I am getting 431v at pin 3 6L6gc.
also using 2X 12au7 and getting 167v at pin 6 plate there.
and getting 5.85v at pin 8 cathode of 12au7
and getting 35v at pin 8 cathode of 6L6gc
any thoughts.
also using 2X 12au7 and getting 167v at pin 6 plate there.
and getting 5.85v at pin 8 cathode of 12au7
and getting 35v at pin 8 cathode of 6L6gc
any thoughts.
Last edited:
The 6L6GC looks like it's in a good spot. Drawing 62.5mA. The plate voltage of 395V (plate to cathode) is good. That means it's dissipating 26.8 watts, which is well below the max of 30.
The 12AU7 voltages seem OK. It's dissipating about 1 watt, which is fine.
The voltages look OK. How does it sound?
The 12AU7 voltages seem OK. It's dissipating about 1 watt, which is fine.
The voltages look OK. How does it sound?
Rongon
Thanks for taking the time to crunch the numbers for me.
it sounds really good very clean and very black background
very warm full from top to bottom.
what is weird I tried a brand new pair of Shuguang KT88
and they did not sound near what the 6L6 sound
they where very bright sounding almost harsh at times
very hifi clinical sound,also very compressed I don't
know maybe it is the difference cathode bias at 35v being to low
for KT88 what do you think.
I know I generally have always preferred EL34 6L6 6550
with a warmer overall presentation.
Thanks for taking the time to crunch the numbers for me.
it sounds really good very clean and very black background
very warm full from top to bottom.
what is weird I tried a brand new pair of Shuguang KT88
and they did not sound near what the 6L6 sound
they where very bright sounding almost harsh at times
very hifi clinical sound,also very compressed I don't
know maybe it is the difference cathode bias at 35v being to low
for KT88 what do you think.
I know I generally have always preferred EL34 6L6 6550
with a warmer overall presentation.
Last edited:
These measurements where taken with tubes installed.
you did take that into consideration right Rongon.
you did take that into consideration right Rongon.
Yup, sure did.
You mentioned you didn't like the sound of the Shuguang KT88 in the amp. Did you take a fresh set of measurements with that tube in it? I'll bet the voltage at the cathode (pins 1,8) was different. I wonder where it wound up.
KT88 likes to have quite a bit of current through it. If you just pop a KT88 (or 6550A) into a circuit designed for 6L6GC, that KT88 might be running with a fairly low level of plate current, which can change the sound. Then again, I have no experience with the Shuguang KT88, so probably shouldn't comment.
I would expect an EL34 to run colder (less current through it) than the 6L6GC with that 560 ohm cathode resistor. But that's a good thing, as its max plate dissipation (plate volts times plate current draw) is 25 watts. A good EL34 can sound really nice in triode.
I think you're good to go. Enjoy the amp!
--
PS - What kind of 6L6GC are you using? Just curious, as you seem to like its sound...
All the credit goes to you Blair.
thanks for being so patient with me on the build
I will post some pictures soon as I can find a little time.
Scott
thanks for being so patient with me on the build
I will post some pictures soon as I can find a little time.
Scott
He did
Congrats Scott! You should post a picture of your work. It looks very nice!
Blair
I just thought of something... In a cathode-biased KT88, if pins 1 and 8 are shorted, if something was to short the metal base of the tube to the chassis (or some other ground at 0 volts), that would remove the bias voltage from the KT88 and cause it to overheat.
The 6550A has a phenolic wafer below the metal shield, which puts an insulating layer between the tube, the socket and the chassis. I assume the KT88 has the same. I suppose that's why I've never had a problem plopping a 6550A into a tube socket wired for 6L6GC.
Just figured I should warn you of that.
--
Hi Alex
I used your revised schematics for the build
and used diode bridge rectifier.
I pretty much followed it except I used 3x100 ohm resistors in series
where you show the 5H 250r choke,
also using Transcendar 3.5k 20 watt SE output Transformers,
it looks like a lot of guys use 5K I figure not much difference there
I really did not like the sound of the KT88 in at all.
so I would be more apt to call it a SE EL34 or 6L6gc or 5881 amp
seems to be more geared to those without a lot more tinkering with
B+ voltage and Cathode bias resistor,
Any thoughts are welcome
Scott
I used your revised schematics for the build
and used diode bridge rectifier.
I pretty much followed it except I used 3x100 ohm resistors in series
where you show the 5H 250r choke,
also using Transcendar 3.5k 20 watt SE output Transformers,
it looks like a lot of guys use 5K I figure not much difference there
I really did not like the sound of the KT88 in at all.
so I would be more apt to call it a SE EL34 or 6L6gc or 5881 amp
seems to be more geared to those without a lot more tinkering with
B+ voltage and Cathode bias resistor,
Any thoughts are welcome
Scott
Nice amp, pictures please.
I did that build in diyaudioprojects.com.
I like the EL34 best in that amp, somehow the vocals and highs are better, also I like 470R on the cathode of the EL34.
Last edited:
Alex
So with the 470 ohm cathode resistor are you running around 40mv there.
seems like that is a good bias for EL34 from what I can recall.
right now I am running 35mv there with 560 ohm and probably need to be closer to 40mv as well.
