What Tubes Can I Use?

[Helmholtz]

Now the (original) KT77 is a very special tube. Like all of the KT family is was manufactured only by a single company (Osram > MO-V = Marconi-Osram-Valves > GEC > Genalec ("Gold Lion"): all the same manufacturer). Some of its data looks similar to the EL34 but it is a completely different tube. Like all KT tubes it is a beam power tetrode (KT meaning "kinkless tetrode") and not a pentode like the EL34. It was designed/optimized for UL ("ultralinear") operation and seems to have a delicate screen structure. The spec limits screen voltage to 300V if not used in UL:
KT77.pdf

Nevertheless it was used as Marshall OEM equipment in the 80s for a short period, of course with screen voltages considerably exceeding the 300V limit. While the sound was great, the tubes soon developed structural rattling.

A schematic showing modifications and measured voltages would be much more useful than the layout.

[gtrplayr1976]

No harm in using a smaller value bass pot. In some cases I've installed as low as 50K. Also, there's no "bright" cap on the master volume. Of course that won't make any difference if it's all the way up. But if you ordinarily have it dialed down some, a cap across hot-to-wiper will bring through the upper frequencies. What value to use? That is something you will have to select according to your own needs. Anything from 47 pF to 0.005 uF might do.

Since it's a dual gang pot which part would get the cap?

[gtrplayr1976]

I'm a 40 year pro guitarist and run a JTM-45 regularly that has an 8K OT impedance. I have tried every plausible iteration of octal power tube and made every necessary circuit adjustment (resistors, OT taps, etc) to give them the best possible operating environment in which to survive.

In my experience the 5881 delivers the goods better than the KT77 or the EL34. For my tastes I don't prefer the rounded (BIG bass) tone of the KT66 in that amp and the 6L6GC is close but no cigar, for me.

Try a pair of Tung Sol (New Sensor) or EH 5881 with moderately high current draw numbers. I believe this tube type would mitigate the excessive bass and provide the best overall tone for the JTM45, IME.

Excessive bass can also be a function of your speaker(s) system. What speaker(s) are you using?

I was testing with a 1x12 Hellatone. I will be using a 2x12 , or pair of 2x12s most likely with Mojotones ,and Celestions ,or WGS.

[gtrplayr1976]

I'm willing to bet that Marshall combo with 5881s has Sovtek 5881WXTs in it, which it's worth pointing out are a lot closer (or possibly exceeding) to 6L6GCs, not true 5881s. Just an FYI to be aware of.

Justin

My combo had 6l6s in it.(probably added by previous owner) I have JJ 5881s in it now.

[gtrplayr1976]

Now the (original) KT77 is a very special tube. Like all of the KT family is was manufactured only by a single company (Osram > MO-V = Marconi-Osram-Valves > GEC > Genalec ("Gold Lion"): all the same manufacturer). Some of its data looks similar to the EL34 but it is a completely different tube. Like all KT tubes it is a beam power tetrode (KT meaning "kinkless tetrode") and not a pentode like the EL34. It was designed/optimized for UL ("ultralinear") operation and seems to have a delicate screen structure. The spec limits screen voltage to 300V if not used in UL:
[ATTACH]55305[/ATTACH]

Nevertheless it was used as Marshall OEM equipment in the 80s for a short period, of course with screen voltages considerably exceeding the 300V limit. While the sound was great, the tubes soon developed structural rattling.

A schematic showing modifications and measured voltages would be much more useful than the layout.

Good to know. I will avoid those with this amp now. I was using JJs.

[Helmholtz]

I was testing with a 1x12 Hellatone. I will be using a 2x12 , or pair of 2x12s most likely with Mojotones ,and Celestions ,or WGS.

The best, most authentic JTM45 sound I have heard up to now was with a 4x12 cab and Celestion G12M-20 Heritage speakers. Sounded better to me than 2X12 Celestion Alnico Golds in an open cab. (The 2 Alnicos seemed much louder, though).
BTW, the output with 8k primary impedance will hardly exceed 30W.

[Leo_Gnardo]

Since it's a dual gang pot which part would get the cap?

I'm not seeing a dual gang mv in the layout you posted. If you have converted your amp to have ppimv instead, then the answer is, both.

A word about JJ's 5881: I'm a big fan! It seems more sturdy than JJ would have you believe. I've been installing them in all sorts of places 6L6GC would usually go. Delivers good power, and in 3 years I haven't had to replace one yet, maybe 100ish of them so far. And no complaints from customers either. I'd recommend not setting bias way high on these. 30 mA plenty enough in amps with a 450-500V B+ and similar screen grid like most Fenders.

