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i have a 78' DR504 2 inputs with tag board and i'd like to convert the bias circuit.

i saw the M Huss mod but don't have some UF1007 diode (couldn't find some here) , so as i have some UF5408 and saw a 2001' hiwatt DR504 schematic where it was used in place of the UF1007, i was wondering if i could use it instead ?

the thing is that i don't have the BXY94 diodes in my amp but 3x BY227 and teh resistor before the diode is the 110 k /1w (that M Huss replaces with 82k/1w) and i saw in the 2001' schematic that the two other diodes they use are BY127 and the resistor in front of the UF5408 diode (they use for the bias circuit) is just 1k /1w (compared to the 110k or 82k M Huss uses) .

so, can i put this UF5408 diode for the bias , keeping my two other BY227 diodes and do i have to use the 82k /1w or 1k / 1w resistor with it ?

so, if i understood it well, the two 100k resistors of the bias are replaced with 220k resistors and the 27k resistor with 15k + 25k bias trimmer, isn't it (on top of the diode / resitor replcement + 100uf /100v cap) ?

could i use that UF5408 diode (still with 1k or 82k resistor ?) for the bias circuit mod of a 72' DR504 i have that uses the BXY94 diodes ?

i saw that Aspen Pittman says it's possible to put a 1N4007 diode instead of the BXY94 in his amp book and i found some 1N4007 diodes in my stock of parts (although they look much smaller than the ones i have actualy but i wonder if there are some 1N4007 made for different voltages / A , although it doesn't seem to be the case ) and don't really know what resitsor to put in front either as it's not written in Pittman's book.

the UF5408 seems to be given for 1000v 3A/150Ap 75ns
1N4007 SI-D 1000V 1A/50Ap
BY227 SI-D 1250V 1.75A /50Ap
UF1007 1000v 1A /30Ap 75ns

so, i suppose the UF5408 should be ok and i read it was better (as an ultrafast diode) than the 1N4007 (that seems to show Pitmann in his book)

but, i don't really know what resistor to put in front of the diode !

maybe i should put the 82k as given by MHuss mod (and not teh 1k shown on 2001 Hiwatt schematic ) ?

i noticed the 110k and BY227 diode are reversed compared to the schematic on M Huss website (the 110k isn't between the two diodes ), i suppose it mustn't change much (and it's stock on my amp as it is totaly stock , still with red dye everywhere, except the filter caps i changed).

i tried the Mhuss bias circuit mod except i used the UF5408 diode and two 2w resistors in serie to make the 82k but i when i use a bias tool , IK = 0 with or without power tubes installed (and my plate voltage seems to be higher as i now measure 500 vdc for 220 ac in whereas i think i measured a smaller plate voltage before the mod , like maybe 450 vdc but i'm not sure).

i kept the two 22k resistors (going to the power tube pin) after the 220k resistors added with the mod although i'm not sure they have to stay but i saw on the 2001' hiwatt dr504 schem that they kept them (but they seem to only use a 1K/1W resistor after the diode and not 82k ) .
i measure about - 64 vdc on the grid but can't change it with the trimmer without the output tubes installed at least and as Ik = 0, i'd prefer to resolve the problem before putting them back in.
i suspect some resistor values are too high but don't really know where to start.
unless teh diode combination is a problem but it seems the diode i installed is mounted teh right way

i also left the diode and 82 k resistor , the way the original diode and 110k resistor were mounted stock in my amp, that is to say the 82k isn't in between the two diodes (it was working before the mod) .
not really sure ifit change something !

From the symptoms it seems the output tubes are getting to much reverse bias.

Are you sure the trimmer is connected to the 15k on the bottom, and that it is grounded at the far end? Measure the bias voltage at both ends of the 15k.

--mark

i have one side of teh 25k trimmer connected to a turret that goes to ground , the other side of teh trimmer is connected to teh 15k .
i measure -39vdc between the trimmer and the 15k resistor.
on teh other side of teh 15 k resistor i have -65vdc.

i only used one 25k trimmer (between ground and the 15k resistors , that are both connected instead of the previous 27k resistor ) and the 220k resistor i talk about bellow instead of a 25k trimmer + 220k resistor per power tube as i suppose it's just to be able to bias the power tubes independantly if the ones we have are not matched .

at first i had replaced both 100 k resistors that go to the 22k resistors (that go to the power tubes grid pin) with 220k resistors but i put back the 100k 's before the 22k but disconnected one end of the 100k resistors that went to the turrets (one each) and connected a 220k to the two 100k resistors free ends and connected the other side of the 220k resistor to one of the turrets where the 100k reisistors were connected, so one side of the UF5408diode i installed, as well as the negative side of the 100uf/ 100v and one side of the 15 k resistor,are all connected to the 220k (that is connected to the two 100k resistors on the other end) .
it was the most simple way i found to make your mod (if i understood it all ! LOL

the 2001' hiwatt dr504 schematic have two 220k resistors going to tetheh 22k resistors that are both connected to the power tubes grid pin though !

as on your bias mod it 's not written to replace the two 100k resistors with two 220k , i didn't do it.

but i don't have any thing else than Ik = 0 'with weber bias tool or compu bias !)

my bias trimmer has two feet (not 3) , it's not the stand tall model but the one lying (the one sold as jtm45 bias trimmer kit on metroamp).
one end goes to ground and the other goes to the 15k resistor.
when measured each side of the trimmer / 15k resistor combination, it was going from 15k to 39k depending if the trimmer was full clockwise or anticlockwise, so i suppose it was working.

i thought this mod would take 1 hour or two but i've been working on it for all teh evening up until 1 o' clock ! LOL

i wonder if i replace the 82 k resistor (2 in serie) with a 1k as on eth 2001' schem, maybe it will work .

i had Ik = 50 to 60 mA (depending of the pair of tubes i put in)- before i made the mod ( that 's why i made the mod ! LOL)

after i tried with the 110k instead of the 82k, i had IK=5ma , so as i thought i had a problem in my resistors !
so i checked teh trimmer and realised despite i was sure i had measured it ok, it was not mounted right ! LOL
so once it's been mounted correctly, i could finally setup the bias.
i have now 480 vdc on B+ with 225ac in tap selected, so i biased about 32 mA corresponding to 50% on the groove tubes bias tool manual.

i still have one or two questions left .

is it only one 220k resistor leading to the two 100k resistors (each going to a 22k and to the power tubes grid pin) , if i only use one trimmer ,as i did ?

it seems to work but i can't push the amp, it's 2:30 AM here ! LOL

i suppose i can leave that UF5408 diode in place even if it's a tough one, i'm not sure it will make a difference with a UF4007.

i saw on the 2001' hiwatt dr504 schem that the 1k resistor after the UF5408 is a 1k / 1w as well as the 110k / 1w on the DR 50w power supply schematic and bias mod i found on your website Mark , whereas on the hiwatt 50w output power supply layout of your website, it's a 2w , is it safer to use the 1w or 2w (i suppose it's not a big deal) ?

thanks for your help Mark and thanks very much for all the infos on your website; it's been a few years i look at it from time to time !

Cool, I'm glad you got it working. The extra 220k is not needed in this case, and that diode will be fine.

--mark