JTM45 RI Bias

[Alabama Thunderpussy]

I asked Marshall what the factory fitted Sovtek 5881's bias should be set to & this was the reply...

"Thank you for your e-mail regarding your question the bias on the JTM 45 we recommend when fitting 5881 the bias is set to -48VDC."

This obviously doesn't refer to the 1 ohm resistor method, could anyone explain it to me please.

Thanks.

Yeah nevermind the plate voltage or current. Just set it to -48VDC

 

[Birk]

I've replaced 5881 for KT-66 in my JTM45RI some time ago. Belive I set bias for about 45 mA, if memory serves me right.

 

[MajorNut1967]

I've replaced 5881 for KT-66 in my JTM45RI some time ago. Belive I set bias for about 45 mA, if memory serves me right.

Hi Birk,

Do you remember what your plate voltages with the KT66?

 

[musicman]

I tried Svetlana Winged C 6L6 but they are way too clean.

Currently waiting for a pair of JAN Philips 5881's.

 

[Birk]

Hi Birk,

Do you remember what your plate voltages with the KT66?

Checked my notes now, it appeared to 45mA and 391 volts.
I can not remember if I adjusted the bias lower after that.

 

[musicman]

25w / 391v = 64ma x 70% = 45ma

Your voltage is quite a lot lower than mine @ 424v

 

[MajorNut1967]

I bet it sounds great though! It's not necessary to have massive plate voltages to have great tone.

 

[MajorNut1967]

I tried Svetlana Winged C 6L6 but they are way too clean.

Currently waiting for a pair of JAN Philips 5881's.

Having tube GAS huh? those are good tubes.

 

[musicman]

Yes, Derek at Watford Valves recommended them and has given me a decent discount.

The = C = are for sale if anyone is interested.

Very well matched and only been used for an hour or so.

 

[Birk]

25w / 391v = 64ma x 70% = 45ma

Your voltage is quite a lot lower than mine @ 424v

Well, first time around, the voltage was 351 volts.
After some reseach, I found out that my 8 ohm cab wasn't 8 ohm at all, and used another cab instead, then I changed the rectifier tube.

I don't know why the voltage is that low, but according to my friend (who is an amptech), it's ok.

The JTM45 is now for sale btw...

 

[MajorNut1967]

Don't worry guys my 1968 Model 1987 read only 383vdc on the plates, and it sounds tits!

 

[musicman]

So the JAN Philips 6L6WGB (5881) arrived today.
I was surprised how small they are.
Anyway, put them in and biased to 38ma (23w / 424v x 0.7 = 38ma).
I'm not overly impressed, they're quite clean.
I actually prefer the sound of the Sovtek 5881's that were in the amp.
Is 38ma right or do the JAN Philips need to be biased "hotter" ?

 

[MajorNut1967]

So the JAN Philips 6L6WGB (5881) arrived today.
I was surprised how small they are.
Anyway, put them in and biased to 38ma (23w / 424v x 0.7 = 38ma).
I'm not overly impressed, they're quite clean.
I actually prefer the sound of the Sovtek 5881's that were in the amp.
Is 38ma right or do the JAN Philips need to be biased "hotter" ?

If they are real JAN Philips 6L6WGB, they are a 26 watt tubes. lets bump up to 44mA and it is a 6L6 variant not a true 5881; so it will be very clean.

 

[musicman]

This is the blurb from the Watford Valves website...

"There are the original small bottle American Military JAN 6L6WGB/5881 that was made for the USA Military in the mid 1980S. These are Full Jan Specification which means that they are long life and superbly made with the highest grade materials. No modern day 6L6GC valve is made to this specification
These are the ultimate 6L6 type for use in any Fender amplifier but paticularly the Fender bassmans as no other valves gives a 59 bassman the early breakup and that rich midrange twang as this classic.The bass response is big and tight with a smooth warmth. It provides all the classic Fender sounds with ease. Solo and riffs are punchy and fat It also does not get muddy and fizzy under extreme satuaration. So it is equally at home in modern high gain amplifiers.
This is why this one of my all time favourites and the one that I have used in my own amps for many years.
I have run these myself in a series of Silveface Fender Twins, Mesa Boogie MK 4,Boogie 400 plus and Fender bassmans including Silverface 100 head.
The specification of these valves are also different from the Early Tung-sol and R.C.A 6L6WGB valves as these can supply 30 watts plate dissipation without any problems and can run with 6l6GC anode voltages."

So maybe safe to bias up to 30w / 424v x 0.7 = 49ma ?

 

[MajorNut1967]

This is the blurb from the Watford Valves website...

"There are the original small bottle American Military JAN 6L6WGB/5881 that was made for the USA Military in the mid 1980S. These are Full Jan Specification which means that they are long life and superbly made with the highest grade materials. No modern day 6L6GC valve is made to this specification
These are the ultimate 6L6 type for use in any Fender amplifier but paticularly the Fender bassmans as no other valves gives a 59 bassman the early breakup and that rich midrange twang as this classic.The bass response is big and tight with a smooth warmth. It provides all the classic Fender sounds with ease. Solo and riffs are punchy and fat It also does not get muddy and fizzy under extreme satuaration. So it is equally at home in modern high gain amplifiers.
This is why this one of my all time favourites and the one that I have used in my own amps for many years.
I have run these myself in a series of Silveface Fender Twins, Mesa Boogie MK 4,Boogie 400 plus and Fender bassmans including Silverface 100 head.
The specification of these valves are also different from the Early Tung-sol and R.C.A 6L6WGB valves as these can supply 30 watts plate dissipation without any problems and can run with 6l6GC anode voltages."

So maybe safe to bias up to 30w / 424v x 0.7 = 49ma ?

HMM! Sounds like a bunch of HooYa why don't we stick with the original 6L6WGB SPEC. to be safe! try it at 44mA.

http://www.mif.pg.gda.pl/homepages/frank/sheets/087/6/6L6WGB.pdf

 

[musicman]

Will give it a go tomorrow.
Thanks for your advice.

 

[demonufo]

I guess these must be tubes that were made in the Sylvania plant then?
Quite a lot of "different" tubes coming out of there in the 80's.

 

[musicman]

Reset the bias to 44ma - well 44 on one tube 41 on the other - the pair is not particularly well matched & the difference follows the tube.
Sounds better but I'm not blown away.
Think I'll try some KT66's at some point but I wouldn't spend £72 on a pair of JAN Philips again.
Thanks for everyone's help and advice.

 

[MajorNut1967]

Reset the bias to 44ma - well 44 on one tube 41 on the other - the pair is not particularly well matched & the difference follows the tube.
Sounds better but I'm not blown away.
Think I'll try some KT66's at some point but I wouldn't spend £72 on a pair of JAN Philips again.
Thanks for everyone's help and advice.

Well mate, I wish he would've gone with the KT66 power tubes in the first place! If the 5881 power tubes were a few milliamps off spec wise you could ask to return them and get credit towards the purchase of some KT66’s? Anyway the KT 66 will give you a nice clean tone similar to what a 6L6/5881 would, but when pushed hard they break up really nice and would give you that old classic Marshall sound. With that being said I can't believe they charged you 72 quid for a pair of 5881 tubes.

 

[musicman]

The amp has a sticker on the chassis saying replace with 5881 valves only.
Don't get me wrong I like the amp, I just prefer the Sovtek valves.
Whatever you pay in $ we pay in £ so the JAN Philips are £30 each plus 20% VAT (Value Added Tax) - what a rip off.