Different audio amp for Bc-610h

[ve8xj]

Though I am fairly satisfied with the B+W BC-614h I can't help but think that one could do better. After all it was designed for military communications audio in mind.

I was thinking about getting away from tubes all together for this but am open for suggestions .

Questions like ;

  How many watts of audio would I need to drive the 2A3's?

I imagine 15 watts or so would be plenty ,but I am not sure.

I found this on EBAY but I suspect it would be way too powerful . Would something like this but in a less powerful package be ideal? Maybe something like this could be used to directly bypass the 2A3 stage and go write to the 100th's through a coupling transformer.I like the idea of the built in mixer and the ability such as this one of  being able to add more to the audio chain.

http://www.ebay.com/itm/110767730141?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_3944wt_974


  What impedance would be best to feed the 2A3's? 600ohms? Or if one was to directly feed the 100th's what impedance ?

Has anyone played with this before and if so what did you use ?



 

[W2PFY]

I use a backwards configured 60 watt transformer driven by a 100 watt amplifier. I use the 16 ohm tap to limit HV kick back. It works great and people say it sounds great. The plate to plate impedance of my transformer is 6000 ohms but I suspect it could be lower. I think you should use at least a 60 watt amplifier so you will have plenty of head room. That eBay amp looks nice. Thanks for posting that link. I may buy one and use it instead of my present amplifier. I am using a BC-610D at this time.

[k4kyv]

It doesn't take much power to drive the 2A3s. They operate in class AB1.  I use a quad of 2A3s to drive my 810 modulator, and for a long time drove them with a pair of small triodes (type 37), similar to a pair of 6C5s or 6J5s, or both sections of a 12AU7 in push-pull. Then I discovered that the broadcast peak limiter I use has enough 500-ohm output to drive the grids of 2A3 driver stage directly, so I took the line amplifier out of the circuit, eliminating a couple of transformers in the audio chain.

The BC-610 input transformer to the 2A3s is already set up for balanced 500/600-ohm line input.

The 2A3 driver circuit in the 610 is an integral part of the modulator bias system, so completely replacing the driver stage would require completely re-designing the bias supply as well.  You need a pair of a low-plate resistance tubes and an appropriate driver transformer for the class-B 100TH modulator.  Since you already have all that in the 610, I wouldn't fool with something JS like a speaker amplifier working into a backward-connected output transformer. Without substantial negative feedback, a pair of screen grid tubes like 6L6s, 6V6s (which have a high plate resistance) will not drive a class-B modulator without distortion.

A speaker type amplifier loaded down into an 8-ohm or 16-ohm swamping resistor should provide sufficient audio voltage to the line input to the 610.

[W2PFY]

The 2A3 driver circuit in the 610 is an integral part of the modulator bias system, so completely replacing the driver stage would require completely re-designing the bias supply as well.  You need a pair of a low-plate resistance tubes and an appropriate driver transformer for the class-B 100TH modulator.  Since you already have all that in the 610, I wouldn't fool with something JS like a speaker amplifier working into a backward-connected output transformer. Without substantial negative feedback, a pair of screen grid tubes like 6L6s, 6V6s (which have a high plate resistance) will not drive a class-B modulator without distortion.

The way I have mine set up works and sounds fine. The bias can be brought out from the wiper blade on the rheostat and you hang a 50,000uf low voltage cap on there.That bias  goes to the center tap of the output transformer.

Yes, it's a bit JS and it's not for the purest, but if you want to get on and sound rather good using a solid state amplifier as a driver, you can get there with little time spent doing it this way. I know that certain people who know what I am doing don't approve but I'm not living for them

[K3YA]

I'm on my second BC-610, both were used without the BC-614 speech amp.  In both cases I had a small external panel that contained a modulator cathode current meter and an audio attenuator to replicate those functions of the 614.  In both I used the original 2A3 driver stage and associated transformers.

One issue is that the 2A3 stage takes a lot of voltage to drive it.  In my first BC-610 I added a PP 12AU7 stage under the modulator to make the transmitter easier to drive with audio.  On my current 610 the audio distribution amp will just barely drive the transmitter.  You may need a small amplifier or step up transformer between your audio chain and the transmitter.

A lot of folks have used an external hi-fi or PA amp to drive the 100TH's directly through a backwards connected output transformer and it works out well too.  Just remember to maintain the 100TH cathode return that the 614 provided.

[W2PFY]

Just remember to maintain the 100TH cathode return that the 614 provided.

You need not do that with the C and D models. At least I didn't and it worked. I think the E and later models used the meter on the 614 as a modulation indicator. Whenever I used the later model 614's on my C & D models, the modulation indicator would not work but the meter on the 610 did work.

[K3YA]

I'm not as familiar with those old buzzard BC-610 C&D.  My experience is with the modern F and I models.

