Partridge PT with double the voltage on one HT leg.

[overtone]

I am concerned about a PT which is putting out out twice as many AC volts on one side of the HT winding as the other. 10% difference maybe OK, but double?

The amp has been dropped at some point, the PT went into orbit and stabbed the filter caps, there is a melted impedance selector and someone has replaced a socket and wired it with household ground wire… so I'm being cautious. Have not fired it up. The OT tests fine so far...

About the amp: it is an early 1980s Burman 4000HD with 4 x KT77s. I have another 4000 in mint condition to compare to and a few more Burmans to keep them company when I'm out at the pub. I play these things since the early 80s and below is the schematic of the mint 4000 which I drew up last year with much help from Tracy Norton.

So back to this wreck, which was a pot-luck fleabay purchase and rather nice because the old KT77s were still all in it:
- the measurements between the PT connections match on both amps, no problems.
- the HT measures 20,8 Ohms in the good one and 20,6 in the wreck.
- did the RG Keen neon-bulb internal short tester from geofex and it flashed on, maybe a bit dim, but the neon did flash on.
- have quickly fired it up with no load yet, with the bulb limiter giving 112V / 219V HT. No smoke.
I forgot to do a no-load, bulb limiter voltage measurement on the good amp, but I have a 1V AC input comparison:
the good: 1V in = 0,677V / 0,755V HT & 0,476 bias tap
the bad: 1V in = 0,558V / 1,337V HT & 0,673 bias tap

So am I fretting too much about this imbalance on the HT legs? Never come across this before.

Thanks in advance for any advice and opinions,
best, tony

[Tom Phillips]

Tony,
I'm having a difficult time following what you are saying.

..a PT which is putting out out twice as many AC volts on one side of the HT winding as the other.

The amp uses a full wave bridge rectifier and there is no tap on the secondary HT winding. Are you measuring each side with respect to chassis ground?

..I have a 1V AC input comparison:
the good: 1V in = 0,677V / 0,755V HT & 0,476 bias tap
the bad: 1V in = 0,558V / 1,337V HT & 0,673 bias tap

Both the good and the bad HT readings seem strange to me. Can you explain a little more about how you are taking the measurements?
Cheers,
Tom

[Tom Phillips]

Additional thoughts

Since you fired it up with full line voltage and producer no smoke it's likely that everything is OK. Dis you measure the HT secondary AC voltage (i.e. voltage between the brown leads)? Did you measure the resulting B+ rail. (461VDC per the scematic)?

[overtone]

Thanks for the questions, and I apologize: I really should have stated that the measurements are from each HT leg with respect to ground - wrtg. I usually do these low voltage tests on any unknown iron and have never seen two HT legs with such a voltage difference. It is the same, both in circuit and out.

The 1V AC test is injected at 1kHz on the mains input side and, to confuse, this was measured only between the inputs so:

• good amp: adding the 0,677V to 0,755V comes up to 1,432V HT in total which is kind of what I expect.
• wrecked amp: adding the 0,558V to 1,337V comes up to 1,895V HT in total

If I remember correctly the filaments were at 0,012V each leg wrtg.

To your second question: that was mains voltage through the bulb limiter and out of circuit. No bulb-glow at all. The measurements were with a meter on each HT leg wrtg. The total of 112V+219V=331V across the HT legs does seem low. I will try it without the limiter tonight.
The whole PT is out now to replace the wrecked turret board. It will be a few evenings work before I can see how the B+ comes up. All good fun!

Does a s.s. bridge rectifier just laugh at this kind of an imbalance, roll it's sleeves up and get on with work as usual?

Best, tony

[Steve Conner]

Wait a minute. If it has a bridge rectifier then the HV winding doesn't have two "sides". It's one single floating winding. If you measure the voltage from an end of this winding to ground, while the HV winding is disconnected from the circuit, you can get just about any answer because there is no complete circuit.

The only valid measurements to make out of circuit are:

Resistance from either end of the winding to the primary/core/other windings, without power applied, should all show "overrange" on the meter's highest scale.

AC voltage between the two ends of the winding, with power applied, either in circuit or out, should show about the rated voltage. (DC HT voltage divided by 1.4)

With the PT connected to the rectifier, and the rectifier connected to the smoothing caps, and some sort of load on the HT, you should read roughly the same AC voltage from either winding end to ground, but in all other cases it's undefined.

If you get the rated DC voltage under load with no smoke and no RCDs tripping, you don't have a problem.

[overtone]

Thanks Steve, I really got my knickers in a twist on this one!