2020 Monkeymatic: Bogen DB114

[xtian]

Here's an attractive little 6V6 p-p model, incoming!

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[martin manning]

Nice! Clearly the consumer was expected to understand that vacuum tubes run hot, and that the responsibility to avoid touching them was all on him.

[xtian]

Nice! Clearly the consumer was expected to understand that vacuum tubes run hot, and that the responsibility to avoid touching them was all on him.

Didn't everyone learn tubes are hot back in the 50s and 60s?

[brewdude]

I imagine they learned in the fifties, the same way most people do... They touch them and go “damn, that’s Hot!”

[xtian]

And the professionals among us go, "Ouch--that's about right." Or, "OUCH! Bias too hot!"

[Guy77]

I really like this blue faceplate and the control knobs they matched it with!
So retro in the best way! Makes me think of watching a color 50's or 60's movie on TCM with Frank Sinatra or Rock Hudson in it!

Guy

[xtian]

Amp arrived. I checked it visually, and, finding nothing amiss, plugged it in. Perfect!

House voltage has dipped to 110vAC today (107 degrees in Chico!), so it's even more like living in the 60s.

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[xtian]

Can we discuss some "new to me" stuff on this Bogen schematic?

Power tube screens are connected to the 360v HT node, BUT, also connected via 470p > 1K2 to the OT primary leg, and (see dotted line) possibly alternatively to other leg. Is this a local NFB loop?

Power tube screens are also connected to cathodes with a 33K resistor. Why?

Also wondering about the connection of C25 to previous stages. Is this also NFB?

Finally, V2a has a 2K7 resistor on the heater pin 4. Why?

[sluckey]

Don't think of all that stuff as if it's connected to the screen. Think of all that stuff AND the screen as being connected to the 360V source which is at signal ground via a 20µF filter cap. So, there's nothing magical happening between the screen, cathode and that rc filter. Break it down and look at the three things that are connected to the 360V source.

1. Screen connects to 360v source. No brainer here.

2. R29 and R34 form a voltage divider from the 360v source to ground and the cathode is connected to the junction of the resistors. This voltage divider puts 3.5Vdc on the cathode. This voltage provides a bit of fixed bias on the 6V6s. This fixed bias is in addition to the normal cathode bias. I don't know why but I've seen several other amps that operated with fixed and cathode bias.

3. C24 and R31 form a shunt filter from one 6V6 plate (or the other, via the dashed line) to ground. Yes, it's connected to the 360V source, but remember, that 360VDC is at AC ground potential via the filter cap. I suppose this filter is meant to get rid of some hi-freq parasitic oscillation and should be factory connected to whichever tube plate gives the best performance.

The resistor connected to the heater pin 4 of V2 is NOT a 2.7K. It's a 2.7Ω and is used to reduce the filament voltage a small amount for some reason that probably only Bogen knows.

So, I don't clearly understand the whys but that's the way I break down the circuit.

[xtian]

Those are great...educated...guesses, Steve. Thanks!

[xtian]

Ask me if any of my previous questions matters now...

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[pompeiisneaks]

Gutted! literally and figuratively

~Phil

[Phil_S]

The resistor connected to the heater pin 4 of V2 is NOT a 2.7K. It's a 2.7Ω and is used to reduce the filament voltage a small amount for some reason that probably only Bogen knows.

Most likely the heater voltage is off because 1) the winder didn't get it right, or 2) the winding plan was wrong. If you have ever unwound a PT, you'd know the heater winding is just a few turns, so it is easy to be off by a quarter of half a turn. I've used this trick to drop heater voltages that was north of 7V.

[xtian]

Most likely the heater voltage is off because 1) the winder didn't get it right, or 2) the winding plan was wrong. If you have ever unwound a PT, you'd know the heater winding is just a few turns, so it is easy to be off by a quarter of half a turn. I've used this trick to drop heater voltages that was north of 7V.

Interesting theory... in practice, the heater supply seems nicely balanced, now that I stripped everything out and measured secondaries.

Coming along nicely. I adapted the Supro 6424 circuit that I've been building (this will be #4). This time, instead of paralleling the input triodes, I'm cascading them, with a Gain pot between.

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[sluckey]

The resistor connected to the heater pin 4 of V2 is NOT a 2.7K. It's a 2.7Ω and is used to reduce the filament voltage a small amount for some reason that probably only Bogen knows.

Most likely the heater voltage is off because 1) the winder didn't get it right, or 2) the winding plan was wrong. If you have ever unwound a PT, you'd know the heater winding is just a few turns, so it is easy to be off by a quarter of half a turn. I've used this trick to drop heater voltages that was north of 7V.

So why only drop the voltage to the 6U8? All the other tubes are connected directly to the filament winding as usual.