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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Fri Feb 19, 2016 2:23 pm
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tremolo arm wrote:
TimsAudio wrote:
You can pull one power tube at a time. It won't hurt the amp. It will sound funny.
It will reveal differences between the power tubes if only one rattles.


I don't want to doubt what are you saying, but I've heard that by removing one power tube you're bound to unbalance the primary side load the tubes place on the transformer, and it will leave one side of the primary completely unloaded thus putting te transformers at risk.

Is that not applicable to the 68 CVR?


TimAudio know amps.


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Sat Feb 20, 2016 10:53 am
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Update as from a few minutes ago:

You guys have proved once more your Sherlock Holmes skills by pointing in the right direction.
It's definitely coming from the power tube(s) or their slots.

I run a test whereby I would fret with with my left hand at the fret where the offending frequency was produced and with my right hand I would touch the tip of the tube (whilst holding the body of the guitar against the amp to increase the sustain). Well, upon touching the tip of the tube, the rattling would stop. Upon releasing my grip, the rattling would be back. I then swapped the tubes and the same tube would produce the rattle, despite being in the other slot. However, the other tube also seems to rattle a little (but to a much lesser extent).

I also noticed that the tubes would not slot in flush with the edge when inserted the right-hand slot (the left hand one seems OK). You can see some of the tube pins not fully in and the tube itself rocks slightly when you move it inside the slot. Could this be a problem?

Now that we have established the source of the problem as the tubes, should I buy an exact replacement - Groove Premium GT-6L6-s or are there other tubes which I should consider?

Thanks


Last edited by tremolo arm on Sat Feb 20, 2016 12:26 pm, edited 1 time in total.

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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Sat Feb 20, 2016 12:19 pm
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tremolo arm wrote:
Update as from a few minutes ago:

You guys have proved once more your Sherlock Holmes skills by pointing in the right direction.
It's definitely coming from the power tube(s) or their slots.

I run a test whereby I would fret with with my left hand at the fret where the offending frequency was produced and with my right hand I would touch the tip of the tube (whilst holding the body of the guitar against the amp to increase the sustain). Well, upon touching the tip of the valve, the rattling would stop. Upon releasing, the rattling would be back. I then swapped the tubes and the same tube would produce the rattle, despite being in the new slot. However, the other tube also seems to rattle a little (but to a much lesser extent).

I also noticed that the tubes would not slot in flush when inserted the right-hand tube slot (the left hand one seems OK). You can see some of the tube the pins are not fully in and the tube rocks slightly when you move it inside the slot. Could this be a problem?

I see that sometimes, no problem

Now that we have established the source of the problem as the tubes, should I buy an exact replacement - Groove Premium GT-6L6-s or are there other tubes which I should consider?

Other than original tubes need a bias check.
With your tubes issue, I'll check bias anyway.

I like JJ's if you know how to bias a amp.


Thanks


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Sat Feb 20, 2016 1:32 pm
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I find the Electro-Harmonix 6L6EX to be very cost-effective alternative that's both good sounding and durable.

As Stratele (above) recommended, new power tubes regardless of brand should be correctly biased after installation.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Sat Feb 20, 2016 2:03 pm
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Retroverbial wrote:

As Stratele (above) recommended, new power tubes regardless of brand should be correctly biased after installation.

Arjay


Even the original type / brand?
I'm not sure I have te necessary knowledge and certainly don't have the equipment to re-bias.

I hope there's a way to avoid taking the amp to a dealer for a simple tube swap?


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Sat Feb 20, 2016 2:28 pm
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tremolo arm wrote:
Retroverbial wrote:

As Stratele (above) recommended, new power tubes regardless of brand should be correctly biased after installation.

Arjay


Even the original type / brand?
I'm not sure I have te necessary knowledge and certainly don't have the equipment to re-bias.

I hope there's a way to avoid taking the amp to a dealer for a simple tube swap?


Bias is a must if you want longer tube's life and better tone.
Tools for a bias ; a meter and knowledge.

If you put new tubes and they don't red plating you can live them there like many people do.
Red plating; tubes will blow in the next minutes.

No link for a shematic ?


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Mon Feb 22, 2016 6:28 am
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OK, so I ordered the new tibes. I may also be able to borrow a standard voltemeter.
Is it a relatively straightforward thing to bias the CVR?
I would really appreciate if anyone could give me a short idiot proof guide on how to...


