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Relyon KT88/EL34 SE issue, schem attached

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Ian444

Member
Joined 2008
#1
Hi all, new here, have read a lot of great info here, so have come back to post a question. I bought a Relyon KT88 SE Chinese amp about a year ago and had a listen, put it into storage, then pulled it back out the other day. I am not entirely new to valve gear as I have built a few valve guitar preamps, but I am new to valve hifi. This Relyon amp shows promise but is not clear sounding, especially noticeable in the high freq of cymbals, and drums are not clear, or vocals when "s" is sung. Tone is pretty good with EH EL34. Here is a schem that I drew up this afternoon with voltages measured at idle with the EL34's. The distortion sounds like an impedance mismatch type of distortion, maybe the 10K pot on the input should be 100K to start with? There is also some NFB in this amp. I am a bit out of my depth, looking for suggestions. The distortion is present at all volume levels whether I use the KT88, EL34 or 6P3P. There is plenty of power, that is not a problem. If I don't fully understand it, I cannot fix it, just need a push in the right direction I guess. I can use an iron and modify, that is not a problem.

http://i198.photobucket.com/albums/aa160/birdy81260/RelyonKT88SEschem.gif

Any suggestions or comments appreciated.
Thanks,
Ian.
 
#2
Does the amplifier have multiple output taps? (4, 8, 16 ohms or just one?)

What is the impedance of your speakers? If you connect a 4 ohm speaker to the 8 ohm output tap of a typical tube amplifier that will in most cases produce exactly the kind of symptoms you describe.

Amplifiers with single output taps are frequently designed for 8 ohm speakers..

Having eliminated the above issue, it is also possible that you are hearing a poor quality electrolytic cathode bypass capacitor in the output stage. (47uF) There are countless other possibilities - do you have a scope?
 
#3
Hi Ian
The distortion you hear probably is caused by OPT phase shift.
Since the NFB is linear (47k) there is no correction for this phase delay and HF gets boosted.
You can place a snubber ( 1nF + 1k Ohm for a start ) from plate to screen at the OPTs primary to tame down the HF-overshot.
To fix it right a scope is needed.
Klaus
 
#4
Thanks for the replies.

Kevin, it only has one output tap per channel for the speakers, I presume 8 ohms. I have tried 8 ohm speakers and 2 x 8 ohm speakers in parallel per channel, but cannot hear any difference. I have a scope (Fluke scopemeter) and a signal generator.

Klaus, thanks. It looks like from both replies I need to use the scope and put in a sine wave of different frequencies and I should be able to see the distortion on the scope? And use a dummy load on the output...I will try this today, thanks guys for your time.

Ian.
 
#6
Hi Ian,

Just to add to what Kevin and Klaus have already said... and looking at the schematic, a few additional points:

1- Cathode bypass cap on the output tube results in a -3dB down point around 11hZ. Increasing this to 100uF would lower it to around 5Hz.

2- The grid resistor for the output tube is in spec for the EL34 but on the high side for a KT88. I would probably drop it to 249K. The 0.22uF coupling cap into 249K would give you a -3dB point at 3Hz.

3- The input amp has the screen bypass capacitor to ground. In most cases, this is usually tied to the cathode. When tied to ground, any voltage developed across the cathode resistor also appears as voltage shift on the screen, including your feedback. It's small, but it's still there.

I would run the same tests using square waves, 100Hz, 1KHz and 10KHz and see how they look on the scope. If I read your scope values, the output level was only about 1.8VRMS, which is about 400mw into 8 ohms. I would push the output to at least 1-watt RMS output which is 2.83VRMS into 8 ohms for these tests. You could also run to clipping into 8 ohms then go up and down to check the power bandwidth.

Regards, KM
 
#7
OPT ringing

Hi Ian

To find the HF-resonance of the OPT ( the point where the phase turns 180° ) you need to go much higher with a sine wave.
The HF-overshot is more easy to see on a 10khz square wave.
It is not necessary to flatten it down completely, this would rob you a lot of audio information.
In my practice I have had good results with a snubber on the primary side to stop OPT ringing.

Cheers !
Klaus
 
#8
KM, thanks for your advice.
Klaus, I now understand what you mean by overshoot, I can see it on the 10kHz square wave. Is that excess overshoot?

