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arwokc
Posts: 12
(3/13/04 9:39 am)
Hum in bias circuit?
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I just can't seem to eliminate this annoying hum. It pretty much stays constant at any volume. In other words, it doesn't get louder if you turn the volume up or quieter if you turn the volume down. By the way this is a Vibroverb version of the AB763 circuit. The only thing that will make it go away is removing the bias section 12AT7 tube. I've double checked every ground, all the solder joints, I've rewired the grounds to the factory locations, then I put all grounds at one point through a power transformer mounting stud, then I put them back to the layout that is suggested in Doug's grounding schematic. Nothing makes any difference. I switched 12AT7 tubes with the reverb section and that made no difference. I tried the artificial heater tap methods also. All that did was smoke my 470-1watt bias resistor. Yes, I remembered to remove the PT CT. So, I put that back to ground, removed the two 100 ohm resistors, and replace the 470-1w resistor, and installed a new diode just for good measure while I was there.
Any ideas anyone?
Andy
Edited by: arwokc at: 3/13/04 9:40 am
GroundhogKen
Forum Moderator
Posts: 2318
(3/13/04 9:49 am)
Re: Hum in bias circuit?
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Using a 100 ohm artificial heater tap should have no effect on the bias circuit.
Post the voltages of all you tubes:
preamp tubes pins 1,2,3,6,7,8
power tubes pins 1,3,4,5,8.
Besides the hum, can you get actual guitar signal out the speaker?
GroundhogKen
Forum Moderator
Posts: 2319
(3/13/04 9:54 am)
Re: Hum in bias circuit?
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Do you have one wire ( not the negative feedback wire) of the output transformer secondary grounded? ( This would be the black speaker wire on Doug's AB763 layout. )
Ken
arwokc
Posts: 13
(3/13/04 11:19 am)
pin voltages
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WOW! I wasn't expecting to get assistance from the top gun of all things amplifier. Thanks in advance Ken. Hopefully I can keep up with your expertise. Be gentle, I'm new to this.
Anyhow, yes I have P-L-E-N-T-Y of output from this thing. It is way loud. If I could get it to quiet down and get the Tremolo to work correctly, I'd be in heaven. But the hum is my first issue since it's so annoying.
Here are the valve voltages:
Pin Valve 1 Valve 2 Valve 3 Valve 4 Valve 5 Valve 6
1 250 252 433 246 353 326
2 0.0 0.0 0.0 0.0 -14 28
3 1.81 1.79 8.15 2.0 0.0 122
4 AC AC AC AC AC AC
5 AC AC AC AC AC AC
6 255 258 433 247 348 274
7 0.0 0.0 0.0 0.0 -14 53
8 1.9 1.9 8.15 2.0 0.0 122
9 AC AC AC AC AC AC
Fill in the DC voltages for the 8 pin power tubes
Pin Valve Valve Valve Valve
1 -44 -44
2 AC AC AC AC
3 436 435
4 435 435
5 -44 -44
6 436 436
7 AC AC AC AC
8 0 0
I don't know if this means anything or not, but when I probed the voltage on pins 2 & 7 of valve 6, the hum went away. ??
Also, it doesn't matter if the guitar is plugged in or not, the hum is still there.
As for the OPT ground, I initially had it grounded underneath the speaker ouput jack. I have since grounded it to the bolt that mounts the OPT to the chassis along with adding ground leads from the speaker jacks to this same bolt with the OPT ground lead. Neither method made any difference.
I appreciate your help and it's nice to have finally met your acquaintance.
Thanks,
Andy
Edited by: arwokc at: 3/13/04 1:25 pm
GroundhogKen
Forum Moderator
Posts: 2320
(3/13/04 2:35 pm)
Re: pin voltages
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I'd say everything looks reasonable except the phase inverter (V6)
Check your component values around this tube.
Pin 1: 82K ( gray red orange ) resistor--NOT 8.2K
Pin 2 and 7: 1 Meg ( brown black green ) resistors--NOT 100K
Pin 6: 100K ( brown black yellow )
Pin 3 and 8 tied together and connected 470 ohm ( yellow violet brown )
Ken
arwokc
Posts: 14
(3/13/04 4:04 pm)
BINGO!
