Mix and match pre and power amp sections - what to do with the PT?

SunDevil

My Frankensteinian idea is to mate a Trainwreck Rocket preamp, PI and valve rectifier with a pair of KT66. As I understand it, the 66s in a JTM45 are running at plate voltages IRO 450v, whereas the PT in a Rocket is putting out about 280v (over the 4 EL84s) I don't know if I should: 1) spec the PT to give the 66s JTM voltages, 2) stick with the original PT and tweak resistor values to get to 450 3) stick with the original PT and either run at 280 or I'm able to double up on that with there ou being the two tubes and drop it back down to 450 4) ..no doubt there's a '4' ..there's always a '4' I wonder if anyone can please set me straight? Baz

ecc83

If you want the full welly from the KT66s you will need to run them at 450V.

Yes, the preamp/PI supply should not exceed 300V but to get that from 450V is just Ohms law.

Each section of an ECC83 will draw about 1mA so add up all the sections and work out the value (and power rating!) of the dropper resistor needed. It is usual to have a dropper R for each section, each with a 10mfd 450V decoupling cap. This is a "cut and try" calculation. Do the best calculation you can with approximate currents, build it, then measure and adjust.

Dave.

SunDevil

Thanks Dave - understand the concept So it sounds like JTM45 PT then! Best Baz

SunDevil

A couple more questions if I may?

Will the difference in running KT66s at 300v rather than 450v be a question of sonic preference, or are we potentially getting outside of the proper operating range of the valve? - can anyone point me to a 'morons guide on how to read tube graphs'?

I tend to prefer a  AT7s / ECC81 in the PI, but would like the option to put a ECC83 in there too - would I need to consider a switchable resistor load into the PI or is the operating envelop 'wide' enough to cover both with a single value? - I'm assuming this comes down to the operating envelope of the preamp valve (and so I'm back to tube graphs again!)?

Best

Baz

jpfamps

You can run KT66s fine at 300V; you'll probably get around 20W in fixed bias, and probably around 15W in cathode bias.

Bear in mind with a valve rectifier the final HT voltage will depend very much on the current draw at idle.

You may need to experiment with the bias conditions loading for optimum power and sound.

We run a pair at 350 VDC in fixed bias in our amps to get 25W.

The PI in the online rocket schematic will be fine with either 12AT7 or 12AX7.

There is a good calculator here to look at how different circuit values affects the performance of the PI:

http://www.ampbooks.com/home/amplifier-calculators/long-tailed-pair/

MtB

@jpfamps  - have a wisdom on me for the link to an interesting web site for amp circuit calcs.

SunDevil

Any thanks gents - much appreciated

SunDevil

@jpfamps - just been looking at your Regent amp - looks interesting Will give you a bell to discuss if that's cool? Baz

jpfamps

SunDevil said:

@jpfamps - just been looking at your Regent amp - looks interesting Will give you a bell to discuss if that's cool? Baz

Feel free to get in touch.

BowynMadley

I run a pair of kt66s cathode bias in my head which it's a dumble preamp (in which I use 5751s) and a bluesbreaker style output with 450v across the plate. Last measure it was producing 28.6 watts biased slightly hot. I use an ax7 in the PI. It would seem you can run 66s between 300v and 450v, just depends if you like the sound and level of output produced.