Amp PT troubles

hywelg

Cheers guys, I can finish it tomorrow and see how it sounds.

Might still need to tweak the pre-amp voltages, we'll see.

jpfamps

hywelg said:

No but these Chinese bulbs are crap and if I kill it I will be in deep dudu!! We seem to have loads of 60watters but only 1 100watt.

Thanks, I couldn't understand why it would pull so much current if it was still reading logically. Nik did suggest 7R2 wasn't far off standard.

8 ohms is pretty much a dead short.

What would happen to an 8 ohm resistor if you connected to the mains?

It would draw 30A and expire in short order!

So why doesn't a transformer with an 8 ohm primary not expire?

This is due to the back EMF on the primary virtually completely cancelling the input voltage, and hence presenting a very high impedance to the mains.

For this to work you need a large inductance, which requires a certain number of turns. If some of these turns get shorted out then the inductance of the primary may not be enough to prevent excessive currents being drawn and hence fuses blowing etc.

You can't really tell anything about a mains transformer from the resistance of the primary. In fact the resistance is a nuisance, and a perfect PT would have zero resistance on the primary.

hywelg

Update on this, finally got around to installing the filament centre tap R's . Powered up fine but an audible hum from the PT which was damped down a little when I put my hand on it.Set the bias no problem, then set about checking voltages.

V1 and V2 both low but I had a 5751 on v2 so I expected it to be different from my original build voltages. v3 wasOK but the output valves were down around 350v. I was expecting nearer 415-420 from a 320-0-320 PT. I previously had 454v from a 345-0-345 PT. Investigating further one side of the HT was much lower than the other at the rectifier.

So I got a little more time today and I pulled both HT's and the bias supply from the rectifier and checked the raw voltages. 256-0-315. Bias 23v. I suspected I had made a schoolboy error and mixed up the CT and Bias wires, but no, it seems not, and the resistances all tally. Now I am wondering if they could have connected the wires incorrectly internally, the schematic is quite clear, Red and red are HT, Red/Yellow is CT, Red/Blue is Bias (should be 60v)

Not quite sure what to make of it, wondering if my level of knowledge is just inadequate and I'm making a stupid mistake!!

exocet

That does sound like a mix up between the Bias Tap and CT,  60 volts or so difference?

hywelg

Here's what I measured

Red-red 21.7 ohms Spec sheet says 20.15 +- 20% so Ok there

Red 10.2 red/yellow 11.2 red
Red/yellow 2.2 red/blue
Red 12.5 red/blue 9.2 red

Doesn't make any sense to me. Definitely wired as per schematic so could they have mixed up the wire colours? But if Red/Blue was CT thats more out of balance than Red/Yellow as CT

ICBM

What's the primary current draw with the secondaries disconnected?

hywelg

Didn't really want to disconnect all the secondaries just yet. Still got the heaters connected up. Was hoping its a schoolboy error and I (or they ) mixed up the CT and bias taps.

DJH83004

Personally I would measure the secondaries under light load to ensure you are passing enough current through the primary to magnetise the core (magnetisation current). One way would be to pull the output valves, just leaving the pre-amp valves in & reconnect the reds and the centre tap and measure the AC off rectifier valve base and go from there.   

hywelg

Just realised how stupid my comment was. Of course I can avoid disconnecting the heaters just by pulling all the valves. Durr.... WIll get back to it tomorrow.

@DJH83004. I did originally measure the secondaries whilst connected from the rectifier 285-one side 334 the other. All valves installed and biased at 38mA. I was getting 351v on the anode of the 6L6's, which is what alerted me to the problem. 

ICBM

hywelg said:

Was hoping its a schoolboy error and I (or they ) mixed up the CT and bias taps.

If the CT and bias were mixed up you should get higher voltage, not lower - the bias tap to the opposite HT is a bigger winding. It will have more ripple on it because it will be getting closer to half-wave, but at light current draw you won't notice it that much.

If you get much more than about 70-80mA mains draw with the secondaries unloaded I would guess you've got a shorted turn in the HT winding. In theory that should produce a near-short on the primary too, but I've come across some where it doesn't seem to - I don't know why.

