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Post subject: How I Customized my 68 CVR
Posted: Mon Nov 17, 2014 5:19 pm
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The perfect amp for me would be a 40 watt 65 Twin Reverb in a 40 or so pound package. Why? Because I'm 67 years old and a 64 pound REAL Twin Reverb is just too much for me to handle. And I'm not getting younger :lol:

So I bought this 68 Custom Vibrolux Reverb in hopes that it would meet my needs. After a week or two of testing it as it came out of the box, I concluded that the Celestion Ten speakers that come in it from the factory did not meet my expectations. So, I replaced them with Eminence Ragin' Cajun speakers. Instantaneous improvement, but still off the mark.

This morning I pulled the back panel and metal cage off the back and discovered that from the factory, the 6L6's are Groove Tube "Matched 4." So I checked the Groove Tubes web site and discovered the following:
LOW= 1-3 Early distortion; softer attack; good for rock & blues solos.
MEDIUM= 4-7 Normal performance; best all around rating type for all styles of playing.
HIGH= 8-10 Most dynamic range; most clean power; less break up for power players, jazz & bass amps.

Which tells me that the Matched 4 set are barely out of the "early distortion and softer attack category. In order to get the Twin Reverb sound, soft, early breakup tubes won't do it. In another thread I wondered how Fender got a set of 6L6es to sound like 6V6es in the CVR!! By the way, the factory had set the bias current at 27.5 ma, which is wa-a-ay cold. Now I know how they did it!

So I just happened to have a set of new JJ 6L6es that I bought from Eurotubes to try in a DRRI. Took 'em out of the DRRI, replaced them with the original 6V6es and traded the DRRI in on the 68 CVR. And after discovering the above about the Groove Tube 6L6es, I replaced the stock Groove Tubes with the JJ 6L6es, rebiased it for 41 ma at 425 volts and WOW! I think I'm as close to a 40 watt Twin Reverb or a Bandmaster as I'm going to get. It is truly a sweet sounding amp! I'm thinking that with the volume knob on the Vintage channel at 6 to 7 I'm getting a very nice SRV-type sound. It's quite cool!

Now I know that this business of getting "your sound" is very subjective. But I believe I've found mine. I'm sharing this with you in case you think there is something lacking in your 68 CVR and you wonder what you can do. If you like the stock sound with the Celestion Tens, I'm happy for you. They are outstanding amps.


Last edited by Lynn Nicholson on Mon Nov 17, 2014 11:06 pm, edited 1 time in total.

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Post subject: Re: How I Customized my 68 CVR
Posted: Mon Nov 17, 2014 6:45 pm
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That's awesome Lynn Nicholson! I wonder why people almost always change tubes before correcting the cold bias with the existing tubes. This seems to lead to many false conclusions about particular tubes. :wink:

I have read about people changing their GTs, which were made by JJ, and installing JJ tubes, correcting the cold bias, and then rave about how much better the JJs are than the GTs (which were made by JJ). :lol:

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Post subject: Re: How I Customized my 68 CVR
Posted: Mon Nov 17, 2014 6:52 pm
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I don't know if GT uses JJ's or not. They used to use some Russian brand as I recall. But that was a long time ago. But GT does measure tube parameters and then classify them as to whether they are "hard" or "soft." When I ordered my set from Eurotubes I did tell Bob that I wanted maximum headrooom before breakup. So he may have some method of classifying them also. Whatever...it's a bit beyond my knowlege of tubes. I suppose I could have gotten better sound out of the GT brand, but according to their own website, they would not give me what I was looking for. And the JJ's from Eurotubes hit the nail on the head!!


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Post subject: Re: How I Customized my 68 CVR
Posted: Mon Nov 17, 2014 7:08 pm
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GT uses many brands, including JJ and others. You just don't know what the GTs would sound like with a better bias setting unless you try. :wink:

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Post subject: Re: How I Customized my 68 CVR
Posted: Mon Nov 17, 2014 7:13 pm
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Lynn Nicholson wrote:
I don't know if GT uses JJ's or not. They used to use some Russian brand as I recall. But that was a long time ago. But GT does measure tube parameters and then classify them as to whether they are "hard" or "soft." When I ordered my set from Eurotubes I did tell Bob that I wanted maximum headrooom before breakup. So he may have some method of classifying them also. Whatever...it's a bit beyond my knowlege of tubes. I suppose I could have gotten better sound out of the GT brand, but according to their own website, they would not give me what I was looking for. And the JJ's from Eurotubes hit the nail on the head!!


