I had normalized the 235v by reducing the 6k resistor. However the b+ of pentode is stil at 158v and plate at 53v. Double check resistor vslues and thry still good. Is this an indication of weak tube 6an8? Reduced plate resistance of pentode to 115k ang plate voltage reads 69.7v and triode cathode to 73v. Vg2 stil lowat 34,5v. Output bias at -37v andb+ of 422v fo the 6p3s. Tested with music and sounds but limited power. Even direct feed to pentode griid. Next move is to change the 1.5k cathode resistor to 1k with 47uf cathode bypass cap.
I had than those thing if possible no parts wiil ne changed excep the caps. The prolem is the primary line voltage. 230v is adviced but for us 0nly anout 215v. So definitely i have a drop of b+ from 478vdc to 425v actual, thus affecting all supplies gor the plates. Im tweaking at this moment to find out what resly thr ptoblem is and taken to sccount all your vsluable inputs. Hopefully i in close enough to finding out and will try a new tube..
fluke 87V has an input impedance of 10meg, i am using the same as Jun...
another thing is that the plate and cathode resistors should be well matched...
The Valve Wizard
i also use ne2 neons to protect the grid cathode of the concertina on start up....
the raw B+ will determine voltages of downstream stages...
and raw B+ is also a function of mains line voltages...
how you bias the ouput tubes also have an effect on the raw B+,
bias the output tubes hot and the raw B+ is lower, bias them light and the raw B+ increases upwards...
reasonably close is what i aim for...
so in order to make meaningful the voltage indicated in schematics,
first you have to know the line voltage at which the readings were taken, second, that is the bias current fo the output tubes?
when you have ascertained these, then you can start looking downnstream...
another thing is that the plate and cathode resistors should be well matched...
The Valve Wizard
i also use ne2 neons to protect the grid cathode of the concertina on start up....
the raw B+ will determine voltages of downstream stages...
and raw B+ is also a function of mains line voltages...
how you bias the ouput tubes also have an effect on the raw B+,
bias the output tubes hot and the raw B+ is lower, bias them light and the raw B+ increases upwards...
reasonably close is what i aim for...
so in order to make meaningful the voltage indicated in schematics,
first you have to know the line voltage at which the readings were taken, second, that is the bias current fo the output tubes?
when you have ascertained these, then you can start looking downnstream...
.... as should also be the next stage grid resistors. The total value of load resistors (and Cs) should be the same, as intimated in the reference.
Junm,
I am slightly bothered by too much current passing through the 6AN8 pentode - a sign of too good a valve, not the other way round. At a more convenient hour ....
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I may have missed it: has the 30uFd cap at the pentode cathode been removed or replaced? If it is *shorted*, the pentode is underbiased and sucks more than it should. Snipping one lead and re-checking voltages is quick. It will work that way also; just better (more as designed) with a cap there.
Yes i had disconnected it but still doesnt show what voltage reading as it should be. I had used two king of dmm whic both have ahigh input impedance and reading is close to each meter. A new set of6an8 is coming and i will find out. I dont have a tube tester so trying another nos tube might fix thise unusual voltages.
yes, that seems okay now....how is the sound? did you bias the output tubes yet,
if i were you i'd put 1 ohm 3 watt resistors in the 7591 cathodes to monitor cathode current...
or put the 1 ohm resistor in series with the opt B+ line to monitor current at idle...
this is from data sheet...
if i were you i'd put 1 ohm 3 watt resistors in the 7591 cathodes to monitor cathode current...
or put the 1 ohm resistor in series with the opt B+ line to monitor current at idle...
this is from data sheet...
Attachments
Junm,
I read back as well as I could. Last time you found an anode voltage (pin 6, 6AN8) still too low with the grid (pin 8) shorted to ground. Have you since removed the short? As in: Is the undesirable +1.3V still on G1?
Again (apology to sound like a repeating groove LP or slipping track CD), you need to get to the source of that voltage one way or another! Is it the same with the new tubes? I can only caution yet again: Whatever you do or whichever new tubes you fit, do not carry on making changes until that condition is normalised. (Apology that I could not keep my promise to hook up the circuit here; I have no good 6AN8s and nowhere locally to get some.*)
(Also if not mentioned: The screen (pin 7) voltage you measure will read lower even with a 10 meg DVM as you are measuring through a 600K resistor if I remember correctly.)
SO! New tubes, and re-check pin 8 for that unwated +1,3V before doing anything else.
Then: Your question regarding a low anode voltage was handled in post #59. No, not too low right now, but operating close to borderline conditions.
_____________________________________________________
P.S: I see that the ECF80 (6BL8) is not widely different from the 6AN8. Those I use aplenty and will then try hook up your pentode circuit and see what befalls. Let us get to the bottom of this!
I read back as well as I could. Last time you found an anode voltage (pin 6, 6AN8) still too low with the grid (pin 8) shorted to ground. Have you since removed the short? As in: Is the undesirable +1.3V still on G1?
Again (apology to sound like a repeating groove LP or slipping track CD), you need to get to the source of that voltage one way or another! Is it the same with the new tubes? I can only caution yet again: Whatever you do or whichever new tubes you fit, do not carry on making changes until that condition is normalised. (Apology that I could not keep my promise to hook up the circuit here; I have no good 6AN8s and nowhere locally to get some.*)
(Also if not mentioned: The screen (pin 7) voltage you measure will read lower even with a 10 meg DVM as you are measuring through a 600K resistor if I remember correctly.)
SO! New tubes, and re-check pin 8 for that unwated +1,3V before doing anything else.
Then: Your question regarding a low anode voltage was handled in post #59. No, not too low right now, but operating close to borderline conditions.
_____________________________________________________
P.S: I see that the ECF80 (6BL8) is not widely different from the 6AN8. Those I use aplenty and will then try hook up your pentode circuit and see what befalls. Let us get to the bottom of this!
Yes removed short and replace with 250K res. cut connection from Balance pot and inset temporarily a coupling cap0.22uf to eliminate the 1.3v DC.
Yes I'm still trying to find out where is that voltage coming from.
no, If i lower that 600k resistance to get higher VG2 supply, it affecs and pull the plate voltage to lower value. so I stay with 600k.
@sir Tony, I had already replaced the output tube to 6P3S as requested by the owner and bias it around -28 v. Sound is dull and not so loud.
Jun, i was gifted with 8 6p3C tubes and only two was useable, so testing is important...
the 7591 has different connections for grid 1 and cathodes, make sure you got it right..
a weak and dull sound is an indication of problem upstream, or worse your OPT can be bad, but it is hard to conclude just yet, get the preamp stages working properly first...
you can tap the signal from the top of the volume control and feed it to another of your power amp to confirm that the signal is good...
the 7591 has different connections for grid 1 and cathodes, make sure you got it right..
a weak and dull sound is an indication of problem upstream, or worse your OPT can be bad, but it is hard to conclude just yet, get the preamp stages working properly first...
you can tap the signal from the top of the volume control and feed it to another of your power amp to confirm that the signal is good...
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