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Posted: Mon May 17, 2010 3:34 am
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400 volts!!! You have a problem there. You should have more : 450-465.

450 at tubes but 465 at standby switch.

Is your rectifier tube is good ?
And what about you power xfr ? You must have 345-0-345 AC volts

Do you have a good voltmeter ?


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Posted: Mon May 17, 2010 8:33 am
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stratele52 wrote:
400 volts!!! You have a problem there. You should have more : 450-465.

450 at tubes but 465 at standby switch.

Is your rectifier tube is good ?
And what about you power xfr ? You must have 345-0-345 AC volts

Do you have a good voltmeter ?


Yes, my voltmeter is good.

I have at the xfr 345 ac to the rectifier tube. (JJ 5U4) and 400volts dc at the standy switch.

I tried 2 rectifier tubes and the same. The xfr is an original and seems to be good. How can I tell for sure?


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Posted: Mon May 17, 2010 1:51 pm
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Your transfo is good : you read the good AC voltage :345v Do you have also 6.3 AC on filaments ?

Very strange that you lost so manys DC volts.

Your Power Tubes : remove them and mesure at stanby switch. Your DC voltage should go up.
If yes, It looks that your tubes are running too hot, not enought bias.
On your amp you can't ajust the bias, the pot is a balance between the 2 tubes.
If you don't have enough bias, you have to modify the bias circuit.

Do you know what I'm talking about ?


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Posted: Mon May 17, 2010 2:07 pm
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stratele52 wrote:
Your transfo is good : you read the good AC voltage :345v Do you have also 6.3 AC on filaments ?

Very strange that you lost so manys DC volts.

Your Power Tubes : remove them and mesure at stanby switch. Your DC voltage should go up.
If yes, It looks that your tubes are running too hot, not enought bias.
On your amp you can't ajust the bias, the pot is a balance between the 2 tubes.
If you don't have enough bias, you have to modify the bias circuit.

Do you know what I'm talking about ?


Yes, the filaments are 6.5v ac 5v ac for the rectifier.

The tubes are not running too hot. It is a bit cold. 37ma at 400volts plate. This is about 50% dissipation. I do know how to mod the bias circuit. The fact is I don't think the 400v B+ is causing the hum. I put in a 5AR4 and that bumped the B+ to 430v and still hums.

I eliminated the fact that the new power filter caps might be bad. I swapped them out with a new set. Still hums. This is a tough one.

Also, it is not the power tubes as it ONLY hums with the boost circuit on.


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Posted: Mon May 17, 2010 2:35 pm
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You could be right about your output tube are not making hum, but low B+ voltage is not normal. It could be the way to find the hum ? Or another problem.


Yes 37 ma at 400 volts is cold.


Remove your power tubes and mesure the B+


How do you mesure 37 ma ? you put a 1 ohms resistor in serie with the plate of each tubes or you have a bias probe ?


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Posted: Mon May 17, 2010 4:52 pm
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stratele52 wrote:
You could be right about your output tube are not making hum, but low B+ voltage is not normal. It could be the way to find the hum ? Or another problem.


Yes 37 ma at 400 volts is cold.


Remove your power tubes and mesure the B+


How do you mesure 37 ma ? you put a 1 ohms resistor in serie with the plate of each tubes or you have a bias probe ?


Okay, the voltage is 443v without power tubes. I use a bias probe. It is actually the cathode current. The plate current would probably be 2ma less.


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Posted: Tue May 18, 2010 5:16 am
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43 volts drop ! It's a lot of voltage for tubes bias cold. If your tubes was bias hot , 43 volts drop could be ok but cold....No

It's not easy to fix an amp by e-mail like we try. I should be sure of what you do.
I don't know what is you skill in electronics and what is your tools.
When I ask you if you have a good voltmeter. I hope an answer with the name and model of your voltmeter.

I meet often guys with bad or cheap voltmeter and we loose time because that. Those guys was shure they have good voltmeter and was surprise to discover the meter doing bad false reading.
Ot guys doen use voltmeter the right way.

And your bias probe : name and model please.

By the way if you have 37 ma at cathode, you SHOULD have about the same at the plate.
How can you have 2 ma at plate ? How do you mesure that ?


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Posted: Tue May 18, 2010 8:37 am
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stratele52 wrote:
43 volts drop ! It's a lot of voltage for tubes bias cold. If your tubes was bias hot , 43 volts drop could be ok but cold....No

It's not easy to fix an amp by e-mail like we try. I should be sure of what you do.
I don't know what is you skill in electronics and what is your tools.
When I ask you if you have a good voltmeter. I hope an answer with the name and model of your voltmeter.

