What Tubes Can I Use?

[gtrplayr1976]

Hey folks. Apparently I'm not educated on exactly how to tell what tubes can be used in specific amps. I was under the impression that if it could handle the plate volts you could use it. I was just told that the el34s I just put in my JTM45 Marshall build don't match the OT primary resistance ,but was told I could use the 16 ohm tap on my amp with the 8 ohm cab ,and that would give the tubes what they are looking for? I was using KT77s before that ,and they seemed to work ok. I removed those because they are part of a quad I sold with a MusicMan amp. I just thought most Marshalls used el34s. Can someone please shed some light on this?
It started with me replacing these tubes ,and some dropping resistors because I was thinking the voltage was too high. The amp has no noise ,but doesn't sound very good in general. Sounded way better with the KT77s. Along with changing these dropping resistors I changed some others where I had installed metal film ,and metal oxide. (it's all I had when I was building it) I also replaced the preamp tubes with some vintage Amperex. I was thinking this may have some to do with the tone as well. Maybe just one is bad ? I'm planning on putting the dropping resistors back ,and trying the newer preamp tubes to see what she does.
I'm attaching the specs for the OT I am using. Thanks folks.

[Leo_Gnardo]

The old wisdom - change one thing at a time - evaluate the effects - then continue step by step on the changes. That way if you take a step in a direction that's not pleasing you, then you can back up one step and try something else.

If KT77 sounded good and EL34 not, then score some KT77 same brand if possible and use them. FWIW the '45 iirc was originally equipped with KT66 in mid 60's, real Genelex Gold Lions not Russian so called copies*. And the KT's all of them are tetrodes not pentodes like the EL34. As far as impedance match, lots of amps are run with tubes that aren't optimally matched to the OT. You can try different output loads, sometimes that will help, but it's all rather unpredictable as tone is a subjective thing, and different speakers sound different too not simply a matter of impedance.

* FWIW I've had better luck with Slovak JJ's in the KT series than any of the New Sensor varieties, and nobody I know can afford real Genelex they're gold dust money now.

[gtrplayr1976]

The old wisdom - change one thing at a time - evaluate the effects - then continue step by step on the changes. That way if you take a step in a direction that's not pleasing you, then you can back up one step and try something else.

If KT77 sounded good and EL34 not, then score some KT77 same brand if possible and use them. FWIW the '45 iirc was originally equipped with KT66 in mid 60's, real Genelex Gold Lions not Russian so called copies*. And the KT's all of them are tetrodes not pentodes like the EL34. As far as impedance match, lots of amps are run with tubes that aren't optimally matched to the OT. You can try different output loads, sometimes that will help, but it's all rather unpredictable as tone is a subjective thing, and different speakers sound different too not simply a matter of impedance.

* FWIW I've had better luck with Slovak JJ's in the KT series than any of the New Sensor varieties, and nobody I know can afford real Genelex they're gold dust money now.

Thanks Leo_Gnardo. I had JJs in it before. I'm going to try the easiest things first. Replacing the preamp tubes with the new ones ,and go from there. Put the dropping resistors back the way they were. Really nothing else was changed that should affect the tone.
What was strange is that I had to cut the bass pretty much all the way out on the bridge pickup. The neck was horribly bassy sounding ,and the I know it sounds strange ,but when I played past the 12th fret it was just dead sounding?

[The Dude]

Have you tried using the 16 ohm tap as suggested? I don't think it's that critical, but technically it would be a better match. I would not have changed so many things at one time without an audio test after each mod. Not sure why you changed any dropping resistors. The El34's should be able to handle about the same as the KT77's. You changed preamp tubes, output tubes, power supply resistors, etc. Each of those by itself will have an effect on how the amp sounds. Which of those you don't like is anyone's guess. It would also help to see a schematic of the amp and maybe specify which resistors you are changing. And, what does "doesn't sound very good in general" mean- distorted, no bass, no treble, etc., etc.?

[gtrplayr1976]

Have you tried using the 16 ohm tap as suggested? I don't think it's that critical, but technically it would be a better match. I would not have changed so many things at one time without an audio test after each mod. Not sure why you changed any dropping resistors. The El34's should be able to handle about the same as the KT77's. You changed preamp tubes, output tubes, power supply resistors, etc. Each of those by itself will have an effect on how the amp sounds. Which of those you don't like is anyone's guess. It would also help to see a schematic of the amp and maybe specify which resistors you are changing. And, what does "doesn't sound very good in general" mean- distorted, no bass, no treble, etc., etc.?

