Is it possible to sub a 6AQ5 tube for the 6AK6 tube in the R-390? Also would there be any benefit to replacing the 12AU7 tubes in the audio amp with 12AX7?

Also I found a couple mods on pages 6-9 of this PDF document http://www.r-390a.net/r-390_cookbook.pdf

One is to sub 6080 regulator tubes for the 6082 tubes. The other subs a 12BY7 or 12BH7 for the 3TF7 ballast tube.

Would those mods be good to do?

I do know my 6082 tubes check a bit weak and there are a few 6080 tubes for sale in the classifieds section and I am pretty sure I have a 12BH7 or 12BY7 around somewhere.

I also read somewhere that a couple LEDs can be inserted in place of the cathode resistor of the12AU7 section that drives the 6AK6 output tube for the local audio in order to reduce the distortion of the audio, but that was a mod for the R-390A. Would that work with the R-390? If so I should be able to get or maybe have a couple LEDs that will work.

I also read where the audio coupling capacitors can be increased in value to provide better bass response.

As is the audio sounds very good, but if it can be made better without irreversable mods that would be great.

None of those subs sound like a good idea to me.

The 6082 has a 26.5-volt, 0.6 Amp filament whereas the 6080 has a 6.3-volt, 2.5 Amp filament . . . not exactly a direct substitute! The 6AQ5 has a higher filament current requirement than the 6AK6. The 12AU7 and 12AX7 tubes have quite different characteristics and uses and should not be randomly interchanged. And so it goes . . . .

Waste of time and money. Remember that these receivers were designed to drive headphones or distributed audio. You have a terminal strip on the rear panel with line level audio which you can use to drive a powered speaker of your choice. The 3TF7 lasts for years and years and even if yours fails they are readily available. IMHO the only mod that may be worth the time and effort is to add a small fan to remove the heat from the 6082 area. There are plans for that available that require no mods to this fine receiver.

If you need to connect a low Z speaker to the 390 simply use a commercial sound 70 volt transformer.

Here's a link you may find interesting. http://mailman.qth.net/mailman/listinfo/r-390

Figured the 12AX7 12AU7 sub would be questionable.

Wasn't sure about the 6AQ5 and 6080 mods as apparently someone did them and had no problems with those mods unless they did some other stuff they did not document in order to make those mods work. From the PDF file I posted the two 6080 tubes have their filaments wired in series and the pair in series with a diode

I do have a fan to cool the 6082 tubes.

I will look at the audio amp schematic in the manual. I do think I might just replace a few capacitors in the audio amp section.

I have tried to drive a powered speaker using just the local output and still got some sound from the speaker with the local volume control full CCW.

Audio is fed through a 600/8 ohm Hammond transformer to a vintage 15" Jensen Concert series speaker with a 5" Magnavox tweeter added to smooth out the upper midrange where the 15" Jensen alone didn't sound too good.

I suspect that my best bet wil be to feed an audio signal at the local and line level controls then follow the signals through the amplifier with an oscope and see if there is any distortion before doing anything aside from replacing some coupling capacitors. I probably have enough stuff to run the 6AK^ tubes in push pull for more output, but I really don't want to muck up the audio amp that way and plus I would then only have one powered audio output.

If anything I may take another look at the Gray Audograph I restored about a year ago. It uses two 35C5 tubes in push pull and is operated from an isolation transformer given it is a hot chassis design. That would provide plenty of audio, but would still have the issue of audio when the receiver's local volume control is all the way CCW.

Would have been nice if they used something like a 6AQ5 tube in the audio amp.

One thing I just thought of is if I were to replace the 6082 tubes (when they finally go bad) with a solid state device I would then have a free 24 volt filament winding at 1.2 amps and could then operate one 6AQ5 tube heater with a suitable dropping resistor. That would also eliminate a large amount of the damaging heat and the need for a fan.

I replaced the two .01 uF coupling caps with .1 uF caps and replaced the 8 uF local output 6AK6 cathode capacitor with a 25 uF cap and bass response has improved quite nicely.

