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Maximum Screen Voltage for Triode Strapped 1625 or 837
Maximum Screen Voltage for Triode Strapped 1625 or 837
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Old 23rd April 2017, 12:50 AM   #21
Tubelab_com is offline Tubelab_com  United States
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A word of caution from my experience.

It has been stated that the screen grid voltage rating can be ignored when the tube is triode wired. Some tube manufacturers publish higher screen voltage ratings when the tube is triode connected.

When the amp is idle, on but no music is playing, or the volume is very low, the output tube doesn't know if it's triode, pentode, or UL connected. It only knows that it is constantly seeing an excessive voltage on the screen. Most tubes can deal with this, but there are going to be some that may do OK for a while, then slowly start to exhibit "bias creep" (the idle current slowly starts to creep upward over time) leading to eventual tube runaway and blown parts.

This is especially bad in SE (always class A) and class A push pull amps, since idle is the worse case dissipation condition. Triode wired sweep tubes are particularly prone to bias creep and eventual runaway. I built a triode wired 6LW6 amp years ago. It worked great, and sounded excellent at 400 volts, but unexpectedly blew a tube after a few months. The second tube blew up after a few more months, so I reduced the plate voltage to about 340 volts and added a fuse in the tube's cathode connection. Over the next year and a half, two more tubes ran away, but blew the fuse. Each tube was no longer stable and could not be used after the initial runaway event. All runaway events were during idle conditions.

The 1625, 807 and 6BG6 ARE 6L6 types, and may be 6L6GA's, 6L6GB's or even 6L6GC's (rare) internally depending on who made them, and when they were made. I have seen 807's and 6BG6's that were 6L6GC's inside and even a few with 7027A guts. Obviously the ones with newer guts can eat much more screen voltage.
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Old 23rd April 2017, 01:06 AM   #22
TheGimp is offline TheGimp  United States
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So when you are not listening to the amp, leave "Crazy Train" playing at full volume?
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Old 23rd April 2017, 03:11 AM   #23
Tubelab_com is offline Tubelab_com  United States
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Quote:
leave "Crazy Train" playing at full volume?
Inna Gadda Da Vida! The neighbors hated that one when I was a teenager with several hundred watts of audio and huge cabinets with 4 X 12 inch speakers in each one!

For years I had a Tubelab SSE hooked up in my work room. It could be switched to the PC output, the turntable, the CD player, or a Samson Mixpad (small mixer). My workbench had a master switch, with a KILL button. In case an experiment got out of hand I could smack that button and everything except the PC went dead.

The SSE was always on whenever the bench was on, and connected by relay to the PC by default whenever the PC was on. So the amp idled a lot. It didn't seem to mind. I swapped in an SPP, and several other amps as they were built, even a 125 WPC version of Pete Millett's big red board. For the last several years in Florida, I had a switchbox so that I could swap up to 4 amps live by turning a knob, but somehow I always gravitated back to my original "industrial strength" SSE made on diamond plate aluminum. Sadly it was dropped during the move out of Florida, and doesn't look too good right now.

One of the amps that pushed the SSE out of its usual spot, was a bigger not so simple SE that used 6LW6's and made over 15 WPC in triode. The SSE reclaimed its spot after the 6LW6 amp ate its 4th output tube. I got mad and stripped it for parts!
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Old 23rd April 2017, 04:28 AM   #24
BinaryMike is offline BinaryMike  United States
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Just FYI; the respected Sarser/Sprinkle Musician's Amplifier ran a pair of 807s in class A triode mode at 400V.
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Old 23rd April 2017, 05:08 AM   #25
trobbins is offline trobbins  Australia
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The 807 screen voltage in class A spec was uprated circa late 1940's to 400V due to the interest in deploying it as an alternative to the KT66 in the Williamson amp.

The 6CM5 sweep tube has been extensively used for PA amps, and for DIY (although not normally triode connected). Consistent with Tubelab's caution, that sweep tube was prone to runaway if idled anywhere near its supposed plate dissipation spec limit by DIYers. Philips successfully sold a lot of 6CM5 based PA amps up to sextet output stages, but operated them at close to class B, and used a 50% B+ for screen.
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Old 24th April 2017, 07:53 AM   #26
6A3sUMMER is offline 6A3sUMMER  United States
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I am glad Tubelab_com spoke up.

In triode-wired mode, depending on whether a tube is the early or late design-and-construction and model version, some may not work very long when they are set to the maximum plate voltage rating as their quiescent state.

Also, in ultra-linear mode, depending on the percentage of the UL tap, and the plate swing, the plate will be far below the screen voltage. For example, take a 30% tap, with the screen at 405V and the plate at 400V (due to primary DCR drop). If the plate swings down to 150V (250V swing), the screen will only swing down to 330V (75V swing).
Now the plate is 180V below the screen voltage, at a time when the control grid is dictating max current.
Could be a problem.
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Old 24th April 2017, 03:56 PM   #27
rcollege is offline rcollege  United States
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Thank you Tube Lab, 6A3summer and everyone

The 1625's I have are 1944 date of manufacture. The 837's look to be the same Era. I believe both are Military grade. I know it's a stretch, however, I am running low current, around 35ma or less. I have to calculate it out. I'm using 2k for the cathode resistor. The high impedance opt mean less current is needed.
I don't know if the current makes a difference, the dissipation of the 837 and 802 may be near the limit...but far from the limit of the 1625. I may increase the screen resistor...to more than 100ohm to ensure limit dissipation and ensure the screen voltage is always below the plate.

Last edited by rcollege; 24th April 2017 at 03:58 PM.
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Old 25th April 2017, 12:20 AM   #28
rcollege is offline rcollege  United States
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It looks as though the plate dissipation of the 802 and 837 are beyond their maximum limit.. approx 13.5 watts.....Not too surprised seeing as this amp was built for the 1625. That's what you get for not measuring.

Now the question will be... lower the plate dissipation via higher cathode resistor, or stick with the 1625...hhhmmmm...ideas brewing...
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Old 26th May 2017, 05:35 PM   #29
rcollege is offline rcollege  United States
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Quote:
Originally Posted by trobbins View Post
The 807 screen voltage in class A spec was uprated circa late 1940's to 400V due to the interest in deploying it as an alternative to the KT66 in the Williamson amp.

The 6CM5 sweep tube has been extensively used for PA amps, and for DIY (although not normally triode connected). Consistent with Tubelab's caution, that sweep tube was prone to runaway if idled anywhere near its supposed plate dissipation spec limit by DIYers. Philips successfully sold a lot of 6CM5 based PA amps up to sextet output stages, but operated them at close to class B, and used a 50% B+ for screen.
So, does keeping a lower dissipation allow for having a higher screen voltage?

I have been told by manufacturers saying that screen voltage can be higher as long as plate dissipation is kept within rated specs.
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Old 26th May 2017, 07:24 PM   #30
sser2 is offline sser2  United States
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Russian rule: operation of tube at two or more limiting parameters is prohibited.
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