Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on August 09, 2023, 05:25:06 pm
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I was doing a search for the schematic of a Stereo Grundig radio that uses a pair of ecl86 tubes, I got it at a Flea Market some time ago (PT and 2 x OT + Tubes donor @ 20€)
it ended to be a Grundig 3299 Stereo
Doing the search I finded one other Stereo Grundig radio schematic that uses 2 x ecl86 tubes, model so302
The schematic seems to be of a Stereo unit with two SE Power Sections BUT .....
But it is a bit odd to me, the cathodes of the pentodes of the ecl86 are both connected to a single cathode plus a single bypass capacitor
(https://i.imgur.com/DD5SiqH.jpg)
(https://i.imgur.com/kYg0GEN.jpg)
:w2: :w2: :w2:
What is happening there ? I don't remember to have seen such a thing previously, or, better, I've seen it only in an old italian GEM guitar amp, but it was an el84 PP amp but the brand used a pair of SE OT instead of a single PP OT (the friend who restored the amp swapped the two SE OT for a single PP OT)
Have you any idea about this thing ?
THANKS
Franco
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It is bypassed. It gives DC bias but "little" cross-talk on audio.
There were push-pull stereo amps with all four power bottles on a single cathode network.
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It is bypassed. It gives DC bias but "little" cross-talk on audio.
There were push-pull stereo amps with all four power bottles on a single cathode network.
My stereo (a 60s kit amp) does that. It also uses that cathode current to run the phono preamp filament.
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Thanks Friends
But if there is Cross Talk, which as far as I know we don't want, what's the reason .... to save the price of a resistor and a capacitor ?
Franco