This design likely to be my next build- History of favorable reviews.
A bit perplexing are the cathode windings in the OT design- Have not seen this design elsewhere. Anyone here have DCR on this winding?
Please shed light as to the operational aspect, and detriment thereof if omit & sub in some 18 or 22 ohm resistors to ground.
Thanks,
Jim
ps. didnt see spec on primary impedance... think 5-7k is typical, but??
A bit perplexing are the cathode windings in the OT design- Have not seen this design elsewhere. Anyone here have DCR on this winding?
Please shed light as to the operational aspect, and detriment thereof if omit & sub in some 18 or 22 ohm resistors to ground.
Thanks,
Jim
ps. didnt see spec on primary impedance... think 5-7k is typical, but??
Last edited:
That's called "unity coupling" and is basically a 50/50 split between the plate and cathode loading. There is no gain in the output stage, thus the preamp and driver stages have more gain than other tube amplifiers.
Without the original output transformers, which were proprietary designs, I think it's going to be nearly impossible to duplicate this circuit. You can't just substitute resistors, there's a matter of the winding inductance etc. that makes the cathode loading work. There are many fine vintage designs one could duplicate that don't involve the complexity of the McIntosh circuits.
Without the original output transformers, which were proprietary designs, I think it's going to be nearly impossible to duplicate this circuit. You can't just substitute resistors, there's a matter of the winding inductance etc. that makes the cathode loading work. There are many fine vintage designs one could duplicate that don't involve the complexity of the McIntosh circuits.
You might want to look at these before trying to duplicate it.
https://pearl-hifi.com/06_Lit_Archi...01/Sec_03/187_McIntosh_Patent_Compilation.pdf
https://pearl-hifi.com/06_Lit_Archi...01/Sec_03/187_McIntosh_Patent_Compilation.pdf
So then the 7591's are just a unity gain follower, like an op amp? Coupling high impedance to low impedance?
If I knew the composition, then possibility of duplication could be had. There are some quite capable winders on this site,
Thats way, way too long a read, but thx for linking.
I've some experience w/ US PTO - 1 design and 1 utility patent... just high priced frameables unless you have the $$ to defend them in court.
But thats another tangent
Jim
I think the point was that it's a very difficult circuit to emulate, and I'd be very surprised if you could find a DIY winder who would attempt to duplicate that OPT. Now, you CAN buy replica Mac 275 OPTs and circuit boards from China, and by all accounts they are very good. That might be a way to go.
Although the MA-230 schematics look like the "Unity Coupled" Mc designs (primary windings drawn the same length), they're really more like a QUAD II, with lots of cathode feedback and cathode loading (maybe 10 - 20%). But not bifilar wound and not equal anode and cathode loading. Makes the output valves driveable without exotic bootstrapping, and valve operation partial triode / almost UltraLinear. A better choice for DIY all around, but still not something with off the shelf OPTs (AFAIK).
All good fortune,
Chris
All good fortune,
Chris
Are you going to build just the power amplifier, or the whole amplifier?
It will be hard to find the Tone Control Encapsulated Module, Filter Encapsulated Module, and those multi gang input switches.
It will be hard to find the Tone Control Encapsulated Module, Filter Encapsulated Module, and those multi gang input switches.
I wondered if it was "unity coupled" or more like the Quad. Thanks for ther clarification, Chris.
Just the tube section. I'll use my existing input selector and pre-amps.
Plan to do my own PCB as a mono block using (2) 6GM5's and 6U10 for the 3 triodes (2 low mu and 1 hi mu triodes)
Hmm,
Where should I look? The Mac 275 OPT's are compatible w/ 230 design?
You can find them on Aliexpress. But I don't imagine they'd be compatible with the 7591. As I said before, there are a number of excellent vintage amps you can replicate with off-the-shelf transformers--Heathkit, Eico and Dynaco among them. I know of an Eico HF87 design and a 25wpc Williamson tuned and stabilized for Edcor outputs. I myself built 20wpc Williamsons using KT66s and a Peerless S-265-Q copy from Heyboer that is very, very close to the original. These are wonderful amps and I've built several sets for friends. I even commissioned a PCB for them that makes them incredibly easy to build. But regardless, I really think you'd be wasting your time trying to duplicate a 230 without the original outputs.