Did you use a regular meter to test bias and then adjust a trim pot?
Or did you get one of those plug in adapters?

Where is the bias trim pot in the CVR?
The Hot Rod Dlx III has a very say way to adjust bias.
It has easy to access bias test points and an easy to find and use bias trim pot.

I haven't been into my CVR yet, nor on my CDR.
I'd like to check the bias on both, but I don't have any visual guides on where the trim pot is, nor if there is one.
If you have picts please post, that would be awesome.

I have a couple of matched pairs, not a matched quad, of SED Winged C 6L6's and would like to try those.
In all the other tubes amps I've put them into the results were excellent, except for the
Hot Rod Dlx 3, so I'm interested to hear what they may sound like in the CVR.
The C's have a great rich harmonic quality to them.
I have some in my Egnater Vengeance head and they made a great positive change in that amp as well.
I've also got a pair of Sovtek 5881's and those sounded great in the Hot Rod where the
Winged C's didn't.

I'm comfortable with bias adjustments.
But as I said I've not been into the CVR or CDR and I'd really like to check things out.
The Egnater's bias test and adjust couldn't be easier as the test points and adjustment are right on the back panel of the amp. It uses a quad of 6L6 type tubes, but they don't have to be a matched quad because each pair as their own bias adjustment.
In fact, the design is so cool I can run pairs of different tube types like a pair of 6L6 with a pair of EL34, or a pair of KT77 with a pair of 5881's, really cool.

Any info and picts would be really great.
Thanks. :)


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Post subject: Re: How I Customized my 68 CVR
Posted: Mon Nov 17, 2014 8:12 pm
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Rverb,

Since I didn't take photos while the amp was apart, I really don't want to take it apart again. But it's not that hard to get to. HOWEVER, you do have to remove the upper back panel, which has the cage around the power tubes attached to it (4 screws to remove the panel and the cage comes off with it). After you have the cage removed, turn the amp upside down (so the handle is on the ground (or whatever surface you put your amp on) and the bottom of the amp faces skyward. I had trouble finding the slot in the pot through the hole, so I ended up removing all screws necessary to slide the chassis about 2 inches out of the cabinet. From the rear, if the amp is upside down, the power output (6L6) tubes are on the right-hand side. Towards the front of the amp on the chassis near the grid cloth, behind V2 you will see a copper-clad access hole in the chassis. This is just an access hole to what I suspect is a PC-board mounted pot (when I first attempted to adjust the pot through this hole I touched the side of the chassis and saw a spark and heard a pop in the speaker. After understanding which way to turn the pot, I started turning the amp "Off," then tweaking the pot, then turning the amp back on until I got the bias adjusted to where I wanted it. I used the Eurotubes bias probe for only one of the tubes, but has both plate voltage and bias current on digital meters. Very handy as long as you are using matched tubes. And I trust Eurotubes to be smart enough to provide a matched set of 2 or 4, depending on the amp. Yep, I guess I'm a bit of a fool :)

ATTENTION FENDER TSL!!!: You need to mount the bias pot to the chassis and run wires to the PC board. This is much safer and prevents damage to the amp, even if it costs a buck or two more to do it!!


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Post subject: Re: How I Customized my 68 CVR
Posted: Tue Nov 18, 2014 12:59 am
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Something is wrong if you get sparks from touching the chassis and/or bias pot. Maybe when you slid the chassis out, you allowed something inside to touch the shielding. At any rate, there is nothing wrong with the design of the amp that would cause sparks, but something has gone wrong.

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Post subject: Re: How I Customized my 68 CVR
Posted: Tue Nov 18, 2014 8:56 am
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Nothing seems to have been damaged. Everything is working fine. I'm wondering if the tip of the screwdriver slid out of the slot in the pot and touched a nearby component wire or a trace on the PCB. Not having a schematic and board layout, I am only speculating to say it is probably a low voltage area around that pot. If I ever have to adjust bias again, I will definitely completely remove the chassis from the cabinet to ensure I have unobstructed access to the bias pot. Rverb, hope you read this :)


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