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Thread: Help please! 1973 Marshall Super Lead - EL34 to 6550s bias question

  1. #36
    Bent Member Chuck H's Avatar
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    Something is definitely hinky here. You've measured with and without a current limiter in place. You paralleled some resistances (to 6550 spec as per an old Unicord schematic) in the circuit but weren't specific about what the final resistances are. You said in post #24 that you're going to "put it back to EL34 configuration and get readings with the EL34s". But one of your el34's is blown. Then later you said that you're going to put it back to el34 spec and see how it runs with the 6550's. The 6550's have already exhibited unusual current characteristics and we don't know their provenance. And then there's the unusual increase in current through the amp indicated by the limiter brightness with what you've said was a small increase in bias current. And that seems odd. So it seems the limiter is going in and out of the testing parameters and some current figures for later adjustment ARE being taken with the limiter in place.

    This thread is wandering all over the place. I think we all need to slow down and carefully execute some concise and understood measurements with known parameters, then allow time for responses and follow up questions before moving ahead at each step.

    My guess is that one or both of the 6550's is questionable. and it's been missed in the jumpy tests. (Was the limiter there when BOTH sockets were tested? Only one of them? Are you sure? did you move ahead with anything and now aren't sure what your actual test conditions were?) And anyway, "I" also wouldn't adjust the amp for an anomalous set of tubes and call it good because I would always question those tubes. That's like adjusting your car's suspension for strangely wrong sized tires when the right tires are available.

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    Last edited by Chuck H; 10-15-2019 at 03:43 PM.
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    Quote Originally Posted by Chuck H View Post
    Something is definitely hinky here. You've measured with and without a current limiter in place. You paralleled some resistances (to 6550 spec as per an old Unicord schematic) in the circuit but weren't specific about what the final resistances are. You said in post #24 that you're going to "put it back to EL34 configuration and get readings with the EL34s". But one of your el34's is blown. Then later you said that you're going to put it back to el34 spec and see how it runs with the 6550's. The 6550's have already exhibited unusual current characteristics and we don't know their provenance. And then there's the unusual increase in current through the amp indicated by the limiter brightness with what you've said was a small increase in bias current. And that seems odd. So it seems the limiter is going in and out of the testing parameters and some current figures for later adjustment ARE being taken with the limiter in place.
    The final resistances are perfectly in spec with what the schematic calls for with 6550 tubes. I think I posted the figures in my first post. It is possible I've done something wrong. I am just a human novice. But my voltages elsewhere seem to be on point. It seems, right now, like these 6550s just aren't conducting well.
    I can undo what I've done and it will be back to EL34 specs, true. I did have a blown EL34, but I also have a pile of spare EL34s. The amp worked fine with EL34s.
    All of my actual meaningful measurements have been without the limiter in place. Amp plugged straight into the wall and on a dummy load.
    True, the 6550s are questionable and might be bad. I don't have a tube tester.

    This thread is wandering all over the place. I think we all need to slow down and carefully execute some concise and understood measurements with known parameters, then allow time for responses and follow up questions before moving ahead at each step.
    I'm trying to be as systematic and logical as I can. I'm trying to give the best info and implement the responses the best I can. Again, I am a novice. I have a rudimentary understanding of what's going on but I can read a schematic and take readings. I have successfully built, modified, and maintained amps before. I'm no expert, but I'm not totally green either. I'm like...green-ish.

    My guess is that one or both of the 6550's is questionable. and it's been missed in the jumpy tests. (Was the limiter there when BOTH sockets were tested? Only one of them? Are you sure? did you move ahead with anything and now aren't sure what your actual test conditions were?) And anyway, "I" also wouldn't adjust the amp for an anomalous set of tubes and call it good because I would always question those tubes. That's like adjusting your car's suspension for strangely wrong sized tires when the right tires are available.
    I agree. I'm also questioning the tubes, for now. Sadly this is the only set of 6550s I have and honestly I'd rather just put it back to EL34s and have it work than order another set of 6550s. Also, this is a 100 watt amp. Four 6550s.

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  3. #38
    Senior Member vintagekiki's Avatar
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    The value of the bias voltage depends solely on the bias voltage divider (15 kOhm) according to (56 kOhm + 22 kOhm bias trim pot)
    Bias feed resistors (100 - 220 kOhm) and grid leaks resistors (1.5 - 5.6 kOhm) does not participate in the formatting value of bias voltage.

