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UL/Pentode/Triode Switch

Joe Willis

Joe Willis

Active Member
Hey,
If I wanted to add a switch to go between ultraliniar pentode and triode mode on a 6bq5 do I have to worry about the cathode bias resistor value?

I'm pretty sure if I switch between ul and pentode the bias won't change substantially but If i go into triode it might.

Currently the tube is cathode biased with a 270R resistor and idling at 38ma.
 
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I never did that with 6bq5. But what we did (modders) to guitar amps was first find the values of the resistors when switching. but we used grid bias as this is the standard method in guitar amps so we tweaked the grid bias to each mode.

for your situation since you're cathode bias, find the 3 circuits with your current B+ and switch your components out using a ganged rotary switch. Remember to put the amp in standby or turn off before switching modes or damages to speakers might occur.
 
Idle current won't change much, as the screen voltage is almost exactly the same (actually drops a little in triode, as there is the additional drop in the output transformer winding.
 
So Tom, I should be able to just switch without being concerned with the cathodes? Great! that's what I was hoping.
Joe
 
It Depends. (remember screen voltage has more to do with current then plate voltage)

True textbook Pentode operation which runs the screens voltage at something generally
substantially lower then the plate, would change bias substantially if switched to Triode.

So you need to look at any given design an see what the screen voltage is compared to
the plate, if there basically the same voltage then you can switch between any mode yu
want and get the same basic bias/current.

But if going from pentode with a lower screen voltage to triode or ul with a higher screen
voltage then current will raise if you keep the cathode resistor or - grid voltage the same.
(so be careful when going from Pentode to Triode and or UL, check screen voltage)
 
Hiya,

Would it not make better sense to not use a switch but a jumper type setup to switch modes.

Seems like a actual switch could be a problem but I am not totally schooled on exactly what would be getting tied to what when switching.

Frannie
 
hey-Hey!!!,
given the voltage involved, the switch is has to have some pretty substantial ratings. Usually the safety factor applied to such devices allows you to get away with murder. For an EL84 amp, a 240VAC switch should cover you adequately. For higher power amps, the AC across the switch can run to several hundred volts, and that is while simultaneously having to deal with the DC applied by B+.
cheers,
Douglas
 
I kind of like the jumper idea. that would prevent random users from throwing a switch while the amp was running. I was initially thinking that the switch wouldn't have to be rated as high of a voltage because it would only be switching when powered off as long as the insulation and current ratings were appropriate, but a jumper would add an additional safety/idiot factor protection. not to mention it would be easier to wire a jumper circuit than a switch circuit if cathode resistor changes will be made. I'll have to take some voltage readings and see what needs to be adjusted when making the operating mode changes. Its going to be a while because I ordered the transformers from edcor so they won't be here for a month or two. :(

Joe
 
Quote
Tubes4hifi sells the switches rated for the given voltage, they sell them for the dynaco amps, I think this is what you are wanting to do.


The common switch is only rated at 250 volts. Tubes4hifi is not selling anything different so caution should be exerted here. The switch is usable as long as you do not switch it on the fly.
 
Use a "normal" switch and put it under the chassis. Only throw it when the amp is off. The switch won't be under near as much stress as if it were to be thrown under load.
 
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