Relyon KT88/EL34 SE issue, schem attached - diyAudio
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Old 7th November 2008, 02:19 PM   #1
Ian444 is offline Ian444  Australia
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Default Relyon KT88/EL34 SE issue, schem attached

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/a...T88SEschem.gif

Any suggestions or comments appreciated.
Thanks,
Ian.
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Old 7th November 2008, 04:20 PM   #2
kevinkr is offline kevinkr  United States
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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?
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Old 7th November 2008, 04:50 PM   #3
KaDe is offline KaDe  Germany
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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
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Old 7th November 2008, 10:07 PM   #4
Ian444 is offline Ian444  Australia
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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.
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Old 8th November 2008, 12:40 PM   #5
Ian444 is offline Ian444  Australia
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Here are some scope shots at low power level. Is this the phase problem you were suggesting Klaus?

http://i198.photobucket.com/albums/a...1260/100Hz.gif

http://i198.photobucket.com/albums/a...81260/1khz.gif

http://i198.photobucket.com/albums/a...1260/10khz.gif

Ian.
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Old 8th November 2008, 01:29 PM   #6
kmaier is offline kmaier  United States
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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
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Old 8th November 2008, 04:10 PM   #7
KaDe is offline KaDe  Germany
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Default 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
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Old 9th November 2008, 08:34 AM   #8
Ian444 is offline Ian444  Australia
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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/a...0/10khzswA.gif

Channel B 10kHz square wave
http://i198.photobucket.com/albums/a...0/10khzswB.gif

Channel A 1kHz square wave
http://i198.photobucket.com/albums/a...60/1khzswa.gif

Channel A 100Hz square wave
http://i198.photobucket.com/albums/a...0/100hzswa.gif

Channel A 30kHz sine wave
http://i198.photobucket.com/albums/a...1260/30khz.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.
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Old 9th November 2008, 09:39 AM   #9
kmaier is offline kmaier  United States
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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
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Old 2nd December 2008, 10:04 AM   #10
Ian444 is offline Ian444  Australia
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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/a...260/Relyon.jpg
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