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Old 21st December 2001, 03:14 PM   #11
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Join Date: Feb 2001
Location: Columbia, SC
Sleep is not an option...
The 22pF goes to the grid, not ground.

Grey
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Old 24th December 2001, 05:27 AM   #12
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Join Date: Aug 2001
Location: Secunderabad, India


Dear Grey,
I am back after the weekend, with more trouble :-((

Yesterday (sunday), I connected the 100K between grid and ground, and connected the 22pf to the grid side of the 100K, and fed the signal direct the the grid.

Before doing this mod, I noticed that the amp was making a squealing sound whenever it was being powered up. The volume of this squealing sound used to reduce and disappear in a couple of seconds.

One thing I forgot to mention earlier was that, I am using a delay circuit with a relay to switch on the high voltage around 45 seconds after the heater voltage is applied.

I am using a toroidal transformer as the power transformer. I had connected one diode each to the 225V winding on the secondary of the transformer, connected the other ends of the diodes together and then connected this end to one contact of the relay. The other contact of the relay was connetced to the positive ends of the 2200mFd filter capacitors.

I thought that the squealing sound was caused by the sudden rush of current to the filter capacitors, once the relay was closed.

To avoid this, I disconnected the diodes from the relay, and connected the diodes directly to the filter capacitors, and the output from the capacitors, (after the 10-Ohms, 2W resistors) was connected to the relay(s) to be switched on after the preset 45 Second delay, to the amp circuit.

I did this modififcation along with the changes described in the first paragraph above.



I connected the input and the speakers and switched on the amp.
All the noise (hum, motorboating, etc.) had gone away, but the output from the amp is very very low. With the volume control in the preamp set to more than halfway, the amp is producing just audiable sound from the speakers.

Please help.

I would appreciate if you could guide me in troubleshooting the new problem. Please guide me to a step by step process wherein I can save the amp.

Wishing all of you a Merry Christmas and a Prosperous New Year.

Thanks and regards,
rk
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Old 24th December 2001, 11:07 AM   #13
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Join Date: Aug 2001
Location: Melbourne/Australia
Hi Arkay,

I don't know how much I can help, I am only learning myself, so here goes...

From reading your last post, I am not sure I understand how you have connected the input. The way I would expect it to be connected is a follows (ignore the dots),

..........22pF................
...........| |...................
IN -----| |----*--- GRID
...........| |.....|.............
....................\............
..................../............
....................\............
..................../..100K..
....................\............
..................../............
....................|............
................GND..........

Also if that is the case, I would think that 22pF is quite a small value of coupling cap.

Regards,
Tim.
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Old 24th December 2001, 12:20 PM   #14
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Join Date: Aug 2001
Location: Secunderabad, India
Hi Tim,

The connection is as follows:

..................................
..................................
IN ------------*--- GRID
...................|----------------| |-----> anode
....................\............ 22pF
..................../............
....................\............
..................../..100K..
....................\............
..................../............
....................|............
................GND..........

I am also learning. Grey had pointed out that the 22pF is the local feedback component.

regards,
rk
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Old 27th December 2001, 04:28 AM   #15
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Join Date: Aug 2001
Location: Secunderabad, India
Hi,
With the help of Mr. Hedlund's advice, I replaced the 10Ohm resistors in the power supply which had burnt out, and have removed the delay circuit. The amp is working now, and the howling problem has gone away.

The motorboating continues.

Please help me remove this.
Thanks & regards,
rk
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Old 27th December 2001, 10:26 PM   #16
diyAudio Member
 
Join Date: Aug 2001
Location: Melbourne/Australia
Hi arkay,

Regarding the pre-amp. I think there maybe another misprint.

The power supply shows an elevated heater supply for V2 and I would expect that the upper valve could do with an elevated supply, yet is labelled 1/2V1 on the schematic, V2 is not shown anywhere.
I would expect that V1 is the lower valve and V2 is the upper valve and that each valve is shared between the left and right channels (1/2 per each channel). That way the upper valve can be connected to the elevated heater supply.

How did you connected interpret the schematic?

Regards,
Tim.
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Old 5th January 2002, 05:53 AM   #17
diyAudio Member
 
Join Date: Aug 2001
Location: Secunderabad, India
Hi,

During the new year holidays, I did a bit of rewiring, especially the grounding wires, and the motorboating has come down drastically.

Earlier, even though I had a single chassis grounding point, in some places in the circuit, I was running a single wire to connect multiple grounding points, and then connecting the other end of the wire to the common ground in the chassis. I replaced this, and connected individual wires from all the grounding points shown in the schematic, to the chassis common ground.

Now, when I am playing the CD player (connected through the pre-amp), there is absolutely no motorboating or hum, and the audio output is soooo sweet. On the other hand, when I am playing the tape player (again connected through the pre-amp), there is slight hum and motorboating (very very slight, audiable to about 1 foot from the speakers).

I thank all of you who have given suggestions in helping me get rid of the motorboating. Any advice to get rid of the last bit of motorboating would be highly appreciated.

Thanks once again,
regards,
rk
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