3-500Z use

[K9EID]

I have a TL 922A with only one good 3-500.  OK to use around 400 watts with just one tube until I locate another?

Bob Heil, K9EID

[KD6VXI]

Bob,

Shouldnt be an issue with the fan cooling.

Makes a nice 125 watt carrier.

Do you still need the manual for the PT2500?

I moved and still haven't unpacked everything.  Work is moving me again next month, so if you need it, I'll try and find it.

--Shane
KD6VXI

[K1JJ]

Bob,

Sure it will work fine.  Pull out the bad  tube and 400 watts pep output will be reasonable for one.   Hopefully the filament voltage doesn't rise above 5V too much with less load on the fil xfmr. Also, the tank plate impedance will be lower than optimum for one tube since you don't have a roller inductor to compensate. Tape off the unused air holes of the tube socket.

I run a single 3-500Z here with only about 1500V and it will do about 750 watts out max pep.  I usually run it at 100 watts carrier and 500w pep out when properly matched.


T

[WQ9E]

Normally this would be okay BUT the TL-922 runs the filamentary cathodes in series across 10 volts so it needs both tubes in place.  You can probably run 4-400A tubes instead if you can't locate another 3-500Z.

It is a nice amplifier, Kenwood did a good job.  They also designed the nicest sweep tube amp I have ever run across with their predecessor model, the TL-911.  It runs five 6LQ6 tubes in parallel with excellent cooling and protection systems.  It provides individual metering for each tube along with ten individual adjustments to balance each tube both at idle and at full input.

The series connection of tubes is fairly unusual but I recall Henry did the same in one or more of their amps.  It was probably intended to keep the filament choke size/voltage drop reasonable.  For a set with parallel tubes, you would still have to be careful to check voltage with a single tube because the filament voltage will be higher with the reduced I2R drop across the choke and decreased transformer load.

Rodger WQ9E

[K1JJ]

Normally this would be okay BUT the TL-922 runs the filamentary cathodes in series across 10 volts so it needs both tubes in place.  You can probably run 4-400A tubes instead if you can't locate another 3-500Z.

Rodger WQ9E

Yikes, 10V in series. We should have checked that.  Most are 5V in parallel and this is an exception.  The 4-400As wud be a good sub for now.

Yes, the TL-922 schematic below shows series fils:


T

[K9EID]

Thank each of you for yoru help.  I did fire it up this morning for a minute just to make sure nothing else was wrong.  All is well at 300 watts out.   I have a brand new 3-500Z on the way and will wait until I have that to use the beauty.  My main amp is a 922A that I have had since the 80's.  Works great.  I use it on AM most days driving with a Harvey Wells about 45 -50 watts out which drives the 922 very softly around 180 watts out.   

Thank all.  Stay safe and Happy New Year to all.

Bob Heil, K9EID

[WQ9E]

Bob,

You definitely want to wait for that second tube.  When one tube is removed, then the 15 amp return current from the single tube filament is through choke L20 in the center tap which is a small gauge 80uh choke NOT designed for high current. 

High current flow through this results in low voltage to the single tube filament and greatly excessive current through L20 which will quickly take it out.  It is in the center tap to the 10 volt filament winding and normally sees only a slight imbalance current that results from two imperfectly filament matched tubes.  A friend lost that choke when a Chinese 3-500Z in his 922 went filament open due to a poorly made filament connection in the base pin.

Rodger WQ9E

[K9EID]

Hello Roger and thanks for your reply.  Yes, I do understand running only one tube could cause some damage although I know of fellows that have run with one forbears.... I am not in a hurry as I have a second 922 that I have had since the 80's.  It is a wonderfully engineered product.  I already have a new 3-500Z on the way.   Appreciate your thoughts.

Have a Happy New Year .  Stay safe.    Dr. Bob Heil, K9EID

[Bob K4TAX]

Bob:
I would not run the amp with one tube.  Various reasons stated above.   Although differing opinions on this but both tubes should be replaced.   Since one is old and one is new their emissions will be different.  Due to the metering system, the total grid current is displayed.  Thus one does not know if one tube is drawing 80% of the grid current and the other tube drawing 20%.  Now, the reason for saying this, the grid current dissipation is the most critical element of the 3-500 and the more likely cause of tube failures.    I learned the hard way not to like two tube amps unless properly designed and metered.  The TL-922 is not one of those.

Just saying......your amp, your money.

73
Bob, K4TAX

[WQ9E]

You are welcome Bob and Happy New Year to you also!

And your company builds great products.  My new Yaesu FTDX-101MP arrived yesterday and I dug out the Heil headset/mic I bought for my FT-1000MP for contesting back in 1996 and it is still working perfectly.  It is a great setup and I had forgotten how nicely it works.

Rodger