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There's the problem. I have been misinterpreting the print all along. I had a 22K resistor instead of an 82k resistor. The thing is really quiet now. However, I rechecked my tube bias and now it is all out of whack! When one tube is 37.0ma, the other is 29.4ma. That's quite a difference. I remember seeing a post where you instructed someone how to get them to match a little closer, and also how to determine the maximum bias setting based on B+ voltage. I'm trying to find that post again now. I would like to get the amp to break up (overdrive) sooner (at a lower volume setting). If I interpreted the post correctly, currently a higher bias will do that. Please correct me if I've misunderstood.
Thanks,
Andy
GroundhogKen
Forum Moderator
Posts: 2321
(3/13/04 4:35 pm)
Re: BINGO!
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Are these new matched 6L6 tubes?
A 7mA difference is not a huge mismatch, but I would call them matched. If this is an older set of tubes, I wouldn't worry about the difference.
arwokc
Posts: 15
(3/13/04 5:07 pm)
tubes
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Yea, they are suppose to be a matched pair of Svetlana 6L6GC's. I got them from tubestore.com.
GroundhogKen
Forum Moderator
Posts: 2322
(3/13/04 5:38 pm)
Re: tubes
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Any chance you damaged one in the early stages of your amp's life?
Your power tube voltages looked pretty much the same. Are you measuring the current with 1% tolerance 1 ohm resistors? Do you have a decent quality ( > $39 ) meter?
There probably isn't much you can do about the volume level. A pair of 6L6GC running at 450v into efficient speakers is plenty loud enough to play lead or rhythm next to a firm hitting rock'n roll drummer.
If you want power tube clipping at residence volume, try a 50 watt MASS attenuator from WeberVST.com. It's a great attenuator for about $150.
Ken
arwokc
Posts: 16
(3/13/04 6:10 pm)
tubes
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I bought an Amp-Head dual bias tester that allows me to change from one tube to the other with a switch. It is hooked to my Snap On multimeter that I use at work. Being in aviation we are required to have them recertified annually. It was the most expensive model they sell and was made by Fluke.
arwokc
Posts: 17
(3/13/04 6:21 pm)
tubes
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Ken,
Considering that I had the wrong resistor (22K) installed instead of 82k, I'd say its very possible I damaged one of the tubes. Initially my bias was at 36.9/36.1 when I turned the amp on for the first time. The tubes were supposed to be matched Svetlana 37's. Even though I had to swap them in their sockets to get the bias that close. I just swapped out the two 220k resistors for two matched resistors that actually read 220k. It didn't help. My bias is now reading 36.9/28.5. I wish I could get them back where they were before.
GroundhogKen
Forum Moderator
Posts: 2323
(3/13/04 6:58 pm)
Re: tubes
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I don't believe your 22K vs 82K error would have caused any damage to the 6L6s.
As long as both tubes have the same voltage on pins 3,4,5, I wouldn't worry too much. Fender didn't use matched tubes. If one of them dies an early death, then you might be able to get thetubestore to send you a new set.
The plate current will be most sensitive to changes in the screen voltage ( pin 4 ). So if you notice different voltages at pin 4, then check that the 470 ohm screen resistors are the proper value.
Ken
arwokc
Posts: 18
(3/14/04 12:16 am)
Good enough
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Thanks Ken,
I can't tell you how much I appreciate your help. The hum is all but gone, it sounds (and feels) amazing. The 15" JBL is incredible. I'll tackle the Tremolo circuit another day. It really isn't an effect I'll use all that much anyway. Right now, I'm just going to enjoy playing with the amp and see how long it takes to piss off the neighbors. I'm still curious how much having Schumacher transformers would have effected the sound if any. I've learned an incredible amount from you and the other guys, and I have the itch to learn even more.
I don't have a website to link pictures to, and I guess you can't attach pictures to your posts. I'll email pictures of the amp to anyone who asks.
Thanks again,
Andy
mrr3000gt
Posts: 36
Topic: Hum in bias circuit? & (Read 3951 times)