Removing valves for reduced output

ChuckManual

There's an amp for sale in the classifieds that is 100w, switchable down to 25w ...but the owner says if you pull two of the four valves, it'll go down to 60w / 15w.

Now, apart from not being able to work out the maths there; aren't circuits/transformers/whatever all designed to work in a certain way because exact components, in specific quantities, in a very definite order, are all supposed to be there? Won't pulling valves damage the amp? If not, why not? And can it be done to any amp with more than one valve?

ICBM

ChuckManual said:

There's an amp for sale in the classifieds that is 100w, switchable down to 25w ...but the owner says if you pull two of the four valves, it'll go down to 60w / 15w.

50W/12.5W is more likely to be accurate. You don't need to be a maths genius to see why .

(In fact, there may be a *small* rise above the exact half power, but it's not likely to be as much as 20% even on the 50W, and on the lower setting it definitely won't be.)

ChuckManual said:

Now, apart from not being able to work out the maths there; aren't circuits/transformers/whatever all designed to work in a certain way because exact components, in specific quantities, in a very definite order, are all supposed to be there?

Yes and no. In a four-valve amp, it's most common for them to be arranged in two pairs, where essentially each pair is operating in parallel as if it was a single, larger valve. (There is actually only one exception I know of, more about that later.) So by removing two of the valves you still have the same circuit, but with half the power capability.

In order to do it correctly you also ideally need to reset the output transformer's impedance ratio, because that is designed for the four valves - to do that, you need to set the amp to half the impedance of the cabinet, because that retains the correct ratio between both sides of the transformer. If you don't the amp will be running mismatched, although that is not likely to be very harmful.

ChuckManual said:

Won't pulling valves damage the amp? If not, why not?

All of the above relies on the amp being either fixed-bias, or cathode-bias with separate resistors for each valve. If it's a more typical cathode-bias amp with one resistor for all four valves, you can't remove two valves because then the remaining two will bias themselves far too hot and fail - trying to do the job of the normal four - even if you reset the impedance.

So almost any four-valve amp which is cathode-biased (sometimes mis-called Class A, and most often using EL84s - eg Vox AC30) cannot have two valves removed. But almost any amp which is push-pull fixed bias with four or more valves can safely have pairs of valves removed, if you reset the impedance.

The exception to the pairs of valves rule is the Laney Lionheart 20, which has all four in parallel, but you still can't remove two valves because it's also cathode biased with a single resistor.

ChuckManual said:

And can it be done to any amp with more than one valve?

No - most amps with two valves are push-pull, so if you remove one valve you will only get roughly half the waveform, which will sound oddly distorted and not very good. You need more than one *pair* of valves, so it's really limited to 4/6/8-valve amps.

Does that help at all?

tekbow

ICBM said:

ChuckManual said:

There's an amp for sale in the classifieds that is 100w, switchable down to 25w ...but the owner says if you pull two of the four valves, it'll go down to 60w / 15w.

50W/12.5W is more likely to be accurate. You don't need to be a maths genius to see why .

(In fact, there may be a *small* rise above the exact half power, but it's not likely to be as much as 20% even on the 50W, and on the lower setting it definitely won't be.)

ChuckManual said:

Now, apart from not being able to work out the maths there; aren't circuits/transformers/whatever all designed to work in a certain way because exact components, in specific quantities, in a very definite order, are all supposed to be there?

Yes and no. In a four-valve amp, it's most common for them to be arranged in two pairs, where essentially each pair is operating in parallel as if it was a single, larger valve. (There is actually only one exception I know of, more about that later.) So by removing two of the valves you still have the same circuit, but with half the power capability.

In order to do it correctly you also ideally need to reset the output transformer's impedance ratio, because that is designed for the four valves - to do that, you need to set the amp to half the impedance of the cabinet, because that retains the correct ratio between both sides of the transformer. If you don't the amp will be running mismatched, although that is not likely to be very harmful.

