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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
peterf6 OP
87 posts
 
Thanks @atmasphere for the thoughtful concerns. I will definitely consider the alternatives and do some more research on people who have done this mod on the 8b. 
Peter
imhififan's avatar
imhififan
2,217 posts
 
@atmasphere
Thank you for the educational post! Wish to see more posts like this in this forum...

atmasphere's avatar
atmasphere
11,999 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.
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