Gary, your thoughts are appreciated, I have a long way to go with this amp yet. I got my excuse for a scope out and saved these images. The 1kHz square wave has some overshoot. Phase shift on the sine waves looks OK to my untrained eye. The top trace is input, bottom trace is output into 8.3 ohms. I could see visible clipping on the scope at 6.8V rms at the output with a 400mV input signal. Does this mean ~6W at the output? The phase splitter is clipping also at this level putting out ~9V rms. This amp is using no global feedback at this stage. Schematic is here.
10kHz sine
1kHz sine
1kHz square
Morgan Jones book is on the way, but if anyone has any comments or suggestions I'm all ears.
10kHz sine
1kHz sine
1kHz square
Morgan Jones book is on the way, but if anyone has any comments or suggestions I'm all ears.
Last edited:
Ian, were those scoped across the speaker outputs? Dummy load or speaker? I suspect the output loading of this amp to be quite critical especially with no NFB.
Ohms law says 6.8v rms into 4 ohms is 11.5 watts, which would be impressive for this amp I think, and given the amp is specced to run 8 ohm loads from memory, even more impressive. Makes me wonder if it isn't actually happier with 4 ohm loads (did someone make a mistake with the OPT spec?).
Just as an aside, I compared the little SE amp I built with 6P1P-EV and EL84, and given the unknown OPT's I am using come from a ECL86 amp and are supposed to be for 4 ohm speakers, it sounds VASTLY superior with the EL84. The 6P1P-EV made it sound just like my Meng with a little less bass, and that made me even more determined to try an EL84 (probably 6P14 though due to the prices of good EL84s) conversion on the Meng. Only possible problem is a little more heater current needed. Switching from a pair of ECL86s alone in the SE to EL84s with a 6N3 driver certainly raised the temp of the original power transformer.
Anyway, gotta go for now, early start tomorrow
Gary
I used 8.3 ohm dummy loads on the output. The LED arrays are passing 34mA at 10.5V so the 6P1's are only dissipating around 8W at quiescent conditions.
It would be interesting to hear the Meng with EL84. From my experience with Chinese amps, I wouldn't be surprised if the power and output trannies are identical for the 6P1 and EL84 amps. The EL84 have around twice the gm of a 6P1, and they certainly seem to be a favoured choice.
Ian.
It would be interesting to hear the Meng with EL84. From my experience with Chinese amps, I wouldn't be surprised if the power and output trannies are identical for the 6P1 and EL84 amps. The EL84 have around twice the gm of a 6P1, and they certainly seem to be a favoured choice.
Ian.
Don't know if it makes much difference, but I use 6 Ohm loads for testing.
Much thanks for the LED readings. I should be ordering parts today. Thought about it some, guess I should ditch the paraphase too.
Considering the steel chassis, I was going to try raising the transformers on brass hardware and washers to see it it effects hum any.
What's left from Meng is 3 transformers and 4 sockets. I wonder if they are watching. Free design service so they should be able to produce a new version for about the same cost and be a killer!
Much thanks for the LED readings. I should be ordering parts today. Thought about it some, guess I should ditch the paraphase too.
Considering the steel chassis, I was going to try raising the transformers on brass hardware and washers to see it it effects hum any.
What's left from Meng is 3 transformers and 4 sockets. I wonder if they are watching. Free design service so they should be able to produce a new version for about the same cost and be a killer!
Yes, it is a "classic". HK A-300 Elite. It even has the original HK output tubes, quite rare I am told. Schematics all over the WEB. I do hate to cut it up as it is worth some change, but then again, a chassis, transformers, sockets and so on costs more. I guess I could take offers
I have the same dilemma with SS. I want to play with come gainclone or class D amps, so I am going to cut up a perfectly good Hafler 220. I can't buy a transformer, heat sinks and box for what I paid for the amp.
My choice would be the 4xEL84 Meng for your power needs, but I would bet that the best sounding of the three (or the easiest to MAKE sound the best) would be the EL34 SE. The one with the 8 output tubes just scares me
I am a little suspicious of the power claims for the EL34 SE though. I have seen it advertised as 25wpc, which is way too high. I would take all Meng power claims with a grain of salt.
I own a Meng 6P1 PP (modified now), by the way.
Be prepared to ground the chassis to the mains input socket at least, and note the 220v input means just that (not 230 or 240).
