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Problem With 01A Preamp (Losing Rectifiers)
Problem With 01A Preamp (Losing Rectifiers)
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Old 4th December 2016, 05:09 PM   #1
amandarae is offline amandarae  United States
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Default Problem With 01A Preamp (Losing Rectifiers)

Hello guys,
First off, I am building a 01A preamp base on Ale Moglia's (Bartola Valves)design. The schematic is this:

Click the image to open in full size.

The only difference I made is to replace the 47K resistor to a 100k pot(volume control).

For power supply, I am using the same PSU I have been using, with no problems whatsoever, with my Type 26 preamp in a two chassis set up with HV from a tube rectified(AZ1)arrangement, plus two filament regulators(Rod Coleman's) using 16Vac transformer per to each set of diode rectifiers.

An umbilical cord is connected to the PSU to the main preamp chassis. I am getting 202V B+(LCLCLC) and 25V DC for each filament supply measured at the iNPUT of the Rod Coleman boards.

Here is the problem. Looking back at the schematic, I can measure 4,8V at the top of the filament resistor and 8.11 V at the cathode of the 201A, and 118 at the plate with respect to ground(0 V) and 30 mV across the resistor R5. If I flip the SW1, it takes the tube rectifier out with a flash. If I connect an interconnect at the output of the preamp to an input of say another amp who's Ground(0V) is connected to chassis ground(IEC ground), it takes the rectifier again. In other words, as soon as I have the chassis ground (IEC) connected to my 0V, it destroys the rectifier!

Also, the 4.85V is the minimum I can get from the Coleman Reg adjustment (using 3.9 ohms R1) and measuring from the Filament Input(+ and -) on the board, I am getting 25Vdc! Why is it too high?

Chassis is aluminum. Heat sinking is to the chassis, umbilical cord is a Neutrik 8 Pole with 1+ and1- as Filament Supply 1, 2+ and 2- as B+ and 0V, 3+ and 3- as HV 0V and IEC safety ground, and 4+ and 4- as Filament Supply 2.

Trying to seek help as I lost three AZ1's already and this project is getting too expensive

Thanks!

Abe
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Old 4th December 2016, 05:53 PM   #2
Ketje is offline Ketje  Belgium
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M3 (or M2) making contact with the chassis ?
Mona
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Old 4th December 2016, 06:21 PM   #3
mogliaa is offline mogliaa  United Kingdom
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Problem With 01A Preamp (Losing Rectifiers)
Hard to say without some pictures showing layout and wiring. Specially on ground and earthing.
Some suggestions:

1) Measure the isolation of top FET
2) if top FET is damaged will provide a short or low resistance which may be the issue
3) you should test the gyrator with a dummy load and if you have a variac you can slowly bring HT and find out the issue
3) what anode and mu follower voltages do you have?
Ale
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Old 4th December 2016, 08:24 PM   #4
euro21 is offline euro21  Hungary
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Problem With 01A Preamp (Losing Rectifiers)
Quote:
Originally Posted by amandarae View Post
1+ and1- as Filament Supply 1,
2+ and 2- as B+ and 0V,
3+ and 3- as HV 0V and IEC safety ground,
and 4+ and 4- as Filament Supply 2.
IMHO you have some wiring mistake.

What the difference of B+(0V) and HV(0V)?

Why connect preamp chassis to IEC safety ground?

I grounding only PSU box metal chassis, the HV and filament PSUs are independent from earth (floating).

Sometimes connecting preamp metal plate to 0V (aka HV(0V) ) necessary (lesser hum), sometimes not.
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Old 4th December 2016, 09:06 PM   #5
andyjevans is offline andyjevans  United Kingdom
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Quote:
Originally Posted by amandarae View Post

Also, the 4.85V is the minimum I can get from the Coleman Reg adjustment (using 3.9 ohms R1) and measuring from the Filament Input(+ and -) on the board, I am getting 25Vdc! Why is it too high?