Channel A 10kHz square wave
http://i198.photobucket.com/albums/aa160/birdy81260/10khzswA.gif

Channel B 10kHz square wave
http://i198.photobucket.com/albums/aa160/birdy81260/10khzswB.gif

Channel A 1kHz square wave
http://i198.photobucket.com/albums/aa160/birdy81260/1khzswa.gif

Channel A 100Hz square wave
http://i198.photobucket.com/albums/aa160/birdy81260/100hzswa.gif

Channel A 30kHz sine wave
http://i198.photobucket.com/albums/aa160/birdy81260/30khz.gif

There has been a strange twist to this story. I tried triode connection on the output valves and put 44uF (2 x 22uF) in parallel with the 6J8P 2K cathode resistor and removed the NFB. The distortion disappeared but the sound was a bit on the bright side. It didn't matter what output valves I used, it still sounded just a bit too much treble, and bass was less. I then put it back to standard to get the scope shots above, and the distortion was gone! 😕 It sounded very good, to my ears at least.

I see the PCB is double-sided and on some of the components there is no solder on the side I am looking at - just the component lead going through the hole. I suspect that during the change to triode mode I may have repaired a doubtful solder joint by accident? Also the solder on the lugs of the tube sockets looks grey instead of shiny. I will pull the PCB out and go right over it with the soldering iron maybe next weekend. Also this amp is built into a tiny case and gets too hot, there is little airflow around the OPT's (mounted inside the tiny box with little ventilation), the whole lot needs to be pulled apart and remounted on a bigger chassis I feel.

In the scope shots, the top trace is signal input with V RMS and freq displayed top left, and the lower trace is output with V RMS displayed top right. Time division is center bottom. It was driving a 6 ohm load. The 100Hz square wave doesn't look that good, but I've never scoped a valve amp before so I don't know what to expect... Thanks for the help everyone.

Ian.
 
#9
Hi Ian,

Overall the square-wave pics look decent. The 100Hz one is typical as you're now charging/discharging the cathode bypass cap and approaching the lower limit of the transformer response. A larger cathode bypass cap (as mentioned in my earlier post) would probably improve that a bit.

I think you might be right regarding some cold solder joints. In most cases a dual-layer PCB has flow-thru holes so you don't need to solder top and bottom on the component. Also be careful on top-side soldering as you're right on the component and heat damage may be an issue.

Anyway, seems you have corrected the cause... and I would certainly recommend re-flowing the PCB. I would use a solder bulb and suck out the old solder first and then re-solder with known good quality stuff. Just be careful on the heat and de-flux it after you're done. Good luck!

Regards, KM
 
#10
It took a bit longer than expected, reworked all the components, most were removed and refitted. It was a real challenge actually pulling it to bits, this amp was never really designed to be pulled apart or repaired easily. I can see the makers had trouble assembling it even. I moved the OPT's from inside under the power tranny to outside as per the photo in the link below, to keep the heat down. Now I can keep my hands on the sides of the case when it is hot, before I could not. I found the choke is also getting hot, quite hot. What I especially noticed when disassembling the amp is that nothing was clean when the amp was built, the PCB and solder and components all dirty requiring several desoldering/resoldering operations till it came clean. The power tranny laminations were not assembled square, they were lop-sided, etc, etc. Also the heater voltages on the preamp and power tubes are different, so I need to run it on 220V (approx 6.3 and 6.1V) otherwise it will go too high.

I did change the cathode bypass caps on the output tube to 100uF and on the input tubes the screen bypass capacitor was wired to the cathode instead of ground as per your advice KM. I found there were some 4 ohm taps on the OPT's so I added a wire to each tap and connected it to the output post as I run 2 x 8 ohm speakers in parallel per channel. The feedback was still connected to the 8 ohm tap and it didn't sound too good, sometimes a bit of distortion, sometimes sounding a bit hollow, not a very happy sound, so this afternoon I put it back to original with the 8 ohm tap feeding the speakers and it sounds very good.

Here's a pic, bit of a Frankenstein, a bit rough, but running cooler and sounding good. No noticeable distortion either! Sounds better than the other amp so its my first rough reference amp - gotta start somewhere! Hahaha.....I love it.

http://i198.photobucket.com/albums/aa160/birdy81260/Relyon.jpg
 
#12
Thorsten, maybe you could swap the 6J8P valves over to see if is a faulty valve or not.

When you say the left valve dies, do you mean it stops glowing? Does the amp do this consistently, that is, stop working in the left channel after about an hour?

In any case, it should be relatively easy to fix as there aren't a lot of components in there, and a simple DMM is all you need, but the big risk is that of fatal electric shock if you are not familiar with working on high-voltage mains-powered equipment. Its not something worth dying for. If you don't feel competent, take it to someone who is and give them the schematic. If you do feel competent, I will help all I can.

I made a few more changes to my amp since the last post, it is still sounding great, and has been reliable.

Ian.
 
