Hi there,
I've built an amp with a pair of KT66s cathode biased, and seem to have under-estimated the effect the screen voltage has on the current these things pass - they're dissipating more than they should be! Having looked back at the datasheet, I'm a bit confused as to how I missed it, as there's a graph showing the response curves for the screen voltage I have!!
Anyway, obviously, I'd rather not completely rebuild the thing, so wanted opinions on the way to go.
Currently, the OT centre tap is at 465V, screen supply 413V, Rk is 560R per tube, and cathode voltage is about 37V.
Options are:
to raise the cathode resistance to 1k, which should give me about 40mA with a -40V bias, or
lower the screen supply by 100V or so, but it would need to go quite a bit lower than my PI supply should be, and would likely introduce quite a bit of sag...
My preferred option would be the 1k cathode Rs, but I'm a bit concerned that nothing is coming up when I search for that - is it likely to give me unwanted harmonics?
KT66 cathode bias
Moderators: pompeiisneaks, Colossal
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memphis032
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- Location: Suffolk, England
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potatofarmer
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Re: KT66 cathode bias
Start with increasing the cathode resistor, unless your OT's primary impedance still works with the lowered screen voltage.
High plate voltages require higher values of cathode resistor, which leads to pushing the bias even colder when plate current increases, leading to more crossover distortion. This is why a lot of people ignore the 70% "rule" when cathode biasing.
You won't know if you don't like the sound of increased crossover distortion until you try it. There are some tricks to deal with it or prevent it, like the "paul ruby" mod or putting a zener in parallel with Rk in order to prevent the voltage across Rk from getting too high. I like that one; cathode bias at clean output but fixed bias at heavy overdrive.
With over 400 volts on the plates, you might want to consider trying fixed bias instead.
High plate voltages require higher values of cathode resistor, which leads to pushing the bias even colder when plate current increases, leading to more crossover distortion. This is why a lot of people ignore the 70% "rule" when cathode biasing.
You won't know if you don't like the sound of increased crossover distortion until you try it. There are some tricks to deal with it or prevent it, like the "paul ruby" mod or putting a zener in parallel with Rk in order to prevent the voltage across Rk from getting too high. I like that one; cathode bias at clean output but fixed bias at heavy overdrive.
With over 400 volts on the plates, you might want to consider trying fixed bias instead.
Re: KT66 cathode bias
Were you looking to use the conditions at the bottom of p2? http://www.mif.pg.gda.pl/homepages/fran ... 66_GEC.pdf
If so your Vg2 seems too high.
To avoid big sag, it could be dropped with some zeners, cold cathode voltage regulator tube eg OC-3, or the old school method was to add a g2 supply from the B+ winding, single wave rectified with a choke input filter.
Regarding the data sheet table linked above, can anyone enlighten me what is meant by Va(b)(o), Va(b)(max sig), Va(o), ie what condition is being specified by the (b) and (o) indicators?
Maybe (o) indicates quiescent / no signal / static, and (b) indicates the power supply node voltage? But I'd like to know rather than assume!
If so your Vg2 seems too high.
To avoid big sag, it could be dropped with some zeners, cold cathode voltage regulator tube eg OC-3, or the old school method was to add a g2 supply from the B+ winding, single wave rectified with a choke input filter.
Regarding the data sheet table linked above, can anyone enlighten me what is meant by Va(b)(o), Va(b)(max sig), Va(o), ie what condition is being specified by the (b) and (o) indicators?
Maybe (o) indicates quiescent / no signal / static, and (b) indicates the power supply node voltage? But I'd like to know rather than assume!
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memphis032
- Posts: 40
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- Location: Suffolk, England
Re: KT66 cathode bias
I'm thinking the fixed bias option is probably the best - top of p3 of the Genalex datasheet has almost the same conditions.
Shame I'd already ordered the 1k resistors...
Shame I'd already ordered the 1k resistors...
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Andy Le Blanc
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- Location: central Maine
Re: KT66 cathode bias
cathode bias is the most tube friendly way to go.
you can also push the bias to 100% that's anode plus screen rating
put a 1 ohm r in the cathode circuit to measure ma over, just double check what you rigged as a screens supply and keep the screens some odd volts negative to the plates
55w dissipation for the pair ... @465 v.... thats pushing 120 ma for the pair..
double check the rating on your power tranny
it might seem extreme but cathode bias is the most stable way to go.
you can also push the bias to 100% that's anode plus screen rating
put a 1 ohm r in the cathode circuit to measure ma over, just double check what you rigged as a screens supply and keep the screens some odd volts negative to the plates
55w dissipation for the pair ... @465 v.... thats pushing 120 ma for the pair..
double check the rating on your power tranny
it might seem extreme but cathode bias is the most stable way to go.
lazymaryamps
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memphis032
- Posts: 40
- Joined: Sat Apr 21, 2012 7:57 pm
- Location: Suffolk, England
Re: KT66 cathode bias
It's already converted, and seems quite happy now. Or at least it did after I changed the PI output caps - one had gone short somehow, putting +135V at the grid of one KT66...