On/Off switching for preamp tubes

[Kai]

Hi guys,

I just posted another thread concerning power tubes. In this thread I would like to raise a similar problem.

I built all LP preamps during the last years. I have a standard layout with one PSU-PRE and two LP preamp boards in one rack unit. For example there is my "British preamp" rack unit with M-PRE and V-PRE. With a simple SPDT I switch between outputs of M-PRE and V-PRE. The SPDT is connected to the main preamp output jack. 

Most of the time I only use either Marshall or Vox sounds while I'm playing. This means, one preamp board including heat dissipating preamp tubes is permanently turned on but not used.

Is there any mechanical switching that would turn off the B+ plate supply and the heater feed at a time between PSU-PRE and for ex. M-PRE respectively V-PRE? Maybe anything better?

Unfortunately, most DPDT switches can only handle DC voltage up to 250V. B+ plate supply usually is much higher. The heater feed can easily be broken. It is 12 V AC from the power supply.

Can anybody help me?

Best wishes 
Kai

[makinrose]

Hi guys,

I just posted another thread concerning power tubes. In this thread I would like to raise a similar problem.

I built all LP preamps during the last years. I have a standard layout with one PSU-PRE and two LP preamp boards in one rack unit. For example there is my "British preamp" rack unit with M-PRE and V-PRE. With a simple SPDT I switch between outputs of M-PRE and V-PRE. The SPDT is connected to the main preamp output jack. 

Most of the time I only use either Marshall or Vox sounds while I'm playing. This means, one preamp board including heat dissipating preamp tubes is permanently turned on but not used.

Is there any mechanical switching that would turn off the B+ plate supply and the heater feed at a time between PSU-PRE and for ex. M-PRE respectively V-PRE? Maybe anything better?

Unfortunately, most DPDT switches can only handle DC voltage up to 250V. B+ plate supply usually is much higher. The heater feed can easily be broken. It is 12 V AC from the power supply.

Can anybody help me?

Best wishes 
Kai

Kai I'm not certain that what you want is necessary.  Running a pre-amp tube at idle will have negatable wear. I think that's less trouble than the expense and hassle designing and implementing a switching system to turn it off .  Pretty much any multi-channel or amp with effects amp has tubes that are not in use some of the time.  For example the tremolo is rarely used in many amps but no designers have found it important to turn the tube off when not in use.  I hope that helps!

[Kai]

Hi guys,

I just posted another thread concerning power tubes. In this thread I would like to raise a similar problem.

I built all LP preamps during the last years. I have a standard layout with one PSU-PRE and two LP preamp boards in one rack unit. For example there is my "British preamp" rack unit with M-PRE and V-PRE. With a simple SPDT I switch between outputs of M-PRE and V-PRE. The SPDT is connected to the main preamp output jack. 

Most of the time I only use either Marshall or Vox sounds while I'm playing. This means, one preamp board including heat dissipating preamp tubes is permanently turned on but not used.

Is there any mechanical switching that would turn off the B+ plate supply and the heater feed at a time between PSU-PRE and for ex. M-PRE respectively V-PRE? Maybe anything better?

Unfortunately, most DPDT switches can only handle DC voltage up to 250V. B+ plate supply usually is much higher. The heater feed can easily be broken. It is 12 V AC from the power supply.

Can anybody help me?

Best wishes 
Kai

Kai I'm not certain that what you want is necessary.  Running a pre-amp tube at idle will have negatable wear. I think that's less trouble  than  the expense and hassle designing and implementing a switching system to turn it off .  Pretty much any multi-channel or amp with effects amp has tubes that are not in use some of the time.  For example the tremolo is rarely used in many amps but no designers have found it important to turn the tube off when not in use.  I hope that helps!

[K O'Connor]

Hi Guys

Subminiature switches such as the C&K 7000-series and equivalent from Carling easily handle the voltages in question. If the tubes have not been made externally accessible, then the switch for the heaters and plate is functionally useful as in Kai's situation. A DPDT is sufficient.