The McIntosh 275 output transformer is very complex.
3 Center Tapped dual Windings:
Cathode Primary (Unity Coupled to the plate)
Plate Primary (Unity Coupled to the cathode)
Bootstrap Secondary to the driver tube plate circuit
2 Single Windings
Global negative feedback Secondary to the input stage
Output Secondary to loudspeakers.
It is not just a matter of having all these windings at the proper turns ratio.
It is important to have very good coupling of all the windings, not only the Unity Coupled windings.
A "Replacement" output transformer that has different leakage inductance between the windings is going to need special compensation of lead and lag in the various stages, or . . .
You end up with a Power Oscillator, instead of a Power Amplifier.
Just $pend the money, and purchase a working pair of Mcintosh 275 amplifiers.
Or $pend almost as much for a pair of real McIntosh 275 output transformers
The McIntosh 230 does not have all those windings, is Not Unity Coupled, and driver bootstrapping is Not needed (not present).
A 230 Clone is a more realistic project, both the tube circuitry, and the output transformer.
Just my opinions.
Your Mileage May Vary
3 Center Tapped dual Windings:
Cathode Primary (Unity Coupled to the plate)
Plate Primary (Unity Coupled to the cathode)
Bootstrap Secondary to the driver tube plate circuit
2 Single Windings
Global negative feedback Secondary to the input stage
Output Secondary to loudspeakers.
It is not just a matter of having all these windings at the proper turns ratio.
It is important to have very good coupling of all the windings, not only the Unity Coupled windings.
A "Replacement" output transformer that has different leakage inductance between the windings is going to need special compensation of lead and lag in the various stages, or . . .
You end up with a Power Oscillator, instead of a Power Amplifier.
Just $pend the money, and purchase a working pair of Mcintosh 275 amplifiers.
Or $pend almost as much for a pair of real McIntosh 275 output transformers
The McIntosh 230 does not have all those windings, is Not Unity Coupled, and driver bootstrapping is Not needed (not present).
A 230 Clone is a more realistic project, both the tube circuitry, and the output transformer.
Just my opinions.
Your Mileage May Vary
Last edited:
Norman Crowhurst offered an alternative way to build a unity coupled circuit using TWO output transformers.
I have original MC275, clone MC275, and original MA230 OPTs in the house.
The MA230's were a recent purchase. I measured their output impedance at 5.5 kohm as I recall. I can confirm this as well as measure resistances you may need.
I'm probably responsible for the most of the Good Things Said about the chinese MC275 clones.
https://audiokarma.org/forums/index...c275-mk-vi-clone.989756/page-17#post-16670684
https://audiokarma.org/forums/index.php?threads/mcintosh-mc275-clone-build.1032698/
I have also built, using 6GM5 (7591 in nine pin socket), a unity coupled amp here using Vanderveen transformers
https://audiokarma.org/forums/index...rveen-vdv-1070-uc-output-transformers.937649/
In the thread above, you will note that McIntosh utilized a 4800 output impedance on the MC225. Dave Gillespie says this is a value he likes as well. That said, the MA230's would be a good match for 7591A tubes as McIntosh in fact did. I plan to build a 6L6GC amp with them. In my estimation, the transformer cores are good for 60 watts.
Attached is an article using 6550 tubes with these transformers for a touted 70 watts. Be careful. It appears the author kept the original MA230 schematic intact including 270k grid leak resistors. 6550's should have more like 47k.
Keep us posted.
The MA230's were a recent purchase. I measured their output impedance at 5.5 kohm as I recall. I can confirm this as well as measure resistances you may need.
I'm probably responsible for the most of the Good Things Said about the chinese MC275 clones.
https://audiokarma.org/forums/index...c275-mk-vi-clone.989756/page-17#post-16670684
https://audiokarma.org/forums/index.php?threads/mcintosh-mc275-clone-build.1032698/
I have also built, using 6GM5 (7591 in nine pin socket), a unity coupled amp here using Vanderveen transformers
https://audiokarma.org/forums/index...rveen-vdv-1070-uc-output-transformers.937649/
In the thread above, you will note that McIntosh utilized a 4800 output impedance on the MC225. Dave Gillespie says this is a value he likes as well. That said, the MA230's would be a good match for 7591A tubes as McIntosh in fact did. I plan to build a 6L6GC amp with them. In my estimation, the transformer cores are good for 60 watts.
