Max headroom from LTP?

[iknowjohnny]

I have a build with a LTP with common marshall values. 1M grids, 470R cathode with 10k tail and 100k/82k plates. Plate voltages are around 230 on each side, the one being slightly more of course. Pre PI master. I want the PI to stay as clean as possible. What is the best way to accomplish this?

[Roe]

this may help a little: raise plate voltage to ca 300v and try the fender silverface twin values with 12at7

[Jana]

As Roe said, raise the plate voltage.

Also,
increase the size of the plate resistors
decrease the 10K tail to 4K7

Make sure that your max peak AC voltage from the PI is less than the bias voltage or you will drive the power tubes into cutoff. That sounds like crap. Been there, done that.

[iknowjohnny]

I was thinking about that. (supply V) I was thinking i don't want to affect preamp voltages or filtering to the preamp, so leave everything as is and simply cut the connection to the plates at the PI and take a new parallel line from the screen supply and add a new maybe 2.2k dropper (has a 10k now) and the same filter value as before. Right now i have about 230v, any idea how much that might increase with a 2.2k ?

[pdf64]

Make sure that your max peak AC voltage from the PI is less than the bias voltage or you will drive the power tubes into cutoff. That sounds like crap

Jana, there may have been another reason for that.
All regular guitar amps drive their power tubes hard enough to overdrive them, ie to a point where the Vg-k >0 and grid conduction results.
It's pretty much an inevitability that the other side of the drive waveform will therefore send the tube into cut off.
If the previous stage can't supply that level of drive voltage to the power tube grids, then the amp will be unlikely to reach its potential AB1 output power.
The BF Princeton, with its cathodyne fed from a lowish VB+ node, is said to fall into this category, notable by being an exception.
Pete

[Jana]

Pete,

I would respectfully argue the point that "all regular guitar amps drive their power tubes hard enough to overdrive them." I have noticed that when the power tube grid starts to conduct, the PI can't supply the current. When that happens, there is a "nipple" that starts to form on the signal. You can observe this on a scope. This, in my experience, doesn't sound musical or pleasing. While it may be true that many guitar amps do this, drive the tubes into conduction, I would argue that the amps that sound really good don't do that.

I can get 50 to 60 watts out of a pair of EL 34 or 6L6 (480v B+) without driving the tubes into conduction (biased to about 70%).

[Jana]

Pete,
I think I had a brain fart. I said cut off when, which is when the grid goes so far negative that the tube stops conducting, correct? I meant saturation--when the grid is being driven positive (or the PI tries to drive it positive).

[pdf64]

Hi Jana,
I guessed you meant saturation!
Nipples (!) can occur with cathodynes; for LTP I would suspect it may be some sort of oscillation or negative feedback error signal.
Maybe open any feedback loops to see what happens to it.
I've noticed that the small cap (eg 50 or 100pF) often fitted between the LTP plates can cause weird blips at the onset of clipping - amps generally working fine without it, and opening the feedback loop generally stops it anyway.

I can't think of any amps that don't overdrive their power tubes though - other than maybe a Princeton (never checked one), every classic Fender Vox Marshall amp does it.
It's easy enough to eliminate it by use of a LarMar type volume control etc but I've never heard an improvement in tone as one is turned down.

I can see the benefit in the elimination of blocking distortion at the power tube grids that would result. However, that can be mitigated and controlled, and a certain degree of it is part of the classic guitar tone.

Kudos to you for doing it differently. How do you go about eliminating grid conduction?

cut the connection to the plates at the PI and take a new parallel line from the screen supply and add a new maybe 2.2k dropper (has a 10k now) and the same filter value as before

That should help a lot. As to how much, the best answer is to try it out. With just its own 2k2 feed from the screen node, there will be very little voltage drop, so maybe 10 to 20V lower than the screen.

I'm having similar problems trying to drive KT120s.
Pete

[JazzGuitarGimp]

Hi Pete,

Just curious: what anode voltage are you running the KT120's at? And what is the bias voltage?

Thank,
Lou

[pdf64]

Lou, the B+ is just under 600V, 5k P-P UL, the control grids need about -80V. Gives a good clean 100W, once the LTP is set up for such a big voltage output.
Pete

[JazzGuitarGimp]

Thanks, Pete. www.thetubestore.com says (actually, I think it is the manufacturer's claim) that 150+ watts can be had from a pair of these tubes, though at a B+ of 600V, I am guessing a Zpri of about 3k2 would be required. UL suggests Bass or HiFi amp…..

[pdf64]

Yes, if I halve the load, so as to provide 2k5 P-P, then more output is possible, maybe 120W but the B+ sags etc.
I was re-purposing an old KT88 PA amp (TVA PA200) for bass, so was worried about the PT being able to deliver the current, plus there was the hassle of sourcing a lower impedance OT >100W. Edcor offer useful impedances (yes 3k3 looks the best bet) but stop at 100W.
He isn't going to overdrive it, Jack Bruce in Cream style, so I figured KT120 should have an easy life.
It should be sweet once I get around to finishing it.
Pete

[Jana]

Pete,

I don't do anything special--just tweak the values of the PI. A 1K instead of the usual 470 ohm lowers the gain a bit but it also shifts the bias point of the AX7 so that it has a little more swing on one side--the side that goes negative on the plate. To get the max out of a KT120 you might have to have another driver tube after the PI. Or maybe if you really want to go nuts, an interstage transformer.