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
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
I'd leave them alone for now.
You'll need to buy a decent meter, then we can help you measure the step-down ratio.
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.
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
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
Attachments
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
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
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
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
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
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
Please note: changing the output tubes will result in small improvements, bigger improvments are possible changing the driver tube spending less $.
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
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.
Last edited:
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.
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.
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
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
dasa2131,
The link in post #1 says the amp is for 110V AC. You more likely have about 120V AC. That by itself can make the power transformer warmer.
Use your Fluke 87 to measure your power line voltage.
So many of the Chinese amps have power transformers that have 2 primary windings, and are connected in series for 220V, and connected in parallel for 110V.
The link in post #1 says the amp is for 110V AC. You more likely have about 120V AC. That by itself can make the power transformer warmer.
Use your Fluke 87 to measure your power line voltage.
So many of the Chinese amps have power transformers that have 2 primary windings, and are connected in series for 220V, and connected in parallel for 110V.
Also new output transformers will most probably not fit in the location of your current ones due different bell housing/bolt widths. So you will have to modify your chassis to get the new ones to fit, provided there is enough room.
A new, from scratch design would be your best bet if you want a real improvement.
A new, from scratch design would be your best bet if you want a real improvement.
Your fluke meter will be acceptable for the job at hand. You'll want to open the amp up and look at the wires leaving the output transformer that don't go to the speaker terminals. There will either be one wire going to the 8 pin socket or 2, then another wire that connects to the power supply.
What you need to do (with the amp off and unplugged) is to put a small AC voltage across the speaker terminals (a 100Hz test tone off your phone at half volume will do), measure that voltage, then measure the voltage between pin 3 on the 8 pin socket and the wire connecting to the power supply. This voltage will be a lot higher and will give you your transformer ratio.
The suggestion to measure all the DC voltages is also a good one. Sometimes there are some strange things going on in these amps. I'm not sure if it's this particular amp or a similar one that operates the EL34s in ultra-linear but doesn't use any feedback. That is certainly something to remedy as well.
What you need to do (with the amp off and unplugged) is to put a small AC voltage across the speaker terminals (a 100Hz test tone off your phone at half volume will do), measure that voltage, then measure the voltage between pin 3 on the 8 pin socket and the wire connecting to the power supply. This voltage will be a lot higher and will give you your transformer ratio.
The suggestion to measure all the DC voltages is also a good one. Sometimes there are some strange things going on in these amps. I'm not sure if it's this particular amp or a similar one that operates the EL34s in ultra-linear but doesn't use any feedback. That is certainly something to remedy as well.
I have a gemtune x1 which afaik is the same amp but only single 6sl7 shared between channels so no srpp. It's pentode mode but does have nfb back to unbypassed rk on the 6sl7. Outputs are decent size but I haven't measured. It sounds...like a pentode amp without enough ndb. I like it in the system sometimes. Fun but definitely not sota.
Thank you everyone for your responses! I really appreciate it. Based on your feedback, this is what I'm going to do:
1. Try to get the schematic. I e-mailed GemTune and asked for it, but haven't heard back from them. Any other source for getting this schematic, or do I need to figure it out myself?
2. Hold off on expensive new output transformers and power transformer. I'll save up some money to build something else.
3. Open up the amp and start measuring some of the voltages. I'm going to wait and get some help from my dad on this. From what I know, I should measure the plate voltage and the filament voltage. What else do I need to measure?
4. I might be able to borrow a signal generator and an oscilloscope. If I understand correctly, I can hook up a high wattage 8 ohm resistor to the amp's output, connect the signal generator to the audio input, and use the oscilloscope to view the input and output waveforms. I can measure the voltage across the resistor and can calculate the amp's power using V^2 / R. Am I on the right path here?
Audiowize, I don't think I have a low voltage AC source as you suggested. Could I use a 12VAC wallwart or similar?
Thank you everyone for helping me,
Dave
1. Try to get the schematic. I e-mailed GemTune and asked for it, but haven't heard back from them. Any other source for getting this schematic, or do I need to figure it out myself?
2. Hold off on expensive new output transformers and power transformer. I'll save up some money to build something else.
3. Open up the amp and start measuring some of the voltages. I'm going to wait and get some help from my dad on this. From what I know, I should measure the plate voltage and the filament voltage. What else do I need to measure?
4. I might be able to borrow a signal generator and an oscilloscope. If I understand correctly, I can hook up a high wattage 8 ohm resistor to the amp's output, connect the signal generator to the audio input, and use the oscilloscope to view the input and output waveforms. I can measure the voltage across the resistor and can calculate the amp's power using V^2 / R. Am I on the right path here?
Audiowize, I don't think I have a low voltage AC source as you suggested. Could I use a 12VAC wallwart or similar?
Thank you everyone for helping me,
Dave
Dave,
This is the schematic I looked at for the BL-02 that was the basis of my comments. Scroll down towards the bottom of the page to find the schematic.
Single ended triode tube amp - Vinyl Engine
Steve
This is the schematic I looked at for the BL-02 that was the basis of my comments. Scroll down towards the bottom of the page to find the schematic.
Single ended triode tube amp - Vinyl Engine
Steve
Hello Steve,
Thanks for sending me that link. I enjoyed reading through that list of posts, though I must admit I didn't understand a lot of the electronic bits. I copied the picture of the schematic and will compare it to my amp.
I enjoyed hearing about your woodworking modifications to the amp - I had planned on doing the same things to mine!
I previously bought a cheap tube pre-amp buffer off Amazon and built an enclosure for it using left over flooring (Bolivian rosewood). I also machined the knobs and tube rings out of brass. The fan cover is also machined Bolivian rosewood. If you look closely at the feet, you'll see segmented rings (12 segments).
Regards,
Dave
Thanks for sending me that link. I enjoyed reading through that list of posts, though I must admit I didn't understand a lot of the electronic bits. I copied the picture of the schematic and will compare it to my amp.
I enjoyed hearing about your woodworking modifications to the amp - I had planned on doing the same things to mine!
I previously bought a cheap tube pre-amp buffer off Amazon and built an enclosure for it using left over flooring (Bolivian rosewood). I also machined the knobs and tube rings out of brass. The fan cover is also machined Bolivian rosewood. If you look closely at the feet, you'll see segmented rings (12 segments).
Regards,
Dave
Attachments
Plate, grid, and cathode voltages on the output tubes.
Sorta. Without knowing how much distortion the amp is putting out, you may end up seeing power at 20-50% THD. Generally when you just see the wave starting to flatten out on the top or bottom, that's a decent enough estimate of power.
Yes, a 12V AC wallwart will work. Put the wall wart across the primary side (wires that connect to the power supply and tubes), then measure the two AC voltages.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.