Dual OT's for 6V6

[Ed_Chambley]

Is it possible to use 2 output transformers in a mono design for the purpose of half power.  Directly stated, 4 tubes and each pair having its own OT at 8K.  If so, can you simply switch out one of them when it is desirable to go to half power.

I know there is a less expensive way, but I have a load of OT's already.

For instance, if you had a similar amp to a Princeton with a PT sufficient current , but you wanted to run 4 6v6's in it, but keep 8K on each pair.

[eleventeen]

Don't see why not. Depends, of course, on exactly what you mean "simply" switch out. Cut the B+ to the center tap of the OT? Why wouldn't that work? Well...maybe the NFB would undergo some kind of voodoo. I'd have to ponder that. Interesting proposition.

I got this CONN chassis with 4 qty 12V6 (ALL the tubes ran 12 v fils in order to keep current down, I take it...there were 36 qty 12AU7 over in oscillator-land) which sort of asks your very question as it sits. I was planning to make a rather ordinary Princeton Reverb out of it, maybe add an extra 9-pin socket just in case I get ambitious somewhere down the line.

[darryl]

Cut the B+ to the center tap of the OT? Why wouldn't that work?

Because the screens would melt. 

With no voltage on the plates, the screens would attempt to carry the entire current passing through the valve, with disastrous results.

[printer2]

Traynor switches the cathode off in an amp that uses a regular power tube, can't remember if it is 6V6 or EL84, and switches in a 12AU7.

[eleventeen]

"Because the screens would melt."

Well, other than that...~!

Sorry, oops!

[Ed_Chambley]

Certainly you would have to address the screen on the switched.  What I am concerned with is will the B+ increase and cause the remaining tubes to run hot.  I have seen many switch out 2 tubes on paralleled quads, but it always creates a mismatch which should be addressed IMO, but most time it is not.

I am thinking it can be done, but thought I would ask as I am not aware of the potential problems created.

[jojokeo]

Interesting question Ed but I simply would run VVR. Your idea would be simpler running cathode biased. But  thinking out loud here maybe run seperate bias' for each pair?

[DummyLoad]

for half power simply disconnect signal to one pair and AC couple both inputs to ground and you'd let the unused tubes idle - solves the B+ jump issue. YOU STILL NEED A LOAD for the idled pair - either via fixed R or real speaker. yes i know, you can idle without a load with no input signal, however, if any signal leaks into idling stage...better safe than sorry, right?

i attached a schematic describing how you could switch out a pair in PP with a single DPDT switch an additional pair of coupling caps and an additional pair grid leaks. the down side is the 2 x 220K grid-leak load is now seen as 2 x 110K load to the concertina when in switched in full power mode. no biggie - a little less voltage swing to the output stage. you'd switch from either mode with this arrangement when powered off or in standby.

if you're paranoid, use a 4PDT switch and force a real resistor load to output of the idled pair. you're only using 3 poles of the switch; you'd skip a switch space for some proximity isolation between input output signals.

--pete

[jojokeo]

Ed, if you just want a versatile "multi-powered" amp then design it to run on whatever tubes you choose to use that day, ie - 6v6/6l6/5881/6550/kt77/kt88, install VVR, pentode/triode, and throw in Geezer's half power switch and run an attenuator while you're at it too. You'll have all the bases covered! (Sorry, can't help myself)

[Ed_Chambley]

 Your idea would be simpler running cathode biased. But  thinking out loud here maybe run seperate bias' for each pair?
[/quote]
This is what I was planning.  I am only looking at getting about 20 watts out of 4 6v6 and 10 from 2.  I have never warmed up to a VVR.  As popular as power scaling is.  Probably me not fooling with it enough. 

I am working on an idea and just asking questions before I begin drawing.  I am wanting to use relays more and have 3 separate preamps.  Maybe an octal tweed, vox style ef86 and a twin AB763.  The main reason for this configuration is I have everything already.  It will be a long time in the works as I am petty happy with what I have now.

[DummyLoad]

Your idea would be simpler running cathode biased.

same plan works for fixed or auto bias. bias supply does complicate things.

But  thinking out loud here maybe run seperate bias' for each pair?

why? fender twin runs single bias for 4 tubes, so does marshall 100W and many others...

I am only looking at getting about 20 watts out of 4 6v6 and 10 from 2.

ok, will you use lower volt B+?

I have never warmed up to a VVR.  As popular as power scaling is.

neither have i - to me VVR makes amp sound too dark and lifeless when turned down.  

I am wanting to use relays more and have 3 separate preamps...will be a long time in the works as I am petty happy with what I have now.

sounds like a fun project.

--pete

[jojokeo]

There's that "10 watt Twin" idea coming back around again. I was immediately thinking about it before finished reading the post.