ChuckManual said:

Won't pulling valves damage the amp? If not, why not?

All of the above relies on the amp being either fixed-bias, or cathode-bias with separate resistors for each valve. If it's a more typical cathode-bias amp with one resistor for all four valves, you can't remove two valves because then the remaining two will bias themselves far too hot and fail - trying to do the job of the normal four - even if you reset the impedance.

So almost any four-valve amp which is cathode-biased (sometimes mis-called Class A, and most often using EL84s - eg Vox AC30) cannot have two valves removed. But almost any amp which is push-pull fixed bias with four or more valves can safely have pairs of valves removed, if you reset the impedance.

The exception to the pairs of valves rule is the Laney Lionheart 20, which has all four in parallel, but you still can't remove two valves because it's also cathode biased with a single resistor.

ChuckManual said:

And can it be done to any amp with more than one valve?

No - most amps with two valves are push-pull, so if you remove one valve you will only get roughly half the waveform, which will sound oddly distorted and not very good. You need more than one *pair* of valves, so it's really limited to 4/6/8-valve amps.

Does that help at all?

I actually understood all of this, which suprised me more than anyone. A year ago I'd have been "durr..."

Great info, cheers!

ecc83

DO NOT try this with a Blackstar 100 watter, the heaters are in series!

Dave.

ICBM

ecc83 said:

DO NOT try this with a Blackstar 100 watter, the heaters are in series!

So are most modern Peaveys.

No harm will be done, it just won't work at all...

hotpickups

@ICBM so on my badcat black cat 30 which has 4 EL84 valves will not turn two valves off when in half power mode?

ICBM

hotpickups said:

@ICBM so on my badcat black cat 30 which has 4 output valves will not turn two valves off when in half power mode?

I think it does - it has separate resistors for each pair of valves.

hotpickups

ICBM said:

hotpickups said:

@ICBM so on my badcat black cat 30 which has 4 output valves will not turn two valves off when in half power mode?

I think it does - it has separate resistors for each pair of valves.

That's good then. It'll save some wear on the other two as I have it mainly on half power 

ICBM

hotpickups said:

It'll save some wear on the other two as I have it mainly on half power 

Not really. To save wear you need to remove them from the amp, otherwise they're cooking away but with no current going through them, which can cause 'cathode poisoning'.

hotpickups

ICBM said:

hotpickups said:

It'll save some wear on the other two as I have it mainly on half power 

Not really. To save wear you need to remove them from the amp, otherwise they're cooking away but with no current going through them, which can cause 'cathode poisoning'.

I'm sure they'd have thought of that in the circuit design surely if they had a dedicated half power switch would t they I.e. BadCat designers ?

ecc83

ICBM said:

hotpickups said:

It'll save some wear on the other two as I have it mainly on half power 

Not really. To save wear you need to remove them from the amp, otherwise they're cooking away but with no current going through them, which can cause 'cathode poisoning'.

Which is exactly what happens with a conventional stand by switch! But, you don't leave an amp in S/B all the time.

In any event, cathode poisoning is not really a problem in conventional valves, more affected CRTs according to Morgan Jones?

Dave.

ICBM

hotpickups said:

I'm sure they'd have thought of that in the circuit design surely if they had a dedicated half power switch would t they I.e. BadCat designers ?

Really? Given that they don’t reset the impedance, so the amp is running mismatched at half power, and that all their amps run the valves at well over the maximum design ratings - hence their short life anyway - I wouldn’t be so sure of that...

It doesn’t mean they sound bad of course, but proper audio electronics design and good-sounding guitar amps are very uncomfortable bedfellows, pretty much by definition!

ecc83 said:

Which is exactly what happens with a conventional stand by switch! But, you don't leave an amp in S/B all the time.

In any event, cathode poisoning is not really a problem in conventional valves, more affected CRTs according to Morgan Jones?

