Sparking Tubes!

[jaymanaa]

Hi, nothing I have read to this point says anything about sparking inside tubes. I am currently using a modified Dyna 70 with EL 34's. What happens is if I turn the preamp off (a HK concerto) first and then the power amp the tubes will sparkle inside. If I turn the Dyna off first it takes a long time for the music to slowly fade out and the tubes just make the normal cooling down klinking sound. The tubes are biased at 25ma and everything works fine as for as making music. I just don't want to ruin anything. Is there a best way or sequence to operate this setup? Thanks in advance for helping out a newbie.

 

[kathodyne]

[if I turn the preamp off (a HK concerto) first and then the power amp the tubes will sparkle inside.]

so the El34's sparkle??? only when the pre-amp is turned off??
do they do that also without the pre-amp connected????

what do you measure at the dyna's input when you turn off the pre-amp???

 

[Tube_Dude]

Hi jaymanaa

A pre and amp separate combo must be :

- at turn on , the pre turned on first .

-at turn off , the power amp turned off first.

Cheers

 

[kevinkr]

The concerto is most likely generating a small dc pulse at turn off - if this is the case would be the cause for the internal arcing you are seeing in your output tubes.

The proper sequence as indicated in another post is to turn on the pre-amplifier and then the amplifier.

Turn off the amplifier, wait a moment, then turn off the pre-amplifier.

Depending on power consumption you might want to leave the pre-amplifier on most of the time.

The arcing you observed can cause serious damage to your EL34's, and may ultimately result in internal tube shorts.

25mA per tube seems low for any st-70 modded or not. Should be more like 50mA per tube.

Out of curiosity who made the mods in your amplifier, you might want to talk to them about this issue as well.

Kevin

 

[jaymanaa]

thanks everyone, The mods are as follows Kevin, from a circuit published in VTL The output stage is grid biased and set at25ma per tube. The B+ is over 500 VDC due to solid state rectification.The input and driver stages are mil.spec 12AT7's, two per channel. The phase splitter/driver is a long tailed pair The input voltage amp is a paralled 12AT7. Each channel has a control inside the chassi to balance the phase splitter outputs for min. distortion.Each output tube has a seperate bias pot and test point. The power supply is DC for the 12AT7 heaters. B+ is heavily filtered, all electrolytic caps are bypassed with film caps to improve high freq. Supplies are seperate for each channel. With el34's tested with an HP signal gen, HP RMS voltmeter and HP331 distotrtion analyzer it puts out 50 watts per channel. Thats el34's in ultralinear mode. KT 90,s were good for 60 per channel. Due to OPT limitations max power at 20hz is about 45 before magnetic saturation sets in, however full power is available at 25hz and up. These mods were made by the guy I bought it from who suggested the 250mv (25ma) bias to make the el34s last a long time. I'm wanting to try experimenting with different output tubes, any suggestions? This amp drives my Heresys very nicely with tons of bass. I am however looking into building a pair of 300B's. Thanks Again and any advice is greatly appreciated!