Robyn T-123B/T-240D 7189 tubes

[960]

Has anyone changed out the 6BQ5 tubes (both the audio modulator, and the final) to the 7189 tubes? The only difference is the 7189 is rated at 400vdc plate versus 300vdc plate on 6BQ5.
Anyone who has done this, did you see any audio improvements or power improvements or really nothing at all???

[2600]

There is ONE other difference in the 7189. One pin (#6) that is not conected in a 6BQ5 is hooked to the screen grid in a 7189.

No big deal, so long as that pin on the tube socket has nothing connected to it.

But if some frugal engineer uses that empty lug on the tube socket as a tie-point for some other circuit, you'll get a nice, smelly "POOF" from under the radio when you make that substitute.

Before you try this sub, have a look at pin 6 on the 6BQ5 socket. If there's nothing connected, you're good to go.

And if there are two parts from the squelch circuit soldered to it, the fix is simple. GENTLY clip that lug clean off the tube socket. Bend the parts slightly away from the socket's other lugs.

It's that simple.

But only if you do it BEFORE you plug in that 7189.

73

[960]

Pin6 is empty on both tube sockets, good to go!

Has anyone tried the swap out and was there any improvement results? To get a pair of these tubes costs about how much I bought the radio for, but I will spend the money if it takes the radios audio and longevity to a higher level.

[kc8cdz]

I think Jim Goulsby did it once, but I can't be for certain!

[960]

Jim Goolsby has done EVERYTHING atleast once! He is a man of honesty, integrity and dignity!

[2600]

Except for that one pin difference, the 7189 is just an industrial-grade 6BQ5. Don't expect a big difference in performance. Biggest difference was in the reliablilty, not the power you'll get.

73

[960]

thanks 2600 for the info.

I imagine you have some experience with this style of radio? Any instant recommendations you know of off the top of your head as far as restoration is concerned? The receive has really died down the past couple days, and I also noticed that when I turn it on cold, it have better receive and as it heats up the receive dies down. I am going to replace as many capacitors first off with new superior versions (I had to change the .1uf 600v oil caps as two of them where blown apart and I have no receive, it was like permanently squelched. I will do the resistors with 1% 1/2 watt flameproof down the road, that will be alot of work but I am going to recap my T240D first.

[2600]

It's the 240D that has the weak receiver, right?

I'd check R106, the 47k that feeds L201, and R303, a 100k 1/2-Watt feeding pin #6 of V3, a 6BA6.

The same 100k part on the other receiver-side 6BA6 is R404, feeding pin 6 of V4.

My best guess is that a shorted tube is what pops these resistors most often.

The most aggravating part of those radios has to do with the white enamel paint they used to lock the tuning slugs in place.

Used to be able to get solvents that would soften the enamel without meliting the plastic coil form inside the IF-RF transformers and coils. Now all those nice, aggressive CFC and HCFC solvents are history.

Usually we find a handful of slugs cracked, from someone inserting a metal allen wrench and cranking it hard enough to hear that teltale "crack!" sound. Once the slug cracks, it will seize against the inside of the coil form. Twisting the tool inside it forces the core's fragments outwards against the inside of the coil form, locking it in place so it won't turn any more. Kinda like the centrifugal clutch on a go-kart.

Extracting cracked slug is a major undertaking. Finding a new slug that matches the frequency of the 'can' where it goes is not that easy.

The tuning slugs are NOT all the same material. You can't just cannibalize a slug from a 455 kHz IF can and use it in the 27 MHz or 10.695 MHz circuits.

Taking a scorched-earth approach to all the elctrolytic capacitors, and replacing them all at once will reduce aggravations down the road. Fixing them one at a time as they fail is a lot more work.

73

[pathfinder259]

Hello,,No dont expect any power inprovments.I tried this
in a robyn t-240 d. I bought a 7189 new in box. An changed out
the audio tube with it..The old 6bq5 did the same output on
power as the 7189..Surelly not worth the money.Unless like the
guy said,it might be a little more reliable tube..

[kr750]

whats a good brand of tube? my robyn is geting week..

[Tombstone (R.I.P.)]

I've had good luck with JJ Tesla tubes made in Yugoslavia, Lithuania or some such place. You could try www.tubesandmore.com (Antique Electronic Supply). They might have a brand name new old stock tube that was made in the US?

Tombstone

[2600]

I agree. The JJ/Tesla-brand tubes are good. They are made in the Slovak Republic.

The "EI" brand was okay, too. Those were made in the former Yogoslavia.

73