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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 2:53 pm
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Rock Star
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CB, The AB568, AA1069, and the AA270 all came with 5U4GB as OEM rectifiers. At least per schematics:

http://www.ampwares.com/schematics/supe ... _ab568.pdf

http://www.ampwares.com/schematics/supe ... _aa270.pdf

http://www.ampwares.com/schematics/supe ... aa1069.pdf

The AB763 had a GZ34:

http://www.ampwares.com/schematics/supe ... _ab763.pdf


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 3:09 pm
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Location: deep down in Florida...
So it looks like I'll need a 5U4GB. I was told the amp was blackfaced, not sure how to confirm this, I'll have to ask my amp tech to take a look and let me know. If it was BF'ed do I stick with GZ34 then? I have a JJ GZ34 that I'm using in my DRRI, I can try it I guess.

Just did another test, my ears hurt! With the 5U4G in there it's definitely an improvement, it's not cutting out like it was with the 5AR4 but it wants to, so the notes gets distorted more at loud volumes with I hit it hard...

I forgot to mention something, I had V1 removed, I put a 7025 in there and this seemed to calm things down a bit as well.

The tube that's biased at 40 mA, when I'm hitting these loud notes my meter is showing over 100 mA sometimes, like 110 or even 120, but this is just for a split sec...


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 3:38 pm
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Yeah, it looks like a 5U4GB rectifier is what you need. But, trying the JJ GZ34 rectifier out of your DRRI should be fine in the meantime. You will need to re-bias the amp.

Bias values are done at idle: zero volume, no input, & speakers connected. It's common for amp's power tube currents to vary all over the place, as the amp is being put through its paces. Just like amount of gas used in a car. Should be stable (more-or-less) at idle, under no load. But, increases as you need to accelerate. Drops back to idle as you brake, etc.

Glad that 7025 helped some of the issues.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 4:09 pm
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Ok now I'm really confused! I tried the GZ34 in there and there was almost no volume loss! :?

I re-biased everytime to make sure the same tube is always at 40 mA with either rectifier... maybe the Sovtek 5AR4 is going bad. I have a couple more 5AR4 tubes that I tried in my DRRI, a GT and a Ruby, I'll try both in the SR and see how it goes.

I called my amp tech and he recommended using 5AR4, even if it wasn't BF'ed. He said it handles more power than the others and it shouldn't cut out anymore, if it still does then he'll need to take a look somewhere else, not tube related.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 4:44 pm
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Hey! I've used 5U4GB for a long time. With very good service. I usually stick to US made NOS. They were dirt cheap, just a few years ago. Even, the famous NOS 1950's black plate TungSol 5U4GB's. Now, not so...

I kinda went away from using 5AR4/GZ34... as at the time I was doing a lot of GZ34 rectifier amps. I stuck to Mullards or Sylvania's. Had a good stash. But, the level is getting a little low. Hard to find consistent 5AR4/GZ34 new rectifiers. Guess I'll start looking for selected JJ and Chinese made 5AR4 to try out.

Good luck with your amp!


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 7:59 pm
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I'll have to get me a 5U4GB to try, don't have any...

For now either rectifier works, just need to bias it properly. Playing at a fest this weekend and want to make sure amp is ready.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 8:22 pm
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well it looks like you may have a bad rect tube. Rect tubes are usually bullet proof but it does happen. Get a good NOS or used one from ebay. Decent deals to be had.
Hope the suggestions helped. Good luck and have fun at your fest.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 8:43 pm
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Thanks! Any suggestions for a NOS rectifier? Do I go with 5U4GB or 5AR4?


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 8:49 pm
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since your amp is specd for 5u4gb get a couple of those. I just looked on ebay and there are many for very good prices. 30.00 down to 9.99. Look for a seller that you can return. Also the new in box (NIB) and NOS are good bets. The Sylvania ones are nice as are the RCA.
GZ34 are much more expensive. Expect to pay 75.00 minimum. Usually at leat 95.00 tho.
Mullard are the tube of choice there. Look for rebranded Mullards like RCA or Hammond for example. Still mUllard tubes but can be had for less $$. Read up on the net to educate yourself about it. Also heard good things about the new JJs rect tubes as well.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 8:56 pm
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Thanks again! Good to hear the 5U are much cheaper. I've known about the Mullard GZ34 but avoided them because of the prices out there...


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 9:11 pm
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save your pennies and get one or two when you can. Worth it just for the "mojo"....they are killer tubes as well....good luck


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 9:17 pm
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From personal experience, I've used quite a few Mil spec US Navy Approved black plate CBS-CHY 5U4GB with very good success. They are tough as nails. And don't have the kinda voltage sag seen in commercial branded 5U4GB's.

Might want to check these out:

http://cgi.ebay.com/NOS-Matched-Pair-5U ... 27babb2202


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 9:30 pm
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Thanks BMW! I'll keep an eye on those too.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Mon May 30, 2011 5:30 pm
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Here's an update on my SR...

I had my amp tech go over it and replace anything that didn't belong.

First it was the grid resistors

Image

Image

Then after checking everything he found a resistor with the wrong value, that's what was causing the amp to cut out at loud volumes! This is in the PI circuit.

Before

Image

After

Image

This minor change had a big effect on tone! The amp is not as wild as it was before, cleaner and not as loud, much more controllable in a way.

I just tested some rectifiers, got me some 5U4GB since you guys recommended I do so... these were $5-10 each off ebay.

Image

I compared all the 5U4GB to the coke bottle 5U4G and the #s were almost identical :?

With 450 volts I get 36 mA, the 5U4G measured 35.3 mA at 450. When I set the bias at 44 mA they all measure 442 volts. No idea what all that means, but I'm sure someone here can explain it :)

I did notice slight change in tone, and overall preferred the tone with the coke bottle and the GE 5U4GB in there.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 31, 2011 7:25 am
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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.

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.

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.

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!


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