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Old 4th August 2006, 06:51 PM   #11
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I have been searching about SY's version of maida regulator, but I didn't found a concrete solution to my problem.
In respect to the LED Array, yes, i red the article and sounds very interesting.
I think i'm gonna build this amp.......
Yahoo ID: maxfield_rock if anybody wanna help me.
Thanks!
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Old 5th August 2006, 02:19 AM   #12
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Default Re: OPT Project for EL84 SE

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Originally posted by Toober
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Thank you!
That design SUX on many levels. No NFB with a single ended pentode is going to sound truly horrible. The output impedance is going to be too high to adequately damp the woofers, so the bass will probably sound excessively "boomy" and with poor definition as the woofer will be playing its own resonant frequency, not the notes the musicians played. There is no screen regulation, which is essential for good, linear operation. A series dropping resistor isn't going to cut it. The power supply filtering is inadequate, and will be really horrible in triode mode.

Ineither case, one half of a 12AX7 doesn't have the "moxie" to adequately drive the grid circuit of very many audio finals, especially not a triode with its attendant Miller capaciatnce. This thing'll lose the high end due to slew rate limiting.

I might use the durn thing as an AM plate modulator (and for this power level, I'd pretty much go solid state and/or low level modualtion plus linear amplification) but it's pretty useless for anything else.

All things considered: Click the image to open in full size.

I also don't like using those kind of jacks for connecting the speeks: that's just asking for major poofage when that connection fails.
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Old 5th August 2006, 04:24 AM   #13
rdf is offline rdf  Canada
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Miles, you forgot the undersized cathode bypass caps with a corner frequencies somewhere at the bottom of the audio band, just in case the sub-1 damping factor didn't yet have quite enough bass.
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Old 5th August 2006, 05:16 AM   #14
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Quote:
Originally posted by rdf
Miles, you forgot the undersized cathode bypass caps with a corner frequencies somewhere at the bottom of the audio band, just in case the sub-1 damping factor didn't yet have quite enough bass.
I didn't even get that far after seeing what I mentioned. Now that you mention it, a one watt resistor seems way too small. The bypass would be OK for ham radio useage: 300 -- 2500Hz.
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Old 5th August 2006, 12:37 PM   #15
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Quote:
The bypass would be OK for ham radio useage: 300 -- 2500Hz.
It was probably OK (and intended) for a small guitar amp. In many small (read cheap) guitar amps the low end was rolled off to keep the 1 inch OPT out of complete saturation.

I have found another reason. If you build a big guitar amp, with good LF response (mine was SS) and you smack the strings hard, the speaker cone will follow this movement. In my case, I got a loud snap from the voice coil bottoming out. I have seen speakers destroyed this way. Another reason for the purposely lousy DF. It also brings out the speaker cabinets resonant effects (intended).
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Old 5th August 2006, 02:17 PM   #16
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It is a guitar amp. The 1/4 phone jacks in and out are a giveaway. Use the other 1/2 of the 12ax7 for a tone stack, and you have an ax84 P1. Never intended for hifi. Designed to distort at low volumes.
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Old 5th August 2006, 02:43 PM   #17
amperex is offline amperex  United States
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Default I agree with NickC

Place pot in input side of 12AX7 (grid). I used the 12AX7 with both sections in parallel- higher mu, lower noise. With parallel sections, the cathode resistor needs to be 1/2 normal resistance & I did not use a cathode cap. Try the cathode cap yourself to find out if you like it better or not.

Inexpensive Sovtek 12AX7LPS outperformed Telefunken 12AX7 smooth plate & Mullard 12AX7 short plate in the EL84 amp I built- go figure.
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Old 5th August 2006, 04:37 PM   #18
Giaime is offline Giaime  Italy
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Default Re: I agree with NickC

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Originally posted by amperex
I used the 12AX7 with both sections in parallel- higher mu, lower noise.
I'm sure you meant double transconductance, not mu.
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Old 5th August 2006, 05:49 PM   #19
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Quote:
Originally posted by tubelab.com
It was probably OK (and intended) for a small guitar amp. In many small (read cheap) guitar amps the low end was rolled off to keep the 1 inch OPT out of complete saturation.

I have found another reason. If you build a big guitar amp, with good LF response (mine was SS) and you smack the strings hard, the speaker cone will follow this movement. In my case, I got a loud snap from the voice coil bottoming out. I have seen speakers destroyed this way. Another reason for the purposely lousy DF. It also brings out the speaker cabinets resonant effects (intended).
That explains a lot. However, what about drive capability? That 12AX7 doesn't have very much, especially if you operate EL84 as a triode. Maybe a 12DW7 would be better: use the high gain section as a voltage amp, and the other half as a cathode follower grid driver.
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Old 5th August 2006, 09:54 PM   #20
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That explains a lot. However, what about drive capability?
Once upon a time long long ago someone built a guitar amp with a 12AX7 driving a 6V6 or EL-84 (Fender Champ and others) and it has been copied 10,000 times. It probably runs short on HF response and slew rate, but it works. I have seen a 12AX7 as a LTP phase splitter driving 4 EL34's in parallel push pull (old Marshalls). It certainly isn't HiFi, but it sold a lot of amps.

Triode wired guitar amps are not common either. Most are pentode, and some SE amps do not use feedback or stabilized screens. The referenced schematic is pretty common.

When you design a guitar amp today you must use tubes that you can by at a music store if you expect to sell any of them. That used to limit the designer to about 6 different tubes. The 12DW7 was used by Ampeg in the 1970's and they became scarce. Now there is a new production version, and it is available in some music stores, so it is a valid choice for a guitar amp. Still many guitar amp builders just copy a known design, and the old Fenders and Marshalls are the most popular designs to copy. Fender used a 12AX7 for almost everything. Some of the later designs, especially the ones that used 4 6L6's used 12AT7 for drivers which is a better (and valid today) choice.
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