DPDTI
C&K 7201SYZQE
Carling 2M1-DP1-T1-B1-M1QE

Have fun

[makinrose]

Hi guys,

I just posted another thread concerning power tubes. In this thread I would like to raise a similar problem.

I built all LP preamps during the last years. I have a standard layout with one PSU-PRE and two LP preamp boards in one rack unit. For example there is my "British preamp" rack unit with M-PRE and V-PRE. With a simple SPDT I switch between outputs of M-PRE and V-PRE. The SPDT is connected to the main preamp output jack. 

Most of the time I only use either Marshall or Vox sounds while I'm playing. This means, one preamp board including heat dissipating preamp tubes is permanently turned on but not used.

Is there any mechanical switching that would turn off the B+ plate supply and the heater feed at a time between PSU-PRE and for ex. M-PRE respectively V-PRE? Maybe anything better?

Unfortunately, most DPDT switches can only handle DC voltage up to 250V. B+ plate supply usually is much higher. The heater feed can easily be broken. It is 12 V AC from the power supply.

Can anybody help me?

Best wishes 
Kai

Kai I'm not certain that what you want is necessary.  Running a pre-amp tube at idle will have negatable wear. I think that's less trouble  than  the expense and hassle designing and implementing a switching system to turn it off .  Pretty much any multi-channel or amp with effects amp has tubes that are not in use some of the time.  For example the tremolo is rarely used in many amps but no designers have found it important to turn the tube off when not in use.  I hope that helps!

I think I can tackle that one too.  I don't think you can easily find relays that will handle that much voltage.  There probably are some made....all that said a standard DPDT is rated for 250VAC not DC so it is safe to use for DC plate voltage.  Just be sure to use a large body high quality switch like a Carling found in many amps.  Standby switches in amps that switch on and off the B+ are same type.  You may want to position a .1UF plastic or ceramic cap across the B+ contacts to reduce wear on the switch.   Another alternative is to use the cathode connection you described but it will generate idle heat.  You could install a fan to keep things cool too.  That would extend the life of everything in the amp if it's getting toasty in there.  I hope that helps!

[K O'Connor]

Hi Guys

Most relays will work reliably in this situation and have sufficient ratings to do so. You have to look at the entire data sheet, not just the bullet points.Look specifically at the insulation ratings.Relays with footprints the size of a DIP (IC package) usually have good ratings. Even the smaller IM-series at 0.2" width (5mm) has sufficient ratings for this app.

Have fun

[Kai]

Hi Kevin,

you say subminiature switches such as the C&K 7000-series and equivalent from Carling easily handle the voltages in question.

I do not really understand how to calculate VDC ratings from VAC ratings on these switches. For ex. switches have usually ratings such as 250VAC.

According to TOT p. 3-2 the peak VDC as I understand is usually VAC times 1.414. That means it is much higher.

How do I know if a switch can handle more than 300VDC? I have a switch that shows 3A 250VAC. What is the maths behind it?

Best wishes
Kai

[K O'Connor]

Hi Kai

The switch ratings are exactly the same as the relay contact ratings inasmuch as you have to look at the actual data sheet and see the insulation rating. Real distributors will have links to the data sheets for the components.

Have fun
Kevin

[Kai]

Hi Kevin,

the Carling 2M1-DP1-T1-B1-M1QE data sheet lists "Dielectric Strength 1500 Volts RMS".
The C&K 7201SYZQE lists "DIELECTRIC STRENGTH: 1000 Vrms min.".


I suppose dielectric strength means the same as insulation rating. Is this correct?

Best wishes
Kai

[Sherlok Ohms]

Greetings Kai

Yes, those two terms are the same. "Dielectric" is just a twenty pound word for "insulation", as they say.

Cheerio

[Kai]

Thanx for the reply, Sherlok.

Best wishes
Kai