1U power amp 2×20w- any ideas for a build?

[rogb]

Having resurrected my #183 Dumble 1U preamp, I've been looking for a 1U tube power amp but the usual suspects Marshall or Mesa 20 20 are now quite old and still expensive. SS options are Palmer Macht and Matrix but I'm not sure I'll like the sound.
Using a toroidal in a 1U rack case, I wonder what the options for 2 channels and 12-20w per side are, maybe single ended EL84 ? Only 2 power tubes needed = less heat.
Also OT for 8 Ohm, I'm not sure what would fit in the limited space.
I'm in the UK so shipping parts from the USA is out these days, sadly.
Anybody built something similar?
I built my 1U preamp and 1U Dumbleator, so I'd quite like to build the power amp, given it would cost some £400 minimum for an old tube rack power amp.
The Marshall 20 20 has 3x12AX7 and 4xEL84 so a lot of heat to disperse!

[Guy77]

If you want to go 12- 20 watt per side clean headroom with Single ended you will need to use larger power tubes like 6L6GC and at least 2 in parallel per side to get closer to 20 watts. I just finished experimenting with a single ended 6L6 D Style 124 build. Single ended sounds great! You just need larger transformers. If you are looking for less heat and more efficiency go with push pull.

https://ampgarage.com/forum/viewtopic.php?f=6&t=36308

Cheers
G

[rogb]

If you want to go 12- 20 watt per side clean headroom with Single ended you will need to use larger power tubes like 6L6GC and at least 2 in parallel per side to get closer to 20 watts. I just finished experimenting with a single ended 6L6 D Style 124 build. Single ended sounds great! You just need larger transformers. If you are looking for less heat and more efficiency go with push pull.

https://ampgarage.com/forum/viewtopic.php?f=6&t=36308

Cheers
G

Thanks Guy, I just built an EL34 SE 5F2A in a 10" combo. It really sounds great!
The problem with going bigger than EL84 is the 1U rack enclosure.
Here is the Marshall 20 20 with its compact OTs, toroidal PT and plenty of tubes!
Probably SE won't be an option and it would be P-P then.

Screenshot_20230704_134452_Firefox.jpg

[martin manning]

1u is going to be a challenge. p-p is a must, so four outputs tubes, EL84 or 6V6GT, which have 40% lower heater power (0.45A vs EL84's 0.76A). There are 9-pin 6V6 lookalikes, 6973 (expensive) and TV sweep tubes like 6CZ5 (cheap). Looks like you might find a Marshall 20/20 for around $500-$600.

[rogb]

1u is going to be a challenge. p-p is a must, so four outputs tubes, EL84 or 6V6GT, which have 40% lower heater power (0.45A vs EL84's 0.76A). There are 9-pin 6V6 lookalikes, 6973 (expensive) and TV sweep tubes like 6CZ5 (cheap). Looks like you might find a Marshall 20/20 for around $500-$600.

Yes I'm coming to the conclusion that 2 channels = 2x everything- price, heat and the complexity of getting it all in 1U .
I have played through a lot of tube rack power amps and always liked them. I'm not sure I'd be happy with Class D or MOSFETs.
It's looking more like one of these old Marshalls or Mesas, I guess.
Your input is, as always, much appreciated.

[chaccmgr]

the marshall gut shot looks like a reasonable concept for your case. A band member used it and it sounded good.

You could order a custom made toroidal PT. I'm located in Germany, whenever I need a special PT I order from polish http://www.badel.pl/, they always made whatever I nedded at a price that was often lower than standard off the shelf products, even in quantities of 1
They are no audio company though, so you need to give them complete specs...

Another source in Germany is https://www.mueller-rondo.com/, they have audio knowledge and help to spec your requirements. Also not expensive. They make custom OTs as well (standard and toroidal)

A quite compact OT for 15-20W Guitar use is https://www.musikding.de/Output-transformer-Push-Pull, not sure though, if it fits into 1U. At Musikding not available but you can try at the manufacturer at https://www.die-wuestens.de, (very old fashioned site w/o search function), go "catalogue" / "Transformers and Chokes" / "OPTs", search for ATRA0427.

hth
Robert

[Reeltarded]

You might be very surprised by a common class D off the shekf power section for less than the cost of tubes and an enclosure.

