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Old 6th September 2008, 01:39 PM   #41
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Phil,

R14 is 470 Ohms, not 470 KOhms. A Mills MRA-5 is appropriate here too.

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What would you suggest for a good cap. type for C1?
C1 is outside the global NFB loop. Therefore, it has an impact on the amp's voice. A paper in oil (PIO) part gets my nod. Soviet surplus is the best deal in PIO capacitors. Regardless of the cap. construction settled on, a well matched pair of parts (for the 2 channels) is in order.

Quote:
Also you mentioned the voltage rating for C2 to be not less than 6V, am I right in thinking this was a typo? I've not seen such a low voltage rating on an electrolytic.
Low WVDC 'lytics are ubiquitous. Think of all the digital stuff that runs on 5 V. Look at this Nichicon data sheet, for a suitable part. BTW, 6 WVDC is minimum. Using a part that's rated for more Volts is perfectly acceptable.

Quote:
I have kept the incorrect symbol for the 10M45S for now just for clarity for myself when I'm connecting it up.
We use the interlocked circles to represent the conglomeration of parts that comprise the CCS. The box for the 10M45S (by itself) is fine. Only 2 passive parts are needed, along with the 10M45S. A 1 KOhm trim pot. is used to set the current. A 100 Ohm 1% tolerance metal film part is used as a test point. The trim pot. is wired up as a simple variable resistor and connects to the K terminal and the 100 Ohm part. The G terminal connects to the junction of the pot. and test resistor. Start with the pot. set for max. resistance and back off until the drop measured across the 100 Ohm resistor is 0.3 V.
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Old 6th September 2008, 03:52 PM   #42
phil_2 is offline phil_2  United Kingdom
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all being well this could be it! drum roll please...

Click the image to open in full size.

let me know if you spot anything else, its time for me to make a shopping list!
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Old 6th September 2008, 04:32 PM   #43
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Sorry, I'm swamped with yet another task (paying the price for goofing off last night) so I haven't checked the values of your cathode resistors. But the grid stoppers for the output tubes are probably too big for triode mode (lots of Miller effect). 1-2k should be fine there.

You should probably consider bypassing the 470R cathode resistor. And to make the input simpler, you could either direct couple (do you expect any offset voltages from your sources?) or at minimum use standard values for the input cap and grid leak resistor like 100n and 1M. Eli is correct about the fundamental frequencies of bass, but you really want your f3 a decade lower than the lowest frequency of interest to reduce phase shift.

I do not like paper caps at all (they can be leaky and drify, very humidity sensitive), but others do. If it were my amp, I'd use a good polypropylene and foil like Wima FKP or some of those cool Russian surplus Teflon caps.
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Old 6th September 2008, 05:25 PM   #44
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Quote:
Sorry, I'm swamped with yet another task (paying the price for goofing off last night) so I haven't checked the values of your cathode resistors. But the grid stoppers for the output tubes are probably too big for triode mode (lots of Miller effect). 1-2k should be fine there.
Phil, there you have it. R3, R4, R15, and R18 are all 1 KOhm Carbon composition. 733 Ω is the calculated value for R . 732 Ω is the closest "stock" value. 715 Ω is the next lower "stock" value and should be safe to use too. If you bypass the 470 Ω EL42 shared cathode resistor, use a 50 WVDC/150 μF. part.

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And to make the input simpler, you could either direct couple (do you expect any offset voltages from your sources?) or at minimum use standard values for the input cap and grid leak resistor like 100n and 1M. Eli is correct about the fundamental frequencies of bass, but you really want your f3 a decade lower than the lowest frequency of interest to reduce phase shift.
SY, rolling the deep bass off slightly below 30 Hz. is INTENTIONAL. The Edcor O/P trafos Phil is ordering will require a substantial LF error correction signal to extend their bass response. An overly large correction signal will saturate the O/P trafo core. IMO, the selection made is the best compromise. FWIW, we did the same thing in "El Cheapo" and achieved a satisfactory result.
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Old 6th September 2008, 06:15 PM   #45
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OK, that makes sense. I don't like to use tube amps for bass, anyway- in my system, everything below 110Hz is handled by some Sunfire amps.
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Old 7th September 2008, 01:34 PM   #46
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Thanks for all the advice guys, all points gratefully noted.

