More BoM questions...
Is 100V rating fine for the 330uF caps in the grid bias power supply?
Any specific type of trim pots for the bias adjust?
..Todd
Is 100V rating fine for the 330uF caps in the grid bias power supply?
Any specific type of trim pots for the bias adjust?
..Todd
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100V should be fine 60*1.4 =84V as long as the winding isn't too hot. To be safe since the 60V is rated for 800mA, the no load voltage will be significantly higher, I would go to 150V. Maybe somebody could measure the no load voltage.
It certainly wouldn't hurt to add a 470K/3W across C1 and/or C2 in case the 2.5H choke is disconnected. I'm not familiar with the form factor of the hash choke, so it's unclear to me whether that can also be easily disconnected.
I replaced a single 220K bleeder R with a pair of 560K R's to my existing power supplies after Tubelab's post (one on each side of the choke), and the B+ still bleeds down in about 5-10 seconds, so they seem to be a reasonable value as far as the trade between burning ma and bleed-down time.
Even if the winding is 70vac unloaded, we are beneath the rating of 100v caps.
But these are only a couple bucks, so 160V caps is probably worthwhile to maximize their life expectancy.
..Todd
But these are only a couple bucks, so 160V caps is probably worthwhile to maximize their life expectancy.
..Todd
I would put the bleeder resistor before the hash choke. Or basically accross each electrolytic capacitor. The 1000pF hash cap won't hold much of a charge and will be drained by either side should the either choke be disconnected or it opens up.
The hash choke is axial, round ~1/2 diameter and 1"long. Looks like an axial capacitor or large resistor.
160V caps are probably the best selection for C-. Gives more of a safety margin and cost is small. At 800mA I have no idea what no-load voltage would be from them. That coil will essentially operate at no-load conditions.
The hash choke is axial, round ~1/2 diameter and 1"long. Looks like an axial capacitor or large resistor.
160V caps are probably the best selection for C-. Gives more of a safety margin and cost is small. At 800mA I have no idea what no-load voltage would be from them. That coil will essentially operate at no-load conditions.
Yes - but then it should be DE-coupled at the lowest frequency. Actually it is rather more involved. All those 'decouplings' could have a common resonance frequency, at which an increasing oscillatory response going up to instability can result when the 'prompting' signal occurs at the right frequency. (Think of those early suspension bridges that started to oscillate at the right wind turbulence, ever increasing, with catastrophic results. That before mechanical engineering advanced to the stage of being able to do the necessary calculations to avoid such an instability factor.) There exists a formula with which Spice can display such a tendency graphically, but I cannot do that.
Back to the start, such decoupling would invariably have to be effective to very low frequencies. Thus with our R1, the low value of which is forced by other factors, sufficient decoupling would necessitate a C4 of some 6 - 10 mF (as in milly-farad) - clearly not an economical consideration. As said previously, I would chicken out of such calculations and test in practice. I cannot now say whether a practical R1-C4 could be found or whether the solution might be to leave it out. It has to do with the stability margin of the whole circuit. In the past I placed a value, excited the circuit with an impulse and watched on the oscilloscope to what degree there is a hopefully well-damped oscillation on the B+-line, or better, none at all. More academically-advantaged persons might assist here.
Again, actually not quite. Voltage reduction in the first place, but then it is also used for decoupling. Again, what amplitude of signal on the h.t. would set matters into l.f. oscillation would depend on component values, phase shifts and the input stage's sensitivity to such variations on its h.t. feed. (One must remember that at very low frequencies - sub 10 Hz - all capacitors start to behave reactively. It could well be to the extent that quite strong positive feedback can be the result.) Yet again perhaps a subject for study, yet again not for me - easier to sort out on the bench. I am an engineer, not a physicist.
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The bias network is never unloaded - not dependant on tubes heating up - the resistors are always there. In that sense one can have voltage ratings without the otherwise safety factors.
Yes; expensive types! Certainly not carbon miniatures. Plastic, cermet or such. Members with better information on modern materials could comment here.
Johan,Thanks for responding. I guess someone will have to build this circuit and test it. I am a metallurgical engineer which is even further from the physics. I did get LTSpice to load, (but that was at a Holiday Inn Express).
