Morrison 300B SET - where to connect current meters?

[BinaryMike]

I would suspect the big 75R resistor or the filter choke at this point. One of them is probably opening up as it begins to conduct significant current. Have you monitored voltage across each of them as B+ decays during warmup? Have you checked voltage relative to ground at both ends of these components after B+ decays?

 

[davesatu]

I would suspect the big 75R resistor or the filter choke at this point. One of them is probably opening up as it begins to conduct significant current. Have you monitored voltage across each of them as B+ decays during warmup? Have you checked voltage relative to ground at both ends of these components after B+ decays?[

Thanks Mike. OK to call you that?
Really appreciate the help. I have been building this sort of circuit for more decades that I care to remember. The 25W Arcol 75R and Hammond choke are both new. And rated well for this job. So I assumed they would be OK.

Will do the tests suggested but as mentioned are now upgrading the tube tester to DHT status. I had these tube tester boards built in China. If you want one let me know.

regards Dave.

 

[waltube]

Hi

not permanently!
You can use a switch so the meter is engaged when you want to check.
This is the best way for me


W

 

[waltube]

Try to disconnect the chocke and take the Vdc directly from Solen cap only, one channel at time.
It he problem still exist, short the 75 R and check again.
Check also the Vac at pin 4 and 6 of 5U4 to ground, will be the same-


Walter

 

[davesatu]

Hi

not permanently!
You can use a switch so the meter is engaged when you want to check.
This is the best way for me


W[/QUOTE

Thanks for your reply Walter. Can you explain further why the meter(s) should not be permanent at the cathode tails?

Reason asking is I built the JE Labs 45 SET a year ago. And put a single ma meter in the B+ line before the split to L/R. These 45's are becoming rare and expensive so wanted to keep a check on the current in the whole system.

My calcs said 76 ma. And that is where the meter has been since the start.
But to be fair no meters were in the cathode tail area we are discussing here.

No difference in sound quality was heard in the 45 with that one meter in or out of the B+ circuit.

The tests you suggest are in progress. May take a few days.

tks again Dave.

 

[rmb]

What a great idea! Used what I had 1N5408's and with 2 each 6SN7 and 300B inserted here are the results.

1. Line in at half normal 120 VAC the B+ out is 274V and steady.
2. Increasing line in until 300B plate voltage is 430 stays steady for maybe 5 seconds and then slowly decays but has a few goes at increasing by a few volts on the way down.

What next? I am currently upgrading my tube tester to test DHT's. Or maybe that first 39uF Solen cap is faulty. They have some reputation in that regard although I have never had any issues with them. And had a 47uF Solen in there as per original design to start and saw the same problem.

tks Dave.

Any of those electrolytic caps getting hot when the PSU is turned on? Be careful, left hand in pocket, turn OFF PSU before feeling the cases checking for any hot capacitors. Be certain the caps are wired with correct polarity. Warning..if you have a bad electrolytic capacitor, or if it is reverse wired, the capacitor will overheat fast and perhaps explode with possible injury.

 

[waltube]

I think is the best way because 1 ohm inserted has not influence on circuit, then with a switch you can check sometimes the current also for the other channel if you have a stereo chassis.
If you buy a 100 mV ac meter chinese it is cheper and the measurement is good; if you have a good tester you can compare the value always on 1 ohm resistor. With a screwdriver you can adjust the indicator.
The volt you read are the mA flowing
There are other meter, Simpson p.e., but are very expensive but very good. I my ref. pre I use them

Walter

 

[davesatu]

I would suspect the big 75R resistor or the filter choke at this point. One of them is probably opening up as it begins to conduct significant current. Have you monitored voltage across each of them as B+ decays during warmup? Have you checked voltage relative to ground at both ends of these components after B+ decays?

Just spent some more time trying your suggestions. Readings seem to be all over the place.The B+ both channels with all tubes in starts high as usual then stabilizes at around 290 V after 3-4 minutes. At the same time the readings across both R1 and L1 start at zero increasing in a very rapid trending up/down way to around finally 16 V. Try it again and the figures change to as high as 24 V.

Voltages to ground go the same way as the B+ output - down. Taking R1 and L1 out of circuit one at a time makes no difference to the behaviour. And now something blew the main S1A fuse. And the OPT's (I am using One Electron UBT2's) for the first time were a bit warm after all this. But still no current. Could it be the 300B's? Will test then decide what to do. Currently thinking about throwing it all in the bin and starting again. Not really but at least a rebuild.

Again appreciate all the help offered.
Dave

 

[BinaryMike]

16VDC across the 57R choke coil resistance is 280mA, which is a lot of current going somewhere. If you proved that it's not going through the power tubes by metering their cathode current, then it's time to suspect bad (leaky) main filter caps. However, if the OPTs warm up but the power tubes aren't conducting, then the OPT anode connections may be shorted to ground. If all that current is going through the power tubes and you're sure the cathode bias resistances are correct, then the DC blocking caps to the grids may be leaky or the tubes may be defective. You can simply short the 300B grids to ground as a test if you suspect leaky grid caps.

I don't see an "S1A" fuse in the docs you posted. This is a very simple amp and so the failure is likely quite simple, but remote diagnosis can be exceedingly difficult. Especially when reported symptoms are inconsistent. Perhaps the best way to proceed is to seek out a local technician or advanced amateur who can get you oriented without so much guesswork.

 

[waltube]

Try to check the voltage without tubes inserted , if the voltage is ok after some seconds install one tube at time and check again; if the trouble is on the tube in this way you can fix.
If the problem happen without tubes take out the first cap and put one, just 10uF /500 Vdc ( also 2 uF will be good).
Check again.

Walter

 

[davesatu]

Hi Walter. Sorry for the delay in reply.
Yes I have tried that method early in the piece. All good until you insert either one of the JJ 300B's. Then the collapse of the B+ starts. So as these 300B's are expensive I have been converting my tube tester to check these. As I think it must be the problem having spent so much time up stream from there. Not suggesting the JJ 300B's are bad.

The only active components different from several successful builds using this format are:
1. The JJ 300B's.
2. Solens in the power supply and elsewhere. Rather than electros.
3. The 5U4GB

All of which have been checked to death.

Will report back when JJ's checked. A new PCB for the tester is in process.
Dave.