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

[g1]

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?

[The Dude]

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

[g1]

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.

[Greg_L]

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.

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.

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.

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?

[g1]

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."

[vintagekiki]

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)



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.

[Greg_L]

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."

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)

[ATTACH=CONFIG]55582[/ATTACH]

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.

[vintagekiki]

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

[Greg_L]

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?

[Greg_L]

Here's the data sheet for these 6550s.

6550we-sovtek.pdf

[Helmholtz]

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.

[Greg_L]

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?

[vintagekiki]

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)

[Helmholtz]

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

[Greg_L]

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?