JTM45 RI Bias

[MajorNut1967]

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.

Well 72 quid is 149.50 New Zealand dollars ouch.

See at the voltage you are getting in that amplifier and the 8000 ohm impedance is the perfect match for KT 66 power tubes. And I realize there's a sticker on there that says 5881 but that's a legal issue, Marshall has to state what power tubes they have the design for originally.

 

[musicman]

I'll keep an eye open and give some KT66's a try.

 

[stratele52]

Hi first time in this forum.
I want to put KT66 in JTM45 reissue. But you know that KT fillament need much more filaments current ( amperes) than 6L6.
Does anybody mesure if the power transformer can handle this at 6.3 volts at filaments ?

 

[MajorNut1967]

Hi first time in this forum.
I want to put KT66 in JTM45 reissue. But you know that KT fillament need much more filaments current ( amperes) than 6L6.
Does anybody mesure if the power transformer can handle this at 6.3 volts at filaments ?

You are correct that the KT66 draws about 1.3amps which is considerably more than the 6l6 .9amps. but most of the PT Marshall uses should have enough current, because some of the reissues used EL34's and they draw 1.5amps and I doubt there are that much difference for the reissues.

 

[musicman]

I'm waiting for a pair of Valve Art KT66's to arrive from the US and will post an update when they arrive and I get chance to try them.

 

[musicman]

Valve Art KT66's have arrived.
I'm thinking 25w / 424v x 0.7 = 41 mv/ma bias setting
I see the Marshall Vintage Modern 2266 runiing 2 x KT66 bias is recommended to be between 43 & 48 mv/ma
Any suggestions please (plate voltage is 424v)

 

[Lane Sparber]

Valve Art KT66's have arrived.
I'm thinking 25w / 424v x 0.7 = 41 mv/ma bias setting
I see the Marshall Vintage Modern 2266 runiing 2 x KT66 bias is recommended to be between 43 & 48 mv/ma
Any suggestions please (plate voltage is 424v)

Biasing RIGHT at 70% dissipation is not advisable because there's no headroom to allow for wall voltage fluctuation. I'd start at 65% and see how that sounds to you, so let's re-calculate:

25w / 424v x .65 + 5ma (screen current) =43ma per tube.

Remember, bias is NOT the be-all end-all of tone, and way too many people obsess about it. Just set it so it sounds good (within the tubes' design parameters, that is!). That's just my two cents. Take it or leave it.

Good luck!

-Lane

 

[musicman]

That's great thanks Lane

 

[musicman]

Tried the KT66's but the lowest bias I can achieve is 47.9ma on one and 49.0ma on the other.
Is this acceptable please ?
25w / 424v x 0.7 + 5ma screen current = 46ma so I guess not !
So to the use KT66's the amp needs to go to my tech.
What does he need to do please ?
Thanks.

 

[Lane Sparber]

Well, if THIS is the schematic for your reissue:

jtm45-readable

...then I'd change the 180k resistor in the bias supply to a 150k or 160k. This will let more negative voltage through so that you can get those tubes idling coolly enough.

-Lane

 

[musicman]

Thanks Lane - will get this info to my tech

 

[MajorNut1967]

First of all a KT 66 is 30 watt tube! If you don't have enough BIAS voltage change the bias input resistor to 150k to start, you may have to go down as far as 120k. Depending on how the amp is being biased get within the 65% range, then adjust to what sounds best to you. Good luck. Oh by the way those Valve Art KT 66's are great!

 

[musicman]

OK so now...
30w \ 424v x 0.65 + 5ma = 50ma
(alternatively 30w / 424v x 0.70 = 49.5ma)
So my 47.9 & 49.0 is ok ?

 

[MajorNut1967]

OK so now...
30w \ 424v x 0.65 + 5ma = 50ma
(alternatively 30w / 424v x 0.70 = 49.5ma)
So my 47.9 & 49.0 is ok ?

Well are you red-plating? Does the Amp sound good? Are you happy? If you answered "No, Yes and Yes" then you are ok.

 

[musicman]

Will have a look tomorrow (I took the KT66's out and put the Sovtek 5881's back in as I thought the KT66 bias was too high)

 

[yladrd61]

I have noticed that with 470 ohm screen resistors I am getting closer to a 2 ma. screen current with 6L6GC tubes in my Fender Bassman so you may want to measure the voltage drop across your screen resistors for your actual screen current.

 

[musicman]

Will do thanks.
I've actually decided to take it to my tech next week and let him carry out these mods Marshall JTM-45 and '62 Bluesbreaker Reissue Modifications (apart from the new transformers as these are too expensive) so hopefully all will be sorted out.

 

[MajorNut1967]

Will do thanks.
I've actually decided to take it to my tech next week and let him carry out these mods Marshall JTM-45 and '62 Bluesbreaker Reissue Modifications (apart from the new transformers as these are too expensive) so hopefully all will be sorted out.

Careful you go that far with mods and it will sounds like my amp, real mushy and flubby "Like the old ones". you may not like it, so go a little at a time.

 

[yladrd61]

My friend just got an email from George at Metropoulis he just finished building her JTM45 it looks just like the real thing I hope it sounds really mushy and flubby like the Major's

 

[musicman]

OK, the mods I'm asking him to make are:

1. Change power tubes to ValveArt KT66's and rebias Rebias to around 40ma at idle.
2. Change the filter cap near the GZ34 to a 32X32uF. The other one at the other end of the chassis should be changed to a 16X16uF.
3. Make small parts changes according to the schematic and Ken Bran's handwritten notes which are found on the first schematics page in Doyle's Marshall Book.
1.Change (C3) 330uF at V1 cathode to 250uF. A low voltage (25v) electrolytic is fine.
2.Change mix resistors (R16, R18) from 470k to 270k.
3.Change Ch I mix resistor bypass cap (C6) from 220pF to 500--550 pF.
4.1k cathode resistor R15 on V2 was originally 820ohms.
5.The .1uF cap at the Presence pot originally was connected from the pot wiper (center terminal) to ground.

It seems as though changing the filter capacitors will make the biggest difference ?