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Post subject: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 9:41 am
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I would like to upgrade the tubes on my DRRI with something that would have a good vintage - blues tone. After reading some of the prior posts regarding what tubes others prefer, I am no closer to figuring this out. 1.) Should I buy a matched retube set (for simplicity) , or buy individually and assemble the set myself? 2.) It seems that such brands as Tung-sol, Mullard, and NOS RCA are highly thought of. What are the relative merits of each of these brands? I know, it's all very subjective. Also, it seems that this gold pin business is alot of bull. 3.) Lastly, what dangers lurk in doing this yourself? I've run across a few disclaimers along the way saying that even if unplugged, the amp is dangerous to work on. When changing tubes, what precautions should be taken? Thanks to all!


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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 9:50 am
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First.

If your talking about power tubes, you will need a bias adjustment any way, so.......
find a good tech who understands what your after and understands Fenders and have them swap out the big bottles.

if its preamp tubes, (little tubes) it won't matter, turn it off, unplug and wiggle while you gently pull downward, make sure you alighn them (missing pin is the keyway).

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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 10:40 am
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blues bondsman wrote:
First.

If your talking about power tubes, you will need a bias adjustment any way, so.......
find a good tech who understands what your after and understands Fenders and have them swap out the big bottles.

if its preamp tubes, (little tubes) it won't matter, turn it off, unplug and wiggle while you gently pull downward, make sure you alighn them (missing pin is the keyway).


+1

The NOS tubes, especially those designated as "JAN" or "mil-spec" are the hardiest tubes ever produced. Most sound great and will last for decades. They are quite expensive though. Your power tubes should be replaced by matched sets only, accompanied by a bias check (and adjustment if necessary). The rectifier tube (V9 on your chassis) is the hardest-working bottle on the chassis -- if it fails, your amp fails. Thus, use only the most durable type you can afford. The V1 thru V6 pre-amp tubes allow for some lattitude based upon what your ear tells you. You can mix-and-match brands to suit your druthers. When experimenting with different pre-amp glass, change out only one tube at a time then play the amp and note any sonic changes. It's important to understand the signal chain within the amp so you have an idea of which tubes affect which channel. V1 and V2 are, respectively, the first and second stage of the "normal" and "vibrato" channels. Changing V1 will have no effect on the sound of the vibrato channel, as an example. V3 and V4 handle the driver and recovery functions of the reverb (thus, no effect on the normal channel). Likewise, V5 handles the tremolo chores. V6 is the phase inverter for the final drive (output stage). It should be selected based on balanced transconductance to maintain symmetrical interaction between the power tubes as they pass their load through the output tranny.

Best of luck, HTH

Arjay

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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 11:05 am
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I read that the Blues Jr. III is fixed bias and you can put in different power tubes and not have to re-bias. Is this 100% accurate? I just don't want to mess anything up when I re-tube it.


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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 11:39 am
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Ok... "fixed bias" is sometimes misunderstood. It means that the amp has a separate BIAS POWER supply that insures the signal grid of the power tubes "sees" a FIXED negative voltage, at idle (no signal, volume at zero).

This sets the "idle" bias current flow of the power tubes. Kinda like setting the idle of your car. What idle you set the tubes to depends on the Class of amp (ex: Class AB1--most Fenders, Class "A"---most SE amps, etc). AND the specific electrical character and tonal response of your power tubes.

To add to the confusion, many Class "A" single ended amps and push-pull amps have another type of biasing: Cathode-Bias. Now these amps usually have a set (or fixed) resistor(s) tied from the power tube's cathode to ground. In other words, no adjustable pot. But, the tube can somewhat adjust idle current draw, within a range. Thus, self-leveling, to an extent (or self-biasing).

What is confusing is that many "fixed biased" amps DO HAVE adjustable potentiometer(s) to adjust the amount of negative voltage (and thus the idle bias current flow) of the power tubes.

For power tubes... I recommend ALWAYS checking and readjusting idle bias.

For input, gain stage, EFX, and phase inverter tubes (smaller usually 9-pin mini tubes), their bias is set by a set resistor tied to ground (and usually run in Class "A"). They are self-biasing within a range and usually do not need to be rebiased--- if you replace them with new tubes. Or experiment with various tube types (as long as the pinouts are the same. EG: subbing a 12AT7 for a 12AX7), you do not usually need to worry about the cathode-tied resistor. The new tubes will self-level, at idle at least.

Only caveat here... And isn't there always some crazy exception? Onboard Reverb Units are often driven by 12AT7 or 12AU7, because they can idle at higher currents than 12AX7's. The tanks needs this extra current to work. So, if you sub in a 12AX7 in the reverb driver position... the extrra current needs may burn out the 12AX7 rather quickly.

Sorry, for all the yaddy-yaddy. But, it never hurts to ALWAYS rebias new or different power tubes.


