Look carefully at the "El Cheapo" schematic. Please notice that it's an "integated" amp. The high μ, high gm, low RP 12AT7/ECC81 provides sufficient gain to drive both the "finals" and the NFB loop, when only a CDP is "upstream". The other types mentioned can't do the job. Another point in favor of the 'T7 is powerful synergy in combination with PP O/P tubes. Its skewing of the HD spectrum towards 2nd order blends very well with the internal even order HD cancellation of PP "finals".
Like a bad penny, the 12AU7/ECC81 keeps turning up.
JMO, there are only 2 audio jobs the 'U7 is any good at: cathode follower and "concertina" phase splitter.
Like a bad penny, the 12AU7/ECC81 keeps turning up.
JMO, there are only 2 audio jobs the 'U7 is any good at: cathode follower and "concertina" phase splitter.Yep, but the El Cheapo uses global negative feedback.. so just wondering what happens if you use an XPP transformer (rated at 70Hz min), and it runs out of 'oomph' at bass frequencies..
What's wrong with NFB PROPERLY done?
If you look carefully at the schematic you'll see an input cap that is designed to limit the low frequency bandwidth in case a cheap OPT is used. The value there in the schematic was for the guitar OPT originally spec'd. With a change to an Edcor or Dynaclone that cap can/should be increased in value.
Adjust that cap value to what's appropriate for the OPT you choose.
Let me reinforce what Eli said about the 12AT7. You CAN'T look at the tube in isolation, you have to look at the entire circuit. The 12AT7 not only offers enough mu/gain to allow the amp to be driven to clipping with line-level sources, coupled with the P-P output stage and the low/moderate NFB it provides low THD that is NOT dominated by odd-order products as is often the case in P-P amps. No other tube I know of has the combination of attributes needed to make the El-Cheapo sing. People who've built it (especially with Dynaclone OPTs) found it to be really good, way beyond what they expected.
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Edcor's XPP25-8-7.6K should do, as long as you stick to triode mode "finals". That trafo's 25 W. rating means sufficient magnetic headroom is present to cope with the NFB induced deep bass error correction signal. Even at 56 nF., the high pass corner lies below the lowest note a double bass can produce. High pass filtering infrasonic noise, along with properly employed GNFB, allows "budget" O/P trafos to sound pretty darned good. Of course, the better the "iron, the better the amp. Built with DynaClone Z565s, an "El Cheapo" is a very good amp.
High pass filtering infrasonic noise, along with properly employed GNFB, allows "budget" O/P trafos to sound pretty darned good. Of course, the better the "iron, the better the amp. Built with DynaClone Z565s, an "El Cheapo" is a very good amp.Edcor's XPP25-8-7.6K should do, as long as you stick to triode mode "finals". That trafo's 25 W. rating means sufficient magnetic headroom is present to cope with the NFB induced deep bass error correction signal. Even at 56 nF., the high pass corner lies below the lowest note a double bass can produce.
Hmm.. I was thinking about using ultralinear mode and maybe even paralleled 6V6s as output (using XPP-16-7.6k as OPT). Maybe that's just asking too much.
What's wrong with NFB PROPERLY done?
If you look carefully at the schematic you'll see an input cap that is designed to limit the low frequency bandwidth in case a cheap OPT is used. The value there in the schematic was for the guitar OPT originally spec'd. With a change to an Edcor or Dynaclone that cap can/should be increased in value.
Yeah, you are right, should have looked at it more carefully..
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I would like to get this question answered. I would like to know the voltages for each the B+, B2+ and B- in the El Cheapo amp.
eli-duttman If you happen to have those values that would be great.
Guys, no need to put in an additional trannie for the bias. Just AC couple a voltage doubler to the secondary then filter it and you are good to go. Adjust the value of the cap to get the desired voltage. With all due respect to Mr Prower I adhere to the late Charlie Kittleson's motto 'USE A TRANSISTOR GO TO JAIL'
Selling and promotional posts belong in the Swap Meet of Vendor section. Self promotion is frowned on in the regular threads.
Generally speaking you can consider Fivre tubes as American tubes made in Italy under license with quality materials.
However I have seen crazy prices around. As always cost is subjective. IMHO, I could buy them in case I would want to restore an old Fender Amp to its original state. For DIY I would just buy the black coated Russian GT version which is more than good enough and very strong. Tested up to 450V plate voltage in pentode mode and it worked flawlessly. Having said that I have special 6V6 types in my collection but I bought when their price was still reasonable....
Yes, I built that amplifier several years ago. It was a fun project . Easy, and sounded excellent. Click on the reference to el-84 or 6v6 tube use on top of page. You can substitute el-84's for the 6v6 if you change output stage cathode resistors to 250 ohms. I used a heavy duty, dpdt toggle switch to make the cathode resistors switchable to use either 6v6 or el-84 tubes. I believe I used tube socket adapters for changing from 6v6 to el84. I can not remember if I preferred the amp with el84, or 6v6. I used it for a couple years, then sold it to finance another amp project. Have fun. Its worth yhe time and effort in my humble opinion.
Mg16
Mg16
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