Tim Shoppa wrote:
cliff wright wrote:

wrote:

On Sep 20, 4:38 pm, cliff wright wrote:


Hi guys!
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need a
bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?
Regards Cliff Wright ZL1BDA ex G3NIA.


i'm considering to go for the same do u hav any ckt idea as well as
documents to create my own as of now i quiet new to this field dear
cliff wright.....


Just an update for those who might be interested.
It turns out that my reservations about valve (tube) Testers were proved
to be correct.
I used a single 6883b in the final amp and got the following very
interesting results.
Tube 1 for a dc input of 84 watts the RF output was a miserable 25 watts
measured with a calibrated Marconi RF power meter.
Tube 2 for a Dc input of 63 watts the RF power was the same, vis 25 watts.
25 Watts is the maximum power rating of the Marconi test equipment.

So it is very obvious that damaged or low emission finals can have the
effect of drastically reducing the efficiency of the Power stage.
Fortunately I hope to source some more 6883B's or 6146B's this weekend.

So if your transmitter gets "feeble" but the Dc input remains the same
suspect your finals. Especially if it tends to red hot anodes!!!


Red hot anodes still doesn't sound right, Cliff. Did you have to turn
the bias pot to any unusual positions to get them to glow like that?

Weak emission tubes just don't draw much current, and even if I
cranked the bias current up to normal while their output would be
feeble I generally find their plates to be sucking current anemically
when they're low emission. Now, gassy tubes, especially if there is
enough grid current to upset the grid bias, that is a problem I've
been having with NOS tubes lately.

What's the grid voltage when you're done biasing? For my "good"
6146B's, it's almost always -50V in AB1. -75V or so is class C. If you
are setting the bias to something closer to -30V or -40V, I think
you're getting closer and closer to class A.

I remember spending a few weeks allowance on some known good 6146B's
when I was a kid. Are known good final tubes that hard to come by down
under? I'm starting to feel like I should send you some of mine just
to help you in your struggles :-). Besides, I'm mostly using 807's
here except for my Eico 720 these days!

Tim.


Hi Tim.
Yes the bias is correct as are all the other electrode voltages. I think
that you are correct in thinking that the tubes are no longer "Vacuum
Tubes" but more akin to 1920's soft types.
I'm pretty sure that the problem originally began with a loose ground
connection on the UHF antenna output cable.
This meant that the final was running into a terrible mismatch.
BTW over my 50 years+ as a Ham I've built several linear amps, including
ones using 811A's, 807's, 6146B's and even TT21's. I'm still of the
opinion that all these transceivers with class AB1 linears need a meter
position to measure grid current. In my experience the best way by far
to ensure proper operation in AB1 is to keep it from flowing!
As to relacements, well I hope to get some from a local "junk sale"
on Saturday next. The 6883B's have 12 volt heaters and were often used
in old local AM and SSB mobile radios by small ships and the forest
service. That means that there are quite a few around in NZ. Just as well!
The Kenwood is designed for a Japanese equivalent to the 6146B but a
simple mod to the heater wiring on the final PC board makes it possible
to use 6.3 volt or 12 volt heater tubes.
&#'s Cliff Wright ZL1BDA ex G3NIA

On Oct 9, 10:53 pm, cliff wright wrote:
TimShoppawrote:
cliff wright wrote:

wrote:

On Sep 20, 4:38 pm, cliff wright wrote:

Hi guys!
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need a
bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?
Regards Cliff Wright ZL1BDA ex G3NIA.

i'm considering to go for the same do u hav any ckt idea as well as
documents to create my own as of now i quiet new to this field dear
cliff wright.....

Just an update for those who might be interested.
It turns out that my reservations about valve (tube) Testers were proved
to be correct.
I used a single 6883b in the final amp and got the following very
interesting results.
Tube 1 for a dc input of 84 watts the RF output was a miserable 25 watts
measured with a calibrated Marconi RF power meter.
Tube 2 for a Dc input of 63 watts the RF power was the same, vis 25 watts.
25 Watts is the maximum power rating of the Marconi test equipment.

