What Tubes Can I Use?

[Helmholtz]

Yes, the standard 1k shared feeding 470 ohm individual. I tried the 1k shared feeding 100 ohm individual, and couldn’t see/hear/measure any benefit.

Thanks. I always thought that the extra shared screen resistor is an interesting feature of the JTM45. It is a deviation from the BM circuit and not all JTM45s are reported to have had it. It protects the KT66 screens from over-dissipation (which is an issue with 8k OT primary impedance) and also reduces output power. I guess it was either introduced to prevent KT66 failures and/or to protect the two 15W Alnicos in the early Bluesbreaker combos.
Even more surprizing that the direct successor to the JTM45, model 1987 with EL34s, had no screen resistors at all.

I may have already reported it in an earlier thread: My original 1966 JTM50 (model 1963 PA version) came with a shared screen resistor of 11k (2x22k in parallel) + 470 Ohm individuals. The solder joints and carbon compound resistors looked original.

[pdf64]

Yes, I’ve read other accounts of very high value shared resistors being used, eg 8k2; they must have crippled the amp’s power output!
I wonder if they had trainees assembling them and poor QC, hence the introduction of those initialled stickers assigning responsibility / blame for each stage of manufacture?
The 1k shared + 470 individual arrangement noticeably change (round out) the large signal output waveform, compared to the later 1k/tube only arrangement (I regret not taking pics of the scope screen whilst I was tinkering with that stuff).

[pdf64]

Mine is 1K as well as the individual ones.

Ok, good, I noticed it was missing from one of layouts you posted, so assumed that was how your amp was.
One day you may feel our frustration at the absence of an accurate schematic!
Just a smartphone photo of a hand drawn one is fine :-)

[gtrplayr1976]

Ok, good, I noticed it was missing from one of layouts you posted, so assumed that was how your amp was.
One day you may feel our frustration at the absence of an accurate schematic!
Just a smartphone photo of a hand drawn one is fine :-)

I'm not that talented to be able to draw out a schematic. ..I can follow them though for troubleshooting. I believe the Ceriatone is the layout I used for most of it except for the MV ,and I increased the screens to 1k because someone at some point said it was safer? Would those be better at 470?

[pdf64]

...I increased the screens to 1k because someone at some point said it was safer? Would those be better at 470?

As you've got a 1k shared resistor feeding the 2 individual screen grid resistors, it's fine for them to be 470.
470 ohm may be too low if there wasn't that shared 1k resistor feeding them.
With them at 1k, you may be losing a little power output, and the overdrive tone may be a little softer.
If you were happy with the amp then there would be no point changing them, but as you're not, it's your choice really.

FYI the standby arrangement shown in the tube depot layout of post #11 is nasty, to avoid stressing the GZ34 it would be best not to use standby mode. The original 5F6a / JTM45 standby was no better, perhaps back then tubes were more resilient to surge currents.

Also the use of PT transformer bolts for electrical connections to the chassis is shoddy, and in the case of the mains ground, dangerous and not compliant with modern electrical safety standards.
I wish that competent engineers were used to draw, or at least review and authorise, such layout drawings

[gtrplayr1976]

As you've got a 1k shared resistor feeding the 2 individual screen grid resistors, it's fine for them to be 470.
470 ohm may be too low if there wasn't that shared 1k resistor feeding them.
With them at 1k, you may be losing a little power output, and the overdrive tone may be a little softer.
If you were happy with the amp then there would be no point changing them, but as you're not, it's your choice really.

FYI the standby arrangement shown in the tube depot layout of post #11 is nasty, to avoid stressing the GZ34 it would be best not to use standby mode. The original 5F6a / JTM45 standby was no better, perhaps back then tubes were more resilient to surge currents.

Also the use of PT transformer bolts for electrical connections to the chassis is shoddy, and in the case of the mains ground, dangerous and not compliant with modern electrical safety standards.
I wish that competent engineers were used to draw, or at least review and authorise, such layout drawings

I'm getting closer to having it where I want. I have installed a set of old Mesa branded 6l6GCs in it ,and they sound pretty damn good. I have a couple of other sets I want to try just to compare.

I can just leave the standby on if that's an issue. I know I have heard this before. I bypassed the standby on my Vibrolux because the switch crapped out. I am currently using a Weber copper top rectifier in this.

I didn't use their grounding scheme. I have separate grounding points. I know some of the amps I have worked on have had grounds on the xfmr bolts , but I changed them.

[pdf64]

...I am currently using a Weber copper top rectifier in this...

Note that your HT voltage at full load will increase with a real GZ34, and with that, the amp's power output increases, significantly so when overdriven.
I found that out the hard way with a Fender combo type build; it worked fine with my favourite 50 W Weber 12F150 speaker, but burnt it out when I swapped the WZ34 copper cap to a real GZ34, as its overdriven power output increased from ~45W to nearly 70W.

[gtrplayr1976]

Note that your HT voltage at full load will increase with a real GZ34, and with that, the amp's power output increases, significantly so when overdriven.
I found that out the hard way with a Fender combo type build; it worked fine with my favourite 50 W Weber 12F150 speaker, but burnt it out when I swapped the WZ34 copper cap to a real GZ34, as its overdriven power output increased from ~45W to nearly 70W.

Wow,I didn't know they did that. Not sure I want to change to a real one unless maybe go to a 5V4?

[gtrplayr1976]

Yaaay!



OK, keep us posted.

I have put this thing back to near stock. I left the cap value changes because I thought they helped. Removed the PPIMV. I just couldn't get it to sound good. I will focus now on selecting tubes. The last ones I tried (JJ e34L) sounded really good as did the kt77. I'm trying to set all the controls the same to compare ,but I know I may have to tweak some.

[Leo_Gnardo]

I have put this thing back to near stock. I left the cap value changes because I thought they helped. Removed the PPIMV. I just couldn't get it to sound good. I will focus now on selecting tubes. The last ones I tried (JJ e34L) sounded really good as did the kt77. I'm trying to set all the controls the same to compare ,but I know I may have to tweak some.

OK, that's progress I s'pose. I figured KT77 would be the hit.

[Helmholtz]

As soon as Marshall switched from KT66s to EL34s they also changed the OT for a primary impedance of 3.2k.
As said above, for KT77s your screen voltage is probably too high.

[Leo_Gnardo]

I have put this thing back to near stock. I left the cap value changes because I thought they helped. Removed the PPIMV. I just couldn't get it to sound good. I will focus now on selecting tubes. The last ones I tried (JJ e34L) sounded really good as did the kt77. I'm trying to set all the controls the same to compare ,but I know I may have to tweak some.

With a primary OT impedance of 8K, and you seem to have been going for a low-power situation with the PPIMV, I had the wild-hair idea you might give 6V6 a go.

[gtrplayr1976]

With a primary OT impedance of 8K, and you seem to have been going for a low-power situation with the PPIMV, I had the wild-hair idea you might give 6V6 a go.

Would 6V6 work? I have some.

[gtrplayr1976]

As soon as Marshall switched from KT66s to EL34s they also changed the OT for a primary impedance of 3.2k.
As said above, for KT77s your screen voltage is probably too high.

I'm using JJ KT77s. Specs look the same as KT66 if I'm not mistaken?

[The Dude]

Would 6V6 work? I have some.

What's your plate voltage?