Bias test points and HV fusing question

[Tonegeek]

On my Twin to #124 conversion I want to use one bias pot and only 2 test points, one for each phase. Is this the right way to do the test points? (see attachment). What wattage resistor should I use? I am thinking 5W should work seeing how I used 2W ones on my 50 watter with one per tube.

ALso I want to add a fuse on the high voltage. What circuit should I fuse? I was thinking the connection between the rectifier and the first filter.

[ChrisM]

Those test points will work. You'll be measuring the current for both tubes though, keep that in mind.

You probably wont be pulling more than 100mA across that resistor (for a pair). Maybe more than 100mA but most ODS are biased kinda cool.
P = VxI (V and I and are the same here)
= 100mA ^2

You can use 1/2W, less even. I generally use 1W or 2W 0.1% Dale.

[kd]

I don't know if it is considered correct or not but I usually place the fuse between the first filter cap and the center tap of the OT.

Ken

[M Fowler]

here is another way. Oh yeah I forgot you only wanted two points sorry.

Mark

[martin manning]

On my Twin to #124 conversion I want to use one bias pot and only 2 test points, one for each phase. Is this the right way to do the test points? (see attachment). What wattage resistor should I use? I am thinking 5W should work seeing how I used 2W ones on my 50 watter with one per tube.

The layout drawing Mark posted is the same as yours electrically, it's just missing jumpers to the neighboring cathodes for a quad.

With a 400V supply, one 6L6 can pass close to 350mA on something a bit more than a 50% duty cycle when driven to square wave, so being conservative and using 350mA to get a power rating for the resistors I^2*R is 0.35A^2*1Ohm*2tubes=0.25W. I think most people use a 1W, even if there is one resistor for each tube.

Also I want to add a fuse on the high voltage. What circuit should I fuse? I was thinking the connection between the rectifier and the first filter.

HT fuses are usually on the AC side of the rectifier, between the transformer CT and ground, or better yet in line with both sides of the secondary (using two fuses) to break all current paths beyond the PT secondary. Putting the fuse after the rectifier won't protect the PT from a rectifier short. Fusing between the power amp plate supply and the OT center tap will provide protection for the power section and protect the power supply from problems in the power amp, but not from a short in the pre-amp. This should be only considered as an extra precaution, in addition to the HT being fused at the PT secondary, which already protects all of the HV circuitry. Merlin's Valve Wizard site has some good discussion on this.

[llemtt]

consider also 0.25 - 0.5 W flameproof testpoint resistors -> they act like a fuse if something goes wrong

cheers

[Structo]

consider also 0.25 - 0.5 W flameproof testpoint resistors -> they act like a fuse if something goes wrong

cheers

This was what I was going to mention.

There are a couple schools of thought on these resistors.
Using a lower wattage resistor can cause it to act as a fuse if there is a catastrophic failure of one or more tubes.

If you have a power tube that goes dead short, a 1/2 watt 1 ohm resistor may react faster than a slo blo fuse, possibly saving the amp from more damage.

If you make that resistor so large that it does not open under any circumstance, then a lot of things can happen before a 3-5 amp slo blo fuse goes open.

I slipped with a meter probe while measuring power tube plate voltage once on my amp.
I started an arc with the probe and it probably took three seconds for the fuse to let go. I don't know for a fact how long the resistor would take.

Currently on my amp I have 1 watt 1 ohm Dales bias check resistors.

[ChrisM]

Well if you used the bare minimum power rating wise it would take less time then say a bigger power rating. You are pushing your margin for error though. You don't want to take out that little resistor just anytime (say when you play a big chord loud). You only want to take it out when something goes wrong.

I don't know... I was always told you should never have resistors acting like fuses. Then again new production tubes are ya know, kinda sketchy.

[VacuumVoodoo]

Fusible resistor. Look it up. It's what you want.

[Structo]

http://www.vishay.com/docs/31031/cmffuse.pdf

[Tonegeek]

On my Twin to #124 conversion I want to use one bias pot and only 2 test points, one for each phase. Is this the right way to do the test points? (see attachment). What wattage resistor should I use? I am thinking 5W should work seeing how I used 2W ones on my 50 watter with one per tube.

The layout drawing Mark posted is the same as yours electrically, it's just missing jumpers to the neighboring cathodes for a quad.

With a 400V supply, one 6L6 can pass close to 350mA on something a bit more than a 50% duty cycle when driven to square wave, so being conservative and using 350mA to get a power rating for the resistors I^2*R is 0.35A^2*1Ohm*2tubes=0.25W. I think most people use a 1W, even if there is one resistor for each tube.

Also I want to add a fuse on the high voltage. What circuit should I fuse? I was thinking the connection between the rectifier and the first filter.

HT fuses are usually on the AC side of the rectifier, between the transformer CT and ground, or better yet in line with both sides of the secondary (using two fuses) to break all current paths beyond the PT secondary. Putting the fuse after the rectifier won't protect the PT from a rectifier short. Fusing between the power amp plate supply and the OT center tap will provide protection for the power section and protect the power supply from problems in the power amp, but not from a short in the pre-amp. This should be only considered as an extra precaution, in addition to the HT being fused at the PT secondary, which already protects all of the HV circuitry. Merlin's Valve Wizard site has some good discussion on this.

Math looks good.

Fusible resistor. Look it up. It's what you want.

http://www.vishay.com/docs/31031/cmffuse.pdf

Interesting. It appears Mouser does not have a 1 ohm version.

I will look around for one, but otherwise I think I will use a flameproof 1 watt resistor. Also I will put an HV fuse after the rectifier like Marshall does it although I agree that 2 at the PT secondary would be better. Not too worried about the PS diodes shorting. If so, the AC fuse should open up.

[Structo]

I wonder if a 1/2 amp on the CT would be better?

[Tonegeek]

I wonder if a 1/2 amp on the CT would be better?

I meant 2 fuses, not 2 amps . 250 ma each or 500ma if using one. So you are saying don't worry about the pre-amp and just protect the output? I suppose it is pretty unlikely that the preamp would short out unless a plate feed came loose. On the other hand, if you fuse between the diodes and the first filter cap, you are protected if something in the PS shorts out, although I suppose the AC fuse might open up in that case. I just don't know.

[martin manning]

I wonder if a 1/2 amp on the CT would be better?

I meant 2 fuses, not 2 amps . 250 ma each or 500ma if using one. So you are saying don't worry about the pre-amp and just protect the output? I suppose it is pretty unlikely that the preamp would short out unless a plate feed came loose. On the other hand, if you fuse between the diodes and the first filter cap, you are protected if something in the PS shorts out, although I suppose the AC fuse might open up in that case. I just don't know.

I believe Tom is saying fuse the PT CT with a 1/2A fast-blow. Diodes do fail to shorted, and sometimes in a soft kinda way.

[Structo]

Yeah sorry, 500ma on the HT CT.

That seems to be the way Marshall did it on a few amps.

On the 1987 Plexi 50 watt they put a 500ma on the B+ line right before the choke.

I think the secret is to use a fast blo there because the current usually isn't much there so if it does go up something dreadful is wrong.