Here is my Rever search string.

https://reverb.com/marketplace?query=Mo ... ower%20amp

I used a bunch of different power amps in almost 40 years. Crest, Crown, Peavey.. all the mosfet 80/90s stuff. Some sound sloghtly better but in many ways I am indifferent to models as long as they have massive headroom and bandwidth.

[Reeltarded]

Type Ohs For Free

[rogb]

the marshall gut shot looks like a reasonable concept for your case. A band member used it and it sounded good.

You could order a custom made toroidal PT. I'm located in Germany, whenever I need a special PT I order from polish http://www.badel.pl/, they always made whatever I nedded at a price that was often lower than standard off the shelf products, even in quantities of 1
They are no audio company though, so you need to give them complete specs...

Another source in Germany is https://www.mueller-rondo.com/, they have audio knowledge and help to spec your requirements. Also not expensive. They make custom OTs as well (standard and toroidal)

A quite compact OT for 15-20W Guitar use is https://www.musikding.de/Output-transformer-Push-Pull, not sure though, if it fits into 1U. At Musikding not available but you can try at the manufacturer at https://www.die-wuestens.de, (very old fashioned site w/o search function), go "catalogue" / "Transformers and Chokes" / "OPTs", search for ATRA0427.

hth
Robert

Thanks Robert, I also came across suitable toroidals, they are out there also some compact OTs. When costed out it came to much the same as finding a Marshall 20 20.
In the UK even a reasonable 1U aluminium case would be some £90 these days, never mind 6 or 7 tubes...
I have now got a Palmer Macht 402 on order. I'm not that keen on Class D but I'm hoping at under £300 it will be usable. Let's see.
https://www.palmer-germany.com/en/produ ... /macht-402

[rogb]

You might be very surprised by a common class D off the shekf power section for less than the cost of tubes and an enclosure.

Here is my Rever search string.

https://reverb.com/marketplace?query=Mo ... ower%20amp

I used a bunch of different power amps in almost 40 years. Crest, Crown, Peavey.. all the mosfet 80/90s stuff. Some sound sloghtly better but in many ways I am indifferent to models as long as they have massive headroom and bandwidth.

Thanks Miles, much appreciated. Please see my response above, mirroring what you have suggested.

[Reeltarded]

I like it. Probably a really good choice! Would like to know how it works out.

[rogb]

I like it. Probably a really good choice! Would like to know how it works out.

I will do that for sure. I'm hoping that the 2x12AX7 in the preamp and 1x 12AX7 in the Dumbleator will fool the Class D into tube like behaviour

[Reeltarded]

Dumble things are usually played at volumes where it is 99% about the preamp and the output is damned near idlling.

You will be surprised is my bet.

This is a 50w amp on a Palmer PGA-04 into a 2200w Class-D stereo amp and a quad of Marshall cabs.

https://www.dropbox.com/s/9hok0lymcfwmm ... 0.MOV?dl=0

I set it up right at the edge of taking off and played it slow to show how reactive it is. It does the same thing with +3db on totally clean setting. Crazy good.

[TZRsteve]

My setup is a vox VT40+ and a modded KLD marshall 18 clone into a stereo mosfet. Simply the best setup ive had in 40 plus years. The DIY route is always fun but there are so many good preamps available and going second hand could save you a furtune. I have an old unused peavey TGRaxx knocing around. This has no OT but simply takes the output from one of the 12ax7's via a capacitor and a couple of resitors. These sound ok but are better known for the clean settings. The schematic is online and worth looking at. Sorry i aint got the hang of posing pics here yet.

You could modify the first stage as a bassman, early orange, etc, etc.

[Reeltarded]

Sorry i aint got the hang of posing pics here yet.

Hey Steve!

It's sort of convoluted, but when replying if you look below the message you see "attachments" tab. Select that, select image, it uploads and you click "place inline". You can always use the preview button below the message pane when trying to post images to see if it is what you intended before posting.

Gross

IMG_7794.jpeg