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Given one 12AX7 how would you run it given the rest of the amp is SS?
Given one 12AX7 how would you run it given the rest of the amp is SS?
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Old 26th November 2017, 02:48 AM   #11
nigelwright7557 is offline nigelwright7557  United Kingdom
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I used a 12au7 on the front of a TDA7294 chip amp.

Had a few problems with oscillation and noise but fixed it with LP filter on front end.

I had reduced gain in chip amp so 12au7 could make up for gain.
Then someone told me the tda7294 has a min gain for stability hence why it was oscillating. Managed to tame tda7294 with a 1000pf across inverting and none inverting inputs.
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Old 26th November 2017, 03:16 AM   #12
Printer2 is offline Printer2  Canada
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Quote:
Originally Posted by thoglette View Post
Waste of a triode. Might as well use a MOSFET.

Or do you have a clever scheme to drive the CF to clipping/cuttoff?
The CF is an imperfect machine, it colors the sound. If you want clean then a MOSFETis the way to go. Actually a clever Russian got a LND150 to act like a tube.
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Old 26th November 2017, 04:47 AM   #13
bwaslo is offline bwaslo  United States
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Given one 12AX7 how would you run it given the rest of the amp is SS?
as a pilot light?
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Old 26th November 2017, 01:04 PM   #14
Printer2 is offline Printer2  Canada
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I thought of that but not using the LND150. Drive a mosfet with the rectified signal to modulate the supply voltage to introduce some sag. Need a load, a pilot light might do it.
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Old 26th November 2017, 01:56 PM   #15
AquaTarkus is offline AquaTarkus  Canada
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Quote:
Originally Posted by thoglette View Post
Or do you have a clever scheme to drive the CF to clipping/cuttoff?
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Originally Posted by Printer2 View Post
The CF is an imperfect machine, it colors the sound. If you want clean then a MOSFETis the way to go.
Valve Wizard Merlin discusses this in his article on CFs. As I mentioned previously, the CF grid must be directly coupled to the anode of the preceding voltage amplifier stage. This is a common arrangement in guitar amps and the resulting 2nd harmonics are perceived as tube warmth.

Last edited by AquaTarkus; 26th November 2017 at 02:00 PM.
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Old 26th November 2017, 03:09 PM   #16
JMFahey is offline JMFahey  Argentina
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If you want the tube to be more than a Marketing point (which I guess is the case with the Joyo amplifier), then it must "do something audible" to the sound.

Which means driving it hard, to clipping or at least to very high signal levels, >10 or 20V RMS.
Then it will add noticeable harmonics which will colour the sound.

A simple cathode follower, specially at the front end (where sigmal level is lowest) or output , where "everything is already cooked and flavoured" and it only has to provide about 1V RMS or less, will do very very little; detectable on a Lab but practically not audible, specially for Guitarheads who are used to lots of angry harmonics.
We are not talking a Hi Fi guy who detects small fractions of 1% in his signal chain.

Cathode followers driven hard and driving a complex load (typically the Tone Control stack) add lots of interesting distortion , unsymmetrical so lots of even harmonics, which Guitar players like, so te unwritten rule (at least most follow it) in Valvestate type amplifiers and its derivatives is to have a transistor/Op Amp front end, driving a single triode gain stage, direct coupled to a low idle current cathode follower which will starve when trying to drive the negative swing into the TC.
Best bang for the buck.

Signal output will be reasonable for a tube power stage (the design comes straight from Tweed Fenders and Marshalls) and must be attenuated to drive SS amps, any Class, but the point is to drive it hard first.
If you set signal levels so it just passes around 1V RMS or less and straight the SS amplifier, it won´t do that much either.
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Old 26th November 2017, 04:48 PM   #17
AquaTarkus is offline AquaTarkus  Canada
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That's exactly why my first post in this thread said to have some SS amplification in front of the first triode, to enable it to be overdriven. Then the direct-coupled CF adds its additional warmth. Yes, we're both thinking along the same lines here.
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Old 2nd December 2017, 02:27 AM   #18
Printer2 is offline Printer2  Canada
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Just threw a SS section before the tube circuit, think if I did a one tube one this might be a place to start.

Click the image to open in full size.

Oh yeah, S1 with a center off position.
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Old 3rd December 2017, 05:54 AM   #19
thoglette is online now thoglette  Australia
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Quote:
Originally Posted by AquaTarkus View Post
Valve Wizard Merlin discusses this in his article on CFs.
That'd be The DC Coupled Cathode Follower

Quote:
Originally Posted by Printer2 View Post
Oh yeah, S1 with a center off position.
Interesting. But how do you get output in the centre position

Last edited by thoglette; 3rd December 2017 at 05:59 AM. Reason: Ensuring quotation is correct.
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Old 3rd December 2017, 01:45 PM   #20
Printer2 is offline Printer2  Canada
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I actually have a four pole switch with no center position. Or at least I think I do, will have to find it yet. I thought about the open centre position of the other switches I have, then I quickly put that thought out of my head. I did make use of a two pole three position switch yesterday to change the voicing of the tone stack. Slope resistor 33k with two 250p caps, 56k with one 250p, then open position three resistors in series, 33k, 22k , 120k with the 250p, which gives a reasonable BF curve. Will be fun to try.
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