So it is very obvious that damaged or low emission finals can have the
effect of drastically reducing the efficiency of the Power stage.
Fortunately I hope to source some more 6883B's or 6146B's this weekend.

So if your transmitter gets "feeble" but the Dc input remains the same
suspect your finals. Especially if it tends to red hot anodes!!!

Red hot anodes still doesn't sound right, Cliff. Did you have to turn
the bias pot to any unusual positions to get them to glow like that?

Weak emission tubes just don't draw much current, and even if I
cranked the bias current up to normal while their output would be
feeble I generally find their plates to be sucking current anemically
when they're low emission. Now, gassy tubes, especially if there is
enough grid current to upset the grid bias, that is a problem I've
been having with NOS tubes lately.

What's the grid voltage when you're done biasing? For my "good"
6146B's, it's almost always -50V in AB1. -75V or so is class C. If you
are setting the bias to something closer to -30V or -40V, I think
you're getting closer and closer to class A.

I remember spending a few weeks allowance on some known good 6146B's
when I was a kid. Are known good final tubes that hard to come by down
under? I'm starting to feel like I should send you some of mine just
to help you in your struggles :-). Besides, I'm mostly using 807's
here except for my Eico 720 these days!

Tim.

Hi Tim.
Yes the bias is correct as are all the other electrode voltages. I think
that you are correct in thinking that the tubes are no longer "Vacuum
Tubes" but more akin to 1920's soft types.
I'm pretty sure that the problem originally began with a loose ground
connection on the UHF antenna output cable.
This meant that the final was running into a terrible mismatch.

"Yeahbut" the tubes probably took that mismatch in style.

It was probably another failure caused by the mismatch that started
the cascade of problems. I'm guessing an arcover in the plate blocking
capacitor and/or the final choke. After that happens I could easily
believe the tubes glowing a nice bright blue inside and other stuff
going bad (usually visibly and stenchily but not always) in the final
compartment.

An -820 is in my head still an "almost new" radio, BTW :-). I am
willing to believe some grounds failing somewhere. If I saw a burnt-
through PL-259 or piece of coax I'd be willing to believe a bad ground
in a cable.

You will note that you keep on coming back to the tubes... and I keep
on fingering other stuff in the final compartment.

My gut feeling is that if you can get the tubes to draw enough current
to make their plates glow a bright cherry red, then it's hard to blame
tube emission as being deficient. You've still got something else
going on that's preventing you from loading up nicely, and while
you've traced out some problems with your DC metering already I'm not
convinced that those were the only problems in the final compartment.

Tim.

On Oct 9, 10:53 pm, cliff wright wrote:
TimShoppawrote:
cliff wright wrote:

wrote:

On Sep 20, 4:38 pm, cliff wright wrote:

Hi guys!
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need a
bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?
Regards Cliff Wright ZL1BDA ex G3NIA.

i'm considering to go for the same do u hav any ckt idea as well as
documents to create my own as of now i quiet new to this field dear
cliff wright.....

Just an update for those who might be interested.
It turns out that my reservations about valve (tube) Testers were proved
to be correct.
I used a single 6883b in the final amp and got the following very
interesting results.
Tube 1 for a dc input of 84 watts the RF output was a miserable 25 watts
measured with a calibrated Marconi RF power meter.
Tube 2 for a Dc input of 63 watts the RF power was the same, vis 25 watts.
25 Watts is the maximum power rating of the Marconi test equipment.

So it is very obvious that damaged or low emission finals can have the
effect of drastically reducing the efficiency of the Power stage.
Fortunately I hope to source some more 6883B's or 6146B's this weekend.

So if your transmitter gets "feeble" but the Dc input remains the same
suspect your finals. Especially if it tends to red hot anodes!!!