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Mon Feb 22, 2016 6:53 am
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tremolo arm wrote:
OK, so I ordered the new tibes. I may also be able to borrow a standard voltemeter.
Is it a relatively straightforward thing to bias the CVR?
I would really appreciate if anyone could give me a short idiot proof guide on how to...



Please answer to my words in blue

Schematic is a must to start.


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Mon Feb 22, 2016 7:40 am
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stratele52 wrote:
tremolo arm wrote:
OK, so I ordered the new tibes. I may also be able to borrow a standard voltemeter.
Is it a relatively straightforward thing to bias the CVR?
I would really appreciate if anyone could give me a short idiot proof guide on how to...



Please answer to my words in blue

Schematic is a must to start.


I am not sure I understand you, Stratele.
Do you mean that I should supply a schematic of the amp so that you can guide me where I need to connect the voltemeter?
I looked online, but I could not find a schematic for this amp.
I guess I could take some photos of the circuit and maybe you can guide me from there?
That said, I read the manual and it says that the amp needn't be re-biased if the same tubes (as the original are used). This is on page 9 of the manual: http://www.fmicassets.com/Damroot/Origi ... manual.pdf


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Mon Feb 22, 2016 8:54 am
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tremolo arm wrote:


Schematic is a must to start.


I am not sure I understand you, Stratele.
Do you mean that I should supply a schematic of the amp so that you can guide me where I need to connect the voltemeter?
Yes and more


I looked online, but I could not find a schematic for this amp. Same for me . :(
I guess I could take some photos of the circuit and maybe you can guide me from there?
I need more than pictures, schematic first

That said, I read the manual and it says that the amp needn't be re-biased if the same tubes (as the original are used). This is on page 9 of the manual: http://www.fmicassets.com/Damroot/Origi ... manual.pdf[/quote]

That is not always true. First , can you buy the original tubes ?


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Mon Feb 22, 2016 9:56 am
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The groove tube classification system will get you a safe bias with no resetting.
That being said, a safe bias does not necessarily sound good. Like putting cheap gas in your car, it may not perform up to it's capability.
Investing in a bias job allows you to use more musical tubes, such as JJ, wingedC, or other brands. Owning a tube amp signs you on to this kind of maintenance. The tube tone does cost more. What is your tone worth?
The difference between a cold, glassy tone and a rich vibrant tone is only the difference of 10 milliamps of plate current. Fine tuning the bias ensures your amp is musical, not harsh.
While the GT system will give you a safe bias, it doesn't guarantee the correct bias. Only a meter will do that.
Before I can comment on the Bias circuit, as Stratle said, we need a schematic or you need a bias probe and a meter.


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Mon Feb 22, 2016 10:26 am
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TimsAudio wrote:
The groove tube classification system will get you a safe bias with no resetting.
That being said, a safe bias does not necessarily sound good. Like putting cheap gas in your car, it may not perform up to it's capability.
Investing in a bias job allows you to use more musical tubes, such as JJ, wingedC, or other brands. Owning a tube amp signs you on to this kind of maintenance. The tube tone does cost more. What is your tone worth?
The difference between a cold, glassy tone and a rich vibrant tone is only the difference of 10 milliamps of plate current. Fine tuning the bias ensures your amp is musical, not harsh.
While the GT system will give you a safe bias, it doesn't guarantee the correct bias. Only a meter will do that.
Before I can comment on the Bias circuit, as Stratle said, we need a schematic or you need a bias probe and a meter.


Thank you for the explanation.

Given that the instruction manual does not have a schematic and neither Stratele nor I have been able to locate one on the net, it sounds like this may not be an easy job (especially for a biasing novice like me).

I wonder if anyone on this forum has biased their CVR and can provide pointers.

I will ask Fender if they can send me a schematic. Failing that I will have to "risk" compromising on tone or simply take the amp to a tech.


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Post subject: Re: 68 CVR rattling noise in certain frequencies
Posted: Thu Feb 25, 2016 4:26 am
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I Put 2 x TAD 6L6 WGC short bottles in mine, biased them roughly to 70.0 ma on the test point on the circuit board (test point reads both together) I also used a octal socket tester i bought off ebay to get a reading for each tube one was 34.8 and the other one was 35.1 so its def the right test point. I preferred these power tubes to the factory groove tubes. the factory power tubes were biased to 58 ma at the test point, 29 each.. no wonder I prefer the short bottles!


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