goldline thanks for Your sugestions. Low B+ is the main problem to achieve real power. But I wonder if pushing ecc82/driver tubes a little harder could increase it? Generally 180V on anodes and common 1,2k cathode resistor should be fine to almost full power of kt88. I also wonder if driver change to 6n6p with 200V on anode and 20mA could rise the power. As for changing power transformer, for 560V B+ i would need about 420VAC on secondary. This indel You proposed is 313V (or 481V two secondary windings connected in series). Is there a possibilty to connect existing power transformers to achieve about 560VDC? I'm thinking of it and can't figure out anything with sense.
To ruben alstad, what are the rating of your new power transformer? Have You left dual mono power configuration?
To ruben alstad, what are the rating of your new power transformer? Have You left dual mono power configuration?
I'm running single rail b+ with a unholy amount of filtering, my amplifier came with just a single rail and two transformers, one for the b+ and bias and the other one for two 6.3v rails, the heater transformer is the same dimensions mounting vise but had pretty much half of the iron/laminations in it, also dropped to 5.9v on the heaters so I swapped it out with a toroid with a self wound secondary to get stable 6.3v, you can now also keep your hand on the trany without it getting uncomfortable.
Every amplifier that leaves their shop in china is its own unique beast it seems.
Every amplifier that leaves their shop in china is its own unique beast it seems.
That's truth. My friend same MA have B+ at about 435Vdc, so it's lower than mine.
Today I measured main trafos. Results:
primary resistance 10ohm for 110Vac so for 220Vac in series about 20ohm
secondary resistance 18.7ohm
secondary voltage without load 188Vac, with load (bias set to 56mA each) 179Vac which gives 454Vdc B+, under full load it gets under 390Vdc B+, so it is unacceptable low.
I wonder if it will be an option to rise the input voltage from 230Vac to about 300Vac by 230/300V 400W transformer. Every section would have +30% extra voltage. I already have 6Vac on input/driver heaters so it would even help. Ofcourse extra voltage could be lowered by resistors. For bias section it wouldn't be a problem also. The main B+ capacitors should be for at least 600Vdc. This is only my thinking.
I didn't take off MA transformer enclosures. Can You tell me what kind of trafos are under them? Will EI102 trafos fit? Or maybe something else?
Today I measured main trafos. Results:
primary resistance 10ohm for 110Vac so for 220Vac in series about 20ohm
secondary resistance 18.7ohm
secondary voltage without load 188Vac, with load (bias set to 56mA each) 179Vac which gives 454Vdc B+, under full load it gets under 390Vdc B+, so it is unacceptable low.
I wonder if it will be an option to rise the input voltage from 230Vac to about 300Vac by 230/300V 400W transformer. Every section would have +30% extra voltage. I already have 6Vac on input/driver heaters so it would even help. Ofcourse extra voltage could be lowered by resistors. For bias section it wouldn't be a problem also. The main B+ capacitors should be for at least 600Vdc. This is only my thinking.
I didn't take off MA transformer enclosures. Can You tell me what kind of trafos are under them? Will EI102 trafos fit? Or maybe something else?
I've converted input/driver stage to full ussr version. Inputs are 6n1p-ev and drivers are 6n6p. I heard about this combo before and I must admit that these cheap tubes plays really very good. Here is the link to schematic:
http://images78.fotosik.pl/299/ab375884d3e28034.jpg
I will have to fit somehow 10k 6n6p plate resistors with higher power, because there is more than 3,5W on each and with 5W ones they are getting really hot. Maybe 6n1p for input and driver also should be nice, but I think 6n6p sounds better than 6n1p.
About the sound. The improvement is almost in all aspects. The lows are now in, before the bass was so weak. 6n6p is also more neutral than ecc82 that is for sure. The sounds are more vital and alive. With the original circuit the music was annoying, now it is much more pleasure.
Interesting... so changing the feedback networks did the trick? Does the crossed-out caps (6800pF) mean they were bypassed or simply removed from the circuit altogether? Did you take any measurement before and after the modifications were made?
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Dankal6, your wanted B+ voltage of 560 volts you can get from a standard 230/400 volt
industrial transformer a 1/2 amp. 200watt core will give you 100ma bias to all four powertubes. just forget the original powertransformer
Other modifications could be electronic choke coils (Gyrator) in the b1 rail.
