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Noob questions about Chinese tube amp
Noob questions about Chinese tube amp
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Old 17th April 2018, 01:08 AM   #1
dasa2131 is offline dasa2131  United States
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Default Noob questions about Chinese tube amp

Hello everyone,

I'm a noob when it comes to tube amps. I just got a Chinese single ended EL34 amp from Amazon (Amazon.com: GemTune BL-02 Class A Integrated tube amplifier with tube 5Z3PX1,EL34-BX2,6N9PX2 Tube: Electronics).

I've started using it, and so far it sounds okay. I only have about 5 hours of time on it. My speakers are DIYSOUNDGROUP Aggressive 1299's (99db efficiency). I was previously using a QSC GX5 amplifier for the speakers.
I have a bunch of questions I'd like to ask. Please forgive me if these sound dumb or have already been answered before a hundred times.

1. This kit came with PS VANE EL34 tubes. Can anyone recommend an EL34 tube that will sound MUCH different than what I've got already? I'm not looking to spend a fortune here. I don't have audiophile-quality ears!

2. I've heard the output transformers are the first things that should be improved on these amps. How do I figure out what model Edcor output transformers and power transformer I should get to replace the Chinese ones?

3. So far I've been careful with pushing the amp (nothing loud yet). How do I know when I start getting distortion? What does tube distortion sound like? Can I hurt the amp if I play something too loud? As I said, I'm a noob here.

Thank you very much,

Dave
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Old 17th April 2018, 01:58 AM   #2
audiowize is offline audiowize  United States
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Quote:
Originally Posted by dasa2131 View Post
1. This kit came with PS VANE EL34 tubes. Can anyone recommend an EL34 tube that will sound MUCH different than what I've got already? I'm not looking to spend a fortune here. I don't have audiophile-quality ears!
I'd leave them alone for now.
Quote:
Originally Posted by dasa2131 View Post
2. I've heard the output transformers are the first things that should be improved on these amps. How do I figure out what model Edcor output transformers and power transformer I should get to replace the Chinese ones?
You'll need to buy a decent meter, then we can help you measure the step-down ratio.

Quote:
Originally Posted by dasa2131 View Post
3. So far I've been careful with pushing the amp (nothing loud yet). How do
I know when I start getting distortion?
You have really efficient speakers, I'd recommend putting some small bookshelf speakers on the amp and crank away. You'll hear the distortion quite clearly. You won't hurt the amp doing this.
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Old 17th April 2018, 02:12 AM   #3
dasa2131 is offline dasa2131  United States
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Mr. Audiowize,

Thank you very much for your fast response - I really appreciate it. As you suggested, I'll leave the tubes alone for now. And thank you for telling me about using small inefficient speakers to hear the distortion. I'll try that.
Regarding your second comment about getting a meter, I assume you're referring to a multi-meter? I have a Fluke 87, a craftsman multi-meter, and a Simpson 260. If these aren't sufficient, please let me know what I need. To be clear, I'm a mechanical engineer - my knowledge of electronics is very weak. Any help you can offer is greatly appreciated!

Thank you again,

Dave
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Old 17th April 2018, 02:25 AM   #4
anatech is offline anatech  Canada
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Noob questions about Chinese tube amp
Hi Dave,
Your Fluke is a good meter with a high enough frequency response. Forget the other meters exist for now. The 260 isn't as accurate and it's HF response is poor. The Craftsman meter isn't very reliable and will not have a decent HF rating at all. So it's best if you just use them for basic, low accuracy type work.

Output transformers are typically rated for impedance. Your tubes would be happy with about 6,600 ohms and the speaker end is probably a 4 ohm tap - could be 8 ohms or you might have all of them brought out on terminal strips for the speaker connection.

Output tubes. I think the 6CA7 sounds a lot better than the EL34, and they are interchangeable. Not singly, in pairs! One of the best new tubes would be Electroharmonix, so a 6CA7EH or EL34EH. New Sensor in New York is the importer and many shops will carry them. Let the amp burn in with what it has for a bit, just as audiowize suggested.

-Chris
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Old 17th April 2018, 02:33 AM   #5
dasa2131 is offline dasa2131  United States
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Chris,

Thank you very much for your response. I'll use the Fluke as you suggested. My speakers are 8 ohm impedance (I should have listed that in my first post - sorry). I won't ever be using this amp for 4 ohm speakers.
Would you be willing to clue me in on how to measure the output transformers? Also, do I need to replace the power transformer as well? I didn't mention this earlier, but the thing gets REALLY hot. Is this normal for tube amps?
I probably sound like an idiot for asking all these questions when I only have ~6 hours of time on the amp. Any help you can offer is greatly appreciated. This site is WONDERFUL!

Thank you again,

Dave
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Old 17th April 2018, 02:43 AM   #6
anatech is offline anatech  Canada
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Noob questions about Chinese tube amp
Hi Dave,
Transformers are expensive. If you are going to replace all that, you would be a lot further ahead starting fresh and building your own amplifier.

