tube CCS below cathodes, startup issues?

Rundmaus · 31st December 2010, 09:39 AM

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

Doz · 31st December 2010, 09:55 AM

Andreas,

Is it likely the B+ will come up after the tubes are warm?
A cold tube won't conduct anything anyway.

Rundmaus · 31st December 2010, 10:11 AM

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

atmasphere · 31st December 2010, 04:36 PM

Quote:

Originally Posted by Rundmaus

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?

piano3 · 31st December 2010, 06:25 PM

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.

tomchr · 31st December 2010, 07:46 PM

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

Alastair E · 31st December 2010, 08:18 PM

You could use a Neon-bulb...

Some strike at around 60-70V, so will keep Vh/k under control....

Rundmaus · 1st January 2011, 11:33 AM

Quote:

Originally Posted by tomchr

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

Bandersnatch · 1st January 2011, 12:30 PM

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

Rundmaus · 4th January 2011, 05:57 PM

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

31st December 2010, 09:39 AM	#1
Rundmaus diyAudio Member Join Date: Aug 2005	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

1st January 2011, 12:30 PM	#9
Bandersnatch diyAudio Member Join Date: Jul 2003 Location: Ann Arbor, MI	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 __________________ the Tnuctipun will return

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31st December 2010, 09:55 AM	#2
Doz diyAudio Member Join Date: Jul 2010 Location: Sat Down	Andreas, Is it likely the B+ will come up after the tubes are warm? A cold tube won't conduct anything anyway.

31st December 2010, 10:11 AM	#3
Rundmaus diyAudio Member Join Date: Aug 2005	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

31st December 2010, 06:25 PM	#5
piano3 diyAudio Member Join Date: Aug 2010 Location: London,UK	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.

31st December 2010, 07:46 PM	#6
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	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

31st December 2010, 08:18 PM	#7
Alastair E diyAudio Member Join Date: Feb 2004 Location: South Wales	You could use a Neon-bulb... Some strike at around 60-70V, so will keep Vh/k under control....

4th January 2011, 05:57 PM	#10
Rundmaus diyAudio Member Join Date: Aug 2005	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