[Boatanchors] sweep tubes (was Re: Galaxy V)

[Rodger Singley]

I have also had good luck with sweep tubes having a long life.  I ran a set of Drake C line twins for several years including a lot of contest and DX activity and the tubes still produce normal output without issue.  Properly treated they can work very well and last a long time in transmitter service.  Originally they would have lived in a poorly ventilated TV set running many hours per day so careful operation in amateur service can be much easier on them than their originally intended service.

I have a number of sweep tube rigs varying from a couple of lower power single tube Japanese rigs intended for their local market through the high power Swan, Galaxy, Yaesu, and Trio rigs.  The Kenwood TL-911 2KW PEP amplifier is the ancestor of the well known TL-922 and the 911 is the nicest sweep tube type amp I own.  It uses five 6LQ6 tubes in a modified grounded grid design with the screens grounded for RF but fed with regulated 70 volts for lower distortion on SSB with the control grid biased to -35V, for CW control grid bias is increased to -70 volts for Class C operation.  The cathode current meter has positions to read total current and also individual current for each of the five tubes and there are two adjustments per tube with one balancing idling current between tubes and the other balancing current at full input.  A protect circuit trips instantly for severe overload and also trips if average current is too high.  The final is nicely shielded with a large fan providing directed air flow around the tubes.

For long sweep tube life:

1.        They need a cooling fan and if one wasn’t provided by the manufacturer add one.  If space is available a heat dissipating plate cap will also help.  This cooling leads to longer tube life and can also help reduce the odds of instantaneous failure from a tuning mistake.

2.       Tuning should be done quickly. Use a log/index sheet so that you can preset the controls to near the final setting.  If a reduced power tune position is provided then use it.

3.       A common and serious mistake made with sweep tubes is to load them too lightly trying to reduce power and increase tube life.  Light loading leads to increased distortion but more critically it also causes  screen grid current to soar and in these tubes the screen grid has very little power dissipation capability and is very easy to damage.  All input to the screen is going to be dissipated by that element.  Slightly reducing power is an excellent way to increase tube life and a 10 or even 20 percent power reduction is likely to be unnoticed on the receiving end but it can add tremendously to tube life.  The proper way to do this is to tune and load to rated input and then reduce power via the mic gain/drive/carrier level control depending upon mode.

4.       The power rating for SSB for these rigs is based upon a light duty cycle so trying to run heavy speech processing via an outboard processor will overheat the finals, power supply, and possibly tank components.  Most digital modes, RTTY, etc. are far worse and you must severely de-rate the rig if running high duty cycle modes.  In many cases the safe high duty cycle power rating is going to be 30% or less of rated PEP.

5.       When putting a new rig into service or repairing a rig make sure that the final plate caps are secure and there are no bad/broken connections where these connect to the RF choke/HV supply feeding them.  If power is applied and the plate doesn’t receive voltage then the screen grid acts as the plate and it will momentarily turn incandescent before vaporizing and turning your tube into a non-functional version with one less grid than it had when it left the factory.

Sweep tubes aren’t cheap anymore but they really will last a long time if not abused.

Rodger WQ9E

Sent from Mail<https://go.microsoft.com/fwlink/?LinkId=550986> for Windows 10

From: Dennis DuVall<mailto:duvallddennis at gmail.com>
Sent: Thursday, September 15, 2016 12:15 PM
To: Marvin Match<mailto:mvmatch at ece.utah.edu>
Cc: boatanchors at mailman.qth.net<mailto:boatanchors at mailman.qth.net>
Subject: Re: [Boatanchors] sweep tubes (was Re: Galaxy V)

With reasonable care sweep tubes can last a long time.  The original pair of 6JS6C sweeps in my Yaesu FT-101B are still going strong after 40 years. I suspect most failures occurred due to
maltreatment such as holding the key down while thumbing through the manual looking for the next tune-up step.  Saw stuff like that happening at a club station particularly with younger members.

Dennis D.  W7QHO
Glendale, CA

****************