DL - when I'm "thinking out-loud" I'm usually not thinking all that straight!? 

One thing I've noticed with VVR is that if you don't need or want it then don't use it. I partially agree with your comment BUT there is a point to where it's beneficial too. It's not IMHO - a full yes or no vote in that it's either good or bad. It's both really depending on how you use it. Not sure if it makes sense but you don't build an amp to turn the VVR all the way down. However with judicial use it's pretty darn good in a number of ways. Also there's more than a couple ways to use it too.

Question 1: what type do you prefer as a master volume control if you had to have one?
Question 2: do you prefer to have a MV or No MV in your main amp?

[Ed_Chambley]

Ed, if you just want a versatile "multi-powered" amp then design it to run on whatever tubes you choose to use that day, ie - 6v6/6l6/5881/6550/kt77/kt88, install VVR, pentode/triode, and throw in Geezer's half power switch and run an attenuator while you're at it too. You'll have all the bases covered! (Sorry, can't help myself)

I have an amp like this.  It even has a disabled VVR in it.  No half power switch tho.  I have not built many 6v6 amps, but I do have lots of iron for them.  I have scaled the whole amp and just the power section and to me it doesn't sound very good, but I do like running high voltage on Power Tubes.  I even bought one from London Power to see if it makes a difference.  To me, they sound like 200 watt speakers being pushed by a Crate 15 watt SS amp.  I am not being sarcastic either.  I do not own an attenuator.  I could just get a Line 6 modeler and have perfect tone at any volume.

I think I am stuck in the 70's.  Seriously, I spend a lot of time with speakers which IMO is where over half of the tone resides.  If the speakers are not moving lots of air, no amp sounds good to my ears.  Like cabinet simulators for recording or to send to the console.

On your amp with a VVR, does it really sound the same with the only difference being a volume reduction?  If you had a 15 watt amp would you use 100 watt speakers?  This may be an extreme example, but I have never attempted scaling an amp under 40 watts which is where it seems they are the most beneficial.  I am not sure, but I think 20 watts and 10 watts would both sound great with a 12 Celestion blue.

Maybe I just don't know who to use a VVR.  I think I am like a kid with my first distortion pedal.  Remember when all your pedals were dimed until you learned a touch of this and that were better and used to enhance your amps tone.

[Ed_Chambley]

for half power simply disconnect signal to one pair and AC couple both inputs to ground and you'd let the unused tubes idle - solves the B+ jump issue. YOU STILL NEED A LOAD for the idled pair - either via fixed R or real speaker. yes i know, you can idle without a load with no input signal, however, if any signal leaks into idling stage...better safe than sorry, right?

i attached a schematic describing how you could switch out a pair in PP with a single DPDT switch an additional pair of coupling caps and an additional pair grid leaks. the down side is the 2 x 220K grid-leak load is now seen as 2 x 110K load to the concertina when in switched in full power mode. no biggie - a little less voltage swing to the output stage. you'd switch from either mode with this arrangement when powered off or in standby.

if you're paranoid, use a 4PDT switch and force a real resistor load to output of the idled pair. you're only using 3 poles of the switch; you'd skip a switch space for some proximity isolation between input output signals.

--pete

Thanks Pete, this is better than what I was thinking.

[jojokeo]

I think I am stuck in the 70's.  Seriously, I spend a lot of time with speakers which IMO is where over half of the tone resides.  If the speakers are not moving lots of air, no amp sounds good to my ears.  Like cabinet simulators for recording or to send to the console.

On your amp with a VVR, does it really sound the same with the only difference being a volume reduction?  If you had a 15 watt amp would you use 100 watt speakers?  This may be an extreme example, but I have never attempted scaling an amp under 40 watts which is where it seems they are the most beneficial.  I am not sure, but I think 20 watts and 10 watts would both sound great with a 12 Celestion blue.

Maybe I just don't know who to use a VVR.  I think I am like a kid with my first distortion pedal.  Remember when all your pedals were dimed until you learned a touch of this and that were better and used to enhance your amps tone.

We were lucky to go through the 70's for more reasons than the great tone the amps had; many great original bands & guitar-driven music! Ed, you are so right about the speaker and moving the appropriate amount of air! Low wattage speakers for low wattage amps 
Also my favorite current "house" amp is my single ended el34 w/ VVR driving a Blue Celestion. Great call on that one!
Here's something of interest for you? The amp is very quiet at idol even when things are turned up but there is always going to be some floor noise associated with this condition BUT, when the VVR is turned down a bit, the llittle bit of overall floor noise also drops to almost being imperceptible. The overall volume, headroom, and tone is all there just slightly reduced.