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Noob questions about Chinese tube amp
Noob questions about Chinese tube amp
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Old 17th April 2018, 10:26 AM   #11
6A3sUMMER is offline 6A3sUMMER  United States
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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.
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Old 17th April 2018, 02:01 PM   #12
kstagger is offline kstagger  United States
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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.
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Old 17th April 2018, 03:08 PM   #13
audiowize is offline audiowize  United States
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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.
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Old 17th April 2018, 06:57 PM   #14
jdarg is offline jdarg  United States
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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.
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Old 17th April 2018, 06:59 PM   #15
audiowize is offline audiowize  United States
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If someone near Seattle has one of these, I'd be happy to measure it.
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Old 18th April 2018, 01:34 AM   #16
dasa2131 is offline dasa2131  United States
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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
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Old 18th April 2018, 03:05 AM   #17
nigelwright7557 is offline nigelwright7557  United Kingdom
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I suspect changing output transformer and valves will give little difference in sound.
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Old 18th April 2018, 07:38 AM   #18
ArcticBrew is offline ArcticBrew  United States
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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
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Old 18th April 2018, 01:00 PM   #19
dasa2131 is offline dasa2131  United States
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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
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Old 18th April 2018, 04:10 PM   #20
audiowize is offline audiowize  United States
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Quote:
Originally Posted by dasa2131 View Post

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?
Plate, grid, and cathode voltages on the output tubes.

Quote:
Originally Posted by dasa2131 View Post
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?
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.

Quote:
Originally Posted by dasa2131 View Post
Audiowize, I don't think I have a low voltage AC source as you suggested. Could I use a 12VAC wallwart or similar?
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.
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