I don't quite understand how the lower bass can improve like it did since the transformers only go low as 200 Hz. The volume from that 15" Jensen speaker fills my 12' X 12' building to an uncomfortable volume level at full undistorted audio output and once I put the speaker in a proper wall mount box like the old wall mount jukebox speakers I suspect the sound will improve even more.

If y'all need an excellent sounding speaker for your boatanchors go get you an old wall mount wood jukebox speaker with at least a 10" speaker inside or a vintage speaker of at least 10" and build a wall mount box.

I run one on my Hammarlund SP-600 JX-17 and the audio sounds much like a console radio. Decent bass and excellently sounding mellow midrange up until the capacitor in parallel with the primary of the output transformer starts rolling off the highs.

Replacing the caps did what I wanted so that is the only mod I will do until the 6082 tubes go bad. I found a nifty regulator circuit using two zeners and two transistors that according to the author meets the specs for the original regulator circuit. Will post link when I get home.

You can simply use a TV horizontal output transistor to replace the 6082s. Be sure and heatsink it well. You need a transistor with plenty of dissipation and decent (probably 500-700V) V(CE).

I did that yyears ago on a R-390 I owned back in the 1980s, when I couldn't find 6082s anywhere. As I recall I didn't use any zeners, I just connected grid-->base, plate-->collector, cathode-->emitter. It worked fine for 6 months or so til I had to sell it for tuition money.

Oh I see. Just using the transistor like that allows the rest of the regulating circuitry to still function.

I may look into that in the future. I suppose that when my 6082 tubes go south I can simply salvage one of the bases then get a heatsink to properly mount the transistor on and put it all in the base which will make it a plugin replacement.

I just thought of something. If I do want more power from the receiver can't I simply use a 6BQ5 or similar push pull stage and use the balanced 600 ohm line or local output to drive the tubes? If so I could simply hide the circuit in the wall mount speaker box I will build for the speaker. I have a push pull 14 watt SEP 6BQ5 amp I can use to test my theory to see if it will work. I would simply unplug the driver tube of the amp and connect the receiver to the grids of the output tubes. If it works I have a Magnavox SEP stereo 6BQ5 amp chassis that I can use for the amp. All I would need to do is mount a push pull audio output transformer on the chassis then construct the amp circuit. IIRC the chassis still has the power transformer on it.

I tried it using a transformer and it worked very good, but I still get some volume when the local volume control is set full CCW which is why I didn't use an amplifier with the local output before.

You could engineer a replacement for the 6082's, and reengineer the audio chain, but IMHO, why mess with success? Is it likely you're smarter than the Collins engineers?

You could slap in a MOSFET and heatsink and it might work as a 6082 replacement, but here are plenty of pitfalls. Its high grid (gate) capacitance is going to mess up the regulator's output impedance at medium to high freqs. It's going to be susceptible to shorting out unless you surround it with several zeners and TVR's. And it should probably have a small fan. A bit too rube-goldbergy IMHO.

Same with the audio output-- you could rejigger it with a heftier tube, but then you'd also need a bigger output transformer. A bit too much hackery IMHO.

Getting rid of the 6082s is a good plan. They make a LOT of heat and since they're mounted upside down they tend to char the audio/PS chassis.

I'll note that the R-390A (nor any other radio I can think of) does not have a pass regulator like this. I think Collins learned an important lesson on reliability and didn't go down that path again.

Looks like the EL-95 tube would work provided I put a resistor across any tube heaters in series with it to take up the extra 50 mA increase of heater current. Would work with the original output transformer and should get me about 3 watts of audio provided the output transformer can handle the slight increase in plate current. Only wiring change will be moving pin 7 to pin 2.

http://www.mif.pg.gda.pl/homepages/fran ... e/EL95.pdf

Well, you'd save about 32 watts of heater juice. The B+ dropping heat will be exactly the same. I'm not sure 32 watts of savings is worth that much hackery. Maybe adding a small fan to cool off the area would be an adequate compromise.

If you think this power supply is a bit overmuch, the original specs required the radio to run off 115VDC volts too. The guys at Collins designed up a thyratron power supply that did the trick, but it was a real rube-goldberg device.