Red hot anodes still doesn't sound right, Cliff. Did you have to turn
the bias pot to any unusual positions to get them to glow like that?

Weak emission tubes just don't draw much current, and even if I
cranked the bias current up to normal while their output would be
feeble I generally find their plates to be sucking current anemically
when they're low emission. Now, gassy tubes, especially if there is
enough grid current to upset the grid bias, that is a problem I've
been having with NOS tubes lately.

What's the grid voltage when you're done biasing? For my "good"
6146B's, it's almost always -50V in AB1. -75V or so is class C. If you
are setting the bias to something closer to -30V or -40V, I think
you're getting closer and closer to class A.

I remember spending a few weeks allowance on some known good 6146B's
when I was a kid. Are known good final tubes that hard to come by down
under? I'm starting to feel like I should send you some of mine just
to help you in your struggles :-). Besides, I'm mostly using 807's
here except for my Eico 720 these days!

Tim.

Hi Tim.
Yes the bias is correct as are all the other electrode voltages. I think
that you are correct in thinking that the tubes are no longer "Vacuum
Tubes" but more akin to 1920's soft types.

Actually Cliff I'm not really convinced the tubes are bad. If you
trace out the DC plate and screen current with respect to grid voltage
(I'm sure you're very familiar with the bias pot by now) I think you
might convince yourself this isn't a soft tube problem.

If you can momentarily change the grid bias from -50V to -20V or so
and see the plate current go from fifteen-twenty mA up to 200mA, then
I'm sure that the tubes aren't soft. Keep the plate current at 200mA
for more than ten or so seconds, though, and the anodes might start
melting, so this is purely a momentary exercise!

Tim.

Tim Shoppa wrote:
On Oct 9, 10:53 pm, cliff wright wrote:

TimShoppawrote:

cliff wright wrote:

wrote:

On Sep 20, 4:38 pm, cliff wright wrote:

Hi guys!
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need a
bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?
Regards Cliff Wright ZL1BDA ex G3NIA.

i'm considering to go for the same do u hav any ckt idea as well as
documents to create my own as of now i quiet new to this field dear
cliff wright.....

Just an update for those who might be interested.
It turns out that my reservations about valve (tube) Testers were proved
to be correct.
I used a single 6883b in the final amp and got the following very
interesting results.
Tube 1 for a dc input of 84 watts the RF output was a miserable 25 watts
measured with a calibrated Marconi RF power meter.
Tube 2 for a Dc input of 63 watts the RF power was the same, vis 25 watts.
25 Watts is the maximum power rating of the Marconi test equipment.

So it is very obvious that damaged or low emission finals can have the
effect of drastically reducing the efficiency of the Power stage.
Fortunately I hope to source some more 6883B's or 6146B's this weekend.

So if your transmitter gets "feeble" but the Dc input remains the same
suspect your finals. Especially if it tends to red hot anodes!!!

Red hot anodes still doesn't sound right, Cliff. Did you have to turn
the bias pot to any unusual positions to get them to glow like that?

Weak emission tubes just don't draw much current, and even if I
cranked the bias current up to normal while their output would be
feeble I generally find their plates to be sucking current anemically
when they're low emission. Now, gassy tubes, especially if there is
enough grid current to upset the grid bias, that is a problem I've
been having with NOS tubes lately.

What's the grid voltage when you're done biasing? For my "good"
6146B's, it's almost always -50V in AB1. -75V or so is class C. If you
are setting the bias to something closer to -30V or -40V, I think
you're getting closer and closer to class A.

I remember spending a few weeks allowance on some known good 6146B's
when I was a kid. Are known good final tubes that hard to come by down
under? I'm starting to feel like I should send you some of mine just
to help you in your struggles :-). Besides, I'm mostly using 807's
here except for my Eico 720 these days!

Tim.

Hi Tim.
Yes the bias is correct as are all the other electrode voltages. I think
that you are correct in thinking that the tubes are no longer "Vacuum
Tubes" but more akin to 1920's soft types.


