8950 tube ??

[nomadradio]

Okay, RJB, I'm gonna suggest you start a whole new thread for this linear problem.

Below is pasted the PM you sent about this. Right there it answers the question I just asked above, about the relay going "click".

Hi
I was given a Pal 351BDX and told that the tubes were changed from a 8950 to 6JM6A and the caps needed to be
replaced (4) 30uf 500v... So i replaced the caps ..... but the problem is in Xmit mode i don't get any output signal if the amp is in standby mode there is an output of only the 5w of the cb radio...
any help please thanks
RJB 73

So we know the relay is going "click" since the radio's wattage that passes through the amplifier on standby goes away when it's in operate mode.

We still don't know why the tubes are not amplifying. But we know the relay is going "click". Might start with cleaning the contacts. Cutting a strip of paper you can thread between the contact points, getting it wet with a solvent, and drawing the paper under the contacts while you gently press down on them is a start.

And if you see a streak of dark schmoo on the paper aftwerwards, it was time.

Just don't use any kind of abrasive. Good chance this would damage the contact points on a small relay.

Might not help, but won't hurt.

73

 

[Tokin]

hi everyone,
I was give a PAL 351 BDX and the tubes were swapped out from 8950 to 6JM6A
the problem that i am have is i can't get the amp to amplify in xmit mode. i hope someone can help me

Welcome to the forum! As NomadRadio said it would be best to start a new thread on your problem at hand. Makes it easier to find info in the future.
Something else i know folks here helpful in trying to diagnose issues is pictures.
For cleaning contacts and switches ive been using Deoxit Gold series. An old trick is to use a crisp dollar bill to drag thru the relay contacts. Its tougher than paper and less likely to tear.

 

[RJB]

ok thanks Tokin and Nomadio

 

[Danzik]

To convert from a 12v filament tube to a 6 volt filament tube two of the 6 volt will be wired in series.

6+6=12

Two 6lb6 will use 12v for filament
One 8950 will use 12 volt for filament.

Since there is an odd number of tubes, not even. You will need to keep one 8950 at 12v filament.

then the other eight 6lb6 can be wired in series. Using two 6lb6 wired together for 12v filament from transformer.

Hope that did not confuse you as I re read it may be confusing.

I would sell the 8950's and then you would have enough $$ to buy 6lb6 to last a lifetime for that amp.

Pin 1 and pin 12 are heaters.
8950 pin 1 will have 12v and pin 12 will be at ground.

to make it work for 6lb6 two tubes are required.
Pin one will have 12 volts on tube A.
then on tube A pin 12 jumper it to pin 1 of tube B.
Pin 12 of tube B will go to ground.

I would put a cap at tube A pin one to ground for RF bypass.

hope that clarifies it better.

If you are unsure of how to do this modification then please take it to your technician.

Hi, I am also wanting to do this conversion and have a Siltronix LA-550 and I am just slightly confused and or concerned on wiring the 6lb6's in a series, could you possibly help? The brown wire in the photo is the 12v and it is going to the first tube on pin #12, then it goes to tube 2 on pin#1, then to the 3rd tube on pin #1 again then on the 4th tube to pin#12 ....... so to do the modification wouldn't I just have to work on the top 2 tube sockets and move the brown wire from #1 pin and place it on #12 pin on the top right socket in the photo, then move the wire on the other top left socket to pin#1????? Also as you see in the photo there are the ceramic caps that tie pins 1 and 12 together and go through a ground, wont these need to be removed and turned around and soldered back in ??? Thank you and I hope you or somebody can clear this up for me

 

[Dmans]

Not an easy way to type this out but I will try.

For 12v filaments, all 4 tubes will have a single 12v wire to pin1 (or 12-doesn't matter). The pin without the 12v feed will be grounded.

In your picture above, it appears the 12v feed starts at the lower left tube in the picture. The other 3 tubes are just "daisy chained" in the circuit.

