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Post subject: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 11:23 am
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Need some help/advice... trying to figure out what the differences are between 5AR4 and 5U4G tubes and which is the right one to use in my '73 SR. In plain English please :)

The amp came with a Sovtek 5AR4 and a badly matched pair of tall bottle Sylvanias, but it sounds very sweet! I recently replaced the Sylvanias with a pair of TAD 6L6GC-STR (tall bottle), perfectly matched. The amp still sounded very sweet, very similar to the Sylvanias... I also tried a pair of Tung-Sol and short bottle Sylvanias, both didn't sound as good.

One of the TAD tubes is already dead I think. I noticed that at loud volumes (6+) the amp would cut out when I hit a hard note or chord. So I thought I would check the bias again, I had it around 36 mA. I hooked the tubes up to the Bias Rite, turn amp on, let the tubes warm up, switch standby on and get a loud hum so I turn it off and notice a bit of smoke coming from around one of the tube sockets! I also noticed that only one of the tubes was glowing blue while the other stayed the same as it was when in standby.

I let the amp cool, put the Sylvanias back in, all back to normal. Put the TADs back in, same story. I moved the tubes from one socket to the other, same thing, the same tube would glow and the other wouldn't, still get loud hum. The TADs are going back and I'm getting a replacement.

A very generous young man sent me a beautiful JAN 5U4G. So I took out the Sovtek 5AR4 and put that big coke bottle in, check the bias on the Sylvanias and notice a drop of about 12 mA. Is this normal behavior? I also noticed a drop in volume.

I'm wondering if the 5AR4 is not a good choice for my amp. Even though I didn't have the tubes biased hot, it seems that at loud volumes I was overpowering these 6L6 tubes. But with my lack of electronics knowledge I really have no clue what's going on... so who's willing to explain what happened and suggest the right tubes and bias? :)


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 11:51 am
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The 5U4GB is the tube spec'd in the print for your amp. The GZ34/5AR4 will increase your amps voltage supply to the power tubes maybe 20 to 30 VDC. You can use either but I wouls stick to the spec'd tube and rebias.
You may be right about one of the TAD tubes going south. It happens. Seems like the problem followed the tube. Get replacements. The TADs are functionally the correct tubes for your amp. They will work. Teh tonal character is up to you to decide if you like it.
It may be a good idea to pull the chassis and check the screen grid resistors that go from pin 4 to pin 7 on the power tubes. They are usually a 470 ohm 1 watt resistor. Only get into the chassis if you are comfortable and carefull. These resistors if not healthy will cause a tube to get way hot and run away so to speak. Good idea as maintenance to have them replaced anyway.
All things being healthy though a 6L6GC biased at 36mA should be happy in your amp.
IMO...try the Winged C SED 6L6GCs....great tubes for a SR....


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 12:08 pm
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Thanks for the explanation! I will remove the chassis and re-bias with the 5U4G (not 5U5GB) in there. I'll take a look at the resistors and take a couple pics...

So is it possible that with 5AR4 in there and biased around 36 mA, which is a lot less than 70%, will still cause problems when playing loud? Will using the 5U4G prevent a spike and the cutting out I was experiencing?


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 12:53 pm
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cedarblues wrote:
A very generous young man...


"Young man"

:lol:

You're too kind, sir!

:mrgreen:

Arjay

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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 1:37 pm
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:mrgreen:

We're all young at heart :)

So I took some measurements...

With 5AR4:

Tube A: 29.5 mA / 481 v

Tube B: 40 mA / 481 v

With 5U4G:

Tube A: 21.7 mA / 458 v

Tube B: 30.4 mA / 457 v

This is with the Sylvanias and no bias adjustment. I also noticed that with the 5AR4 rectifier Tube B will drift from 40 to about 44 mA after a few mins, is that ok?

Using the 5U4G rectifier and plate voltage around 458, what's good bias number? At 70% that puts it around 46 mA. Is it ok to bias tube B at 44-46 to get tube A up in the 30s range?


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 2:03 pm
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Here's a couple pics:

This is V8, looks ok

Image

But V7 doesn't! I believe that's where the smoke came from...

Image

Image


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 3:08 pm
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I think you may have an out-of-spec screen or cathode resistor, which would account for the substantial swing in the idle bias current between the tubes.

