Bias Test Points

[Scott_F]

Okay, poll time again...

what do you think about bias test points that allow you to just plug in a multimeter and take a measurement?

I can see how it could be good, but then again, in the wrong hands, someone fries themselves. Maybe an amp tech is the way to go.

What do you think?

 

[theodie]

Re: Bias Test Points

Personally They are a bit more convenient for me. I like them but, someone that dosent know what they are doing can easily fry them selves. I say its a toss up. I want mine with them though!!!!!!!

 

[DeadSkinSlayer3]

Re: Bias Test Points

I personally love them in my XXX. I use the amp so much that I couldn't take it to a tech, and no local tech I've found has a decent turn around time.

If I had an amp without them, I'd have to have a spare amp around. Wouldn't mind that too much, though...

 

[STRATDELUXER97]

Re: Bias Test Points

1 ohm cathode resistors(2 watt or better)inside the amp on the cathodes of each tube is good enough...That forces a guy to peek around in the amp to make sure all is well inside...

 

[Xeromus]

Re: Bias Test Points

explain this to me, how it works

 

[STRATDELUXER97]

Re: Bias Test Points

explain this to me, how it works

On the 6L6/6V6 BF amp we'll use as an example,you do the following...

On pin 8 of each output tube inside the amp and at the tube socket...You disconnect the braided ground wire and you place a 1 ohm 1% resistor rated for 2 watts or higher in series and in between the ground wire and pin 8 of each output socket...This resistor is in "series" between pin 8 and ground...Pin 8 is the cathode...

So...You now have one end of the resistor soldered to pin 8....The other end of that resistor gets soldered to the grounded braided wire...

The resistor changes milliamps to millivolts....You place the red lead on pin 8 on one end of the resistor and the black lead on the grounded end of the resistor,set you meter to read in millivolts......Set your amp to a value in millivolts normally within the range of 30 to 40 millivolts....Use the tube giving you the highest reading as the one to set to....

There is a specific formula based on tube plate dissipation and plate voltages that I have written down,but I've been biasing my own amps for many many years now and so I know what to set my amp bias to....

Be careful because you can shock the piss out of yourself or die if you don't know what you're doing! :wink:

You can not do this mod on every single tube amp out there because other tubes like EL84s have different pin outs....Some amps are actually cathode biased already and they self bias once you stick your output tubes into your amp....

 

[Alvin Lee Fan]

Re: Bias Test Points

I like the option of external test points, but for the amps that don't have them I've got a Weber Bias-Rite that also reads plate voltages. Works great! Instead of, or in addition to the external test points, provide a pot to adjust the bias. That's more important IMO. You can always get a Bias-Rite or similar to take your readings.

 

[Scott_F]

Re: Bias Test Points

I like the option of external test points, but for the amps that don't have them I've got a Weber Bias-Rite that also reads plate voltages. Works great! Instead of, or in addition to the external test points, provide a pot to adjust the bias. That's more important IMO. You can always get a Bias-Rite or similar to take your readings.

agreed, a bias test point doens't mean much if you can't easily get access to the trim pot.

 

[casblah]

Re: Bias Test Points

I think that they are a novel idea. :dance:

 

[STRATDELUXER97]

Re: Bias Test Points

agreed, a bias test point doens't mean much if you can't easily get access to the trim pot.

Scott.....I don't like drilling holes in my 66 amp and it takes less than five minutes to pull the chassis to set my bias....Adding external jacks on a concept amp,is a great concept though...I'm not recommending drilling holes in any vintage amp however.. :wink:

Your way is safe and conveinant....I've been biasing every tube amp I Own this way for quite awhile now... :dance:

 

[Scott_F]

Re: Bias Test Points

I've been hacking up my 68 Pro Reverb on the inside, but the outside looks completely stock and well... the inside can be stock again at some point if it *had* to be...

 

[STRATDELUXER97]

Re: Bias Test Points

I've been hacking up my 68 Pro Reverb on the inside, but the outside looks completely stock and well... the inside can be stock again at some point if it *had* to be...

Having the test points underneath the chassis would be ok though,but not on the back panel...

You can mount a nice 10k Cermet pot under there also...Mouser sells these metal housings that mount to the chassis and the cermet pot goes inside that little housing...It's strong,safe,and would hold up well...No JB Weld,Silicone globs,or Krazy Glue to hold that pot onto the chassis... :dance:

 

[Scott_F]

Re: Bias Test Points

Having the test points underneath the chassis would be ok though,but not on the back panel...

You can mount a nice 10k Cermet pot under there also...Mouser sells these metal housings that mount to the chassis and the cermet pot goes inside that little housing...It's strong,safe,and would hold up well...No JB Weld,Silicone globs,or Krazy Glue to hold that pot onto the chassis... :dance:

I think just a standard dual sided screwdriver adjusted bias pot is plenty. Not sure I want to just put a knob on it. Too many wankers would twist it just to see what it did.

 

[STRATDELUXER97]

Re: Bias Test Points

I think just a standard dual sided screwdriver adjusted bias pot is plenty. Not sure I want to just put a knob on it. Too many wankers would twist it just to see what it did.

But you'd have the adjustment on the outside of the chassis or on the inside? With it on the inside,you'd still have to pull the chassis to adjust bias? :wink: The adjustment I'm talking about requires a small jeweler's screwdriver that has to be inserted into a sought of small tunnel to make your bias adjustments...

Maybe I'm not following you Scott? :smack: :laugh2:

 

[Scott_F]

Re: Bias Test Points

doesn't your pro reverb have a double sided adjustable pot on it? you don't have to pop the chassis out if you either use a weber biasrite or a some test points...

 

[Gearjoneser]

Re: Bias Test Points

I think every amp should have them on the back of the chassis. If anything, it makes it safer for the non-tech to bias their amp, since they don't have to have a live amp exposed. Marshall DSL's and Fender Pro Tube Series amps already have them.

 

[Rich_S]

Re: Bias Test Points

Test points are great, but need to be designed right, i.e. "finger safe". On some amps, they would have enough voltage on them to shock someone, but at least it's not B+.

In today's litigious society, some moron could just as easily argue that you are liable because you DIDN'T include bias points, thereby encouraging him to open up the amp and cause himself grievous harm.

 

[STRATDELUXER97]

Re: Bias Test Points

doesn't your pro reverb have a double sided adjustable pot on it? you don't have to pop the chassis out if you either use a weber biasrite or a some test points...

Yes....It has the stock style pot that you can adjust from inside or outside of the chassis....I was thinking more on the lines of what I did for a buddies JCM 800 a few years ago....We converted the board mounted pot to the outside chassis and 2 test points so he could plug his meter in. The cermet pot was hard wired off the board and over to the new chassis mounted pot...My mindset was on that and not the fender style pot... :duh:

The pot we used was a multi-turn type and allowed for nice precise adjustments...Still workin great(Thank God!) :laugh2:

 

[chcjunior]

Re: Bias Test Points

All I know is I loved the set up my DSL 50 had. Three little prongs in the back, and the bias pot to tweak. ALL amps should have this set up. I need to figure out what the bias is on my Rivera so I can get some JJ's for it, and I'm quite honestly afraid to try and wing it.

 

[STRATDELUXER97]

Re: Bias Test Points

All I know is I loved the set up my DSL 50 had. Three little prongs in the back, and the bias pot to tweak. ALL amps should have this set up. I need to figure out what the bias is on my Rivera so I can get some JJ's for it, and I'm quite honestly afraid to try and wing it.

Maybe you could call and ask Rivera himself?