For 6v filaments (from a 12v feed) tie Pin 1 (after removing the ground tie-via the capacitor soldered to the rim of the tube socket) of the lower left hand tube (in your picture above) to Pin 1 of the lower right hand tube (in your picture above). Pin 12 of the lower right hand tube is grounded (as pin 1 was) with a capacitor tied to ground at the tube socket.

Repeat the process on the 2 upper tubes (in your picture above) and you are done. You can feed your 12 v from Pin 12 (of the lower left hand tube in your picture above) You will need to add a wire at Pin 12 to feed the 2 upper tubes.

Two 6v tubes in series = 12v.
Tube 1---12v feeding Pin 12-filament-Pin 1 will measure 6 volts.
Tube 2---6v (from Pin 1 of tube 1) to Pin 12-filament-Pin 1 to ground. You will measure 0 volts on Pin 1 of tube 2.

:Edited to add the magnificent artwork!!
I hope this is not too confusing.

73
David

 

[Danzik]

Not an easy way to type this out but I will try.

For 12v filaments, all 4 tubes will have a single 12v wire to pin1 (or 12-doesn't matter). The pin without the 12v feed will be grounded.

In your picture above, it appears the 12v feed starts at the lower left tube in the picture. The other 3 tubes are just "daisy chained" in the circuit.

For 6v filaments (from a 12v feed) tie Pin 1 (after removing the ground tie-via the capacitor soldered to the rim of the tube socket) of the lower left hand tube (in your picture above) to Pin 1 of the lower right hand tube (in your picture above). Pin 12 of the lower right hand tube is grounded (as pin 1 was) with a capacitor tied to ground at the tube socket.

Repeat the process on the 2 upper tubes (in your picture above) and you are done. You can feed your 12 v from Pin 12 (of the lower left hand tube in your picture above) You will need to add a wire at Pin 12 to feed the 2 upper tubes.

Two 6v tubes in series = 12v.
Tube 1---12v feeding Pin 12-filament-Pin 1 will measure 6 volts.
Tube 2---6v (from Pin 1 of tube 1) to Pin 12-filament-Pin 1 to ground. You will measure 0 volts on Pin 1 of tube 2.
View attachment 61019


:Edited to add the magnificent artwork!!
I hope this is not too confusing.

73
David

Hey man, I really appreciate that ! Thanks a ton ! One last question, am I supposed to remove all those caps that are ground tied if they are in pin 1 or 12 on all 4 sockets ?

 

[Danzik]

Not an easy way to type this out but I will try.

For 12v filaments, all 4 tubes will have a single 12v wire to pin1 (or 12-doesn't matter). The pin without the 12v feed will be grounded.

In your picture above, it appears the 12v feed starts at the lower left tube in the picture. The other 3 tubes are just "daisy chained" in the circuit.

For 6v filaments (from a 12v feed) tie Pin 1 (after removing the ground tie-via the capacitor soldered to the rim of the tube socket) of the lower left hand tube (in your picture above) to Pin 1 of the lower right hand tube (in your picture above). Pin 12 of the lower right hand tube is grounded (as pin 1 was) with a capacitor tied to ground at the tube socket.

Repeat the process on the 2 upper tubes (in your picture above) and you are done. You can feed your 12 v from Pin 12 (of the lower left hand tube in your picture above) You will need to add a wire at Pin 12 to feed the 2 upper tubes.

Two 6v tubes in series = 12v.
Tube 1---12v feeding Pin 12-filament-Pin 1 will measure 6 volts.
Tube 2---6v (from Pin 1 of tube 1) to Pin 12-filament-Pin 1 to ground. You will measure 0 volts on Pin 1 of tube 2.
View attachment 61019


:Edited to add the magnificent artwork!!
I hope this is not too confusing.

73
David

I guess what I am asking is if those caps connections on pins 1 0r 12 that are grounded are supposed to be removed completely or just off the pin because they run across other pins in the tube also

 

[jtrouter]

Very good advice, if you are not sure how to proceed, you can wind up doing damage to the amp, or your new tubes, or you, these old amps have lethal voltages in them.