    Bias voltage value depend from anode supply voltage and specified working classes of service (A, AB, B) for a given value quiet anode current.

    In order to avoid an accident when adjusting bias voltage (quiet current), with the bias trim pot set bias voltage to the highest -Ug1 value (quiet current min), and gradually decrease the -Ug1 to set the quiet current to the desired value.
    If the bias voltage cannot be adjusted with bias trim pot 22 kOhm, increase or decrease resistor 56 kOhm.

    Everything has its price.
    The higher value of the quiet current gives a aggressive powerful sound but shortens tube life.
    A smaller quiet current value gives a gentle soft tone but a longer tube life.

    Useful links

    https://music-electronics-forum.com/showthread.php?t=49473

    http://www.marshallforum.com/threads/biasing-a-100-watt-superlead.54981/

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    Quote Originally Posted by vintagekiki View Post
    The value of the bias voltage depends solely on the bias voltage divider (15 kOhm) according to (56 kOhm + 22 kOhm bias trim pot)
    Bias feed resistors (100 - 220 kOhm) and grid leaks resistors (1.5 - 5.6 kOhm) does not participate in the formatting value of bias voltage.

    Bias voltage value depend from anode supply voltage and specified working classes of service (A, AB, B) for a given value quiet anode current.

    In order to avoid an accident when adjusting bias voltage (quiet current), with the bias trim pot set bias voltage to the highest -Ug1 value (quiet current min), and gradually decrease the -Ug1 to set the quiet current to the desired value.
    If the bias voltage cannot be adjusted with bias trim pot 22 kOhm, increase or decrease resistor 56 kOhm.

    Everything has its price.
    The higher value of the quiet current gives a aggressive powerful sound but shortens tube life.
    A smaller quiet current value gives a gentle soft tone but a longer tube life.

    Useful links

    https://music-electronics-forum.com/showthread.php?t=49473

    http://www.marshallforum.com/threads/biasing-a-100-watt-superlead.54981/
    Thanks for the links.

    It seems I had a good negative bias voltage at the grids with the 6550s, but they just weren't passing current.

    I removed everything I'd done and it's set to run EL34s again. The EL34s are kind of all over the map because they're a hodge podge of unmatched tubes, but they are conducting and my voltages seem very good. I'm getting bias readings from 30-50ma from tube to tube. I know 50ma is high, but that's just one tube. The other three are all around 30ma. The high current moves with the tube. My plate voltage is back up to 455vdc. Grid bias voltage is -44vdc. Screens are around 448vdc. All of this was measured with the amp plugged into the wall- no limiter. So back at EL34 spec, the amp seems fine (not counting the mismatched tubes).

    Do I dare put the 6550s back in just to see what happens???? What if they work all of the sudden with everything at EL34 specs?

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  5. #40
    Senior Member vintagekiki's Avatar
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    Quote Originally Posted by Greg_L View Post
    Do I dare put the 6550s back in just to see what happens???? What if they work all of the sudden with everything at EL34 specs?
    To start, put one tube 6550 in socket.
    Previously with the bias trim pot set to the highest -Ug1 value (- 55 V min), put one 6550 and gradually decrease the -Ug1 to set the quiet current to the desired value (30 - 40 mA).

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    https://el34world.com/charts/Schematics/files/Marshall/Marshall_charts.pdf
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    Edit 191016
    http://web.archive.org/web/20110621211356/http://diyguitarist.com/Images/BiasChart-KT88.jpg
    KT88/6550 Bias Chart

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    Last edited by vintagekiki; 10-16-2019 at 02:02 AM. Reason: Edit 191016
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    Quote Originally Posted by vintagekiki View Post
    To start, put one tube 6550 in socket.
    Previously with the bias trim pot set to the highest -Ug1 value (- 55 V min), put one 6550 and gradually decrease the -Ug1 to set the quiet current to the desired value (30 - 40 mA).

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    Marshall Voltages

    Edit 191016
    http://web.archive.org/web/20110621211356/http://diyguitarist.com/Images/BiasChart-KT88.jpg
    KT88/6550 Bias Chart
    Wow really? Just put one tube in? That's not gonna melt the OT? This is safe?

