6L6GC to KT66

[Jason deBroux]

Have to clarify, I will not be the one making these mods. I would leave that up to a good amp tech.

 

[william vogel]

The advice I was given is to relabel the impedance selector positions if it's a KT66 amp.

4, 8, 16 becomes 8, 16, 32. Match up the cabinet impedances to the new markings.

A 1962 model JTM should have an 8k output transformer.

 

[william vogel]

Can you tell me more about this solid state plug in rectifier?

Solid State Rectifier

This octal based solid state rectifier is a cost effective replacement for any of the following tubes: 5AR4 / GZ34, 5Y3, 5U4G / 5U4GB, 5R4 / 5V4 / 5Z4 Not only is this an inexpensive alternative, but this solid state rectifier will firm up your low end. If you have too much sag in your low end, then

tubedepot.com

 

[myersbw]

Everyone has given you the electrical rundown on the tube selection to a degree. I'm going to come at it from pure tone. A friend brought a Dr. Z Antidote prototype (#2) to me. He wanted it converted to 1978 Marshall JMPP 2203 MV MKii specs. This prototype was setup with dual bias circuits (EL34/KT66)...just flip a switch and change tubes. So...we did! He hated the KT66's and loved the E34L's as Marshall is his goto. I loved the KT66's! They gave a huge bell-like mid-upper range tone. You cannot mistake them...you will hear a huge difference. I treated the KT66 bias like I would a 25w tube to capitalize on tube life. I was not focused on power tube breakup with a master volume preamp. We used a pair of Gold Lions...they're nice tubes. Not for everyone though.

 

[Pete Farrington]

When amp is "designed" for KT66 then "correct" output/speaker options should be used.

I suspect Marshall never used normal valve power amp design processes (eg 'through the knee' loadline).
Rather it may be that a Fender 5F6A schematic and roughly suitable off the shelf transformers etc available locally were used for the initial prototype, and as that sounded and worked well, production followed that 'blueprint',
A few changes seem to have been made incrementally, eg the initial 5881 mil surplus output valves were replaced with KT66, HT fusing, shared screen grid resistor added.
An RS Deluxe OT, typically wired for 6k6 to 15 ohms, seems to have been used for the first couple of years, then a 784-103, just under 8k primary.
Both 6k6 and 8k are somewhat below the 6L6 / 5881 or KT66 knee, eg

Possibly KT66 produce more distortion if/when 6L6GC OT impedance is not "optimized" for KT66

About 4k seems good for 2nd generation and later 6L6 at typical valve guitar amp loaded HT levels eg KT66

6L6GC
1st gen 6L6 have a 19W design centre 22W design max anode limit, 6L6, G, GA, GB.
2nd gen were uprated about 20%, with a 23W design centre anode limit, 5881, 6L6WGB. I suggest that KT66 are considered as being closely similar in many ways, a bit different in others, to these.
3rd gen were further uprated about another 13%, with a 30W design max, (probably about 26W design centre) anode limit, 6L6GC, 7027A, 7581.

using too high OT impedance definitely increase distortion

My understanding is that as load impedance increases, from right of knee, to 'through the knee', to below the knee, linearity improves. Take that far enough and what was an AB amp will become class A. Note that as the loadline is moved away from 'through the knee', max power output will tend to reduce.
eg from https://frank.pocnet.net/sheets/084/k/KT66_GEC.pdf see

 

[william vogel]

Marshall definitely missed the marks when they chose the output transformer for the JTM45 if they made their decision based on power output and minimal distortion but I think they just winged it with an available product and it was a hit with the audience. I’m sure later they shook their heads once they figured out it could have been better. Along came the EL34 and yet another output transformer and it was right in every way. None of the math matters if you like the sound of a JTM45.

 

[AlvisX]

GE 7581a will physically fit in a 70's JMP 2x12 combo cab , which is about the same depth of an early RI BB ,is it not ?

 

[Jason deBroux]

Solid State Rectifier

This octal based solid state rectifier is a cost effective replacement for any of the following tubes: 5AR4 / GZ34, 5Y3, 5U4G / 5U4GB, 5R4 / 5V4 / 5Z4 Not only is this an inexpensive alternative, but this solid state rectifier will firm up your low end. If you have too much sag in your low end, then

tubedepot.com

Since the product says it increase will B+ (plate voltage) 20-30% any competent tech should be able to make the necessary change to the filter caps, correct?

 

[william vogel]

Since the product says it will B+ (plate voltage) 20-30% any competent tech should be able to make the necessary change to the filter caps, correct?

Your amp won’t have that much increase in voltage. It won’t need any modifications to the capacitors. It’ll give you about 9 volts at idle and then it’ll maintain a more stable sag than with the GZ34 but nothing like 20% increase.

 

[anitoli]

20% no. 460 +20% = 552.

 

[Jason deBroux]

20% no. 460 +20% = 552.

I’m not great with electrical theory, do you mind expanding on this ?

