streetwise
Active Member
I have vintage RCA tubes, the efb trimmer changed the .89ma up to around .95ma from what i recall
Fisher 400 both channels distortion
- Thread starter streetwise
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Replied from the phone without glasses and missed decimal point. Sorry. So, are those readings a result of measurement across the cathode resistors in millivolts and then divided by the value of the resistor? Or this is the reading in millivolts across cathode resistors?
streetwise
Active Member
without the tubes installed im getting a range of 1.58-1.78 ma adjusting the trimmer on the EFB. this measurement is taken at the test points on the IBBA board.
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streetwise
Active Member
the tube test points on the IBBA board connect to the pin3 10 ohm resistor. the other testpoints are the heater supply connection from the EFB to the IBBA. and third the ground. I adjusted the trimmer on the efb to supply a steady -25.00v to the IBBA. but im not getting any measurements from across the 10 ohm resistor or the the test points to ground.
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streetwise
Active Member
Checked again with tubes installed im not getting "ma" readings across the 10 ohm resistor but im getting roughly 68.0-78.0mv across the resistor. My previous measurements might have been in error as I was measuring across the pin3 test point to the ibba supply voltage. stupid me I forgot that was a test point on my board.
streetwise
Active Member
without tubes installed pins read
pin 1 290v
pin 2 -17
pin 3 nothing
pin 9 398v
pin 1 290v
pin 2 -17
pin 3 nothing
pin 9 398v
You divide the pin 3 mv readings by 10, your resistor value, (mv / Ω) to get the ma.
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streetwise
Active Member
so im shooting for 210 mv on each tube. The EFB trimmer should still supply -25v at the tubes? Why can't my meter read ma across pin 3? did I always bias this thing wrong in the past?
I
The only way the amperage can be measured with the meter is when your meter is installed in place of the (removed) cathode sensing resistor. Your meter would need to be set on mA DC scale. For simplicity, we are measuring mV DC drop across the resistor in mV DC scale and than divide mV reading by the value of the resistor to get the mA value.
streetwise
Active Member
ahh shite, well my guess is my tubes are about fried than, im only getting 165mv maxed on the trimmer and thats with the efb trimer cut all the way down too -23.5v. Im guessing I really messed this thing up, these were decent rca's I bought maybe 1or2years ago they had reading of 9200 written on them
165mV makes sense. Now, what is the negative voltage reading on the grid of that tube?
streetwise
Active Member
where do I check that, what pin 1 Screen Grid, 2 Control Grid, 3 Cathode, Suppressor Grid
streetwise
Active Member
13.7-14v
streetwise
Active Member
yes all between 13.7 - 14v on pin2 and the most I can get is 15-16ma on pin 3. Its possible that the EFB issue helped them fail. Hopefully when I can find a decent set of tubes they last longer with the EFB doubler fixed. I think I have my original set of mismatched tubes they probably still have more life than these ones. Hate myself for screwing these tubes up but oh well thats how I learn
I have several other more important projects to sort out now i.e. volvo xc70 with bad transmission valve body, evap leak, catalyst inefficiency another volvo s60 with bad motor mounts, chainsaw with a vacuum leak when hot, washing machine with a bad control board! The list goes on thats just some of my day to day projects when I have time. All this developed in the past month or so, I have a long 2023 to look forward too luckily I know what I need to do to get the music back. Thanks grindfix, Dave & Larry it takes a village to help me figure things out. I can't thank you all enough.
Thanks Paul
streetwise
Active Member
Final questions for this thread
Should I build a bucking transformer to cut the 120 supplied to 110? Will this help things run smoother and what are the benefits?
Also what should I be shooting for with the efb trimmer when and if im ever able to find a set of good output tubes for this receiver? I know Dave said anywhere between - -23 to -25v but is there a ideal value I should set it at im able to go anywhere between -23.5 to -25v.
Should I build a bucking transformer to cut the 120 supplied to 110? Will this help things run smoother and what are the benefits?
Also what should I be shooting for with the efb trimmer when and if im ever able to find a set of good output tubes for this receiver? I know Dave said anywhere between - -23 to -25v but is there a ideal value I should set it at im able to go anywhere between -23.5 to -25v.
You don't need to lower the input voltage unless the output tube heater voltages are too high. If they are within -0% +5% of nominal 6.3v - AC voltage reduction is not needed.
That may depend on the tubes. Verify that all voltages in the receiver are correct and EFB operates properly and leave it until you have better tubes to install. You mentioned that you have a mismatched set. Those may be capable to conduct more current. Adjust the EFB trimmer with worn-out tubes to the lowest mV reading across cathode resistors before you plug in other tubes to make sure they don't redplate on initial startup.
Remember that with EFB installed in late model receivers, the Screen Grid voltage is dropped to around 300 vdc (+/-), so a lower control grid voltage (versus the stock design) is normal. In my receiver my very good NOS tubes required about -15.0 vdc, but your screen voltage is a little lower than mine was, so less control grid voltage will be required to achieve the target quiescent current flow.
Dave
Dave
Dave, can control grid voltage potentially go as low, or even lower than in OP's unit?
I guess the only way to find out if tubes can stay is to reduce grid voltage in an attempt to achieve 21mA per tube and run a full-power test, right? Output results will tell the story.
I guess the only way to find out if tubes can stay is to reduce grid voltage in an attempt to achieve 21mA per tube and run a full-power test, right? Output results will tell the story.