Also, some experience with JJ's KT77. I'm a fan of those too. Found JJ had just started making them in 2006 when a customer brought in 3 rare Burman amps that called for '77s. B+ about 500V & screen grids not far behind. They worked flawlessly. I've also used them in Dyna Mk II hi fi amps I use in my living room stereo & they gave me a good long run before starting to get flakey, say 8 years. If JJ KT77 sounded good in your '45 I'd say no need to avoid them, score another pair, they're affordable enough. "If it sounds good, it is good." Duke Ellington. "Sounded great, just before it blew up." Joe Walsh.

BTW if you installed them in a typical MusicMan, they put 700V on the plates & 350 on the screen grids. Unless your MM was one of those later models that used lower voltage.

[gtrplayr1976]

I'm not seeing a dual gang mv in the layout you posted. If you have converted your amp to have ppimv instead, then the answer is, both.

A word about JJ's 5881: I'm a big fan! It seems more sturdy than JJ would have you believe. I've been installing them in all sorts of places 6L6GC would usually go. Delivers good power, and in 3 years I haven't had to replace one yet, maybe 100ish of them so far. And no complaints from customers either. I'd recommend not setting bias way high on these. 30 mA plenty enough in amps with a 450-500V B+ and similar screen grid like most Fenders.

Also, some experience with JJ's KT77. I'm a fan of those too. Found JJ had just started making them in 2006 when a customer brought in 3 rare Burman amps that called for '77s. B+ about 500V & screen grids not far behind. They worked flawlessly. I've also used them in Dyna Mk II hi fi amps I use in my living room stereo & they gave me a good long run before starting to get flakey, say 8 years. If JJ KT77 sounded good in your '45 I'd say no need to avoid them, score another pair, they're affordable enough. "If it sounds good, it is good." Duke Ellington. "Sounded great, just before it blew up." Joe Walsh.

BTW if you installed them in a typical MusicMan, they put 700V on the plates & 350 on the screen grids. Unless your MM was one of those later models that used lower voltage.

I think I posted the wrong layout.
I put the dropping resistors back to what they were ,and tried the impedance thing switching the amp to 16 ohms. Didn't hear much if any difference. Going to try a different cab ,and possibly try the 5881s. I think the set I have though are fairly cold.
Yes that set of KT77s is currently in the MusicMan with 700 plate volts. I was told they would handle it?
My JTM has the post PI MV like the one attached.

[Helmholtz]

Also, some experience with JJ's KT77. I'm a fan of those too. Found JJ had just started making them in 2006 when a customer brought in 3 rare Burman amps that called for '77s. B+ about 500V & screen grids not far behind. They worked flawlessly. I've also used them in Dyna Mk II hi fi amps I use in my living room stereo & they gave me a good long run before starting to get flakey, say 8 years

Don't know about the Burmans but AFAIK the Dyna MK IIs are UL amps. With UL operation the KT77 spec allows for an absolute max screen voltage of 650V. The screen voltage limit for UL is often higher than for standard operation as the transformer coupling forces the instantaneous screen voltage to follow the plate voltage, both changing in-phase. Consequently plate voltage never gets lower than screen voltage. In standard PP designs high instantaneous plate current causes the plate voltage to be lower than the screen voltage making the screen an attractive target for the electrons. The consequent increase of screen current and dissipation is the reason for the lower screen voltage limit.
(Absolute plate voltage limit of the KT77 is 850V.)

Of course the JJ KT77 may be more rugged than the original GEC tube - but I tend to doubt it.

[Tone Meister]

EDIT FOR CORRECTION: Bias the reissue Tung Sol 5881 as 23 watt tubes.

30 - 32 mA is plenty @ 450Vp.

[gtrplayr1976]

Bias the Tung Sol 5881 as 26 watt tubes

The set I have are relatively cold? Will I need a hotter set?

[gtrplayr1976]

Bias the Tung Sol 5881 as 26 watt tubes

With the 5881 do I need to use the 16 ohm setting with the 8 ohm cab?

[Helmholtz]

The set I have are relatively cold? Will I need a hotter set?

No, but you need to set/adjust the grid bias using the bias pot.

[Helmholtz]

With the 5881 do I need to use the 16 ohm setting with the 8 ohm cab?

Not necessarily. The 16 Ohm setting will allow for more clean output power but may like the sound with the matched 8 Ohm setting better.

[Tone Meister]

With the 5881 do I need to use the 16 ohm setting with the 8 ohm cab?

Start with the 8 ohm output. The 5881 are similar to 6L6 with regard to reflected impedance they want to "see."