Ebay Tube amp?

[Patches9412]

These things are popping up everywhere.
KT88 Amps
And some 300B amps and El34s.

They are from 324$ to 700$ or so, which is cheap for a KT88 and real cheap for a 300B. I think they use Electro Harmonix which are pretty good, no the best but will do the job and are dead solid.
I was wondering about your opinions. The way i see it is going in there and changing the out some of the crappy caps bight make this a nice amp. ITs got power, or so it says. There is a pic here of the inside, so you can see some of the bug caps that are used, not Solen stuff, obviously.
My next consern would be the Output transformer.

Thoughts? IT is worth it?

Hank

[50 watt head]

Re: Ebay Tube amp?

I might guess the person who soldered that PCB was drunk :D

[Face]

Re: Ebay Tube amp?

I might guess the person who soldered that PCB was drunk :D

Or high from solder fumes.

[Patches9412]

Re: Ebay Tube amp?

Ahaha, Well, it does look that way, that is easy enough to fix, but as long as the joints dont bleed into one another, or break, it doesn't have to look pretty. But what does that say about build quality huh? ha.

H

[bkeane1259]

Re: Ebay Tube amp?

I guess I must be drunk when I solder as well (or high off the fumes)....oddly enuf my XO soldering skills are similar to that PCB.

I do enjoy a glass or 4 of wine when working on my crossovers. I abstain when using the power tools tho.

[dcgold2000]

Re: Ebay Tube amp?

looks like the caps are in series which doubles the voltage and thats OK, but generally they should have a resistor voltage divider across them to equalize the voltage across each cap. I don't see the resistors in the pics.

[keiths]

Re: Ebay Tube amp?

The real cost of a tube amp is in the transformers/chokes. It's the copper that gets you. When you consider how cheap things can be made in volume, it's quite reasonable that products from China can be that inexpensive and not necessarily bad. A tube amp is a very simple device. There are only about 16 components in a 300B mono block.

I had been thinking of building a 300B. I have the research, the plans, the skills, etc. I just didn't have the $$$ for the parts. Then I realized that I could buy a 300B cheaper from many many sources than I could buy the parts. Are they inferior? I don't know. I know what the EL84 based tube amp I did by sounds like. It sounds great with a pair of Overnight Sensations! $30 in tubes and it sounded even better!

Just to add fuel to the fire, here's a N.A. based tube amp supplier I've been looking at.
http://www.musicalparadise.ca/mp/

Oh, and it looks like there might be a boxing day sale. :D
http://www.musicalparadise.ca/wordpress/

That 300B is about half of the price of the parts.

Edit:
Just looked at the inside picture from the original post. Those insides look better than many commercial offerings I've seen. Nothing scary to me. The joy of such a simple amp is you can tune and tweak to your hearts content. It's like DIY speakers in that regard only simpler as long as you don't shock yourself on the 300V.

[djg]

Re: Ebay Tube amp?

Ask the seller to kindly show you a photo of the transformers under those pretty boxes. You might be surprised.

Things aren't always what they seem.

[50 watt head]

Re: Ebay Tube amp?

In all seriousness, I wouldn't hesitate to buy something like that, as long as I was confident the following conditions were met:

(1) high-quality transformers
(2) high-quality chassis (preferably aluminum)
(3) roomy (i.e., non-compact) circuit layout

If you're handy (and that includes *safe*) to work on tube electronics, then you're in even better shape. A well-built tube amp is often very easy (and fun :D) to repair/modify.

However, if any of the above conditions fail ... I wouldn't bother. Not when you can get lots of good, clean, solid state power for ~$299.

[Patches9412]

Re: Ebay Tube amp?

I would loveto build a 300b, but again the transformers are so outrageous. Edcor has some good pushpull type transformers for around 50$ best deal i could find that is full spectrum. Edcor is pretty aswell. I was planning on seeing if i could get some specs on the out transformers used in this amp or others like it. It would determine its quality greatly.

H

[tom_s]

Re: Ebay Tube amp?

Build a PP 2A3 amp. You can use the Hammond PP OPTs, I think it's the 1620 and a simple 6SN7 splitter. I had one running years ago and it sounded pretty good.

If you want to start with a Dynaco ST-70, drop me a line. I have all the parts for a rebuild just sitting here in a box.

[weinstro]

Re: Ebay Tube amp?

My next consern would be the Output transformer.

Thoughts? IT is worth it?

Hi,

My first concern is that the eBay ad states that this is a Class A amplifier rated at 50W. A cursory review of a couple of KT88 datasheets cover Class AB1. There are several ways to get to 50W, but some of these will run the tubes hot, or be high distortion operating points, but none are Class A. A high distortion output can be addressed somewhat with more negative feedback, but then the output transformer will have to have high bandwidth, and that's unlikely at this price point.

To me, the first component of importance is the output transformer. The tubes will be 10-20% of the cost of a good OPT.

Last, rare is the amp that can be "fixed" by simply replacing some capacitors.

I would pass on this thing. For some ideas, try tubelab.com or turneraudio.au.

Regards,

Rob

[Wushuliu]

Re: Ebay Tube amp?

The musical paradise has a lot of fans and has very few reported issues. The direct-from-china ebay amps invariably end up with people spending more to fix them up than the actual cost of the amps; although they look very slick and shiny, horror stories abound.

Better to build a simple but extremely good sounding amp like the Tubelab SimpleSE or SimpleSE Push Pull or Tubelab SE (300B) to name a few. Outstanding, and I mean *outstanding* sound for <$500 easy. High quality transformers and power can be had from Edcor inexpensively. They come with PCBs and down to the last resistor manuals on the website and are easy to put together. If you can build a speaker, you can build a tube amp.

Plenty of tube amp info at diyaudio and other forums.

[djg]

Re: Ebay Tube amp?

www.tubes4hifi.com

[PWR RYD]

Re: Ebay Tube amp?

I am currently in the gathering parts stage to build a "from scratch" KT88 push-pull amp (DIY case and point-to-point wiring). I concur with everyone else's comments. The output transformers and power transformer are by far the most expensive and critical parts. But equally important is having a solid circuit design. Doesn't matter how high the quality of parts that are used, if the circuit is unstable, drifts, etc, it's junk! Other parts such as the tubes, resistors, and caps matter, but not nearly as much as the output trannys and the circuit design. The most expensive tube could sound like junk (or fail) if it's run at the wrong operating point. For tubes you can start off with cheap made in China tubes and step up to more expensive tubes later. That's my plan at least.