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Old 8th June 2002, 02:08 AM   #1
PassFan is offline PassFan  United States
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Default 6bq5

Perhaps someone knows the answer to:

1) In a push pull class AB circuit, the plate to plate resistance off the data sheet shows 8K. What effect will using a plate to plate of 7.6K have in the long run? Is the 400 ohm difference close enough not to worry about?
2) In that same class the data sheet shows a max. plate voltage of 300 volts. The curcuit I am using has a B+ of 335 volts and I have an RCA circuit with a B+ of 355 volts. How can this be, and how high above the rating can one go?

I'm playing catchup here when it comes to tubes and I'm slowly figuring a lot of stuff out on my own (old habit that people who know really appreciate) but every now and then I need a gentile nod. Where are all the tube heads at?
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Old 8th June 2002, 07:08 AM   #2
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7.6k? 8k? Show me a loudspeaker that actually stays put at 8 ohms! This variation is of course reflected back to the toobs. I can't imagine that it would make the slightest audible difference. But then again, I can't hear the difference between a normal and a "hifi" power cord either.

GP.
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Old 8th June 2002, 07:59 AM   #3
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Default Theys a mighty fine tube ya got thar boy

I like 6BQ5s, Very sweet tube. I always run them in class A PP.

<b>1) In a push pull class AB circuit, the plate to plate resistance off the data sheet shows 8K. What effect will using a plate to plate of 7.6K have in the long run? Is the 400 ohm difference close enough not to worry about?</b>

None and yes.
The Dyna ST35 and SCA35 (basically the same amp stage) used a transformer with a 7k6 primary. There were a bazillion of these made, and heaps are still working fine today. There are many Heath and Fisher amps amongst others that use similar transformers too.

A transformer is an impedance converter amongst other things. The primary impedance (Zp) is determined by multiplying the secondary (Zs) by the square of the turns ratio (TR). In this case, you have a 30.8:1 turns ratio Zp/Zs=(30.8)^2. Voltage and current are inversely and directly proportional to the turns ratio, respectively.
Vs=Vp/TR and Is=Ip x TR
Power is the same on both sides accounting for losses in the transformer.

If your speaker were 8 ohms flat, then the output tubes would always see 7k6 as a load. But say at port resonance of a BR box, you might have 30 ohms. Now the tubes are seeing 28460 ohms, and if they dipped to 5 ohms somewhere, they'd see 4743 ohms. A higher plate-to-plate load gives less power and distortion, and a lower one more. But it's not a direct relationship, especially in class AB. So 400 ohms isn't significant.

<b>2) In that same class the data sheet shows a max. plate voltage of 300 volts. The curcuit I am using has a B+ of 335 volts and I have an RCA circuit with a B+ of 355 volts. How can this be, and how high above the rating can one go? </b>

If you exceed the max ratings the tube life is greatly reduced. I suggest not to do it. Best compromise in my experience between tube life and sound is around 80% of max plate dissipation.

The 7189(A) is usually directly substitutable into the circuit and has a max plate V of 440V, and 13W dissipation. Try some of them. A matched quad of Sovteks is $US50 from Triode.

Feel free to ask more questions or for clarification.

<b>I need a gentile nod</b>

I'm a gentile. Consider yourself nodded.
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