Gary
4xEL84 good but shipping to me about $200 - worst spending money
Did You satisfied You Meng, which modifications You planning to do?
First, I want to buy Miniwatt amp. Also Suppo El84, but manufacturer remains silent. Now I look on Meng.
220V is good for me, our outlet work in same voltage.
PS Your 6P1 PP has separate volume controls?
Last edited:
Doing some baselines. As it warms up, I can watch pin 7 of V1 climb to .25 and eventually to .5 VDC. The other three remain right at ground or so. I lifted the cap from the splitter to be sure it was not leaking. Nope. So, I am guessing this is a bad tube. No big deal. Just wandering what is this mechanism?
Before I think about EL84's any recommended source for higher quality 6P1's? Russian I guess.
Just a few numbers for everyone to compare theirs against. Seems there is some creativity between amps.
Plate of splitter sits at 108 V, cathode .66.
No grid isolation resistor in either stage
b+ at 259, dropped to 227 to feed the plate resistirs of the splitter. So .66 mA quiescent current per 6N2.
THe cathode resistors are not two 180's but two 260's.
I have 15V on the cathodes.
My heaters are running at 6.02V.
The symmetry pot is 20K, not 50 as marked.
Before I think about EL84's any recommended source for higher quality 6P1's? Russian I guess.
Just a few numbers for everyone to compare theirs against. Seems there is some creativity between amps.
Plate of splitter sits at 108 V, cathode .66.
No grid isolation resistor in either stage
b+ at 259, dropped to 227 to feed the plate resistirs of the splitter. So .66 mA quiescent current per 6N2.
THe cathode resistors are not two 180's but two 260's.
I have 15V on the cathodes.
My heaters are running at 6.02V.
The symmetry pot is 20K, not 50 as marked.
What is the cathode doing on that tube that the grid is climbing, and what is the actual voltage on the plate?
Before I brought the power supply under control (stock 220vac transformer running on 235vac), I had one of the 6P1s in my amp that "took off" thermally if the anode voltage gets over 250v at all. The cathode climbed to a nominal 12-15v on all valves. 3 stopped at 15v, and one kept going to around 20v possibly more, and it sounded BAD. The self bias is holding it from going bang, but it's not good all the same. If this amp used fixed bias, the tube would glow a pretty color I bet
If your plate voltage is over 250v, get it down and I'll bet the tube stabilizes, and your sound will probably improve as well.
I'm actually still using the bad tube in mine. With the 317 CCS and B+ at 250v, it's still OK. No change in sound going to Russian 6P1P-EV, and just for amusement I ran the voltage back up to how it was originally and no runaway or bad sound quality.
By bad sound I don't mean obvious clipping or other distortion. It was like this:
Imagine you are listening to a good quality system. Now turn around 180 degrees and cup your hands behind your ears so you are hearing more of the reflected sound rather than direct. That's what my Meng sounded like when I got it. I couldn't listen to it for more than 15 minutes or I actually felt ill (like a hangover).
Since bringing the voltages under control, I can listen for hours without much fatigue.
Couple of other things:
Mine had 2 270 ohm cathode resistors for the 6P1s.
Heaters were 7v on mine before introducing a dropper resistor on the primary of the PT. You won't need to do that obviously, so your B+ adjustment will have to be done with the resistors just after the doubler (51 ohms on mine). My heaters are 6.2v now.
My B+ was 281v before, with 261v on the plates (hence the runaway).
Symmetry pots were 50k on mine, now they are not there
Interestingly, the heater on my "bad" 6P1 extends out the top of the tube internals by a good 1/8". Other tubes at various heights, with one below the top and extending out the bottom. With the Russian tubes, you could put a ruler across them. Quality control is obviously different, yes?
Mine had 2 270 ohm cathode resistors for the 6P1s.
Heaters were 7v on mine before introducing a dropper resistor on the primary of the PT. You won't need to do that obviously, so your B+ adjustment will have to be done with the resistors just after the doubler (51 ohms on mine). My heaters are 6.2v now.
My B+ was 281v before, with 261v on the plates (hence the runaway).
Symmetry pots were 50k on mine, now they are not there
Interestingly, the heater on my "bad" 6P1 extends out the top of the tube internals by a good 1/8". Other tubes at various heights, with one below the top and extending out the bottom. With the Russian tubes, you could put a ruler across them. Quality control is obviously different, yes?
