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Post subject: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 7:07 am
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Is it ok to swap the GZ34 for a 5u4gb?...i did and it mellows the amp but the it goes from 35MA on the tubes to 24 MA...can i run it up to 30 or so with the 5u4gb?..any help would be appreciated..


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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 7:15 am
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The 5U4GB requires approximately 30% more heater current than a GZ34/5AR4, which may be overtaxing the unregulated 6.3 volt tap on the P/S's tranny. That may account for your decreased bias current measurement on the power tubes. What plate voltage are you seeing on the 6L6's with the 5U4GB?

Arjay

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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 7:26 am
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Retroverbial wrote:
The 5U4GB requires approximately 30% more heater current than a GZ34/5AR4, which may be overtaxing the unregulated 6.3 volt tap on the P/S's tranny. That may account for your decreased bias current measurement on the power tubes. What plate voltage are you seeing on the 6L6's with the 5U4GB?

Arjay



its showing in the mid 4 range...


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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 7:56 am
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"Mid 4 range" ?? What kind of meter do you have?

Measure the DC volts at the Plate, you should get something close to 490 volts. It will be a specific number. :idea:

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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 8:04 am
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According to Fender's Bassman LTD owner's manual, the 5U4GB is a legitimate substitute for the GZ34/5AR4 (I wanted to check to be certain). The schem indicates a B+ of 491 VDC but I would calculate the max plate disappation based on your observed plate voltage and bias accordingly.

HTH

Arjay

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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 9:31 am
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I am curious too, I presumed that both rects would be close in voltage output, but if one is much lower, I agree that either way, just set bias according to the Plate voltage measured. That means that with lower Plate voltage with the new rect (if it actually is lower), the idle current of the output tubes can be a little higher in order to still run at the same idle wattage as before. Maybe the OP could compare the two rect tubes and measure the Plate voltage with each? :)

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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 9:42 am
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shimmilou wrote:
I am curious too, I presumed that both rects would be close in voltage output, but if one is much lower, I agree that either way, just set bias according to the Plate voltage measured. That means that with lower Plate voltage with the new rect (if it actually is lower), the idle current of the output tubes can be a little higher in order to still run at the same idle wattage as before. Maybe the OP could compare the two rect tubes and measure the Plate voltage with each? :)


+1

Such data might be useful to anyone else contemplating a Bassman LTD rectifier swap.

Thanks for mentioning that, shimmilou.

Arjay

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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 12:23 pm
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After reading up more..its fine to swap to a 5U4GB...it has a better sound to me for blues...i have my tubes running at 30ma..and its sounds purty..hope this helps anyone wanting to switch


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Post subject: Re: Bassman ltd rectifier swap
Posted: Tue Apr 05, 2011 12:42 pm
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Not really any help, Fender already shows it as a legit replacement. Any difference in voltage between the two? If you run the same current on the output tubes with a lower voltage on the Plate, the idle wattage will be lower than it was with the higher Plate voltage. This isn't necessarily a problem, but it would be very helpful to others to know the voltages, if you have 5 minutes to check. :)

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Post subject: Re: Bassman ltd rectifier swap
Posted: Fri Apr 08, 2011 3:16 pm
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I ran a 5U4GB and 5881's in mine for a couple of years with no negative problems.

I also traded out the speakers for webers with excellent results.

I got beat up when I first suggested it here also, it greatly improved the tone !!! and gave me a sweet and slightly earlier break up.

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Post subject: Re: Bassman ltd rectifier swap
Posted: Thu Feb 02, 2012 2:26 pm
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Hi,
I;m thinking about buying a Bassman '59 Reissue. I'm hearing a lot of people saying it sounds nothing like the original so can anyone here put me straight? Also, I would like to swap out the rectifier for the 5U4GB. Aside from biasing, what else needs to be doen with regard to that swap-over?


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Post subject: Re: Bassman ltd rectifier swap
Posted: Thu Feb 02, 2012 7:49 pm
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"sounds nothing like the original" Well........

They sound Very Good !!! Original according to the experts every Bassman made in 59 might sound a bit different and so ya never know yours may well sound just like one of the originals.

The 5U4GB and a nice set of 5881's sweetened up my 59 LTD, but the real sweetness came from selling off the fake jensens and putting weber alnicos in there !!!!

Don't let the reissue fact bug ya, unless you have deep pockets and can go buy an original, my guess is in a blind sound test half the "experts" would pick the RI over the original any way, and few could tell the difference.

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Post subject: Re: Bassman ltd rectifier swap
Posted: Fri Feb 03, 2012 1:06 am
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A 5U4GB is going to sound more "Bluesy" than a GZ34/5AR4 in most amps. Because the 5U4GB "sags" or has more forward voltage drop than a GZ34. Like 15-20 VDC for GZ34/5AR4 with a cap input versus 50 VDC for the 5U4GB.

http://www.audiokarma.org/forums/archiv ... 85660.html


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Post subject: Re: Bassman ltd rectifier swap
Posted: Fri Feb 03, 2012 1:50 am
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Being a greenie about tube rectifiers, some guy over here has told me that the 'newer' Bassman amp will not handle the 5U4G! Check this out and gimme your views:-

'THE NEW LTD'S CANT TAKE A 5U4G WHICH ALLOWS FOR GREATER SAG, DUE TO THE LTD'S USING LEAD FREE SOLDER,THEY CANNOT BE RUN FROM A 5U4G,ONLY A 5AR4'.

Ok, I dunno what solder's got to do with the performance of a tube but I need some collective advice here before I go spend money on the wrong kit.


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Post subject: Re: Bassman ltd rectifier swap
Posted: Fri Feb 03, 2012 2:13 am
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You got me. First time I ever heard of solder being an issue. If it is... just suck up the RoHS stuff and resolder with Lead/Tin stuff. Since both recitifiers have their internal wires soldered to the pins with similar compounds--- this cannot be an issue.

The ONLY thing that you need to worry about is the heater current draw. 5U4GB being about 1-amp more than a 5AR4/GZ34. Nearly all Fender amps use overbuilt trannies (why they last so long). So, this isn't an issue either. What serial number PT does your amp have?

I have it as "036483". See bottom of bottom link. Which is close to a vintage Tremolux/Vibrolux PT (125P6A). You are safe.

Wow... solder... amazing what you'll find on the Net. :mrgreen:

http://www.webphix.com/schematic%20heav ... a_h-fa.pdf

http://www.svvintageamps.com/dating.php


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