Thanks Dude. I'm going to go back ,and get those dropping resistors back where they were. Then start from there. The amp just seems really bassy. I had to cut the bass almost all the way down. Playing on the neck pickup is ridiculously bassy ,and playing past the 12th fret just sounds dead if that makes any sense? May try another cab as well. I'm using a 1x12 with a Hellatone. Should handle it though.

[TomCarlos]

gtrplayr -

Have you tried a different guitar on this amp? I ask because you mention a couple things.... the neck pickup is ridiculously bassy and the notes above the 12th fret are dead. I wonder if you need to adjust the string height and perhaps the pickup position. Maybe the neck pickup is too high on the lower strings?

You might also try the guitar on a different amp to see if you have the same issues. If "yes", then perhaps you could review the guitar setup.

[Mick Bailey]

I've been using the JJ 5881 in some JTM45s and they work out nicely. 5881 were fitted in some early amps and I've had some reissue amps with them fitted from the factory. Here's a chart outlining the various tube types etc.

http://www.marstran.com/Historic%20Data%20Base.htm

[gtrplayr1976]

gtrplayr -

Have you tried a different guitar on this amp? I ask because you mention a couple things.... the neck pickup is ridiculously bassy and the notes above the 12th fret are dead. I wonder if you need to adjust the string height and perhaps the pickup position. Maybe the neck pickup is too high on the lower strings?

You might also try the guitar on a different amp to see if you have the same issues. If "yes", then perhaps you could review the guitar setup.

Thanks Tom. The guitar is fine. I use it regularly.

[gtrplayr1976]

I've been using the JJ 5881 in some JTM45s and they work out nicely. 5881 were fitted in some early amps and I've had some reissue amps with them fitted from the factory. Here's a chart outlining the various tube types etc.

http://www.marstran.com/Historic%20Data%20Base.htm

Thanks Mick. I actually have a Marshall combo with 5881s in it. I may try those if I can't get this to sound the way it should.

[Helmholtz]

Your OT has 8k primary impedance. This is what was typically used in JTM45s with KT66s and the Drake OT. 8K is too high for EL34s and will cause excessive screen dissipation and low output power - might even cause the amp to oscillate. EL34s are fine with 3k to 4k. So it is a good idea to connect your 8 Ohm speaker to the 16 Ohm tap as this will reduce the primary load to 4k.
But the EL34 (a pentode) has characteristics different from the KT66 (a beam-tetrode) and will never produce the original JTM45 sound.

The KT66 has some similarity to the 6L6 and 5881 (both also beam-tetrodes), so these will get you closer and you may use the 8k primary impedance.

Of course bias needs to be adjusted when changing tubes.

It is no surprise that the JTM45 has lots of bass as it is the copy of a bass amp, the '59 Fender Bassman.

Showing your actual circuit and voltages may help to give further advice.

[gtrplayr1976]

Your OT has 8k primary impedance. This is what was typically used in JTM45s with KT66s and the Drake OT. 8K is too high for EL34s and will cause excessive screen dissipation and low output power - might even cause the amp to oscillate. EL34s are fine with 3k to 4k. So it is a good idea to connect your 8 Ohm speaker to the 16 Ohm tap as this will reduce the primary load to 4k.
But the EL34 (a pentode) has characteristics different from the KT66 (a beam-tetrode) and will never produce the original JTM45 sound.

The KT66 has some similarity to the 6L6 and 5881 (both also beam-tetrodes), so these will get you closer and you may use the 8k primary impedance.

Of course bias needs to be adjusted when changing tubes.

It is no surprize that the JTM45 has lots of bass as it is the copy of a bass amp, the '59 Fender Bassman.

Showing your actual circuit and voltages may help to give further advice.

I would use KT66 ,but the won't fit physically due to chassis layout. I knew the JTM45 was taken from the Bassman ,but this thing is ridiculously bassy. I'm going to go back ,and putting the dropping resistors back as in the schematic/layout I used ,and will then get voltages again. I will also try the load change. I have some 6l6s I could use if needed ,or I will go back to KT77s. They seemed to work much better. This attached layout is pretty much what I used. Thanks Helmholtz

[Leo_Gnardo]

this thing is ridiculously bassy.

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.

[Tone Meister]

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?

[Tone Meister]

Edit: I see you're using a 112 with a Hellatone? Which Hellatone are you using?

If you brought it to me with those issues, I'd push you toward a pair of 5881s and a Celestion H75 Creamback speaker. Done.

[Justin Thomas]

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