Future plan for 300B XLS amp

What people are working on at the moment
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IslandPink
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#196

Post by IslandPink »

Well the plot thickens, and my confusion mounts as I try to make sense of the filament supply and driver modifications.
Having used a voltage divider from B+ to ground to supply G3 on the 4P1L drivers with something slightly above cathode ( only about 3V above due to unforeseen circumstances ) I did some more faffing tonight .
I got some 3V lithium batteries from RS ( should have looked more extensively , there are some reasonably small 9v batteries ! ) and used them instead, to put G3 a little above cathode . I hoped this would tighten up the bass timing and extension vs. the voltage divider , but still give me some of the reduced H3 by raising G3 .

So I did this and also put some planar (BI) 22R resistors into the filament supply ( virtual centre-tap ) instead of the 2W carbon films .

So, unhelpully, the resulting sound is no more incisive than before, but is really soft and 'classic' SE , with good tone and space . I did get a great deal of pleasure listening to the first side of Joni's 'Hejira' , it must be said .

So the next step is unclear.
It would be good to the press the Microsoft 'undo' button several times at this point .

Watch this space for a 'Future plans for GK-71 amp' thread !
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#197

Post by JamesD »

Interesting! Very interesting!...

But... puzzling - I'm not sure what soft means in this context? Too much 2nd harmonic?

And, as you say, difficult to ascribe a definitive character to any one change...

J
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#198

Post by IslandPink »

As I mentioned today on e-mail ( just to let the rest of you know ) ; soft as in poor bass focus and timing - woolly bottom end .

More mods to be done with another Lehane current-reg board, which I believe is on its way to me. I'll keep you posted.

I might also buy some 6CB5A which, ( mentioned in the 'valves' section ) triode-connected, are an IDHT alternative to 300B - as promoted by Thomas Mayer.
This would get us another refernce point and take out nearly all of the potential filament interaction effects .
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Paul Barker
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#199

Post by Paul Barker »

Found my RCA Receiving tube manual in the man cave, dusted it off and thumbed through. Found this.

Image

regarding the 4P1L

May be relevant in connection with what you are trying with the +ve ref on the beam forming plates.

I know you love your VR regulated g2 voltage, but this may be worth trying aswell, even if only to eliminate it.

'cause I know you haven't got enough variables to try :D

I'll get my coat.
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IslandPink
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#200

Post by IslandPink »

OK, that's interesting . Thanks.
What's with the chokes ? - is it some sort of oscillator or RF circuit ?

I was wondering today about a feed from B+ and a zener to the cathode.
Hopefully if the current modulationon G3 isn't high, the Zener would be inaudible.
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#201

Post by JamesD »

The chokes are probably the primaries of ITs :D :D :D :D

More Iron!!!!!!
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#202

Post by Paul Barker »

It was the phrase "the cutoff characteristic of a pentode is extended by this method of supply".

I used this method in the guitar head amp on the 6sj7 input valve, but at that time hadn't read this.

The point is we sometimes see things done in a way we don't expect but this finding shows there was a reasoning at the time, which is lost until someone finds a statement like this.

This form of connection therefore has a validity worth evaluating.

My uTracer is still crippled (awaiting parts) or I would run a new set of curves on the 4P1L setting the screen supply at 300v then intercept it with a resistor, so we can see graphically what effect this has.
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#203

Post by IslandPink »

I'm with you now.
I had it in mind you were thinking of adapting this form of supply to the G3 connection. However it's definitely of interest if this can be used as an alternative to the OA2 setting of G2.
I am a bit discouraged though, as it's introduced as a scheme that can be used if the range of anode or screen grid currents is small . In this case, driving a 300B , I think that is definitely not the case.
However it's a bit confusing as it's noted as a way of handling a larger range of signal than normal ....

It would be very good to see test curves when you get the uTracer going again . However there may be dynamic rather than static reasons why it could be problematic - only testable by listening, I guess.

Well spotted, though .
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#204

Post by Paul Barker »

IslandPink wrote:I am a bit discouraged though, as it's introduced as a scheme that can be used if the range of anode or screen grid currents is small . In this case, driving a 300B , I think that is definitely not the case. .
Yes good point.

I'll trace it when I can but also I shall trace it and compare it to fixed g2 voltage for the 6SJ7 which is low current. It was with the 6SJ7 that I used it and it sounded great. But that was intentionally a guitar amplifier, the purpose I had in mind was good tone not perfect linearity. Tone was great. I just copied it from an old book, I didn't then have the understanding. This RCA Receiving Tube Manuel gave me the understanding.

It reminds me of that odd PX25 circuit diagram in the Mullard Tube Manual. Sadly Mullard didn't give any text to explain the rationale, and we are left guessing. But surely it must be tried.
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Nick
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#205

Post by Nick »

If someone wants to send me a valve I will trace it if it helps, or you can borrow my tracer
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#206

Post by Paul Barker »

Yes I'll send a pair of 4p1l and try to dig out a 6sj7 also.
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#207

Post by Nick »

Fine. Will test when they arrive.
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#208

Post by Paul Barker »

Couldn't find 6SJ7 so sending two 4P1L's did a triode trace on them before sending both identical see the Valves thread for the trace of one and the similarity to the PX25.

I have removed the nasty quality alloy locator because I nearly killed the uTracer trying to remove one from the Loctal base. Since we aren't using them in an aircraft which does a loop the loop the possibility of them falling out is none. Only problem is no locator pin. I have put a red line where the pin goes. You can double check correct pinout by putting ohm meter on pin 1 and pin 7 (outer filament connections) on the uTracer to confirm correct orientation.

You can keep these two to build with, I have enough left for my needs.

By the time I find 6SJ7 my IC's for uTracer should arrive.

The uTracer heats them alright on the 4.3v setting. I know it should be 4.2 but you can quickly change the 6 to a 4 and bob's your uncle.

If I read Ronald's heater discussion correctly , when you heat dht's from the uTracer it biases them at 0v because he switches the heaters off at the same time he pulses the anode and screen voltages. So the curves should be true. I stand ready to be corrected if necessary.
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Nick
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#209

Post by Nick »

Ok, give me instructions what to trace, do I need to do something special with g3?
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#210

Post by IslandPink »

I don't think so , probably the usual connection to k, unless Paul thinks otherwise.
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