Compare the sound of 6550 amp to a 6L6 amp
Hi all. I'm new to the forum and am thinking about cloning a Fairchild 260 in order to build a couple of mono blocks that I can (hopefully) afford.
I just bought a "Photofact" for the 260 on eBay for $5.00 that has the schematics, internal views, test voltages, parts list and descriptions.
I would like to know your opinions on the sonic differences of 6L6 tubes compared to the 6550. I'm trying to figure out if there would be any advantage in adapting the circuit to the 6550 in order to boost the output. I've noticed that the 6L6 is on average less expensive than the 6550 which steers me to it. But, I've also noticed that Conrad Johnson has gone to the 6550 to power their high output amps.
I currently run Maggie MMGs with a Parasound HCA 1500A. My goal is to power the Maggies with the mono blocks and use the 1500A to power 10" subs (that I plan to build) below 100 Hz to accentuate the bottom end.
Thanks for your ideas!
I wish you luck in your quest. I must ask a few steering questions. These questions will hopefully lead you into some discovery...:)
Have you done any searching for the components listed?
Have you reached any conclusion on which ones you may have to acquire substitutes for?
Have you reached any conclusion about parts you *CANNOT* substitute for?
What have you learned about the cost for the BoM based on the answers you have gotten for the first three questions?
I've been researching this idea for several days looking for a way to build the tube amp I could never afford, the MC 60 While doing so, I discovered the Fairchild, then found info on its reputation and then found the tech manual yesterday on eBay! Whew!
I haven't received the manual yet so I don't have the parts list. I'm just hoping the transformer won't be as impossible, or expensive, as the McIntosh's to acquire.
Today, while looking around, I came across this site:
which shows a pretty interesting (and recent) 30W push/pull design that I want to research some more.
After receiving my eBay loot and having a look, I'll be better equipped to answer your questions.
Cheers for now!
To change from 6L6 to 6550 will give you more power and should sound just as good, as long as your power supply and output transformer can support the additional current without loss of B+ voltage, overheating or damage.
I'm not all that impressed with Claus Byrith's article on improving the Mullard 5-20, to be honest, because IMHO he overlooks some important points. For example, changing the operation of the EL34s from cathode bias to fixed bias is fine, but it entails reductions in in both input and output impedance, which he does not address. This will affect the suitability of both the driver/splitter tube and the output transformer. He also does not address the fundamental unbalance in the phase splitter, which could be overcome easily by including a CCS in the tail.
One thing to keep in mind about comparing tubes...
When comparing different output tubes in the same circuit you will get some noticable difference...but thats because your using the same circuit for different tubes...
The tubes don't make bass or treble.... The sourounding circuit works with the tubes to make the sound....
The output transformer inductance, leakages..plate loading...ect..ect work with one tube may work differently with other tubes...the input sensitivity can also be different....
You really need to target the circuit for the choosen output valve...
The 6550 will be able to make more power output than the 6L6 tube..... Usually when you sub a 6550 in a 6L6 circuit you will get more headroom and the sense of more bass colorization....
... modifying the Mullard 20W to run on 6550s. Poking around with that circuit to run on fixed bias is tempting; But
I tried running 6550s in lieu and found thd at 10kHz over 5% probably caused by the ECC83 unable to dump o/p tube Miller effect. In addition the 6550 requires lower grid leak resistances than either 88's or 34's. Despite the 5881 / 6L6 having lower dissip than either, they may sound better in such an amp as both have easier drive requiremnts.
In the 1960's there was a 50W version of the Mullard 20W amp running 34's in fixed bias....a circuit must be somewhere .....this beefed up version was commonly used in MI guitar amps.
You're right, 12AX7 is a poor choice to drive 6550s (or even EL34s with fixed bias, IMHO).
A Mullard-based amp with 6550s would call for a different driver tube for sure (meaning lower plate resistance, higher current and lower gain) but I'm sure it could be done successfully. The Mullard 5-20 had excessive gain anyway but I think you would want to keep the EF86 operating in pentode mode in the first stage, to make up for the reduced gain in the drivers.
Run a pentode LTP for the phase splitter. Source the B+ for that stage from the respective screen taps.
Try the 6AC7...it's got enough gm to do away with the input SE stage. Even with plate loads that leave reasonable output Z ( like 15k ) for the stage.
You'll not need any loop FB if you do it that way too, and that's a fine benefit.
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