Hi, I'm restoring a couple of Quad 2 monoblocks. The amps worked a long time ago and I have replaced all the rsistors with Kiwames, the main psu caps with F and T 15 + 15uf caps and the other electrolytics with decent quality Nichicons. Coupling caps have also been replaced. On turning on the amps the rectifier and output tubes all flash as if the amp is oscillating wildly. The cathode resistor (5 watt) is getting very hot quickly so the tubes are dissipating much more than they should. Both monoblocks are behaving identically so it cannot be a case of a poor /missed connection. I tried adding two x 22pf caps to the grids of the output tubes which made no difference. So my questions are:
1. Any pointers on the cause given these amps are generally stable?
2. Any pointers on the best way to test given I don't want to have the amp operating with this level of excess dissipation?
Thanks in advance
David
1. Any pointers on the cause given these amps are generally stable?
2. Any pointers on the best way to test given I don't want to have the amp operating with this level of excess dissipation?
Thanks in advance
David
David - you're an experienced tech. I'm sure you'll figure it out. The first place I'd look is the grid leak resistors on the tubes. Could you have missed something out inadvertently?
The only other electrolytic is the cathode bypass capacitor, 25uF 50V. Did you observe the voltage rating?
Yes, that one I replaced with 100v rating. The coupling caps are new and these amps normally run stably without grid stoppers
Hi Andy,
These amps normally run stably with no grid stoppers. I'm pretty certain I couldn't have missed anything identically on two amps.
David
What you describe is usually caused by leakage of the two big coupling caps - but you replaced them. To be sure check power tube grids are at zero volts all the time before you switch off. Are the big tubes still good ? This thread is missing pictures.
Agreed. My next step is to check the power tube grids with output valves removed. I'm wary about having the amp powered up in this condition to measure or take pictures!
Are you sure, that gnfb correctly installed?
Is the output snubber (4R7-390nF) installed, and it's in the right place?
Try amp without (removed) coupling capacitors.
Which version is this QuadII?
Is the output snubber (4R7-390nF) installed, and it's in the right place?
Try amp without (removed) coupling capacitors.
Which version is this QuadII?
Last edited:
I havn't touched the gnfb so no reason to think it's incorrect (if as conveyed to me the amps worked previously). There's no output snubber on this version (version 2 I believe). Trying without the coupling caps is a good idea!
Thanks
Thanks
I mean the grid leaks - grid to ground - on each stage. I would imagine you check all the resistances on the tube pins to ground before firing up.
What is your mains supply voltage?
Maybe the amps were set at 220V or lower. Where does the HT settle at, at switch on?
Also have you verified how the earth is configured? Is the chassis earthed?
A long time ago one of my amps motorboated very badly due to a poor chassis earth.
I think the cathode resistors will run hot as 5 watts dissipation is maybe too small.
With modern resistor's being so small, even for 5 watt dissipation they do seem to run hot.
Maybe the amps were set at 220V or lower. Where does the HT settle at, at switch on?
Also have you verified how the earth is configured? Is the chassis earthed?
A long time ago one of my amps motorboated very badly due to a poor chassis earth.
I think the cathode resistors will run hot as 5 watts dissipation is maybe too small.
With modern resistor's being so small, even for 5 watt dissipation they do seem to run hot.
Hi, my mains is quite high-246v but the amps are set on 240v. HT was correct around 340 but of course fluctuates with the pulsing. I have implemented an earth lift system with the chassis correctly safety earthed. the correct dissipation for the cathode resistor is 3watts, which I agree would be reasonably hot, so 5 watts should be OK, not the enamel roasting I'm currently getting!
You have implemented an "earth lift system" What else is modified in the quads ? Chances are that
mods are the cause of your problems ...
mods are the cause of your problems ...
No mods apart from the earth lift for safety /hum reasons. Many have implemented this arrangement without issue and there's no reason for it to cause these symptoms. Anyway its the easiest thing to short it out leaving an earthed chassis
Read with tester on pin 5 to ground of each 66 if there is the value as schematic
Also pin 4 are in common for 66 and goes to c4
Hi, these values are correct. I did manage to measure the voltage at Pin 5-it's huge around 40v + so way out
Ok did you disconnect the OPT and now have the windings reversed I know you did say you did not touch the GNFB.