I meet often guys with bad or cheap voltmeter and we loose time because that. Those guys was shure they have good voltmeter and was surprise to discover the meter doing bad false reading.
Ot guys doen use voltmeter the right way.

And your bias probe : name and model please.

By the way if you have 37 ma at cathode, you SHOULD have about the same at the plate.
How can you have 2 ma at plate ? How do you mesure that ?


I have a decent bench. My electronics skills are good enough to have fixed about 24 tube guitar amps and I also restore Nakamichi cassette decks. I am not an electrical engineer, though.

My meters are Fluke 289, I have a capacitor meter, Weber Bias Rite. The voltage reading is good as I even tried another meter and got the same.

I meant the plate will be about 2ma less than the cathode. So 35ma or so at plate .


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Posted: Tue May 18, 2010 1:12 pm
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Thank's for answering jazzgene.


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Posted: Wed May 19, 2010 9:35 pm
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Thanks for asking the personal questions, Stratele ! Actually, a little info in the profile section can help(no offense, Gene). But, here we are with a lowish B+ voltage. I can live with that, if it is clean(little or no AC). Gene, all of the other voltages checked out OK, right ? The coupling and bypass caps & resistors for V2, V3, and V4 are OK, right ? And all of the grounds are good, so I guess I'd like to know what kind of activity, we're seeing off the closed switch on both sides of the 10k and the 220k resistors. Have you guys been flipping back and forth between this schem and the AA270 schem ? It looks like the gain of V3 is dropped down through the unbypassed cathode resistor(680ohm). Just wondering if it may be useful to increase it up to eliminate some gain, but it would be at the expense of the reverb signal. But we also have to know that the step down reverb trans is doing it's job. I mean we're looking at almost a 1 watt amp sending it's output into the circuit !!!This is quite an interesting problem. Art

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Posted: Fri May 21, 2010 10:54 pm
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Gene, I've been wondering about the 500PF reverb cap. This is also a governing factor of this circuit. Art

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Posted: Sat May 22, 2010 9:37 am
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aclempoppi wrote:
Gene, I've been wondering about the 500PF reverb cap. This is also a governing factor of this circuit. Art


Hi Art,

Thanks for your reply. I have some ideas but can't get to my bench until next weekend. I will definitely check the 500pf cap off pin 2 and 7 on V3.

Gene


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Posted: Sat May 22, 2010 9:41 am
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aclempoppi wrote:
Thanks for asking the personal questions, Stratele ! Actually, a little info in the profile section can help(no offense, Gene). But, here we are with a lowish B+ voltage. I can live with that, if it is clean(little or no AC). Gene, all of the other voltages checked out OK, right ? The coupling and bypass caps & resistors for V2, V3, and V4 are OK, right ? And all of the grounds are good, so I guess I'd like to know what kind of activity, we're seeing off the closed switch on both sides of the 10k and the 220k resistors. Have you guys been flipping back and forth between this schem and the AA270 schem ? It looks like the gain of V3 is dropped down through the unbypassed cathode resistor(680ohm). Just wondering if it may be useful to increase it up to eliminate some gain, but it would be at the expense of the reverb signal. But we also have to know that the step down reverb trans is doing it's job. I mean we're looking at almost a 1 watt amp sending it's output into the circuit !!!This is quite an interesting problem. Art


My amp does not have the boost pedal jack. So the 10k/220k resistor junction is 1k/220k. I did replace these guys with no effect on hum.

The bypass caps are new. I did hear from someone that does have a vibrolux with pull boost and his amp does not hum loudly with the boost on. (although the hum does get louder a bit)


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Posted: Sat May 22, 2010 9:18 pm
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Whoa there Hoss! I've been looking at the wrong schem! OOPS !!!
I've been using, schematic heaven's AA270 Vib Rev, page 1 & 2. Can you give me a link to your schem? Well, it's a good thing that you're in touch with another Vibrolux owner. That can really help the situation. Yeah man, me and mistakes have a real solid relationship. Keep at it, Gene. Maybe some larger gauge ground wiring or better locations will be necessary. Also, a quiet tube combination might be the key. Art

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Posted: Sat May 22, 2010 9:43 pm
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aclempoppi wrote:
Whoa there Hoss! I've been looking at the wrong schem! OOPS !!!
I've been using, schematic heaven's AA270 Vib Rev, page 1 & 2. Can you give me a link to your schem? Well, it's a good thing that you're in touch with another Vibrolux owner. That can really help the situation. Yeah man, me and mistakes have a real solid relationship. Keep at it, Gene. Maybe some larger gauge ground wiring or better locations will be necessary. Also, a quiet tube combination might be the key. Art


I don't have a link but I can email you a copy. What is your email?

This one is the closest I have seen to my amp. It shows the boost pedal which my amp does not have. Next weekend, I will have some time to go at it.


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