Switching Plate Load

[Colonel_Sanders]

Hello,

I'm a newbie doing my first design and have a question. If I was going to add a parallel plate load switch with a DPDT relay to the 5F6A bassman preamp (12AU7 + 12AX7), what voltage relay would I require? I think the B+ is about 325VDC and the voltage drop across Ra is about 175V. Do I need a relay that will cover the B+ plus spikes, or is there something magical I don't understand?

Thanks,
C_S

[tbryanh]

I assume you are using a DPDT switch because you plan to switch between two resistors. In other words, with the switch in one position, R1 will have both leads in the ciruit, and R2 will will have both leads out of the circuit. Vice versa for the other position of the switch.

You might consider leaving one resistor in the circuit at all times and use a SPST switch to add another resistor in parrallel when needed. I don't know if this would help with the voltage rating of the switch.

Switches I believe are rated at a specific maximum voltage they can interupt with a specific current flowing through the contacts. (I know contactors are rated this way.) This can be an AC or a DC rating, or it might have both types of ratings. Generally, AC ratings are meant fo 60 cycles. Not sure if higher frequencies matters.

There is also the issue of inductive and non-inductive loads, and I think switches can be rated for this too. (I know contactors are rated this way.)

Since your dealing with preamp tubes, the amount of current is probably not that much of an issue, and you don't have an inductive load, so you got that on your side too.

I bet a switch for the typical standby circuit of a power amp would work fine.

[Enzo]

Yes, tbr, the one permanent resistor with a parallel method will greatly reduce the voltage needs of the contacts. You would never be interrupting the voltage or current.

In fact a SPST will be all you need. Leave the parallel resistor connected at one end. Now switch the other end in or out.

[tbryanh]

Yeah, it would reduce it from the full B+ to just what is accross the plate resistor. Good point.

[Colonel_Sanders]

Hey guys, thanks for the replies.

I am planning to have one resistor in the DC path at all times (thanks to Kevin O'Connor!). The DPDT switch is to switch a parrallel Ra in on both input stages of the 5F6a design. Would it be better for me to use 2 SPST relays driven from the same control voltage?

Thats great news on the current and voltage requirements! I'm still a little new at this, but does that mean that I'll only need a relay with a maximum rating of 175VDC/90VAC (V drop across Ra)?

Cheers,
C_S

[Enzo]

Sorry, I thought you were using DPDT to use a switch at both ends of the part.

If you are switching two separate circuits at once, my only remaining concern would be the two plate circuits would then be in very close proximity - within the switch - but I don't think that is an issue with the plate loads. After all they are right next to each other on the eyelet boards.

[Colonel_Sanders]

No worries man. Thanks for the tip, I'll be extra extra careful with the soldering.

And the relay will only need to be half the voltage of the Ra V drop because the load is in parrallel huh?

Thanks again,
C_S

[tbryanh]

Actually if the the guitar is plugged in and the amp is cranked up, the voltage accross the plate resistor can be the full B+ if your unlucky and the contacts open when the tube is saturated.

If you first mute the amp and then switch, it would always be 1/2 B+.

[Enzo]

Do you really find typical 12AX7 preamp circuits with 300v p-p across the load resistor? I don't see nearly that much signal.

Switch and relay contacts are voltage rated more for what they can interrupt than for what flows through them. SO unless you are switching while cranking a max signal through the amp, the signal amplitude won't matter.

[Colonel_Sanders]

Enzo, thanks for the info. Makes the ratings heaps clearer.

I am planning this switch, and a myriad of others in the amp, for live use, so its entirely possible that the parrallel Ra will get switched in while the tube is in saturation. That means I WILL need a relay to potentially stop all the B+? Eep, confusing.

If anybody can recommend some reading material on the subject that'd be great as well.

Cheers,
C_S

[tbryanh]

Getting the relay might be a little tough.

Relays are used alot in guitar amps, but you see them mostly in the lower voltage areas around the cathodes and tone stacks, etc.

I am sure there is something out there. Please post what you find. You've got me a little curious now.

[Enzo]

What stage in the amp is this going to be used in? You can easily measure the amount of signal on the plate of such stage. The tube only has so much gain, so if it is the input stage, I might expect as much as 30v signal rather than 300.

[Colonel_Sanders]

Just the input stage, but I'll be using a 12AX7 instead. Attached a snippet of my mock up schematic.

The amp has 3 options, but I'll just run with 2 for now.

So, for V1a when 3a is closed, the AC30/5F6A JFET's are on, making:

Ra=104K
Rk=1.5K
Ck=25uF

V1a When 3a is open:

Ra=220K
Rk=1.5K
Ck=25uF

I'd be really interested in getting the formula to calculate the stage gains and p-p voltage swings.

Cheers,
C_S

[Enzo]

I don't think it really makes much difference, but you are aware that the JFETs do not go to zero when ON? SInce they are ON until turned OFF, you can hold one in your hand and measure the ON resistance - I'd expect maybe a couple hundred ohms.

[loudthud]

There are JFETs that are suitable for switching. The J105 comes to mind. The on resistance is something below 3 ohms. It takes about 9V to turn them off and the breakdown voltage is only 25V. Should work well in cathode circuits.