Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on March 01, 2016, 05:50:26 pm
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I'm not able to understand C6 placement (or function)
I think the schematic is wrong and the minus of C6 is to be connected to ground, isn't it ?
or not ? C6 seems to be .......... a capacitor trimmer, and this is more odd !
(http://i.imgur.com/fBwZBpa.jpg)
Also I find odd that there are 270v AC and B+ is 270v DC .... and the moltiplication factor + drop of the 6X5 ???
Thanks
Franco
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Yes that does seem rather odd. :w2:
An adjustable E-cap? I don't recall seeing one of the before.
It does seem that it's - leg should continue on down to the main Grounding line. :dontknow:
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I'm not able to understand C6 placement (or function)
I think the schematic is wrong and the minus of C6 is to be connected to ground, isn't it ?
or not ? C6 seems to be .......... a capacitor trimmer, and this is more odd ! ...
This looks like hum-neutralization to me.
The OT and 6V6 get B+ from filter cap C7B; C6 is not a filter cap, and at 5-80pF (the new "micro-micro-Farad") it's way too small to filter B+ ripple on its own.
The 6AU6 screen is at 27vdc which is supplied through the 2.2MΩ dropping resistor (R9). The 6AU6 screen also has a 0.05µF cap (C3) to ground. That means there is a capacitive voltage divider made up of C6 and C3, with the screen taking an "a.c. signal" from this divider's midpoint. If C6 is adjusted to 50pF, then the 50pF/0.05µF divider is a 1000:1 voltage divider.
Make the relationship simple by finding Xc for each cap at 120Hz:
Xc = 1/(2*π*f*C)
Xc (50pF) = 1/(2*π*120Hz*0.000000000005F) = 26.5MΩ
Xc (0.05µF) = 1/(2*π*120Hz*0.000000000005F) = 26.5kΩ
Seen in terms of Ohms, you can see how the smaller upper cap working against the larger lower cap makes the voltage divider. Equal-value caps would be a 1/2 voltage division.
Injecting from the B+ to that 6AU6's screen has the effect of counteracting a voltage change at filter cap C7B:
Assume the voltage at C7B is rising; this means the 6V6's plate voltage is rising, and there rising-voltage input through C6 to the 6AU6 screen. With rising screen voltage, the 6AU6 will conduct more plate current, increasing the drop through R6 and pulling the 6AU6's plate voltage downward. This is a downward-going input to the 6V6 control grid, which will cause it to conduct more plate current. More alternating plate current through the 6V6 creates a larger voltage drop across the OT's primary impedance, causing the 6V6 plate voltage to go downward. Therefore, C6 is providing negative feedback to reduce ripple heard at the speaker.
This doesn't affect the audio output, because it's on the C7B side of the OT primary, which is also a.c. ground. If the connection of C6 were moved to the other side of the primary, at the 6V6's plate, then the audio output would also be fed back to the 6AU6.
I think the cap is a trimmer to be adjusted by ear for minimum hum, not unlike a hum balance pot.
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... Also I find odd that there are 270v AC and B+ is 270v DC .... and the moltiplication factor + drop of the 6X5 ??? ...
This is a perfect example of why I sometimes object to the simple "multiplication-factor" charts for tube rectifiers.
- The 6X5 is not a big, beefy rectifier and I'll assume it has a fair amount of voltage drop on its own.
- The first filter cap (10µF) is not overly large, and will allow its voltage to sag with significant current draw from the circuit.
- The PT secondary resistance (which surprises me to be marked on this schematic) is fairly high at 375Ω per half-winding.
- The PT primary resistance is 22Ω, but when reflect through the impedance ratio of the PT ( [(270v+270v)/117v]2) looks like an extra 234Ω per half-winding. This with the above means the PT presents 609Ω of series resistance to the rectifier/filter cap circuit, and limits the charging current which can be pulled by the filter cap.
The net result of all of the above is a believable 1:1 ratio of applied a.c. to rectified d.c. The process of proving it would be a complicated multi-part step with a lot of reference to tables in place of math equations (which you can look up yourself in RDH4's chapter on rectification).
The bottom-line is this shows the impact of using a small PT using high-resistance/skinny-wire windings. However, all the other factors figure in, as well.
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Thank you HBP! :worthy1:
It all makes a lot more sense now.
So C6 is not an E-cap, more like an air gap metal plate adjustable signal cap?
Now I have seen those before. :l2:
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"mmF" is pFd. This is a super-small cap. What HBP said.
