Bias voltage question

I have owned my St70 for many years. It has a capacitor board and a cloan board I installed several years ago. It was been working perfectly. I saw, on Ebay, a pair of LED bias voltage meters that fit in the front socket. I thought that was a good idea and bought it. Before I installed the LED meters I check the bias voltage and set it 1.50 volts. Installation was easy and took about an hour. To my surprise they read 5.9 volts! I rechecked my work and made sure the ground was good. No errors were found. Put the tubes back in and connected up the speakers- same results.
At first, I thought I am reading filament voltage but the senor wire is on pin 1. Either the meters are wrong or I really screwed up when I disconnect the front socket.
My question-do I remove all the tubes and test for bias voltage at pin 1 to ground? I think I read that I should see .40 volts with no load. Am I correct? All ideas gladly accepted!

Interesting idea. Do you have the link or instructions, maybe we could see what goes where from then.

Just a thought. If you remove the tubes won't the voltage across the cathode resistor drop to 0v as no current will be flowing?

If you can set the bias voltage to the correct value across the resistor with the tubes in then the amp is ok and it is the meter that is reading wrongly.

How are they supplied. I only say this as the heater winding is normally floating and to get a good voltage reading something must be grounded at the meters as a reference unless they have an isolation circuit.

Depending on how the plug in meters are designed, I'd think there's an adjustment you need to make to compensate for the additional resistance added by the meter circuit. That was a real head scratcher here when I was adding meters to my custom build. I ended up adding trim pots to the meters. I still have test points to use a VOM, set the bias that way, the adjust the trim pots to get the same reading on each tube.

That's a one shot process for initial setup. As mentioned, all you're doing is calibrating the add on circuit to match the amp.





I'd assUme that'd be easy enough to do with what you're using if it's a simple plug 'n pray installation IF there's an adjustment on the meter modules. Can't really get more specific without knowing exactly what you're using and how it does what it does.

PS, when I say meter error, I meant meter error ... it was what some might call ... significant.



(And ya, I know I got the VOM on the wrong range ... so shoot me already)

The diagram supplied with the kit has some known errors. It shows the bias voltage sensor wires going to pin 5. The written instructions tell you to tie the wire on pin 1, which makes sense. The kit is very easy-only four wires to connect. One wire connects on the switch, one wire to the fuse and each voltage sensor wire to pin 1 on the front tube sockets. The meters are grounded thru a two terminal strip (and I am sure it is grounded). I feel I need to find out if the meters are wrong or did I foul up the bias voltage. The twisted pairs of wires are soldered to their mate from the socket and the the resitors from the pc board to the sockets are removed. Simple. How can it go wrong?

Any more info that might allow us to assist? With only four wires and a mention of connecting to the fuse this would indicate that the meters are line powered.

Yes the meters are line powered thru a small transformer. The meter is towards the front of the picture. The purple wire is the voltage sensor wire from the kit. I soldered it (and shrink tubed it to the wire that went to the front socket terminal one. Simple. Since, I lost the easy way (front socket) to test bias voltage how do I test bias voltage to prove if the meter is incorrect or the bias is wrong?
url=https://servimg.com/view/19545657/3][/url]

But your bias was correct before you started so it shouldn't be wrong now. In your shot you can see a 15.6 ohm resistor. One side of this is grounded to the chassis. The other side should be your bias voltage of approx. 1.56 volts. If you really had an incorrect bias giving you 5.9 volts then I would expect to see some smoke signals.

If your meters are line powered then two of the wires will be supply and the other the input. Without more details of them, which despite requests, you have not given, it may be assumed that these are the input lines and one should go to ground and the other the cathode side of the 15.6 ohm resistor.

Just to rule out the meter, I would disconnect one, and connect it to a 1.5V battery. If it reads about 1.5V, the meter is good!

Good idea. Just be aware that 1.5V alkaline Duracells typically read 1.6V+ when new.

Excellent idea! I'll give the battery a try.

Connect the sensor lead to a D cell battery. Unsoldered the power leads from the chassis so I could power the meter without the amp being on. My meter claimed the voltage from the battery to be 1.599.  To my surprise the LED meter read 5.71 volts. Something is wrong with the meter! Thanks guys got it figured out.
url=https://servimg.com/view/19545657/4][/url]

It is interesting to note, that the ad on eBay, seems to show the blue LED meters displaying 5.01 volts, whereas the grren and yellow LEDs shows a proper 1.38 volts!

http://www.ebay.com/itm/Visual-Led-Bias-Monitor-Kit-In-4-Colors-For-The-Dynaco-Dynakit-ST-70-/282187470550?hash=item41b3af16d6:g:~5IAAOSwQupXUPxZ

Perhaps, you just need to subtract 5.01 volts, and multiply the remainder by 2, to get the correct bias!!!

Hmmm good spot... And what happened for the Red LED ones, guess the amp was still warming up!!

You would have thought the advertiser would try to at least represent their product giving a sensible reading.

If the meters read 0.00 when disconnected the you could always put in a potential divider to get them to read right.

I have contacted the seller-he is looking into the issue. I can't be the only one that has a problem.

Hi! were you able to make it work with the seller? will you recommend this solution?

Yes, the seller worked thru several possible items. It turned out the transformer was wired to the board backwards. I reversed the leads and it worked! The meters have been flawless ever since. The sellers paperwork poor but he was quick to respond to questions and kept working with me.

Hi Eric,
I am looking into aquiring these led bias displays for my future ST-120 build and am wondering how they have fared since.
Also, would it not be possible to make this plug into the front sockets instead of replacing the sockets with these entirely?

Eschew needless complexity.

William of Occam, 1285 - 1347

Roy D wrote:Hi Eric,
I am looking into aquiring these led bias displays for my future ST-120 build and am wondering how they have fared since.
Also,  would it not be possible to make this  plug into the front sockets instead of replacing the sockets with these entirely?

They have done very well. I don't think the brightness has dimmed at all. I check the accuracy with a multi meter just before Christmas and they were spot on. They are set up as a screw attachment I don't see how to make them a plug in.

Good to hear they remain accurate - a little less brightness I would appreciate though I think lol