How messy booger achieved switchable power tubes?

[Nicko_Lps]

Greetings everyone,

Im wondering, how exactly did they manage to make a power tube section work with 2x6L6 or 4x6L6 or 2x6L6+2xEL34 or 4x6L6+2xEL34.
I can see from their schematic that they use relays to switch on/off the cathode connection to ground, yet my question is...

How exactly the output transformer impedance is working in this case? By disconnecting the cathodes on 2 out of 4 tubes for example, just because the plate and screen grid remains connected to the output transformer, the impedance remains unaffected ?

Does that mean(for example) that you can add a switch to a 4 power tube marshall or fender and simply disconnect the 2 tubes cathode and operate in a 50 watt mode?

In their case cant post any schematic or name for the obvious reasons, im pretty sure everyone can get a roadking2 schematic from the internet and take a look on what im trying to understand!

I do see in their schematic 1 tap in their output transformer but their schematics are mostly for relevance, Also it seems that EL34's have the outer(higher impedance) tap that adds more confusion to the subject.

 

[printer2]

It's complicated.

 

[Jon Snell]

Nicko wrote; "Does that mean(for example) that you can add a switch to a 4 power tube marshall or fender and simply disconnect the 2 tubes cathode and operate in a 50 watt mode?"

The answer is ... almost.
Removing two of the output valves from service will double the drive impedance and in turn produce a higher rate of harmonic distortion in the output stage. Now, if there is plenty of negative feedback, that will tend to cover that up and perform nicely, unless it is a HiFi amplifier and we won't go there.
Four 6L6/EL34 etc require about 2k impedance to work into for an efficient transfer of power. I use the word efficient, loosely.
The output transformer will therefore have an anode to anode load or impedance of around 2k Ohms.
If a pair of valves require 4k Ohms anode to anode, (a quad will require 2k), there will be losses because the impedances won't match the requirements. A lot of players like the result. Earlier break up etc.
So yes, lifting the cathodes of one pair will reduce the power output and it is more convenient than removing said pair, physically.

Half the power does not mean half the volume!

 

[Lowerleftcoast]

Mesa Roadking 2 manual has examples of switch settings to achieve the various tube combinations. It is not willy nilly. Switching the rectifier mode will change the B+ voltage which in turn has an impact when looking at load lines for speaker impedance.

Does that mean(for example) that you can add a switch to a 4 power tube marshall or fender and simply disconnect the 2 tubes cathode and operate in a 50 watt mode?

Short answer... yes, or as Jon wrote "almost".
Keep in mind tube amplifiers and the relatively large output transformers are forgiving. They tolerate an impedance mismatch 4 to 8 Ohm, 8 to 16 Ohm, and vise versa. To maintain a similar impedance match, when removing two tubes, use a 16 Ohm load in the 8 Ohm tap etc. Mesa's manual talks about matching and mis-matching speaker loads to maintain *correct* impedance or to hear the difference of mis-matching.

Also it seems that EL34's have the outer(higher impedance) tap that adds more confusion to the subject.

Just another excuse to create this Swiss Army Knife of an amp. lol. "Because you can doesn't mean you should" comes to mind.

 

[radiocaster]

Using a relay to switch the cathode connections? That's a pretty complicated amp. You could use switches for something simpler.

But in any case, you're referring to a cathode biased amp, which is somewhat forgiving. You'd be fine with the same bias for EL34/5881/6L6, more or less. Probably need something else for 6V6/EL84. Or maybe some slight adjustment for each specific type, some people are into that.

And also, you likely wouldn't be able to get 50W out of two power tubes with cathode bias, you'd likely need fixed bias, which requires precise adjustments for whatever you put in there.

 

[joebloggs13]

Greetings everyone,

Im wondering, how exactly did they manage to make a power tube section work with 2x6L6 or 4x6L6 or 2x6L6+2xEL34 or 4x6L6+2xEL34.
I can see from their schematic that they use relays to switch on/off the cathode connection to ground, yet my question is...

