Another Noob with a tube question.....

[Cygnus X1]

Not to hijack the thread but...

Cyg, if I have a schematic for my amp, how can I tell if it's self-biasing or not?

With help from some of the amp experts around here
(I am NOT one of them) we ought to work up a sticky about biasing, what is what, and tube selection.

I know what I know, and one of the biggest problems is a very well known book (cough Aspen Pittman...Groove Tube Sales Guy...cough) has an otherwise fine book that has put the tube world upside down for years upon years.

For now let me link two very good, dependable...reliable sources for everyone to study.

http://www.aikenamps.com/

(Go to "Tech Info").

And:

http://www.tone-lizard.com/Table_Of_Contents.htm

Peruse at will.
Highly opinionated but I have not found a single fault in his reasoning.
I designed my SEL partially on his theory and found he is correct by tweaking, modding, fiddling, changing things until it came around to exactly what he is describing.
Read "Marshall Myths" very closely, and read between the lines.

http://www.tone-lizard.com/Marshall_Myths.htm

[DonP]

Not to hijack the thread but...

Cyg, if I have a schematic for my amp, how can I tell if it's self-biasing or not?

It's fairly easy. The bias feeds the power tubes with a negative voltage. You start at the power tube grids and work your way back to the power supply. If you see a variable resistor (or more) in the circuit, look to see if it's controlling the bias voltage.

[Cygnus X1]

It's fairly easy. The bias feeds the power tubes with a negative voltage. You start at the power tube grids and work your way back to the power supply. If you see a variable resistor (or more) in the circuit, look to see if it's controlling the bias voltage.

Absolutely.
I'm still reluctant to tell people where to run down the rabbit hole without the proper safety procedures in place.

Could you help work up a safe sticky?

Thanks,
Cyg

[Scooter]

Absolutely.
I'm still reluctant to tell people where to run down the rabbit hole without the proper safety procedures in place.

Could you help work up a safe sticky?

Thanks,
Cyg

Great idea. Especially how to complete discharge of stored power safely. Most will say tubes are relative to taste, but I'd say many opinions are valued. Thanks for the links, I'm heading there now.

[rat]

A self-biasing MOS linear amplifier is provided with improved means for maintaining the circuit biased to its linear amplification range. That means comprises a clocked feedback field effect transistor operatively connected between the output and input terminals and adapted to charge a storage capacitor at the input during a first clock interval, the bias level applied at the input being a function of the operating characteristic of the switching FET. The storage capacitor is effective to maintain the device so biased throughout the remainder of the clock cycle. In a second embodiment the circuit comprises a plurality of inverter stages, a single clocked feedback FET being operatively connected between the output and input terminals whereby the circuit is designed to stabilize in its operative linear amplification range during the first clock interval.

[Spivonious]

A self-biasing MOS linear amplifier is provided with improved means for maintaining the circuit biased to its linear amplification range. That means comprises a clocked feedback field effect transistor operatively connected between the output and input terminals and adapted to charge a storage capacitor at the input during a first clock interval, the bias level applied at the input being a function of the operating characteristic of the switching FET. The storage capacitor is effective to maintain the device so biased throughout the remainder of the clock cycle. In a second embodiment the circuit comprises a plurality of inverter stages, a single clocked feedback FET being operatively connected between the output and input terminals whereby the circuit is designed to stabilize in its operative linear amplification range during the first clock interval.

Uh.....what? :think:

:ROTF:


I took a look at the schematic and there are a couple variable resistors in between the grids and the power supply (if I'm following the circuit correctly).

I also never knew that a 6L6 was a tetrode until looking at this schematic. You learn something new everyday!

Oh, another question. There's a diamond shape with arrows going around it...what is that? It's labeled "BD1" with what I'd guess are pin numbers 1-4 coming off of it.

[Hellbat]

I'm going to guess bridge diode. It's for converting AC to DC.

[Spivonious]

I think you're right. After some more research, it's four diodes hooked together. I thought the transformer handled the conversion from AC to DC?

[rat]

I'm going to guess bridge diode. It's for converting AC to DC.

