Hiwatt DR201 improved bias circuit?

[Balticnoiseforge]

I just want to ask if there is anyone that has designed another bias circuit for the dr201 x4 KT88.

Picture from the psu from Mark Huss hiwatt page feels like there is room for improvement, mainly some failsafe resistor on the potentiometer etc?

Seen the Reeves 225 someone did with 6 bias potentiometers, but iam just looking for a 2 pot one.

I have tried designing one in ltspice but i fail to rectify it to negative voltage, so ill ask here since some of you could probably tell what to do ontop of your head.

Also i have read some threads about people trying to do the bias suited for both KT88 and 6550 - but would the latter really handle the voltage?

So what would be a modern way to design it in 2021?

[Williamblake]

This works for me.

[JZRepair]

This works for me.

I too am looking to make this adjustment to a DR201.  Could you tell me more about what your modification is doing?  Also, I don't see any voltage values for the caps, just that they are 10uF.  With this circuit, what does it allow for range for the negative voltage?

[Williamblake]

I used 100V caps just to be on the safe side of things, dont remember the output voltages but i remember i was happy with the range.

[JZRepair]

I used 100V caps just to be on the safe side of things, dont remember the output voltages but i remember i was happy with the range.

I'm curious, because the 1N4001 is only capable of reverse blocking 50V.  I need to get the bias voltage to -61 or more.  My current max is -51.5 and it's still running the tubes way too hot.

[sluckey]

Picture from the psu from Mark Huss hiwatt page feels like there is room for improvement, mainly some failsafe resistor on the potentiometer etc?

I'm a believer in KISS especially when dealing with a bias circuit. Simple is more reliable. Lotta expensive stuff depending on a reliable bias circuit.

The mhuss circuit is fine as is but failsafe resistors would be better. I would also add a 10µF cap to the wiper of each pot and lower the 100µF to about 50µF.

I'm curious, because the 1N4001 is only capable of reverse blocking 50V.  I need to get the bias voltage to -61 or more.  My current max is -51.5 and it's still running the tubes way too hot.

Just use a 1N4007. Same price.

[JZRepair]

Picture from the psu from Mark Huss hiwatt page feels like there is room for improvement, mainly some failsafe resistor on the potentiometer etc?

I'm a believer in KISS especially when dealing with a bias circuit. Simple is more reliable. Lotta expensive stuff depending on a reliable bias circuit.

The mhuss circuit is fine as is but failsafe resistors would be better. I would also add a 10µF cap to the wiper of each pot and lower the 100µF to about 50µF.

I'm curious, because the 1N4001 is only capable of reverse blocking 50V.  I need to get the bias voltage to -61 or more.  My current max is -51.5 and it's still running the tubes way too hot.

Just use a 1N4007. Same price.

Thanks for the input sluckey.  I agree with KISS.  My stupid @__ needs exactly that!

As for the 1N4007, that's exactly what I was thinking about swapping in.  2 Amps that mhuss calls for seems quite excessive since the bias circuit is currently under a 10th of a watt, I can't imagine it even getting close to 1W even with this doubler.

[Williamblake]

Thanks for the headsup, fortunately just a typo and those are 7s. Sorry about the mistake.
I did include fail safe resistors so i am still feeling safe. But now yall have me wondering how this could be any simpler.

[JZRepair]

Thanks for the headsup, fortunately just a typo and those are 7s. Sorry about the mistake.
I did include fail safe resistors so i am still feeling safe. But now yall have me wondering how this could be any simpler.

Works like a charm.  I have the turret board version in the amp on my bench, so I had to do some slight rewiring to accommodate for the updated bias circuit, but it is working great.  I'm peaking at -60.3V, which is giving me plenty of headroom for adjustments.

Thank you for sharing this circuit and for clarifying those components!

[Williamblake]

Well i learned about the Mike Huss information when i was finished. Sorry for doubleposting. https://hiwatt.org/

[JZRepair]

Well i learned about the Mike Huss information when i was finished. Sorry for doubleposting. https://hiwatt.org/

Honestly, I liked your circuit better, especially knowing it was put into a DR201.  My customer was happy with it too!  My issue with the MHuss one was that it was designed for a different amp, with clarification as to whether or not it would work with this one.  I didn't think there would be an issue, but I still like being sure and I needed to find something quick that had been verified.  I can't thank you enough for sharing this and being willing to discuss it!  Thank you.

[JZRepair]

Well i learned about the Mike Huss information when i was finished. Sorry for doubleposting. https://hiwatt.org/

Ok, well, this amp found it's way back onto my bench.  The owner sold it and it was damaged in shipping.  He wanted the remaining electrolytics replaced, all new tubes, and everything repaired.  Well, the thing got pretty beat up.  The original choke was damaged due to the PT breaking off it's brackets and flopping around.  The only replacement I could find for it was directly from Hiwatt's suggestion.  It's a 300mA 800V 5H fixed inductor rather than the 4H inductor that was on it.  The new choke mixed with the new caps has caused my working voltages to drop a bit.  Now, the bias circuit doesn't allow me to adjust it any more.  Max I can get is about 20W across the plates, which is still under 50%.

Is there any modification I can make to this doubler to get that bias voltage lower? 

[PRR]

Is there any modification I can make to this doubler to get that bias voltage lower? 

Reduce the 22k (or is it 12k?) ?

Simpler: tack 100k across the 22k/12k and see where that gets you. A few trials should get a pot-range which won't allow idiots to quickly burn-up the amp but still get out of the dead zone. Either leave the trims, or figure the parallel equivalent and use a resistor that value.

[JZRepair]

Reduce the 22k (or is it 12k?) ?

Simpler: tack 100k across the 22k/12k and see where that gets you. A few trials should get a pot-range which won't allow idiots to quickly burn-up the amp but still get out of the dead zone. Either leave the trims, or figure the parallel equivalent and use a resistor that value.

I was curious about the effects of changing that 12k.  In my mind, it made sense that I could adjust this value for a bit more play.  I have a 2W 50k pot, so maybe I'll throw that in there without the output tubes and is series with a 1k resistor to see what we get for bias voltage swing.

Another issue I'm dealing with is some "static" that wasn't present before, but I think I'll start a new thread for that.  During shipping, the PT bounced around after breaking free and destroyed the original choke, internally.  It is oriented the same as the original, however, it's much larger, so in theory, the magnetic field should be larger and it's very close to one of the output tubes.  I think that's what's picking up the noise and transmitting it to the speaker.  What would be really nice is if I could find someone who can rewind the original choke.  To my knowledge, there's no one in my area and searches haven't produced much.