Bad Cap?

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Wanabejimi
Junior tube assistant
Posts: 34
(2/4/04 7:21 pm)
 Bad Cap?
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 I've got a brand new build that has the following symtoms... 1) It sputters...I guess you could say it was motorboating, but its a bit slower than that. 2)The sputtering speeds up and slows down as voltage is applied and reduced with a variac. 3) The voltages are a bit high in the preamp section. In other words, the voltages are hardly reduced as they pass through the filters.

This is amp has a bunch of caps. All of them are new. Any chance I could have gotten a bad one?

Is there any way that I can test them with a DMM?










 
GroundhogKen
Forum Moderator
Posts: 2240
(2/4/04 7:37 pm)
 Re: Bad Cap?
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 What type of amp?

Can you make the noise change by spinning the knobs? Can you make the noise go away by yanking preamp tubes?


Ken
 
Wanabejimi
Junior tube assistant
Posts: 34
(2/4/04 8:18 pm)
 Cap
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 The preamp is the Ampeg preamp from Kevin O Connor's Tonnes of Tone. The Power section is from an Ampeg V4 with 7027's. For fear of blowing up high dollar new tubes, I've got worn out 6l6's in there now.

The "clack clack clack clack" sound is not effected by volume, bass, or treble knobs. In fact, the amp has a preamp out and it's all good there. I can play it through another amp. The noise does stop when I pull the phase inverter.

The amp has 3 pairs of 40 uf/450 v caps in series, followed by 3 40 uf/500 v caps

Any thoughts or comments are appreciated!
 
Wanabejimi
Junior tube assistant
Posts: 35
(2/5/04 6:07 pm)
 what the ?????
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 OK. In an attempt to test the coupling caps (40 uf/500v), I unsoldered the neg side from ground, set the DMM for dc to see if any dc was passing through. THE WHOLE B+ was passing through!!! On all three caps. Just for giggles, I soldered in a 4th one and got the same results. I am totally stumped. Please help a stupid tube freak!
 
GroundhogKen
Forum Moderator
Posts: 2240
(2/5/04 6:41 pm)
 Re: what the ?????
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 40uF 500v electrolytics are not coupling caps, they are power supply filter caps.

If these or any other electrolytics in the amp are more than 15 years old, you should change them.

The results of your little experiment may have been different if you had connected a 1 meg resistor between the lifted leg and ground.


Ken
 
gammelnalle
Junior tube assistant
Posts: 2
(2/6/04 12:33 pm)
 Re: what the ?????
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1.Do you usa a variac on the primary of tre transformer??? Then you alters the 6,3V for the heater too. ???
2. If you run the signal from the preamp to anoter amp,(no tubes in the PI) do the sputtering continues?
3. I think you done some bad grounding somewhere, have you followed Mr KevinO'Connors very exelent advises on grounding?

Cheers!

 
pzung
Junior tube assistant
Posts: 6
(2/8/04 12:12 pm)
 Re: what the ?????
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 check what these capacitors are supposed to be doing . it sounds like they are the filter capacitors for the speaker transfor and the b+ to all the tubes ? if so they are stages of filtering for dc ripple and any voltage drops between them are controlled by the resistors that seperate the capacitors . there should be one or two for the speaker transformer those first ones are usually the biggest . they have to withstand the voltage coming off the rectifier circuit so they are rated 500-700V or some combination that adds up to that and around 40-60mfd . if you are using and solidstate rectifier the capacitance value can be higher which filters more . after this initial stage there is one stage for the output tubes , one stage for the driver and then maybe two or three for the preamp tubes . if too many preamp tubes are fed by the same stage something bad happens . if the capacitor values for any of the preamp stages is too high it will boost the bass response beyond what your amp can amplify . they are usually closer to 5,8,10mfd not 40mfd . and since the voltage is usually lower by those stages they can be rated for less voltage . if you leave out the resistor between stages(capacitors) you are wiring the caps in parallel . in parallel the capacitors capacitance values are added together and the voltage rating stays the same . in series the capacitance halves and the voltage rating doubles . even after the voltage gets dropped between these stages it usually gets dropped again right before the preamp tube by another resistor . this is because if you drop the voltage of an earlier stage to the correct voltage for which ever tube it supplies , you then wouldn't have enough voltage left over to supply the next tube off the next stage of filtering .
i must qualify that i am new to this , but i'm pretty sure that is right .
 
Tiny Daddy
I will work on all amps
Posts: 483
(2/9/04 8:22 am)
 Re: Cap
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 '"clack clack clack clack" sound'
Maybe your feedback is reversed. Have you tried disconnecting or reversing the feedback?
 
Wanabejimi
Junior tube assistant
Posts: 36
(2/9/04 9:25 am)
 Re: Cap
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 I have NOT tried reversing the OT, but that is a great idea! I've had it reversed before and and gotten the "squeel" sound, but I guess it could "clack" just as easily huh?

Along those same lines, I have a bias gauge that I bought a few years back....I noticed that the bias voltage "jumps" about 5 ma with each "clack"...if that provides any clues to you gurus out there. That had me more convinced of a bad cap, but I substituted all the caps that I could (with the caps I had available) over the weekend, and there was no change.
 
Wanabejimi
Junior tube assistant
Posts: 37
(2/9/04 6:05 pm)
 cap
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 Ok, I came home, fired up the soldering iron, and reversed the OT primaries and got the same results. I then wired up another OT. Same results.

Here are all my symptoms. A slow, "clack clack clack" motorboating. I am getting a guitar signal through the amp, but it is very distorted and nasty and without much volume, but I have not cranked it up.

It sounds to me, like there is something wrong with the power supply, but I've changed out all the caps that I have available to me. I still have not changed out the 3 pairs of 40 uf/450 caps because I don't have anymore on hand. Is there any alternative to buying six more caps and putting them in there? I am officially throwing up my hands. Any ideas will be appreciated!  
 
ganzonimx
Hey get your own solder
Posts: 357
(2/9/04 6:37 pm)
 Re: cap
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 Do you have a preamp schematic?

What are the voltages?

Cris

PS:

Just out of curiosity: The Ampeg V4 power amp, shows a connection of the plates of the PI over a 820k and 1M resistor, from their union a 0.1 uF cap to the lower input of the PI, kind of a feed back loop. Whats the purpose of that circuit? could that be the culprit of the sputtering?

Edited by: ganzonimx at: 2/9/04 7:10 pm
 
Wanabejimi
Junior tube assistant
Posts: 38
(2/9/04 8:01 pm)
 cap
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 Nice try with the cap on the PI! I tried taking it out. No dice. I can't tell if the "clacking" was still there or not. The volume dropped off considerably and the sound was more distorted than ever.

I don't have any way to post the schematic, its pretty simple. Just two halves of a 12ax7 with a tone stack in the middle...very ampeg-ish without all the mid select/hi-lo switches. Then I went into the 1/2 of the 12dw7 that is on the power amp schem of the v4. Then into the PI.

I have negleted to note that this amp is built into the carcass/chassis of a v4. Because of the layout of the actual chassis, the preamp lead dress is nice, clean, and neat. The lead dress to the power section is a bit lengthy . Any advice on how to route wires to the power tubes might prove helpful. Unlike the schem, I do have 1 ohm resistors on pin 8 for biasing.
 
 
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