I am at my wits end. I jeep trying to bias this amp and to no avail. First I did the 1N4007 on the 5ar4 mod. This stopped the fuse from blowing but now the tubes glow orange and then the fuse blows after about a minute. When it blows the voltages measures 4v on both left tubes. I only have in the 5ar4 and the 2 left channel EL34's. I switched out the 22k resistor on the SDS cap board for a 2.2k resistor for R9 and bridged R10. [/size]
4 posters
VTA upgrade board with SDS cap board bias problem.
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
[size=31] PLEASE HELP!
I am at my wits end. I jeep trying to bias this amp and to no avail. First I did the 1N4007 on the 5ar4 mod. This stopped the fuse from blowing but now the tubes glow orange and then the fuse blows after about a minute. When it blows the voltages measures 4v on both left tubes. I only have in the 5ar4 and the 2 left channel EL34's. I switched out the 22k resistor on the SDS cap board for a 2.2k resistor for R9 and bridged R10. [/size]
I am at my wits end. I jeep trying to bias this amp and to no avail. First I did the 1N4007 on the 5ar4 mod. This stopped the fuse from blowing but now the tubes glow orange and then the fuse blows after about a minute. When it blows the voltages measures 4v on both left tubes. I only have in the 5ar4 and the 2 left channel EL34's. I switched out the 22k resistor on the SDS cap board for a 2.2k resistor for R9 and bridged R10. [/size]
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
Last edited by tharhai on Wed Feb 04, 2015 8:17 pm; edited 1 time in total (Reason for editing : error in image)
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
audiobill- Posts : 425
Join date : 2014-03-13
Location : Albany, NY
Do all the tubes red plate, or just one?
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
They all red plate after about a minute. I tried different tubes that were also brand new on both the left and right channel. They all exihibit the same behavior of overheating after they are fully warmed up. Voltage climbs fast up to 4v then stops and fuse blows about 5-10 seconds later. I measured the pins #5 and #6 of each output tube socket last night. Whereas they should be between -45 to -65 (I believe), they all measured closed to zero. (For example 0.03v) This seems to be related to my problem but I don't know where to proceed from here. I also checked all solder joints again and reflowed a couple that looked like they might be cold but no change.
Bob Latino- Admin
- Posts : 3260
Join date : 2008-11-26
Location : Massachusetts
tharhai wrote:They all red plate after about a minute. I tried different tubes that were also brand new on both the left and right channel. They all exihibit the same behavior of overheating after they are fully warmed up. Voltage climbs fast up to 4v then stops and fuse blows about 5-10 seconds later. I measured the pins #5 and #6 of each output tube socket last night. Whereas they should be between -45 to -65 (I believe), they all measured closed to zero. (For example 0.03v) This seems to be related to my problem but I don't know where to proceed from here. I also checked all solder joints again and reflowed a couple that looked like they might be cold but no change.
If you get no negative DC voltage on pins 5 and 6 on the output tubes then the bias system is not working. Put up a photo of the complete back side of the driver board so that we can look at the board. One of us may be able to find something in the photo that would cause a loss of bias voltage.
You should be getting -30 to about -45 VDC on pins 5 and 6 with an VTA boarded ST-70 with the 4 bias pots set near the center of their rotation.
Bob
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
I just resoldered the diode on the driver board. I now get -55v with no tubes and was able to bias the left channel at .4V each tube. Is the -55v from pin#5 & #6 to ground an okay measurement (without the tubes).
audiobill- Posts : 425
Join date : 2014-03-13
Location : Albany, NY
Tharhai, that sounds more like it!
I note you're in Downingtown; I'm in Chester Springs.
I note you're in Downingtown; I'm in Chester Springs.
Bob Latino- Admin
- Posts : 3260
Join date : 2008-11-26
Location : Massachusetts
tharhai wrote:I just resoldered the diode on the driver board. I now get -55v with no tubes and was able to bias the left channel at .4V each tube. Is the -55v from pin#5 & #6 to ground an okay measurement (without the tubes).
Yes - that -55 VDC that you get will vary with the setting of the bias pot .. Try biasing the amp again now that you have bias voltage ..
Bob
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
audiobill- Posts : 425
Join date : 2014-03-13
Location : Albany, NY
Cool - I'm just off 100 near Eagle.
Congrats on the fix!
Congrats on the fix!
colin86325- Posts : 61
Join date : 2014-04-01
Another local guy here--I'm in Glen Mills.
audiobill- Posts : 425
Join date : 2014-03-13
Location : Albany, NY
Hey, Colin!
tharhai- Posts : 8
Join date : 2015-02-04
Location : Downingtown, PA
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