Clearly I need to spend more time in Fremont. I haven't seen any of that stuff down there, just traffic, traffic, and more traffic.
Sorry if I am getting a bit over-technical here, but perhaps further clarification is needed regarding the relative unsuitability of very high-mu triodes as first stages.
SY is correct as far as direct attenuation is concerned, but there remains a further factor. Most of us use feedback of 15 - 20 dB, in case of which one usually adds an h.f. pole (serie R.C in parallel with anode resistor of 1st stage) for complete stability. Even if its effect is just outside the audio range, the input capacitance with such tubes can rob you of the ability to shape this, as it forces a permanent pole where one would want one under control, as well as one that will vary with input resistance. Because there are alternatives, I normally avoided this. (An acceptable alternative is of course not to have a volume control as the very last item in your pre-amp, feeding your main amp. You can follow this by a low output impedance stage in which case the objection falls away.)
But this just for completeness sake and not to bewilder folks new to this. Start off as you intend, Navin, you can always refine later.
Wayne, my very best wishes for your family. You are being thought of.
Regards.
SY is correct as far as direct attenuation is concerned, but there remains a further factor. Most of us use feedback of 15 - 20 dB, in case of which one usually adds an h.f. pole (serie R.C in parallel with anode resistor of 1st stage) for complete stability. Even if its effect is just outside the audio range, the input capacitance with such tubes can rob you of the ability to shape this, as it forces a permanent pole where one would want one under control, as well as one that will vary with input resistance. Because there are alternatives, I normally avoided this. (An acceptable alternative is of course not to have a volume control as the very last item in your pre-amp, feeding your main amp. You can follow this by a low output impedance stage in which case the objection falls away.)
But this just for completeness sake and not to bewilder folks new to this. Start off as you intend, Navin, you can always refine later.
Wayne, my very best wishes for your family. You are being thought of.
Regards.
Johan, I think that the pole created by the volume control and the Miller capacitance is outside of the feedback loop, so won't have much to do with the dominant pole, which is really there for stability reasons. But likewise, it won't be corrected by the feedback, so the point that you're making (and the one that I made in #142) remains valid- a high mu tube and significant source impedance is a bad juju combo.
SY,
As I see it, the feedback signal sees a grounded grid stage, with a variable resistor in the grid circuit and a capacitor from anode to grid. I did find visible changes in the square wave response when varying the input resistance in such a case.
But again as said, more for completeness sake or where high Rls and perhaps input resistors are used. I have not done this for years - the experience came from an existing commercial amp most frugal of components, with expected compromises. When idle again I will hook something up and revisit. Until then, not to worry anybody. Thanks for your comment.
As I see it, the feedback signal sees a grounded grid stage, with a variable resistor in the grid circuit and a capacitor from anode to grid. I did find visible changes in the square wave response when varying the input resistance in such a case.
But again as said, more for completeness sake or where high Rls and perhaps input resistors are used. I have not done this for years - the experience came from an existing commercial amp most frugal of components, with expected compromises. When idle again I will hook something up and revisit. Until then, not to worry anybody. Thanks for your comment.
thanks a bunch!😀 fyi, there are still lots of tv tubes that can be bought here, many 9pin and 12 pin compactrons.
Johan, even with the pole outside of the feedback loop, you'd expect that, no? Or did you actually see stability change, not just rolloff?
wayne,
sorry to hear that, wish everything would be ok for your family.
navin,
i plan to use 2W for all the resistors except for the el34 cathode resistor which would be 3W. this should give us some safety margin i guess.
sorry to hear that, wish everything would be ok for your family.
navin,
i plan to use 2W for all the resistors except for the el34 cathode resistor which would be 3W. this should give us some safety margin i guess.
SY said:
OOps. I left Fremont in 2003. Haven't been there since. I should have upgraded my profile. Now I live 100% of the time in Bombay (Mumbai).
JojoD818 said:
2W! wow! do they make metal film in that wattage? Have nto seen them here. Anyone else can offer suggestions onteh wattage of the resistors?
JoJoD818, you live in Manila right? You find these parts easily there? Importing into India is a big hassle and tube parts are hardly available locally.
navin said:
Yup, I live in Manila, Philippines. And yup, these are available here locally, even the tubes are.
Maybe you can buy online? If there's a will, there's a way!
😀
JojoD818 said:
Wow. It is the customs that drives me nuts. They want bribes for every little thing.
That's what we're trying to get rid off here too. 😡
Those bad corrupt guys... always preying on audio guys like us!
Those bad corrupt guys... always preying on audio guys like us!
navin look,
ricken resistors, vishay (hiding underneath) illinois caps, solen caps, sprague orange drops, bournes switches and pots, made to order 2mm aluminuim chassis.
all bought locally navin.
ricken resistors, vishay (hiding underneath) illinois caps, solen caps, sprague orange drops, bournes switches and pots, made to order 2mm aluminuim chassis.
all bought locally navin.
An externally hosted image should be here but it was not working when we last tested it.
navin said:
thanks!
this isn't the one we are talking about here but it sure is a new one. 😀 when I finish my work on this amp then my next project will be the EICO hf87 in fixed bias and CCS. 😎
JojoD818 said:
And while you do that I will collect parts as per your schematic. Only difference is that you are using teh 7025 and I am using the 6SL7.
BTW how many EL34 amps are you building? The one in the photo + the HIF87 (modfied) + ???
navin said:
I'm already in the process of collecting parts too for the amp. 😀 Just got some old caps from a friend's amp (he upgraded) that I can reuse (saves me money).
The one in the photo is in the works, planning to build next is the modded HF87, and maybe midyear a ul pp 6bq5 for my wife.
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