Adam Pritchard
16 posts
Dec 27, 2013
11:04 AM
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Quick couple of questions to you amp experts out there... the recommendation I've read for the rectifier tube in a Bassman re-issue LTD is the 5U4 rather than the stock 5AR4. Is there any difference between a 5U4, 5U4G and 5U4GB as the latter two seem to be the more commonly available? Also will I need to get the amp re-biased after changing the rectifier or is this only required when changing power tubes?
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tmf714
2304 posts
Dec 27, 2013
1:38 PM
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No discernible difference between tubes 5U/G/GB-
The 5U4 has more sag,and forward voltage drop. The heater current is higher,therefore taxing the power transformer-but those are built fairly tough,so I don't see any issues there.
A re-bias would be in order-30 MA would be a good start.
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Rick Davis
2796 posts
Dec 27, 2013
5:43 PM
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The www.GreenBulletMics.com website has a lot of good info about how to tweak a Bassman RI amp for harp. I got a lot of advice there when I was modifying my Bassman. Here is what he says about the rectifier:
"The 59 Bassman reissue as it comes out of the box is a decent sounding harp amp, but it can definitely be improved upon. As they used to come with a solid state rectifier, in the form of a solid state plug that fits into the octal tube socket, you can replace it with a tube rectifier, which is the first thing you can do to improve it's tone. I suggest using a 5U4G or a 5U4GB rectifier to get the most tube sag, and having the fixed bias replaced with an adjustable bias control so that you can adjust the amps bias."
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-Little Rick Davis
The Memphis Mini harp amp
The Blues Harp Amps Blog
The Mile High Blues Society
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tmf714
2305 posts
Dec 27, 2013
6:46 PM
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Those specs are for a 59 RI-Adam owns an LTD-the LTD comes stock with a 5AR4 rectifier and a bias pot.
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Rick Davis
2797 posts
Dec 27, 2013
9:15 PM
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Tom, it offers an answer to Adam's direct question: What rectifier tube is recommended for the Bassman RI. And he happens to agree with you.
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-Little Rick Davis
The Memphis Mini harp amp
The Blues Harp Amps Blog
The Mile High Blues Society
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Kingley
3342 posts
Dec 28, 2013
1:07 AM
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I could be completely wrong here, but I was always led to believe that if you were going down in value (GZ34 to 5U4G) you could do it without rebiasing. If you're going up in value though (5U4G to GZ34) then you have to rebias.
My best advice as always when to comes to amplifiers is to ask someone like Mark Burness or if you're in the States maybe Skip Simmons or Bruce Collins. Because the problem with things like amp mods/valve swaps is that there is loads of information out there and most of it is simply wrong. It's far better to wait and get professional advice than risk damaging an amp that may turn into an expensive repair job to my mind.
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5F6H
1724 posts
Dec 28, 2013
4:22 AM
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Switching to a rectifier with a bigger voltage drop (e.g. from SS to GZ34/5AR4/5U4g/GB & from GZ34/5AR4 to 5U4G/GB), reduces plate voltage and subsequently idle current (biggest generator of heat in the amp). This won't damage your amp, if you drop too far the amp may not hold together on long drawn out notes, in extreme circumstances (unlikely) it will cease to make sound (switch back to the last thing that worked).
Kingley is right, in that when switching to hotter rectifiers (5U4G/GB to 5AR4/GZ34/SS, or from 5AR4/GZ34 to SS), caution should be excercised as voltage & currents increase, so monitor your idle current...which is also a very good idea anyway when subbing rectifiers & power tubes.
When Fender launched the 59 RI there were no current production 5AR4/GZ34 tubes (nor 12AY7).
Pre & post LTD 59 RIs can all take the 3Amp 5U4*G (*Glass) or *GB (*Glass, B update designation).
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Last Edited by 5F6H on Dec 28, 2013 4:25 AM
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tmf714
2306 posts
Dec 28, 2013
6:09 AM
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" I could be completely wrong here, but I was always led to believe that if you were going down in value (GZ34 to 5U4G) you could do it without rebiasing. If you're going up in value though (5U4G to GZ34) then you have to rebias."
Its always a safe idea to at least check the bias-therefore you have at least some idea where youre at-rebiasing may not be needed,but its a good idea to check it while youre back there.especially if you have a bias probe or tool-takes me all of five minutes.
From the 59 Bassman RI owners manual-
For those who want their
amp “stock,” the solid state rectifier can be
replaced with a 5AR4 or 5U4 GB tube, although this
modification will slightly reduce power output and
reliability.
Hence the re-bias.
Last Edited by tmf714 on Dec 28, 2013 7:34 AM
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Rick Davis
2798 posts
Dec 28, 2013
8:07 AM
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The swap to a 5U4G rectifier tube in a Bassman RI amp has been done thousands of times. I have never heard of an amp failing or suffering because of it. It is a safe and proven mod. The next time you have the amp in to a tech you might ask him/her to check the bias, but there is no reason to worry about it.
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-Little Rick Davis
The Memphis Mini harp amp
The Blues Harp Amps Blog
The Mile High Blues Society
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Adam Pritchard
17 posts
Dec 29, 2013
7:52 AM
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Thanks everyone for the benefit of your knowledge on this. I think I'll play it safe and get the bias checked by a tech. Don't really know what I'm doing myself so probably best not to fiddle and end up spending the New Year in hospital (or worse).
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Rick Davis
2802 posts
Dec 29, 2013
7:56 AM
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LOL, Adam, good call. The Bassman RI can be a very good harp amp. It is the amp most commonly used by pro players. There are lots of online resources out there with info on how to get the most out of the amp.
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-Little Rick Davis
The Memphis Mini harp amp
The Blues Harp Amps Blog
The Mile High Blues Society
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