Which powertube to pull in my Bluesbreaker?

[Guest]

Okay! If I pull a tube in my bluesbreaker to reduce wattage, which one to pull?

Does it differ when having just two powertubes?

It runs on 8Ohm now, should I switch that to 4Ohm when pulling one tube?

Thanks!

 

[loudriver23]

Re: Which powertube to pull in my Bluesbreaker?

I thought there were some serious issues that could result from pulling out Power Tubes if your amp is not specifically designed to do so. As in overloading/underloading the Transformers.
I'm sure the true AMP Guru's will chime in soon, but I wanted to give my .02 to make sure you don't do something that could seriously damage your amp.

 

[Lewguitar]

Re: Which powertube to pull in my Bluesbreaker?

Don't pull either if you only have two output tubes. If you have four you can sometimes pull the two on the outside or the two on the inside. I used to do that with my 100 watt Marshall JCM-800. You have to change the setting of the speaker impedence selector switch too...I forget whether you go higher or lower. I think higher but wait until someone who remembers for sure posts...

Lew

 

[Guest]

Re: Which powertube to pull in my Bluesbreaker?

Thanks Lew and Lou! No pulling here then

 

[the guy who invented fire]

Re: Which powertube to pull in my Bluesbreaker?

Like Lew said you can only pull tubes if you have 4...and if you do pull 2 of the tubes the impedence goes up. Example...if you have a Fender Twin you can pull the inside or outside 2 tubes the amp goes from 100 (or so) watts at 4 ohms to 50 (or so) watts at 8 ohms.

Either way if you only have 2 power tubes make sure you are always runngin both of them...if you want to reduce the wattage look into a set of THD Yellow Jackets...they plug in where current power tubes go and they run EL84's so you amp woudl then be around 15 or so watts...

 

[Scott_F]

Re: Which powertube to pull in my Bluesbreaker?

What tubes are in there now? You can always pull whatever is in there and try to find a tube that breaks up earlier.

Example if you are runnign KT66's and they're not what you like, you can try some small bottle 6L6WGB's or maybe some 5881's. The bias range is probably about the same. If you drop EL34's in there, you'll probably need a new resistor in the bias circuit.

 

[shredaholic]

Re: Which powertube to pull in my Bluesbreaker?

www.londonpower.com:

Q: In a magazine Q-A, a player wanted to pull tubes to reduce power, but the "expert" said this would cause a meltdown of the remaining tubes. Of course, it was suggested that the expert's attenuator product was the preferred way to go. Is any of this true?

A: This is a person who should know better!

Removing tubes from a multi-tube fixed-bias output stage is never a problem. You can remove any number of tubes, and yes, that means you can take one tube out of a two-tube amp; one, two, or three out of a four tube stage, et cetera. This sounds heretical to techs stuck in the mire of convention, but it is something that has been known since tubes were invented.

The even-number tube extractions reduce power symmetrically. Neither the tubes nor the transformer will be damaged. Power will be reduced and so will frequency bandwidth - you will lose some bass and some treble. This is the point that switching the impedance selector to a less-than-load setting is supposed to correct, but it is completely subjective whether you should. The only 'should' of the matter, is do I like it this way, or do I like it that way?

In the uneven tube extractions, asymmetric power reduction occurs. Conventional thought says "the one tube on one side of the circuit will be trying to match the output of the two tubes on the other circuit half". This is wrong. The single tube can only produce so much power, and that's all it does. It doesn't melt down. The transformer does not blow up.

So, what's missing from conventional thought? The realization that tubes are "self-limiting power governors", which was stated in The Ultimate Tone (TUT), and explored in more detail in TUT2 and TUT3. TUT4 explores all of this in great detail. Our "expert" should get a copy.

In the end, you can pull tubes to reduce power, unless the amp is cathode biased - then you have to split the bias resistor. In any case, you do not have to worry about the impedance selector either.

