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Old 16th January 2011, 03:11 PM   #1
g(f(e)) is offline g(f(e))  United States
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Default using existing pre with this circuit

Hello,

I posted last week about using the grounded grid pre I built - and what circuit might be good with it. I wasn't very specific. Let me state what I have and what my goal is.

I have a handful of 5U4 tubes and EL84 tubes. I want to build a SE amp and use my pre amp to feed it. I found this circuit searching diy audio.

RH 84 - Tube Audio ...... RH DESIGN

It would seem to fit my criteria. The problem is I do not have enough knowledge to strip away the things I do not need from this circuit being I plan on using my pre amp. Any help would be appreciated.

I will have many other questions after I get started also, so I could use a mentor that can tolerate newbie questions.

Thank you.

Gary
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Old 16th January 2011, 04:11 PM   #2
DF96 is offline DF96  England
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What were you thinking of stripping away? I can't see any superfluous components.

One possible snag with this circuit is that the ECC81 may be biased rather too near the grid current region. Others will be able to tell us what the typical cathode voltage is. If less than about 1V then this amp might have a non-linear input impedance so would need to be driven from a low impedance preamp, otherwise you might get too much "musicality".
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Old 16th January 2011, 05:00 PM   #3
g(f(e)) is offline g(f(e))  United States
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My thinking was, if this is a stand alone amp circuit, then the ECC81 being fed by the input must be the pre amp stage for the EL84. So, I thought I might not need it if I was using the pre.

Is this not a good idea?
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Old 16th January 2011, 05:27 PM   #4
SY is offline SY  United States
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Quote:
Originally Posted by g(f(e)) View Post
I found this circuit searching diy audio.

RH 84 - Tube Audio ...... RH DESIGN
Caution, that site appears to have viruses.
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Old 17th January 2011, 04:20 AM   #5
DougL is offline DougL  United States
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I like your idea in theory. Using the right amount of gain stages can increase system performance.
Looking at the circuit you chose, it may not be a straightforward conversion because it uses plate to plate feedback, labeled Rfb to lower the output Z of a pentode output stage.
Without feedback, you would have an amplifier with characteristics similar to a Firstwatt F2.
If this does not meet your goals, converting the output to triode or finding a suitable feedback point on your GG pre are your options as I see them.

Good Luck.

Doug
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Old 17th January 2011, 10:04 AM   #6
DF96 is offline DF96  England
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Without feedback, you basically have the output stage from an early 1960s wireless set! The ECC81 is not a preamp, but a voltage amp/driver, and in this particular circuit is tied quite tightly to the output stage via feedback. The EL84 grid needs 10-15V of signal - rather high for a preamp! The ECC81 adds enough gain that you might not need a preamp, but this does not mean that you can replace the '81 with your preamp as they are doing different things.

In a typical set-up, the voltage amp/driver stage is needed; the preamp is optional.
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Old 17th January 2011, 10:48 AM   #7
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As found in many AF output stages of vintage (60's) radio receivers using EL84,as well in old vademecuums (handbooks), right value of EL84 cathode resistor was (is) 135 ohms producing - Ug1 of 7.3 Volts at plate current of 48 mA + 5.5 mA of Ig2 (all with plate voltage of 250 Volts). Such resistor value was readily found 45 years ago. Now you can use 2 resistors of 270 ohms tied together in parallel. So, max input voltage is 14,6 Vpp for class A operation, equals 5.18 Veff. Suggested plate load was (is) 5,2 KOhms, so with 5 KOhms you are not so far away of ideal load. Max output power with such operating points is 5.7 Watts with 10 percent of THD. As DF96 stated, Rk of ECC81 is to low. Be aware of various "designers", instead use tube data easily found on web, i.e. Frank's Electron tube Pages Frank's Electron tube Pages
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Old 17th January 2011, 11:57 AM   #8
DF96 is offline DF96  England
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OK, my 10-15V was an overestimate. That is what happens when I use my memory!

The circuit is strange, in that the feedback linearises the output stage but increases distortion in the voltage amplifer by loading it heavily. The cathode degeneration will help. On balance the distortion will be lower, but possibly more complex?
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Old 17th January 2011, 04:32 PM   #9
g(f(e)) is offline g(f(e))  United States
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Thanks to those that replied. Some of the comments were very helpful.

Gary
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