And a autobias circuit like Prima-Luna.
Nice work on the pre and driver stage, did you still have enough gain for the overall
feedback ?
industrial transformer a 1/2 amp. 200watt core will give you 100ma bias to all four powertubes. just forget the original powertransformer
Other modifications could be electronic choke coils (Gyrator) in the b1 rail.
And a autobias circuit like Prima-Luna.
Nice work on the pre and driver stage, did you still have enough gain for the overall
feedback ?
To jazbo8, I didn't changed anything in feedback circuit. I put it on schematic just as it is. These 6800p caps were in place of resistors. I don't have oscilloscope yet, but I have to buy it soon, because now I can only count on my ears, but I'm not a bat ofcourse 
To goldline, gain is about the same as was before changing anything. Now because of bass it even supposed to be louder, but I think it's almost the same. I've got 230/400v trafos in my stuff, but after I red some posts about really parameters of kt88, I wonder what voltage would be accurate for my kt's. First I wanted to go for about 600vdc to get about 550vdc when fully driven, but these EH kt88 seems to be too weak to force this. First of all I will pull out existing trafos and check is there maybe a possibility to put some more wire into them, without changing them. Secondly I thought about put another small toroid trafo into chasis and wire it in series with existing two. I was thinking about 2x40 or 2x50vac 2x0,8A it is about 95mm diameter and to 40mm height, so I should place it in chasis somehow. Both propositions are cheap, first dual covered wire for trafos about few euro, second one about 18euro. I know I could wind trafos by myself on EI102 but this would take me too much time. The most proper way wound be to let winding both trafos to someone who do this for living. Polish winding guru told me 55euro/each. So there are many ways to achieve more power, from diy to custom made. I don't know now which to choose. I don't know either what are maximum safe voltages for my electroharmonix kt88? Maybe You know how high I could reach?
To goldline, gain is about the same as was before changing anything. Now because of bass it even supposed to be louder, but I think it's almost the same. I've got 230/400v trafos in my stuff, but after I red some posts about really parameters of kt88, I wonder what voltage would be accurate for my kt's. First I wanted to go for about 600vdc to get about 550vdc when fully driven, but these EH kt88 seems to be too weak to force this. First of all I will pull out existing trafos and check is there maybe a possibility to put some more wire into them, without changing them. Secondly I thought about put another small toroid trafo into chasis and wire it in series with existing two. I was thinking about 2x40 or 2x50vac 2x0,8A it is about 95mm diameter and to 40mm height, so I should place it in chasis somehow. Both propositions are cheap, first dual covered wire for trafos about few euro, second one about 18euro. I know I could wind trafos by myself on EI102 but this would take me too much time. The most proper way wound be to let winding both trafos to someone who do this for living. Polish winding guru told me 55euro/each. So there are many ways to achieve more power, from diy to custom made. I don't know now which to choose. I don't know either what are maximum safe voltages for my electroharmonix kt88? Maybe You know how high I could reach?
Sorry, it is still not clear to me what you did... perhaps I should ask where the schematic came from? And who replaced the resistors with the capacitors (6800pF). Finaly, do you have the capacitors in the circuit or not?
Sorry if I explain not clearly. Goldline published schematic of his music angel. I checked what is in mine. The differences I've put on this schematic. On original schematic there are two 150k feedback resistors in series. In my MA there is 470k resistor in series with 6800p. That's it. I didn't modified feedback circuit in any way for now. The reason is I don't have scope and I don't have any experiences with feedback circuits. I hope this time I managed to explain how it is.
I can tell that when I measured my friends xd800 I found 300k feedback resistor, exactly as on goldline schematic. This month I will go to my friend and compare my modified version with his one. On my monitors I could not hear differences so easily, so I have to listen to my friends speakers, which are bigger and higher class.
