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Posted: Tue Dec 14, 2010 4:23 pm
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:lol: That is why I hang one of those tree shaped air fresheners on my amp. It is "New Amp Smell". :lol: BTW, I just got a response from GT about the specs for the GT 6L6 S, they "are unable to share any more information at this time." :(

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Posted: Tue Dec 14, 2010 4:29 pm
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MrGreenGenes wrote:
...I will probably just replace R87 with a pot and put a resistor in series, like shimmilou said....


Have to credit stratele52 for that. We discussed this before, and I would have just replaced the resistor with a pot, but he recommended the series resistor, I only agreed. :)

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Posted: Tue Dec 14, 2010 4:50 pm
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shimmilou wrote:
:lol: That is why I hang one of those tree shaped air fresheners on my amp. It is "New Amp Smell". :lol: BTW, I just got a response from GT about the specs for the GT 6L6 S, they "are unable to share any more information at this time." :(


Hmmmm. Maybe they think you are trying industrial espionage. 8)

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Posted: Tue Dec 14, 2010 6:04 pm
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shimmilou wrote:
MrGreenGenes wrote:
...I will probably just replace R87 with a pot and put a resistor in series, like shimmilou said....


Have to credit stratele52 for that. We discussed this before, and I would have just replaced the resistor with a pot, but he recommended the series resistor, I only agreed. :)


I will give credit where credit is due: thanks stratele!!

Using the Fender Blues Deluxe-Deville 6L6GC tubes, would you even recommend spending the time and effort to install the bias pot? I emailed Fender support about this issue and I got a simple response that the amp is fixed for a reason, so just keep the same tubes and do not rebias. I think I will just try those tubes bluesky mentioned and see what I get. I can at least test them out to see what the bias is before I decide what I want to do.

Have any of you had problems with tubes being out of tolerance? As in, labeled as a 60mA pair, but actually lower or higher?


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Posted: Tue Dec 14, 2010 7:29 pm
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I like to modify amps, so I would do it just because I can. The adjustable bias will allow you to get the best out of any tubes that you might want to use. You might even like the sound of your stock tubes better with a higher idle wattage. I put an adjustable bias in my BJr, and it works great and gives me the flexibility that I wanted. I actually needed to reduce the idle on the BJr. You can read about it here, just click "amps" at the top on the site:

http://shimmilou.mysite.com/

But, replacing the tubes with the same ones would give you the best odds of keeping your sound the same as when new, and having the idle wattage be the same. You also have the best chance to match your originals by using the GTs. I have never had a problem with any GTs being unbalanced (not matched), but I have had with other tubes, including what were supposed to be "premium" tubes. I believe this has to do with the difference in the GTs dynamic tests vs the others static tests of tubes. I have had a couple of GT preamp tubes go microphonic within 6 months (one came that way new), so I returned them for replacements. Of course, I'm not a professional musician so I only use my amps a couple of hours a day for practicing and jam sessions with friends. So far, I have over 10 years on my HRDlx with the original GTs and it still sounds great.

However, I don't think that you will hurt anything by trying some other tubes without re-biasing. Like others have pointed out, the bias on your amp is very likely on the cool side (low idle wattage), so there is virtually no chance of it being too hot for any tube that you might want to try (except a 6V6). You could check your Plate current and voltage to see what idle wattage your tubes actually are. You have a unique opportunity to compare GTs and another brand under the exact same conditions in the amp, with the fixed bias, and see which sounds better to you. :)

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Posted: Tue Dec 14, 2010 9:22 pm
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shimmilou wrote:
I like to modify amps, so I would do it just because I can. The adjustable bias will allow you to get the best out of any tubes that you might want to use. You might even like the sound of your stock tubes better with a higher idle wattage. I put an adjustable bias in my BJr, and it works great and gives me the flexibility that I wanted. I actually needed to reduce the idle on the BJr. You can read about it here, just click "amps" at the top on the site:

http://shimmilou.mysite.com/

But, replacing the tubes with the same ones would give you the best odds of keeping your sound the same as when new, and having the idle wattage be the same. You also have the best chance to match your originals by using the GTs. I have never had a problem with any GTs being unbalanced (not matched), but I have had with other tubes, including what were supposed to be "premium" tubes. I believe this has to do with the difference in the GTs dynamic tests vs the others static tests of tubes. I have had a couple of GT preamp tubes go microphonic within 6 months (one came that way new), so I returned them for replacements. Of course, I'm not a professional musician so I only use my amps a couple of hours a day for practicing and jam sessions with friends. So far, I have over 10 years on my HRDlx with the original GTs and it still sounds great.

However, I don't think that you will hurt anything by trying some other tubes without re-biasing. Like others have pointed out, the bias on your amp is very likely on the cool side (low idle wattage), so there is virtually no chance of it being too hot for any tube that you might want to try (except a 6V6). You could check your Plate current and voltage to see what idle wattage your tubes actually are. You have a unique opportunity to compare GTs and another brand under the exact same conditions in the amp, with the fixed bias, and see which sounds better to you. :)



I went ahead and ordered the tubes bluesky recommended: JJ 6L6GCs with ECC83Ss for the preamps. I also bought a bias probe so I can verify that the tubes are matched. If they match and it sounds good, I will probably still do the amp mod at a later date. I am going to replace the power tubes alone first to see what I get tone-wise, but I am sure I will eventually just swap them all in the same day. I have quite a few issues with the sound of this amp, so an all new set will make all the difference IMO.