Not sure. I’ve also heard it used as an argument against standby switches too...

ecc83

ICBM said:

hotpickups said:

I'm sure they'd have thought of that in the circuit design surely if they had a dedicated half power switch would t they I.e. BadCat designers ?

Really? Given that they don’t reset the impedance, so the amp is running mismatched at half power, and that all their amps run the valves at well over the maximum design ratings - hence their short life anyway - I wouldn’t be so sure of that...

It doesn’t mean they sound bad of course, but proper audio electronics design and good-sounding guitar amps are very uncomfortable bedfellows, pretty much by definition!

ecc83 said:

Which is exactly what happens with a conventional stand by switch! But, you don't leave an amp in S/B all the time.

In any event, cathode poisoning is not really a problem in conventional valves, more affected CRTs according to Morgan Jones?

Not sure. I’ve also heard it used as an argument against standby switches too...

Oh! it has but not by me (despite what you know of my stance on the things!) . Cathode poisoning, as I understand it, affects valves that run with zero or almost zero anode (beam in the case of CRTs) current for very long periods of time. At a gig and amp might be put on standby for 20mins for a P and beer break? If it is  not going to be used for much longer, turn the fekker off!

Dave.

bacchanalian

Apologies for hijacking but could two of the valves from a Dr Z Jaz 20/40 be safely removed? 
 A friend recently bought one from a fretboard member and likes the tone but even at the 20 watt setting might be too loud for home use.

The Jaz 20/40 has  'the front end of the MAZ series coupled to a cathode biased 4 6V6 output section'

From @ICBM 's comments above I think the answer is yes as long as each valve has a separate resistor.

Any advice that will prevent blowing the amp up will be gratefully received

ICBM

bacchanalian said:

Apologies for hijacking but could two of the valves from a Dr Z Jaz 20/40 be safely removed? 
 A friend recently bought one from a fretboard member and likes the tone but even at the 20 watt setting might be too loud for home use.

The Jaz 20/40 has  'the front end of the MAZ series coupled to a cathode biased 4 6V6 output section'

From @ICBM 's comments above I think the answer is yes as long as each valve has a separate resistor.

Any advice that will prevent blowing the amp up will be gratefully received

No, because its a near certainty that at 20W it's already turned off two of the valves. There are other ways of halving the power, but that's by far the most common. Each *pair* of valves will have its own resistor.

You can't pull one of the remaining valves because it's both half of a push-pull and also shares a resistor.

20W will be *way* too loud for home use if you want to crank it up, and so would 10W be even if you could get that. You need an attenuator.

bacchanalian

Thanks @ICBM ;

bacchanalian

I currently am running NOS 6G6 in my Victoria

Would it be possible to bias the Dr Z for 6G6 or 6K6?

Thanks again

ICBM

bacchanalian said:

I currently am running NOS 6G6 in my Victoria

Would it be possible to bias the Dr Z for 6G6 or 6K6?

It would theoretically be possible to bias it correctly by changing the cathode resistors, but it’s very likely 6G6s or 6K6Gs wouldn’t survive - the plate and screen voltage ratings are far too low, and the maximum current for the 6G6 as well - although 6K6GTs might.

http://www.r-type.org/exhib/aaa0141.htm

http://www.r-type.org/exhib/zaa0067.htm

http://www.r-type.org/exhib/aas0031.htm

What model of Victoria is it?

bacchanalian

Thanks @ICBM ;

The Victoria is a Regal II which is designed for valve swapping and sounds great with the 6G6s installed

ICBM

bacchanalian said:

The Victoria is a Regal II which is designed for valve swapping and sounds great with the 6G6s installed

That’s specifically designed to take different valve types, and I’m sure it will have much lower voltages given that the maximum output with 6L6s is only 35W.

Don’t try it with the Dr.Z, it will very likely fry the valves.

bacchanalian

Once again @ICBM , many thanks