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Old 25th March 2011, 04:39 AM   #1
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Default Confused about SSE "Tubes and Transformers"

I'm wanting to try KT-88s in my Simple which is now 450 volts on the plates with an 560 ohm cathode resistor and 5K outputs driving 6CA7s.

Looking at the matrix for 450 volts I see a range of 429 to 436 ohms Rk which can't be terribly crucial (?) and the numbers in red which I really don't understand.

I see Mills has a 402 and a 453 ohm, I could always combine a pair of 15s with the 402 to make 432. What happens if I just leave the 560?
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Old 25th March 2011, 04:42 AM   #2
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I run my Gold Lion KT88s with the stock 560ohm resistor. I didn't want to run these tubes too hard so I have usually stick to that value. I have found with many 6l6gcs I have to up the cathode resistor to around 800ohms otherwise I get some nice red spots on the plates
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Old 25th March 2011, 05:07 PM   #3
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Look at the Diss column relative to the distortion numbers. You can see that as Diss goes up, distortion tends to go down because the tube is being pushed into a more linear operating region. The red Diss numbers are technically exceeding the plate dissipation spec of the tube type...pushing the tube hard. The orange numbers are leaning on them but still within the limits. You are trading tube life for better distortion numbers.

Note that these numbers are simulated and being compared with published tube specs. Modern tubes tend to vary quite a bit. Some models will glow red at less than the published Diss limits and some can go well over the published specs and still work fine. As Paul said, some Chinese 6L6 tubes seem to glow at fairly low plate specs while I found that JJ 6L6GCs can take as much as or more than some 6550s.

In practice, I've found that the New Sensor 6550 and KT88 types can handle more than average dissipation without plate glow, but this varies from batch to batch.
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Old 25th March 2011, 06:02 PM   #4
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Quote:
Originally Posted by BillEpstein View Post

Looking at the matrix for 450 volts I see a range of 429 to 436 ohms Rk which can't be terribly crucial (?) and the numbers in red which I really don't understand.

I see Mills has a 402 and a 453 ohm, I could always combine a pair of 15s with the 402 to make 432. What happens if I just leave the 560?
Bill: Look at the 450V table again.....the 429 to 436 are Vp (Plate voltage) values.

The cathode R values (Rk) range from 330R to 560R for various power out and distortion, etc.
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