http://nervenet.info/HT_delay/

 

I bought 2 of these to delay the HV startup of the megatron, I'll let you guys know how they turn out. Seems like a pretty nice drop-in

Just an idea since your circuit is already 555 based .... how about feeding the heaters from a CCS  ....and have the 555 coupled as a Schmidt Trigger ...starting the HV when ever the voltage drop across the ´heater is less than some voltage?

Then the heaters will not pull excessive current and warm up gently ...and when ever warmed up the 555 will trigger the HV...

Edited July 1, 201511 yr by sorenb

Looks like a cool part and something that should be easy to retrofit into an existing build.  I'll be curious to hear your experiences.

for megatron this may not be a great idea because

the boards will trigger at slightly different times causing

a huge output bump into the headphones if they are connected.

duly noted. I guess I need to implement the HV mains delay then? without the delay there's some swishing sound that takes around 5 minutes to settle out

 ....is it a bad Idea to feed the heaters from CCS and trigger on the voltage across the heaters? ....I measured the voltage drop across the heaters in the KGST ....(I have .47 ohm in series) it jumped right up at 1,6V and then slowly dropped to 0,5V and stays there ...and it seems to translate into the outputs swinging up to +rail ...and then begins to drop to zero after a while

I guess feeding the tubes from a CCS feeding 1amp will make the voltage across the heaters go from ~2V to ~ 6V .... then use a simple latch that triggers at ~6V to turn on the HV 

 

if several heaters of different tubes are at play one might use a master trigger controlled by some AND-gates making sure that all heaters has reached working state

 

....is it just a bad idea from some reason?

duly noted. I guess I need to implement the HV mains delay then? without the delay there's some swishing sound that takes around 5 minutes to settle out

does it take five minutes for the heaters to reach a steady state? 

You could try this. 2 holes to secure the base. You could select the module for the timing range and the actual time can be set manually.

http://www.ebay.com.sg/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=h3y-2&_sacat=0

They come in ac or dc. You will need the ac for mains delay.

Edited July 1, 201511 yr by Victor Chew

does it take five minutes for the heaters to reach a steady state? 

that's the implication but it could just be the effects of the startup sequence. I don't think others with the Megatron are delaying the HV by 5 minutes

Edited July 1, 201511 yr by nopants

that's the implication but it could just be the effects of the startup sequence. I don't think others with the Megatron are delaying the HV by 5 minutes

Just had a go at the KGST ....it swings to +rails at turn on, after 8-10s it drops quickly to some 20-40V ....and reaching 0V in additional 10s

have you measured the behavior of your (magnificent Megaton btw)  from startup until it settles?

thanks, waiting on the knob to complete everything.

my balance doesn't go over 40 and levels to -13

offset is similar level but I didn't measure right at power on

for the megatron, the two tubes are in series and warm up together

(well close) so there is not going to be a big bump

You could try this. 2 holes to secure the base. You could select the module for the timing range and the actual time can be set manually.

http://www.ebay.com.sg/sch/i.html?_from=R40&_trksid=m570.l1313&_nkw=h3y-2&_sacat=0

They come in ac or dc. You will need the ac for mains delay.

m

do they accept mains by default, independent of the dc rating?

So I have a similar issue with my SRX Plus, which uses a hybrid tube/solid state rectifier bridge.  The positive side is done by the tube and the negative side is solid state with a delay and soft start circuit to roughly mimic the rectifier tube turn-on time.  The output voltages go though some gyrations on turn- on, however the voltages on each channel track each other closely so there isn't a big differential voltage (which is what the headphones see as signal), and there isn't a big bang on turn-on - the gyration is subsonic anyway.

 

Tube filaments generally warm up in 10-30 seconds so that degree of delay should be enough - tubers actually reach steady state faster than transistors, which turn on quickly but then take awhile to heat up their heatsinks to operating  temperature.  That is exactly what sorenb is seeing with is KGST.  When it turns on, the output is connected to the positive rail through the current sources, but the output tubes aren't conducting because the filament is cold.  Once the filament warms up enough the output tubes start conducting and the output voltage drops and after another 10 seconds the tube is fully operational and the output drops to zero.  If you disconnect the servo, you can see that there is a relatively fast voltage change as the tubes warm up, followed by a slow drift as the solid state heatsinks reach operating temperature.