B+ is a steady 258. When I was running the spectrum analyzer on this, sure enough, this side had twice the 3rd order distortion as the other. Let me see if I have any high wattage resistors to up the 51 Ohms slightly to get down to just under 250. I am only 6.05 V on the heaters, so I don't want to drop the main any. US power.
So, by "bad" tube, it is one that can't take being run over spec. Bad design, marginal tube.
If I order some Russian valves, should I pay douple the price ($4) for 6P1-EV over the 6P1P? Numbers are slightly different, but reading neither Chinese nor Russian, not really sure what I am guessing at. Still spec at 250V though. Limiting the screen seems like a good idea too. And adding grid guards, and........
So, by "bad" tube, it is one that can't take being run over spec. Bad design, marginal tube.
If I order some Russian valves, should I pay douple the price ($4) for 6P1-EV over the 6P1P? Numbers are slightly different, but reading neither Chinese nor Russian, not really sure what I am guessing at. Still spec at 250V though. Limiting the screen seems like a good idea too. And adding grid guards, and........
Measure the actual plate voltage before you do any changes to the B+ resistors. Remember, this is going to affect the 6N2 stages as well.
I don't know whether the EVs are worth the extra, but they are more ruggedly built which means they would survive the postal system better.
They obviously have thicker glass, as the do the "clink clink" thing for a while after turning the power off. The Chinese tubes didn't make a sound.
I am working from memory about the actual plate voltages as I didn't write them down and so many changes have been made since then. But I do know they were well over the 250v limit.
4xEL84 good but shipping to me about $200 - worst spending money
Did You satisfied You Meng, which modifications You planning to do?
First, I want to buy Miniwatt amp. Also Suppo El84, but manufacturer remains silent. Now I look on Meng.
220V is good for me, our outlet work in same voltage.
PS Your 6P1 PP has separate volume controls?
My Meng is earlier model (Mengyue Mini) with single volume control. I don't know if it's still available. For what I paid I am happy enough, and there is enough room underneath to do some changes (Suppo is very small).
Suppo wasn't around when my amp was new. I would probably prefer the Suppo now, but they have gone silent as you say (maybe long holiday in China?). I was waiting for their new EL34 amp, but no announcement yet.
Read back through this thread and you will see the modifications that Ian444 has done to good effect.
Last edited:
Yes, I think that is what happened. Only they obviously didn't leave enough margin to allow for normal variations in AC line voltages, let alone the gross difference when shipping a 220v amp to a 240v market.
Measure the voltage and current across the output tubes. That will tell you if they are being run over their rated anode dissipation wattage. It's not just plate voltage that's the killer here.
tvr, something to consider before doing an EL84 mod on yours. Your Mini 6P1 and mine (and Ian's) are physically very different from memory. Mine has the bigger chassis (and hopefully iron) of the "Meng EL84" model of the time.
I believe yours was marketed as "Meng Series 6P1 Headphone and Integrated Amplifier".
Mine is shown here: Tube Amp online shop - YAQIN, MHZS, XDSE, EL34, KT88, 300B, 845, 2A3, Valve, Amplifier, CD Player
Go to Integrated Amplifier from the menu, and scroll down to Meng Series EL84 to see a pic of mine.
I believe yours was marketed as "Meng Series 6P1 Headphone and Integrated Amplifier".
Mine is shown here: Tube Amp online shop - YAQIN, MHZS, XDSE, EL34, KT88, 300B, 845, 2A3, Valve, Amplifier, CD Player
Go to Integrated Amplifier from the menu, and scroll down to Meng Series EL84 to see a pic of mine.
Last edited:
OK, I just pulled the Meng out of listening duty and did some voltages:
B+ = 251v (HT before the 51 ohm droppers is 255v)
Vp = 244v (only measured one tube because of 38mA CCS in all cathodes)
So I have 7v drop across the output transformer, suggesting a different OPT iron to yours or you are not drawing anywhere near 38mA per tube. My OPT primary measures about 185 ohms from each side to CT in circuit if you want to do a comparison. That figure agrees with the V & I measurements.
Vk varies from 11.2 to 11.8v across the various output tubes.
My driver stage voltages are meaningless for you as I have ditched the original circuit in favor of Ian's (but still on original B+ to the 22k dropper).
Gary
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.