Actually Cliff I'm not really convinced the tubes are bad. If you
trace out the DC plate and screen current with respect to grid voltage
(I'm sure you're very familiar with the bias pot by now) I think you
might convince yourself this isn't a soft tube problem.

If you can momentarily change the grid bias from -50V to -20V or so
and see the plate current go from fifteen-twenty mA up to 200mA, then
I'm sure that the tubes aren't soft. Keep the plate current at 200mA
for more than ten or so seconds, though, and the anodes might start
melting, so this is purely a momentary exercise!

Tim.

Hi Tim. Well some more to consider!
However I am at a loss to explain it through any final Tank circuit or
switch problems. I have had the final assembly all apart twice now and
gone through every component. Fotunately I have a reasonably accurate
digital LCR bridge to even check the inductor values. All sems fine.
The antenna T/R relay was another suspect, but this too checks out OK.

The big "Tube" thing of course is that my 2 examples of 6883B's give
such a difference in efficiency. one giving 25 watts out for 84 watts in
and the other 25 watts out for 62 watts. Neither of these tubes are new
or of known provenance, though they both are alledgedly OK on my Heath
TC-3 valve tester. This is with one final operating in Kenwood's "novice
mode". I probably didn't make this clear in my recent posting.
Since the final heaters are now in parallel I can very readily run this
mode whenever I want too.
The fact that they are both inefficient, but to widely different degrees
keeps bringing me back to the tubes being the problem.

Based on my experience with AB1 linears that I've built over the years
I'd expect an output of ~55 to 60 watts with a ~90 watt DC input.
My old parallel TT21 (KT88 with tc Anode) linear was 67% efficient on 14
MHz measured with state of the art test gear in the mid 1970's.
Now from QST and other sources I know that the losses in the Pi network
on the 820S are greater than those I obtained in my "home brew" finals,
but even so I would expect ~60% efficiency if all is well.

I have tried bias changes BTW and the odd thing is that the plate
current tends to rise after a short interval. This brings to my mind the
possibilty of some kind of secondary emission in the 6883B's as the
electrodes heat up. But so far the mystery remains.

In any event I hope to come home at lunchtime this Saturday with a
collection of either 6146B's or 6883B's (or both) and finally prove the
point! I will post a message as soon as I have more data.
73's Cliff Wright ZL1BDA ex G3NIA.

Tim Shoppa wrote:
On Oct 9, 10:53 pm, cliff wright wrote:

TimShoppawrote:

cliff wright wrote:

wrote:

On Sep 20, 4:38 pm, cliff wright wrote:

Hi guys!
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need a
bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?
Regards Cliff Wright ZL1BDA ex G3NIA.

i'm considering to go for the same do u hav any ckt idea as well as
documents to create my own as of now i quiet new to this field dear
cliff wright.....

Just an update for those who might be interested.
It turns out that my reservations about valve (tube) Testers were proved
to be correct.
I used a single 6883b in the final amp and got the following very
interesting results.
Tube 1 for a dc input of 84 watts the RF output was a miserable 25 watts
measured with a calibrated Marconi RF power meter.
Tube 2 for a Dc input of 63 watts the RF power was the same, vis 25 watts.
25 Watts is the maximum power rating of the Marconi test equipment.

So it is very obvious that damaged or low emission finals can have the
effect of drastically reducing the efficiency of the Power stage.
Fortunately I hope to source some more 6883B's or 6146B's this weekend.

So if your transmitter gets "feeble" but the Dc input remains the same
suspect your finals. Especially if it tends to red hot anodes!!!

Red hot anodes still doesn't sound right, Cliff. Did you have to turn
the bias pot to any unusual positions to get them to glow like that?

Weak emission tubes just don't draw much current, and even if I
cranked the bias current up to normal while their output would be
feeble I generally find their plates to be sucking current anemically
when they're low emission. Now, gassy tubes, especially if there is
enough grid current to upset the grid bias, that is a problem I've
been having with NOS tubes lately.