 

[anitoli]

Say your amp is getting 460VDC B+ with a tube rectifier, if changing to an SS rectifier increases the B+ 20% your B+ voltage is going to increase to 552VDC. Nope. More realistically the B+ voltage will increase 2-3%. 460 + 2% = 469.2. This is more in the ball park. The advantage in some eyes with an SS rectifier is you will see less voltage sag as you start driving the amp harder.

 

[Pete Farrington]

the product says it increase will B+ (plate voltage) 20-30%

Err, it says an increase of 10-20%.
That degree of increase will be at full load, when the cap voltage would be lower than at idle anyway.

 

[Jason deBroux]

So it’s sounds like a plug n play type mod and should be safe to try?

 

[knulp]

My two cents.

Find a tech and have him to do the work

Kt66 are worth it,really worth it.

Marshall only use 6l6 when there’s no other option ,you ‘ll know why if have the chance to listen to the difference in the same amp

The only reason to prefer 6l6 is they are cheaper,and better only if you play very tight and fast metal with only preamp gain and some kind of scooped cleans

For classic Marshall tones kt66 are what you want

 

[Jason deBroux]

My two cents.

Find a tech and have him to do the work

Kt66 are worth it,really worth it.

Marshall only use 6l6 when there’s no other option ,you ‘ll know why if have the chance to listen to the difference in the same amp

The only reason to prefer 6l6 is they are cheaper,and better only if you play very tight and fast metal with only preamp gain and some kind of scooped cleans

For classic Marshall tones kt66 are what you want

Yea I don’t play anything scooped. Nor do I play metal.

 

[Pete Farrington]

Me neither, but 6L6 / 5881 (real and Russian) sound fine in my JTM45 and JTM50 based builds.
Granted I've got cloth ears but I play loud enough to notice if something sounds cr@p.
As per above, 5881 ARE a 6L6, and with a slight rebias, KT66 are too. ie for given anode & screen grid voltage and anode current, they all share the same Ra and gm. And initial JTM45 were fitted with 5881 (probably military surplus) then switched to KT66, with no design change I'm aware of (though the bias voltage must have been increased).
To characterise 6L6 as being only suitable for scooped tones and metal doesn't seem right to me.
Bear in mind Jim Marshall described the sound of that first prototype (probably with 5881) as being (to paraphrase) the best sounding amp he's ever heard.

 

[leofender55]

I put Gold Lion KT66 tubes in my BB Reissue (it had 5881's). Ben Fargen also put a Mercury Magnetics transformer and upgraded the caps and tweaked the tone stack. The amp is just crazy good. Mine said 50 W Tremelo on the back plate, and my buddy has the same amp that says JTM45 in the same place. Fargen had heard that the KT66's might not fit in the space for the tubes, but we didn't have a problem.

 

[knulp]

Me neither, but 6L6 / 5881 (real and Russian) sound fine in my JTM45 and JTM50 based builds.
Granted I've got cloth ears but I play loud enough to notice if something sounds cr@p.
As per above, 5881 ARE a 6L6, and with a slight rebias, KT66 are too. ie for given anode & screen grid voltage and anode current, they all share the same Ra and gm. And initial JTM45 were fitted with 5881 (probably military surplus) then switched to KT66, with no design change I'm aware of (though the bias voltage must have been increased).
To characterise 6L6 as being only suitable for scooped tones and metal doesn't seem right to me.
Bear in mind Jim Marshall described the sound of that first prototype (probably with 5881) as being (to paraphrase) the best sounding amp he's ever heard.

Kt66 are a different tube in the same family.
They are a kinkless tetrode.
6l6/5881 are not a kink less tetrode.

Different construction and properties.
Different tone.

No need to have me explain..it’s a known reality since …80 years???

Jim Marshall was a drummer and an enterprising sales man.
In 1962 he copied one of the very few decent amps available and said whatever he wanted
He only worried about cheap availability of parts.
I didn’t say 6l6 are only suitable to scooped metal.

Indeed I love 6l6 in fenders of all kind.

But this happens to be the Marshall forum
As you’ll never see a debate about which is the best 6l6 for a plexi for example

Obvious choice:el34 and kt66 in same specific jtm and plexi,as did Hendrix.

Maybe on some modern Marshall you may find 6l6 to be the best..particularly the hi gain 900 ..they do well with metal

With a rebias you can put 6l6 in a el34 amp.
this doesn’t mean they are the same.

The classic Marshall tone is el34 and kt66 again.

But since we are details obsessed rockers too,let me clarify that a 6l6 jtm45 sounds great and I’m talking metal..

 

[knulp]

Even a drummer like Jim Marshall only interested in making a profit in selling amps did quickly the switch to kt66.

Nobody complained that the tone suffered

Instead a new classic was born

You could say that in Uk kt66 were more readily available..
But Hendrix was in in the USA for example and never did the switch to 6l6.
Instead he kept playing 6l6 fenders and el34 Marshall.
But he was known for for having his plexis loaded with kt66.

I have a small single ended 10 watt Marshall in which I tried every power tube and I never thought that 6l6 are the best choice.

Jim Marshall also said his amps sound better with el34