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Old 7th September 2012, 09:00 PM   #1
fotios is offline fotios  Greece
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Question Cathode preheating???

How much significant is the preheating of cathode before applying the called "B+" or HT on the plate of tube?
In my parallel 300B SET project, i've checked again and again all voltages and currents and i can confirm that are completely stabilized, at the same level allways, after 8 to 10 minutes from Power On. At the same time, i use a pair of cheap headphones connected in output to hear the noise. Indeed the noise dissapears after the 10 min voltage-current stabilization.
It is necessary to include a "8 minutes delay relay" to activate the HT rectification circuit after the filaments supply?
To be sincere i don't like to insert relays and switches into this "chaste" no-feedback single ended class A amplifier, wired at whole with UPOCC Silver wire. If i had the possibility i could not include neither the power-on switch.
I am a bit confused because i've read in a thread that the bombardment of a cold cathode with electrons can reduce the life of tube. But how much?
Thanks for any advice.
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Old 8th September 2012, 04:59 AM   #2
AJT is offline AJT  Philippines
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if under 1000volts B+, i'd say don't bother....but please look here, this topic has been discussed to death here.....

Lets settle the b+ on cold tubes issue!
To switch B+ or not?
B+ time delay circuit - your comments
NTC THERMISTOR as SOFT START for TUBES
about 83 mercury rectifier
Power-On Time Delay Circuit
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Old 8th September 2012, 04:11 PM   #3
fotios is offline fotios  Greece
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Quote:
Originally Posted by Tony View Post
Thank you Tony for the advice and the useful links. Fortunatelly max B+ is 500Vdc as much for all tubes of my SET amplifier (two parallel 300B/c in output, an EF86 at input and a 5687 as buffer between input - output tubes and a 5C3S rectifier) so no reason to... die reading endless threads.
BTW my lesson for today was: EF86 and 5687 include Heaters, while 300B and 5C3S include Filaments. Good spot! I have to study more frequently my "RCA Radiotron Handbook" purchased from e-bay in CD-R.
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Old 8th September 2012, 04:28 PM   #4
ChrisA is offline ChrisA  United States
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Quote:
Originally Posted by fotios View Post
How much significant is the preheating of cathode before applying the called "B+" or HT on the plate of tube?...
This was a big deal with kilowatt sized, tube based ham radio amplifiers but is a non-issue with audio tubes that work at voltage less than about 900V.

The real use of a "standby" switch on the B+ of old tube power amps was because the B+ would be very high before the tube started conducting. The high voltage would go over the rating of the filter caps. Years ago caps were very expensive. So you'd heat the tubes first and then switch on the B+ and the voltage would not be to high for the caps. But today we simply buy filter caps with the required voltage rating.
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Old 8th September 2012, 04:47 PM   #5
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As valves are becoming rarer and expensive, Snake Oil is relatively inexpensive.

In this case I would feed the heaters with an LM317 CCS using a slow start and apply the B+ a few seconds later. It costs pennies to achieve and will give you piece of mind.

I've attached the PSU that I used with my Borbelly Pre-Amp.

VERY TRUE that 60s and 70s amps didn't use or need any form of protection, but then valves were ten a penny.

If you take a normal light bulb, they rarely fail in operation, they nearly always "pop" when you turn them ON. Hence the slow start on the valve heaters.

Cathode stripping is SNAKE OIL, but its easy to implement a delay in applying B+ without compromising sound quality.
Attached Images
File Type: jpg Borbelly PSU.jpg (473.0 KB, 90 views)

Last edited by KatieandDad; 8th September 2012 at 04:51 PM.
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Old 8th September 2012, 06:44 PM   #6
fotios is offline fotios  Greece
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Quote:
Originally Posted by ChrisA View Post
This was a big deal with kilowatt sized, tube based ham radio amplifiers but is a non-issue with audio tubes that work at voltage less than about 900V.

The real use of a "standby" switch on the B+ of old tube power amps was because the B+ would be very high before the tube started conducting. The high voltage would go over the rating of the filter caps. Years ago caps were very expensive. So you'd heat the tubes first and then switch on the B+ and the voltage would not be to high for the caps. But today we simply buy filter caps with the required voltage rating.
Thank you so much - again - Chris for your advices. I realy feel a big obligation to you because you stood by me from the begging, i.e. in my other threads on tube forum. You know very well my SET project!
The filter caps i use in the HT supply are "Mundorf M-Lytic HV+" rated at 550Vdc. The rectification tube i use is a Russian 5C3S with solid carbon plates and is some slow to obtain the nominal anode voltage of 500Vdc, i've checked it just now with the scope. While the voltage for the 300B filaments is provided by DC regulated supplies which are very fast. I think this could be an additional protection for the HT filter caps.
Thank you again.
Fotis
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Old 8th September 2012, 07:03 PM   #7
fotios is offline fotios  Greece
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Quote:
Originally Posted by KatieandDad View Post
As valves are becoming rarer and expensive, Snake Oil is relatively inexpensive.

In this case I would feed the heaters with an LM317 CCS using a slow start and apply the B+ a few seconds later. It costs pennies to achieve and will give you piece of mind.

I've attached the PSU that I used with my Borbelly Pre-Amp.

VERY TRUE that 60s and 70s amps didn't use or need any form of protection, but then valves were ten a penny.

If you take a normal light bulb, they rarely fail in operation, they nearly always "pop" when you turn them ON. Hence the slow start on the valve heaters.

Cathode stripping is SNAKE OIL, but its easy to implement a delay in applying B+ without compromising sound quality.
Thank you so much "KatieandDad" (has your user name any relation with this beautiful puppy on your avatar? ) for your informations. I realy appreciate very much your attention in my query. Very smart your expression SNAKE OIL for cathode stripping! Will take into account.
BTW thank you for the schematic, i have in my plans to bulid and a preamplifier for the system. Still I haven't decide for the circuit and tubes. If you have the kindness can you drop me a link to take a look on your Borbely preamplifier schematic? I realy appreciate very much Erno Borbely work.
Thank you again
Fotis
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Last edited by fotios; 8th September 2012 at 07:09 PM.
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Old 9th September 2012, 03:49 PM   #8
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I've still got PCB layouts and schematics for the Borbelly.

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