B- voltage for tube preamp

[Potato Cakes]

Hello, everyone,

Earlier this year I acquired a pair of Collins 356B-1 program amps that I was trying to wire up before I headed back out on tour. In the original manual, it shows a "B-" for one of the connector pins, which I have never seen. For the voltage requirements, it just says voltage is 250-300 volts DC and 6.3 volts AC or DC. I was referencing some other tube preamp designs and it seems that what the Collins schematic refers to as B- is tied to ground through a resistor. Would the connection labeled B- just be tied to ground? I don't see anywhere else in this specific circuit where the tubes have any ground reference connection of any sort.

Thanks!

Paul

 

[Potato Cakes]

Here a jpeg of the schematic so you guys don't have to download it to look at it.

 

[ruffrecords]

B+ is the HT voltage and B- is it 0V reference. B+ and B- are a left over from the days of battery power when the terminals were marked _+and -. Note tha in this module the heaters are completely isolated from the HT supply. Somewhere, usually back in the external power supply, the heaters would be directly or indirectly referenced to B-.

Cheers

Ian

 

[PRR]

> it seems that what the Collins schematic refers to as B- is tied to ground through a resistor.

I don't see any path B- to ground.

B- connects to Amplifier Common. B+ goes to the usual places. Put your 300V battery across B+ and B-. Put your 6V supply at 6V screws. Connect ground (chassis, not amplifier) to your rack and general dirty ground system.

All audio in and out are transformer, floating.

As Ian says, "somewhere" the 300V, 6V, and dirty ground come together. Usually at a power supply. However the amplifier WILL work with an un-grounded 300V battery or isolated power supply. The tubes do not need to "see ground", they just need to feel both ends of a 300V supply. Possible increased hum and lightning-risk due to no path to room ground or heater supply, which is why we normally bond A B and chassis somewhere.

 

[Potato Cakes]

Thanks, Ian!


Paul

 

[Potato Cakes]

PRR,

Sorry, what I was trying to say is that what is called B- on the Collins schematic is connected to a ground reference on other tubes designs that I compared to this one. B- is indeed not connected to ground in the 356B schematic.

Thanks,

Paul

 

[ruffrecords]

Ground is a very ambiguous term. The B- on the schematic you posted is connected to signal 0V. There are various 'earth'  symbols on the same diagram and these will refer to the chassis.

Cheers

Ian

 

[Potato Cakes]

Thanks for clearing that up! It'll be a couple of weeks before I source the case and a couple of other parts to put this all together. I'll post some pics when it's done.

Thanks!

Paul

 

[Potato Cakes]

Just a quick update on this project. I finally got a case for it, but I'm lacking a number of switches and some other small parts. These modules are very big and they just seem to fit while allowing enough room to do all the wiring. Hopefully one more mouser order and I'll be set.

 

[Potato Cakes]

And the Layout

 

[TimG]

How did this work out for you? I just saw a deal on a 356b?

 

[rmburrow]

How did this work out for you? I just saw a deal on a 356b?

The Collins 356 series mic preamp and program amp are perhaps the best quality tube modules in the old Collins radio station consoles. The 5879 tubes are easier to get and probably less expensive than the EF86's used in the Gates M5215A mic preamp. (The earlier Gates M5215 mic preamp used 5879's.) There is a two section 30-30 uF aluminum can capacitor on the M5215 boards used for decoupling; the Collins module probably includes one of these. Check this, replace if necessary.

 

[Mihobro]

> it seems that what the Collins schematic refers to as B- is tied to ground through a resistor.

I don't see any path B- to ground.

B- connects to Amplifier Common. B+ goes to the usual places. Put your 300V battery across B+ and B-. Put your 6V supply at 6V screws. Connect ground (chassis, not amplifier) to your rack and general dirty ground system.

All audio in and out are transformer, floating.

As Ian says, "somewhere" the 300V, 6V, and dirty ground come together. Usually at a power supply. However the amplifier WILL work with an un-grounded 300V battery or isolated power supply. The tubes do not need to "see ground", they just need to feel both ends of a 300V supply. Possible increased hum and lightning-risk due to no path to room ground or heater supply, which is why we normally bond A B and chassis somewhere.

Please note: the schematic has heater supplies labeled 6.3VAC, this is usually a separate 6VAC (there is no polarity here like a DC circuit) secondary on the power transformer (pretty much on most tube designed transformers). No need for either side to be grounded. But some people will opt for a DC heater voltage as being quieter under some conditions. But it is better to start from the original to establish a base line for any modifications to the original design.