5881s in place of 6L6s?
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soundgardener75
New member
Re: 5881s in place of 6L6s?
That is what I was told. One would have to get the amp rebiased because of plate voltages (?).
Now that would be okay.
That is what I was told. One would have to get the amp rebiased because of plate voltages (?).
Now that would be okay.
guitfiddle
firstlessonologist
Re: 5881s in place of 6L6s?
The 5881 is a US military version of the 6L6. The british equivalent would be the KT-66.
The 5881 is a US military version of the 6L6. The british equivalent would be the KT-66.
Re: 5881s in place of 6L6s?
originally the 5881 was a military 25w version of a 6l6gb. then they came out with the 6l6gc which is what we now think of as 6l6 which should be a 30w tube. the kt66 is a british version of a 6l6 with a 25w rating.
now days none of this means anything. a 5881 and a 6l6 are basically interchangeable. a real kt66 pulls more heater current than a 6l6 or 5881 and so do some reproductions so you need to check your power tranny to make sure it can handle the load
originally the 5881 was a military 25w version of a 6l6gb. then they came out with the 6l6gc which is what we now think of as 6l6 which should be a 30w tube. the kt66 is a british version of a 6l6 with a 25w rating.
now days none of this means anything. a 5881 and a 6l6 are basically interchangeable. a real kt66 pulls more heater current than a 6l6 or 5881 and so do some reproductions so you need to check your power tranny to make sure it can handle the load
Lake Placid Blues
New member
Re: 5881s in place of 6L6s?
The original RCA 6L6 dates to about 1934. It was the first major beam tetrode audio tube. An advantage of the beam tetrode design was that it was less noisey than a pure pentode design. A pentode is more efficient for power output though. The 6L6 was very successful and was used among several new circuit designs.
In about 1937 the British firm of Marconi-Osram, or M-O Valves, invented the KT-66 as a more capable 6L6 alternative. The KT-66 quickly became the gold standard of high output audio power tubes, and many would contend that it still is. It's plate dissipation was 27 watts when the regular 6L6 of the era had a PD of 18-watts. The KT-66 demonstrated that could handle plate voltages in excess of 500 volts.
In the 50's M-O Valves and GEC merged, and the KT-66 became a GEC standard as well. Mullard then came out with their own 6L6 alternative called the EL37. The EL37 sounded very American 6L6, but these don't handle high plate voltages. 400 V max.
In America, the 5881/6L6 were developed to handle greater voltages and to produce greater outputs. All of the major tube companies, such as Sylvania, Tung-Sol, GE..ect.. had their own versions and RCA was no longer the only game in town for 6L6/5881. The military grade 5881 had to have a smaller bottle per millitary specifications. Later the 6L6GC was further developed into the 7581 as a more capable mil spec version. The 7581 is probably the most powerful and cleanest 6L6 of all. Its the king of clean for Fenders IMO.
As Jeremy has stated, the 6L6 of today and the 5881 are pretty much interchangeable. Even during the late 50's and early 60's Fender specified 5881, but often installed 6L6GC anyway. Different plate dissapation simply require a different bias adjestment if the 6L6GC has 30 watt PD and the 5881 has 25 watt PD. Bias should always be adjusted to each particular set of tubes anyway IMO. The Sovtek 5881 that became the standard 6L6 during the 90's is pretty much a regular 6L6. I like the =C= 6L6GC for a strong American style 30 watt PD 6L6GC/7581.
KT-66's of today run from 6L6's simply re-labelled as KT-66, to real serious attempts at real deal KT-66's, to regular 6L6 guts installed in big curvey KT-66 looking bottles, to KT-88s or 6550's re-labelled as KT-66. One must be careful about the correct specs of the Kt-66 labelled tube in question.
The original RCA 6L6 dates to about 1934. It was the first major beam tetrode audio tube. An advantage of the beam tetrode design was that it was less noisey than a pure pentode design. A pentode is more efficient for power output though. The 6L6 was very successful and was used among several new circuit designs.
In about 1937 the British firm of Marconi-Osram, or M-O Valves, invented the KT-66 as a more capable 6L6 alternative. The KT-66 quickly became the gold standard of high output audio power tubes, and many would contend that it still is. It's plate dissipation was 27 watts when the regular 6L6 of the era had a PD of 18-watts. The KT-66 demonstrated that could handle plate voltages in excess of 500 volts.
