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Hi: The most I can get is around 40ma at each screw and that is with the negative voltage at around 11. The schematic and tube manual call for neg 15. If I set it at neg 15, the current at the test screw is under 30ma. This is with brand new russian 6p14p-eb tubes. The B+ and screen are within 5% of spec. Rectifier is new and tests strong. Total current draw from wall socket is 121W. What am I missing here? Thank you.
Follow Ups:
who doesn't he installed 1 ohm or 10 cathode resistors on each of the output tubes and that way he can set the DC balance for a 0vdc potential between the two cathodes in each channel.This way the tubes will be balanced and then he can set the bias where he wants.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
As I learned with my Scott 299A, the schematic bias voltage is approx and it needs to be adjusted for the actual tubes used. But you'd think the -11 would be on the low side seeing that they did show -15 volts on the schematic.
You didn't say how you derived at the current reading. If you took a voltage reading and converted it to ma, is it possible you forget that the cathode resistor per output pair shown in the schematic is 18 OHM and not 10 ohm?
The other question would be: how did you balance the output pair?
Hi and thank you to all for weighing in. The scott instructions are to remove the little clips on top of chassis and place current meter from what is a cathode lead to ground. There is a 18 ohm resistor in parallel with the test point and ground I guess that is to make sure the tube is conducting even when you don't have the test lead on the screw. There is a little math problem to do using ohm's law but I can't figure out how the presence of that resistor changes the interpretation if at all. One person asked how I balanced the two tubes. I put my DVM on each grid and set the DC balance pot until they were equal. On both channels, the optimum setting is around 11 volts negative. But even then, the measured current is quite low which is just opposite of expectation.
--Tom in Tucson
You need to install 10 ohm cathode resistors on each of the EL84 cathodes to ground.This way you tell if you are drawing current and how much.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
First thing,was the selenium bridge rectifier changed out to a silicon bridge? If so,you now want to take your voltage source readings at bias supply and dc filament source..Here is a Sams for your unit.Tell what you are getting off the bridge.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
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.
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.
--------------------------
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.
because I'm seeing more blue at the top then normal..What you most likely need to do is take out the old selenium bridge rectifier and put in a silicon bridge to get the negative bias voltage higher.If it was changed,look at the 75v filter caps on that one can.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Edits: 07/13/16
If he changes the set bias voltage to -15VDC, he can only bias to 30mA per pair of 7189. That seems to be too cold if 420VDC is the plate voltage (6.3 watts per tube).
What is the plate voltage onto those output tubes?
I thought he said it was only minus 11vdc,that would make it too hot.
"For every complex problem there is an answer that is clear, simple, and wrong" H. L. Mencken
Each adjustment pot is for two tubes? Or are you talking 40mA per tube? Shouldn't it be closer to 20-22mA per tube at 420VDC on plates?Those maybe weak testing new tubes. It happens. Do you have another strong set that you can try?
Edits: 07/13/16
Hi: The only info I found online about this is cryptic copy of scott instructions which seem to require total of 55ma for a pair on a given channel. These are brand new tubes which test at 100% on a eico 667. One thing I do not know is if reading is to be taken with no music. It does spike up much higher with music turned up. To answer other reply; the image does not reflect true colors. Those in fact are the old JJ tubes which were not red plating or blue aura-ing. Any appearance of same is illusory. And the negative supply is quite adequate; it has been replaced with all new parts. There is plenty of negative supply; the problem is that the numbers do not add up. Thank you.
Quiescent current is always measured when the tubes are quiescent e.g, at idle with no signal input to the grid.
You can adjust the bias supply for less negative voltage by replacing R113 with a 10k ohm resistor and adding a 5k resistor at the more negative end of the bias pot. In so doing you are biasing the amp more towards class A, but reducing the output power. Don't forget that as you reduce the grid voltage you are simultaneously reducing the input voltage available to the output tube.
It might be helpful to replace R60 and R108 with 10r resistors and measure cathode current across the resistors with a voltmeter. Use ohm's law to determine current. This would at least double check the current readings you are getting with the Scott method.
It is quite possible that your new 6P14P tubes just don't bias up to the same current as the 7189 under the same operating conditions.
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