Attached is an article using 6550 tubes with these transformers for a touted 70 watts. Be careful. It appears the author kept the original MA230 schematic intact including 270k grid leak resistors. 6550's should have more like 47k.
Keep us posted.
Attachments
The 'Poor Man's' McIntosh. I built two versions about 20 yrs ago. This one uses a PP pair of 6LU8s,
The included triodes are used as CFs to drive the output pentodes, thus avoiding blocking.
The PS is all SS with a built in startup ramp circuit. The two-stage Diff Amp can be switched between
Crowhursts version & full differential. Its been here on DIY several times & published in Glass Audio.
The included triodes are used as CFs to drive the output pentodes, thus avoiding blocking.
The PS is all SS with a built in startup ramp circuit. The two-stage Diff Amp can be switched between
Crowhursts version & full differential. Its been here on DIY several times & published in Glass Audio.
Attachments
Showing amp section only so we can focus on just the amp circuit. I have thought of acquiring a unit but adding a bypass switch so I can use it as a pure stereo tube amp. Obviously the input grids need grid leak resistors between 100K to 1M or you can keep the volume control and bypass the solid state line stage.
Nice idea from a great vacuum tube amplifier expert.
But the Norman Crowhurst alternative "Unity Coupled windings" have two leakage inductances that might degrade the performance.
N. Crowhurst:
With the two secondaries connected to each other, the leakage inductances are:
Leakage inductance # 1: Cathode windings to Secondary # 1.
Secondary # 1 electrically connected to Secondary # 2 (no leakage inductance on this connection).
Leakage inductance # 2: Plate windings to Secondary #2.
That means there are 2 leakage inductances In Series from the cathode primary to the plate primary.
McIntosh:
The Cathode windings and Plate windings are Bifilar wound.
Bifilar wound has the lowest leakage inductance known to man.
Whether you like it or not, McIntosh was not stupid (and I do not ever want to purchase or clone a McIntosh amplifier).
Just my $0.02
But the Norman Crowhurst alternative "Unity Coupled windings" have two leakage inductances that might degrade the performance.
N. Crowhurst:
With the two secondaries connected to each other, the leakage inductances are:
Leakage inductance # 1: Cathode windings to Secondary # 1.
Secondary # 1 electrically connected to Secondary # 2 (no leakage inductance on this connection).
Leakage inductance # 2: Plate windings to Secondary #2.
That means there are 2 leakage inductances In Series from the cathode primary to the plate primary.
McIntosh:
The Cathode windings and Plate windings are Bifilar wound.
Bifilar wound has the lowest leakage inductance known to man.
Whether you like it or not, McIntosh was not stupid (and I do not ever want to purchase or clone a McIntosh amplifier).
Just my $0.02
Thx for replies-
Although fetching project on the surface, OT's seem to be unobtanium.
Don't mind spending some coin to get a pair.. but not feasible current prices for those or to buy a 230 core.
But can't open said diagram in the link- please help
Thx again,
Jim
Although fetching project on the surface, OT's seem to be unobtanium.
Don't mind spending some coin to get a pair.. but not feasible current prices for those or to buy a 230 core.
Interested in this..
But can't open said diagram in the link- please help
Thx again,
Jim
In my Post # 17, I was talking about real Unity Coupled McIntosh amplifiers.
In Post # 13, The Norman Crowhurst is also a Unity Coupled amplifier.
The McIntosh 230 is Not a Unity Coupled amplifier.
I expect it is easier to get a quality well wound substitute for a Mac 230, But very hard to get a quality well wound substitute for a Mac 275.
In Post # 13, The Norman Crowhurst is also a Unity Coupled amplifier.
The McIntosh 230 is Not a Unity Coupled amplifier.
I expect it is easier to get a quality well wound substitute for a Mac 230, But very hard to get a quality well wound substitute for a Mac 275.