What's the grid voltage when you're done biasing? For my "good"
6146B's, it's almost always -50V in AB1. -75V or so is class C. If you
are setting the bias to something closer to -30V or -40V, I think
you're getting closer and closer to class A.

I remember spending a few weeks allowance on some known good 6146B's
when I was a kid. Are known good final tubes that hard to come by down
under? I'm starting to feel like I should send you some of mine just
to help you in your struggles :-). Besides, I'm mostly using 807's
here except for my Eico 720 these days!

Tim.

Hi Tim.
Yes the bias is correct as are all the other electrode voltages. I think
that you are correct in thinking that the tubes are no longer "Vacuum
Tubes" but more akin to 1920's soft types.


Actually Cliff I'm not really convinced the tubes are bad. If you
trace out the DC plate and screen current with respect to grid voltage
(I'm sure you're very familiar with the bias pot by now) I think you
might convince yourself this isn't a soft tube problem.

If you can momentarily change the grid bias from -50V to -20V or so
and see the plate current go from fifteen-twenty mA up to 200mA, then
I'm sure that the tubes aren't soft. Keep the plate current at 200mA
for more than ten or so seconds, though, and the anodes might start
melting, so this is purely a momentary exercise!

Tim.

Hi Tim. Well some more to consider!
However I am at a loss to explain it through any final Tank circuit or
switch problems. I have had the final assembly all apart twice now and
gone through every component. Fotunately I have a reasonably accurate
digital LCR bridge to even check the inductor values. All sems fine.
The antenna T/R relay was another suspect, but this too checks out OK.

The big "Tube" thing of course is that my 2 examples of 6883B's give
such a difference in efficiency. one giving 25 watts out for 84 watts in
and the other 25 watts out for 62 watts. Neither of these tubes are new
or of known provenance, though they both are alledgedly OK on my Heath
TC-3 valve tester. This is with one final operating in Kenwood's "novice
mode". I probably didn't make this clear in my recent posting.
Since the final heaters are now in parallel I can very readily run this
mode whenever I want too.
The fact that they are both inefficient, but to widely different degrees
keeps bringing me back to the tubes being the problem.

Based on my experience with AB1 linears that I've built over the years
I'd expect an output of ~55 to 60 watts with a ~90 watt DC input.
My old parallel TT21 (KT88 with tc Anode) linear was 67% efficient on 14
MHz measured with state of the art test gear in the mid 1970's.
Now from QST and other sources I know that the losses in the Pi network
on the 820S are greater than those I obtained in my "home brew" finals,
but even so I would expect ~60% efficiency if all is well.

I have tried bias changes BTW and the odd thing is that the plate
current tends to rise after a short interval. This brings to my mind the
possibilty of some kind of secondary emission in the 6883B's as the
electrodes heat up. But so far the mystery remains.

In any event I hope to come home at lunchtime this Saturday with a
collection of either 6146B's or 6883B's (or both) and finally prove the
point! I will post a message as soon as I have more data.
73's Cliff Wright ZL1BDA ex G3NIA.

Tim Shoppa wrote:
On Oct 9, 10:53 pm, cliff wright wrote:

TimShoppawrote:

cliff wright wrote:

wrote:

On Sep 20, 4:38 pm, cliff wright wrote:

Hi guys!
Am still going over the old TS-820 and found another problem re the
meter. It is important when replacing the meter to check the condition
of the multiplying resistors on the underchassis "HT" board and adjust
to suit the actual sensitivity of the meter.
Mine was reading 730 volts off load for final anode voltage but my
trusty old Avometer tells me that it was actually 1,040 volts.
Now this is WAY above the recommended anode voltage for even a 6146B
but since the tansformer secondary is 800 volts its probably correct.
I've now got the voltage reading correctly on the meter, but I'm now
very convinced that the reason for flashover in some 6146's is simply
excessive HT voltage. The finals are running in very dangerous
territory. Just a bit of SWR then Poof!!!
I'm now considering modifying the case and using a TT21 as a final.
These can stand the voltage with ease, are very easy to neutralise
and can give a good 65% efficiency in AB1 up to 30 Mhz. Will give me
around 80 watts out I reckon.
Just that my final box will be a bit taller and the old case will need a
bit of chopping about.
But then its an old rig and it might as well work properly, even
modified. Anyone elese tried major final tube changes in the Kenwood
hybrids?
Regards Cliff Wright ZL1BDA ex G3NIA.

