Jamoke Direct to LPT/CCS PP EL34

[jrenkin]

Finally, after much hemming and hawing through several prior threads, having a child (actually my wife did the bulk of that) and the first month of her life, I have put together a draft of this PP EL34 rebuild/remodel I have had in front of me for a year.

This is my first schematic, I think it should fly, but I throw it on the mercy of those who know more than I do. Any comments are appreciated. Mostly, does this poor Jamoke have a chance at working?

[Nelson Bass]

Originally Posted by jrenkin

Finally, after much hemming and hawing through several prior threads, having a child (actually my wife did the bulk of that) and the first month of her life, I have put together a draft of this PP EL34 rebuild/remodel I have had in front of me for a year.

This is my first schematic, I think it should fly, but I throw it on the mercy of those who know more than I do. Any comments are appreciated. Mostly, does this poor Jamoke have a chance at working?

Check the PP1C power amp.

Build it and send photo and sound report to Allen and ask for the upgrade to the PP1CS (Constant Current Source at the EL34B cathodes).
You'll have a hard time to better the PP1CS... ;-)

[analog_sa]

Your amp is very similar to Monobill.

[SY]

A few things things jump out at me:

1. The CCS needs room to breathe. That means you need to return it to a negative rail to give it enough voltage compliance. The bias supply could work for that.

2. You don't need the RC on the grid of the "bottom" 12AT7- it can return straight to ground.

3. You might want to rethink the current through the 12AT7. 4mA per side will work and be advantageous for driving the high Miller capacitance of the trioded EL34, but surprisingly, the linearity is a bit better at a somewhat lower current (2mA).

[jrenkin]

Thanks for the responses!

I have seen the PP1C and like Allen's work. It is just that I am working from an already built amp and this is already a drastic change, besides, it has one more tube per channel than I want right now, and it uses an entirely different biasing scheme. Maybe the next modification...

I had not seen the Monobill. I will look into that, the info on it will probably help me learn more about my amp design. Also I have a PP KT88 I will eventually rebuild...

Sy, thanks for the comments. May I ask a few additional questions?

I see some info on using a negative rail, and others to ground. The monobill looks like a positive 15V. I am not really understanding this too well. Is the negative rail so the CCS has a larger potential across it than would be provided by cathode to ground allowing it to actually work inside its parameters?

somewhere, I can't find where now of course, I read that the capacitor from the bottom grid to ground was helpful if this is a direct input stage. I am inputting directly from my DAC (2VRMS).

I have heard that lower currents are better for the 12AT7, but I keep getting influenced by TubCAD, which would suggest otherwise. With that lower current, and B+ of 380V, what do you think about my Ra values, which i forgot to write in...33k, and the Rl of 200kish?

Also, just advice about the operating parameters of the EL34's. B+ 430, bias -38 and I am thinking 65-68ma per tube. OPT is 6.7K.

Lastly, I reworked the bias current adjustment so I can individually adjust all 4 tubes, rather than as pairs only. I just want to make sure I didn't screw it up...

Thanks so much, I am still new at the actual implementation of these things, and am super compulsive, so want to make sure it is ok. Really, I suppose I should take the experimenters approach, try it out and blow a few things up to learn....but that might make the baby cry...or me...

[Mosquito]

Originally Posted by jrenkin

I have heard that lower currents are better for the 12AT7, but I keep getting influenced by TubCAD, which would suggest otherwise

...

I have noticed this thing too. Many well known and proven circuits, when passed trough TubeCad, received a "not enough current" warning

[Tekko]

Possibly caused by bad simulation models in tubecad, or by the tubecad creators taste ?

[jrenkin]

My guess is it is due to taking the tube curve too literally. Again and again we find, especially in tube circuits, perfect numbers are not what makes the best listening experience.

[Andrewbee]

Again and again we find, especially in tube circuits, perfect numbers are not what makes the best listening experience.

I agree and would add that some tubes which are known for their linear curves sometimes do not sound as good as others which are less linear.

Now as to whether or not this experience is due to personal taste or to circuit interactions I do not know.

[analog_sa]

Originally Posted by jrenkin

I see some info on using a negative rail, and others to ground. The monobill looks like a positive 15V. I am not really understanding this too well. Is the negative rail so the CCS has a larger potential across it than would be provided by cathode to ground allowing it to actually work inside its parameters?

The positive rail is irrelevant to this discussion. Some CCS topologies require a positive voltage for bias. Or you can simply use a 2-terminal CCS. The absence of a negative rail means that the ECC99 cathode has to be at least 10-12v above ground or that the tube works with the same negative grid bias. Not all tubes will have usable characteristics with such high negative bias.

The inclusion of a negative rail is quite annoying but in most cases essential.