# DIY low power Class A rackmount tube power amp?



## zilla

Hi all,

I've been hunting for what seems like forever to find a decent low power (50 watts or less) 1u tube power amp that won't break the bank but have had limited success. 

Does anyone make a DIY kit for this? I was thinking a 20-30 watt power amp. Class A would be nice, but i could deal with AB.

Tim


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## keeperofthegood

Any home brew tube amp will cost you:

Chassis + hardware + Power Transformer + Output Transformer + tube(s) support + tube(s)

You will find the price nominally breaks down like this:

($25 to $200) + ($10 to $40) + ($30 to $250) + ($20 to $250) + ($10) + ($2 to $50)

Now, you can do your own chassis for fairly cheep, 25 dollars or so, some sweating swearing and probably at least one metal cut thumb. Pre-purposed chassis can easily hit 100 and even 200 just for the piece of metal.

If you do knobs and lights and power cord and fuse and switches on the super duper eBay cheep you can keep this closer to the 10 end, going with "name brand" parts you will easily go to 40 if not more.

If you go with Antek and find a way around their 25 dollar shipping to Canada fee, they sell a $30 transformer suitable for a tube amp. Hammond is everyone's cats meow, and theirs is 250 dollars. There is a broad range between for pricing, but thats the high and low end of that.

OT's on the low 10W end start at about 20 dollars. You can do two 10W channels and use 2 10W OT's for less than 1 20W OT depending on who you go with. Hammond again is the cats meow and again is the highest priced part there.

No matter who you buy your tube sockets/resistors/caps from you will find that you will average 10 dollars per tube.

If you get your tubes from the exUSSR you will be on the low end of the spectrum as low as 2 dollars for the bottle, and honestly tubes can go into prices so high that it no longer becomes a fun hobby. However, I think I see tube prices maxing around the 50 dollars each mark at L&M, me personally, I buy exUSSR.


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## zilla

Right, so this could be done at a reasonable cost.

Are there any DIY schematics or kits for a Class A amp?


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## keeperofthegood

Made kits the lowest prices I think are here Tube Amplifiers Kits and Gear however these ones do not provide everything needed. I think they are all without chassis or tubes and some other bits and pieces.

Next up is Trinity, there is a thread on a course we are trying to get people interested in jumping in on with both feet: http://www.guitarscanada.com/amp-building/34904-build-your-own-amp-toronto-course.html and here is a look at what they have on their sale page Trinity Amps - Parts and Kits Not sure where they have the Tramp spec'd and sale'd. The better deal here is you do have everything you need to plug and play (other than tubes ... not sure).

From these guys up, you will find that the next "lowest" priced kit is 450USD and I think the most expensive I have come across was in the low 1000's. There are a fair number of "kits" on eBay, but competition is pretty stiff if you have to bid on those and those prices can get out of hand fast.


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## greco

Zilla...Welcome to the forum. Enjoy and post often !!

I just want to clarify some things: you want only a power amp and you want it to be 1u in height ?
(about how tall is 1u...1.5 inches or so?)

You might be able to make this (physical) size of amp without tubes and with some type of low profile power transformer. 
It would be Class A , but might only produce up to 10 watts or so. I am thinking of a Hood amp. 
The Hood amp is ONLY a power amp and would need some type of preamp.

BTW...I don't know of any kits for the Hood amp, but there are not many parts involved. 

I am somewhat of a beginner in amps & electronics as a hobby....I'm not a tech. 

Again, welcome to the forum.

Cheers

Dave


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## zilla

thanks for ht ewelcome.

I was thinking something like this (physically)










I'd prefer tube, otherwise a class D solid state amp.


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## Guest

I'll go on record and say: there is no way you can build a one off, 1U tube power amp cheaper than you can buy one. Something like a Mesa 20/20 is already pretty value priced and even better on the used market around the $400 mark.

Do this because you want the challenge (and it *will* be a challenge -- that is a tiny amount of space to build a power amplifier in to), not because you're looking to do it cheaper than you can buy it.

It's rarely the case that building < buying in electronics.


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## zilla

$400?

Please tell me where you've seen one for $400 and I'll buy it right now 

every one that i've seen has gone for at least double that on ebay and CL.


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## mrmatt1972

zilla said:


> $400?
> 
> Please tell me where you've seen one for $400 and I'll buy it right now
> 
> every one that i've seen has gone for at least double that on ebay and CL.


They go for 500-550 USD on ebay (do a search of completed listings). 1U is pretty tight, the space for the transformer probably isnt there. 

The Lexicon Signature 284 is 2U and in the 450 USD range - but they're harder to find... It's a full featured mathcless style omap and stereo power sections. You can drive the output sections (about 3 watts each) via the fx loops though.

Why do you need a tube power section? 1U solid state (often "tube emulated") power amps are cheap and plentiful, as are larger tube power amps like the Peavey 50/50.


