Cheap and Chinese KT88 SE amp

[andrew_whitham]

Just returned from China with a new toy!

"HI-FI" (trade name? not a definition) KT88 amplifier similar to the ones currently coming in from HK and regularly advertised on e-bay (item #130059099147 is typical)

Initial impressions are that its very small... The OPT are shoved under the main power transformer (which is HUGE) Sadly they are not big so the Bass is limited...

There is also a faint hummm on both channels, probably because The power supply is very simple (all the heaters are AC)

Having said all that, for the money, it aint half bad as an introduction to single ended.

The schematic for one channel is attached - yes it really IS that simple.

plenty of room for the tweaking.

Andy

[Kent Smith]

Well, that is pretty simple! I wonder why they chose to hook up a pentode in triode mode for the driver instead of just using a triode? (maybe they got a deal on those??)

I use AC filaments in my amps and I have to get very close to the speakers and barely hear it. I have found that I usually need a choke on the power supply though. I also end up using a few hundred ohms between the UL tap and screen (adjust for cleanest highs).

Should be a great performer!

[ArtG]

I've noticed decent, appearing, anyway, Chinese amps for sale on eBay from time to time, at very attractive prices, shipped directly from China. From reading a few relevent posts on other boards, I get the impression that they aren't too bad, at least considering the price.
Would I be prying to ask the price you paid? How is the workmanship?

[rdf]

Between the UL, no feedback connection and the small output tube cathode bypass I'ld be surprised if the damping factor exceeds 1. Plenty of room for experimentation!

[dsavitsk]

Is it me, or are the diodes in the rectifier backwards?

[Sherman]

Originally posted by dsavitsk
Is it me, or are the diodes in the rectifier backwards?

Doug,

It ain't you, electricity flows backwards in the Eastern Hemisphere.

[Klimon]

Is it me, or are the diodes in the rectifier backwards?

definitely (the diodes that is)

Did you draw that schematic yourself or is there something like a source for 'Chinese tube-amp' schematics?

Simon

[andrew_whitham]

Originally posted by ArtG
I've noticed decent, appearing, anyway, Chinese amps for sale on eBay from time to time, at very attractive prices, shipped directly from China. From reading a few relevent posts on other boards, I get the impression that they aren't too bad, at least considering the price.
Would I be prying to ask the price you paid? How is the workmanship?

Pry away, It was about £90, (1400RMB) - should have been less but I'll admit my bargaining skills were not on top form.
Workmanship, internally is fine, the box is pretty poor close up, obviously been re-worked. Its high on the list of things to change :-)

Originally posted by Klimon
definitely (the diodes that is)

Did you draw that schematic yourself or is there something like a source for 'Chinese tube-amp' schematics?

Simon

oops, wasnt paying attention there was I?

Drawn by me, its reasonably straight forward, Join the dots in cad...

For Chinese tube amps generally, they tend to be really conventional (I had a Yarland FV-34C III previously and a BADA tube headphone amp). Indeed often the seller will be quite blunt about the origins of the design ;-)

If anyone is thinking about getting one sent, just watch the postage. Now that is frightenening.

[andrew_whitham]

Originally posted by rdf
Between the UL, no feedback connection and the small output tube cathode bypass I'ld be surprised if the damping factor exceeds 1. Plenty of room for experimentation!

Suggestions?

Given that I'm very short of room the PSU will need to wait

but the list otherwise is
New volume pot. - I have a 100k Alps Blue. I remember though something about the need to 'drive' volume pots, so is that sensible?

Input cap - I dont care, I'm not THAT brave.

Not shown in the diagram are the headphone outputs - gotta go!

I'd like to change the 6J8P too. See the attached (with right way facing diodes)
I think the Mu follower is correct. But if anyone can point me in the direction of some informtion on dimensioning them it'd be appreciated. But as I type this, I realise I better check the gain vs the original tube too, so forgive me if its blatantly the wrong tube

Later maybe a new box and some new OPT's and, and, and...


Andy

[rdf]

How about the Fi Primer 6L6SE? Can't personally vouch for it but it's been floating around a very long time. I think someone here (Sherman?) likes it a lot.

Warning, a Fortunecity site:

http://tinpan.fortunecity.com/saints/668/primer/