SY, are you happy with the cathode resistors?
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Old 7th September 2008, 02:52 PM   #47
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I'd still rather see an LED there. I hate electrolytics in series with the input signals, though admittedly it's common practice. I haven't pulled out the datasheets and done a loadline analysis, so I'll assume that Eli (a very competent guy) did it right. Worst case, the plate voltage you get is somewhat off and you have to change two resistors. No big deal.

Also, you will probably have to add some compensation circuitry to get stability, but that's something you'll have to determine empirically. One school of thought (mine) is to not worry too much about squeezing out the last bits of HF badwidth, so the amp is compensated without a feedback capacitor, just by using a series RC step network across the plate resistor of the first stage. Starting values would be something like 10% of the plate resistor value and a capacitance to give a time constant corresponding to 10kHz. You'll want to use a scope and square wave generator with a resistive and a reactive dummy load; this is well covered in MJ's "Building Valve Amplifiers."

Others prefer using a capacitor across the feedback resistor. Crowhurst argues against that in several of his books and his arguments look sound (no pun intended). In "BVA," MJ recommends a combination of the two methods, and since he's smarter than me, he's likely to be right.
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Old 7th September 2008, 03:46 PM   #48
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Quote:
Also, you will probably have to add some compensation circuitry to get stability, but that's something you'll have to determine empirically. One school of thought (mine) is to not worry too much about squeezing out the last bits of HF badwidth, so the amp is compensated without a feedback capacitor, just by using a series RC step network across the plate resistor of the first stage. Starting values would be something like 10% of the plate resistor value and a capacitance to give a time constant corresponding to 10kHz. You'll want to use a scope and square wave generator with a resistive and a reactive dummy load; this is well covered in MJ's "Building Valve Amplifiers."
"Perfect" phase compensation is very tricky business and requires a sophisticated test bench. I favor a brute force approach to the problem that requires a builder own only a good multi-meter.

I like to "peak" the voltage amplifying block's HF behavior, by the use of inductive wire wound load resistors. Unfortunately, the CCS loading necessary in this design rules that out. Please notice that the 1800 pF. cap. from "hot" speaker lead to ground shorts the NFB loop out beginning at approx. 80 KHz. Instead of trying to squeeze too much performance out, the natural roll of the O/P trafo is allowed to set in. The measures I advocate hold the HF error correction signal to (hopefully) a reasonable level and avoid slew limiting. The high gm of the 6922 small signal tube in this design is definitely an asset.
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Old 7th September 2008, 05:11 PM   #49
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Yeah, I forgot about the CCS, but I still think a step network across it will do a fine job of stabilizing things. I like to adjust for stability on a resistive load, then hang a 2u cap across the load resistor and see if the ringing gets to be too much. Typically, with a moderate feedback design like this, I'd aim for an open loop -3dB point of 5-10 times lower than the transformer's HF resonance; it's a variation of dominant pole compensation, and one is much more likely to end up with an amp that will remain stable with a real-world speaker load. The trade-off, as I mentioned earlier, is HF bandwidth; the evidence that high bandwidth is a significant factor in audibility is sketchy at best, but without question, and amp that is marginally stable will be annoying to listen to.

Although it is possible to roughly adjust stability without a scope, I wouldn't recommend it. Why work blind?
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Old 11th September 2008, 07:35 PM   #50
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Hi all, not had much to say for a while because I've been busy collecting parts for this project... I'm having trouble finding the 10M45S's in the UK though, does anyone have a couple spare they would be willing to sell? please?

BTW thanks for the advice on possible stability issues, my copy of 'building valve amplifiers' is in the post, I will be reading up on these issues and although I hope it won't be necessary, I do have access to a 'scope at work if needed

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