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Old 26th August 2005, 12:55 PM   #1
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Default Question re Allen Wright PP-1cs

I have a donor amp that can easilly become the PP-1.

However I'm a little confused by the B+ indicators
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On the schematic there are two B+2 at the top and a B+3 on the anode of the lower ECC88.

Shouldn't the lower one be B+2 and the right B+2 at the top be B+3?

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Old 27th August 2005, 10:52 AM   #2
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No, there is no B+3. The place marked B+3 should be B+2. All the other B+2 are correct.
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Old 27th August 2005, 02:58 PM   #3
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Sorry if I am being dense here.

So will both B+2 points in the PSU schematic connect to all three B+2 points in the amp?
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Old 28th August 2005, 07:10 AM   #4
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Does someone actually know what the voltages at points B+1 and B+2 are?

Especially B+2 as a feed for the Hedge would be interesting: The constant current sink evaluates to about 8mA, thus the voltage drop across the 4k7 resistor between points B+1 and B+2 would be about 40 volts. It would be reasonable to assume about 400V at point B+1, thus B+2 would be at about 360V.

But that would exceed the ECC88 ratings in that Hedge circuit:

8 mA across the 25k plate resistor gives a drop of 100V, thus 360V - 100V = 260V at upper tube plates. Assuming the least worst case scenario each tube section has to deal with 1/2 * 260V = 130V plate voltage which exceeds ECC88 specs a bit...

So, again, does someone actually really know what the voltages at points B+1 and B+2 are?

Tom
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Old 28th August 2005, 10:18 AM   #5
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Quote:
Originally posted by lilolee
So will both B+2 points in the PSU schematic connect to all three B+2 points in the amp?
Yes. If you look, you'll see that the B+2 adjacent to B+1 is the supply; the other B+2 connection is a circuit that needs B+2.
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Old 28th August 2005, 12:58 PM   #6
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RE: voltage versus max Va for ECC88, is this a stereo amp? If so, double the drop across the 4k7. Also, I'd expect the B+ to be under 400V given the DCR one might expect from a 10H inductor.

130V is within the max Va spec, but it is admittedly pushing things.

edit: Using a PSUD sim, I get about 390V B+1 for a monoblock, 375V for a stereo pair running off a single supply
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Old 28th August 2005, 02:32 PM   #7
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Quote:
Originally posted by lilolee
Sorry if I am being dense here.

So will both B+2 points in the PSU schematic connect to all three B+2 points in the amp?

Yes.

The "B+2 point" that is the lower one, coming off the filament supply isn't actually "B+2" at all, it should have an arrow that indicates that it connects TO the "B+2" point. The purpose is to inject 6.3vac antiphase (one would hope) back into the B+ that runs the input stage - thus cancelling the hum component. I am guessing this is the idea.

I'd just switch to DC regulated filaments for these indirectly heated tubes and eliminate the problem at the source. Much better idea.

There are a number of other 'issues' with this circuit.

I don't see any application of fixed bias here. Imho you'll get rather fat and sloppy sound by using self bias and setting the tubes up on top of large electrolytic caps in the cathode. At least bypass them with a largish polypropylene?

Also, the phase inversion balance is likely to be somewhat off if you merely ground the (-) input for single ended input. There are some circuits that provide a solution for this, some are in the Radiotron Designer's Handbook as well as other places. Mostly it is a change to what you do with the (-) input, although there are some things that can be done with the long tail pair's cathode side....

At least I think that's how it might go.

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Old 28th August 2005, 02:34 PM   #8
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Quote:
Also, the phase inversion balance is likely to be somewhat off if you merely ground the (-) input for single ended input.
Why would that be so if the current source is a good one?
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Old 28th August 2005, 03:10 PM   #9
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Well,

I am also working on "my" version of the pp1, so the questions discussed here have my full interest...

Quote:
Also, the phase inversion balance is likely to be somewhat off if you merely ground the (-) input for single ended input.
If this is really a problem, what possibilities of working around it do i have (no, I do NOT own sources with symmetric output )?

Quote:
I don't see any application of fixed bias here. Imho you'll get rather fat and sloppy sound by using self bias and setting the tubes up on top of large electrolytic caps in the cathode. At least bypass them with a largish polypropylene?
Am I free to do so? I only build solid state circuits before, where it is usually no problem to bypass a large electrolytic by a foil... If yes, I will do this, should in no way make it worse, i think...

Quote:
The constant current sink evaluates to about 8mA [...]
Does anyone have _exact_ values for the current delivered by the ccs. I am going to replace this part of the circuit by a tube based ccs, but I don't have the value of the current needed, it is not given anywhere. Furthermore, as I am not very familiar with tube circuits, what "don't do that!"-statements do I have to care about when building a tube ccs as replacement for the PP1-ccs?

And, one final thing, although it is a bit OT for this thread: In solid state design I always used to build fully separate power supplies for both channels, perhaps drawing current from the same transformer, but separate rectification and filtering. How do I handle this in tube circuits? a) one PS for both channels b) one transformer, one rectification, separate filtering c) one transformer winding, separate rectification and filtering or d) two separate PS ???

Sorry, so many questions... Gladly looking forward to your answers

Greetings
Andreas
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Old 28th August 2005, 05:42 PM   #10
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Quote:
The "B+2 point" that is the lower one, coming off the filament supply isn't actually "B+2" at all, it should have an arrow that indicates that it connects TO the "B+2" point. The purpose is to inject 6.3vac antiphase (one would hope) back into the B+ that runs the input stage - thus cancelling the hum component. I am guessing this is the idea.
I think the idea is actually to elevate the DC potential of the 6.3v AC heater supply so that it is between the cathode potentials of the two halves of the cascode, as protection against exceeding the heater-cathode voltage limit of the upper triode.

Quote:
I don't see any application of fixed bias here. Imho you'll get rather fat and sloppy sound by using self bias and setting the tubes up on top of large electrolytic caps in the cathode. At least bypass them with a largish polypropylene?
I believe Allen Wright has brought out a later design, which has a shared CCS in the cathodes of the OP tubes.

Quote:
Also, the phase inversion balance is likely to be somewhat off if you merely ground the (-) input for single ended input. There are some circuits that provide a solution for this, some are in the Radiotron Designer's Handbook as well as other places. Mostly it is a change to what you do with the (-) input, although there are some things that can be done with the long tail pair's cathode side....
I've never heard of that before! Can you explain it? I use a 6SL7 LTP splitter as the input stage in my amp, with one side grounded and a pentode CCS (6AU6) in the tail providing 2mA. Balance seems OK.

Quote:
And, one final thing, although it is a bit OT for this thread: In solid state design I always used to build fully separate power supplies for both channels, perhaps drawing current from the same transformer, but separate rectification and filtering. How do I handle this in tube circuits? a) one PS for both channels b) one transformer, one rectification, separate filtering c) one transformer winding, separate rectification and filtering or d) two separate PS
Allen Wright designed this amp as a monobloc. For stereo, just build two of them. Any of the other three options you mention would work but some redesign/component changes would be called for.
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