BC 610 F modulator and audio section

[N5AEA]

My BC 610 F rescue and restore project is now at the audio section.  Need to replace one or both Audio drivers (2A3's).  I don't need tubes with gold grids and pins at a cost of $100 to $500.  So my plan is to use alternate tubes at $12 each.  That will be 1619 type tubes with about the same characteristics and using a socket adaptor.  However, they are not triodes as they have two grids.  To make them act like a 2A3 I need to do something with the screen grid.  So the question is:
A.  Open (NC)
B.  Connect it to the plate.
C.  Connect the two grids together.
D.  None of the above. Will not work at all.
73
Ed

[W3RSW]

They should work well. Determine from what stage gain you need how to hook up the screen depending on whether you want a low mu 'triode' or high mu 'triode'
 -- tie Screen to plate or tie screen and control grid together. Another variation is to use a 20 k or so resistor between the elements.  You'll have fun researching and trying out what is correct and works best.

[w4bfs]

I'm not so sure .... the 1619 operates at twice the load impedance of a 2A3 ... this after a VERY cursory look at the spec sheet ... application of negative feedback may make this operable ... a 2A3 is not an easy critter to replace ... you should be able to find new tubes in the $40 range ....a triode connected 807 is a closer match ... you are already looking at a socket change and bias circuit change, just how far are you willing to re-design?

nb:  just checked retail prices on 2A3 .... wow ... another tube to consider is a 6080 if enough drive swing is available 73

[W3RSW]

The 1619 being a slightly derated 6L6 (15 watts Pd) with a 2.5 volt filament will not have quite the power from mismatched transformer load and I don't think we're talking super Hi-Fi for distortion characteristics. More important for any driver is to be a low and constant current impedance into the modulator grids. 

The 807 though does sound like a better alternative, complete with plate caps on top too. 

[WD5JKO]

   Could add an above board plate to accommodate two 2A3 4-pin tube bases to a quad of Octal sockets. My vote would be a quad of 6W6's in triode connection. Of course adding a 6.3V filament transformer would be necessary. The 6W6 is very linear in triode mode, and a quad of them should get the plate resistance down low enough to be as good or better than a pair of 2A3's. The best part is the 6W6 is a VERY inexpensive tube.

Jim
Wd5JKO

[N5AEA]

Thanks so much to all for the very good ideas.
I selected the 1619 because it does not have a plate cap on top, and also has a 2.5 volt heater.  That way I don't have to pull the audio deck.  At this time I just want to confirm that the modulator has a good chance of working without investing a lot in 2A3's.  Also, if any one has tried the $29 2A3 from China I would like to know how they worked out.  So for now I think I will connect the screen too the plate of the 1619 and watch for smoke.
73
Ed 

[w4bfs]

it has been awhile since I've looked at BC610 driver but as I recall the plates are near ground and the cathode is at -300V from the bias supply ....unusual but fb .... the 2A3 being low u uses about 40 or volts of bias which may not be the correct amount for your low u connected 1619 tube (screen to plate) ....  don't recall if there is a bias adjustment to set idle current of the driver or not .... the whole point of this is that you need some method to be able to check this current BEFORE you goose up the audio ... inadequate conduction current can produce Tesla coil effects in the driver transformer .... not good

[Opcom]

   Could add an above board plate to accommodate two 2A3 4-pin tube bases to a quad of Octal sockets. My vote would be a quad of 6W6's in triode connection. Of course adding a 6.3V filament transformer would be necessary. The 6W6 is very linear in triode mode, and a quad of them should get the plate resistance down low enough to be as good or better than a pair of 2A3's. The best part is the 6W6 is a VERY inexpensive tube.

Jim
Wd5JKO

The 6W6 and 6080 look good. The 6W6 looks really good if Triode connected. The 6080 looks like overkill. Onthing wrong with that. No matter what's done the bias and heater will have to be worked out.
2A3 is tough to replace @ 800 Ohm plate resistance!

[N8ETQ]

Hey Ed,

      You could always just hit the 100TH grids with a backwards
voice coil to line transformer and test the high level part with
an external audio amp.
       Some guys just run them that way all the time. I have a
pair of Sovtek jobs in mine. Back then they were about $25 each.
That was 4-DEC-2005... Still working fine.

73

/Dan

Thanks so much to all for the very good ideas.
I selected the 1619 because it does not have a plate cap on top, and also has a 2.5 volt heater.  That way I don't have to pull the audio deck.  At this time I just want to confirm that the modulator has a good chance of working without investing a lot in 2A3's.  Also, if any one has tried the $29 2A3 from China I would like to know how they worked out.  So for now I think I will connect the screen too the plate of the 1619 and watch for smoke.
73
Ed 

[WB4AIO]

I agree with the suggestion to use a hi-fi audio amp as a driver; that's what I did with my 610 with excellent results. Probably 20 watts will be ample. Hook the speaker output terminals of the audio amp up to a four- to sixteen-ohm winding on an old tube-type audio output transformer connected backwards. The former plate leads on the transformer will go to the grids of the 100THs; the center tap to the bias supply.

I used the 16-ohm tap and loaded it with a 32-ohm resistor for some impedance stabilization; and I also used negative feedback from the modulated B+ back to the second stage of the audio amp. But neither of those two extra steps are necessary.

Most old hi-fi audio output transformers will be significantly better-sounding than the 610's driver transformer.

This also liberates you from the not-so-hot BC-614 speech amp; just use a simple-to-build or cheap-to-buy single stage mic preamp to drive the hi-fi amp. If you like, you can insert an EQ and peak limiter or other processing between the preamp and audio amp too.

73,

Kevin, WB4AIO.

[w1vtp]

Boy! I would NOT be in a hurry to go with a non-2a3 config.  I looked on eBay and there is a 2a3 listed @ $29 ea. with more than 2 available.  Start looking for a pair of 2a3's @ say, <$40 each and consider it a lifetime investment.  There is a good chance they will outlive you.

gl  Al

[N5AEA]

Kevin & Dan provided ideas that I like.  I will do the external audio amp at some time just to see how I like it.  I have a Kenwood KR 6170, 150 watt with Mic inputs, preamps and tone controls. So all I need is the output transformer.  Thought about using a filament transformer just for now.   I have one that has 24 v ct and two 6.3 v windings, 100/125 v pri.  I wonder if that will work?
Thanks for all the help.
73
Ed