• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Weird rectifier circuit with 2 diodes.

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#23
trobbins said:
Even though the amp is from circa 1976, based on datecodes, its likely that all those e-caps are better replaced, unless you want to go through confirming they are each ok for leakage current at rated voltage.

Typical application would be as a guitar amp nowadays, but it would need some effort to make it safe and suitable. You may get some more insight from link on restoring such P amps: https://www.dalmura.com.au/static/Renovating PA amps.pdf
Click to expand...
Hi Trobins, Thanks for that link to Dalmura, I am aware and there is some great information on amps from that page.

cheers.
 
#25
Kay Pirinha said:
Are you sure that the green wire is the HV winding's CT? As this wire is grounded, that wouldn't make too much sense. I guess it's the heater winding's CT instead or the bias winding's gnd terminal..

Best regards!
Click to expand...
Hi Kay, Thanks for bringing this up.

Yesterday, after reading your reply I decided to unsolder both HT & LT secondaries to see if the green wire in the photo I posted earlier was indeed a CT......When I put the meter on, There appears to be no connection with the HT or LT secondaries with the green lead, So I look PDF's regarding A&R and Ferguson PT's, both of these manufacturers have specific purpose built 'voltage doubler PT's' So I very carfefully brought the power up with the Variac to 204v mains powers and I measured 120VAC across the HT and 6.3VAC across the LT. I really don't know the purpose of the green lead, I read in the Ferguson catalogue that some PT's come with an 'Electrostatic shield or Flux Shorting strap' I really don't what the green lead out of the PT really is?


I believe the PT in my amp is indeed a purpose built voltage doubler PT.

For an update for everyone that's reading this thread, I re-soldered the HT & LT wires back up last night, Then today, I connected a speaker & slowly brought the power back up via the Variac and measured voltages, No magic smoke 🙂

EL34's Pin1 - 17.8 & 17.8vdc
2 -
3 - 298 & 298vdc
4 - 281 & 281vdc
5 -
6 - 290 & 280vdc
7 - 6.4 & 6.4 vac
8 - 17.2 & 17.2vdc

EF86 Pin 1 - 79vdc
2 -
3 - 1.9vdc
4 & 5 6.3vac
6 - 81
7 -
8 -
9 -

(v2) 12AX7 Pin1 - 108vdc
2 -
3 - 2.2vdc
4 & 5 - 6.3vac
6 - 108vdc
7 -
8 - 2.2vdc
9 -

(v3) 12AX7 Pin1 - 160vdc
2 - 5.9vdc
3 - 9.4vdc
4 & 5
6 - 164vdc
7 - 6vdc
8 - 8vdc
9 - 6.3vac (heater ct)

I've had the amp running for probably 45mins now, Just a very small amount of hum plus some slightly scratchy pots, Doesn't seem to be overheating.

My next steps in this project is draw the circuit out, replace the two main filter caps, The amp also has 3 x mica caps which I believe are prone to failure, Once I replace the main filter caps I think that should take care of the hum. I'm going to try and shield the EF86 & 2 12AX7's as well.

If there is anything else that anyone wants to add please feel free.

cheers.
 
#29
If you can't physically see a flux band then the 'unconnected' lead is likely an electrostatic screen located between inner primary winding, and secondary windings. Two ways to confirm that are to do an insulation resistance test at 1kVdc between that lead and to primary winding, and to secondary winding, and measure the capacitance from that lead to each end of the primary winding, and to each end of the secondary HT winding - the capacitances are likely to be low (tens of pF) so not all meters will read that low - a bit of logic then confirms it is an internal shield.
 
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#30
trobbins said:
If you can't physically see a flux band then the 'unconnected' lead is likely an electrostatic screen located between inner primary winding, and secondary windings. Two ways to confirm that are to do an insulation resistance test at 1kVdc between that lead and to primary winding, and to secondary winding, and measure the capacitance from that lead to each end of the primary winding, and to each end of the secondary HT winding - the capacitances are likely to be low (tens of pF) so not all meters will read that low - a bit of logic then confirms it is an internal shield.
Click to expand...
I agree, My meter wouldn't/couldn't pick up a reading, So I think your right that it is an electrostatic shield between the inner primary and secondary windings.

thanks heaps.
 
#31
I agree. If there's no conduction to any other lead, it most probably is the terminal of a shield winding, copper foil or something like that. Anyway, I guess the capacitance to the adjacent windings is in the nF (10-9) range.

Does your amplifier feature a three prong mains lead, including PE?

Best regards!
 
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#32
Kay Pirinha said:
I agree. If there's no conduction to any other lead, it most probably is the terminal of a shield winding, copper foil or something like that. Anyway, I guess the capacitance to the adjacent windings is in the nF (10-9) range.

Does your amplifier feature a three prong mains lead, including PE?

Best regards!
Click to expand...
Yes, Its a three prong mains lead, Black = Neutral, Red = Active, Green = Earth which is the old colour coding for Australian standards of an amp of the 60's era, I will change the mains lead so is that it up to date.

cheers
 
#34
osci said:
Hi all, Its been around few years since I've posted here and I'm hoping some much more knowledgeable members can once again give me guidance.

I have a valve PA amp that's been sitting collecting dust and finally opened it up, There's no circuit diagram available so I will have to trace this out, I have no problems with everything after the rectifier circuit, However, The rectifier circuit does have me bemused.

I'm not an electronics tech, But I do have a very healthy respect for high voltage, The amp has 5 valves, 1 x EF86, 2 x 12AX7, 2 x EL34, I believe the rectifier circuit may be half wave? 'but its how the diodes are connected that has me confused' (I've attached a photo).

The AC input of neutral active & earth are not a problem, However, The Yellow wires in the photo are the high voltage secondary's with the green CT below the yellow wires soldered directly to ground.

One side of the high voltage secondary appears to be connected correctly through one diode to the first filter cap. The other half of the high voltage secondary is connected directly to the second pin of the first filter cap, The second diode has one leg connected directly to ground with the other leg connected to the first HV secondary mentioned above?

I can't wrap my head around this as to whether the rectifier circuit is a half wave or full wave rectifier circuit. The other colour wires coming out of the chassis are Blue = 6.3v heater, Green = CT for HV secondary, Brown = 240v mains in.

I've attached a photo of how the circuit is soldered in question

So my questions are

1 - Is the rectifier circuit half wave or full wave?

2 - How do I draw this out for schematic purposes, as I'm a bit dumbfounded by it?

other notes: I've not powered the amp up nor made any changes except cleaning off the build up of cobwebs and dust.

I hope someone can give me advice and guidance with this project.

cheers.
Click to expand...
Just an update for everyone.....I ran the amp for just over 3 hours last night without issue and am currently running it again, At the moment I'm watching a movie on the laptop with audio coming from the headphone jack through the Radio/Pickup input with a cable I made up last night.

I was getting hum from the MIC input, However, After I pulled the jack in/out a number of times the hum has now completely gone, The MIC input on this amp is quite powerful and really surprised me, it is also very bright.

the Radio/Pickup input is more subdued and sounds a little bit muffled to my ears.

For the most part the pots don't have the scratchy sound anymore that I referred to earlier in the thread.

The is a dead spot minimum end on the MIC vol pot which I think de-oxit will fix, The BASS/TREBLE pot doesn't seem to be doing much at all except when using the MIC input.

I think I'll need to give the chassis floor a light sandblast, I know a business that uses a sealed chamber to do sandblasting in, But the first priority now is new filter caps.

Thanks to everyone for your input, its been real valuable.

(ps: I'll keep this thread updated)
 
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