I can tell that when I measured my friends xd800 I found 300k feedback resistor, exactly as on goldline schematic. This month I will go to my friend and compare my modified version with his one. On my monitors I could not hear differences so easily, so I have to listen to my friends speakers, which are bigger and higher class.
http://www.thetubestore.com/lib/thetubestore/eh-kt88eh.pdf
Here you find the datasheet from EH, do not exceed the 42 watt plate dissipation
So 560 Va by 75mA = 42W this is the absolute max. so quiescent current will be 45 mA
for safe AB operation.
Here you find the datasheet from EH, do not exceed the 42 watt plate dissipation
So 560 Va by 75mA = 42W this is the absolute max. so quiescent current will be 45 mA
for safe AB operation.
Today I've compared my MA with same one but older, which belongs to my friend. After only few minutes we've discovered that there is something wrong with mine. The problem was partial feedback circuit. The capacitor+470k didn't work at all. Midrange sounds plastic, and the overall is not real. I found article about partial feedback on tubecad, and there was a formula: effective Rfb=(Rfb*Rg)/(Rfb+Rg) so for older version it would be effective Rfb=(300*120)/(300+120)=85,7k. In mine, where I've put 220k grid resistor the Rfb should be about 140k. For now I've just removed these ugly 6800p red wimas and solder 220k resistors parallel with existing 470k, this should give about 150k. Amp sounds better, but I can't measure it professionally without scope. I didn't checked any other resistor values for now, because I should check every option in my friends equipment. Maybe I'll do this next time. If someone could advise me about partial feedback circuit I would be very thankful.
Could you please provide the voltage readings for all the tubes, so we can better understand the current condition of the amplifier, also what is the primary impedance of the output transformer? And could anyone verify whether the schematic posted by Goldline is the same as the factory's version or not?
Ofcourse, I will check everything as soon as possible. I will also try to measure all OT ratings I could. I have new cheap LCR meter from ebay so I'll give it a try.
My MA circuit was only similar to this from goldline. The main difference was capacitor 6800p in partial feedback circuit. But as I told before, I checked my friends MA and it was EXACTLY the same as goldline's.
Many thanks for your interest.
My MA circuit was only similar to this from goldline. The main difference was capacitor 6800p in partial feedback circuit. But as I told before, I checked my friends MA and it was EXACTLY the same as goldline's.
Many thanks for your interest.
I've found some time to measure my MA. Voltages:
6N1P-EV (input)
first input stage Vanode-cathode 112V, Vrcathode 1,5V so I is about 5mA
phase splitter Va-c 138V, Vrcathode 116V so I about 3,5mA
6N6P (driver)
Va-c 190V, Vrcathode about 5V so I about 30mA !!! I have to adjust it, good is these are russian tubes, quite undestructable and cheap
To output transformer measurements. I've bought cheap LC meter from ebay and it is a crap. So I could only measure resistance:
primary
from first anode to second 101,5 ohm
from first anode to center (B+) 47,3 ohm
from second anode to center 54,2 ohm
secondary
ground to 8 ohm tap = 0,33 ohm
ground to 4 ohm tap = 0,22 ohm
I've ordered new LC meter so till weekend I think I'll manage to measure inductance. Should I measure primary inductance with secondary open, resistor between taps or closed?
6N1P-EV (input)
first input stage Vanode-cathode 112V, Vrcathode 1,5V so I is about 5mA
phase splitter Va-c 138V, Vrcathode 116V so I about 3,5mA
6N6P (driver)
Va-c 190V, Vrcathode about 5V so I about 30mA !!! I have to adjust it, good is these are russian tubes, quite undestructable and cheap
To output transformer measurements. I've bought cheap LC meter from ebay and it is a crap. So I could only measure resistance:
primary
from first anode to second 101,5 ohm
from first anode to center (B+) 47,3 ohm
from second anode to center 54,2 ohm
secondary
ground to 8 ohm tap = 0,33 ohm
ground to 4 ohm tap = 0,22 ohm
I've ordered new LC meter so till weekend I think I'll manage to measure inductance. Should I measure primary inductance with secondary open, resistor between taps or closed?
The inductance would be good to know, but more importantly, we need the turns ratio of the OPT, so we can estimate the output power. I was mistaken on the UL tap, the amp only has pentode and triode modes. With the datasheet's recommended 4k primary impedance, 65W for the pentodes is doable, but of course for the triodes, that would be much lower ~30+W.