 

[Gearjoneser]

Re: Which powertube to pull in my Bluesbreaker?

I think the impedence rule is this...

If you pull 2 inner or outter tubes in a 100W amp, you have to set the impedence at half, even though the cab is still the same.

ie....16 ohm amp with 2 tubes pulled gets set at 8 ohms, going into a 16 ohm cab.

 

[Lake Placid Blues]

Re: Which powertube to pull in my Bluesbreaker?

I think the impedence rule is this...

If you pull 2 inner or outter tubes in a 100W amp, you have to set the impedence at half, even though the cab is still the same.

ie....16 ohm amp with 2 tubes pulled gets set at 8 ohms, going into a 16 ohm cab.

This is correct. The tube pairs are connected to the Output transformer's primary coil, and the amount of coil on the secondary side of the Output transformer should be adjusted in compensation.

In a general sense, one can't simply pull one of the output tubes of a two output tube, class A/B amp, such as the JTM45/Bluesbreaker. Any information that says you can is simply wrong! In this type of amp, one tube handles 1/2 of the sine wave, and the other tube the other 1/2 of the wave. There is some overlap depending on how it's been biased, but these type of amps must have a minimum of two output tubes for proper operation. By taking out the other 1/2 of the pair, your taking out the plate that the other end of the OTP primary coil's wire connects to, as well.

Once my 50 watt jubilee combo lost one EL34 at a gig, during a song. In that case the amp still worked, as in it made a muddy distorted sound untill the end of the song, but it wasn't a good sound. Usually with one tube blown out on a two tube amp, it simply hums uncontrolled and loudly.

To reduce the output of a two ouput tube class A/B amp by 1/2, the ability to run the output tubes in triode, instead of pentode is required. A good amp tech could add Pentode/Triode switching. This would allow you to run the amp at 30 watts or 15 watts.

 

[shredaholic]

Re: Which powertube to pull in my Bluesbreaker?

I'd say a pentode/triode switch is one of the easiest things to add to an amp, just as easy as a PPIMV. Cheap and easy way of cutting the volume to about 60%, WAAAAAAAYYY more effective than the almost negligible volume reduction from pulling tubes.

Pulling any number of tubes is safe, and shouldn't sound like ass. When tubes blow, maybe, but not when they're pulled prior to use. VHoholic has one of the most killer guitar tones on this forum, and he's running his 1959slp with only 1 tube.

 

[Lake Placid Blues]

Re: Which powertube to pull in my Bluesbreaker?

.

....Pulling any number of tubes is safe, and shouldn't sound like ass. When tubes blow, maybe, but not when they're pulled prior to use. VHoholic has one of the most killer guitar tones on this forum, and he's running his 1959slp with only 1 tube.

In this case the one tube has to be running Class A. By diffinition Class A is were a tube is amplifying the whole sine wave through the whole duty cycle. The bias setting must be such to allow the tube to remain "on" through the entire duty cycle. That's pretty hot in an amp designed for Class A/B operation on 450+ volts plate voltage. Also the impedance on the primary side of the OPT is way down, and the current is way up compared to the design parameters..... on just half the windings. This doesn't mean the OPT will melt down any old time, but it's running a greater risk.

Many people feel that Marshall OPT's are rather fragile, but I don't. They can usually handle mismatched impedance, and tramatic events such as tube blowing.

Setting an Class A/B amp up to run one tube should be done very carefully if done at all in my opinion though. I would never reccomend pulling down to one tube to just anyone, unless they can or have the amp set up, based on a solid understanding of the things involved.

 

[Guest]

Re: Which powertube to pull in my Bluesbreaker?

Aargh! Why did smicz go belly up :yell: :yell: :yell:

http://www.musictoyz.com/guitar/amps/smicz.php

I'd so love to get these for playing the Bluesbreaker at home!

Is there any other company that makes these really low wattage tube adapters? Please tell me there is...