cathode switching for power tube selection?

[krash]

Hey guys-

I'm considering building an amp with switchable power stage. I want to be able to switch between two pairs of power tubes, either A or B (but never both). The amp is cathode-biased. I presume I can just use a SPDT switch to switch ground to either one tube pair's cathode R or the other, and this will work completely, right? Any gotchas to look out for? Not looking for on the fly switching (power down or standby switching only).

I guess as long as I am using an 8K pri. OT and both tube pairs are a match for 8K load, then this will work ... primary impedance is correct since only one pair of tubes at a time will draw plate current.

[Andy Le Blanc]

there was this trick to reduce the volume of a fender twin by pulling
out half the tubes... a lot of players did this... without paying attention
to the issue of effective impedance matching....without serious distortion
and without serious detriment to the amp.... I've found good reffences
that things can vary 2:1 on either side of the output tranny without serious problems...so your good there.......
and I've seen several examples where tubes are switched to reduce watts or fused as a safety at the cathodes...
so your good there too....but plan ahead for worst case scenarios...
hot switching and switch failure.... plan to run both pairs at the same time
so if the your switch shorts you wont fry the amp and you know if you take the amp out somebodies gonna try the switch

[krash]

well as far as the pulling tubes Fender Twin impedance thing ... if you pull one tube on a P-P amp (which only has two tubes), you are not changing the amount of the OT primary that the one tube uses for a load. it still only uses half.

but i know what you are saying. a 2:1 mismatch is no big deal. and in the amp i am talking about, running both tube sets is also no big deal.

just making sure i was not overlooking something. this will be a one-off personal variation of a Revelation design (let's call it a prototype) with a hidden switch.

see ya-

[Structo]

I've always heard on amps with four power tubes to either pull the two inside or the two outside ones.
That doing so halves the impedance so if you normally run a 8 ohm speaker that you should set the amp to 4 ohms.

[Andy Le Blanc]

I think it was the other way round.....
pull a pair and you had to double the spk imp to end
up with the same effective impedance...

with the cathode switch Id double check if the switch was
break before make...or make before break.....
if your in standby, there still might some voltage soak left
in the power supply caps to make a pop before it bleeds off...
you might indvertantly connecting both pairs for an instant
without wanting...

[gearhead]

with the cathode switch Id double check if the switch was
break before make...or make before break.....

Also referred to as shorting (make before break) and non-shorting (break before make)

[krash]

well this is not a problem with this particular scenario (switching open, short etc) because I am switching cathode R's to ground, and each pair of tubes have their own cathode R. There will be no damage or ill effects if both tube pairs are on.

[davent]

Hi krash,
I did something similar in an 18watter using el84s and 6v6s. (A few threads over at 18watt on doing this.) It was suggested that this type of setup could lead to cathode stripping in the off pair of tubes so I put a 10k resistor (as was suggested) between each pair of bias resistors/caps and ground so the switch then shorts out the 10k resistor for the on pair. This leaves the off pair marginally on tied to ground through it's 10k resistor. Has worked great since I finished the amp in July.

Take care
dave

[krash]

dave, that's a great idea. I think I'll do that.

Thx-

[austinb]

I tried that on a marshall and it didn't work out. You have to disconnect the screen grid also.

[jhaas]

Dave,

Regarding your switchable 6V6/EL84 18 Watter - how much difference is there in the two modes, in practicality? In your opinion does it add a lot of flexibility to the amp to have both available?

I've built EL84-based 18 Watt Amps and would like to try a 6V6 variant next. I'm just trying to determine if it's worth adding complexity to the amp to make it switchable. I like to keep things simple when I can, so I'm interested in hearing your assessment.

[Zippy]

Ever looked at this???

http://schematicheaven.com/boogieamps/b ... eangel.pdf

[Zippy]

Ever looked at this???

http://schematicheaven.com/boogieamps/b ... eangel.pdf

[krash]

well, for a 4x el84 amp I am currently building, in which the customer requested I put a half-power switch, I'm using the 10K resistor method.

but I still have not finished building the one with the two different power tube sets.

[davent]

Dave,

Regarding your switchable 6V6/EL84 18 Watter - how much difference is there in the two modes, in practicality? In your opinion does it add a lot of flexibility to the amp to have both available?

I've built EL84-based 18 Watt Amps and would like to try a 6V6 variant next. I'm just trying to determine if it's worth adding complexity to the amp to make it switchable. I like to keep things simple when I can, so I'm interested in hearing your assessment.

Hello,
Up front, this is my first real tube amp so my ears have very limited experience, never played through el84's or 6v6's before but can now compare with the flip of switch.

It really doesn't have to add a lot of complexity to the amp although the way I went about it certainly did. I seem to always fall into the trap of moving from simple to complicated. For simple, you would just add another pair of power tube sockets and associated components (grid stoppers, screen resistors and cathode bias resistor/cap) a spdt panel switch, a pair of 10k/5w resistor and you're in business. From there you can make it as complicated as you want.

As to flexibility... well they are different... flip the switch, the 6v6 mode is less crunchy, less bright, I suppose smoother, I like it, lots!. For the 6v6 mode the PI hasn't been compensated to up the drive to the power tubes but the amp is power scaled so i can dial back the voltages in the power amp and dial up the drive with the PPIMV to drive the tubes harder if i want. The amp does seem bright in general and i usually play using the 6v6's, (and i have lots of 6v6's on hand to mess with.) That said, the el84's sound great too, different days, different moods, different guitars. I'm afraid not very helpful.

The schematic shows switching between two single output tubes but it would be set up the the same way for two pairs of output tubes.

dave