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
"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
Attachments
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!
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!
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?
Would I be prying to ask the price you paid? How is the workmanship?
ArtG said:
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
Klimon said:
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.
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 said:
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
Attachments
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/
Warning, a Fortunecity site:
http://tinpan.fortunecity.com/saints/668/primer/
RDF, interesting Fi Primer but input pentodes are not for me. somewhere along the way I developed an 'irrational' distrust of 'em. Plus I'm interested to see how this develops - *must* get new OPT's
Anyway, just to carry on the thread - since modding's almost DIY I guess...
Newly installed volume pot seems to be OK, dont know what all the fuss about driving is about. This one is 25X (in ohmic value) the previous & it works, dont actually know what I was expecting.
(ok, so I'm ignorant to the potential effects of getting it right...)
Also added a switch for UL vs Triode mode. Man that's surprising -the loss of volume one vs the other, I'll see if its prefered or not, both seem adequate with my home built speakers. In-between the screen grid and the anode there's a 300ohm resistance is that required with the kt88 or not? I couldn't find a definitive answer.
Increased the cathode bypass on the output tubes to 220uF, not much room for anything larger, not much effect either...
Meanwhile the 6sn7's arrived today (well ok the 6h8c's) so I'm gonna try the input stage over christmas
Like I say if I have dropped one with the mu-follower I'd like to know.
Merry Christmas all
Andy
Anyway, just to carry on the thread - since modding's almost DIY I guess...
Newly installed volume pot seems to be OK, dont know what all the fuss about driving is about. This one is 25X (in ohmic value) the previous & it works, dont actually know what I was expecting.
(ok, so I'm ignorant to the potential effects of getting it right...)
Also added a switch for UL vs Triode mode. Man that's surprising -the loss of volume one vs the other, I'll see if its prefered or not, both seem adequate with my home built speakers. In-between the screen grid and the anode there's a 300ohm resistance is that required with the kt88 or not? I couldn't find a definitive answer.
Increased the cathode bypass on the output tubes to 220uF, not much room for anything larger, not much effect either...
Meanwhile the 6sn7's arrived today (well ok the 6h8c's) so I'm gonna try the input stage over christmas
Like I say if I have dropped one with the mu-follower I'd like to know.Merry Christmas all
Andy
Andrew.
I've been looking at these, with some mistrust, it has to be said. But the prices are tempting.
In what way exactly is the bass weak?
Is it a bit loose and woolly or just not convincingly deep?
Don't like the idea of the hum as i have 96dB speakers.
Anyone tried the 6p13 versions from HK?
I've been looking at these, with some mistrust, it has to be said. But the prices are tempting.
In what way exactly is the bass weak?
Is it a bit loose and woolly or just not convincingly deep?
Don't like the idea of the hum as i have 96dB speakers.
Anyone tried the 6p13 versions from HK?
Bass is just not there, what is, is ok but its sparse.
The OPT's are mounted in the chassis, which is small. (and gets hot because of the 20 watt heating resistors inside..)
So far I've changed the volume pot, and added a switch to flick between triode and UL modes. To my mind Triode mode helps the bass a bit but the amp is being run harder so it might be that?
Hummmmmm
I have 89db speakers (castle Harlech) I Reduced the hum somewhat by removing the headphone socket(s). Really the PSU wants to be replaced with a new one. The Mains transformer is 310VAC o/p for 345Vdc at the top end of the OPT. seems like a big drop...
Incidentally on mine the headphone socket on the front panel wasn't actually connected to anything. Just like the 4ohm/8ohm switch...
Finally the output transformers (which I think are the reason for the poor bass) as fitted to the amp I have are:
5k with an 8ohm reflected load with a 20% UL tap, a DCR of 365ohms and ABOUT 13H of inductance according to my meter (this varied between left and right channels)
Question: Has anyone seen curves for the kt88 in UL with a tapping at 20% (max power?) I'm interested to find out how its supposed to function.
