Randall RT100 burns cathode follower tubes!

[xtian]

I revived this Randall RT100 head for a client. It had a dead DC heater string. I just had to resolder the FWBR to get it to work again. Keep that in mind.

Put in a complete JJ tube set, and returned to client in good working order. He calls me shortly later, says clean channel fine, OD channels very very low volume. I discover V3 (labeled TB3 on schem) is bad. Hmm, bad tube. Ok, fine. I checked heater voltage and associated resistors (R37, 41, 42) and everything seems fine. Plate and cathode voltages also look fine.

I put in a new JJ 12AX7, and amps kicks ass again. Owner comes next day, we plug it in, and...HEY, volume in OD channels is low again. V3 is burned again! I put a solid state Jet City "valve" in place, and sent the owner home to beat on it for a few days and report back.

WTF caused two new JJs to burn up?

[martin manning]

My guess is heater insulation failure. Vk is probably up around 200V on TB3's CF.

[xtian]

My guess is heater insulation failure. Vk is probably up around 200V on TB3's CF.

Yes! Vk on the CF was closer to 250 or 260v. Datasheet says 200v max heater to cathode. I think you called it. Design flaw in the Randall? Solution?

I recently read a discussion here on CFs. Will try to find.

[xtian]

Couldn't find the thread here, but Merlin has a solution (A Useful Mod' for DC-coupled cthode followers), and common lore is to use a Chinese, or non-spiral-heater-element tube in CF position.

[Leo_Gnardo]

Couldn't find the thread here, but Merlin has a solution (A Useful Mod' for DC-coupled cthode followers), and common lore is to use a Chinese, or non-spiral-heater-element tube in CF position.

Another solution is to float your filaments on a positive DC level, say 60 to 100V. That may not be practical if there's a DC supply derived from the filament line.

All too often I see manufacturers install half-watt resistors in the cathode circuit of CFs, when one watt is barely enough. Roasty-toasty!

[pdf64]

I don't see anything on Merlin's dc CF page regarding the heater-cathode insulation issue?
His diode or neon mod addresses excessive grid-cathode voltage at power up, prior to the preceding CC stage drawing current.

[Littlewyan]

That preamp runs on a pretty high voltage. Just before the decoupling resistor (R43) there is 403V!! Try changing R129 from 4K7 to 10K to bring the voltages down a bit or if you don't want to affect the PI change R43 to 10K. Changing R129 will make a bigger difference though. Chances are you may not notice a difference in tone with the slightly lower voltage and your 12AX7s will be happier . The tone may even improve, try it.

You could also try changing R42 to 82K but I don't think it will lower the voltage enough to make a difference.

[pdf64]

Yes, in the absence of being able to elevate the heaters, reducing the voltages around the circuit somehow may help.
As well as the above, consider biasing the preceding stage a little hotter, eg R41 to 820.
Several little tweaks may get it in a reasonable place.

Also (though I hate to say it)), with regard to the grid-cathode voltage at start up, this is one situation where using standby can help.
So check they're using it at start up.

The Marshall SL 1959 and Vox AC50 tend to run their tone stack driver CFs at similarly high levels.

[10thTx]

I think this was the thread that was referred to?

https://tubeamparchive.com/viewtopic.php?t=29101

[xtian]

That was the one. Thx!

[Littlewyan]

I still think lower the voltage. Otherwise you're restricting yourself to what valves you can use. JJs should be able to cope with the CF position and Chinese I think. Russian valves definitely won't be able to.

The fix outlined in Merlin's guide and on that thread only helps with when you turn the amp on as PDF64 rightly pointed out. The standby switch can also help with this.

[rp]

I had to look it up, though the schematic gives it away, and this is a current production amp. I would definitely make a pot of tea, turn on speaker phone and give Randall a call. Tech might even answer quickly. This was built for Ruskie and Sino tubes so you can't be the first one with this problem.

Maybe see if it can work quietly w/ AC heaters? If so you could elevate the heaters in the mean time. But I'd still run it by Randall if only out of (morbid) curiosity.

[xtian]

Thanks, y'all. I'll have to wait to see if owner brings it back after this weekend. As I said, I put a Jet City Retrovalve (MOSFET device) in place of the 12AX7, and he's playing a show tonight.

[JMFahey]

Just thinking aloud: CF follows driving plate voltage, so lowering that might solve problems for good.

No need for mystery kludges: either increase drive tube idle current (as suggested above ) , increase plate resistor, or both, whatever's needed to bring catode voltage below 200V ... even down to 150/160V if you want to play it safer.

That said, I have had problems with various JJ before, so I avoid them whenever possible

Life's too short to argue with angry customers or absorb unnecessary losses; whatever reliably works is a preference nowadays.

To each his own.

[pdf64]

Bear in mind that the regular 200V heater-cathode limit is Vdc+peak Vac, ie don't forget that with signal, the instantaneous cathode voltage will rise above the idle value.
Note that some 12AX7 / ECC83 limit at 180V.
So the max suitable cathode voltage may be less than 150V, the saving grace being the dc coupling limiting the positive going waveform peaks.
http://www.mif.pg.gda.pl/homepages/fran ... 12AX7A.pdf
http://www.mif.pg.gda.pl/homepages/fran ... /ECC83.pdf