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## greco

mrmatt1972 said:


> 1U is pretty tight, the space for the transformer probably isnt there.


Do you think they use toroidal transformers for these 1U amps? 

The Hammond specs for the secondaries of torodial transformers don't seem to be conducive to use with tubes (given my limited knowledge and understanding of this type of thing).

Cheers

Dave


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## Guest

zilla said:


> $400?
> 
> Please tell me where you've seen one for $400 and I'll buy it right now
> 
> every one that i've seen has gone for at least double that on ebay and CL.


Average used prices for Mesa Boogie /// PrePal - 20/20's average sale price has been USD$473 with a very low resale popularity. That aligns with my quick search of completed auctions as well.

There's an active auction on eBay right now and it's going for $325 with 13 bids on it: http://cgi.ebay.ca/MESA-BOOGIE-20-2...tem&pt=LH_DefaultDomain_0&hash=item53e295bd4e


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## Guest

greco said:


> Do you think they use toroidal transformers for these 1U amps?


Nope. Just a small transformer. Almost looks undersized. 

Took me a while to find a schematic for it so here it is for information: http://dl.dropbox.com/u/870088/pictures/20_20_PowerAmp.jpg


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## greco

iaresee said:


> Nope. Just a small transformer. Almost looks undersized.
> 
> Took me a while to find a schematic for it so here it is for information: http://dl.dropbox.com/u/870088/pictures/20_20_PowerAmp.jpg


Thanks Ian...without wanting to derail the thread, I wish I could understand that schematic more easily. Basically, I can't see where they get the source of power for points A, B,C,D & E..does it need a separate power supply (that they don't show) ?

Cheers

Dave


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## Guest

greco said:


> Thanks Ian...without wanting to derail the thread, I wish I could understand that schematic more easily. Basically, I can't see where they get the source of power for points A, B,C,D & E..does it need a separate power supply (that they don't show) ?
> 
> Cheers
> 
> Dave


Yea, there's a separate power supply schematic. 

http://dl.dropbox.com/u/870088/pictures/20_20_PowerSupply.jpg


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## keeperofthegood

greco said:


> Thanks Ian...without wanting to derail the thread, I wish I could understand that schematic more easily. Basically, I can't see where they get the source of power for points A, B,C,D & E..does it need a separate power supply (that they don't show) ?
> 
> Cheers
> 
> Dave


You got it Dave, the power supply itself is not on that schematic. Essentially, if you build a power supply to give you those voltages stated at those points, you will be able to operate that circuit in its design intended fashion. Power supplies tend to get forgotten when it comes to schematics I have found. Probably a 110v to 220v at ~200ma transformer with a voltage double on it should give ~440 at 50ma, that would knock down space and size of a transformer, diode rectifiers so no 5v winding, and 6.3v at 2amps is not that big at all, so the transformer probably wouldn't run much bigger or smaller than the UK voltage converter transformers you get for traveling with.










For a fuller explanation read the wiki here: Voltage doubler - Wikipedia, the free encyclopedia


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## keeperofthegood

iaresee said:


> Yea, there's a separate power supply schematic.
> 
> http://dl.dropbox.com/u/870088/pictures/20_20_PowerSupply.jpg




HAHAHAHAHA IAN

OK let me go get the wash cloth and get the EGG off my face


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## Guest

Keep: no egg on your face there man. It's common practice to reuse design components within a company. So a power supply design might get reused in many different products. Because of that you'll see schematics that leave off the reused part. It's just a "go look at that schematic to see what I mean" type deal. Then you don't have to change *every* amps schematic when you tweak your power supply schematic.

Anyways: your post was great. Much better than my, "read the schematic" post.


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## greco

Many Thanks Ian & Keeps. 

Just one further question (that might also help the OP) . 

Is the power supply also squeezed into that 1U enclosure with the power amp? 
...I'll bet you guys are having a good laugh over this question....LOL 
My first thought would be "NO" (how could I be so stupid)....but I'd like to know for sure. 

Cheers

Dave


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## Guest

greco said:


> Is the power supply also squeezed into that 1U enclosure with the power amp?


Yup. It's all in that 1U enclosure. There's some shielding walls that also help with heat flow.



> ...I'll bet you guys are having a good laugh over this question....LOL
> My first thought would be "NO" (how could I be so stupid)....but I'd like to know for sure.


It's not a stupid question at all. Those 1U tube power amps pack a ton of stuff in to that form factor.


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## keeperofthegood

LOL Dave, transformers can be flat core wound, a toroid would do too. Me, I am still trying to see how they get both - and + voltages out at those 14 and 2.5 points @[email protected] I am getting -6.6v on both 15 and 33 k resistors!

Circuit Simulator Applet

Copy/paste this to the Import menu of the above applet.



Code:


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## Guest

OP: http://www.fractalaudio.com/forum/viewtopic.php?f=21&t=18505 - $450 USD -- no affiliation with the seller. They sell with regular frequency at that price on the Fractal board.


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