Like I said before; I am an electrician, but I haven't any experience modding amps. If I were to remove R87 and install the trimpot and the resistor in series, would using insulated jumpers be the best bet? Soldering the resistor in series on the board wouldn't be feasible being the solder runs are already there, right? What is the best way to do this, in your experience?


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Posted: Tue Dec 14, 2010 10:26 pm
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MrGreenGenes wrote:
Like I said before; I am an electrician, but I haven't any experience modding amps. If I were to remove R87 and install the trimpot and the resistor in series, would using insulated jumpers be the best bet? Soldering the resistor in series on the board wouldn't be feasible being the solder runs are already there, right? What is the best way to do this, in your experience?


I would recommend that you buy the kit

http://www.torresengineering.com/fenbluesdevn.html

that I referenced earlier. It is only $30.00. Or get something similar. Sure, the parts might only cost you $5.00, but the kit comes with detailed instructions which are worth $25.00 in my opinion.

Note:I do not have the kit. My BDRI came with a bias pot already.

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Posted: Tue Dec 14, 2010 10:45 pm
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I wouldn't use clip leads, definitely solder into circuit. On the pot, solder the 1 and 2 terminals together, leaving terminal 1 straight and bending terminal 2 over, and this will be one side to insert into the top hole where R87 was (top on schematic). Solder one side of the new resistor to the remaining pot terminal 3. Then insert the other side of the resistor into the other hole where R87 was and solder both ends into the circuit. A cermet similar to this one will be perfect. Check to make sure that it is a ten turn, 1/4 watt, 25K ohm, cermet with top adjustment. I really don't know how much room that you will have, you'll have to look at your amp to see if it will fit, but this type will be about the smallest that you can get.

http://www.mouser.com/ProductDetail/Bourns/3299Y-1-253/?qs=sGAEpiMZZMuz2E8dTn0VFfo4R8fTsagsu2h4ioC8%2fZ4%3d

It might well be worth getting the kit that bluesky636 recommended, the instructions might be worth the extra $25 since they will be specifically for your amp. Although, I don't like the idea of drilling two holes in the chassis, not sure why that is necessary.

Edit:
Changed link to the correct pot.

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Last edited by shimmilou on Wed Dec 15, 2010 9:34 pm, edited 1 time in total.

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Posted: Tue Dec 14, 2010 11:09 pm
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shimmilou wrote:
It might well be worth getting the kit that bluesky636 recommended, the instructions might be worth the extra $25 since they will be specifically for your amp. Although, I don't like the idea of drilling two holes in the chassis, not sure why that is necessary.


"This totally great kit installs a REAL FENDER FULL SIZED bias pot, not some cheap little trim pot, and a bias monitoring test lead plug to make the whole bias process easy."

One hole to access the pot, another hole to access the test point? If your'e doing the mod anyway, what difference does driling a couple of holes make? :lol:

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Posted: Wed Dec 15, 2010 12:00 am
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Yeah, the Torres kit seems to be a full sized pot mounted to the chassis, for external adjustments. If you're using a tube probe, the kit's test point would be uneccessary. And yes, the pot would use leads running back to the board. So it would just be a one time desolder and solder job on the board. Art

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Posted: Wed Dec 15, 2010 3:06 am
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I build standard bias pot circuit, with multiturn cernet pot and resistor. All mounted on small board.

I use this circuit on many amps. The 15 Kohms is from one side ( opposite to neg volt) of the pot to common ( ground).
The other resistor is connect from the negative voltage side of the pot to the wiper of the pot. It's a 220 K ohms. It's a protection in case of a bad contact at the wiper.
If bad contact appened, this resistor send bias voltage to protect tubes.
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Posted: Wed Dec 15, 2010 5:23 pm
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Why are you all using multi-turn pots? Its not that wide a range of adjustment. Maybe 1/4 turn on my BDRI. I would think you would be twirling a mult-turn pot all day to dial the bias in. :lol:

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Posted: Wed Dec 15, 2010 6:25 pm
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bluesky636 wrote:
Why are you all using multi-turn pots? Its not that wide a range of adjustment. Maybe 1/4 turn on my BDRI. I would think you would be twirling a mult-turn pot all day to dial the bias in. :lol:



First of all, I appreciate all the responses. It is hard to get much advice from ampheads in everyday life because there aren't many people that enjoy this kind of stuff.

I am in the same line of thinking as bluesky when it comes to the multi-turn pots. That is why I asked the question earlier about how to solder the pot in with a resistor because I was thinking of using a standard pot like what comes installed on bias adjustment circuits normally. Using the cermet will make things easier as for space, but I deal with those little suckers all the time and the adjustment to turn ratio is horrible. You can turn one of those guys 40 times and only see a small adjustment.

I may just get the kit so I can have the correct fender bias pot versus the cermet. I like what stratele did though and I may try something like that, but the kit probably calls for something similar. I just don't want to jerry rig a pot and resistor to the board, so I may just go with the chassis mount. There are plenty of options available and I will just have to decide on what I want to do.


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Posted: Wed Dec 15, 2010 7:59 pm
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Not sure if mine qualifies as jury rigged or not. :lol: The resistor is on the other side of the circuit board. I thought that stratele52 did a nice job also.
A 10 turn pot is about accuracy, but "40 turns" would be a bit too much. ;) Let's see some pics from anyone else who has actually done the job. :)

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Posted: Wed Dec 15, 2010 8:16 pm
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shimmilou wrote:
A 10 turn pot is about accuracy


Are you measuring bias current to the third decimal place? :lol:

Really though, I have no problem setting the bias with the stock Fender setup in the BDRI. Even once set, the bias current tends to fluctuate a milliamp or so.

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