i'm considering to go for the same do u hav any ckt idea as well as
documents to create my own as of now i quiet new to this field dear
cliff wright.....

Just an update for those who might be interested.
It turns out that my reservations about valve (tube) Testers were proved
to be correct.
I used a single 6883b in the final amp and got the following very
interesting results.
Tube 1 for a dc input of 84 watts the RF output was a miserable 25 watts
measured with a calibrated Marconi RF power meter.
Tube 2 for a Dc input of 63 watts the RF power was the same, vis 25 watts.
25 Watts is the maximum power rating of the Marconi test equipment.

So it is very obvious that damaged or low emission finals can have the
effect of drastically reducing the efficiency of the Power stage.
Fortunately I hope to source some more 6883B's or 6146B's this weekend.

So if your transmitter gets "feeble" but the Dc input remains the same
suspect your finals. Especially if it tends to red hot anodes!!!

Red hot anodes still doesn't sound right, Cliff. Did you have to turn
the bias pot to any unusual positions to get them to glow like that?

Weak emission tubes just don't draw much current, and even if I
cranked the bias current up to normal while their output would be
feeble I generally find their plates to be sucking current anemically
when they're low emission. Now, gassy tubes, especially if there is
enough grid current to upset the grid bias, that is a problem I've
been having with NOS tubes lately.

What's the grid voltage when you're done biasing? For my "good"
6146B's, it's almost always -50V in AB1. -75V or so is class C. If you
are setting the bias to something closer to -30V or -40V, I think
you're getting closer and closer to class A.

I remember spending a few weeks allowance on some known good 6146B's
when I was a kid. Are known good final tubes that hard to come by down
under? I'm starting to feel like I should send you some of mine just
to help you in your struggles :-). Besides, I'm mostly using 807's
here except for my Eico 720 these days!

Tim.

Hi Tim.
Yes the bias is correct as are all the other electrode voltages. I think
that you are correct in thinking that the tubes are no longer "Vacuum
Tubes" but more akin to 1920's soft types.


Actually Cliff I'm not really convinced the tubes are bad. If you
trace out the DC plate and screen current with respect to grid voltage
(I'm sure you're very familiar with the bias pot by now) I think you
might convince yourself this isn't a soft tube problem.

If you can momentarily change the grid bias from -50V to -20V or so
and see the plate current go from fifteen-twenty mA up to 200mA, then
I'm sure that the tubes aren't soft. Keep the plate current at 200mA
for more than ten or so seconds, though, and the anodes might start
melting, so this is purely a momentary exercise!

Tim.



Well Folks, I got a collection of 6146B's, used unfortunately, and a
small number of S 2001's the original tubes supplied by Kenwood.
Now this is VERY interesting!
The 6146B's apparently have to be VERY closely matched or one tube draws
most of the current. However the S2001's, which seem to have a slightly
lower gain, judging by the drive level required, were much more stable
and happily produced almost 100 watts output on CW without noticeable
distress! This was despite their characteristics being about 10% different.
In my research I had come across those who had had similar experiences
with paralleled 6146B's. Now it occurs to me that the cathodes of both
finals are directly strapped together with a small resistance to ground.
In the case of mismatched tubes perhaps this could be a source of problems.
However at least it is going quite well now. The mystery is still not
quite solved, but as time permits I'll go into it further.
Regards Cliff Wright ZL1BDA ex G3NIA.