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  7. #42
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    Quote Originally Posted by Greg_L View Post
    Wow really? Just put one tube in? That's not gonna melt the OT? This is safe?
    When you set the quiet current with one tube, you can normally put another tube.
    It goes without saying that the tubes are relatively matched.
    With only one tube in the socket you can measure and matched other tubes (one at a time)

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    Quote Originally Posted by vintagekiki View Post
    When you set the quiet current with one tube, you can normally put another tube.
    It goes without saying that the tubes are relatively matched.
    With only one tube in the socket you can measure and matched other tubes (one at a time)
    What do you mean by "quiet current"? The idle bias voltage?

    If I put just one tube in, won't one half of the OT not be doing anything? Isn't this bad for the OT?

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  9. #44
    Bent Member Chuck H's Avatar
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    Quote Originally Posted by Greg_L View Post
    What do you mean by "quiet current"? The idle bias voltage?

    If I put just one tube in, won't one half of the OT not be doing anything? Isn't this bad for the OT?
    Yes, quiet current would be at idle. And it's ok to have just one tube (or one bank) in for testing. Keeping in mind that with the other tubes out you will have an increase in voltage because of the lower current condition through the power supply. You might even remember reading here about testing voltages without any tubes before installing them. So yes, it's ok.

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    Quote Originally Posted by Greg_L View Post
    What do you mean by "quiet current"? The idle bias voltage?

    If I put just one tube in, won't one half of the OT not be doing anything? Isn't this bad for the OT?
    Quiet current or calm current or idle current is defined by bias voltage.
    One tube don't damage OT.
    When adjusting the bias, the loudspeaker must be connected to the amplifier.

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    Something is very strange about those 6550's.
    They were measuring less current than the EL34's, yet they were pulling down the B+ greatly.
    If they were really conducting less, the plate voltage should have been higher with the 6550's than with EL34's.
    What did you end up doing with the pin 1&8 issue?

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    I agree with Chuck's "hinky" and g1's "strange". There are definitely some things not making sense here.

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    Here's a wild guess. The 6550's are conducting hard. This is shown by the low plate voltage compared to the EL34's.
    Something is messing up the measurement at the cathode. Measuring by the OT resistance method would probably show a more correct reading.
    Some 6550's have the base metal shell connected to pin 1 and you can't trust the datasheet pin outs. I'm guessing these are like that, and the bear trap retainers are shorting the cathodes to ground via the pin 1 & 8 connection. Try disconnecting the pin 1's at the sockets when using the 6550's.
    Some small resistance at the clamps is allowing some measurement to show right now but it is in error.

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    Quote Originally Posted by Chuck H View Post
    Yes, quiet current would be at idle. And it's ok to have just one tube (or one bank) in for testing. Keeping in mind that with the other tubes out you will have an increase in voltage because of the lower current condition through the power supply. You might even remember reading here about testing voltages without any tubes before installing them. So yes, it's ok.
    Ok great, thanks.

    Quote Originally Posted by vintagekiki View Post
    Quiet current or calm current or idle current is defined by bias voltage.
    One tube don't damage OT.
    When adjusting the bias, the loudspeaker must be connected to the amplifier.
    Roger that, thank you. Will try tomorrow.

    Quote Originally Posted by g1 View Post
    Something is very strange about those 6550's.
    They were measuring less current than the EL34's, yet they were pulling down the B+ greatly.
    If they were really conducting less, the plate voltage should have been higher with the 6550's than with EL34's.
    What did you end up doing with the pin 1&8 issue?
    Pins 1 and 8 are connected to ground through a 1 ohm resistor for easy bias readings. Is this no good for 6550s? The amp came with 6550s originally and the ground connection was original...as far as I can tell. They still had red assembly paint on them at each tube socket. The amp is remarkably clean and mostly unmolested for a 1973 model. The OT had been changed and it was converted to EL34s before I got it a few years ago.

    Quote Originally Posted by g1 View Post
    Here's a wild guess. The 6550's are conducting hard. This is shown by the low plate voltage compared to the EL34's.
    Something is messing up the measurement at the cathode. Measuring by the OT resistance method would probably show a more correct reading.
    Some 6550's have the base metal shell connected to pin 1 and you can't trust the datasheet pin outs. I'm guessing these are like that, and the bear trap retainers are shorting the cathodes to ground via the pin 1 & 8 connection. Try disconnecting the pin 1's at the sockets when using the 6550's.
    Some small resistance at the clamps is allowing some measurement to show right now but it is in error.
    Interesting! This amp doesn't have the "bear traps". It has the top hat looking long spring tube retainers. The tubes do have a metal base though, and pin 1 of the tube does show continuity to the metal base. Do I need to break the link between pins 1 and 8 at the tube socket?

