92.6 Ohms vs 118.4 Ohms is fairly typical - I would NOT be suspecting the Output Transformer at this point in your investigation.
It is an indication that the Output Tranny is wound with minimum interleave - probably just 1 winding for each half primary.
The 33mA sounds OK but the other (10mA and unstable) suggests a problem with that tube or its screen resistor - or dirty socket pin contacts.
That imbalance will certainly be causing significant hum.
Swap the EL84s and see if the low current follows the tube, if so it is the tube, if not check socket pin contacts (spray some contact cleaner in there) and check the screen resistor for that socket.
Cheers,
Ian
It is an indication that the Output Tranny is wound with minimum interleave - probably just 1 winding for each half primary.
The 33mA sounds OK but the other (10mA and unstable) suggests a problem with that tube or its screen resistor - or dirty socket pin contacts.
That imbalance will certainly be causing significant hum.
Swap the EL84s and see if the low current follows the tube, if so it is the tube, if not check socket pin contacts (spray some contact cleaner in there) and check the screen resistor for that socket.
Cheers,
Ian
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Thanks Ian...it's late here in London, so will carry out your suggestions tomorrow evening
I'm surpised to hear that such varying dc resistances on the coil taps (over 25% difference) is normal.... I'd have imagined they'd have had to be much closer in spec (it makes me wonder why buy matched output valves if the circuit they are going to be dropped into aren't particularly well matched!)
I'm surpised to hear that such varying dc resistances on the coil taps (over 25% difference) is normal.... I'd have imagined they'd have had to be much closer in spec (it makes me wonder why buy matched output valves if the circuit they are going to be dropped into aren't particularly well matched!)
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For Output Trannies with Ultralinear Taps where each 1/2 primary consists of 2 windings (or more) with part seconday windings interleaved, they typically are much closer in DC resistance but for "simple" guitar amp OTs where you will probably have a 1/2 primary winding closest to the centre, then a secondary winding then the 2nd half primary winding on the outside the wire lengths for the same number of turns for the 2 half primary windings become quite different which you see in the different winding DC resistances.
For highest quality HiFi OTs they interleave lots of part primary and part secondary windings. This is to minimise leakage inductance and interwinding capacitance to get extended High Frequency response. Thats just not required for a Git Amp - so they do it the cheapest way - with just the secondary winding stuck between the 2 half primay windings.
The values you gave are (from my experience) quite typical. Not to say that the OT is NOT faulty but I would not be suspecting it yet. There are more likely suspects to be eliminated first.
Good Luck with it.
Cheers,
Ian
For highest quality HiFi OTs they interleave lots of part primary and part secondary windings. This is to minimise leakage inductance and interwinding capacitance to get extended High Frequency response. Thats just not required for a Git Amp - so they do it the cheapest way - with just the secondary winding stuck between the 2 half primay windings.
The values you gave are (from my experience) quite typical. Not to say that the OT is NOT faulty but I would not be suspecting it yet. There are more likely suspects to be eliminated first.
Good Luck with it.
Cheers,
Ian
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Ok, I must have been tired last night (re my DC measurements!), here's the correct(ed) measurements...
Valve V3 (measured across coil tap of 90 Ohms ...red~blue wires) 2.82V = 31mA
Valve V4 (across coil tap of 115 Ohms ...red~brown wires) 3.86V = 33.5mA
after swapping valves between V3 with V4 sockets...
red~blue wires 3.02V (V3) = 34mA
red~brown wires 3.58V (V4) = 31mA
...so nothing too much out of whack there.
Therefore since the hum is being introduced after V2 (which put the output stage in scope) I possibly need to do something about the PSU ripple?
Valve V3 (measured across coil tap of 90 Ohms ...red~blue wires) 2.82V = 31mA
Valve V4 (across coil tap of 115 Ohms ...red~brown wires) 3.86V = 33.5mA
after swapping valves between V3 with V4 sockets...
red~blue wires 3.02V (V3) = 34mA
red~brown wires 3.58V (V4) = 31mA
...so nothing too much out of whack there.
Therefore since the hum is being introduced after V2 (which put the output stage in scope) I possibly need to do something about the PSU ripple?
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What Ian says.
Criteria for happy audio is "low" DCR and "equal" turns.
EL84 will run with 8Kpp load so 2K each side. Anything under 200r DCR is "low". 26r difference is "nothing".
They wind cheap OTs as 600 turns fine wire on core/bobbin, 30 fat turns over that, and another 600 thin turns for the other side of the primary. A turn at the core may be 4 inches and a turn at the outer wrap may be 7 inches. Same number of turns (often within one turn) but different length thus different resistance.
Remember two "matched" tubes will be 10% different somewhere in the working range, and un-matched tubes really do work fine with 30% differences.
As said, much more "balanced" winding techniques exist and are well known to geeks. But 99% of the guitar-amp market is perfectly happy with the cheapest winding techniques.
Criteria for happy audio is "low" DCR and "equal" turns.
EL84 will run with 8Kpp load so 2K each side. Anything under 200r DCR is "low". 26r difference is "nothing".
They wind cheap OTs as 600 turns fine wire on core/bobbin, 30 fat turns over that, and another 600 thin turns for the other side of the primary. A turn at the core may be 4 inches and a turn at the outer wrap may be 7 inches. Same number of turns (often within one turn) but different length thus different resistance.
Remember two "matched" tubes will be 10% different somewhere in the working range, and un-matched tubes really do work fine with 30% differences.
As said, much more "balanced" winding techniques exist and are well known to geeks. But 99% of the guitar-amp market is perfectly happy with the cheapest winding techniques.
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