Re: Dynaco 70 Bias

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In Reply to: Dynaco 70 Bias posted by John H on July 21, 2002 at 04:11:56:

In a stock Stereo 70 when you adjust the bias pot you are changing the bias voltage to the control input grid of _both_ tubes (one channel) at the same time. The path for the bias voltage from the wiper of the pot to the grids of the tubes is via a pair of 270K ohm resistors (located on the PC-3 circuit board, one for each output tube). These output of theses resistors are fed input signal from the driver stage from the output coupling caps (0.1 uF)and therefore leakage on the part of these caps could effect the bias on an individual tube. The DC bias voltage is then coupled to the output tube via a pair of external 1K resistors and finally reaches the output tube control grids (pin 5) via the output tube siockets.
The other elements of the output tube that contribute to the bias current are the plate, screen grid, and cathode voltages. You have already measured the 15.6 ohm resistor (common to both tubes) and therefore this cannot be the issue.
From my experience, in order of probability, I would check first the integrity of the connections from the bias pot through the PC-3 board and then the output tube sockets (corrosion is very common). Next, lift one end of both couppling caps and see if you can establish equal bias when the tubes are reveresed in position. If after you have confirmed proper operation of those items, I would consider having the output transformer evaluated.
Joe Curcio

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Follow Ups Full Thread

Topic - Dynaco 70 Bias - John H 04:11:56 07/21/02 (5)

Re: Dynaco 70 Bias - Jim Carlon 08:43:22 07/21/02 (0)

Re: Dynaco 70 Bias - Joe Curcio 07:14:44 07/21/02 (3)

Re: Dynaco 70 Bias and PAS 3 - John H 13:30:58 07/21/02 (2)

Re: Dynaco 70 Bias and PAS 3 - Joe Curcio 09:12:35 07/22/02 (0)

Bad Tubes - John H 15:51:29 07/21/02 (0)

Follow Ups

Re: Dynaco 70 Bias - Joe Curcio 07:14:44 07/21/02 (3)

Re: Dynaco 70 Bias and PAS 3 - John H 13:30:58 07/21/02 (2)

Re: Dynaco 70 Bias and PAS 3 - Joe Curcio 09:12:35 07/22/02 (0)

Bad Tubes - John H 15:51:29 07/21/02 (0)

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