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In Reply to: RE: There is an easier way to do this. posted by Michael Samra on July 13, 2016 at 19:52:39
Mikey,
He said the bias supply was rebuilt so the -11 volts appears to me to be the bias voltage required for him to get a higher ma reading per output pair and the bias voltage isn't that low because of a failing rectifier.
By his last post he seems to be confused about the cathode resistor and how different valves of it can affect the ma reading per pair when you convert a voltage reading to an ma reading. Hard to say because he didn't say if he read actual current or voltage at the test point and converted it to current.
Also, it sounds like he balanced the output pair by adjusting the DC balance pots for equal bias voltage on each tube in the pair so they probably aren't balanced correctly either.
My take is that he's running the tubes real hot and doesn't know it.
The Weber calculator was my best fiend when I was working with my 299A and getting confused.
The calculator that wants the output tube count per channel, voltage reading at the test point, the cathode resistor value and the plate voltage was a huge help.
Follow Ups:
There is lots of bias voltage available. I can easily crank it to -15 but when I do, the current measured on a through the meter ammeter (digital) is in the 20ma neighborhood and that is for the PAIR of tubes. When the bias pot is all the way CCW (looking down on it) the neg voltage is around -11 and the current across the 18 0hm resistor is barely 35ma. This is direct reading, not interpolated. The tubes are not cherry-ing and the total current draw is 120W. I have seen that current go higher when output tubes are running hot (on other amps, never this one). I grant that setting the balance pot to get equal voltages through all the network of resistors at the grids of the two tubes is not "by the book" so yes I guess I need to cobble together a giant resistor for the output load and pull the phase splitter and hook up my scope. Was trying to avoid all that.
I have used the weber calculator before on other devices but lost link so thank you for that and I will report back. --Tom H
Use the Weber calculator because when you plug in the numbers it does the current for you BUT use the VOLTAGE at the test point, not the current.
Unless I have it wrong, you would only attempt to read current from the tube cathode to ground with an ammeter IN PLACE of the cathode resistor.
WITH the cathode resistor in place, you read the "voltage drop" across the resistor then convert that voltage to ma.
As for DC balance, it sounds like you might be referring to the AC balance. I just set those pots to center range and hope for the best.
Unless there's a cathode resistor from the cathode of each output to ground it's not a straight forward procedure.
A fast method with some Scotts was to turn the volume all the way down, no signal going in, and listen for hum in each channel if you have efficient speakers. Or connect low impedance headphones to the speaker terminals. Adjusting the DC balance in each channel will give you more or less hum in that channel and your shooting for the least amount of hum in each channel. Hopefully no hum.
That method works great with my 299A but I have very efficient speakers.
The Scott manual for the 299A says to connect the leads of a volt meter to each tube plate in the output pair and adjust the DC balance pot for that channel for a minimum voltage reading but you have to be real careful of test leads touching another pin or falling off shorting something out so BEWARE.
Maybe someone else has a better method without replacing the two 18 ohm resistors with four resistors,a resistor from each output cathode to ground so you can see exactly what the voltage drop is across both resistors in each channel and adjusting the DC balance so the voltages are as equal as possible. But then you need to convert those voltages to ma to set the bias voltage so your not running the tubes cold or hot.
If the manual said 55ma per test point then that would equate to about 27-28 ma per tube.
Could have weak Reflektor tubes.
Look at the 299B schematic. The only reasonable O/P tube option is the Russian 6p14p-ev, AKA EL84M. Nothing but a genuine 7189 equivalent belongs in that amp. Scott beat the living guano out of the "finals".
Eli D.
EL84 or 7189, just to see if you can get into proper bias range. It really sounds like he has a weak set of new tubes.
These reflektor tubes are new and test at 95 to 105% of specified plate current on eico 667 tester set for 7189. The thing came with JJ el84 which is not rated for the voltages in this amp. It came to me with a blown JJ rectifier. The JJs are still OK having not been in there very long but I am not leaving them in there for very long. The current and bias voltage relationships I have described for the reflektors apply to the JJs as well.
As to another suggestion that separate resistors be in place for all four tubes I completely agree and might end up doing that. Thank you all for your interest in this project.
All of JJ's Octal production is, at best, inconsistent. The failed 5AR4 is not at all surprising.
I don't know what you put in, but I suggest you scan the archives for the, several times posted, series SS diode tweak. The tweak allows the use of the reasonably priced Sovtek 5AR4, in complete confidence. The weak spot in the Sovtek variant is PIV handling, which is shored up by the "sand". In all other aspects, the Saratov, Russia, product is highly satisfactory.
Eli D.
Or you can simply install a new production Tung-Sol or Genalex - problem solved!BTW, if it was my amp I'd remove the 18 Ohm resistor and install a 10 Ohm 1 watt resistor at EACH EL84 tube cathode so the cathode current goes through the resistor to ground. Then you can measure the voltage at EACH tube's cathode and use Ohm's Law to calculate the voltage:
Cathode current = voltage at pin 3 divided by 10.
So if you wanted 30 ma per tube you simply use the bias and DC balance pot to adjust so you see 0.35 volts at pin 3 of each tube. You'll have to go back and forth with the bias and DC balance pots to get both tubes right since they interact. Set the DC balance pot at midpoint then adjust the bias pot until you see close to .30 volts at the cathodes. Then adjust the DC balance so the tubes show equal voltages at the cathode. Finally a last adjustment of the bias pot should get the tubes spot on - if not repeat the sequence once more and you are set.
If you can't get enough bias current then either the tubes are tired (put some EL84M Sovteks in there, they are VERY rugged and can survive plus they sound excellent), the plate or screen voltages are too low, or the heater voltage is low.
PLEASE read my FAQ post on this topic, the link is below. Item #4 is particularly important.
Take a look and let me know what questions you have. Do this resistor change once and life becomes MUCH easier both now and in the future!
Edits: 07/14/16
Jim, that link isn't working for me.
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Buy Chinese. Bury freedom.
nada aqui
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Buy Chinese. Bury freedom.
It's fixed - it's the Tube Asylum FAQ.
Try the 1 provided below.
Eli D.
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