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Post subject: Crazy tube (?) issue with 1998 Tweed HotRod Deville 410
Posted: Tue Jun 01, 2010 3:38 pm
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Hi All,

Over the past few months my HRD has been regressing in sound quality and I decided it was high time for new tubes (5 years). I guess i should mention that I have done tube swaps and biases for myself several times before, and always do it with care and technique from a local tube repair shop. Had been running 2 6L6WGB powers and switched back to JJ/Tesla 6L6GC.

Well, It made sense that it would be the tubes after this long, so i hate to say i didn't pay a whole lot of attention to the behavior of the old tubes, but swapping in the new ones i got the same moderately low output and very muddy distorted sound on all channels. The GC's were pulling up 20mV on the bias, so I rebiased to 60mV (as per Fender suggested), but no improvement.

Upon closer inspection BOTH the old tubes and new tubes would glow bright blue when standby was off, and one of the two (the left with the new tubes, right with the old, when facing back of amp) would flash bright blue before settling in to a mild glow all around the tube. All the solder joints to the tube sockets checked out OK.

From what i've heard, that means bad tubes? The package was open, so i suppose they could have been a return that someone messed up and stuck back in the drawer, but before i go exchanging and burning up another set, I figured i should see whether you guys think its the amp or the tubes.

To add to the mystery, there is a mechanical clicking, very fast almost a hum, coming from somewhere in the left of the main PCB, but not the bias pot.

Any help would be much appreciated, i hadn't been able to turn anything up in searches but even pointers to a more appropriate forum would be great if this one doesn't belong here. I'll be happy to toss up some pictures if that will help. Sorry for the long post, just stressful when your baby is not doing so hot! :lol:

Thanks!

David


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Posted: Tue Jun 01, 2010 4:10 pm
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I had the same problem in a HRD few weeks ago . It was a resistor in the Phase Inverter circuit . The resistor was open. It was R 58, 100 K ohms.

Do you know how to fix that , solder ? You have the schematics ?


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Posted: Tue Jun 01, 2010 4:27 pm
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Great! Thanks stratele, I do know how to do the fix if it's just re-soldering and I'll check it out tonight. Were your tubes cooked? How did you identify which resistor was bad? Just visual? Also, what's the best (safest) way to make sure I don't take all that nice capacitor energy?


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Posted: Tue Jun 01, 2010 5:05 pm
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First go to schematic heaven. You will see the resitor on the " mechanical sketch"

http://www.schematicheaven.com/fenderam ... deluxe.pdf

I find it with mesuring , (I'm electronic tech). By the look it always seem good.

You have to discharge all the capacitors, the big ones, there are dangerous voltage there.

My english too bad to explain more

Retroverbial or acelmpoppi can help you they know a lot


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Posted: Tue Jun 01, 2010 6:10 pm
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Thanks! I have the schematic and will look in to discharging the caps. So i know whether or not these tubes are bad... does anyone know if the symptoms mentioned above mean that i should go ahead and grab some new tubes ?


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Posted: Thu Jun 03, 2010 5:33 am
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Okay, so i've tracked the problem down to the power amp section (obviously, i suppose, due to the glow) by testing the pre out/power in jacks on the amp. Unfortunately, radio shack is my only local option for resistors big enough to discharge the caps, and they of course don't carry any useful sizes.

I did exchange the "new tubes" as they were not functioning in another amp as well. If theres some issue in a solder joint somewhere could i blow the new, new tubes too?


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Posted: Thu Jun 03, 2010 1:42 pm
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Hi,

If you changed the 6L6 you have to take care about bias at 60mA (=60mV on the test point) after 20mn warm-up.

(It is definitely concervative as far as the 6L6 can oftern be biased at 70-75mA.)

Tissan

8)


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Posted: Thu Jun 03, 2010 2:08 pm
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NO ! Bias at 60 milliamp is too hot and dangerous for your 6L6.

6L6 is 19 watts plate dissipation so at 430 volts average bias point is 24 ma

6L6 GC is 30 watts plate an average bias is 41 ma



If you don't know wat to do go to see Weber Bias calculator

www.webertvst.com/tubes/calcbias/.htm


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Post subject:
Posted: Thu Jun 03, 2010 3:28 pm
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Thanks! The fender schematc I have agrees with the 60mA bias on the GC. Tube swap would be easiest, so could that mechanical hum be just old tubes?could I blow the new ones if it's something else?


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Posted: Thu Jun 03, 2010 4:12 pm
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If your bias is good you can't damage your tube. And if you can do the right bias that is mean your tubes are good.

The hum could be unmached tubes.


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Posted: Thu Jun 03, 2010 6:02 pm
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You guys have me lost. mA milliamps are not the same as mV millivolts. Bob at Eurotubes recommends between 38-42ma which is around 85 to 90mv. His DeVille was running at 460 volts.

Here's Bob at Eurotubes video on the proper biasing technique for HRDv HRDlx.
I had my HRDlx biased at 85-87mv with 6l6 JJ's and it sounded great. No problems whatsoever.
http://www.eurotubes.com/euro-Fender-HRDV.htm
I wouldn't run it that high with GT's. JJ's are pretty sturdy current production tubes.

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Post subject:
Posted: Thu Jun 03, 2010 8:07 pm
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63supro wrote:
I had my HRDlx biased at 85-87mv with 6l6 JJ's and it sounded great. No problems whatsoever.


Till you burned it up. :P


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Posted: Thu Jun 03, 2010 8:15 pm
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Supro- good catch. Somewhere along here we got off on current vs. voltage. The amp is biased to 60 mV right now at the designated test point. Fender does not specify a current at this point.

So even if i have an issue with a plate load resistor, as long as the bias is set i wont hurt the new tubes? I'm thinking with one of those tubes glowing like a lightbulb and the mysterious hum (the tubes are a matched duet) there could be something else big... should i just pop the new ones in and see?


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Posted: Thu Jun 03, 2010 8:51 pm
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GL, If one side is red plating, swap them with each other to see if it's a tube problem or an amp problem. If just one is glowing normally, and one isn't lit, check the solder joints. You don't have to go to 85mv, try it at 80. I wouldn't go much lower. The amp only needs to warm up for 5 minutes or so before you check the bias.

GTG, mine had problems long before I turned up the bias up 85mv. My was a turd from about a month and a half into it. 60mv is way too cold. All that that does is make lousy tubes last a lot longer but the trade off is lousy sterile tone. It's no secret a lot of them are dogs and some are fine. A tech I know said that Fender put a slightly better board in the HRD series amps. The traces are just a tiny bit heavier but none of the heat related issues were addressed. The 5 watt power resistors still rest right on the board and the tube sockets are still poorly mounted to the PCB. Hopefully the heavier traces will help the cold solder joint issues on the tube socket connections.

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Post subject:
Posted: Fri Jun 04, 2010 2:53 am
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GLjazz, You are better to find a good tech ( not anybody in music store) to do a good bias to your amp.

Right now in this forum, to much incomplete or wrong informations to help you .

Don't forget , there are some very dangerous voltage in tubes amp it can kill you.


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