Recommend me a KT88/6550 + 7N7 (6SN7) Schematic!

[john65b]

Hey all,

Getting ready for my next project. I plan on gutting an existing 6L6GC tube amp I bought years ago since it had some nice components...pics of the amp and internals (yuk) are attached, Generally, here is what major parts I have.

Nice Chassis with Input/Output Transformers installed with 8 Octal Socket holes
Pair of Mil Spec no name Power Transformer 400 0 400, unknown VA but HUGE
Pair 1650N Hammond Trannies
Pair Chokes
Quad Ruby 6L6GC
Quad 6L6
Quad KT88
Quad GE 6550
4 pairs 7N7 (Loctal 6SN7) with sockets
2 Pairs GE and RCA 6SN7
Pair Red base 5691 (6SL7)
pair 1200uf 450V Caps
Plenty 1N4007
2 pairs 5R4 rectifiers

I can use Tube rectifier or SS.
Power Tranny does not have Bias Taps. Don't really want to add another tranny for Bias, so Cathode bias is preferred
I plan on LC filter (choke input)to keep B+ within 450V of my caps
Wanted to do Quicksilver 8417 design, but want to use Octals (already have the holes in Chassis) and don't have bias taps on power tranny

Any recommendations? I have attached a KT88 design and another 6L6 design for starters...

Want to ideally use the amp on my Magnepan Tympani IV's, but may not be possible with a tube amp. Leaning toward a KT88 design due to high-ish power requirements of the maggies...

[Andrewbee]

The Acro 6l6 version I built something similar many years ago, well the front end was the same but used EL84s as outputs. After many frustrating days I got one channel conditionally stable but gave up as I have enough work to aggravate myself with and one should enjoy hobbies

Perhaps it needs a special opt for the stability or I just could not do it properly?

The second circuit appears to be the G.E.C. 30W UL design which uses KT66 or KT88 as output tubes, differing with the opt primary and the B+. The KT88 version runs in "almost class A". Never built it. Its a traditional design which should work well.

I don't know how well either of these tube amps would perform with Maggies though there is one way to find out.

Good luck

[Eli Duttman]

While power hungry, "Maggies" present a tube friendly "flat" impedance curve.

You will have to squeeze every last Watt out of what you have. Unfortunately, the Hammond O/P "iron" needs some linearizing help from GNFB and you don't have magnetic headroom to spare. You will have to tolerate the shortcomings of the "iron". Since both of the circuits you posted have GNFB loops, they are ruled out.

If you SS rectify the B+, you can use "fixed" bias. Phase up the 2 X 5 VAC windings, wire them in series, and voltage multiply the composite.

The filter chokes shown clearly lack the requisite heft for choke I/P service. Also, as the ARRL Handbook states, LCLC is the way to go and you don't have 4 inductors.

The 1N4007 is noisy guano. IMO, that diode should not be in any HIFI equipment, tubed or SS.

"Fixed" bias and a 450 V. B+ rail should get you to 60 WPC. SS rectification into a CLC filter, where the 1st filter cap. is on the small side, will do the job. Don't forget the CL150 inrush current limiting thermistor.

Since O/P power rules the roost, use full pentode mode "finals" and Maida regulated g2 B+. The "exolinear" topology favored by Doug Piccard (Bandersnatch) gets my nod. You should get the gain you need from a differential cascode made of 7N7 sections, as the triode in that type is reasonably high gm.

Consider rolling the response of the amp off around 28 Hz., at its I/Ps. You are very tight on power O/P and 31 Hz. is the lowest note a double bass produces. IMO, there's no point in trying to handle stuff that can be viewed as infrasonic noise, especially when no spare power is available.

[john65b]

Hello Eli,

OK, I can visit PSU designer and make sure I will be at 450V B+ with the CLC (tweak 1st C around 1 - 4uf to get 450V).

What do you recommend on the SS Rectifier if not 1N4007?

Looks like the fixed bias can be made from the 5V (forgot about that). If so I guess I could do the attached (modded Quicksilver 8417 by someone here) - but I don't have the 8417 or the 12BH7...maybe a few tweaks to run a 6550 or KT88 in place of the 8417 and the other half of the 6SL7 for the 12BH7 Phase Splitter?

The QS trannies are 4200 and the 1650N is 4300, close enough?

[analog_sa]

Seems modded by the Diana Kraal brigade Do the Maggies enjoy 15W?

I may try a QC variation using KT88 in pentode with regulated G2, a 6H30 splitter and a D3A front, either pentode or triode. Salas shunt reg for the front end and tube rectifiers.

[Eli Duttman]

John,

Forget about the 3rd schematic you posted, as it too has a GNFB loop. Your O/P trafos handcuff you, sorry.

The UF4007 and UF5408 are drop in replacements for their 1Nnnnn counterparts. UFnnnn diodes are much quieter than 1N diodes, to begin with, and they are easily snubbed into near complete silence. Schottky diodes are naturally "noiseless", but high PIV Schottkys are costly. Scan the archives for "cockeyed bridge" and you'll find a scheme for employing Schottkys, without going to the poorhouse.

Scan the archives for "exolinear". That topology is a sophisticated form of plate to grid short loop NFB. The O/P trafo is not inside the loop. So, your limited magnetic headroom goes towards music, not an error correction signal. To use GNFB containing topologies, the "iron" needs to be (sic) 100 W. capable.

[john65b]

Eli,

I have searched EXOLINEAR on here and google and only find references from you to others.

Do you have a simple schematic to give me the gist of what your saying?

If I take the maggies out of the equation, is one of the previous three schematics I posted worth building in your opinion?

One other thing - I was told the following regarding Magnepans from the SS side:

"They have zero phase shift, Maggies, in sharp contrast to other drivers, and this makes them an ideal choice for a global negative feedback amplifier."

Do you not agree? Or may it only be true for SS amps??

I appreciate your assistance....

[Eli Duttman]

John,

The speakers are not the problem. The O/P trafos you possess lack the magnetic headroom necessary for GNFB to be employed.

I've uploaded a schematic of "Exolinear" style circuitry. While the drawing is very crude, sloppy, and overly simplistic, it drives the key points home. Notice that the O/P trafo is not inside a NFB loop. If you look carefully, you will see short loop NFB from O/P tube plate to O/P tube grid.

[greenvalve]

Originally Posted by Eli Duttman

I've uploaded a schematic of "Exolinear" style circuitry....

Eli, I like your schematic What are you using for CCS B-?

[Eli Duttman]

Originally Posted by greenvalve

Eli, I like your schematic What are you using for CCS B-?

That drawing is not an actual implementation. In the OP's case, the splitter will be a differential cascode and both negative voltages will come from "taps" on a voltage multiplier energized by 5 VAC windings made spare by SS B+ rectification. The OP has a pair of power trafos.

If you want to get into the nuts and bolts of "Exolinear" style circuitry, contact Doug Piccard, AKA Bandersnatch. Doug has done considerable work with the concept.