73
Jeff

The 8950 tubes are rated at 33 watts a pop and the 6lb6 tubes are rated at 30 each.
I have converted a few Palomar Skippers and the difference between the two is not noticeable at all.

 

[Dmans]

One decoupling cap at the "hot" end where 12 Volts feeds the two series-connected heaters, plus one at the jumper point between them should be fine.

Copied from nomadradio's post #23 of this thread.
The decoupling capacitor should be in parallel to the 12v feed of your first tube and in parallel to the (now) 6v feed of the second tube in the series pair.
My drawing above does not show the correct configuration of the decoupling capacitor. It is drawn in series with the voltage feed.
The pin 1 or pin 12 lug you feed the 12v to will have two connections on the lug. One end of the decoupling capacitor, and one 12v wire feed(or 6v if it is the 2nd tube in the series). The other end of the decoupling capacitor will go to ground.

73
David

 

[Danzik]

Copied from nomadradio's post #23 of this thread.
The decoupling capacitor should be in parallel to the 12v feed of your first tube and in parallel to the (now) 6v feed of the second tube in the series pair.
My drawing above does not show the correct configuration of the decoupling capacitor. It is drawn in series with the voltage feed.
The pin 1 or pin 12 lug you feed the 12v to will have two connections on the lug. One end of the decoupling capacitor, and one 12v wire feed(or 6v if it is the 2nd tube in the series). The other end of the decoupling capacitor will go to ground.

73
David

Ok, I do not know much about these decoupling capacitors to be honest I just know that they act as a filter to keep the DC noise out of the AC current when keying the mic, so with that being said what are the values of the decoupling capacitors that I will be needing? I see in the schematic it says 001 500v ? So is that like uf or farad or what I am kinda lost there, also will these have a polarity I will have to go by and install the correct direction like a regular capacitor? Sorry as I am learning here

 

[Dmans]

The value is not critical. There is no polarity on a ceramic capacitor and you can probably use the ones already in the amp.
For example your lower left tube in the picture you posted above has a .002 uF (microfarad) capacitor tied to pin 12, pin 1 and ground. Lift the soldered connection at pin 1 and leave the solder connection to ground. Then tie a wire from pin 1 to the 2nd tube in your first series pair. Repeat the process on the upper 2 tubes in your picture.

73
David

 

[Danzik]

The value is not critical. There is no polarity on a ceramic capacitor and you can probably use the ones already in the amp.
For example your lower left tube in the picture you posted above has a .002 uF (microfarad) capacitor tied to pin 12, pin 1 and ground. Lift the soldered connection at pin 1 and leave the solder connection to ground. Then tie a wire from pin 1 to the 2nd tube in your first series pair. Repeat the process on the upper 2 tubes in your picture.

73
David

I got the job all finished and it turned out well but the units keying relay is not working, I also notice a buzzing sound and it sounds like it is coming from my driver tube and I see a very slight blue color in the bottom of the tube, I have 5 tubes toltal so I am using a 8905 tube for the driver,,, this is ok isn't it ? The rest are the 6 volt tubes

 

[jtrouter]

I got the job all finished and it turned out well but the units keying relay is not working, I also notice a buzzing sound and it sounds like it is coming from my driver tube and I see a very slight blue color in the bottom of the tube, I have 5 tubes toltal so I am using a 8905 tube for the driver,,, this is ok isn't it ? The rest are the 6 volt tubes

Im not a expert but seeing blue does not mean he tube is bad, it is not uncommon and they can still be good. But i do not see a 8905 in my tube data sheets?

 

[Danzik]

Im not a expert but seeing blue does not mean he tube is bad, it is not uncommon and they can still be good. But i do not see a 8905 in my tube data sheets?

8950 I meant sorry

 

[Danzik]

8950 I meant sorry

And the tube is NOS

Im not a expert but seeing blue does not mean he tube is bad, it is not uncommon and they can still be good. But i do not see a 8905 in my tube data sheets?