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Marantz 8B Conversion to Triode Mode Help

Does anyone know of a resource showing this conversion with pictures, video, or drawings for a novice. The instructions sound simple, but I am not sure I know the parts the simple instructions are referring to and would love some pictorial reinforcement. The instructions I've seen that are most clear are not identical, but are below. The manual seems to say move a resistor on each tube base, but there's more: 

  • It is necessary to unsolder one end of each of the four 100 ohm resistors (R20A, R 20B, R21A, and R21B) from the solder terminal steps mounted near the output tube sockets and reconnect them to pin 3 of their socket base. (This will leave resistors from pin 4 to pin 3 on each output tube socket. The screen tap wires — green and green/white — remain dead-ended at each terminal strip.) (Marantz instruction manual)

  • To convert this amp to triode mode, unsolder the screen grid resistors from the taps of the output transformer. Find those terminal lugs, and attach the now-unused wires to SEPARATE lugs. These wires must remain unconnected to anything! If you have to put some heat shrink on the ends of the wires (don't do anything rash like cutting them short in case you change your mind or sell the amp on to a collector) & tuck them out of harm's way instead, do that. Connect the screen resistor (100 ohms?) wired to pin #4 to the anode, which is pin #3. (Joe Rosen on Tube Asylum)

  • The 8b can be converted to Triode by moving a resistor on each EL-34 output tube. Output will drop to around 18wrms p/ch. The original manual shows how to do this. (Tube Asylum)
peterf6
atmasphere's avatar
atmasphere
11,996 posts
 
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.


jetter
3,064 posts
 
You never hear this on Audiogon, but a Dynaco ST-70 is a good amplifier in and of itself so have fun.
peterf6 OP
87 posts
 
Thanks again @atmasphere and @jetter for the tips on the Dynaco. I had a PAS2 and "graduated" to other vintage and modern preamps. It's good to hear from experienced ears that the ST70 is worth exploring.
atmasphere's avatar
atmasphere
11,996 posts
 
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!
peterf6 OP
87 posts
 
Ok @atmasphere you got me intrigued about building an amp! I hope I can find the guidance I was looking for with this post on that project!
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