How exactly the output transformer impedance is working in this case? By disconnecting the cathodes on 2 out of 4 tubes for example, just because the plate and screen grid remains connected to the output transformer, the impedance remains unaffected ?

Does that mean(for example) that you can add a switch to a 4 power tube marshall or fender and simply disconnect the 2 tubes cathode and operate in a 50 watt mode?

In their case cant post any schematic or name for the obvious reasons, im pretty sure everyone can get a roadking2 schematic from the internet and take a look on what im trying to understand!

I do see in their schematic 1 tap in their output transformer but their schematics are mostly for relevance, Also it seems that EL34's have the outer(higher impedance) tap that adds more confusion to the subject.

The same way Steve Carr makes the Telstar...Magic... it uses 1 EL-84(which cannot be changed) and a second power tube, which can be a 6L6, 6v6, KT-66, EL-34 or a 5881. You can swap between any of those tubes without rebiasing. It's a class A push/pull circuit based on the old Harvard Tweeds. Amazing amp.

 

[Lowerleftcoast]

But in any case, you're referring to a cathode biased amp, which is somewhat forgiving.

Nope. The roadking2 is fixed bias.

The EL34's have a separate bias voltage from the bias voltage for the 6L6 group. They change B+ with different means of rectification. They also suggest speaker impedance changes.

 

[ChicknPickn]

Started to read, but then, no.

 

[Nicko_Lps]

So yes, lifting the cathodes of one pair will reduce the power output and it is more convenient than removing said pair, physically.

I dont know how to say this correctly because im not the best english speaker:

So either removing the 2 out of 4 tubes either lifting the cathodes, its still mismatching?

To maintain a similar impedance match, when removing two tubes, use a 16 Ohm load in the 8 Ohm tap etc.

Yep im aware of this, i actually use this method when i swap to 6V6 to double the primary but still the question remains the same: By lifting cathodes youre still mismatching like removing 2 tubes?

Just another excuse to create this Swiss Army Knife of an amp. lol. "Because you can doesn't mean you should" comes to mind.

That is 100% correct though but honestly i love the EH EL34's for their high mids/treble response and i do actually love the way 6L6GC's/5881 fatten up my tone.

I have 2 Heyboer's, a 50 watter and a 100watt one back from 2005. I think it would be a great idea do it IF there is no mismatching, i dont like idea of it despite i know its OK.

Also im aware that 6L6 can handle mismatching, from a 3400 primary. The exactly opposite ,the EL34 on a 4200 ive seen people stating that is not a good idea. Luckily my Heyboers OT's are 3400 and 1700, the JCM 800 type

 

[Jon Snell]

How to lift one pair from the quad output stage.
This is taken from my design the Snell 66 switchable 6L6 with or without 6V6.

 

[Nicko_Lps]

How to lift one pair from the quad output stage.

Thanks for the second reply Jon and as i said, my English is not the best and you might have lost me there.

Im aware on how to do it, the question was if its actually mismatching the OT reflected impedance when you pull the ground from 2/4 power tubes!

 

[2L man]

Also im aware that 6L6 can handle mismatching, from a 3400 primary. The exactly opposite ,the EL34 on a 4200 ive seen people stating that is not a good idea. Luckily my Heyboers OT's are 3400 and 1700, the JCM 800 type

Yes it is not too important that OT load match tubes "anode resistance" when operating point and loadline are set reasonably. This means that for higher load=lower OT impedance the anode and screen voltages are lowered.

So if only two of four power tubes are removed the loadline comes much more upright and healthy AB class tubes can pull so high RMS peak current that they overheat. Upright loadline pass even 2x max anode dissipation. Thats why manipulating OT impedance higher is recommended.

 

[Lowerleftcoast]

By lifting cathodes youre still mismatching like removing 2 tubes?

Yes.
Essentially you are *turning off* or disabling the tubes when the cathode connection is broken. It is like unscrewing a light bulb or *turning off* a light bulb. The load is gone. It is no longer in the circuit.