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High Powered Twin or Dumble?
High Powered Twin 75%  75%  [ 3 ]
Dumble 25%  25%  [ 1 ]
Total votes : 4
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PostPosted: Mon Jul 16, 2018 8:00 am 
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Hi All,

I've been getting a lot of experience in analog design in recent months, and I want to do my second Amp build, the first being an 18W bluesbreaker clone.

I've had my eyes on some different options, and the Dumble OD special as well as the Fender Twin call my name. Seeing that I have most of my hard overdrive needs covered with the Marshall, I want an amp on the cleaner side, but with a good mild drive. I understand that the twin is one of the kings-of-clean, but has a great drive sound as well. In your folks opinion, is the Dumble really as versatile as the fans claim? I love the clean sound and the drive, but there seems to be a whole lot of $$$ and hot air thrown out there about it...

I'm playing a Fender strat and a Gibson Les Paul. My styles include classic rock, stoner rock, ambient guitar, a bit of fusion, and prog. I will take excellent bass response over ear-piercing treble any day. :lol:

Which direction do you all think I should go, if either?

And by the way, here's the schemas I'm looking at to modify to taste:
https://schematicheaven.net/dumbleamps/97fullschem.pdf
http://ampwares.com/schematics/twin_5f8a.pdf

Thanks,
Grant

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PostPosted: Wed Jul 18, 2018 4:27 am 
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Dumble is about as difficult as a build can be. I know that even once it's built, it takes a while to dial it in. A very ambitious 2nd build. Part of the rights of passage in amp building is doing a Fender circuit. I'd do the Twin first.

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PostPosted: Wed Jul 18, 2018 9:19 am 
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geekmacdaddy wrote:
Dumble is about as difficult as a build can be. I know that even once it's built, it takes a while to dial it in. A very ambitious 2nd build. Part of the rights of passage in amp building is doing a Fender circuit. I'd do the Twin first.

^ This.

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PostPosted: Thu Jul 19, 2018 10:37 am 
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I bow to the masters. However, there are projects from Trinity and others that are not terribly complicated and do the OD justice. The switching is a bit of a pain but overall it's a modded Fender :) I stick with my (unpopular) vote...


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PostPosted: Thu Jul 19, 2018 11:11 am 
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duhvoodooman wrote:
geekmacdaddy wrote:
Dumble is about as difficult as a build can be. I know that even once it's built, it takes a while to dial it in. A very ambitious 2nd build. Part of the rights of passage in amp building is doing a Fender circuit. I'd do the Twin first.

^ This.


So, should I go with a normal Tweed Twin design, or the high powered twin design and add a master volume?

Also, in the high powered twin schema, there is this cathode follower. Is this purely for buffering, or is there some bootstrapping going on? I assume its also using the low output impedance to drive the tone stack more easily.
Attachment:
cathodefollower.PNG
cathodefollower.PNG [ 31.39 KiB | Viewed 4062 times ]


Thanks,
Grant

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PostPosted: Thu Jul 19, 2018 11:13 am 
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mmarsh wrote:
I bow to the masters. However, there are projects from Trinity and others that are not terribly complicated and do the OD justice. The switching is a bit of a pain but overall it's a modded Fender :) I stick with my (unpopular) vote...


I would be doing my own design from scratch. Get a chassis and combo cab from tube depot or somewhere similar and go from there. I guess they're right, the twin will be more entry level for this go around. If I were doing a kit though, the Dumble clones would call my name...

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PostPosted: Thu Jul 19, 2018 11:14 am 
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I guess my point was that if it's not a complicated kit, then a ground up design would not be that bad either. Plus you end up with a stellar (not that the twin is a slouch).


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PostPosted: Thu Jul 19, 2018 12:21 pm 
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Also, how does this 12AX7 work as a phase inverter? I understand the cathodyne inverter but this isn't making clear sense to me. Is it the RC filtering that happens before the 12AX7s? I see the significant difference in the caps right before the 12AX7s. But there's also feedback from the output transformer as well. :?
Attachment:
phaseinverter.PNG
phaseinverter.PNG [ 163.28 KiB | Viewed 4054 times ]

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PostPosted: Sat Jul 21, 2018 11:32 am 
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Search long tailed phase inverter if you want a detailed explanation. That’s the classic big-boy amp phase inverter used by just about all manufacturers after 1960. Your Marshall 18 watt probably uses one too.

I don’t really understand how some guys get away with using 80-100 watt amps. I’m always being told to turn down and I play 20 watt amps. So, that being said, I’d go low powered twin. But maybe that’s just me.

The hpt is basically a tweed bassman with 2x12s and 2 extra output tubes. You could build it as a 2x12 bassman with a 45 watt output.

The lpt is a different circuit, akin to the tweed super, pro, and bandmaster except with the ridiculous dual rectifiers. They didn’t have solid state rectifiers back then, that’s why they used 2.

I like the lpt better, but I’m a weirdo. A hpt will give you a very classic sound with great cleans and great breakup.

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PostPosted: Sat Jul 21, 2018 6:29 pm 
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Morgan wrote:
Search long tailed phase inverter if you want a detailed explanation. That’s the classic big-boy amp phase inverter used by just about all manufacturers after 1960. Your Marshall 18 watt probably uses one too.

I don’t really understand how some guys get away with using 80-100 watt amps. I’m always being told to turn down and I play 20 watt amps. So, that being said, I’d go low powered twin. But maybe that’s just me.

The hpt is basically a tweed bassman with 2x12s and 2 extra output tubes. You could build it as a 2x12 bassman with a 45 watt output.

The lpt is a different circuit, akin to the tweed super, pro, and bandmaster except with the ridiculous dual rectifiers. They didn’t have solid state rectifiers back then, that’s why they used 2.

I like the lpt better, but I’m a weirdo. A hpt will give you a very classic sound with great cleans and great breakup.


Okay, did a bit more reading. I'm working through Valve Wizard's book on guitar pre-amps (excellent btw).

Let me make sure I have this right...the cathodes of the long tailed phase inverter are connected at the top of the 470ohm resistor. I'm not exactly sure what the resistor network is for aside from biasing the phase inverter, but I can worry about that later. The ".1-2.00" cap from the lower triode in the long tail grounds ac signal on the lower triode grid, letting the push-pull nature of the long tail do it's magic.

As for the 80W HPT, I agree that I would pretty much never use that much power unless I take it home to the barn and play for the cows. Would it be too sinful (or difficult) to do a half power switch? I've seen a lot of modern amps that do this. This is a big assumption, but based on the hpt schematic, I suppose I could more or less wire a switch such that the heater voltage would be removed from two of the four power tubes when enabled. With a master volume, that would give quite a bit of control.

Thanks,
Grant

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PostPosted: Sun Jul 22, 2018 10:08 am 
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There are lots of examples of half power switches out there. Or you can just remove two of the output tubes: https://www.premierguitar.com/articles/ ... duce-power

Or you can just use the 5F6A bassman circuit (essentially the same circuit with half the power). 5F6A parts are much more readily available as well. And the early Marshall JTM45 is a clone of that circuit, so there are even more possibilities for modding and form factor (heads, combos, etc).

Playing a high powered twin for the cows sounds pretty blissful to me though. :wink:

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