I think the original audio output transformer is going to saturate with that much DC current going thru its primary.

Ancient, there's really no hacking required. The mod I did years ago just plugged into the 6082 socket. Depends on how you approach it, but mine was borne out of sheer necessity (no 6082s available anywhere).

I wonder if I can convert the 6AK6 stage to cathode follower operation which would lower the output enough to where I can drive an external amplifier without having much if any audio output with the local level control set to full CCW? If necessary I can convert the 6AK6 stage to triode mode then use it as a cathode follower if it will not work good as a pentode in cathode follower configuration.

Looks like I will need to install a constant voltage transformer or a 5 volt bucking transformer to ensure the receiver is operated at around 115 Vac.

The amplifier I had planned on using with the receiver does have a 5 volt bucking transformer installed as it's power transformer is designed for 115 Vac operation so I could easily add a power outlet in parallel with the transformer's primary winding which would then provide the necessary 115 Vac for the receiver and also power for the cooling fan. That way everything can be turned on and off by one switch.

Tried cathode follower and got hum. Will bypass the 12AU7 stage for reduced gain. I can then use an external amplifier for the speaker.

The easy way to get good audio out of the R-390 is to just connect an audio amp to the diode load output, thru a blocking capacitor and high impedance attenuator, to knock the level down.

Only downside of this is the vol control on the radio doesn't affect the audio output at that point. No mod needed.

That is true, but I do still want the volume knob on the receiver to control audio output. I tried putting the transformer in series with the 12AT7 plate and it worked pretty darned well except it had reduced bass response. I then tried bypassing the 12AT7 stage and got more distortion than usable audio. I then put it back the way it was originally and it works good.

By looking at the schematic I don't see any reason for there to be a small bit of audio on the local output when the local volume control is turned full CCW. Is a small bit of audio on the local audio output normal whent he volume control is turned full CCW or is there a problem with my receiver?

If I can get the audio down to zero when the conterol is turned full CCW I can then drive a more powerful external amplifier. I even tried just using a 15K balanced/unbalanced transformer driving a pair of 6BQ5 tubes in push pull and still got audio with the volume control turned full CCW. I'll have to study the schematic and see why I still get audio.

Also I noticed that under certain conditions the voltage reference tubes will glow and not glow as though the tube was connected across a capacitor and being charged through a high value resistor. Is that normal or is there a problem?

I also used a variac to knock down the voltage to 115 Vac.

I do notice that if I have the volume control down to full CCW and I turn the RF control up full I get more hum in the speaker and also the Ac voltage on the 180 Vdc test jack increases to around 7-8 Vac. Is that a normal indication or is there a problem?

I know the vitimin Q caps are still good, but are there any other capacitors that need to be replaced?

Also are there any resistors that should be replaced?

Here's the schematic of the audio amp. Maybe y'all can explain to me why the audio from the local out is not completely zero when the local audio volume control is full CCW. Also I noticed that with the local audio control set to full CCW I can turn up the line audio control and hear some audio through the speaker connected to the local audio output. Sounds to me like maybe a bypass capacitor somewhere is going bad. I can see maybe one thing I need to do. The 12AT7 section used in the local amp has its 180K plate resistor connected to the same line that feeds B+ to the output tubes. Maybe I need to insert a 5K resistor or a little higher and then a filter cap after that. Also looking at the schematic if the 1uF filter cap in the plate circuit of the 12AU7 buffer stage or the 10uF filter cap on the line that feeds the output tubes were bad I suppose that I could get crosstalk between both audio channels.



I used a 15K resistor in series with the 180K and a 22 uF 450 volt cap to ground and the audio no longer bleeds over from the line output to the local output.

I just noticed and one of the rectifier tubes is no longer lit up

Would it be ok to simply sub diodes or should I look for another 26Z5 tube? I can get the proper diodes at Radio Shack tomorrow. Thinking about it that tube being bad (not sure when it went bad) could have been the cause of the audio issue and certainly is the cause of the increased hum from the speaker.