Help! 2A3 SET quit after hot restart

Mercury99 · 4th November 2010, 10:50 PM

I have a home-built 2A3 SET amplifier. The circuit is very simple. Two stages capacitor coupled with AC filament heating. The 2A3 are EH gold pin.

The amp runs fine except if I shut it off and turn it back on while still "hot". There is no sound. I measured the 2A3 and the plate voltages are around 400V and grid voltages are around 300V and the heater around 40V. I checked the coupling capacitors. There is no leak. The plate voltages of the previous stage are around 170V.

It sometimes refused to start up until I moved tubes around a few times. Can someone help to provide possible root causes? The 2A3 grid leak resistor values are 499K. Could this be a problem?

Ty_Bower · 5th November 2010, 12:26 AM

Is it vacuum tube rectified, or solid state? Vacuum tube rectifiers often do not tolerate hot restarts.

edit: Never mind. You say you've got 400 volts B+ available.

Mercury99 · 5th November 2010, 01:07 AM

The rectifier is 5U4G.

45 · 5th November 2010, 09:32 AM

Quote:

Originally Posted by Mercury99

I measured the 2A3 and the plate voltages are around 400V and grid voltages are around 300V and the heater around 40V.

Can you explain how do you measure these voltages? They look a bit odd...
A 2A3 doesn't have a heater but a 2.5V filament. Is 40V the drop through the cathode resistor?
How do you measure 300V on the grid? What's the reference terminal?

azazello · 5th November 2010, 02:43 PM

40 v on the cathod means that mode of tube is OK, and current is about
55 ma. I think 300v on the grid is wrong measure. If Ua is 400 v,according Your measure, that means that maybe Ua = 400 - 40 = 360 v??

M Gregg · 5th November 2010, 04:37 PM

Quote:

Originally Posted by Mercury99

I have a home-built 2A3 SET amplifier. The circuit is very simple. Two stages capacitor coupled with AC filament heating. The 2A3 are EH gold pin.

It sometimes refused to start up until I moved tubes around a few times. Can someone help to provide possible root causes? The 2A3 grid leak resistor values are 499K. Could this be a problem?

Please post a schematic of your amp so we can get more information and ask for specific measurements.

Regards

M. Gregg

4th November 2010, 10:50 PM	#1
Mercury99 diyAudio Member Join Date: Apr 2010	Help! 2A3 SET quit after hot restart I have a home-built 2A3 SET amplifier. The circuit is very simple. Two stages capacitor coupled with AC filament heating. The 2A3 are EH gold pin. The amp runs fine except if I shut it off and turn it back on while still "hot". There is no sound. I measured the 2A3 and the plate voltages are around 400V and grid voltages are around 300V and the heater around 40V. I checked the coupling capacitors. There is no leak. The plate voltages of the previous stage are around 170V. It sometimes refused to start up until I moved tubes around a few times. Can someone help to provide possible root causes? The 2A3 grid leak resistor values are 499K. Could this be a problem?

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5th November 2010, 12:26 AM	#2
Ty_Bower diyAudio Member Join Date: Mar 2007 Location: Newark, DE	Is it vacuum tube rectified, or solid state? Vacuum tube rectifiers often do not tolerate hot restarts. edit: Never mind. You say you've got 400 volts B+ available.

5th November 2010, 01:07 AM	#3
Mercury99 diyAudio Member Join Date: Apr 2010	The rectifier is 5U4G.

5th November 2010, 02:43 PM	#5
azazello diyAudio Member Join Date: Aug 2008 Location: Bulgaria	40 v on the cathod means that mode of tube is OK, and current is about 55 ma. I think 300v on the grid is wrong measure. If Ua is 400 v,according Your measure, that means that maybe Ua = 400 - 40 = 360 v??

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