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Old 31st December 2010, 09:39 AM   #1
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Default tube CCS below cathodes, startup issues?

Hi there,

just some thoughts that came up last night...

I am planning to put a pentode ccs powered by a negative rail below the cathodes of a LTP. What I am now wondering about is what will happen if negative HT comes up earlier than B+ on startup.

Without B+, the cathodes of the LTP will allow no current, so if the ccs tries to sink the set current, will it pull them far into negative potential? This might cause problems with max cathode/heater voltage in the LTP.

Hope you understand my point, if not I will attach a simple drawing later...

Greetings,
Andreas
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Old 31st December 2010, 09:55 AM   #2
Doz is offline Doz  United Kingdom
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Andreas,

Is it likely the B+ will come up after the tubes are warm?
A cold tube won't conduct anything anyway.
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Old 31st December 2010, 10:11 AM   #3
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Hi Doz,

both rails will use tube rectification, so it depends on the characteristics of the GZ34 (B+) and EZ81 (HT-) which rail is up first...

Greetings,
Andreas
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Old 31st December 2010, 04:36 PM   #4
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Quote:
Originally Posted by Rundmaus View Post
I am planning to put a pentode ccs powered by a negative rail below the cathodes of a LTP. What I am now wondering about is what will happen if negative HT comes up earlier than B+ on startup.

Without B+, the cathodes of the LTP will allow no current, so if the ccs tries to sink the set current, will it pull them far into negative potential? This might cause problems with max cathode/heater voltage in the LTP.
You won't have problems with the LTP, IME, as those tubes will not be able to conduct until the CCS can. OTOH you may well have issues with startup with the CCS itself.

A pentode might manage CCS duties quite well. Can we see a schematic?
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Old 31st December 2010, 06:25 PM   #5
piano3 is offline piano3  United Kingdom
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I use pentode CCS on my balanced linestage; assuming you have the pentode screen directly connected to ground then if the pentode conducted before the LTP the screen would pass the entire programmed current which might destroy it.
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Old 31st December 2010, 07:46 PM   #6
tomchr is offline tomchr  United States
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If the CCS starts up first, it will pull the cathodes of the LTP down as far as it can. If this violates the Vhk spec of the LTP tubes, you should probably deal with it.

If the LTP inputs are expected to swing a few volts about ground, you could conceivably connect a zener diode between the cathodes of the LTP tubes and ground. This zener diode would need to be off during normal operation but turn on when the Vhk approaches the breakdown voltage. With this setup, the zener will conduct the current from the CCS during start-up. Then as the LTP "wakes up" they'll start taking over that current. You may have to use a diode or a second zener in series with the first to allow the cathode to swing positive with respect to ground.

Makes sense?

~Tom
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Old 31st December 2010, 08:18 PM   #7
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You could use a Neon-bulb...

Some strike at around 60-70V, so will keep Vh/k under control....
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Old 1st January 2011, 11:33 AM   #8
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Quote:
Originally Posted by tomchr View Post
If the CCS starts up first, it will pull the cathodes of the LTP down as far as it can. If this violates the Vhk spec of the LTP tubes, you should probably deal with it.
~Tom
Thanks Tom,

you confirmed what I already suspected. Luckily, discovering errors and problems in the design phase is cheaper than finding them by sparcs and smoke *g*

Right now, I do not see a problem if the situation is the other way round: If the CCS comes up late, the LTP cathodes are floating and unable to conduct until the CCS sets in - but this should not lead to any undesired effects so far?

Am I right in that?

Greetings,
Andreas
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Old 1st January 2011, 12:30 PM   #9
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hey-Hey!!!,
The EZ81 will beat the GZ34 to conduction. There is another issue you have not yet seen; if the LTP conducts first it could take your main B+ filter negative. Tie a zener anode to ground and its cathode to the LTP cathodes. Make its Zener voltage about double what you expect the LTP bias voltage to be. IFF you've got film caps to filter B+ then this efect does not matter one bit.
cheers,
Douglas
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Old 4th January 2011, 05:57 PM   #10
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Oh oh,

the issue seems not to be solved yet - if I summarize the above answers, it comes to the following:

a) If negative rail and CCS is up before B+, it will pull the LTP cathodes negative, probably violating V_hk of the ECC88

b) If, in addition, the (preheated) LTP is able to conduct before the B+ is up, it will probably pull even the B+ rail negative, blowing my electrolytic filter caps

Argh.

Solutions:

The Zener diode seems the simplest and best solution, but it violates my design rule to do completely without semis in this project.

Ideas:

1) Turn on negative rail manually after B+ is up.
+ avoids situations a) and b) if used correctly
- leaves the LTP cathodes floating for long times (Is this a problem?)
- no protection against B+ failure during operation -> catastrophic failure possible

2) ground LTP cathodes via resistor, open ground connection after B+ is up (Relais)

+ if CCS is up before B+, programmed current can be pulled from grounded resistor, pulling cathodes only slightly negative
+ protects against a) and b) if B+ fails unter operation
- ?

Question is I) will the LTP be in a safe state with the resistor to ground and II) will the thump wenn opening the ground connection kill my ears, speakers or both?

Desperately hoping for comments,
Andreas
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