Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TerryD on November 22, 2016, 07:28:08 am
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Ok.
My name is Terry
I am cheap.
I love RCAs
I have a 100 watt Fender PA that I turned into a tone monster with the help of you folks. (1 normal channel, 1 64 Blackface twin, and 1 chain link with the final two channels)
I had a pair of old RCAs. I just bought another set off EBay from a reasonable radio guy. The quad sets of RCAs, I will remind you are astronomical. So I thought I'd shoot craps and try and get my present set of 2 RCAs to work with this set of 2 RCAs I got off of ebay.
Finally, the question. Given my present situation, in what order would you put my two sets into the 100 watt amp, with the number 1 being the power tube position on the right? I know that the two inside tubes go together and the two outside tubes go together, but still not sure in what order I would use my two sets of 6L6s. I can bias as well.
What else can I do to get these sets to be compatible ?
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Plug all 4 tubes in. Check bias (idle current for each tube). Juggle tubes around until each side of the OT has same idle current.
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That sounds goooodt.
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"I know that the two inside tubes go together and the two outside tubes go together,"
Oh I seriously doubt that. That would be different from any other 4 * 6L6 Fender. I don't have a layout in front of me but I still doubt that, bigly.
[nobody can know what you mean "from the right"]
From the right ....... as viewed with the chassis upside down (eg; as normally installed in the cabinet) looking at the back?
From the right ....... sitting on my bench pulled out of the cabinet as viewed from the rear?
Regardless of how you count, it is virtually certain IMHO that 6L6 #1 and #2 are lashed together, and 6L6 #3 and #4 are lashed together.
Easy enough to confirm. With power off and off for a strong 1 minute+, check ohms between pin 3 of any 6L6 and another 6L6. Find the pin 3's that have zero (or super low) ohms between them. Those two are a pair. Mark them. If you get ~~250-300 ohms, those two ARE NOT a pair. Keep one probe where it is, move the other probe to another pin 3. Find the pair that are "shorted" together.
Me? I would install the 6L6 tubes: set A #1, set B #1, set B#2, set A #2. Eg; old, new, new, old. (or N, O, O, N) This makes the assumption that the new set are kinda sorta matched and that the old set are kinda sorta matched, an assumption nobody has any right to make. The only real way is what Sluckey suggested, to measure > juggle > measure > juggle.
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"I know that the two inside tubes go together and the two outside tubes go together,"
Oh I seriously doubt that. That would be different from any other 4 * 6L6 Fender. ...
I think he meant "go together as a push-pull pair" rather than which two tubes are connected to each other and to a single side of the OT.
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Thanks guys! I followed your directions to a tee. Two sets of (2) RCAs One of the sets glass size was bigger than the other but they looked about the same period/use. Put them in and only had to swap two tubes around and wallah! perfectly in tune/balanced/biased. I would never say the Ruby Tubes sounded like crap on a public forum or even imply it, but next to the RCAs they don't even sound Fendery.
Thanks again,
Terry