To give you an idea, the transformers used in the 35 watt per channel Eico HF-87 are about $120 USD each bought new from the original manufacturer. I don't know about Edcor, but Hammond transformers are very good and cost more (but they are larger for that power). The power transformer is similar, for the Eico is $100 USD and more from Hammond. Better parts = lower temperature.

Transformers in tube products will run hot because they are running close to the maximum power out, unlike a solid state amp where the transformer is close to idling most of the time (unless you're pounding it). You should be able to hold your hand on any transformer 'till the count of 10.

Before jumping in with both feet, read some other threads on tube amplifiers. There are some excellent books on the subject too. You really need to learn something about them before you start changing things.

-Chris
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Old 17th April 2018, 03:16 AM   #7
FullRangeMan is offline FullRangeMan  Brazil
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Quote:
Originally Posted by dasa2131 View Post
1. This kit came with PS VANE EL34 tubes. Can anyone recommend an EL34 tube that will sound MUCH different than what I've got already? I'm not looking to spend a fortune here. I don't have audiophile-quality ears!
Please note: changing the output tubes will result in small improvements, bigger improvments are possible changing the driver tube spending less $.
Quote:
2. I've heard the output transformers are the first things that should be improved on these amps. How do I figure out what model Edcor output transformers and power transformer I should get to replace the Chinese ones?
Chances are the Edcor, Heyboer and Hammond OPTs are in the same sound quality than the original OPT, the original OPT will lasts 10 years or more, dont fix what is not broke. Usually EL34 SE Triode is Primary 2.5KΩ OPT. M6 vs regular GOSS Lam have no large sound improvments.
If you insist in change the OPTs I suggest Hashimoto HC-203U, very nice Ei Hi-B lamination 2kg core to 20W. HC-203U
Quote:
3. So far I've been careful with pushing the amp (nothing loud yet). How do I know when I start getting distortion? What does tube distortion sound like? Can I hurt the amp if I play something too loud? As I said, I'm a noob here.
Usually small trafos are not suited to run around the clock, I think 4 hours max in winter to a long life.
My 2 cents.
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Last edited by FullRangeMan; 17th April 2018 at 03:23 AM.
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Old 17th April 2018, 06:45 AM   #8
Alllensoncanon is offline Alllensoncanon  United States
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Noob questions about Chinese tube amp
While rolling tubes are like changing limited circuit parameter randomly, not scientific. But they can be fun as long as you don't spend too much money on ridiculously priced NOS tube. I would take a look at a Russian made 6H9C. They shouldn't cost much and all you need is one for this amp. If you could find it cheap, consider a 6SL7 also.

I also think you should reconsider changing the output transformer. The last time I looked at this amp, the copy I had, there were number of issues here and there. Change the OPT alone won't worth the $ and efforts. If your copy is like the one I looked at, my guess is you should be already hearing some tube distortions. This amp I think uses a 5K primary SE (Single-End) OPT with no UL (ultra linear) tap. But I would put the new transformer budget on a new design and new chassis.
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Old 17th April 2018, 08:04 AM   #9
ColinA123 is offline ColinA123  United Kingdom
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Your first job with a Chinese amp should be to check (or have checked by someone who knows what they are doing)
That voltages and components are within spec and that safety earthing is correct and working.
Then enjoy it while saving up for something better.
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Old 17th April 2018, 08:46 AM   #10
ArcticBrew is offline ArcticBrew  United States
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I bought a used BEZ 6SL7 EL34 SE on the cheap because the owner was unhappy with the sound. Its schematic is almost identical to the Gemtune schematic. The only real difference is probably the quality of the BEZ OTs. This is a popular design for many Chinese EL34 SEs.

I used it in a bi-amp system powering tweeters that are 99db efficient. It sounded okay but not what I was accustomed to from BEZ. When I tested it a 1khz waveform began to go asymmetrical at 1 watt output, grossly by 2 watts and clipping at 3+ watts.

The cathode resistor was too large and it was biased very cold. I added resistance in parallel to lower the cathode resistor until I got an idling current closer to the data sheets. The asymmetry improved.

It was wired as a pentode but with no feedback. It did not have a screen resistor. The screen supply was taken from the same tap as the Plate. The screen voltage was 10 volts higher than the plate. I added rather large screen resistor and got the screen 10 volts below the plate. Again, a noticeable improvement on my scope.

I hooked up a 500k pot for plate to grid feedback and dialed it in until I liked the look of the waveform. Then I replaced it with fixed resistors of the same value (~300K). The 6SL7 SRPP stage had more than enough gain to handle the schade style feedback.

Then I tested various different values on the cathode resistor until I got the best looking sine waves. I believe I ended up with 211 ohms for the cathode resistor and 72 ma.

End result 6+ watts before clipping. Excellent symmetry on sine waves from 30-18khz at all power levels. Very good frequency response.

The only change I would do in the future is to add another separate tap to the power supply for the screens. But, it sounds so good that it has been in my system for 6 months and I have had no desire to change it.

BEZ makes very good transformers. If I owned your Gemtune I would do all of the changes I mentioned above except I would also try global NFB from the transformer.

Steve
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