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Old 9th May 2011, 12:46 PM   #161
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excellent site for beginners on the basics of tube guitar amps:

http://www.freewebs.com/valvewizard/index.html
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Old 12th May 2011, 07:36 PM   #162
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Quote:
Originally Posted by coffeedj View Post
PS: My last reply was general and may not have answered the specific question asked.

This looks like an instrument amplifier input. What is the purpose of this pre-amp?

For the output gain control to work properly it must go to a high impedance input--read tube amplifier. A standard 1-10K input impedance for transistor amps would be incorrect for this amplifier, but the high value output capacitor in series with a 20K resistor suggests that such is the target. In addition the huge bypass capacitor suggests a very low Rk which is not consistant with the output design. A 100 ohm Rk gives a high pass at .7Hz--very unusual and potentially unstable with a choke input power supply. A Rk of10 ohms would not be a medium mu tube design. It would be more in line with a power tube like the 6AS7.

Input low pass could be around 50KHz at low volume, depending on the tube--again, somewhat lacking for an audio amplifier today.
Ok ive studied your text. I have an Class D TA2024 over here wich needs a higher input ( ok i know the resistor mod but would be nice to design a tube pre amp for it).

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:
Click the image to open in full size.

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 ?
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Old 13th May 2011, 06:46 AM   #163
oibacsi is offline oibacsi  Hungary
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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.
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Old 13th May 2011, 09:35 AM   #164
UnixMan is offline UnixMan  Europe
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Quote:
Originally Posted by TubeManiac View Post
If im wright you have to calculate the parallel resistance of Rp and 1M ?
no. The 1M is almost negligible. Output impedance is approx. equal to the parallel of Rp and rp.

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 by-pass...
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Old 13th May 2011, 06:23 PM   #165
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Originally Posted by UnixMan View Post
no. The 1M is almost negligible. Output impedance is approx. equal to the parallel of Rp and rp.

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 by-pass...
Aah ok... but i ment 1M with Rp to calculate the amplifie factor

Can you explain why 2200uF with 3.3K is too high ?
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Old 17th May 2011, 06:44 PM   #166
UnixMan is offline UnixMan  Europe
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The Xc of the by-pass 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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Old 18th May 2011, 04:25 PM   #167
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Originally Posted by UnixMan View Post
The Xc of the by-pass 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.
I can calculate 1/2*pi*R.. if you use 3300ohm C would be like 48uF.

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 ?
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Old 18th May 2011, 05:37 PM   #168
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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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Old 19th May 2011, 04:42 AM   #169
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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
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Old 24th July 2011, 02:49 PM   #170
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is only linearity supermost reason for vaccum tubes in an amp??
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