Such a great amp. Why bother with perfect and just muck around with tubes, sources and speakers?
It's like messing around with a 53 Corvette.
It's like messing around with a 53 Corvette.
It seems sacrilege to me to do this conversion.
I am curious as to why you are comptemplating this change.
The output transformers have been optimised for ultralinear operation. When a designer lays down the design parameters for an output transformer, he has to decide what type of output stage is going to be used.
By converting the output to triode mode you are compromising the amplifier.
If you want to use triode you should get an amp that has its output transformers designed specifically for triode.
Thanks for the clarity, @imhififan !
As for why would I consider doing this to an 8b, I would try it to see what it sounds like. This exact mod is described in the consumer Instruction Manual that came with the amp in the 1960s when consumers owned soldering irons, so it's not really a sacrilege, just an option. I've read about numerous people doing it and reporting interesting sound. It can be reversed, and amps this old from this era have typically been worked on to stay at spec. Thank you for the opinions, I assure you I'll be careful, if I go forward. I'm very nice to this amp as I don't have the budget to buy various amps for comparisons.
Some history from a regular on the Steve Hoffman forum:
The triode switch was first implemented commercially by Marantz in 1955 with their Model 2. Sid Smith had studied the different topologies extensively and determined that this mode gave the most satisfactory and musical sound despite cutting the available power in half. A lot of skeptics viewed any triode or triode connected amp with disdain back in the day (considered outdated at the time), so the manual only referred to this switch as halving the power to avoid damaging speakers.
Later on, with the Model 9, it was said to reduce power from 70 watts to 40 watts with "excellent characteristics" I, too, was skeptical about triode mode until I tried it on my Model 9's per suggestion of John Curl. It was a complete revelation.
In Triode mode, the sound is incredibly lifelike, rich & smooth, with no trace of harshness or artificial hi-fi brightness. Bass is actually tighter and better defined than Ultralinear.
It's the real deal. And best of all, 40 watts is still more than plenty of power for my Chartwell LS3/5A's. I'm not going back to Ultralinear any time soon!
As for why would I consider doing this to an 8b, I would try it to see what it sounds like. This exact mod is described in the consumer Instruction Manual that came with the amp in the 1960s when consumers owned soldering irons, so it's not really a sacrilege, just an option. I've read about numerous people doing it and reporting interesting sound. It can be reversed, and amps this old from this era have typically been worked on to stay at spec.Ultra linear, if done properly, causes the output section to behave as if it is triode rather than pentode or tetrode. Marantz did a pretty good job of getting the taps right on their output transformers (there was a patent in the area that prevented them from getting them perfect without infringement).
Depending on how the output section is set up (UL has more gain than triode operation) the amount of feedback that is present will change and may not be ideal. I think this is why Saul didn't offer the triode operation as a switch: it simply wasn't worth it given all the variables that arise! I have heard the model 9 in both UL and Triode; with that amp triode is the way to go. One thing to keep in mind: if you are operating the power tubes in triode mode, the correct plate to plate load you'll need will be lower. This **might** mean that you would use the screen taps for the plates of the power tubes. The correct cathode current will be different too, since there is no longer screen current with which to contend. Then there is the issue of whether you are driving the tube with the traditional control grid, or if you are using the screen grid as input, or both.
Put another way, but simply wiring the tubes in triode and otherwise making no other changes will be 100% inconclusive. You won't be able to tell if triode is better, you'll just be hearing how the amp works with that mod and not otherwise optimized.
If I were you, I'd build up an amp from scratch that allows you to work with these variables rather than risk damaging an output transformer, a power transformer or otherwise damaging the value of an amp such as this! If you want to mess with stuff like this, do it with a Dynaco ST-70, which uses the same power tube and is also UL operation. ST-70s are a lot easier to find, have much wider aftermarket support and are less expensive to own and repair. I can also guarantee that if you mess with the innards of a Marantz, unless you really have your chops down, the fact that you were in there will be patently obvious to anyone looking at the amp at a later date!
I will definitely consider the alternatives@peter16 if you are really set on doing this, really consider getting a Dynaco ST70 as your test bed (even though they are getting pricey these days, they are a lot cheaper than a Marantz 8). You can set them up triode and see what you think; explore which tap to use, mess with the cathode current, see how the amount of drive needed affects things, see how the feedback value changed even though you didn't mess with the feedback network.
BTW the feedback is a non-trivial matter. In any tube amplifier it is a set of compromises, even it it appears to be simply a resistor. It can have far more effect on the sound of the amp than the difference between UL or triode mode! Quite literally without sorting all that out you really won't know what it is that you are hearing.
You might find a few people that have done this, but if they didn't cover all the bases their experience will be purely anecdotal and really won't tell you anything at all. Take this from an amplifier designer as a FWIW: triodes are very different from other power tubes- they have less gain and lower output impedance, which may well mean that if you want to hear what they are all about an entirely different output transformer might actually be the only way you can sort this out.
You never hear this on Audiogon, but a Dynaco ST-70 is a good amplifier in and of itself so have fun.You'll hear it from me! The ST70 was a lot less expensive than the Marantz but in terms of power, bandwidth and distortion totally gave the 8B a run for the money! You can get lower distortion if you replace the 7199 driver tube with a 6GH8. The latter is inexpensive, and adapters to do so are available cheap on eBay. Obviously the filter caps should be replaced, including those in the bias supply. I also replace the bias rectifier with a silicon device. I've used V-Cap ODAM capacitors in the audio circuit with excellent results.
The weakness of the ST70 is the 5AR4 rectifier, which is not rated properly feed stereo 35 watt amplifiers at full power. Dynaco should have either designed with solid state rectifiers as Marantz did or designed for dual 5AR4s. Triode Electronics in Chicago offers a power transformer that allows for duel 5AR4 operation (converting the space where the filter can is to an octal tube socket). To do that you have to get an alternate multiple filter capacitor from CE Distribution that can fit beneath the chassis...
At some point you might as well build up the amp from scratch rather than mod an existing piece to death!
I started off by building a couple of kits from bottlehead dot com. They all had very good and concise instructions, are fairly easy to build even for a complete novice, and they all worked and sounded great - way better than components costing 5 to 10 times the cost of the kit.
Once you're up to speed, I'd recommend you look at Audio Note Kits.
Note. I am not affiliated in any way with either company.