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    Yes. Leave pins 8 connected and disconnect the pin 1's. Retry your measurements. It's possible the 6550's they were using originally did not have the shell connection. Do the springs or their retainers touch the base shell anywhere?

    Alternately, if you don't want to disconnect, try using the OT resistance method: (quoting Aiken's article found here: https://aikenamps.com/index.php/the-...ord-on-biasing)

    "The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side."

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  16. #51
    Senior Member vintagekiki's Avatar
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    When measurement the cathode current, in addition the anode current, screen current g2 also enters.
    Measuring by the OT resistance is perhaps the more reliable variant since only the anode current are indirectly measured, the disadvantage is that the measurement results are obtained by calculation.

    The fastest and most accurate measurement (adjustment) of quiet current is possible when resistor 1 Ohm instead of in the cathode, serially connect middle OT terminal with to + HV.
    The voltage drop on 1 Ohm resistor that reads on 200 mV DVM is the value of the anode (quiet) current. Logically, if there are more tubes, anode (quiet) current is proportionally higher

    When the quiet current is set for one tube, and when all output tubes are put in the amplifier, the quiet current falls due to the PT load, so it is necessary "fine tune" (quiet current for one tube, multiply by the number of tubes)

    Click image for larger version. 

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    Edit 191016
    Many times pin 1 is used as an auxiliary carrier for some resistor, a considering that the 6550 has a metal base, disconnect pin 1 (no connection or base sheell) from pins 8 (cathode) to prevent a short circuit some voltage on pin 1 over springs to chassis.

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    Last edited by vintagekiki; 10-16-2019 at 10:35 AM.
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    Quote Originally Posted by g1 View Post
    Yes. Leave pins 8 connected and disconnect the pin 1's. Retry your measurements. It's possible the 6550's they were using originally did not have the shell connection. Do the springs or their retainers touch the base shell anywhere?

    Alternately, if you don't want to disconnect, try using the OT resistance method: (quoting Aiken's article found here: https://aikenamps.com/index.php/the-...ord-on-biasing)

    "The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side."
    Quote Originally Posted by vintagekiki View Post
    When measurement the cathode current, in addition the anode current, screen current g2 also enters.
    Measuring by the OT resistance is perhaps the more reliable variant since only the anode current are indirectly measured, the disadvantage is that the measurement results are obtained by calculation.

    The fastest and most accurate measurement (adjustment) of quiet current is possible when resistor 1 Ohm instead of in the cathode, serially connect middle OT terminal with to + HV.
    The voltage drop on 1 Ohm resistor that reads on 200 mV DVM is the value of the anode (quiet) current. Logically, if there are more tubes, anode (quiet) current is proportionally higher

    When the quiet current is set for one tube, and when all output tubes are put in the amplifier, the quiet current falls due to the PT load, so it is necessary "fine tune" (quiet current for one tube, multiply by the number of tubes)

    Click image for larger version. 

Name:	quiet current.jpg 
Views:	8 
Size:	17.9 KB 
ID:	55582

    Edit 191016
    Many times pin 1 is used as an auxiliary carrier for some resistor, a considering that the 6550 has a metal base, disconnect pin 1 (no connection or base sheell) from pins 8 (cathode) to prevent a short circuit some voltage on pin 1 over springs to chassis.
    Wow, thank you guys. This might be the tip that solves the whole thing. I will un-bridge pins 1 and 8 at the tube socket and see what happens.

    Before I started all this I read elsewhere about how pin 1 on metal base 6550s is connected internally and wondered how the pin 1/8 bridge at the tube socket would affect it.

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  18. #53
    Senior Member vintagekiki's Avatar
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    Basically at octal power tubes except 7027 and EL34 pin 1 is no connected and is used for various purposes.
    At EL34 pin 1 (screen g3) must be connected with pin 8 (cathode)
    7027 has a different pin layout.

    For (power) tube datasheets and socket layouts look

    https://drtube.com/en/library/tube-datasheets

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    Quote Originally Posted by vintagekiki View Post
    Basically at octal power tubes except 7027 and EL34 pin 1 is no connected and is used for various purposes.
    At EL34 pin 1 (screen g3) must be connected with pin 8 (cathode)
    7027 has a different pin layout.

    For (power) tube datasheets and socket layouts look

    https://drtube.com/en/library/tube-datasheets
    Interesting, thanks. Pardon my ignorance, but if pin 1 doesn't do anything in a 6550, and it's tied to the tube base, then why does it matter if it's grounded or not?