BUT.... make your first main filter caps a lot bigger than 10uFd (like 40uFd) and the hum goes down without adjustment. You are not trying to sell thousands at $19.95.
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... So C6 is not an E-cap, more like an air gap metal plate adjustable signal cap? ...
Sounds like you're describing a tuning cap:
(http://www.mtmscientific.com/cap3.jpg)
No, no, no! Those are waaaaay too much money for this amp!
More like a cheap ceramic trimming cap:
(http://www.futurlec.com/Pictures/Ceramic_Trimmer.jpg)
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Many Many thanks for the explanation HotBluePlates
So that seems to be very fine engeenering
a good hum elimination using brain instead of BIG capacitors (that I suppose were very expensive at the time)
difficult (very difficult) solution to be replicated, but effective
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According to the capacity (5-80 pf) it is no longer likely to be a trimmer similar to this ?
https://symkrilag.uk/shop/passive-componentscapacitors/variable-trimmer-capacitor/variable-ceramicmica-paddertrimmer-capacitor-4-50pf/ (https://symkrilag.uk/shop/passive-componentscapacitors/variable-trimmer-capacitor/variable-ceramicmica-paddertrimmer-capacitor-4-50pf/)
(https://i.imgur.com/Q4QPai8.jpg)
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Again THANKS
Franco
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The original would be like these:
https://www.rfparts.com/capacitors/capacitors-certrimmer/capacitors-46series.html (https://www.rfparts.com/capacitors/capacitors-certrimmer/capacitors-46series.html)
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Thanks PRR
Franco
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... According to the capacity (5-80 pf) it is no longer likely to be a trimmer like this ? ...
The capacitance range is fine on the part you linked. However, I've never seen an air-insulated rotor/stator type cap built as a small trimpot like that. I personally wouldn't use it, because it would be very fragile, and likely to short between the plates.
That's why I said this type was "more expensive" and linked the picture... Typically, quality air-insulated variable caps are quite large and sturdy to minimize risk of arc-over or shorted plates.
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I agree with you about It is not appropriate to use one of those, a ceramic type is more adequate
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I've never seen an air-insulated rotor/stator type cap built as a small trimpot like that.
Oh, I've seen a lot (and may be I've some on stock), I've a lot of friends that are ham radio and those kind of stuff is (was) really common
Ciao
Franco
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If you ever scrapped an old radio for parts,
then you probably have a few of those adjustable metal plate caps in your junk bin, like me.
Knowing exactly what capacitance they are is another issue.
I don't have a capacitance meter yet.
And they are quite large compared to the cheaper ceramic alternative.
In a simple small circuit like that Airline,
I wouldn't want to put one of those big honkin metal plate caps in,
unless it was the only part I could get my hands on. :l2:
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I've find one other example of the use of a Capacitor Trimmer on a guitar amp (on the Silvertone 1431)
(http://i.imgur.com/Rlq1rAG.jpg)
it is cool to be surprised about something but not lost on the high seas
and this is thanks to you HotBluePlates :grin:
Franco
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Knowing exactly what capacitance they are is another issue.
I don't have a capacitance meter yet.
Most DMMs have these built in, yours doesn't?
On the cap - I'd just throw in a 50pF and be done with it. I highly doubt that this is very critical in an amp like this?
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> Silvertone 1431
That (with unbypassed point B) is a really cheap buzz-fix.
Better to use larger filter caps even if it costs a buck more.
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Ciao PRR
That(with unbypassed point B) is a really cheap buzz-fix.
Better to use larger filter caps even if it costs a buck more.
No doubt about, probably as we were saying at that time e-caps were very expensive
Franco
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I've find one other example of the use of a Capacitor Trimmer on a guitar amp (on the Silvertone 1431) ...
I think you found the same amp, built by the same company, but sold by 2 different stores (Montgomery Wards for the "Wards Airline" and Sears & Roebuck for the "Silvertone").
It doesn't help that one is hand-drawn (and has a part or two of a different value).
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Ciao HotBluePlates
I'll call you Hawkeye :bump1:
Seems you are right :thumbsup:
Franco
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> at that time e-caps were very expensive
Time *and market*.
If a Fender Champ sold for $29, and the small Gibson sold for $24, then Sears (SilverTone) had to have a Low Price model at $19. And still make a profit.
It isn't that caps were so expensive. It is that the electric guitar fad had already sold a LOT of amps, and to get more sales you had to price your amplifier stupidly cheap to get the stupidly cheap buyers.