A rectifier bridge/diode would be correct..allows current flow in only
one direction. Capacitors are for storing a set sum of electrical current and
also used for filtering or cleaning DC current from AC current. Rectification is
also a way to convert AC to DC.

[Spivonious]

Okay, now bear with me here.

My amp (B-52 AT-100) has a switch on the back to switch between Class A, Class AB, and Diode rectification. What I see on the schematic is the mains going into a transformer, and then three circuits coming off of the transfomrer. One appears to be just for the power light. Another goes to the 5AR4 tube, to pin 2, which I read is the filament. Is this just for the heater?

Finally the third section appears to be where the actual power to the amp goes. Part of it goes to the bridge diode, and the other part goes to the standby switch, then on to another switch that appears to switch between the 5AR4 tube and a pair of diodes.

Something smells fishy here, and how exactly could a one tube rectifier ever be considered Class AB?

[Cygnus X1]

Okay, now bear with me here.

My amp (B-52 AT-100) has a switch on the back to switch between Class A, Class AB, and Diode rectification. What I see on the schematic is the mains going into a transformer, and then three circuits coming off of the transfomrer. One appears to be just for the power light. Another goes to the 5AR4 tube, to pin 2, which I read is the filament. Is this just for the heater?

Finally the third section appears to be where the actual power to the amp goes. Part of it goes to the bridge diode, and the other part goes to the standby switch, then on to another switch that appears to switch between the 5AR4 tube and a pair of diodes.

Something smells fishy here, and how exactly could a one tube rectifier ever be considered Class AB?

That's correct for switching between diode and tube rectification.
Pin 2 is the filament, the power light is usually 6.3 volts also.

The A/B is class of amplification (normally)...and I hate getting into those discussions.

[MetalDaze]

Everyone has their favorite tube vendor, but check out these guys:

https://ssl.eurotubes.com/cart/index.php?page=view_products&category_id=8&sub_category_id=158

They have a retube kit especially for the VK100. Plus, they are really responsive to questions about what will work best in a fixed bias amp depending on the type of sound you are going for.

[marcus]

Okay, now bear with me here.

My amp (B-52 AT-100) has a switch on the back to switch between Class A, Class AB, and Diode rectification. What I see on the schematic is the mains going into a transformer, and then three circuits coming off of the transfomrer. One appears to be just for the power light. Another goes to the 5AR4 tube, to pin 2, which I read is the filament. Is this just for the heater?

Finally the third section appears to be where the actual power to the amp goes. Part of it goes to the bridge diode, and the other part goes to the standby switch, then on to another switch that appears to switch between the 5AR4 tube and a pair of diodes.

Something smells fishy here, and how exactly could a one tube rectifier ever be considered Class AB?

The power amplifier operating class has nothing to do with the rectification strategy. They are unrelated. And transformers are used to change voltages, not convert ac to dc. As was stated by a previous poster, this is done by the 4 diodes connected in a diamond shape, which is a bridge rectifier.

[Spivonious]

The power amplifier operating class has nothing to do with the rectification strategy. They are unrelated. And transformers are used to change voltages, not convert ac to dc. As was stated by a previous poster, this is done by the 4 diodes connected in a diamond shape, which is a bridge rectifier.

It's not the power amp class. The manufacturer calls it "tri-mode rectification". It has the aforementioned bridge rectifier, which is used when the switch is in SS mode and a 5AR4 tube which is used in A and AB mode. I had my friend who fiddles with amps look at the schematic and he pointed out the correct switch. I'm still confused how a single tube can be operated at anything but class A.

[tomanyjacksons]

FWIW I contacted Peavey about my VK 212 and asked about replacing the power tubes and they told me they have to be a matched set of 4. I also joined their forum and talked to as many people that would answer me and most of them said that JJ's would be a vast improvement in sound, giving it more headroom. Also I have read a bunch of different things on the internet about doing any kind of work on a tube amp and most of them said to do the work with one hand in your pocket, which sounds kind of tuff to me but I guess it will keep any electricity that you manage to shock the snot out of yourself with from going through your heart. Good luck.