    Or is it more about pin 8 being tied to pin 1? Pin 8 needs to be grounded, right?

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    Here's the data sheet for these 6550s.

    6550we-sovtek.pdf

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    it's tied to the tube base, then why does it matter if it's grounded or not?

    Or is it more about pin 8 being tied to pin 1? Pin 8 needs to be grounded, right?
    Pin 1 is only connected to the metal base sleeve. If pin 1 and pin 8 (cathode) are connected at the socket, the metal sleeve gets connected to the cathode. So if for some reason the metal sleeve connects to the chassis ground, your 1 Ohm current sensing resistor at the cathode is shorted to ground. Without a cathode resistor it wouldn't matter.

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    Quote Originally Posted by Helmholtz View Post
    Yes pin 1 and pin 8 (cathode) are internally connected and the metal base sleeve is connected to pin 1. So if for some reason the metal sleeve connects to the chassis ground, your 1 Ohm current sensing resistor at the cathode is shorted to ground.
    Thank you. So, let me see if I'm understanding this....

    Since pin 8 - cathode - in the 6550 tube is connected to pin 1 internally, and pin 1 is grounded through the base, the tube could actually be working properly but the internal connection doesn't allow me to get a bias reading through my usual bias probe and/or 1 ohm resistor methods? Is that what's happening here?

    And if that's true, does it hurt anything to just leave pins 1 and 8 bridged at the tube socket, if it's redundant anyway, and take my bias readings via another method?

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    Pin 8 is cathode and must be grounded.
    Screen g3 in some tubes is connected to the cathode in the tube itself, or as EL34 derived as a separate electrode that must be grounded.
    Look at layouts tubes socket in drtube datsheets pin 1 is mostly free N / C (no connected)

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    Quote Originally Posted by Greg_L View Post
    Thank you. So, let me see if I'm understanding this....

    Since pin 8 - cathode - in the 6550 tube is connected to pin 1 internally, and pin 1 is grounded through the base, the tube could actually be working properly but the internal connection doesn't allow me to get a bias reading through my usual bias probe and/or 1 ohm resistor methods? Is that what's happening here?
    Sorry for confusion. Please see my edited text. Pin 1 and pin 8 are not internally connected

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    Quote Originally Posted by vintagekiki View Post
    Pin 8 is cathode and must be grounded.
    Screen g3 in some tubes is connected to the cathode in the tube itself, or as EL34 derived as a separate electrode that must be grounded.
    Look at layouts tubes socket in drtube datsheets pin 1 is mostly free N / C (no connected)
    Yes, I see that. Thanks.

    If I'm understanding this correctly, my connections are fine, it's my method of reading the bias that's misleading me. Is that correct?

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    Quote Originally Posted by Helmholtz View Post
    Sorry for confusion. Please see my edited text. Pin 1 and pin 8 are not internally connected
    Oooooh, right. Pin 1 truly does nothing.

    So.....reboot......only pin 8 should go to ground through the 1 ohm resistor? Pin 1 should be left completely isolated?

    If pin 8 is to be grounded, then does it matter if it goes through pin 1 since pin 1 does nothing?? All that really does is force me to use a different bias reading method, correct?

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    For the 6550, the cathode and screen g3 are connected in the tube itself and executed at pin 8.
    To adjust the quiet current connect a 1 Ohm resistor from pin 8 to the chassics.
    Pin 1 is neutral (N / C)

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    it's my method of reading the bias that's misleading me. Is that correct?
    Only if something shorts all metal sleeves to ground. If so, pin 1 should be isolated but only with 6550s!
    Just use your Ohmmeter to check. I consider it improbable, given your cathode voltage readings.

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    Quote Originally Posted by vintagekiki View Post
    For the 6550, the cathode and screen g3 are connected in the tube itself and executed at pin 8.
    To adjust the quiet current connect a 1 Ohm resistor from pin 8 to the chassics.
    Pin 1 is neutral (N / C)
    Quote Originally Posted by Helmholtz View Post
    Only if something shorts all metal sleeves to ground. If so, pin 1 should be isolated but only with 6550s!
    Just use your Ohmmeter to check. I consider it improbable, given your cathode voltage readings.
    Ok, good information. This clears some things up. Thank you very much. I'll implement these ideas later and report back.