Not to get OT, but does a CCS, especially as the anode load, improve decoupling?
Not to get OT, but does a CCS, especially as the anode load, improve decoupling?
If the bias winding is rated for 800ma, and since this is AB1 we are pulling a negligible amount of grid ma, isn't it essentially unloaded? It looks like about 1 ma to ground total from the -80V.
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Yes, it does. A triode/CCS construct exhibits a much better PSRR than a triode/resistor construct. Decoupling networks are used to isolate small signal circuitry from power circuitry induced PSU modulation. A CCS isolates B+ rail variation too.
It's quite likely that decoupling is unnecessary in a 2 stage design, with a CCS loaded voltage amplifier.
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Boywonder, even in AB2 it is essentially unloaded. In either AB1 or AB2 you want to have a low impedance driver so that if grid current flows it is sunk without causing distortion. Grid current can flow even if grid voltage isn't positive.
I'm wondering how suitable 6Н1П (6N1P) tubes are for use in place of the ECC88 here. The Russian eBay vendors tout them as equivalents, but they clearly have differences (beyond just the heater current draw).
The reason I ask is because they are being sold by the case-load from Russia and surrounding area for VERY reasonable prices, and it would be worthwhile to put some to good use.
..Todd
The reason I ask is because they are being sold by the case-load from Russia and surrounding area for VERY reasonable prices, and it would be worthwhile to put some to good use.
..Todd
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Be careful, as specimens of the 6Н1П vary considerably. You want specimens that are 6Н1П-EB (highest military grade).
In this application, the 6Н1П may be better than the 6922. Quite a few knowledgeable people like the 6Н1П in driver "holes". Compared to the 6922, RP is higher and gm is lower, while μ remains 33. However, the 6Н1П is somewhat tougher, with big cathodes, and might take more Volts on the plate.
By "take more Volts", did you mean they might require more Volts to provide the same operating point as ECC88? or that they might withstand more Volts?
Is it possible to "audition" them by, say, removing the output tubes and measuring their (?) output? Or would I have to [somehow] produce a whole set of curves to determine whether I have one close enough to our circuit requirements?
I'm curious about the negative impact of dropping in a tube whose specs are way off. Will the input or output tubes be in jeopardy, or just the off-spec tube itself?
..Todd
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The tube marketed under the EH label as a 6922 is, in fact, 6H23П-EB (6n23p-ev). IMO, its more like a 7308 in its ability to take abuse. The Russky is a good tube: decent sonics, quiet, and outstanding toughness.
Hey Taj, hows the progress on the BOM? I'm waiting for transformers to arrive before buying anything else.
I'm curious if anyone besides me decided to start a build now?
I'm curious if anyone besides me decided to start a build now?
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Well, I've written down everything folks have mentioned so far, but that's it. You don't want me guessing about these things. I'll haul it out and send it up here this weekend sometime when I get adequate spare time, so we can get more input and wrap it up.
My matched quad of Genelex G.L. KT88's and a couple of bench-test quality KT88's arrived from J.M. yesterday, so I guess that means I have officially begun accumulating parts for this build. I'm conflicted about OPTs though. I'd like to order a set from BudP but the budget may restrict me to Edcor. Not that there's anything wrong with Edcors, it's just that I had my heart set on O-Netics OPTs.
TubeMack... I think we're all waiting for you to remove all the smoke first, and assure us that Dianna Krall doesn't sound like Nickelback. 😉
..Todd
My matched quad of Genelex G.L. KT88's and a couple of bench-test quality KT88's arrived from J.M. yesterday, so I guess that means I have officially begun accumulating parts for this build. I'm conflicted about OPTs though. I'd like to order a set from BudP but the budget may restrict me to Edcor. Not that there's anything wrong with Edcors, it's just that I had my heart set on O-Netics OPTs.
TubeMack... I think we're all waiting for you to remove all the smoke first, and assure us that Dianna Krall doesn't sound like Nickelback. 😉
..Todd
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taj, you're looking at 140$ a pop for the first level ONetics, I believe. If you can afford Genelex Gold Lions, you should have money for decent iron, cause otherwise you might just have a little twist in your prioritys...