[ve8xj]

Thanks so far for the great answers

So if I have this straight  In the spirit of keeping it simple ,and doing the least mods, one could use an audio amp such as the one in the EBAY ad I posted ,using a 8ohm swamping resistor across the output and then run that through a 8ohm to 600ohm transformer and feed it to the 2A3's.

I would probably just disconnect the audio line from the bc-614 output socket(located on the front) and put a plug for external audio on the back with  perhaps a switch to go back and forth. That way one still has the mod plate meter and the bias return . This would be fairly versatile . 

The next question is what size swamping resistor is going to give me the proper level of audio to the 2A3's ?  50 watt,75 watt ?I don't want to ruin my nice 2A3's !! The audio amp I posted from EBAY actually is a 100watt amp even thought the ad specifies 300watts . I checked an online PDF file of the manual.

 The main reasons I want to get away from the 614 is that it is very old and even though it does work very well it will just be a matter of time before I have to do a recapp ,resistor swap etc. My 614 has the fungicide treatment so I am not relishing the idea of doing any solder work on it. As well the limiter circuit though it does a good job makes using any other type of compression difficult . It always a chore to get the attack and threshold balancing between it and the mixer I use. 

 

[W2PFY]

If I remember correctly, the 614 uses a 6SN7 in push pull as a driver for the 2A3's. Perhaps you could measure the output at that stage with a 400 cycle tone when it correlates to 100 percent modulation on the 610.

So if I have this straight  Huh In the spirit of keeping it simple ,and doing the least mods, one could use an audio amp such as the one in the EBAY ad I posted ,using a 8ohm swamping resistor across the output and then run that through a 8ohm to 600ohm transformer and feed it to the 2A3's.

Or you could just run the 70 volt line directly into the socket on the 610  which is 600 ohms.

I bypass all that and go directly to the 100TH grids. I also leave the 2A3 tubes in there.

Another thing to consider is having some sort of a limiter ahead of the audio amplifier. If you do not, it will be very easy to over modulate and cause arc overs etc. That's why I use a pro audio limiter/compressor.

[N8ETQ]

Yo'


     I can't quite recall but seems to me, my 614 was fairly clean
and broad when I played with it last. Did you recap it? I think
the "restrictions" show up down wind of the 614.  You could
just use a T-17 and not worry about it  hi hi.

73

/Dan

Though I am fairly satisfied with the B+W BC-614h I can't help but think that one could do better. After all it was designed for military communications audio in mind.

I was thinking about getting away from tubes all together for this but am open for suggestions .

Questions like ;

  How many watts of audio would I need to drive the 2A3's?

I imagine 15 watts or so would be plenty ,but I am not sure.

I found this on EBAY but I suspect it would be way too powerful . Would something like this but in a less powerful package be ideal? Maybe something like this could be used to directly bypass the 2A3 stage and go write to the 100th's through a coupling transformer.I like the idea of the built in mixer and the ability such as this one of  being able to add more to the audio chain.

http://www.ebay.com/itm/110767730141?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649#ht_3944wt_974


  What impedance would be best to feed the 2A3's? 600ohms? Or if one was to directly feed the 100th's what impedance ?

Has anyone played with this before and if so what did you use ?



 

[k4kyv]

If the 614 works well, I wouldn't worry about it.  It is no more apt to fail than the low-level audio and rf stages in the 610 itself, which is also full of ageing caps, resistors and other components and probably also has the fungicide treatment. If both units are of similar age and condition I can see no reason why something in the 614 would be likely to go before something in the rest of the transmitter. Unlike the low-level stages in the 610, at  least you would be able to get at the failed components of the 614 without being a contortionist.

If you  do decide to drive the 610 directly, using the 100-watt amplifier with 8-ohm load, you would not need an 8-ohm to 500-ohm step-up transformer. The 8-ohm output into the load is likely to produce enough voltage to excite the audio line directly without any step-up transformer.  Maybe even too much; you might need additional resistors to further attenuate the signal.

One way to find out would be to use an a.c. voltmeter to measure the audio voltage at the line input from the 614, using something as close to sine-wave tone as you have available, set to modulate 100% (careful - keep the tone test brief with the HV on, lest you overheat the 100THs, since most modulators were designed for voice, not steady tone).  Once you have determined the approximate audio input voltage required to drive the modulator to 100% modulation, run a similar tone through the outboard amplifier, with the voltmeter across the 8-ohm resistor, and see if you achieve at least that same voltage before the volume level is all the way up. If you can substantially exceed that voltage before the amplifier begins to saturate, you should be OK.  No need to worry about impedance matching here, since the outboard amplifier is serving as a voltage source to drive the line input to the 610.  Most of the 8-ohm load is being absorbed by the resistor, with only about 1% of the audio power drawn by the transmitter's input transformer. Since the driving amplifier has far more power available than what you need for the purpose, the only thing an 8-ohm to 500-ohm matching transformer would do would be to  require additional attenuation resistors to prevent blowing the 610's input transformer to smithereens and/or destroying the grids in the 2A3s.

