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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 11 Dec 2018, 06:05 
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Joined: 29 Mar 2013, 20:37
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Location: Italy
Hi, i am fixing and old radio, it was given to me by a friend. he showed me a video of him plugging in the radio (without output transformer connected) and all the tubes heaters coming up and then this small tube went all internal short with sparks and chaos until he removed the plug. the tube is a 35A3 half wave rectifier. i ordered a new nos one but before i let the radio kill this one too, could it have damaged some other components? i see no short on the outputs of the tube with my multimeter but i am afraid of ruining another tube/ this valuable radio. I never fixed something that is older than my granpda before so any help would be valuable :D


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PostPosted: 11 Dec 2018, 08:11 
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Unplug the radio, drain all capacitors and check the filter capacitors in the power supply. Remove all tubes and check any transformers for shorts or open windings. If you have a field coil speaker check the coil for continuity as the coil is often used as a choke.
If all checks out (especially the transformers and speaker) then you can proceed further.
The filter caps will probably be bad.

Pictures would help


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PostPosted: 11 Dec 2018, 08:18 
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That won't be pretty with the complete chaos of the point to point construction of this thing. But thank you for the instructions, will let you know what I find


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PostPosted: 28 Dec 2018, 18:04 
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Update, I got my new rectifier. I replaced the main filter caps and added the new tube, the radio works without killing anything now. The magic eye is dead, you can only see a thin green line, I already ordered one. The reception is awesome but the volume is very low even with it set to max. Schematics say that it uses an ul84 as amplifier but the one inside is an el84. I want to try to replace it to see if it is the cause of my low volume. Should I use an ul84 or an el84?


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PostPosted: 29 Dec 2018, 08:53 
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El84 and Ul84 tubes are diffenately very different. El84 uses a 6.3 volt 750ma heater where as UL84 uses a 45 volt 100ma heater. Pinout appears to be different at least where unused pins concerned. Substituting an El84 is not an option. Look up tube specs on both and you will see the differences. You will need to go back to the Ul84. Hope this helps


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PostPosted: 29 Dec 2018, 09:00 
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I couldn't find anywhere the ul84 heater filament voltage, thank you for the info. This confirms my idea that someone in the past replaced the power transformer with something more readily available and modified the circuit to use different and more common heater voltage tubes.


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PostPosted: 29 Dec 2018, 09:48 
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Or it well could be that they substituted the Ul84 with an El84 not knowing the difference in the tubes and that was the beginning of the failure. You could check the heater voltage and determine if has 6.3 volt heater supply. Have you got a schematic? How about make and model number?


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PostPosted: 29 Dec 2018, 10:02 
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Both tubes use pin 4 and 5 for filiment. These pins are your measurement point for heater (filiment) voltage. 6.3 +/- for el84 or 45+/- for Ul84. I'm guessing you know that but wanted to mention it just in case.


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PostPosted: 29 Dec 2018, 11:20 
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Yeah it is a Wundercart FM950 post ww2, yeah I know the filaments pins but I did not got around measuring them honestly. I will do once I come back home. I did try with an old Philips miniwatt ul84 but it did not work. No noise at all, I assumed it was due to it being a bad tube since I found it on the bottom of the drawer at grandparent's house


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PostPosted: 19 Jan 2019, 12:46 
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hi, i am happy to announce that i completely restored the radio and it works amazingly well!

ended up recapping the whoooooooole bloody thing, new magic eye and rewiring some coils. plus lot of woodwork, the mesh is shamelessly stolen from a Marshall guitar amp. it sounds incredibly clear (beside some hum) even if it is so old


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