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DC coupling on a LTP? Opinions and help needed on Fred's design.


2008-07-04 10:35 am
Trying to understand this concept of DC coupling in this design.

He has 215 B+ on the 12AT7 LTP and -300 volts on the cathodes.

Does this mean there is about -72 volts on the anodes of the 12AT7 drivers which sets the bias on the grids of the output tubes??

So -72 on the anodes and +215 on the other side of the anode load resistors? There will also be a significant negative voltage on the grids of the driver tubes, hence the need for AC coupling on the input side.

I was rigging up a breadboard and testing it out but with a different CCS rather than the pentode.

I had to use a separate psu to supply the negative voltage on the cathode.
If I tried a bi-polar B+ and the negative supply drags down the positive side.

Output sound was not good to say the least.
input tube: 12AX7, 200V anode, 1.75 LED bias (paralleled 1 channel)
driver LTP with 12AU7 DC coupled to 6080 , OTL circlotron output.

Also what is the purpose of the 100K(R502) + 47K(R501) resistors before the capacitor(C501) on the grids of the LTP? Is this to set the correct voltage reaching the grid of the LTP?

Previously I used a FET source follower between the LTP and 6080 output tubes and the result was pleasing. FET source follower was DC coupled but LTP was AC coupled.

RA-100: Drivers and Power Amplifiers

Note that the cathode of V8 is sitting almost at -590 volts, and the cathodes of V7 will typically ride at about -300 volts.

Note that PA tubes' bias can either be set to a given voltage at the grid of each output tube (typically -72 volts), or it can be set by adjusting for equal voltage on the cathodes (representing equal current draw in each section) - typically 40-50 mA. The latter method is probably the better, unless the tubes are reasonably well-matched. Incidentally, this provides an easy way to check the match between two tubes, at least as far as emission (and to a lesser degree, transconductance) are concerned. Set the grids to be equal, and the cathode voltages should be equal also (within reasonable limits).


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#1 Yes,The 12AT7 anodes will be approx. -72V,in relation to ground.

#2 No,R501 and R502 do not set the grid voltage of the 12AT7,any DC is blocked by C501.
I believe these are for part of the balance pot RV2,and IIRC,I omitted R501 and RV2 in my build.
The grid voltage is set with R503.

I am interested in replacing the pentode with a SS CCS (and using a 'lower' voltage -V supply.),but I haven't gotten around to fiddling yet,I still need to get a proper workbench set up here in the new place.

I found that decent matching of the two 12AT7 sections is fairly important.Any minor adjustments can be made with the balance pot.


2008-07-04 10:35 am
Thxs guys.

I'm using a simple sink DN2540 with a low negative voltage (off the top of my head I think it came to about -170 volts). Small 220/6-0-6 tranny in reverse off the heater voltage.

I have a trimmer from the negative voltage to the DN2540 to set the bias voltage on the anodes of the LTP so I can vary it between -40 and -80 more or less in this range.

Also the trimmer on the right cathode of the LTP works out very well to trim off any DC offset and to balance the phases. Bit sensitive though.

Once the balance is set then you just use the 1 trimmer to set the bias ( instead of playing with 2 individual trimmers for each rail.

The idle current stability of the output stage is very dependent on the stability of ALL of the voltages in the circuit, including heater voltages

Good regulation is essential in this set up then I assume.

The grid voltage is set with R503.

470K is the norm for a LTP.

What voltage I'm I expecting or should achieve on the left grid of the LTP here? It should be within the specs of the bias for the LTP tube chosen (12AU7 in my case). (this is the confusing part as I have a negative voltage on the anode in respect to ground).

(assuming I have -170 on the cathodes - have to check that again tonight, can't quite recall, but obviously this changes according to the bias I need on the output tubes).
A rather 'idiosyncratic' amplifier - at first sight I assumed it was a guitar amp, with 'clean' and 'classic' switching! Very bold (risky?) to have DC coupling but no DC feedback. If the tail or LTP valves lose emission you cook the output valves. What happens if the bias pot slider (or the balance spot) loses contact with the track?

Even with a 12AT7 as LTP it has quite high output impedance (damping factor = 1). A 12AU7 would have less gain, so less feedback, so higher output impedance. Also, a little feedback can be worse than no feedback. It would sound strange with almost any speaker.

If you start from a classic design, you can probably get away with modding it a little as it will have some slack in it. Starting from an 'idiosyncratic' design you have no slack, as the designer is already right on the edge of what is possible (or possibly over the edge).