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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
72 posts
 
Thank you very much @imhififan . I assume this is repeated at each tube base and what the resistor was attached to is then the "dead end" referred to in the manual. LMK if I've got that wrong. I appreciate you marking up the photo for me!
Peter
imhififan's avatar
imhififan
2,136 posts
 
https://ucarecdn.audiogon.com/118ac148-1298-42ff-be0d-fbaeb8a3648a/-/autorotate/yes/
dover
2,191 posts
 
@peterf6

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.


peterf6 OP
72 posts
 
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.
peterf6 OP
72 posts
 
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!

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