Martin,
Off-load means: with the load disconnected, ie without the valve.
You can compare both monoblocks without the 300b inserted.
Have you got any power resistors? Eg 5 to 10 ohm 5watt up.
You could use one in place of the valve heater as a "dummy load". Alternatively use a bulb from a car (not a headlight
)
Off-load means: with the load disconnected, ie without the valve.
You can compare both monoblocks without the 300b inserted.
Have you got any power resistors? Eg 5 to 10 ohm 5watt up.
You could use one in place of the valve heater as a "dummy load". Alternatively use a bulb from a car (not a headlight
)Allright, here's what I did. First, I rebuild the DC circuit: 4 schottkies, two capacitors and a 0,22 Ohm restor. I measured the whole chain, first only AC without the rectifying circuit : 5,4 V. Then after the schottkies without caps or load : 4,85 V DC, then with capacitors 7,5 V without load and 6,23 with load. I checked all the voltages in both amps without tubes and they were almost exactly the same. In the monobloc that's working fine the DC voltage is exactly 5 V. It looks as if there's something wrong in the cap-resistor-cap part.
Martin,
Back to the hum:
You said you had the power transformer on rubber grommets and its shields grounded. But is the transformer core grounded?
The following is a longshot but anyway. Try reversing the coupling capacitors to the 300B grids. The logic behind this is that foil capacitors have one of the foils exposed in the outermost turn.
Now the heater supply. 300B needs 5V at around 1.2-1.4A depending on manufacturer. With 5.4V AC, a schottky bridge and a pi filter of 10,000uF + 0.22 ohm + 10,000uF you should get around 6V DC at 1.3A. Changing the 0.22 ohm to 1 ohm should bring it close to 5V.
Back to the hum:
You said you had the power transformer on rubber grommets and its shields grounded. But is the transformer core grounded?
The following is a longshot but anyway. Try reversing the coupling capacitors to the 300B grids. The logic behind this is that foil capacitors have one of the foils exposed in the outermost turn.
Now the heater supply. 300B needs 5V at around 1.2-1.4A depending on manufacturer. With 5.4V AC, a schottky bridge and a pi filter of 10,000uF + 0.22 ohm + 10,000uF you should get around 6V DC at 1.3A. Changing the 0.22 ohm to 1 ohm should bring it close to 5V.
Jan,
using a 1 ohm resistor brings it to 5,8 V . What I don't understand is why this monobloc should have a 6 or 7 times higher resistance in the heater supply than the other one. The voltages from both powertrannies measure exactly the same.
I've looked into earthing the powertrannies core before but there's no way to do that. The bolts that keep the core together is not making electrical contact with the core and there just isn't anywhere else to tie a wire to.
Martin
using a 1 ohm resistor brings it to 5,8 V . What I don't understand is why this monobloc should have a 6 or 7 times higher resistance in the heater supply than the other one. The voltages from both powertrannies measure exactly the same.
I've looked into earthing the powertrannies core before but there's no way to do that. The bolts that keep the core together is not making electrical contact with the core and there just isn't anywhere else to tie a wire to.
Martin
Martin,
Heater voltage:
If they are different, there must be an error somewhere. Are you measuring the voltage actually on the valve pins?
You are right that there should not be a large difference between the monoblocks. I would consider 5% to be acceptable.
Hum:
Have you checked whether there is continuity between the power transformer core and chassis? Maybe the shield earth wire does this...
Looking at your pictures, I notice that the choke on it's own, is orientated at 90 degrees to the others. That will mean that there will be magnetic radiation in 2 different planes. It could be the reason why turning the output transformer was ineffective.
However, there could be other effects. You could try reversing the connections on that choke. There may be a slight difference.
Heater voltage:
If they are different, there must be an error somewhere. Are you measuring the voltage actually on the valve pins?
You are right that there should not be a large difference between the monoblocks. I would consider 5% to be acceptable.
Hum:
Have you checked whether there is continuity between the power transformer core and chassis? Maybe the shield earth wire does this...
Looking at your pictures, I notice that the choke on it's own, is orientated at 90 degrees to the others. That will mean that there will be magnetic radiation in 2 different planes. It could be the reason why turning the output transformer was ineffective.
However, there could be other effects. You could try reversing the connections on that choke. There may be a slight difference.
Martin,
The dummy load should be connected to the valve socket, and the measurement must be taken there. 10 ohms is fine for the comparison.
Sorry, I should say core-bolts. Of course the core laminations will all be isolated from each other. Anyway, don't sweat over this one, the heater voltage is the most important to get right first.
The dummy load should be connected to the valve socket, and the measurement must be taken there. 10 ohms is fine for the comparison.
Sorry, I should say core-bolts. Of course the core laminations will all be isolated from each other. Anyway, don't sweat over this one, the heater voltage is the most important to get right first.
Just had a thought: 
Why don't I take the rectifying circuit from the first amp which is working fine and put it in the second amp ? ( sorry, I'm new to this, probably something very obvious ) So, I did and measured 5.85 V. So this means that the problem is not in the rectifying circuit, I guess.
Why don't I take the rectifying circuit from the first amp which is working fine and put it in the second amp ? ( sorry, I'm new to this, probably something very obvious ) So, I did and measured 5.85 V. So this means that the problem is not in the rectifying circuit, I guess.
Valve heater voltage.
Martin,
Let's use some logic here:
2 Identical circuits will behave identically.
If they behave differently they are not identical.
It's up to you now to check (and swap parts) each against the other, to find the difference.
As I mentioned in an earlier post, things can sometimes get messy like this due to a mis-reading or mis-assumption.
Start at the beginning:-
Heater windings - disconnected - unloaded, and then with 10 ohms dummy load.
Connect bridge rec: Check, unloaded, then with 10 ohm dummy load..
etc.
If it seems like too much of a task, then you're not ready for it. Put the project down for a few days until you're itching to do it again.
Martin,
Let's use some logic here:
2 Identical circuits will behave identically.
If they behave differently they are not identical.
It's up to you now to check (and swap parts) each against the other, to find the difference.
As I mentioned in an earlier post, things can sometimes get messy like this due to a mis-reading or mis-assumption.
Start at the beginning:-
Heater windings - disconnected - unloaded, and then with 10 ohms dummy load.
Connect bridge rec: Check, unloaded, then with 10 ohm dummy load..
etc.
If it seems like too much of a task, then you're not ready for it. Put the project down for a few days until you're itching to do it again.
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