In the 50's M-O Valves and GEC merged, and the KT-66 became a GEC standard as well. Mullard then came out with their own 6L6 alternative called the EL37. The EL37 sounded very American 6L6, but these don't handle high plate voltages. 400 V max.
In America, the 5881/6L6 were developed to handle greater voltages and to produce greater outputs. All of the major tube companies, such as Sylvania, Tung-Sol, GE..ect.. had their own versions and RCA was no longer the only game in town for 6L6/5881. The military grade 5881 had to have a smaller bottle per millitary specifications. Later the 6L6GC was further developed into the 7581 as a more capable mil spec version. The 7581 is probably the most powerful and cleanest 6L6 of all. Its the king of clean for Fenders IMO.
As Jeremy has stated, the 6L6 of today and the 5881 are pretty much interchangeable. Even during the late 50's and early 60's Fender specified 5881, but often installed 6L6GC anyway. Different plate dissapation simply require a different bias adjestment if the 6L6GC has 30 watt PD and the 5881 has 25 watt PD. Bias should always be adjusted to each particular set of tubes anyway IMO. The Sovtek 5881 that became the standard 6L6 during the 90's is pretty much a regular 6L6. I like the =C= 6L6GC for a strong American style 30 watt PD 6L6GC/7581.
KT-66's of today run from 6L6's simply re-labelled as KT-66, to real serious attempts at real deal KT-66's, to regular 6L6 guts installed in big curvey KT-66 looking bottles, to KT-88s or 6550's re-labelled as KT-66. One must be careful about the correct specs of the Kt-66 labelled tube in question.
StillLearnin'
New member
Re: 5881s in place of 6L6s?
Well, 5881's have a lower plate voltage than a 6L6GC. Most Fender amps will easily accept either one as the B+ is generally 400 or a little less. 5881's (even the new Tung Sol's are rated at 400V while 6L6GC's are typically 450-500. I like the sound of 5881's but before you swap them in the B+ in your amp needs to be checked. They will run on a higher rail, but their life will be cut short - I know, because I've done it.
Well, 5881's have a lower plate voltage than a 6L6GC. Most Fender amps will easily accept either one as the B+ is generally 400 or a little less. 5881's (even the new Tung Sol's are rated at 400V while 6L6GC's are typically 450-500. I like the sound of 5881's but before you swap them in the B+ in your amp needs to be checked. They will run on a higher rail, but their life will be cut short - I know, because I've done it.
dr.barlo
New member
Re: 5881s in place of 6L6s?
Thanks for the info guys. It is great to learn the differences in detail.
In my case, I replaced the 5881's with 6L6's, and there was not much biasing needed. But because that I like more clean headroom and basically higher voltages, after checking the power and output transformer, the guy I go to for my amps (he is incredibly knowledgeable, the man wound a transformer for my needs on my Peavey C30, he is great. But he is not into guitar amps that much, because guitar players are poor he says.
Instead he is locally known as the one and only go-to guy for hi-end audio stuff), changed the value of some caps and resistors, and gave more voltage to the circuit.
I dunno what I currently have, yet it is a JTM30 with a bit higher voltage and 6L6s.
To check whether or not I understand the thing well, let me ask the following for a confirmation:
So if I were to replace 6L6's with 5881's, it either must be hot biased (as StillLearnin' said) or the plate voltage needs to be decreased. No?
B
Thanks for the info guys. It is great to learn the differences in detail.
In my case, I replaced the 5881's with 6L6's, and there was not much biasing needed. But because that I like more clean headroom and basically higher voltages, after checking the power and output transformer, the guy I go to for my amps (he is incredibly knowledgeable, the man wound a transformer for my needs on my Peavey C30, he is great. But he is not into guitar amps that much, because guitar players are poor he says.
Instead he is locally known as the one and only go-to guy for hi-end audio stuff), changed the value of some caps and resistors, and gave more voltage to the circuit. I dunno what I currently have, yet it is a JTM30 with a bit higher voltage and 6L6s.