You would need a step-up transformer only if the audio voltage across the resistor at full output is less than the input voltage the 610 requires for 100% modulation. Even then, a 2:1 or 3:1 step-up ratio would likely be sufficient.

[KE6DF]

Well let's try to do a back-of-envelope calculation of the input power needed to drive the 2A3's using an 8 ohm amp output.

According to the manual, you need to bias a 2A3 to -62 volts for AB1 operation.

Assuming a push pull circuit, and assuming you want to drive the 2A3 grids right up to 0V (but no more to stay in AB1) then that's a peak-to-peak drive voltage of 124V,

Divide by 2.8 to get the RMS drive voltage giving 44 volts.

A typical impedance step up ratio for a 600 ohm to grid driver transformer might be 100:1 (or a secondary of 60000).

The square root of the impedance ratio gives the turns ratio. The square root of 100 is 10.

So you need 4.4v RMS input to drive the 2A3's to the limit of AB1 operation given the above assumptions.

Now to get power needed to get 4.4V RMS across an 8 ohm swamping resistor is E squared over R or 4.4*4.4/8 = 2.4 watts.

So that 100W amp has way more than enough power and you don't need another step up transformer.

In fact, if you crank up the gain you would probably  blow the 2A3's.

Best to use a voltage divider after the swamping resistor to reduce the drive to the 2A3 stage.

[N8ETQ]

    What about the Modulator Current Meter? How are we gonna get
side tone? Where do we plug in the EE-8's?  As provisioned the
SCR-299/399/499 were pretty sweet. It's a "System" not just a
radio IMHO.

    Load the output of your 614 with a 600 ohm R and sweep it.
I think you'll find it's up to the task.  Hum can be an issue but
nothing the 610 won't mask. Hi

GL

/Dan

[k4kyv]

The earliest BC-610s and the original Hallicrafters HT-4 had a modulator current meter mounted on the upper right side of the main transmitter panel.  Those first 610s were probably unsold stock of HT-4s with minimal modifications and a Signal Corps nameplate riveted on. The military version quickly removed the Art-Deco chrome trim and eliminated the dedicated modulator current meter to save costs and scarce material.  There was another meter on the upper left side, which was also eliminated. There was such a meter shortage at the beginning of WW2 that the government was even begging the off-the-air amateur community to donate unused panel meters to the cause, the subject of one month's QST cover early in the war.

I remember a local ham who acquired a BC-610E that came without the 614.  He homebrewed an outboard mic pre-amp/speech amplifier, and modified the 610 by cutting a hole and mounting a dedicated modulator current meter on the front of the transmitter, approximately where the original HT-4 meter was, even though he was unaware that Hallicrafters originally had one there.

[ve8xj]

Hey guys

 Thanks for making this an easier task by giving your 2 cents worth. It makes a lot more sense to me now and the task seems pretty straight forward when and if I go about doing it.
   Yes the 614 is doing a fairly good job right now and there is no major reason to ax it yet but I know there will be a day when I decide to do this just for the sake of doing it.
  I am certainly glad that this forum exists or I would have no place to go for some of these important life questions. Even though I was licensed 24 years ago I have not always kept up with my electronics skills. But thanks to great guys like you I am learning more than I can possibly absorb everyday by reading this forum.
WB6IYH thanks for doing the math for me . My math teacher in school wound up committing suicide trying to teach me math so having someone competent figure that out for me really makes things simpler. You make it look easy .The voltage divider will be a must I see .
 Dan I love the photo of your set up A little of the new and a lot of the old.

Don once more thanks for your wisdom ,would have made a few mistakes if you had not pointed these things out. For sure do nor want to blow my input transformer on the 610 to smithereens .

 
 

[W2PFY]

Here's a couple pictures of my D model 610. The A through D models were essentially the same except for the D model, the fuses were then located up front where before, they were located on the rear of the transmitter. You can understand why they did this with the transmitter used in the field. It would have been a real hassle to move it from it's shock mounts to replace the fuses and then re-locate it back again in such a restricted shelter.

The meter on the left is the PA plate meter and yes, it's in the plate circuit and that is why it is mounted on a bakelite sub panel. The meter on the right is also mounted the same way but it's in the cathode circuit and poses no danger to the operator. Many people recessed the plate meter and covered it with a protective screen.


Don is certainly correct with his suggestions for designing a driver amplifier properly. I just wanted to show that there is a cheap and dirty way to do it if you find yourself with a dead speech amplifier and can't repair it in a satisfactory time frame.    

The second picture shows my haywire JS hook up. Yes it works very well This will all be replaced by a neat  tube style cathode follower.  

Did you notice the old 100T's  in the modulator? I can't believe they still work as they are about 75 years old.

[K3YA]

Those earlier BC610's certainly look cooler than my later one.

[k4kyv]

But the later ones, with the roll-out modules, are a lot easier to work on.

[W2PFY]

This thread is too young to die