 

Now, if you don't want to be worried about voltage gyrations at turn-on, one way to do it is to have all the filament supplies on one transformer which turns on immediately, and the HV supply from a second transformer with an AC delay so it doesn't turn on until the filaments warm up, then all the high voltages turn on at the same time.  Does need two transformers, though.  Stax did it manually with two power switches in the T1S, one for heater warm-up and one for HV, which was labeled as the power switch.

I don't really see why it shouldn't work feeding with a CSS (that should even protect the tubes) and trigger on the voltage across the tube heater ... rather than using a timer delay???

if you're talking about the megatron it's because of the "auto" biasing for the output stage

I don't think there is anything wrong with the idea of feeding the filaments with a constant current source and using the voltage generated across the filaments to trigger the HV - actually as the filaments heat up their resistance increases, and hence the voltage across them increases, so the idea would be to trigger the HV when the filament voltage rises to a certain value.  But, it is probably significantly more complicated to implement than a simple time delay, for which reliable, time-tested circuits are already available.  Why go with a more complex method when a simple one does the job, aka KISS?

the real kiss would probably be a separate switch

Just had a go at the KGST ....it swings to +rails at turn on, after 8-10s it drops quickly to some 20-40V ....and reaching 0V in additional 10s

have you measured the behavior of your (magnificent Megaton btw) from startup until it settles?

.

The 6S4A used in the KGST has a controlled delayed warmup for the heater. The A revision was released with this engineered in, to lower the turn on power surge in TVs.

Should be kept in mind when considered as a reference for conduction in other tubes/circuits.

m

do they accept mains by default, independent of the dc rating?

Yes, its just an ac timer. Wire the mains to it and when the time is up it turns on automatically. Eggil, Ristar and myself are using it.

http://www.ebay.com/itm/1Set-AC-DC-H3Y-2-DPDT-Power-on-Time-Delay-Relay-PYF08A-Base-110V-220V-12V-24V-/131001434359?pt=LH_DefaultDomain_0&var=&hash=item1e804b40f7

 

Wire both the heater transformer and the timer directly to the mains. The timer will delay the PSU power up, allowing the heaters to work first.  

nopants said:

 

"the real kiss would probably be a separate switch"

 

Yup, that's what the old Stax T1 had, a pre-heat switch and a power switch.  Now what you want with that system is an interlock so that the power switch won't turn on the HV unless the heater switch is on.  Since I've never seen a T1 in the flesh I don't know if it had that built in or not, but it shouldn't be difficult to do, just have an AC relay on the primary of the HV transformer, in series with the power switch, that is controlled by the heater switch.

 

Laowei said:

 

"The 6S4A used in the KGST has a controlled delayed warmup for the heater. The A revision was released with this engineered in, to lower the turn on power surge in TVs."

 

Shows how important tubes were in TVs in the 50s - versions were designed specifically for them.  For example, the 6SN7GTA was redesigned to the GTB version which had the controlled warmup.  Actually I believe the other big reason for controlled warmup was so that the heaters could be wired in series, e.g. use a 24 volt filament supply and wire 4 6V heaters in series - only works if they all warm up at the same time, otherwise the voltage isn't distributed evenly between the filaments in the string.

Edited July 2, 201511 yr by JimL

On my Atmasphere OTL amp, there are two power switches. One powers the first stage tubes (6SN7) fully and only the filament of the output section tubes (6AS7). The second one supplies the B+ to the output section 6AS7. The procedure is to turn on the first switch and wait at least 60 seconds before turning on the second switch. The switches are wired such that the second switch is only effective when the first switch is on. Turning off the first switch cuts off the power to the second switch. Seems very similar to what JimL described about the T1.

It was implemented this way to preserve/extend the life of the 6AS7.

It's manual but it works very well for me.

Edited July 2, 201511 yr by mwl168

Another way to do it without a relay is have the filament switch with one set of contacts to turn on the heaters, and another set in series with the HV switch.

Yes, its just an ac timer. Wire the mains to it and when the time is up it turns on automatically. Eggil, Ristar and myself are using it.

As I understand the sheet I need to supply 12V to get it to switch properly. how are you running yours?

Edited July 9, 201511 yr by nopants