emartine
Active Member
Glen, what cathode resistors you ended with for EL34, and which one for KT88 ?
KT88 SE Abdellah (modified by Alex Gendrano) + AK..
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Glen, what cathode resistors you ended with for EL34, and which one for KT88 ?
They're both in there, the switch just connects the 3k resistor in parallel for a total of 472R. This is for use with the KT88's. I use just the 560R resistor for EL34, 6CA7, 6L6GC, KT66, Etc. for a little lower plate dissipation.
Glenn
Glenn
Last edited:
Kegger
R.I.P. 1/12/1966 - 6/1/2017
If you really want to run the 6DJ8 in there then 2 adjustments I'd make are.
(the other drivers I mention are still just fine as well after this)
Make R4 10K 2w
Make R3 200ohm 1/2w
I would also make C2 at least 400v rating
And since your feeding 2 screens you can make that 4.7k resistor 2.7k 1w.
An I'd really forget about running in full pentode mode, you'll run out of steam
with that feedback needed to drive the amp to full power with a 6DJ8 in there.
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Is that 2 amps on one chassis? Just curious.. :scratch2:
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Now guy's just a little breakdown here an the cathode resistor size issue.
----
IF your running a 400v supply, using an EL34 an 500ohm cathode resistor.
You'll loose roughly 10v through the output transformer, so 390v at plate.
You'll have roughly 28.5v at the cathode, so 362v plate to cathode there.
(that would be the Actual Plate voltage you use to calculate dissipation)
Now 28.5v on 500ohm resistor is 57ma total plate an screen, subtract oh
say 4ma for the screen and you have 53ma of plate current.
Now 362v times 53ma = 19.186 watts of dissipation on a 25w tube there.
(quite good, especially for cathode bias)
---------------
Me personally...
That is a fine place to be, an I wouldn't raise the cathode resistor unless
I was running a higher B+, an then I would Only do so to keep dissipation
near the same for best sonics, an the extra power the design affords you.
Kegger
R.I.P. 1/12/1966 - 6/1/2017
I would say with this design not really. (that is my personal opinion)
You really need a stouter driver, like the other design I posted where it's using
a high transcon pentode triode strapped and picking one with high gain to boot.
The best tube in the design you are running would be to drop a ECC85 in there.
(and leaving the feedback resistor to 220k would be an decent "compromise")
emartine
Active Member
Kegger, you're saying that the pentode mode is not worth to try with this amp ?? :tears:
How the changes in the feedback resistor would modify the performance in pentode ?
Kegger
R.I.P. 1/12/1966 - 6/1/2017
If you make the feedback resistor smaller you'll get more feedback that the
pentode rigged output would benefit from, but you'll be putting More Strain
for more drive and gain on the driver to where it won't quite get there.
In UL or Triode we need/want less feedback, so we can get away with less
gain in our driver stage, and running a KT88 in pentode with an fair amount
feedback an still have the amp drive to full power we need some good drive.
The ECC85 is most gain tube we have available for this design, and it's just
a touch lite on it's abilities for KT88 in pentode with good feedback in there.
(It will work but just come up a bit short, 6DT8 is the other one to use to)
-----
Personally if I have single ended output transformers with UL taps I don't do
the outputs in Pentode mode, I've experimented and find I prefer how UL Or
Triode perform in these circuits better. (I really prefer/like Single Ended UL)
pentode rigged output would benefit from, but you'll be putting More Strain
for more drive and gain on the driver to where it won't quite get there.
In UL or Triode we need/want less feedback, so we can get away with less
gain in our driver stage, and running a KT88 in pentode with an fair amount
feedback an still have the amp drive to full power we need some good drive.
The ECC85 is most gain tube we have available for this design, and it's just
a touch lite on it's abilities for KT88 in pentode with good feedback in there.
(It will work but just come up a bit short, 6DT8 is the other one to use to)
-----
Personally if I have single ended output transformers with UL taps I don't do
the outputs in Pentode mode, I've experimented and find I prefer how UL Or
Triode perform in these circuits better. (I really prefer/like Single Ended UL)
emartine
Active Member
Suppose what you described for KT88, is the same situation for my EL34..
What I do not get if why is so important to increase feedback when I run
in pentode.. can I just have the same ammount, keeping the same feedback resistor used for UL ?
May be because this way will result in large amount of distortion ???
Just thinking. :scratch2: Please explain
What I do not get if why is so important to increase feedback when I run
in pentode.. can I just have the same ammount, keeping the same feedback resistor used for UL ?
May be because this way will result in large amount of distortion ???
Just thinking. :scratch2: Please explain
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Yes same situation for EL34 (but not quite as bad)
When running output tubes in Pentode, yes they have a LOT more distortion
then when ran in Triode or UL, so to try an keep them in check you increase
the feedback, also output impedance is much higher an creating bad damping
factor without increasing the feedback as well with the Pentode output stage.
---
Now in this design without even changing that feedback resistor you actually
get more feedback when running the outputs in Pentode beacuse it has more
gain which will increase the feedback coming from it's plate.
Which is why I said to leave the feedback resistor the same size as to form a
compromise between gain and feedback if you did want try output in Pentode.
(as apposed to lowering the resistor an using a different driver stage to do it)
When running output tubes in Pentode, yes they have a LOT more distortion
then when ran in Triode or UL, so to try an keep them in check you increase
the feedback, also output impedance is much higher an creating bad damping
factor without increasing the feedback as well with the Pentode output stage.
---
Now in this design without even changing that feedback resistor you actually
get more feedback when running the outputs in Pentode beacuse it has more
gain which will increase the feedback coming from it's plate.
Which is why I said to leave the feedback resistor the same size as to form a
compromise between gain and feedback if you did want try output in Pentode.
(as apposed to lowering the resistor an using a different driver stage to do it)
Kegger
R.I.P. 1/12/1966 - 6/1/2017
Nope that would make it even worse.
A 6SN7 has a gain of only 20, the ECC85 or 6DT8 have gains of 50-60!
(you would need a double 6SN7 gain stage, 1 after the other to get it)
emartine
Active Member
Kegger
R.I.P. 1/12/1966 - 6/1/2017
emartine
Active Member
Thanks!
I cannot find ECC88 tube here, but this JJ 6922/E88CC :
http://www.tubesandmore.com/scripts...vclients/cemirror/ELEVATOR.FXP?item=T-6922-JJ
Noticed some spec are different but I can use it the same without changing anything, right ?
Kegger
R.I.P. 1/12/1966 - 6/1/2017
The ECC88/6DJ8/6922 are all the same basic tube, some pending what they actually
are(not just marked) are an beefier version and can take more, this design here I've
accounted for that and any should be just fine.
Now I find it interesting your pretty resigned to use them but don't actually have any
of them, if it were me and this design I would be looking to purchase ECC85/6AQ8's if
I were buying tubes in nice European brands along with same for an ECC88/6DJ8/6922.
An no I wouldn't buy any new production small signal tubes, to me the old ones Spank
the snot out of them in the sound department, with pretty much any manufacturer to.
For these types I prefer, Amperex(made in holland) and Mullard, as my favorites there.
are(not just marked) are an beefier version and can take more, this design here I've
accounted for that and any should be just fine.
Now I find it interesting your pretty resigned to use them but don't actually have any
of them, if it were me and this design I would be looking to purchase ECC85/6AQ8's if
I were buying tubes in nice European brands along with same for an ECC88/6DJ8/6922.
An no I wouldn't buy any new production small signal tubes, to me the old ones Spank
the snot out of them in the sound department, with pretty much any manufacturer to.
For these types I prefer, Amperex(made in holland) and Mullard, as my favorites there.