Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mxrshiver on February 03, 2022, 02:39:23 am
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this is the Rev. B, green PCB, and has different labels & voltage references than the later models. see attached schematic. i've also attached the loadline & bias point my amp is currently running at, plotted on mutual characteristics.
like many, i'd like to cool these power tubes down a bit so they last maybe more than maybe 8 mo of heavy use 😅 ideally into the 80% or lower plate dissipation range, so long as it sounds right. but to drop it down that far on the loadline by just lowering the grid voltage, would substantially limit the amount of input headroom i have before hitting some IMD in cutoff. i DO think this amp sounds great this close to a center bias, it's just too damn hot overall! i give JJ's credit for putting up with 100-100% plate dissipation for this long...
so, my thought is, instead of changing the grid bias, why not drop the B+ only to the power tubes (somehow)? sliiiiiiide the whole loadline to the left 30-50 V, maybe drop the screen voltage a tad to keep it hitting the same place on the knee... all the amp's most expensive parts are a bit happier, and i just gained some sorely needed compression headroom on the OT!
worth noting all preamp DC voltages are stellar, which is why i'd like to isolate this drop to only the power tubes' supply.
so my question is... can i more or less drop a choke of appropriate current handling capabilities and DCR directly into the B+ path, between the input filter & the OT center tap?
or, would it be wiser to install it directly after the bridge rectifier, possibly with another filter cap preceding it, and then lower the resistance of the first dropping resistor before the preamp appropriately to compensate, so that the EL84 plates and screens are dropped but the preamp runs at relatively the same voltages?
interested in practicality, expense, and effects on tone/response. my thanks in advance!
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A choke won't drop B+
To reduce B+, I use zener diodes stack ;
https://el34world.com/Forum/index.php?topic=17595.0
https://www.tedweber.com/v-dump
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage
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A choke won't drop B+
To reduce B+, I use zener diodes stack ;
https://el34world.com/Forum/index.php?topic=17595.0
https://www.tedweber.com/v-dump
https://robrobinette.com/Generic_Tube_Amp_Mods.htm#B+1_Voltage
these are excellent links, thank you!! what i'm mainly looking for is a way to only drop the plate voltage to the power tubes. can i use a string of zener diodes directly in between the high voltage rail before the dropping resistors, and the OT center tap, to achieve this?
i thought chokes would drop a little bit of voltage? like, considering the high voltage rail is running at maybe 90mA, a 450 ohm DCR choke would drop about 40 volts, correct? or does the DCR change while it's in operation? either way, diodes sounds a mite cheaper :laugh:
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A chocke with that current capability and resistance will be about as big as yout OT.
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This is a well known issue for all revs of Fender Blues Juniors.
The bias type is fixed. Unlike most other amps which use a pot in the bias circuit to allow adjustment to the bias voltage, Fender uses a voltage divider that is well and truly fixed!
So, the EL84s are set to run hot, and yup they wear out pretty quickly.
It seems to me Bill Machrone was the first to publish a mod that adds a trimmer pot to the circuit so that you can set your bias point as you wish, but it is essentially what you see in Hoffman's Blues Junior Circuit design.
This mod works great -- I very much like BJs for what they are, and since I modded my circuits, I went from replacing tubes every 6 months, to . . . well, I haven't had to change out the EL84s since I biased them to 60% maxPDiss.
Don't even think about a choke. Adding a trimmer is a little finicky, but not a big challenge.
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This is a well known issue for all revs of Fender Blues Juniors.
The bias type is fixed. Unlike most other amps which use a pot in the bias circuit to allow adjustment to the bias voltage, Fender uses a voltage divider that is well and truly fixed!
So, the EL84s are set to run hot, and yup they wear out pretty quickly.
It seems to me Bill Machrone was the first to publish a mod that adds a trimmer pot to the circuit so that you can set your bias point as you wish, but it is essentially what you see in Hoffman's Blues Junior Circuit design.
This mod works great -- I very much like BJs for what they are, and since I modded my circuits, I went from replacing tubes every 6 months, to . . . well, I haven't had to change out the EL84s since I biased them to 60% maxPDiss.
Don't even think about a choke. Adding a trimmer is a little finicky, but not a big challenge.
thanks! i've installed adjustable bias pots on several amps, definitely wouldn't be a challenge here. just trying to approach another way of adjusting the bias by decreasing only the plate supply voltage, so that the operating point stays at the same place on the loadline. wondering if there's any practical way to do that, such as the zener diodes.
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"....just trying to approach another way of adjusting the bias by decreasing only the plate supply voltage,..."
Bias pot is the easiest ( and the best ? ) way.