transformer question??

[guitarsnguns04]

I was reading about an amp with a 2xel84 output section with a switch on the back to go from 2xel84's to a single el84?? Is this a special tranny to do both push pull and single ended or is the amp probably just class a with a single ended tranny with the 84's in series rather than parallel?

[paulster]

If it's cathode biased then you can shut off the grid to one tube in a push-pull pair (i.e. ground it) or have a pot to dial it down to nothing, so you can dial in SE to PP (although that's Mesa's patent you'd be infringing ).

You can do conceptually the same with fixed bias but it's more complicated as you need to keep the DC bias to the grid or the tube will go into meltdown.

[guitarsnguns04]

can you explain to me what they are doing here? It says half power on the switch etc?

http://www.ceriatone.com/images/layoutP ... Brulee.jpg

[paulster]

That's a pentode/triode switch.

Basically you put a switch that selects whether the screens get their usual B+ screen grids supply or whether they are tied directly to the plates, which makes the tubes act as quasi triodes and just about halves the power. It does change the tone as well, and whether for better or for worse is up to you.

If you want to see it schematically, take a look at the Hi/Lo power switch on this:
http://www.drtube.com/schematics/marshall/2550amp.gif

[guitarsnguns04]

is see it now..thank you. So its called triode /pentode...high/low..or half power. I guess I was looking at what were are switching rather than tying together. Makes sense now..thanks again

[swarty]

If it's cathode biased then you can shut off the grid to one tube in a push-pull pair (i.e. ground it) or have a pot to dial it down to nothing, so you can dial in SE to PP (although that's Mesa's patent you'd be infringing ).

You can do conceptually the same with fixed bias but it's more complicated as you need to keep the DC bias to the grid or the tube will go into meltdown.

Seems like you could also doing it by having seperate cathode resistors and lifting one (maybe that is the way around that patent? ).

[Wayne]

Not quite - the way I understand it, the "unused" tube still has to pass DC current through it's side of the OT to keep it from saturating.

W

[guitarsnguns04]

im going to have to look at some more schematics on this topic. I just have to think of some amps that do this...mesa boogie simulclass maybe?

[rfgordon]

In Mesa Boogie simulclass amps, both sets of tubes are grid/fixed biased--they just have different bias points. I used to have a Mark IV, and, for the life of me, I couldn't hear much difference between them. I'm not sure how a two pairs of tubes in Class A--one pair cathode biased and the other grid biased--would compare in tone/feel.

[pinkmarkos]

the orange tiny terror also has a 2xel84 output with what they call a 7/15 watt switch. i have never seen an explanation as to how this is accomplished in this amp, but i can say that the 7 watt option gives a little more clean headroom with better low end response, while the 15 watt option is only slightly louder and slightly more mushy to my ear. this sounds like it could be eqivalent to a triode/pentode switch. not sure if this amp is SE or PP though, as orange has made small SE amps in the past (AD-5).

[billyz]

The schematics I have show a PT with two different secondary HV taps to accomplish the power switching from 7 to 15 watts.

[paulster]

I'm fairly sure the Tiny Terror has a switchable B+ taps on the PT to get the different power outputs. I did see a schematic for it once but I don't think I ever bothered to keep a copy.

[pinkmarkos]

ok- now i know how my TT works... another amp i have with 2xel84 output is the egnater rebel. this amp claims to have scalable power output from 1-20watt via either the el84's or 2x 6v6's or a blend of these. is this just a version of a VVR idea? i haven't seen a schematic, so i am curious how they achieve this feature.

[Voodoo_Man]

ok- now i know how my TT works... another amp i have with 2xel84 output is the egnater rebel. this amp claims to have scalable power output from 1-20watt via either the el84's or 2x 6v6's or a blend of these. is this just a version of a VVR idea? i haven't seen a schematic, so i am curious how they achieve this feature.

This is what Mr. Power Scaling (KOC) had to say about the Rebel.

"The Rebel "Watts" control is not Power Scaling. Rather it is a variable current resistor for the splitter, which has the usual flaw of being scratchy over some or all of its sweep." End quote.

Since the VVR appears to be a takeoff of Power Scaling, I would say "no" it's not eveven close. Although I could be in left field. (Or right field, depending of your perception.)

God Bless,

John


Don't shoot the messenger.

[pinkmarkos]

so if i am understanding this correctly... a decrease in wattage can be obtained by using a variable resistance from the PI to the output tubes. Or would it be the resistors that feed the PI? If P(watts)= V*V/R, as you increase resistance, your wattage will decrease, assuming current remains constant. which resistors would you replace with a variable resistor and what value variable resistor would be used? hmmm...