everything before and after is just waiting. To test this just disconnect the driver to grid, then put a 2.2 meg from each of those to chassis. Kill one grid and you have your low power single-ended amp. If that's true you are very close to your goal already. Looks to me like you might have a push-pull class A amp there. 48 plus 2 for screens would give us 50 Ma total for two tubes measured at common cathodes.Ī blackface schematic shows a Champ with 350v plates running at about 40 Ma. The print shows about 36Ma per tube.Ī Class AB1 6v6 at 300v would be set at about 24 Ma for 70% dissipation at idle. That would give us about 38 Ma per output tube plate measured. Deduct screen current from that to get 76.2 Ma. Your common cathodes current appears to be 78.3 Ma. About 1.54 Ma of that is to the preamp plates, leaving 2.1 Ma for the screens. You have about 3.64 Ma going through the screen resistor. You gave these figures: Plates 285v, Screens 281 v, Cathodes 18v. Let's say the 1K screen resistor measures 1100 ohms, and the 220 cathode resistor measures 230 ohms. Man I love it when simple fixes solve everything Nylon washers on both sides of the cooling fins and the amp is silent. How hot does your output tranny get?Īccording to the readings you made you are running PP Class A already, or close to it. everything before and after is just waiting." Steve McQueen in 'Le Mans' If I had time I would experiment on my Princeton. Hi Kev Right about letting the inactive tube just sit and idle. I think a schematic will develop here, I hope. Maybe someone will see some pros and cons of these two ideas or offer a different approach. I think I would leave the cathode connected and just switch out the signal and switch the cathode resistor value. What will happen to the signal on that inactive grid with cathode lifted? Will it see an impedance? Will it matter? Will the screen or plate try to conduct through the control grid G1, I think not, but never tried that, so I don't really know. This also lifts the beam-forming grid G3. I think I have seen stanby schemes that lift the cathodes. That cathode resistor value has to come down some for Class A. But you want to get to class A operation where your dc bias point will be approx 1/2 of the rms audio signal give or take some when you check the current. That may not matter much to your sound, and to your bias. If we lift one cathode we get a higher B+. What does everybody think? Tech help needed. Having said this, there are a lot worse guitars out there, and as well as being historically important, the 1820 bass can certainly provide the goods when required.All you need is your hand-tools, a meter and a stick. Over the course of the 70s, the Japanese output improved dramatically, and in many ways these early 70s models are a low point for the brand. These new Epiphones were based on existing Matsumoku guitars, sharing body shapes, and hardware, but the Epiphone line was somewhat upgraded, with inlaid logos and a 2x2 peghead configuration. The Matsumoku factory had been producing guitars for export for some time, but the 1820 bass (alongside a number of guitar models and the 5120 electric acoustic bass) were the first Epiphone models to be made there. By the end of the 1960s, a decision had been made to move Epiphone guitar production from the USA (at the Kalamazoo plant where Gibson guitars were made), to Matsumoto in Japan, creating a line of guitars and basses significantly less expensive than the USA-built models (actually less than half the price).
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