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400+ Tube removal.

#41
lem8r said:
Yes it is safer, but not infinite load (no cab connected)
Click to expand...

So if the OP decides to try and pull tubes to see how it sounds, at which point would you need to change loads?
If you pulled 1 or maybe even 2 pairs, I can't imagine the impedance requirement would need to be changed. But if he pulled half, i.e. 6 tubes, would that equal half-ing the load and going from 4 to 8?
 
#42
Tualatin said:
So if the OP decides to try and pull tubes to see how it sounds, at which point would you need to change loads?
If you pulled 1 or maybe even 2 pairs, I can't imagine the impedance requirement would need to be changed. But if he pulled half, i.e. 6 tubes, would that equal half-ing the load and going from 4 to 8?
Click to expand...
In my understanding of maths, half of 4 = 2 if we are discussing suitable speaker loads for a nominal 4 ohm tap.:ninja:
 
#43
Tualatin said:
So if the OP decides to try and pull tubes to see how it sounds, at which point would you need to change loads?
If you pulled 1 or maybe even 2 pairs, I can't imagine the impedance requirement would need to be changed. But if he pulled half, i.e. 6 tubes, would that equal half-ing the load and going from 4 to 8?
Click to expand...
Transformation of impedance is not linear but quadratic. But for half tubes pulled you'll get near matched impedance if you'll plug:
- 16 Ohm cab into 8 ohm amp out
or
- 8 ohm cab into 4 ohm amp out
or
- 4 ohm cab into 2 ohm amp out
 
#44
I tried looking in the FAQ sticky but couldn't find a clear answer since this seems to be a pretty specific option to the 400+

I think a clear understanding of the OP's question would be great since we're three pages in and have conflicting opinions. I thought I was confident when I first read it, but now I'm second guessing myself.

I'm still confident about pulling by pairs, i.e. 1,1 2,2 3,3...
But, the impedance is still confusing me. Wouldn't the transformers impedance requirements super cede that of the tubes in this type of circuit, negating any need to alter what output load (up or down) is used?
 
#45
lem8r said:
Transformation of impedance is not linear but quadratic. But for half tubes pulled you'll get near matched impedance if you'll plug:
- 16 Ohm cab into 8 ohm amp out
or
- 8 ohm cab into 4 ohm amp out
or
- 4 ohm cab into 2 ohm amp out
Click to expand...

Thanks, that's what I was trying to say.
Can we confirm the OP's answer as being:
Pull in pairs 1,1 2,2 ,3,3...
When you get to 3 pair/6 tubes, change your impedance to what lem8r listed above?
 
#49
lem8r said:
yup. you can even pull any combination of three pairs: 1,1' 2,2' 3,3' or 4,4' 5,5' 6,6' or 1,1' 3,3' 5,5' or 2,2' 4,4' 6,6' etc..
Click to expand...

you can pull any pairs front row to back row. The front row are all in paralell, back row are all in paralell.

One thing about transformer primary impedance that may not be clear. The primary impedance is determined by the turns ratio and the load (impedance) attached to the secondary. Said another way, the primary impedance of a given amp is determined by the speaker thats connected to the amp, and where the impedance swicth is set, or which output jack the load is plugged into.

The impedance selector switch or output jack selection is changing the turns ratio.

Additionally, Every output tube has a specified load that it likes to work into for maximum power transfer from the tube to the load(speaker). There's a catch though. There's usually a lot of distortion with this "perfect match". By increasing the primary impedance, distortion can be lowered, and sound quality improves.

So the amp designer chooses their poison, more power or less distortion.

Finally, the tube impedance is dynamic. It changes with different operating conditions that the tube see's. So that impedance goes up and down through a range of impedances. For that matter, the primary transformer impedance moves through a range, depending on what frequency is being sent through to the speaker. That's why speakers are rated with "nominal" impedances. An 8 ohm speaker has an average impedance of about 8 ohms. Lower in its frequecy range the impedance goes up. As its frequency goes up, impedance may drop, then rise again. So the impedance of the primary on the output tranny goes up and down with in a range as well.

My point is that tube amps are designed to work within a range of conditions. By removing tubes and changing impedance selections, your essentially moving your operating ranges around, and its OK.

Will it damage the amp? Well, define damage. As I said before, runniing any system towards its maximum wears it out faster. You'll replace tubes more often. Are you gonna pull some tubes and cause the thing to go BOOOOOOM!. No.
 
#52
I dont remember much of my transformer theory as to why, but this is also what I believe to be the facts.
Half the tubes = 2 X higher theoretical primary total impedance and this reflects across the laminates into the secondary windings as the following requirement for ideal matching and transfer:
Connect an eight ohm OP trans winding to a four ohm loudspeaker load if you pull half the tubes.
Anyway thats what I have done lots of times in the past and I haven't had any bother yet.
I don't do it anymore as I have a very beautiful motherload power soak custom built for me by Rick at Sequis so my screwdriver stays conveniently in the tool box these days.:bassist:
 
#53
Bassmec said:
I dont remember much of my transformer theory as to why, but this is also what I believe to be the facts.
Half the tubes = 2 X higher theoretical primary total impedance and this reflects across the laminates into the secondary windings as the following requirement for ideal matching and transfer:
Connect an eight ohm OP trans winding to a four ohm loudspeaker load if you pull half the tubes.
Anyway thats what I have done lots of times in the past and I haven't had any bother yet.
I don't do it anymore as I have a very beautiful motherload power soak custom built for me by Rick at Sequis so my screwdriver stays conveniently in the tool box these days.:bassist:
Click to expand...

Hmmmm...maybe I'm confused. What are we talkng about here? I thought we were discussing how to connect the speaker and and choose secondary taps so that we could pull half of the tubes, but still maintain relatively close tube to trans primary impedance relationships. Yes. No?
 
#58
megadan said:
There was a post a while ago on TB where someone had modded their 400 or 400+ (I think it was the 400) with some custom switches in the rear that took pairs of tubes in or out of the circuit.
Really neat stuff, but I can't find the thread now...
Click to expand...

Yeah that was my 400+ Cathode lift, amp-mod thread, Its got lots of info pertaining to this threads discussion.
Here's the link: http://www.talkbass.com/forum/showthread.php?t=238662&highlight=

By the way my modded 400+ is still working fine several years on down the road and is still running the same power tubes. When I use the amp with 6 power tubes lifted/(half power), I plug my 4-ohm cab into my amps 2-ohm speaker jack.
 

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