Who Has Tried Pulling Two Tubes ?

[PLX]

I asked ChatGPT about the method, safety concerns , and technical details of pulling two tubes from a 1959 circuit, and I'm starting to think ChatGPT is full blown retarded..

Anyone tried pulling two 6CA7's from the same side ?

Does this make the amp no longer a push/pull ?

 

[Deadpan]

I'm not brave enough.

 

[Chocol8]

I only pull tubes in pairs. Pulling two from the same side will make it pull - ___ or push - ___ which is going to be an extreme case of asymmetrical distortion. I really don't know what the dangers are if any, as it never crossed my mind to even think about it.

As far as the ChatGPT version of pulling two tubes, I have done that in a younger dumber past and I do not recommend it. It can be pretty dangerous to have two of anything back there if they do not have steering control. Two capable skiers, sure. Two tubes, bad.

 

[PLX]

I only pull tubes in pairs. Pulling two from the same side will make it pull - ___ or push - ___ which is going to be an extreme case of asymmetrical distortion. I really don't know what the dangers are if any, as it never crossed my mind to even think about it.

I have an amp that I hate.. so I tried pulling out 3 6CA7's and running it with only 1 output tube.

Sounds like Van Halen I

 

[Chocol8]

I have an amp that I hate.. so I tried pulling out 3 6CA7's and running it with only 1 output tube.

Sounds like Van Halen I

Pulling three is probably safer as there is less load on the OT. Still beyond my expertise, but if it sounds good and doesn't blow up, why not?

 

[PLX]

Pulling three is probably safer as there is less load on the OT. Still beyond my expertise, but if it sounds good and doesn't blow up, why not?

You variac it down to like 69 volts too.

 

[Chocol8]

You variac it down to like 69 volts too.

Unless you have a way to keep the heater voltage up, that kills your tubes, including the preamp tubes. If you are going to do that long term, I recommend using voltage control on the B+ only or using a separate transformer to power the heaters.

 

[PLX]

Unless you have a way to keep the heater voltage up, that kills your tubes, including the preamp tubes. If you are going to do that long term, I recommend using voltage control on the B+ only or using a separate transformer to power the heaters.

I've talked with Jim Gaustad, Curt Mitchell, and Rudy Leiren and they all say that's bunk.

 

[Chocol8]

I've talked with Jim Gaustad, Curt Mitchell, and Rudy Leiren and they all say that's bunk.

So you asked an electrical engineering question to a pair of musicians and a guitar tech, and you are confident in their answer?

 

[PLX]

So you asked an electrical engineering question to a pair of musicians and a guitar tech, and you are confident in their answer?

I asked guys who have run old Marshall amps this way for decades.

 

[HomespunEffects]

I used to run my Marshall Super Bass with the middle tubes out. There’s some way you have to match the impedance differently but i forget which way. It works fine. A Super Bass is still insanely loud though.

 

[newholland]

i pulled two tubes from my 60w mesa. it didnt sound like anything.

 

[newholland]

I used to run my Marshall Super Bass with the middle tubes out. There’s some way you have to match the impedance differently but i forget which way. It works fine. A Super Bass is still insanely loud though.

iirc its halving the impedance- so you run it on 4 ohm instead of 8 frinstance.

 

[HomespunEffects]

iirc its halving the impedance- so you run it on 4 ohm instead of 8 frinstance.

Yeah. I knew It was something like that but couldn’t remember.

 

[PLX]

I used to run my Marshall Super Bass with the middle tubes out. There’s some way you have to match the impedance differently but i forget which way. It works fine. A Super Bass is still insanely loud though.

Yeah, I've run my JCM800 like this for years now - variac'ed down also.

 

[Chocol8]

I asked guys who have run old Marshall amps this way for decades.

How often do they change tubes? Have they been tracking how many hours before performance degrades or the tube fails?

When NOS tubes were plentiful and affordable, reducing a 10,000 hour tube life to 1,000 hours was no big deal. Depending on the tubes you are using now, it might make more of a difference. Controlling the B+ is relatively cheap and easy and fits right in the amp chassis so I recommend it as a better method. YMMV.

 

[PLX]

Controlling the B+ is relatively cheap and easy and fits right in the amp chassis so I recommend it as a better method. YMMV.

I have that scheme in my Ceriatone, and it's pretty cool !

There's a separate tap off the PT at 14VAC, and it runs into a rectifier & filter circuit, then a 12VDC regulator - which then powers the heaters.

 

[Chocol8]

iirc its halving the impedance- so you run it on 4 ohm instead of 8 frinstance.

That's maybe backwards...kinda sorta...depending on what you are talking about halving. It can be confusing when people just say half or double.

Half the tubes can push half the current so they want to see twice the impedance. If talking about a Fender or other amp with a single output impedance, something like a Twin Reverb that expects a 4 ohm load with 4 tubes would want to see an 8 ohm load with 2 power tubes. So double not half.

BUT...

If you have multiple output impedances, you would run an 8 ohm cab off the 4 ohm tap and a 16 ohm cab off the 8 ohm tap. in that case you are halving the impedance of the tap you use.

 

[Chocol8]

I have that scheme in my Ceriatone, and it's pretty cool !

There's a separate tap off the PT at 14VAC, and it runs into a rectifier & filter circuit, then a 12VDC regulator - which then powers the heaters.

That is definitely the best way to do it IMHO. Built in from the start it is also much cheaper than a variac.

 

[FractalAudio]

If you pull a pair of tubes you want the primary impedance to double. Therefore you should connect the speaker to the output with half the impedance of the speaker.

I.e., connect an 8-ohm speaker to the 4-ohm output.

One of the dangers with pulling tubes is that the quiescent B+ voltage will increase due to the decreased current draw and lower resulting IR drop. This can cause the tubes to easily exceed the SOA.