KT88 Quad Bias Point for Renegade 65?

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85dot335

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I am retubing my Renegade 65 head with a quad of Ruby KT88STR's. What is the recommended bias point for this application? I know that the Renegade manual doesn't list KT88's as among the power tubes compatible with this amp, but I also understand that the KT88 swap is a not-unheard-of-to-fairly-common "mod". Thanks in advance.

85dot335
Louisville, KY
 
Hi 85dot335,

Using a KT88 in this amp is not a good idea, and you're probably not going to be able to bias the KT88 to an acceptable level for good performance anyway, on top of the fact that they will draw more heater current than even a KT77, so there is no good reason to use KT88s, no advantage whatsoever, and many disadvantages, most importantly is that they will probably sound bad and/or possibly damage your amp. The 6L6, EL34, KT77 are in the range of 25 watt to 30 watt tubes vs the KT88 42 to 44 watt, and you will never see the increase in wattage in this amp. Since using KT88s would be a waste of time and tube, I would stick with the recommended tube types.
 
Just my two cents, it's your amp. If you bias the KT88s at such a low level, well below where they should be, then they should work. I can't imagine that such under-biased tubes would sound good, but apparently others think so.
 
Dear Dot335,
I notice your redirect post was from 3 years ago.
Does the amp still work? Still sound good? After the novelty wore off?
The kt88 can easily drum up 600vdc at the plate.
I worked on these in the navy. They were used in hi powered radio transmitters. Totally different circuit than audio. Idle current in excess of 100ma. I'm not an engineer but with a fixed bias supply as in the Egnater amps, will be hopelessly under biased.
Also not sure if the primaries on the o/t could supply sufficient voltage.
Hope this muddies the waters a bit..
 
Thanks to all -- while I have not heard directly from anyone at Egnater (e-mailed them 4 days ago), I've now seen enough concern to create reasonable doubt as to the KT88 application in an Egnater Renegade. I'm going with a quad of the new Tung Sol 7581's. My goal is to increase clean headroom. Any ideas on whether they should be biased like a 6L6 or like a 6550 (again, the Egnater manual doesn't list them)? They are a 30 watt tube.
 
You got it, bias them as you would any 30 watt tube. IMO, the Egnater recommendation for bias settings are still on the cold side, you can probably bump that number up another 10 mV to 20 mV or so. Typically, to find a good range for biasing any tube just use a simple formula. Take 50% to 70% of the tubes max Plate watt rating, divided by the measured Plate voltage of the amp with the tubes installed, times two, equals the desired range to look for at each test point. In the case of most amps, such as this one, each test point reading is across a 1 ohm resistor, and is for two tubes, so multiply your calculated range of current by two, and this is for each test point. So, (15 watts/Plate volts) x 2 = desired test point reading, and (21 watts/Plate volts) x 2 = desired test point reading. The two answers to the above formula, one for 50% (15 watts) and one for 70% (21 watts) in this example will give you the range to shoot for at each test point.

If you don't measure the Plate voltage, just use the recommended range for any 30 watt tube listed in the manual.
 
Thanks a bunch. I have the data sheets for most of the tubes I want to try, so it will be interesting to see how the formula you've quoted compares to the Egnater recommendations. I've always wondered how bias points are determined and it's certainly less of a mystery to me now.

M. White
Louisville, KY
 
Hot bias is only good for vendors who sell tubes (don't do it) IMHO
 
UPDATE

I have now conferred with an Egnater rep on this topic and he has cautioned that running KT88's at a bias point greater that 45mV in the Renegade could lead to damage in the long term. He also indicated that 7581's should be biased in the 60 - 70mV range. This confirms all of the good advice I received above.
 
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