JTM45/100 complete !! Pics.

[Wayne]

I guess its hard to find decent new preamp tubes that can stand the HIGH preamp voltages in this amp. for instance, v2 in mine has a voltage of 360v - something which will easily kill the tung sol and many sovteks. Also, I must admit that I cannot stand the jj 83s

Are you using the HV taps on the 43 transformer ?

Are you saying that pin 6 (and 1) of modern ec83's may not be able to take over 300v ??

I wrote my values down somewhere.
Might have to re - check.

I used the 490 B + tap instead of the 560 and i KNOW it was exactly that.

You've got me worried now.

I'd better go check.

I think the JJ's sound fine - guess it's a matter of taste.

Wayne

[Wayne]

Hi all.

The data sheets for all 12AX7's i could find put max plate voltage at 300v.
Georges own 43 PT voltage chart shows pin 6 on V2 at 360 volts which is exceeding the max plate voltage - right ??

How can this be right ?

There's nothing like checking for real with a meter though !

I can't find my notes so i will have another look but i'm pretty certain that my plate volts on V2 were well under 360v.

Nothing seemed out of the ordinary - BUT - i did not compare them to any chart.
I just had a quick check.

Mines the "Dickinson" version so I'll do a complete test and post all my readings for the "Dickinson" version in a layout similar to Georges standard JTM45/100 voltage chart in the next few days.

I used the Marstran 43 PT version - which had slightly lower (not by much) B + than the 500v recorded on Georges Voltage Chart.

Any thoughts ?

Wayne

[Roe]

Hi all.

The data sheets for all 12AX7's i could find put max plate voltage at 300v.
Georges own 43 PT voltage chart shows pin 6 on V2 at 360 volts which is exceeding the max plate voltage - right ??

How can this be right ?

...

Any thoughts ?

Wayne

the old tubes would stand very high voltages. the new ones dont - the cathode follower (v2) is the main problem. JJs seems to handle much higher voltages than the tung sol "RI"s.

I use the 230v primary and the 490v secondary on brians 1202-43. biased at 65% diss that gives me a 505v b+

[Big Mike]

Yup. I dislike the sound of JJ's so I generally use a Nos GE12AX7 in my CF stages, or better if I can find it. My 50 has a telefunken there.

My main gig amp is a cathode bias design with only 2 preamp tubes (No CF) so I generally use TungSol or Tungsol/EH or sovtek mix.

They do sound good with NOS stuff too though.