Thanks for the complement.
So with the 470 ohm cathode resistor are you running around 40mv there.
seems like that is a good bias for EL34 from what I can recall.
right now I am running 35mv there with 560 ohm and probably need to be closer to 40mv as well.
Thanks for the complement.
Gorgeous!
Last edited:
A smaller value resistor would actually run the tube "hotter". You would get a little more current going through the tube. The problem is that you have +430V at the plate, and are getting 70mA (current) through the 6L6 already. That's a pretty high current (which probably sounds good). But you do not want to exceed the maximum plate dissipation rating for any tube you'll be using in the amp.
To figure the plate dissipation in watts, the equation is P = I(V), so you figure...
Plate voltage minus voltage at the cathode = plate to cathode voltage, so...
430 - 39 = 391V plate to cathode
Now you multiply the plate-cathode voltage by the plate current (in amperes), so...
391V x .07A = 27.37 watts plate dissipation
That is actually close to the max dissipation for a 6L6GC (30 watts). You're fine with that tube.
What happens if you drop an EL34 in there? It will probably draw less current. I don't have curves, etc. available to me right now, but let's say you get 33V at pins 1+8 (the EL34 cathode). That would be 33/560 = 59mA
If your plate-cathode voltage is now 395V, you have 395V x 59mA going through the tube, so 23.3 watts plate dissipation. The max plate dissipation of an EL34 is 24 watts (or sometimes spec'd as 25 watts). So you're right up there.
If you try a 470 ohm resistor, and you get 32V at the EL34 cathode, that would be 68mA. That would be 395V x .068A = 26.9 watts, and now you're over the max dissipation rating of the EL34 -- but not so far that it wouldn't work. It would just run hot, and possibly wear out the tubes a bit faster. Or the tube might melt down. Hard to say. With today's EL34's, I'd be conservative and back off a watt of dissipation from the published maximum.
You mentioned that you don't want to worry about running the amp at the edge. So I think you probably shouldn't go any lower in value than the 560 ohms cathode resistor with your power supply. Your power supply has a higher B+ voltage than the original design with tube rectifier, so that would explain the need for the higher value cathode resistor.
Make sense?
--
PS - The Sovtek 5881 appears to be good for 30W dissipation, or about 5 watts more than an EL34.
PPS - Before changing any resistor values, I'd drop in an EL34 and take voltages, then figure out if the EL34 is being run too hot or whatever. Then make changes if you want.
PPS - ...And nice job on the build. It does look very nice.
--
To figure the plate dissipation in watts, the equation is P = I(V), so you figure...
Plate voltage minus voltage at the cathode = plate to cathode voltage, so...
430 - 39 = 391V plate to cathode
Now you multiply the plate-cathode voltage by the plate current (in amperes), so...
391V x .07A = 27.37 watts plate dissipation
That is actually close to the max dissipation for a 6L6GC (30 watts). You're fine with that tube.
What happens if you drop an EL34 in there? It will probably draw less current. I don't have curves, etc. available to me right now, but let's say you get 33V at pins 1+8 (the EL34 cathode). That would be 33/560 = 59mA
If your plate-cathode voltage is now 395V, you have 395V x 59mA going through the tube, so 23.3 watts plate dissipation. The max plate dissipation of an EL34 is 24 watts (or sometimes spec'd as 25 watts). So you're right up there.
If you try a 470 ohm resistor, and you get 32V at the EL34 cathode, that would be 68mA. That would be 395V x .068A = 26.9 watts, and now you're over the max dissipation rating of the EL34 -- but not so far that it wouldn't work. It would just run hot, and possibly wear out the tubes a bit faster. Or the tube might melt down. Hard to say. With today's EL34's, I'd be conservative and back off a watt of dissipation from the published maximum.
You mentioned that you don't want to worry about running the amp at the edge. So I think you probably shouldn't go any lower in value than the 560 ohms cathode resistor with your power supply. Your power supply has a higher B+ voltage than the original design with tube rectifier, so that would explain the need for the higher value cathode resistor.
Make sense?
--
PS - The Sovtek 5881 appears to be good for 30W dissipation, or about 5 watts more than an EL34.
PPS - Before changing any resistor values, I'd drop in an EL34 and take voltages, then figure out if the EL34 is being run too hot or whatever. Then make changes if you want.
PPS - ...And nice job on the build. It does look very nice.
--
Ron
That is fantastic information.
are saying I am closer to safe readings with 5881
probably is that correct. and it would be pushing EL34 right to the max.
I will see how the Sovtek 5881 sound and make
a judgement call if I will wait a little while to get EL34.
I will report back on 5881 they should be here tomorrow.
Scott
That is fantastic information.
are saying I am closer to safe readings with 5881
probably is that correct. and it would be pushing EL34 right to the max.
I will see how the Sovtek 5881 sound and make
a judgement call if I will wait a little while to get EL34.
I will report back on 5881 they should be here tomorrow.
Scott
Technically yes, but the -3dB frequency is at about 1.5Hz as designed, so changing the cap to 0.68uF will not make any difference audibly in the bass response.
Last edited:
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.