Arjay

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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 6:02 pm
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make sure those big ceramic resistors are 470 ohms. The 1500 ohm resistors across pins 5 and 1 for grid are the right value. The split in bias current readings on those svets may just be the tubes them selves. the split is ok but if it was my amp I would like to see the bias current to be a much closer match. Like no more than 3mA apart for a given setting. But thats just me.
The plate voltage split between the 5u4 and the GZ34 is fine. It is what it should be.
If you are having spikes and cut outs there the 5u4 wont help that. That is an idication of a problem somewhere else. If you mean when you had the TADs in then thats most likely the issue. One of those TADs was toast it seems like.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Tue May 10, 2011 9:01 pm
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mxvin wrote:
Like no more than 3mA apart for a given setting.


+1

The closer the better to equalize the load upon each of the tubes.

Arjay

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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 8:48 am
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mxvin wrote:
make sure those big ceramic resistors are 470 ohms. The 1500 ohm resistors across pins 5 and 1 for grid are the right value. The split in bias current readings on those svets may just be the tubes them selves. the split is ok but if it was my amp I would like to see the bias current to be a much closer match. Like no more than 3mA apart for a given setting. But thats just me.
The plate voltage split between the 5u4 and the GZ34 is fine. It is what it should be.
If you are having spikes and cut outs there the 5u4 wont help that. That is an idication of a problem somewhere else. If you mean when you had the TADs in then thats most likely the issue. One of those TADs was toast it seems like.


The resistors say 470 ohms 5 watts, do they need to be replaced with 1 watt resistors?

Yeah the mismatch has to do with those Sylvanias, the TAD were within 1 mA, one of the reasons I replaced the tubes... but even with the mismatch those tall bottle Sylvanias sound wonderful! I have another set of TADs on the way, I'll make sure to have the amp looked at before putting them in.

I will test the amp again with the 5U4G in there and see if it still cuts out at loud volumes... I re-biased the Sylvanias, 40 and 28 mA, gonna leave it there for now. I do love the way the amp sounds with both the Sylvanias and 5U4G in there, tone to die for! The highs are so sweet and amazing tight low end!


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 9:46 am
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no...you can leave those resitors in there. The 5watt are fine. Makes no dif to tone or function.
That mismatch with the Sylvanias wont hurt anything. Just a bit unblanaced. But if you like the tone then thats waht matters. Its more that you like the tone of the sylvanias than a product of the matching. If you find some sylvanias that are more closely matched you may like the tone even moreso!!

You say the amp cuts out at high volumes. You have to clear that up for me.
Which tubes are or were in there when this happens. If it always doesa with a particular 5U4 then that recto tube may be weak. You can good 5u4 on ebay kinda cheap.
Anyway....explain that scenario when the amps cuts out so we are on the same page.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 9:57 am
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Yeah I do prefer the Sylvanias, but the TADs are not bad at all, very close in tone.

Here's what I experienced, I had the amp vol around 6 to get a nice break-up tone, usually keep the guitar vol down, 6-8. If I turn it all the way up and hit a note hard, like a bend or slide it sorta cuts out... this was with the TAD or Sylvanias and the 5AR4 in there, haven't tried it with the 5U4G yet. I'll report back as soon as I do that test.

What could cause this to happen in an amp? The volume is overpowering something, no idea what though...


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 10:57 am
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hmm...If I had to guess it could possibly be a weak rectifier tube. If you have your amp bias at 40mA when you strum hard the mA will go up to like 60mA or so for example. Those numbrs I used were just for arguement sake
Its possible the if the rect tube is weak it cant handle the sag and rebound. I could be way off base by thats my guess. Could be other things of course but I would start there.


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 11:32 am
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I'm with Mxvin on this one. Are these 5U4G tubes or 5U4GB? RMS steady state voltage supply are the same on both = around 450 VDC. But, the current is around 275mA for the 5U4GB and around 225mA for the 5U4G. The 5U4GB has more reserve power.

Prolly most important is the max peak current of the 5U4GB is about 1000mA (1amp) per plate. Whereas the 5U4G is around 800mA. It'll be hard to bias a strong set of Svetlana (SED) 6L6GC's, with a 5U4G. Which could be why those screen resistors popped. As the plate voltages applied to the Svet's were a tad too low. And under heavy load, the current through the cathode tried to compensate and popped the resistors hanging on the screens.

Plus, you run the risk of flashover & early failure in that 5U4G.

Just a WAG....


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Post subject: Re: Tube experts, need some help... '73 SR rectifier
Posted: Wed May 11, 2011 1:32 pm
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I'll check mA values when I'm playing... I remember seeing #s in the 80s or 90s.

BMW, I'm using a 5U4G retifier now. I don't have any svets, I was using TAD and Sylvanias 6L6. The TADs went bad with the 5AR4 in there NOT 5U4G.

So what is the right rectifier for a SF SR? 5U4G or B?


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