Adam Pritchard
23 posts
Jan 06, 2014
3:07 PM
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Hi guys,
I've just picked up my Bassman re-issue LTD from an amp tech who was checking the bias for me after I swapped the stock 5AR4 for a JJ 5U4GB. He told me that the 5U4GB requires 3amps heater current compared to 2amps for the 5AR4. He said he didn't think that this was likely to cause a problem and he couldn't find anything online to the contrary.
He also said that R28 and R29 were running slightly warm (0.6 watts when they're only 0.5 - I've got a 12AU7 in v3). He said he could change them to 1 watt resistors but that he didn't think the current ones were likely to fail, and even if they did it wouldn't be a big job to sort out.
Does it sound like this guys talking sense or do I need to worry?
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tmf714
2317 posts
Jan 06, 2014
3:46 PM
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Sounds like he knows what he is doing.
Most change the R28 and 29 when they run a 12AU7-the resistors in those locations will run hot with the 12AU,and are not of the best quality-therefore the upgrade to 1 watt.
Did you get a bias/ma reading from the tech? 32-38 ma per tube is fine.
Last Edited by tmf714 on Jan 06, 2014 5:20 PM
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Rick Davis
2828 posts
Jan 06, 2014
5:23 PM
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Adam, yes, as Tom said, he sounds like he knows what he's talking about.
Have fun with the Bassman RI. A very nice harp amp. You will get lots of advice on how to mod it for harp.
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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
24 posts
Jan 06, 2014
11:05 PM
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Thanks tmf714 and Rick. The bias is set to 32. The amp's sounding pretty good. I've got it set up with 5U4GB:6L6:6L6:12AU7:12AT7:12AY7. I might get the resistors upgraded just to be on the safe side and I've been thinking about swapping out a couple of the speakers. Overall though I'm pretty happy with the sound I'm getting from it.
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