schem question

[trem]

My questions.....

1. Bottom right, near Power Lamp, RY-101 part of the "stand by switch".
What is that coil and diode in // doing.?

2. Power tube cats. R128 and R129. What are those 1k doing.?
Thank You

[Malcolm Irving]

I think the 10k resistors R128 and R129 are to allow an 'average' bias for the two output tubes to be read by looking at the voltage between TP1 and TP2. The 10k resistors are needed to avoid shorting out the 10 ohms resistors. I think a better way would be to have three test points, so that the bias balance between the two output tubes could also be measured.

Sorry, no idea about the coil and diode!

[trem]

Thanks Malcolm -
I THINK I follow you.....so that the "bias" adjust on the outside, back of the chassis can be used by the consumer...probably set to some mV range suggested by the manufacture(er).?

[Malcolm Irving]

The user would read the voltage between TP1 and TP2. That would be the average voltage drop across the two 10 ohms resistors in series with the cathodes. So the average bias current per output tube would be (I=V/R) i.e. that reading divided by 10. There's negligible volt drop across the 10k resistors because the current drawn by the voltmeter is very small.

The manufacturer would probably suggest different readings to aim at for EL34s or 6V6s.

[Mark Black]

Coil & diode = the coil of relay 101 (RY-101) shunted by a diode to damp 'flyback' transients when the coil is de-energized (common practice).

[trem]

Hey Mark -
I understand what you are saying, In a textbook way.
But what is that relay for, in the heater circuit.?
At my level of understanding, maybe a better question is.....what would happen if that relay were not there.? Is it the switch that causes the need for the Coil/Diode.?
Thank You
Thanks

[Mark Black]

Far left of schematic is the switch which is part of relay-101 (RY-101 MUTE, normally open). Looks like they are using the relay to ground the input of IC-101B at the same time the other part of the standby switch is cutting off high voltage to the power tubes. I'm guessing this is to eliminate the turn/off thump or snap you get when operating the standby in a more classic simpler design. The relay keeps the two circuits isolated from each other and keeps the low-voltage sensitive input of the op-amp physically removed from the high-voltage arc in the standby switch when it is operated which itself could cause a noise on standby operation.

Not actually a part of the heater circuit, other than deriving power for the relay from there.

Edit: I'm a slow typist, and G1 beat me to it. :-)

[g1]

Your explanation was much better, so I deleted that.
I missed IC101B.

[Dave H]

The relay is not in the heater circuit. One pole of the Standby switch connects the bottom of the relay coil to ground. (There's a ground symbol connected to that side of the filament winding). The relay contacts mute the preamp.

Edit: Ditto g1, You can still type faster than me Mark

[trem]

Man...OK.....so RY101 (both parts) are also tied up with the SB Switch...SW101-A and SW101-B.
I will ask another question separately.
Mark
Malcolm
g1
Thanks Again