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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 31, 2011 9:09 am
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BMW2002Ti wrote:
But I do not like long leads on grid stopper and screen resistors. They sometimes can contribute to instability, esp when really cranking the amp for any length of time. See how Fender soldered the grid stoppers onto the socket tangs, originally? Almost zero lead length. Just for that reason.


+1

Same type of "instability" that crappy lead-dress in many of the later silverfaces is responsible for.

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: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 31, 2011 9:59 am
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Thanks again BMW!

BMW2002Ti wrote:
Glad it worked out. That 100 ohm resistor he replaced with a 47 ohm (in phase inverter circuit), was a 100 ohm OEM from the factory. It sets the grid to ground on one half of the phase inverter. But, if it the amps sounds ok... go with it.

Sorry I'm confused, my tech replaced the 47 ohm resistor with 100 ohm, not sure why the previous owner did that... I did like the tone with the 47 ohm resistor in there but it made the amp unstable and louder somehow.

BMW2002Ti wrote:
5U4GB take awhile to break in. I use the USN mil spec CBS-Hytron 5U4GB (far right of your photo). It takes forever to break-in and "loosen" up, but lasts forever. GE's are great 5U4GB too. In fact their 1950's 5Y3GT's are some of the best. I have yet to find an NOS 5U4GB that is bad. The worst are JJ with their tiny pins. They sometimes make poor contact with older OEM sockets.

All the tubes I got off ebay were labelled NOS, but only the Sylvania and CBS tubes sounded a bit harsh, so maybe they do need some time to loosen up...

BMW2002Ti wrote:
Caution on the coke bottle 5U4G. It puts out a tad less current than a 5U4GB. So, you may want to rebias the power tubes down a bit (225mA versus 275mA output). So maybe drop the idle bias down about 80-85% idle current when using the 5U4G versus the 5U4GB.

When I was swapping tubes I noticed that the voltage was always the same, 450 volts. I only noticed a difference when I used the GZ34, voltage went up to 480 I believe.

BMW2002Ti wrote:
A little peccadillo of mine. And it may mean nothing (prolly does). But I do not like long leads on grid stopper and screen resistors. They sometimes can contribute to instability, esp when really cranking the amp for any length of time. See how Fender soldered the grid stoppers onto the socket tangs, originally? Almost zero lead length. Just for that reason.

If you have issues with power tubes "runaway" or short life, you may want to shorten those resistors (470-ohm) lengths, on the socket tangs. Sorry, just another thing to worry about. But, really--- if the amp sounds and plays well, let sleeping dogs lie.


Good luck with the amp!

Thanks! I'll look into shortening the leads if that might cause headaches in the future... the amp sounds wonderful and dead quiet!


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 31, 2011 4:54 pm
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Sorry about that resistor (47 versus 100-ohm), but the type of resistor replaced looks like the same kind he put in the screen position. And the "new" 100-ohm looks like one of those original 10% tolerance type you have a hard time finding these days.

Yes, the VOLTAGE will remain the same between the 5U4G and 5U4GB. But, the -GB can push more current than the -G. Idle bias current draw is important for power tubes. As well as the plate voltage. Just be aware that max output of the coke bottle is around 225mA versus the -GB's 275mA. So, if it seems that the amp "runs outta gas" at high volumes or during heavy bass runs --- or the 5U4G rectifer seems to running hotter than the 5U4GB, after a long session, the current limitation maybe the reason.

If not... don't sweat it. All of these rectifers were overbuilt like Mack Trucks. And the mil spec ones (like that CBS-Hytron) are made like Tiger tanks. It can take a good week or two of daily use before they loosen up and actually give you the "sag" everyone talks about in these rectifiers.

A good NOS 5U4GB built like a brick@hit house. Puts nearly every other rectifier type, except the Mullard GZ34 and Bendix 6106 (5Y3WGT) to shame. Oh, and that crazy 5R4WGB "potato masher."


https://www.tubeworld.com/5r4wgb.jpg


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 31, 2011 6:30 pm
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BMW2002Ti wrote:
Sorry about that resistor (47 versus 100-ohm), but the type of resistor replaced looks like the same kind he put in the screen position. And the "new" 100-ohm looks like one of those original 10% tolerance type you have a hard time finding these days.

So do the "new" resistors that replaced the old ones with the wrong values look ok to you? I believe the two resistors he used are 1 Watt, they replaced the 5 Watts that were in there before.

BMW2002Ti wrote:
Yes, the VOLTAGE will remain the same between the 5U4G and 5U4GB. But, the -GB can push more current than the -G. Idle bias current draw is important for power tubes. As well as the plate voltage. Just be aware that max output of the coke bottle is around 225mA versus the -GB's 275mA. So, if it seems that the amp "runs outta gas" at high volumes or during heavy bass runs --- or the 5U4G rectifer seems to running hotter than the 5U4GB, after a long session, the current limitation maybe the reason.

Didn't notice the amp acting weird in any way with the coke bottle... I did put the GE 5U4GB in there for now, just to be safe.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 31, 2011 8:34 pm
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The 5U4GB prolly will last longer. But the 5U4G is mil spec... so it's tough. For the screen resistors and grid stopper resistors, I use 1/2-watt resistors. You shouldn't need too much more (1-watt is fine). You want the resistors to pop before allowing too much power to go into the power tube and damage it. And worse, damage the output transformer.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed Jun 01, 2011 8:58 am
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BMW2002Ti wrote:
You want the resistors to pop before allowing too much power to go into the power tube and damage it. And worse, damage the output transformer.

Yeah, my tech warmed me about the OT, but didn't know it could damage the power tubes as well... wonder if that's what took out one of the TAD 6L6 I had in there :?:

Was just wondering, is it possible to use a 5U4GB in my DRRI? Just like to experiment and see what it does to the tone...


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Thu Jun 02, 2011 4:20 am
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Yes, you prolly can use the 5U4GB in place of the stock 5AR4/GZ34 in your DRRI. As you know, plate volts may drop --- so good idea to check idle bias current draw. You'll lose the slow warmup of the indirectly heated GZ34.

One other drawback is the 5U4GB draws about an amp more current than the GZ34. But, I'm pretty sure that the DRRI can handle the extra load. Check the PT heat with the GZ34 and with the 5U4GB. If it's really hot with the 5U4GB, you may want to not use it.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Thu Jun 02, 2011 5:51 am
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I've never seen the advantage of using a 5U4GB over a 5AR4/GZ34,myself.
I tried it one time in an amp of mine but don't remember which one,but wasn't impressed with the results.
I do have a very good selection of 5U4's that I'm saving for the right amp.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Thu Jun 02, 2011 9:06 am
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I'll give the 5U4GB a try in the DRRI one of these days and report back.


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