Help with load line for LTP phase inverter

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SixStringBender
Posts: 93
Joined: Sat Nov 01, 2014 4:17 am
Location: USA

Re: Help with load line for LTP phase inverter

Post by SixStringBender »

Tubeswell, I very much appreciate your help. I took notes and already used your notes to calculate the plate dissipation in the preamp because I thought the B+4 supply was going to be way too high at 410V-415V. Now I understand how the 12AX7's last forever in Super Reverbs.

I'm going to study the constant current source for the PI. I want better balance and the ability to drive KT88s. I'll be using everything from JJ's 6V6S to KT88s in the new amp (multi tap OT).
tubeswell
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Joined: Thu Mar 27, 2008 6:42 am
Location: Wellington. NZ

Re: Help with load line for LTP phase inverter

Post by tubeswell »

If you want to drive 6L6s, they like a bit more current, and a 12AT7 is great for this as per Fender BF AB763-type amp values

If you want to drive KT88s well, you need lots of current* and good balance, and a better type of phase inverter is a 12AX7 cathodyne with a couple of 12AU7s - one on each side after the PI - providing a driver and cathode follower for each side of the output stage - take a look at the Ampeg SVT.

*KT88s require lower grid leak resistance than 6V6s or 6L6s, so more grid signal current gets 'sunk' through the grid leak path, and you need a driver stage that supplies high current to compensate for this. You won't get this from a typical LTP.
He who dies with the most tubes... wins
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jazbo8
Posts: 311
Joined: Sat Jul 23, 2011 12:59 am

Re: Help with load line for LTP phase inverter

Post by jazbo8 »

martin manning wrote: Sat Jun 24, 2017 11:35 am Looks like the plot above came from Blencowe's load line plotter. Link for those who want to play (scroll to "downloads" at the bottom of the page): http://www.valvewizard.co.uk/links.html
I think Merlin's load line plotter is intended for generic common cathode stages, which gives a good approximation of the load line for the LTP. The actual load lines for the two sides are different in practice, because only one side is driven, so the Vg-k's are a bit different for the two sides. Also, I don't know if it has been mentioned, but without using a CCS on the tail, the balance can be improved by reducing the plate resistor on the driven side, such as the 82k/100k combo used in the Fender.
SixStringBender
Posts: 93
Joined: Sat Nov 01, 2014 4:17 am
Location: USA

Re: Help with load line for LTP phase inverter

Post by SixStringBender »

I forgot about this topic. lol

Yeah, I have a pair of IRF820 MOSFETs source followers after the phase inverter. I have a -177VDC (125VAC) supply tied to 47K MOSFET source/control grid leak resistors. I have 1.5K stoppers. On the drains I have a 28VDC (20VAC) power supply. I had a custom power transformer wound by Heyboer to obtain these power supplies.

I got to thinking. I didn't think about current draw with my power tranny's HV secondary for 6550 and KT88 when I had Heyboer wind the HV plate supply. It is rated for 200mA. Will that handle a pair of KT88 or 6550? Also, I don't like huge iron for the output tranny. I don't like the way large iron affects the treble (too clear and piercing). How will the KT88 do with a 40W OT? The impedance will match because I'll use a multi tap Vibrolux size OT to reflect any primary impedance I need. I may even test a Super Reverb multi tap OT, but I doubt I'll like it.

The phase inverter is totally balanced now. I'm using a 12AX7.
435V B+3 with 110V across the tail. So325V.
100K A half and 105K B half.
619R cathode
30.9K tail

Excellent balance in LTSpice.
tictac
Posts: 610
Joined: Fri Mar 31, 2006 4:42 am

Re: Help with load line for LTP phase inverter

Post by tictac »

Do you mean "125VAC" or "125VA" ?

TT
SixStringBender
Posts: 93
Joined: Sat Nov 01, 2014 4:17 am
Location: USA

Re: Help with load line for LTP phase inverter

Post by SixStringBender »

It is a two wire secondary at 125VAC at 50mA. It is full wave full bridge rectified to -176.75V, doing the math. In LTSpice I think it is around -169V. It has a Pi filter using 100R resistors and 100uF caps. This is the power supply for the source of the two MOSFETs that drive the output tube control grids.

The 20VAC 50mA secondary is full wave rectified with a full bridge and Pi filter. It is used for the drain of these two MOSFETs. I also have the artificial center tap of the filament supply going to this power supply.
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