas!

Justin

[bob p]

The FAFG is wrong on this -- the B-10 isn't really CB, it's ported. And when you play bass with it, it tends to generate a pumping sound through the ports for the LF fundamentals. It has neither the benefits of an OB or CB cab, and being poorly ported, it's worse than either one. The cure is to plug the ports to make it CB or to cut open the back. I don't know if you've noticed it, but the pumping thing is definitely annoying with a P or J. Dunno about a Mustang though.

I put casters on mine. They make it easy to move but even more prone to tipover. I'm thinking a pair of tilt-back legs might be in the amp's future if I open the back...

Frederick, no offense intended -- thread hijacking is par for the course around here.

[Justin Thomas]

I only ever saw two in person (not to work on, either) and the backs were definitely closed. The ports are in front, I take it, and covered by the grill cloth? I guess if the whole cab is sealed it's called "infinite baffle?" Never saw the ports on either, though I remember you talking about the pumping now... I like to be accurate, so I'll try to remember to change my descriptions!

Justin