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???
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.
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.
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.
Post by pathfinder259 on Jun 12, 2008 20:21:12 GMT -5
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..
Post by Tombstone (R.I.P.) on Sept 26, 2008 16:33:02 GMT -5
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?