HELP! After installing a second output transformer from "laboratorio musicale " I started to obtain too high bias in put 1-8 and now I got "0" bias.
4 posters
ST-70 NO BIAS
joseramonnavarro- Posts : 9
Join date : 2022-07-09
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
joseramonnavarro wrote:HELP! After installing a second output transformer from "laboratorio musicale " I started to obtain too high bias in put 1-8 and now I got "0" bias.
If you are getting zero bias in your amp try another rectifier tube .. A bad (not working) rectifier tube is the most common cause of no bias on all output tubes. Note that just because the rectifier "lights up" is no guarantee that it is producing the high voltage that the amp needs.
Bob
Gabriel and joseramonnavarro like this post
Gabriel- Posts : 38
Join date : 2022-01-23
I purchased a Genalex GoldLion 5AR4 which lasted me approximately one month!
Switched the amp on this afternoon and no sound!
Checked the internal components, including the resistors. All good.
I swapped it out for my trusty 5u4gb electro harmonic, which despite its lower price, was told is more reliable?
Switched the amp on this afternoon and no sound!
Checked the internal components, including the resistors. All good.
I swapped it out for my trusty 5u4gb electro harmonic, which despite its lower price, was told is more reliable?
joseramonnavarro- Posts : 9
Join date : 2022-07-09
Thanks. I just replaced the previous rectifier and obtain the same results, zero bias.
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
joseramonnavarro wrote:Thanks. I just replaced the previous rectifier and obtain the same results, zero bias.
See if you can post a photo of the inside wiring of your amp after you have been a member 1 week. Read the sticky on how to post a photo on the Dynaco Tube Audio Forum.
If a new rectifier doesn't fix a "no bias" issue then it could be a bad selenium rectifier (if the amp is an original Dynaco ST-70) or a bad diode if the selenium rectifier has been replaced by a diode.
Bob
joseramonnavarro- Posts : 9
Join date : 2022-07-09
This is my st70, almost original. I do have negative dc voltage on the left side of the diode (-69).
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
If you do have a negative DC voltage of -69 VDC at the start of the bias circuit then try this > With the amp OFF, check the value of the two resistors and two capacitors on the 7 lug terminal strip. The resistors should be 10K resistors and the capacitors should be 50 uF to 100 uF.
Bob
Bob
joseramonnavarro- Posts : 9
Join date : 2022-07-09
Correct, both resistor are at 10K and capacitors 75uf 100vdc.
Bob Latino- Admin
- Posts : 3263
Join date : 2008-11-26
Location : Massachusetts
In some cases people measuring bias accidentally don't have their meter set on the proper setting OR the meter is not working. Make sure that you have your meter set to the 0 to 2 volt DC scale to measure bias. If your meter is autoranging > I had a customer whose meter stopped autoranging for some reason and would not measure bias anymore.
Try a different meter ? It is sometimes helpful to have an alternate meter in case your daily meter starts giving improper readings.
Bob
Try a different meter ? It is sometimes helpful to have an alternate meter in case your daily meter starts giving improper readings.
Bob
joseramonnavarro likes this post
joseramonnavarro- Posts : 9
Join date : 2022-07-09
Thanks: I ordered another voltimeter with capacitance measure also. I also notice to have the correct resistance ck at pin 1-8 been 15.6, also the negative -69 volts is present on pins 5-6 of all output tubes, el34.
joseramonnavarro- Posts : 9
Join date : 2022-07-09
I just received my new meter wih Capacitance Reading and attemptedto measure the capacitance inside the circuit, but, also just read that i need to remove all wires in the capacitor to test each section to ground. i am about to do this but first. Its anyway my no bias at pin 1-8 could be related to the quad capacitor? All sectuions are producing over 350vdc, please explain. than
thks.
thks.
joseramonnavarro- Posts : 9
Join date : 2022-07-09
I desolder all points and measure capacitance from ground to square, triangle, blank and semicircle and read this values 42uf 33uf 24pf and 86uf respectivelly.
Any ideas?
Any ideas?
joseramonnavarro- Posts : 9
Join date : 2022-07-09
Correction.... my capacitor is 80/40/30/20 starting with the halfmoon.
Could the faulty item be the choke? how can i test it?
Could the faulty item be the choke? how can i test it?
joseramonnavarro- Posts : 9
Join date : 2022-07-09
I AGREE 200% It does looks like a bad meter, not able to measure up to 2 volts. BUT...
I used another meter and there is no DC voltage on pins 1-8, and there is -69 vdc on pin 5, where does the voltage escaping to?
I used another meter and there is no DC voltage on pins 1-8, and there is -69 vdc on pin 5, where does the voltage escaping to?
Roy Mottram- Admin
- Posts : 1839
Join date : 2008-11-30
follow the wires from the -69vdc voltage divider, they go to the two bias pots. Make sure you have good connections on the two bias pots.
The center connection of each pot goes to the PCB and out to pins 5 & 6 of each output tube.
Under normal operating conditions, with the bias pots set near midrange, the voltage on each output tube pin 5 & 6 will be around -40vdc.
If you don't have that, you've got a bad connection somewhere. Once you have that, check the bias across the 15.6 ohm resistor, should be set to around 1.30vdc (not 1.56vdc as manual says)
The center connection of each pot goes to the PCB and out to pins 5 & 6 of each output tube.
Under normal operating conditions, with the bias pots set near midrange, the voltage on each output tube pin 5 & 6 will be around -40vdc.
If you don't have that, you've got a bad connection somewhere. Once you have that, check the bias across the 15.6 ohm resistor, should be set to around 1.30vdc (not 1.56vdc as manual says)
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