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Dirty-South Blues Harp forum: wail on! >
Adjustable Bias - Good or bad idea?
Adjustable Bias - Good or bad idea?
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Oxharp
498 posts
Apr 26, 2012
10:02 PM
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Hi to all you guys out there in the know about adjustable bias in an amp.
I have read and understand that the most tonal difference in an amp come from the pre amp tube chain and not so much from say a different rectifier. What about the power tube output?
I am doing up a Fender Pro Junior at the moment and wanted to know if you change from fixed bias in the amp to an adjustable one and run the power valves at a colder voltage how would this affect the tone of the amp? Do you get earlier break up and will the valves last longer?
Is this a good idea and is it something that a lot of amp guys do for harp?
I also have a Fender Bassman 59 RI that I would like to do the same to.
Hope I made sense.
Cheers
Russ
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Oxharp
Last Edited by on Apr 27, 2012 9:35 AM
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5F6H
1170 posts
Apr 27, 2012
2:15 AM
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Rectifiers affect all the voltages in the amp, you will find that the difference between a SS rectifier and, say a 5Y3, or 5R4 in the same amp (if you have that option - you don't on either the Pro Jr or really on the bassman) is massive.
Adjustable bias, or at least the ability to read your bias and sub tubes (another method of rebiasing) is pretty essential in a bassman style/fixed bias 6L6 amp. Pro Jr's are usually under biased (too much current) from the factory, I don't have experience of rebiasing them for harp but have found that lower than late 20's mA for guitar can be cold.
The cooler you bias the longer the tubes last, but if biased reasonably they should last plenty of time anyway. More crunch the cooler you go, less feeedback, if you go too cold the amp might struggle to hold a signal on long notes or sound thin & sterile.
I don't know what point this would happen on the Pro Jr, but as long as you are above 5ma per tube on the bassman it should work fine, fine tune to taste.
Higher currents are smoother sounding, but more prone to feedback, very high currents will sound greasier. Don't exceed 17.5W per tube for 6L6/5881.
<15mA might be considered cool for a bassman/6L6 fixed bias amp, late teens to 30mA middle of the road, above 35mA hot.
Preamp tubes make a big difference because people aren't typically subbing like for like (e.g. one brand of 12AX7 for another) when retubing for harp, they are subbing completely different tubes with totally different characteristics (plate voltage, bias, gain)...there isn't really a parallel regarding power tube swaps. You can in some circumstances (check with manufacturer, or me, before attempting) sub 6V6 for 6L6, 6L6 for 6V6, or 6L6 for 6550/KT88/KT90 but subbing one power tetrode for "the next nearest" thing is nothing like subbing a 12AX7 for a 12AU7 or 12BH7.
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www.myspace.com/markburness
Last Edited by on Apr 27, 2012 6:22 AM
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5F6H
1171 posts
Apr 27, 2012
3:14 AM
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Looked through my notes & found this...
Fender Pro Junior bias pot installation
1) Clip the ungrounded end of R29, leaving enough lead length so that another wire can be attached here.
2) Take a 10K PC style, single turn, top adjust cermet pot and solder a ~1” length of solid core 20/22 gauge wire to the left hand terminal, looking from the adjuster side of the pot.
3) Place the pot on to the circuit board so that the 22/20 guage wire you have just soldered to the pot meets the free end of R29, which should be bent away from the board to avoid shorts. The centre terminal of the pot should sit against the ‘-‘ (ungrounded) lead of C13.
4) Solder the pot wire to R29, remove any excess wire length.
5) Solder the middle pot terminal to the ‘-‘ side of C13.
6) With the power tubes removed, power up the amp and read the negative voltage at the junction of R21 & R22. Adjust for -11v and power down.
7) Now install the power valves (recommend Harma E84L), power up and read the power valve’s bias current using the transformer shunt method, or TAD bias probes. Adjust plate current to 30mA.
I’ve lost count of how many times I’ve read on the internet that Pro Juniors are “class A” or “self-biasing”. Rubbish! They are class A/B fixed-bias and, just like every other class A/B fixed-bias amp, bias current must be checked/adjusted when installing new power valves!
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www.myspace.com/markburness
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Oxharp
499 posts
Apr 27, 2012
9:44 AM
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@ Mark,
Thank you so much for your very informative reply.
I should have just emailed you and asked directly but I did not want to hassle you.
So far I have swaped the speaker for a Weber Sig 10 ceramic which has made loads of difference and changed the EL84 valves for NOS Mullards and put in 2 EC882 for the Pre Amp chain (Mullards again)
I am very pleased the way it is progressing and need to do a few more tweaks to get it sounding right.
Once again thank you so much for your excellent knowledge and experience.
Russ
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Oxharp
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5F6H
1174 posts
Apr 27, 2012
12:12 PM
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You're welcome Russ, as is anyone with any amp queries, to e-mail me direct...if I'm snowed under it'll just take me longer to respond, but no amp mails go unanswered. Well, it's not like it's "work" is it? :-)
Keep us posted on the Pro Jr progress, it'd be good to have some datum points for a "harp friendli-er" version.
Regards, Mark.
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www.myspace.com/markburness
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