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Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum 

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9th May 2011, 12:46 PM  #161 
diyAudio Member
Join Date: Aug 2008
Location: Tel Aviv

excellent site for beginners on the basics of tube guitar amps:
http://www.freewebs.com/valvewizard/index.html 
12th May 2011, 07:36 PM  #162  
diyAudio Member
Join Date: Feb 2010

Quote:
Ive have a couple of 6FQ7 tubes over here.. and they seem have a low rp and mu. rp: 6700 mu: 20 lets say we take this diagram: So we have for example: rp: 6700 mu:20 Rk:3300 Rp:220k Output resistor 1M If im wright you have to calculate the parallel resistance of Rp and 1M ? Thats about 180K. Amplification factor would be: mu 180k/(rp+180k+Rp (mu+1) = 14 times. Is this right ? But about the problems you write about... so you calculate the fequency of in this case 180K (Rp+1M) and 2uF ? 1 / 2 pi R C = 0,5 hertz ? 

13th May 2011, 06:46 AM  #163 
diyAudio Member
Join Date: Sep 2010

Dear All,
I'm new at this forum and I'm currently building an RH84 se amp: RH 84  Tube Audio ...... RH DESIGN I would like to know the overall voltage gain of the circuit, because my cd player has only 1V rms output. Is it enough to drive it to the clipping point or no? My transformer ratio is 1:25 of course. (square root of 5000/8) Thank you very much. 
13th May 2011, 09:35 AM  #164  
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Join Date: Apr 2005
Location: L'Aquila, Italy

Quote:
For the input, I'd use a 100K volume pot (with no extra R in series), and likely one or more LED or a Zener diode (whatever fits better the required Vgk) in place of the R//C for cathode BIAS. Oh, anyway, for the "automatic" bias case with cathode resistor, 2200uF WRT 3.3K is way too high a value for the bypass...
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13th May 2011, 06:23 PM  #165  
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Join Date: Feb 2010

Quote:
Can you explain why 2200uF with 3.3K is too high ? 

17th May 2011, 06:44 PM  #166  
diyAudio Member
Join Date: Apr 2005
Location: L'Aquila, Italy

The Xc of the bypass cap at the minimum frequency (would be 20Hz, but it's better to be conservative and use 10Hz instead) must be "negligible" (<= 1/10) WRT the value of the R. That is:
Xc = 1/2*pi*f*C <= R/10 @ f=10Hz Thus: C >= 1/2*pi*R With R=3.3K, C ~= 47uF. You may use a somewhat bigger C to be even more conservative (say 100 or 220uF), but there is really no point on using a much larger value such as 2200uF.
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18th May 2011, 04:25 PM  #167  
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Join Date: Feb 2010

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But i don't get the other calculation. The capacitive reactance (Xc)= 1/2*pi*f*c = 1/2*pi*10*47uF = 338.6 ? right ? for 20hert example.. its would be Xc of 169.3 ? And for 2200uF with 10herts it would be Xc 7.23 what is the idea behind these value's ? 

18th May 2011, 05:37 PM  #168  
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Join Date: Apr 2005
Location: L'Aquila, Italy

As said, the idea is that you want Xc << R at the minimum frequency of interest (20Hz).
About your calculation, ~170 ohm is << 3.3K, so that value is ok. Of course so it is ~7 ohm, but there's no point in goin' so far. A ratio of 1:10 if enough. Using too big a capacitor only brings in more troubles, as e.g. it will have worse performance at higher freq. Oh, I'd use a much higher operating voltage cap, at least ~50V. As for 47uF vs. 48uF, that does not really make any difference! Consider that capacitors (and particularly electrolitics) do have typ. 20% tolerance. A nominally 47uF cap. can be in fact anything between 38u and 56u, a 100uF one may range from 80u to 120u and so on! I said 47u 'cause that's the closest standard value.
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19th May 2011, 04:42 AM  #169 
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Join Date: Feb 2010

Aaaahhh right ! The picture becomes clear now !
I calculated 48uF and posted it to show it if it was correct Thank you very much for explaining Unixman 
24th July 2011, 02:49 PM  #170 
diyAudio Member
Join Date: Jul 2011

is only linearity supermost reason for vaccum tubes in an amp??

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