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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 12:02 pm
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im really up for an upgrade, my tubes have the fender groove tubes stamped on them.
im going with TAD's 6v6s v7,v8 and 7025 for v1,v2,v4, v5 is getting a jj 12ax7,
v3 is getting a jan phillips 12at7wc, and a balanced one for v6
v9 is the sovtek 5ar4/gz34
https://www.kcanostubes.com/content/fen ... mendations
this gives a really good explanation from v1-v6

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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 4:37 pm
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That sounds like a tasty tube menu, BST! Let us know how they sound.

Arjay

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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 4:55 pm
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BMW2002Ti wrote:
What is confusing is that many "fixed biased" amps DO HAVE adjustable potentiometer(s) to adjust the amount of negative voltage (and thus the idle bias current flow) of the power tubes.


This statement is confusing. If it has a pot to adjust negative voltage, then it would seem to me that it would be an adjustable bias amp. "Fixed bias" to me always meant a fixed resistor on the bias supply to provide a fixed bias voltage.

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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 5:13 pm
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bluesstrattone wrote:
im really up for an upgrade, my tubes have the fender groove tubes stamped on them.

C'mon man, what are you waiting for? I thought you'd have 'em by now :)

I got tired of waiting on you and ordered some TAD myself... :mrgreen: the 6V6s are nice, very similar to the Tung-Sol, I'll stick with my RCAs, nothing comes close so far. If the TS are cheaper I'd just get those.

The TAD 7025 are really nice, but I just recenty tried the re-issue Mullard 12AX7 and really liked 'em! They're a bit warmer sounding than the TAD, both are very nice preamp tubes, so it depends if you want brighter or warmer...

To the OP, either get the NOS RCA or save money and go for the Tung-Sol or TAD 6V6. For preamp, lots of examples were given... get you 4 12AX7 or 7025 and make sure you get the best 12AT7 you can afford, you'll need 2, one balanced. These are excellent IMO, just tried them in my SR.

http://thetubestore.com/mul12atcv.html


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Post subject: Re: Tube upgrade for DRRI
Posted: Sat Apr 23, 2011 8:35 pm
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Cedar,lmao, I've been waiting to order the tubes, because
Researching, meanwhile ordered the bias probe, this week
I'll order them.
So you think tung sol and tad 6v6 are great choices.
On the mullard 12at7 read these were great but also read the
Jan Phillips were good too and 20 bucks less for each.

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Post subject: Re: Tube upgrade for DRRI
Posted: Sun Apr 24, 2011 7:55 am
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"Fixed Bias" means that the amp places a fixed negative voltage onto the signal grid. It is supplied by a separate Bias Power Supply (with it's own rectifier and filter cap(s). By placing a pot in this circuit, it allows you to change the fixed amount to negative voltage on the power tubes. Thus, this is how you bias these types of amps.

The negative voltage is constant or "fixed" at idle. Just like changing the fixed idle point of your car. The idle should not vary within a certain tolerance. In these types of amps and your car.

Cathode bias amps operate using a different circuit philosophy. Prolly best left for another thread.

I know the whole thing is kinda confusing. Terminology gets discombobulated.

Check this link for more yaddy-yaddy:

http://aikenamps.com/

Click onto TECH INFO, then INTRODUCTORY, then "What is Biasing?"


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Post subject: Re: Tube upgrade for DRRI
Posted: Sun Apr 24, 2011 8:40 am
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BMW2002Ti wrote:
"Fixed Bias" means that the amp places a fixed negative voltage onto the signal grid. It is supplied by a separate Bias Power Supply (with it's own rectifier and filter cap(s). By placing a pot in this circuit, it allows you to change the fixed amount to negative voltage on the power tubes. Thus, this is how you bias these types of amps.

The negative voltage is constant or "fixed" at idle. Just like changing the fixed idle point of your car. The idle should not vary within a certain tolerance. In these types of amps and your car.

Cathode bias amps operate using a different circuit philosophy. Prolly best left for another thread.

I know the whole thing is kinda confusing. Terminology gets discombobulated.

Check this link for more yaddy-yaddy:

http://aikenamps.com/

Click onto TECH INFO, then INTRODUCTORY, then "What is Biasing?"


I use Aiken alot. I have him in my favorites. He fell a little short on this explaination, however. The term "self biased" could have been used I think. Interesting. Learned something today.

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Post subject: Re: Tube upgrade for DRRI
Posted: Sun Apr 24, 2011 10:47 am
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Here is an excellent explaination between fixed and cathode.

http://musicalilluminism.wordpress.com/ ... operation/

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Post subject: Re: Tube upgrade for DRRI
Posted: Sun Apr 24, 2011 2:51 pm
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Thanks, for the link. RG Keen used to have a nice page dedicated to basic amp designs. God only know where it went to.

But, your link describes the importance (and often overlooked) of that 220K-ohm resistor that goes from the power tube grid to ground in most Champs. This resistor and the cathode-tied 470-ohm resistor helps set the idle bias. I like to use at least 5%, if not better tolerance resistor for the 220K-ohm, as well as the 470-ohm resistors. Keeps the 6V6GT happy.

http://www.ampwares.com/schematics/champ_5f1.pdf

http://www.ampwares.com/schematics/cham ... _aa764.pdf


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