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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Skunkie Designs 300B

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#141
Resuscitate the delayed breadboard of this project.
I am using Meanwell ac dc 5V smps to power filament of 300B.
Is there any need for the 100R balancing potentiometer across pin 1 and 4 of output tube ? Any advantage or disadvantage ?
BTW, l have the 100R potentiometer already
 
#144
The Skunkie schematic has 414V dropping to 222V across a 47k resistor going to the second plate. This gives about 4mA. The current running to the first half of the 6SN7 should be about the same. If I'm interpreting this correctly, you get another half mA going to the second grid.

Note that the current draw from the power supply is 4mA, since the plates and cathodes are wired in series.
 
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#145
Ybolg,

You said: "another half amp going to the second grid". I am sure you meant another half mA, not another half Amp.
But, there is no grid current there at all . . .

If that circuit is Skunkie's cascode circuit, then there is no grid current for the bottom tube, and no grid current for the top tube of the cascode.

If you get any grid current in either of the cascode tubes, it means one of 4 things:
Either you are sending too many volts of signal to the amplifier input,
Or, one of the tubes is faulty, so replace it.
Or, the wiring is incorrect,
Or, the resistor values are incorrect.

Just a few hints for Newbies.
 
#146
ou said: "another half amp going to the second grid". I am sure you meant another half mA, not another half Amp.
But, there is no grid current there at all . . .

If that circuit is Skunkie's cascode circuit, then there is no grid current for the bottom tube, and no grid current for the top tube of the cascode.
Click to expand...
Oops, yeah I meant mA. Corrected it.

I was imprecise in my wording there. I got the half mA based on the 414V to 91V drop across the 680k resistor connected to the grid, but I assume most of that current draw is going across the 200k resistor to ground (and 94 across 200k shows about the same current). I did want to point it out as it is another (albeit tiny) current draw coming from that leg of the power supply.
 
#147
@6A3sUMMER, there is an extra 0.5 mA in the supply circuit for the grid of top tube. It is necessary to bias the tube correctly but there is no grid current of course. It is a voltage divider and the current goes to ground. See schematics at post #1. They were just trying to understand the total current draw from the supply and it is about 4.5 mA for each channel. That's all.
 
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#149
I am building a 300b amp using the HV from this design, my caps are 40u, and 150u after the chokes. I am finding that I blow a 2A fuse on the primary of the transformer at startup.
The funny thing is that the fuse blows as the DC voltage is at 41V. That happens to be close to the 5AR4 plate/fil drop if I remember correctly. I see the filament glowing and then the fuse blows at 41VDC which I am guessing is where the rect tube starts to conduct.
Is this because of excessive capacitor charge current? I double and triple cheched everything. Basically I am trying to thes the HV section without load to make sure everything works, do I need to put a load to test it?
First tube amp build and I am loving it already....🙄! No really, so far the layout has been alot of fun.
thanks
 
#150
If you have checked for short circuits, the rating of the primary fuse may be worth checking.

The transformer can take a big pulse of current at startup, so the fuse should normally be a T rated (time delay type). Even with that, 2A(T) seems a bit close for a 115V line fuse.

Modern 5AR4 tube rectifiers are not great quality, look up the trick of adding series uf4007 diodes to protect them.
 
#153
Thanks again Rod,
I found the posts about the backup diodes and the thermistor. Amazing that most schematics out there do not include these as default! These should be standard issue IMO. Now after reading of 'sparked out' JJ GZ34 and Svetlana 5AR4 on FIRST POWER UP, I am worried I may be in that unfortunate camp! In fact, it just so happens that these are the very rectifier tubes I purchased! I wonder how many more surprises I have in stock and how many tips and tricks these plug&play projects remain undisclosed. 🤣
 
#156
NOS GZ37 fits the socket, but it drops more voltage and costs too much. All the modern types have a more-or-less spotty track record.

I got tired of all varieties of octal rectifier, on account of short life and faillures modes that can cause damage elsewhere. The NOS damper diodes (6CJ3, other 6C*3 and 6D22S) drop very little voltage and are very tough, and last for decades without problems; but the heater power is greater, the sockets are odd, and you usually need two bottles - but I found them worth all of this hassle.

The other thing that works is Silicon Carbide 1200V diodes and a slow-rising cap-multiplier with TO-247 packaged FET on a big heatsink. This also allows the use of big Kemet ALS or ALC caps for better low-frequency performance.
 
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#159
That's exactly why I mentioned the slow rise capacitor multiplier.

I designed mine to show a rise-time of 30s, to match the 6CJ3 dampers I was using up until then.

The multiplier needed a 10uF 500V film capacitor to get there, with a source resistance of 3M approximately.

It has been working perfectly for many years, and sounds quite like the damper solution.

The bulk capacitors are two 820uF 550V Kemet ALC, which provides an edge for low frequencies.
 
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