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Old 4th August 2008, 11:12 PM   #11
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I don't like putting 500V on modern 6L6GCs, and even less on EL34s.

I explain this weakness to my customers, and many like to have a mod installed to drop the screen voltage - since the screen is the problem (even the modern ones can take it on the anode/plate).

Use a 900V TO-220 ISO packaged PowerMOS device bolted to the chassis and a potential divider (use high voltage resistors: 220..560K to gate, about 2M to ground). 47nF or so at the gate is enough in many guitar amps to do without a choke, if the incoming supply is not too rough. Feed the FET drain through a 5W+ 100ohm resistor and 1uF or more of adequately rated capacitor (MKP best here). At the source, 22uF to 68uF depending on the amp's power. A 10V gate protection zener (don't ever omit this) and 390ohms or so of gate resistor (noninductive, pref. chip resistor, right up at the gate pin) rounds off the design.

The pic shows such a gyrator nestling into a JCM800. The "PCB" is just blank copper FR4 with hacksawed tiles.

Set the voltage at 450V or less for good 6L6GCs, 400V or less for other 6L6/5881, and 390V for EL34s.
Even with 390V Vg2 on EL34PP (fixed bias) and 450V anode (plate) volts, you still get over 50W if the trafo is OK.

Dropping the screen volts hugely improves the durability of old high volt guitar amps, and if you use the protection components listed, the reliability of the dropper/gyrator is perfect. And, players are very happy with the sound, most importantly!
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Old 4th August 2008, 11:46 PM   #12
gofar99 is offline gofar99  United States
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HI, Check out the new manufacture JJ KT77s as replacements. They bun about a $1 more, but seem to hold up well.
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Old 5th August 2008, 12:21 AM   #13
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Quote:
Originally posted by gofar99
HI, Check out the new manufacture JJ KT77s as replacements. They bun about a $1 more, but seem to hold up well.
They pull 1.4A to light instead of .9Am and bias differently. The lower plate resistance might be a plus though.
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Old 5th August 2008, 12:48 AM   #14
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Thanks for the suggestions. I think Im gonna give the Sovtek 6L6WXT+ a try since they are so cheap. The SED's look nice but are too expensive at $60/pair!!


Rod,
Your comment about the screen grid makes sense as these are usually connected to the almost the same potential as the plates on most guitar amps. Btw, doesn't the EL34 handle 800 volts(plate only)? I think it also uses an extra pin that's unused in the 6L6's. But EL34's on a Fender, heressy
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Old 5th August 2008, 03:17 AM   #15
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The suppression grid is brought out to a separate pin in the EL34 while in the 6L6, it's internally connected to the cathode.
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Old 5th August 2008, 03:47 AM   #16
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Try the generic Sovtek 5881's they will handle that amp easily they sound ok. I really liked the old Sevetlana 6L6's in Fenders like that one but I've also had good luck with the JJ 6L6 maybe you had some bad ones? The newer Sevetlanas sold by New Sensor are just not as good as the old ones were I've had those blow up and fail quickly in a Fender, I have not tried the SED tubes.
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Old 5th August 2008, 04:26 AM   #17
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"The suppression grid is brought out to a separate pin in the EL34 while in the 6L6, it's internally connected to the cathode.
"

so I just need to short those two pins?
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Old 5th August 2008, 04:33 AM   #18
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If you want to use an EL34, yes. Pins 1 and 8. Just make sure pin 1 isn't being used for anything else.

http://tdsl.duncanamps.com/show.php?des=6L6GC
http://tdsl.duncanamps.com/show.php?des=EL34
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Old 5th August 2008, 11:29 AM   #19
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Quote:
Rod, Your comment about the screen grid makes sense as these are usually connected to the almost the same potential as the plates on most guitar amps. Btw, doesn't the EL34 handle 800 volts(plate only)? I think it also uses an extra pin that's unused in the 6L6's. But EL34's on a Fender, heressy
Yeah, I agree, you could never put EL34s in a Fender, any more than 6L6s in a Marshall!

The high 800V spec on the EL34s just shows that anode volt limits are NOT the problem. In fact the modern EL34 is usually reckoned to be weaker than 6L6GC in guitar amps - check out the notes in the Mesa-Boogie manual for instance.

The Mullard EL34 spec was Anode 800V max, Screen 500V max. Compare the new JJ EL34: 800V/450V - and if you play your JJs hard, you will quickly see why they reduced that spec.

For 6L6GCs the ratings are 500V anode, 450V screen for most -GC versions.

Overrunning the screen voltage is always going to run you into short life and sudden failure territory, especially if you like to turn it up. Keep in mind that screen overheating, below the level of outright failure, can release contaminants into the vacuum, ultimately killing tone. I reckon the screen dropper pays for itself fairly soon against frequent valve replacement.
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Old 5th August 2008, 02:39 PM   #20
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Nothing wrong with using a 6L6 in a Marshall and many of their amps came with them or similar type KT66's. For what it's worth I've tried a 6V6 in a Marshall 50 watt just to try to reduce power and to experiment. Believe it or not it still sounded very "Marshall".. EL34's in a Fender can be done but be careful about drawing too much heater current.

Your options are many on the 6L6 amp. Try a set of the Valve Art KT66's and see how you like them in the Fender. I use them in Marshalls and they are quite nice. The Saratov version that Groove Tubes used to sell as the KT66HP was great but I don't have any more of those.
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