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Old 26th June 2012, 08:12 PM   #11
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I've had good results from a cascode LTP. This, however, I have doubts about. The whole idea behind the cascode is to improve the gain and Cmiller problems of a straight triode stage. The characteristic is rather much like that of a tetrode (where the name comes from: cascade + tetrode) less the secondary emission "kinks", and a less extensive saturation region. You also have the same large Rp of the tetrode.

Cascodes for small signal audio amplification: good idea. For power amplification, not so much.

As with anything else, though, it's a try and see (listen?) situation.
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Old 27th June 2012, 01:40 AM   #12
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Quote:
Originally Posted by DF96 View Post
The output impedance of a cascode is roughly mu(top) x Ra(bottom). This may or may not be higher than a pentode, but it will be much higher than a triode on its own. An unbypassed cathode resistor will add mu(bottom) x mu(top) x Rk.

It will get you into A2 mode, but at the expense of a high cathode voltage which limits efficiency. Interesting as a learning tool, but in most cases I would think that more conventional outputs are better.

The main benefits of a cascode are low input capacitance and pentode-like gain without partition noise. Both are very useful for video and RF, but less so for audio.
Thank you, I was having trouble figuring out for sure the impedance of a cascode in this application since every resource for them shows being loaded by a plate resistor for voltage gain. The spec sheet for 6bq5/el84 says 19 for mu with respect to g2. So if 19 is mu for trioded el84 and 250 is plate resistance of one side of 6080 then my output z should be about 4750. Is that right?

This is very fun for me, I just wanted to see if I could do it. I was actually a little surpised it actually worked the first power up with the limited parts I had laying around.
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Old 27th June 2012, 03:27 AM   #13
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Old 27th June 2012, 09:27 AM   #14
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My amplifier today and all last year, the purest sound.
last edition -Eliminate the resonance rise of frequency characteristic if it is above 20 kHz, easier, increasing the value of the resistor (180Om) , in gate transistor, up to 3-5 kOm, which, together with an input capacity of transistor 1 NF, creates a filter that is 30-50 kHz, smoothing over the resonance, and limiting the bandwidth of the input signal.
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Old 28th June 2012, 07:16 AM   #15
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Default what is an easy way to bias this top half?

I don't need to go for power right? I should aim for low parts count...

If I run the cascode from the top rail then I could bias the lower tube easily with the plate resistor of the preamp 12ax7. The grid for the el84 colud be biased with batteries rock solid. So no high value cathode resistor and no coupling cap. If the grid stopper resistors were removed and pure silver wire for signal path could I use my tube amp as a linear cb amp? I'm just kidding of course.Click the image to open in full size.
SUPERS~1 by Dirtyrottenstinkingbiker, on Flickr

Last edited by Austin Translation; 28th June 2012 at 07:20 AM.
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Old 28th June 2012, 09:13 AM   #16
DF96 is offline DF96  England
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You don't need silver wire for CB frequencies! (Or audio)
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Old 28th June 2012, 09:19 AM   #17
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Quote:
Originally Posted by DF96 View Post
You don't need silver wire for CB frequencies! (Or audio)
I really do have some though, for making colloidal silver... This wire I ordered on ebay for making it.. You use a resistor to limit the current to one miliamp with three or four nine volts connected to each other in series in some hot distilled water for couple hours..
I just thought it funny to remove the grid stoppers and insert something of less resistance than even copper wire.

Last edited by Austin Translation; 28th June 2012 at 09:24 AM.
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Old 2nd July 2012, 04:38 AM   #18
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It works! I think I can make it better by playing with the bias a little bit but it acomplishes the goals I had in mind, direct coupling, class a2 and cascode output all with low parts count. Any suggestions and comments are welcome

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Old 24th July 2013, 01:56 PM   #19
aldovan is offline aldovan  Brazil
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Has anybody ever tried this to see what it sounds like? I think it's pretty interesting, especially as a super easy way to drive the top tube into grid current.


Power cascode - Power Cascode
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I made this projects and sound really fine.

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