Andy
The OPT's are mounted in the chassis, which is small. (and gets hot because of the 20 watt heating resistors inside..)
So far I've changed the volume pot, and added a switch to flick between triode and UL modes. To my mind Triode mode helps the bass a bit but the amp is being run harder so it might be that?
Hummmmmm
I have 89db speakers (castle Harlech) I Reduced the hum somewhat by removing the headphone socket(s). Really the PSU wants to be replaced with a new one. The Mains transformer is 310VAC o/p for 345Vdc at the top end of the OPT. seems like a big drop...
Incidentally on mine the headphone socket on the front panel wasn't actually connected to anything. Just like the 4ohm/8ohm switch...
Finally the output transformers (which I think are the reason for the poor bass) as fitted to the amp I have are:
5k with an 8ohm reflected load with a 20% UL tap, a DCR of 365ohms and ABOUT 13H of inductance according to my meter (this varied between left and right channels)
Question: Has anyone seen curves for the kt88 in UL with a tapping at 20% (max power?) I'm interested to find out how its supposed to function.
Andy
Thanks Andrew.
Sounds like the transformers' low inductance could be the root of the problem.
13H seems low for 5K.
As for the rest of it. A rebuild into a bigger box would help but then it's not such a cheap amp by the time you've finished.
Its a shame these are so compromised. A few more dollars spent in the right areas would make it a much better proposition.
However, there's always the chance that the bass might improve with time.
Sounds like the transformers' low inductance could be the root of the problem.
13H seems low for 5K.
As for the rest of it. A rebuild into a bigger box would help but then it's not such a cheap amp by the time you've finished.
Its a shame these are so compromised. A few more dollars spent in the right areas would make it a much better proposition.
However, there's always the chance that the bass might improve with time.
Just some thoughts (as I am not really qualified to offer serious advice on amp re-design):
Have you measured the output impedance of the amp in operation. I believe this could help to evaluate the need for any additional feedback scheme.
The thread 807 E-Linear might provide some interesting idea for improvement. Or maybe You could try the more conventional (?) plate to grid feedback.
Your amp having a penthode driver, may be suitable for modification to such feedback schemes.
SveinB
Have you measured the output impedance of the amp in operation. I believe this could help to evaluate the need for any additional feedback scheme.
The thread 807 E-Linear might provide some interesting idea for improvement. Or maybe You could try the more conventional (?) plate to grid feedback.
Your amp having a penthode driver, may be suitable for modification to such feedback schemes.
SveinB
Some of the Chinese amps sound fairly good and they are very cheap.
I have not bought one because the voltage difference. In China they have 220V main and in OZ we have 240V. I sometimes found it to be 250V!
I suspect those Chinese amps would last for no longer than a few months. The AC heater would be at 6.9V for a 6.3V heater!
Unless I use a step down transformer, or replace most components with different values. However, regulating the heaters to provide the right voltage is the problem. It is not hard to design and make it but it won't fit into existing chassis. I am not interested in adding in a series resistor because the transformer may not have the reserve to deliver the current or it can get hot.
What a pity!
Any solutions without using a step down transformer?
I have not bought one because the voltage difference. In China they have 220V main and in OZ we have 240V. I sometimes found it to be 250V!
I suspect those Chinese amps would last for no longer than a few months. The AC heater would be at 6.9V for a 6.3V heater!
Unless I use a step down transformer, or replace most components with different values. However, regulating the heaters to provide the right voltage is the problem. It is not hard to design and make it but it won't fit into existing chassis. I am not interested in adding in a series resistor because the transformer may not have the reserve to deliver the current or it can get hot.
What a pity!
Any solutions without using a step down transformer?
HiFiNutNut said:
there is always possibility that you can wind severall additional turns to primary (10% looks not that much in turns) if at least some of window is still available;in that case all sec voltages will be smaller 10% than intended,but-who cares
or you can wire in series secondary of some small 20V xformer and hide it somewhere under the hood
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