Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => EZ Board postings => Topic started by: Geezer on June 27, 2005, 07:26:15 pm
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aerojazz
Senior tube assistant
Posts: 163
(3/6/04 6:41 pm)
Feedback, hum noise comming from PI.
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If I place my standby switch after the bridge rectifier,(I have to) Do I also have to switch the ground as well ?
I currently have the B+ only going to the switch and I'm seeing big voltage in the circuit even when just on standby, the center tap is connected to the junction of the first two main filtercaps(Marshall 1959 Superlead). I don't know if that has anything to do with it.
I just don't understand why I'm seeing voltage while on standby.
Edited by: aerojazz at: 3/9/04 9:19 am
Tiny Daddy
I will work on all amps
Posts: 530
(3/6/04 7:35 pm)
Re: Plexi style standby switch?
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It's 'cause the standby switch only disconnects two of the diodes in the bridge. The other 2 diodes charge the lower filter cap. One way to fix it: remove the center tap and tape it off and install a 270K 3 watt balancing resistor across each filter cap.
aerojazz
Senior tube assistant
Posts: 164
(3/6/04 8:15 pm)
Re: Plexi style standby switch?
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Ok, I did that, it did work, and brought down B+ 475 Volts after going off stand by But while on standby my voltmeter was showing 1016 volts waiting at the switch. Is that normal ? the bridge is rated at 1000. Am I exceeding specs or is current an issue here?
Tiny Daddy
I will work on all amps
Posts: 531
(3/6/04 8:50 pm)
Re: Plexi style standby switch?
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Not sure what your meter is reading, maybe there are voltage spikes on the AC line? Add a 1M resistor to ground across the bridge +/- and see if the voltage is still there.
jbrew73
Junior tube assistant
Posts: 103
(3/6/04 10:20 pm)
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what power transformer are you useing? are you sure its for a bridge rectifier?
ok i just re-read some of your old post regaurding this amp. it seems you've had lots of problem with this power supply. i would disconnect the power transformer secondaries from the amp and check the voltages and post them. use the center tap for your ground lead and read the voltages on the HV taps you should have something like 175-0-175 but i bet you get 350-0-350. let us know.
Edited by: jbrew73 at: 3/6/04 10:29 pm
Tiny Daddy
I will work on all amps
Posts: 532
(3/7/04 1:48 am)
Re: Plexi style standby switch?
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If it should be that you need to cut the B+ in half you will need to disconnect the - (neg) side of the bridge, do not connect the bridge - (neg) to anything, it is not used.
Then ground the transformer tap and you are there.
aerojazz
Senior tube assistant
Posts: 165
(3/7/04 6:12 am)
Re: Plexi style standby switch?
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That did the trick, and it's NOT blowing fuses anymore.
I left the transformer CT at the junction of the two main fiter caps, as Model 1959 shows. Is this acceptable?
I know, doctor says "do this', I do that.
110 AC in, voltages are as follows:
B+ 413
V1 pin 1 188
V1 pin 6 180
V2 pin 1&7 162
V2 pin 6 273
V3 pin 1 311
V3 pin 6 311
V4 thru V7 pin 3 406
pin 4 394
pins 1&8 are all 35.2 mv. to 35.8 mv.
Trannies are nice and cool, I'm getting a sweet almost inaudable hum thru the spkr.
I must have wired my guitar jack wrong,
I aint got no volume.
Thanks for your help
Edited by: aerojazz at: 3/7/04 6:30 am
jbrew73
Junior tube assistant
Posts: 105
(3/7/04 8:38 am)
Re: Plexi style standby switch?
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you will need to disc
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you will need to disconnect the CT from the cap junction and ground the CT to the tranny mounting bolt. i'm not sure what could be happening the way you have it now but it's probably not good.
aerojazz
Senior tube assistant
Posts: 166
(3/7/04 10:59 am)
Re: Plexi style standby switch?
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Ok, Did that, voltages looking even better now.
Still no sound.. let me look a little more
EDIT:
Ahh found it, no wire from 470 ohm to PI cathodes.
EDIT:
Total Feedback, hum noise comming from PI. I totaly diconected input jack wire by wire to see if it was the wiring, No, Something is screwing up at the PI.
Any Ideas?
I even replaced the socket and tube.
Edited by: aerojazz at: 3/7/04 2:30 pm
PHATamps
I only work on tube amps
Posts: 422
(3/7/04 10:37 pm)
Re: Plexi style standby switch?
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Maybe you have positive feedback--try swapping the OT leads at the plates.
You'd have never known without hooking up your PI cathodes!
aerojazz
Senior tube assistant
Posts: 167
(3/8/04 5:13 pm)
Re: Plexi style standby switch?
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I'll try that as soon as I get home.
Thanks
aerojazz
Senior tube assistant
Posts: 168
(3/9/04 9:18 am)
Re: Plexi style standby switch?
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That was the problem, Thanks PHAT