• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Easy DIY tube amp


Paid Member
2011-10-31 1:00 am
In my older designs I used an 8A fast blow fuse... Now I use a 3A slo-blo.

For the SPP at 230V, I'd say 3A fast acting or 1A slo blo. if 1A isn't large enough go to 1A5.
The only thing that will blow the fuse besides a fault is the initial surge current on turn on, and where it is in the waveform when you hit the switch.
Double protection:

1. A Fast Blow Fuse in the power transformer primary for the inrush current of a cold amplifier at power up.
Example: Cold filaments (1/4 of hot resistance, 4 times the current at turn on), and if the B+ uses solid state rectifiers, the charging of the filter cap is very quick - a large turn on transient current.

And . . .

2. A Slow Blow Fuse in the power transformer primary so that if a warmed up amplifier has a fault . . .
An output tube that has thermal run-away current; an output tube that has the screen get hot and deform badly, so that it touches the control grid (short).

All my amplifiers have the power transformer primary fast blow and slow blow double protection:
One of my amplifiers has a 1.5 Amp fast blow fuse for the inrush, and a 0.5 Amp slow blow fuse in case of an output tube fault.

Your fast blow fuse and slow blow fuse ratings will vary (measure the maximum inrush current; several times; it happens on a cold amplifier, when the power switch comes on at the crest of the sine wave).
Start with a small value fast blow, if after a few cold start ups, it blows, then use the next higher current rating.
Then, after finding the correct fast blow fuse, put in a small value slow blow in series. If it blows, then try the next higher current rating, etc.

One amplifier I had would power up and run for many hours; it had a 0.5 Amp slow blow fuse; but after a few weeks the slow blow fatigued and opened up. A 600 mA fuse now always works (0.6 Amps, just a litter hard to find a source for one). It never blows now.

Just my opinion.
I have a pcb based on a wiring diagram of the audio research VT60 SE, in the list of components are indicated RIKEN 2watt resistors - 4 x 200ohm, 4 x 10ohm and 1 x 1ohm, it is specified that they must be selected with a minimum deviation from the indicated value. Unfortunately they are almost impossible to find.
I bought vishay dale 2watt 200ohm 1% tolerance (CPF2200R00FKB14) - dale 2watt 10ohm 0.1% tolerance (CPF210R000BHE14) and dale 2watt 1ohm 0.5% tolerance (CPF21R0000DKB14).
I think there should be no problems with the operation, but I would like a guru's opinion.
I would like to thank in advance those who do their best to answer my question.
attach wiring diagram


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Well they are exagerating, not only precision, but also al those parallel resistors :ROFLMAO:
5% is close enough exept for R24-25. If you want exact reading of the bias current use 1%.
With all those resistors the importent ones are missing :cautious:
If a bias potmeter fails the tube has no negative grid voltage no more, not very healty.
To prevent a catastrophy put in 4 resistors.


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