To check whether or not I understand the thing well, let me ask the following for a confirmation:
So if I were to replace 6L6's with 5881's, it either must be hot biased (as StillLearnin' said) or the plate voltage needs to be decreased. No?
B
StillLearnin'
New member
Re: 5881s in place of 6L6s?
If you're around 400V with the amp biased for 20-40 mA per output tube, you're safe with the 5881's. You can still bias the 5881's without issue with a higher plate voltage (you don't have to necessarily bias them hot), but the higher plate voltage will degrade them quicker. The physical construction and vacuum level in the tube is what controls what kind of plate voltage they can handle. Lowering the B+ will typically entail changing power transformers unless you use a high wattage resistor right after the primary filter caps. The trade off there is a voltage sag when you start cranking things up (because of the voltage drop across the resistor) Actually, it's a mod that some folks do to simulate the sag of a tube rectifier in a solid state rectified amp. Personally, I don't like that approach.
Thanks for the info guys. It is great to learn the differences in detail.
In my case, I replaced the 5881's with 6L6's, and there was not much biasing needed. But because that I like more clean headroom and basically higher voltages, after checking the power and output transformer, the guy I go to for my amps (he is incredibly knowledgeable, the man wound a transformer for my needs on my Peavey C30, he is great. But he is not into guitar amps that much, because guitar players are poor he says.
I dunno what I currently have, yet it is a JTM30 with a bit higher voltage and 6L6s.
To check whether or not I understand the thing well, let me ask the following for a confirmation:
So if I were to replace 6L6's with 5881's, it either must be hot biased (as StillLearnin' said) or the plate voltage needs to be decreased. No?
B
If you're around 400V with the amp biased for 20-40 mA per output tube, you're safe with the 5881's. You can still bias the 5881's without issue with a higher plate voltage (you don't have to necessarily bias them hot), but the higher plate voltage will degrade them quicker. The physical construction and vacuum level in the tube is what controls what kind of plate voltage they can handle. Lowering the B+ will typically entail changing power transformers unless you use a high wattage resistor right after the primary filter caps. The trade off there is a voltage sag when you start cranking things up (because of the voltage drop across the resistor) Actually, it's a mod that some folks do to simulate the sag of a tube rectifier in a solid state rectified amp. Personally, I don't like that approach.
glassman
Electron Herder
Re: 5881s in place of 6L6s?
The Peavey 6505 has what I term a "user bias adjustment" on it. It's sweep is very small in relation to that of a Marshall or a Fender. The range on a "middle of the road" set of 6L6s is in the 14 to 22 mA range; extremely cold to cold). The plate voltage is around 490 volts at it's warmest setting and 505 volts at it's coldest. It is set up like a Mesa, designed to be very tight and articulate.
If you really are serious about running 5881s, the resistor mod StillLearnin mentioned works, as does substituting a 91 volt, 10 watt zener diode for that resistor. It provides the voltage drop without the resistive sag. The tone will be more "middy" and you would probably want to have the bias network modded to allow for a hotter bias. The amp really isn't set up to sound good this way so proceed at your own risk.
The Peavey 6505 has what I term a "user bias adjustment" on it. It's sweep is very small in relation to that of a Marshall or a Fender. The range on a "middle of the road" set of 6L6s is in the 14 to 22 mA range; extremely cold to cold). The plate voltage is around 490 volts at it's warmest setting and 505 volts at it's coldest. It is set up like a Mesa, designed to be very tight and articulate.
If you really are serious about running 5881s, the resistor mod StillLearnin mentioned works, as does substituting a 91 volt, 10 watt zener diode for that resistor. It provides the voltage drop without the resistive sag. The tone will be more "middy" and you would probably want to have the bias network modded to allow for a hotter bias. The amp really isn't set up to sound good this way so proceed at your own risk.
StillLearnin'
New member
Re: 5881s in place of 6L6s?
I hadn't thought of the zener deal - I may try that on one of my frankensteins. I'll have to hunt down some large value zeners though - those aren't available at my local electronics shop.
I hadn't thought of the zener deal - I may try that on one of my frankensteins
. I'll have to hunt down some large value zeners though - those aren't available at my local electronics shop.