#13
Hi Ian im always very careful. Once touched a cap in a photo flash
when i was a beginner in electronics, ouch. When the tube dies you can smell "electricity" like an overheated resistor before melting. I havent opened the chassis yet. And yes the schematic is very simple, so i will first check if all the components are correctly soldered to the board. I dont know how and where in China they make such amps, but i think they have very low quality standards there. I will let you know what i ve found out.
So far, thx a lot for your help!! Thorsten
 
#14
OK, sounds like you need to remove the base and have a good look in there. Use the DMM to check no dangerous voltage is present on pin 3 of the output valves. The negative speaker post can be used as a ground to measure this against, but check that it is actually grounded first. You should be able to check all the resistor values with the DMM except the 300R on pin 8 of the KT88, which has the 47uF cap across it. Might be a bit difficult to check the 390K on pin 5 of the KT88 too, due to the caps in the circuit. When looking at the valve sockets (from underneath) the pins are numbered clockwise 1 to 8, with pin 1 being the first pin to the right of the keyway. One method to systematically check as much as possible is to print out the schematic, and as you confirm each connection and component, draw over that part of the schematic with a highlighter, that way you know for sure what has been checked. Make no assumptions, there could be mistakes or bad solder joints anywhere. Also, your amp could have different component values, in some factories the QC may not have been that good. I don't know that the values in my schematic are ideal. There are 2 x 1uF caps under the PCB that you can't see, near the front panel, for HT filtering.

There were 3 main changes I made to the circuit:
1. The 390K resistor between pins 6 and 8 of the 6J8. I lifted the end connected to pin 8 and connected it to the other side of the 150K resistor, to the 280V end.
2. The capacitor between pin 6 and ground of the 6J8. It now connects between pins 6 and 5.
3. The capacitor between pin 8 and ground of the KT88. It was increased to 100uF.

If you find the need to remove the PCB from the case I'll give you a brief rundown on that, it's not easy, so best to avoid that if possible.
 
#16
Hi Im back, opened the case of my amp and was shocked.
Safety Earth not connected to chassis
Very simple power switch
220 Volt leads close to the chassis without any insulation
Cheapest possible pot, caps, input switch
NO!!! Fuses at all
And the soldering points, Ive never seen such a poor quality before
No I will not repair this amp, because repairing it will mean rebuild
Too much work, too much money. Thank god i did pay only 110 Euros for it and the german ebay seller added 5 russian tubes for free, so i will disassemble the whole thing, sell the parts and keep the chassis which is really quite nice. It will be perfect for a little PCL86 amp that i will put in. Less power but made by myself with good components. Id rather spend some money in high
effective speakers. Ian thx for your help and good luck with your amp. Best regards Thorsten
 
#17
Mine also had no earth wire, sorry, forgot to mention it. But there is a fuse in the IEC power connector housing.

If you are going to strip it - after a lot of stuffing around, this is how I did it.
1. Remove the choke.
2. Unsolder the 2 large electrolytic caps from the PCB. The PCB cannot come out of the chassis unless they are removed. Be gentle or you will end up ripping out the plated-through holes of the PCB. A big tip in the soldering iron helps.
3. Remove the volume pot, headphone PCB and source selector switch.
4. Undo the nuts securing the PCB. (The screws might need to come out too but I can't remember for sure).
5. Undo the 4 screws securing the front panel, a very long Phillips screwdriver may be required for 2 of them. It is very difficult to access these screws, but my PCB would not come out unless the front panel was removed. The PCB can then slide out the front after the heater wires, speaker wires, headers etc have been disconnected. It may or may not be easier to unsolder the heater wires from the transformer instead of from the PCB - or just cut them and replace later. It is not an easy job, there's a lot of parts crammed into a very small space, as you can see.

It would only cost you 4 valve sockets and a handful of parts to rebuild it with point to point wiring, and there's lots of single-ended schematics on the internet, but something would still need to be done about the excessive heat from the choke (I drilled more holes in the chassis for air flow). My only other doubt with this amp is the output transformers look small, they are a bit smaller than the output transformers on 2 other push-pull amps with similar power output that I have, but the sound quality and power of this amp is surprising, so I am not sure what to think, as I am still new to valve amps and lack experience. I was going to build a PCL86 amp, but was concerned about future valve replacements, as I don't think it is made anymore. I can only find the Polam PCL86 at a good price right now. So I am building up a push-pull with Chinese 6P1 into an old chassis at the moment. With the Relyon, I think it would take a lot less work and money to rebuild it point to point compared to putting different valves in there. I don't think you will be disappointed with the end result 🙂
 
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