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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
 
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
11,644 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
72 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!
pauly
1,012 posts
 
@peterf6
If you are interest in building your own amp, I’d suggest you start off building a kit. There is a lot of things to keep track of and a kit is great place to start.


peterf6 OP
72 posts
 
@pauly Exactly what I was thinking. Any suggestions?
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