Although I had a really odd one a few months ago, an old Fender PT which drew about 300mA but otherwise seemed completely normal, including all the secondary voltages. I would never have noticed if I didn't routinely monitor the current on power-up. I still don't know what it was... the amp ran fine, even on an all-day full power test.

DJH83004

That's fine, I have to say it does look very much like the manufacturer may have got the centre tap and bias tap crossed over in terms of the outgoing wire colour coding, but very unusual in my experience.

hywelg

I swapped the CT and the bias, and it made no difference (well a couple of volts max) to the secondaries.257-0-315 Bias 28v
Swapped back to Red/Yellow CT and get 256-0-314, 22v

@ICBM, current draw with secondaries disconnected (heater wires connected but no valves installed) 0.17mA.

Stumped....

ICBM

hywelg said:

I swapped the CT and the bias, and it made no difference (well a couple of volts max) to the secondaries.257-0-315 Bias 28v
Swapped back to Red/Yellow CT and get 256-0-314, 22v

That's very odd. You would expect the CT to bias tap to be the same, the HT on the same winding as the bias to go down, and the other HT to go up.

hywelg said:

current draw with secondaries disconnected (heater wires connected but no valves installed) 0.17mA.

Are you sure? That's impossibly low. Or do you mean 170mA (0.17A) - which is too high.

At this point I would disconnect the transformer entirely and measure the voltages and the current draw - just in case there is some odd short in one of the valve bases or something. You're going to have to take it out anyway if the only other possibility is correct - that it's buggered.

hywelg

If this is me making a schoolboy error its going to be difficult to live down

ICBM said:

Are you sure? That's impossibly low. Or do you mean 170mA (0.17A) - which is too high.

I desoldered the white primary which was connected to Neutral on the IEC and connected my MM in series at this point (thus removing the current draw , if any, of the relay transformer from the measurement). MM was definitely reading 0.17mA, I changed the range a few times and the decimal point kept moving as expected and the 'm' next to the 'A' was always present. Must admit its a cheap MM and I really do need to get a better one if I'm going to be doing much more troubleshooting!

Transformer is coming out.....

hywelg

Ahem......

Backtracking and checking everything that I hadn't already checked, in order to prevent me looking a fool and sending a perfectly good PT back to the supplier. Fuse on the CT had blown!! When the old PT was blowing HT fuses I checked the other fuse as well, so it was good, now it isn't. Swapped it and I get

337-0-337, bias 61.7 volts Bang on spec.

How do you delete a thread to save ones blushes?  ">

ICBM

That almost sounds like the transformer has an open circuit somewhere and is developing the secondary voltages weakly, which can still happen - although not normally when it's hooked up to the amp!

Does it have a dual 120V primary and could you have wired it out of phase with itself? Long shot…

hywelg

No, single 240v primary. 291FEX

ICBM

Aha! I posted that before I read your reply.

I was right - sort of - the transformer did have an open circuit. In the CT fuseholder .

This is one reason I don't like CT fuses by the way. A fuse in the proper HT supply - after the rectifier - is better, since it leaves the bias supply working normally.

hywelg

This has both. 2A slow blow in the HT supply and a 0.5A Fast blow in the CT.

Anyway, far from being sorted, now the amp is blowing CT fuses regularly, within a couple of seconds of turning off standby, even with no valves installed, and the rectifier board hanging free (I thought I might have made a botch of resoldering the PT wires, but no). WHich is different from when I powered up first time. Then I was able to set the bias to 38mV and between then and checking the internal voltages the CT fuse blew and gave me the weird unbalanced HT supply voltages.

Now its getting it bit beyond my knowledge/patience level so its booked in for an experienced tech to look at.

hywelg

Turned out to be a very simple problem to solve. 

The PT is capable of nearly twice the current required, being for  Fender Twin. When goingoff standby the inrush current is jus too much for a 500mA fast blow fuse. a slow blow/timedelay of the same value is fine. 

Amp sounds fine though the preamp voltages are alittle off, though I do have a 5751 in v2 which isnt stock.