[randytsuch]

Hi guys
I have starting working on a tube amp project, modding one.

It appears to have one of the plates of a 6SN7 tied directly to B+.
I measured B+ last night, unloaded (no tubes) and it was 528VDC.

I don't have much experience playing with tube circuits, but I looked up the max voltage of a 6SN7, and it's 450VDC, so obviously 528 is too  high.  I think my house voltage is on the high side, which is not helping.

The other half of this tube has a 27K resistor between B+ and the plate.  Seems like there should be a 27K on this one too.

I ran it a simulation on PSU designer, and if I set it up for a resistive load enough for 10ma load current, I get a plate voltage of around 250V.

Right now, I only have NOS Ken Rad VT231 tubes, which aren't going to get plugged in until I figure this out.  I might buy some cheap 6SN7's to play with.

Any help or advice would be appreciated.

TIA
Randy

[Steve]

Hi Randy,

The would suspect the tube with the plate measuring 528 volts has a large cathode resistor and is operating as a cathode follower.

If so, the cathode resistor would have a large voltage across it, so one would need to subtract the voltage across the cathode resistor from 528 volts to find the actual plate to cathode voltage.

Hope this helps.
Steve

[randytsuch]

Hi Randy,

The would suspect the tube with the plate measuring 528 volts has a large cathode resistor and is operating as a cathode follower.

If so, the cathode resistor would have a large voltage across it, so one would need to subtract the voltage across the cathode resistor from 528 volts to find the actual plate to cathode voltage.

Hope this helps.
Steve

You mean like 100k 

Thanks, that helps a lot.
Obviously have a lot to learn about tube amps.

Kind of related question.
Anybody out there use tubecad?

Randy

[mgalusha]

I do and have found it quite useful. It does the math for you, which isn't that difficult but it does it immediately so you can see the results without spending the time to calculate everything out.

Anybody out there use tubecad?

[Bill Baker]

Be careful should you decide to play with any NOS 6SN7 tubes. Early productions, before 1953 (I think that's the date) thier volatge capabilities are not what they are today. Many of them could not exceed 300V-350V.

 As Steve mentioned, this is not the voltage "at" the plate but after subtracting the cathode volatge from the voltage measured at the plate.

[randytsuch]

Be careful should you decide to play with any NOS 6SN7 tubes. Early productions, before 1953 (I think that's the date) thier volatge capabilities are not what they are today. Many of them could not exceed 300V-350V.

 As Steve mentioned, this is not the voltage "at" the plate but after subtracting the cathode volatge from the voltage measured at the plate.

Tubecad shows out of stock at antique, so I was playing with ltspice yesterday.  My project has two 6sn7 tubes per side, and ltspice shows that the first one is well below 300, but that the 2nd one is at least 350.  Of course, I could be doing something wrong, I am just learning how to use ltspice.

I guess I will pick up a couple chinese 6sn7 tubes to play with, and check voltages before I try my nos tubes.

Randy

[randytsuch]

So I ebayed some chinese 6sn7gt's, and got them yesterday.

First tube has around 170vdc from cathode to plate.
The cathode follower also has around 170 from cathode to plate
but the other 1/2 of the 2nd tube has a hair under 300vdc from cathode to plate, so I am a little worried about that one.  The actual is lower than my spice model, but I am not sure how accurate the tube spice model is?

So, I can use NOS tubes for the first 6sn7, but I guess not for the 2nd one, unless I make changes to lower the voltage?

Randy

Edit
Couple of other things to mention.
I think my house voltage is high, need to check it again.  The heater voltages are also high, at 6.96 vac, instead of 6.3.  I kind of remember that my house is about 120, so about 10% high, which obviously would cause all of these voltages to read higher then expected.  May try DC heating for these tubes.

Randy

[Frihed91]

I copied this from 6SN7 VT-231 5692 Tube Audio Performance – Chimera Labs Revised 05/2003 on deHavilland hfi's web site:

"I will again caution everyone that various 6SN7 types have different voltage and plate dissipation ratings. The voltage rating refers to voltage across the tube's plate and cathode. To ensure good tube reliability you should probably run a tube at 70% or less of its maximum plate dissipation.

5692 - 275 Volts maximum & maximum 1.75 Watts per section
6SN7GT/WGT -- 300 Volts maximum & maximum 3.5 Watts per section
6SN7GTA/GTB - 450 Volts maximum & maximum 5.0 Watts per section

Using the general audio application of 250 plate volts, a bias of 8 volts results in 9ma of current and a plate dissipation of 2.25 Watts, we find:

5692 - runs 43 % over maximum rating – It isn’t a 10,000 hour tube when you run it this HOT!
6SN7GT - runs 64% of maximum – coasting and liking it
6SN7GTA/GTB - runs 45% of maximum – Hardly turned on, may outlast you."

[tomat]

So I ebayed some chinese 6sn7gt's, and got them yesterday.

First tube has around 170vdc from cathode to plate.
The cathode follower also has around 170 from cathode to plate
but the other 1/2 of the 2nd tube has a hair under 300vdc from cathode to plate, so I am a little worried about that one.  The actual is lower than my spice model, but I am not sure how accurate the tube spice model is?

So, I can use NOS tubes for the first 6sn7, but I guess not for the 2nd one, unless I make changes to lower the voltage?

Randy

Edit
Couple of other things to mention.
I think my house voltage is high, need to check it again.  The heater voltages are also high, at 6.96 vac, instead of 6.3.  I kind of remember that my house is about 120, so about 10% high, which obviously would cause all of these voltages to read higher then expected.  May try DC heating for these tubes.

Randy

i suggest don't use that tube again ,there is too much difference voltage for each triode .and i suggesting also to use DC heating filament for NOS tube ,that will help to reduce hum and noise.

cheers

[randytsuch]

i suggest don't use that tube again ,there is too much difference voltage for each triode .and i suggesting also to use DC heating filament for NOS tube ,that will help to reduce hum and noise.

cheers

Hi
I guess you are talking about this tube
The cathode follower also has around 170 from cathode to plate
but the other 1/2 of the 2nd tube has a hair under 300vdc from cathode to plate

Are you saying there is two much difference between the two sides (170 vs 300), or that the 300 is too high? 

BTW, I ebayed a variac last week, it should be arriving any day now, so I can lower the input voltage to what it should be, and make some more measurements.

Randy