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    Quote Originally Posted by Helmholtz View Post
    Only if something shorts all metal sleeves to ground. If so, pin 1 should be isolated but only with 6550s!
    Just use your Ohmmeter to check. I consider it improbable, given your cathode voltage readings.
    I thought it improbable too, especially with the spring type retainers. BUT it is the only thing I can think of that explains the B+ dropping so much with the 6550's compared to the EL34's, even though the measurement suggests they are running colder.

    Quote Originally Posted by Greg_L View Post
    Oooooh, right. Pin 1 truly does nothing.

    So.....reboot......only pin 8 should go to ground through the 1 ohm resistor? Pin 1 should be left completely isolated?

    If pin 8 is to be grounded, then does it matter if it goes through pin 1 since pin 1 does nothing?? All that really does is force me to use a different bias reading method, correct?
    Just to be clear, you are correct that there is no issue leaving the pins this way, just causes problem reading idle current at cathode.

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    Quote Originally Posted by g1 View Post
    I thought it improbable too, especially with the spring type retainers. BUT it is the only thing I can think of that explains the B+ dropping so much with the 6550's compared to the EL34's, even though the measurement suggests they are running colder.


    Just to be clear, you are correct that there is no issue leaving the pins this way, just causes problem reading idle current at cathode.
    Well friends, I'm not exactly sure what's happened, but with the amp's bias supply back at EL34 specs, these 6550s are working. Working too well actually. They are pulling the juice! I can't get the bias low enough now.

    I put the bias supply resistor back to 15k (resistor in parallel with jumper wires) and I can just get them down to around 50-55ma. About -51vdc at the grid. Plate voltage is holding steady at 455. They're glowing blue-ish with no red plating....so far. Grid stoppers are still 220k though.

    The tube bases are not shorting to ground anywhere that I can tell, and I am getting solid readings across my 1 ohm resistors with pins 1 and 8 still tied to ground through the resistors.

    So what do yall make of this? Any thoughts?

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    Quote Originally Posted by Greg_L View Post
    Well friends, I'm not exactly sure what's happened, but with the amp's bias supply back at EL34 specs, these 6550s are working.
    Sometimes, to make progress, you have to take a step backward. Which step ... that is the question.

    Working too well actually. They are pulling the juice! I can't get the bias low enough now.

    I put the bias supply resistor back to 15k (resistor in parallel with jumper wires) and I can just get them down to around 50-55ma. About -51vdc at the grid. Plate voltage is holding steady at 455. They're glowing blue-ish with no red plating....so far.
    OK now you have to get that bias voltage up a tad. Vb somewhere in the mid -50's looks like your target area. 40 mA on 6550's plenty enough current. I just had a '74 Super Lead come thru here last week, it had Shuguang 6550's, that's where I set it and it appeared to be quite happy turning out a healthy 111 watts.

    It's not the stopper resistors that are 220K I hope, but the bias distribution R's. Stoppers are 5K6 I hope. The ones right on pins 5.

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    Quote Originally Posted by Leo_Gnardo View Post
    Sometimes, to make progress, you have to take a step backward. Which step ... that is the question.
    It's entirely possible, and likely, I just did something wrong before. I put the bias circuit to 6550 specs and got nothing. I put it back to EL34 specs and everything works. Who knows.



    OK now you have to get that bias voltage up a tad. Vb somewhere in the mid -50's looks like your target area. 40 mA on 6550's plenty enough current. I just had a '74 Super Lead come thru here last week, it had Shuguang 6550's, that's where I set it and it appeared to be quite happy turning out a healthy 111 watts.

    It's not the stopper resistors that are 220K I hope, but the bias distribution R's. Stoppers are 5K6 I hope. The ones right on pins 5.
    Yes that's what I meant. Sorry, I don't quite know all the proper terminology and get them confused sometimes. The bias feed resistors are still 220k....supposed to be 150k for 6550s.

    According to my plate voltage and current readings, I'm dissipating about 25w per tube.. But these Sovteks 6550WEs might be 42w 6550s??

    If I'm on the right track, I'd like to just fine tune the bias supply voltage and leave it. How critical is it that the bias feed resistors actually be 150k?

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    How critical is it that the bias feed resistors actually be 150k?
    150k resistors are safe with both 6550s and EL34s. 220k is too high for 6550s.

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    Quote Originally Posted by Helmholtz View Post
    150k resistors are safe with both 6550s and EL34s. 220k is too high for 6550s.
    Okay, thank you.

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