MC3500 with 807 tubes and modern driver tubes

[tryonziess]

I have not found any truly profound negative press related to the original Mcintosh MC3500 amplifier. I have several dozen 807 vacuum tubes and a really good service manual for the Mac amp. Would a clone of the this amps circuitry and the 807 tubes be feasible? One objective here is to avoid spending 10000 dollars on this amp. The 807 tube is plentiful and cheap. To achieve an adequate output 10 tubes in parallel would need to be used.
My main concern is the driver tubes. Would the exact compliments be needed to achieve the desired results or could cheaper more readily available substitutes be used?
Raising the output tube voltage to 750 volts and maintaining most of the driver and input section should yield a very nice amp. The power transformers are not that expensive from Antek and with 10 output tube the overall impedance in the output transformer could be reduced considerable making for much less capacitance.
There are many with much more knowledge on this subject. Do you think this is a viable build. I do not have 20000 dollars for a pair of these amps and would very much like to build something close to this venerable classic.ro
My previous 813 project posed to many output tranny problems.

Tad

[Wavebourn]

I'm afraid 807 are too different from 6LQ6. Some other sweep tubes would be better, IMHO.

[smoking-amp]

A link to Mc3500 OT transformer info:
McIntosh MC-3500 Schematic Information

I definitely agree with Anatolyi on finding some Sweep tubes instead of 807s. The 807s are rated at 110 mA max DC, 6LQ6/6JE6 are rated at 350 mA max DC. Plenty of 18 Watt Sweep (175 or 230 mA max DC rated typical) tubes around (many at $1 each) that could be used (two of these = 6LQ6). ( 17GJ5, 21GY5, 21JZ6, 21KA6, 22JF6 all $1 each for example, 21JV6 for $2, or a 24 Watt 6HJ5 for $4 and 80% of a 6LQ6)

Do you really need to get 350 Watts? Maybe scale the thing down some. And change the OT setup to a Circlotron instead of the Mac bifilar thing. It's equivalent. Much easier to find something or to wind. If you can settle for 120 Watts out, the Hammond 1650T with two 24 Watt (or bigger) sweep tubes (or 4 x 18 Watt sweeps) could work in Circlotron mode using the 16 Ohm output for the 8 Ohm load, so the full primary then looks like 1K Ohm, (freq. response inproves down to 15 Hz too with the modded Z load) Or for 280 Watts use the Hammond 1650W with 4 x 24 Watt Sweeps or 6 x 18 Watt Sweeps. Or order a custom OT from Edcor is another way.

And changing the drivers up from wimpy 6BL7s would be a wise choice too.

[HollowState]

Originally Posted by tryonziess

My previous 813 project posed to many output tranny problems.

Tubes are not the problem. If you think you had output transformer problems with 813s, IMO properly cloning that pentafiler monster will give you way more trouble then you think. Good luck!

[pieter t]

Originally Posted by tryonziess

My previous 813 project posed to many output tranny problems.

Tad

The MC3500 output transformer is a very complex piece of iron; multiple primary windings for power and driver stage. I don't think this transformer is "easier" than what you'd need for an 813 amp.
When 350 watts of power is not what you aim for (I wonder why you would need that much power for a domestic application) I think you could "adapt" the MC3500 design to something more moderate, and you could use 807's to do this, and different tubes for the input and driver stages.
Parallelled output tubes ask much from the driver stage; in the MC3500 the output tubes are driven by cathode followers.
When you are able to calculate what an amp like this requires for it's input/gain stage, driver stage and power stage (feedback included) you might succeed in building an MC3500 clone, but it will be a massive project nonetheless.

[Wavebourn]

Paralleled sweep output tubes is the way to get rid of big number of turns; cathode feedback is the way to expand frequency response that can be obtained from an iron of such size. Cyclotron or parallel feedback like in my Pyramids or Pete's Engineers' RedBoard are another ways to go, but it would not be a Mac clone you want to build, though...

[tubelab.com]

Pete's Engineers' RedBoard are another ways to go,

I have seen 250 WPC out of my red board using 35LR6's and Plitron toroidal OPT's. I used 35LR6's because I had them. Any 35 or 40 watt sweep tube with integral heat radiating fins welded to the plate will work. Full power bandwidth was something like 12 Hz to 50 KHz. A 2500 ohm OPT with a single center tapped primary is a whole lot easier to make than a multi winding monster like the Mac. B+ is 650 volts at over an amp for both channels driven to full power. This amp can run for hours on end driven to clipping on music peaks. If you need to make 250 WPC of sine waves for more than 30 seconds at a time, the output tubes will be over dissipated. If this is the case you need to go PPP using 4 big sweep tubes per channel. If a 2500 ohm load is used you will still get 250 WPC. If the load impedance is reduced you get more power. At 1250 ohms with 4 tubes I was getting 525 watts at the edge of clipping. Again there will be a dissipation issue if full power continuous operation is needed.

For now I am running the amp at the 125 WPC level using 24 watt tubes and 600 volts of B+ into a 3300 ohm OPT. It sounds so good I am making more of them. It is also just too easy to make stupid levels of power with this design.

Posted new P-P power amp design

[bear]

FYI, you can get 200 watts out of 4 x 807 in AB2... it has been done and published.

The key is that the 807 will handle quite a bit more "juice" than the stock values, and also more than the 6L6 in terms of B+ voltages...

Just thought I'd toss that meat into the pit and see what happens...

_-_-bear

[tryonziess]

I did not mention that I also have the transformer winding schematics. In any case I was not going to build these transformers myself. I asked in an earlier thread if anyone had seen these transformer unpotted and got an email from an obscure member with a lot of information. Nice fellow.
I do not need the full 350 watts. Something closer to 275 or 300. Mainly I would like to set these up and see how this classic sounds and to have a somewhat similar tube amp on the shelf. The build schematic is quite detailed and should not be too much harder to put together than anyother multiple tube project == which I will have to work on.
The other big question is the driver and input stage tubes. Are these going to be hard to find and extremely expensive. If I change out too many different components I will no longer have an animal with the same stripes.
I would like to use Miles original design with relatively common tubes, read cheap here, and good quality caps. I think the circuit design is an important element in this construction.

I will look into the tube suggestions proposed by the above members and get something closer. This amp does not use really high B+ voltage and I think Mac was trying to get maximum longevity and reliability from these values. They have always been conservative.

I always like to have something that is not around anymore. Old cars, motorcycles, houses and in this case and old classic huge amp.
I much appreciate the input and will heed all. Tad

[tubelab.com]

The other big question is the driver and input stage tubes. Are these going to be hard to find and extremely expensive.

All are quite common.