Abe
My 01A Gen 2 has a 15v DC supply into Rod's regs and a 4.7K sense resistor. Not sure why you're using different values here.

Nothing to do with your problem, but thought I'd mention it.
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Old 4th December 2016, 09:53 PM   #6
amandarae is offline amandarae  United States
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Hello guys,
Thanks for the responding!

Ketje, no I double-triple check and M1 or M2 does not touch the chassis.

Ale, I realized I do not have replacement rectifiers anymore available(on order) so I cannot do any measurements. I will upload pics later. I was poking around and one thing I did not mentioned was that both L&R channel shares C3 (0.5uF) then connects to the B+. I desoldered it and measure it and I am getting a short. Will that be the culprit? Also, I did test the Gyrator with a 22K 2Watt dummy load before installing it in place to the circuit and it was fine.

Euro21,
Sorry, I made a mistake in my post above. 3+, and 3- is B+ and 0V. 2+ and 2- chassis GND and IEC GND. Let me explain.
For the HV, 0V is the center tap of the transformer secondary. IEC is the safety GND from wall,and chassis ground is the connection of the two chassis. On my 26 preamp PSU, the 0V and IEC ground goes to the umbilical cord to the receptacle connector of the preamp chassis then the IEC GND connects to the middle of the chassis by itself. The OV now is isolated and becomes the GND for the circuit components. The "Chassis ground" is the line that connects the two chassis together.
Filaments 1 and 2 are isolated, after the filter goes to the umbilical cord, then Coleman Reg input, with 1000uF/35V shunt cap, output then to the cathode and the cathode resistor as shown on the schematic. The bottom end of the resistor is then connected to the 0V.
B+ (0V) and HV (0V) are the same right?

Andy,
I emailed Rod and told him I a going to use the regulators for a 01A filament and he replied "...for 01A, simply change R1 to be 3.9 ohm 2W to 5W, wirewound....".
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Old 4th December 2016, 10:54 PM   #7
amandarae is offline amandarae  United States
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Some Pics.
Click the image to open in full size.

IEC Connector to the chassis.: YEL(+) GRN(-) for Filament 1 and BLU(+) GRY(-) Filament 2. ORNG(IEC GND) VIO(Chassis) Ground, RED(B+) BRN(0V).

Click the image to open in full size.

Click the image to open in full size.
There is a copper wire than serves as a bus for the OV where the ff is connected from left to right. Input NEG, Output NEG, GNBD for the VOL pot(100k), GND for both Gyrator boards, bottom end of the filament resistor for both channel, then 0V.


Click the image to open in full size.

Click the image to open in full size.


0V is not connected to the chassis unless switch S1 is close( left of the Neutrik connector in pic 1.
Notice C3 is not in the pictures as I desoldered it to test it.

Abe

Last edited by amandarae; 4th December 2016 at 11:01 PM.
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Old 4th December 2016, 11:34 PM   #8
andyjevans is offline andyjevans  United Kingdom
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For Rod's supply, 3.9R would be correct for an 01A at book values, but Ale's design starves the filaments giving 4.5R, so I used 4.7R. Supply should be around 15v, though.
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Old 5th December 2016, 04:37 AM   #9
amandarae is offline amandarae  United States
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Correction: I meant "Open on C3" not "short" on my reply to Ale. Thanks!
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Old 5th December 2016, 05:06 AM   #10
Rod Coleman is offline Rod Coleman  United Kingdom
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Problem With 01A Preamp (Losing Rectifiers)
Hi Abe,

A couple of quick checks, with the mains disconnected, and B+ capacitors discharged (measured at 0V):

1. With a DMM set to measure resistance on the 200KΩ scale, measure across the B+ capacitor (red lead to + of the cap). I would expect a reading of more than 100KΩ to show.

2. Disconnect the SUPPLY input to the filament regulator. Measure (again on 200KΩ scale) from the raw dc (+ and -) to the system ground (the 0V of B+ capacitor). This must be open circuit - no resistance reading. Don't power the system if there is a leakage here - this must be cleared.
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