... Get*SET*Go...... RESISTOR substitutions ...... SET amplifier Boards 10-02-17

dowto1000

Junior Member
A local audiophile friend in Kansas City, asked me to provide a good set of resistor choices, so that he can re-populate his ....Get*SET*Go.... printed circuit board, to improve both it's sonics, and reliability. What will apply to him, may be freely-used by any others, hence this open-to-all post.

Being reasonably experienced with 6B4G amps, ( designed / built my first 6B4G amp 1983, 34 years ago ) I traded-off Cost versus Performance, and have determined the best resistor choices that "I" am aware of.

The stock amp's Manual / Build suggests using resistors that are typically ¼ of a Watt rated, and are in the 9 cents to 40 cents price range, typically 13 cents, for ¼ Watters.

All of my replacement suggestions come from one supply source, so there will be only one single resistor shipping cost. Michael Percy.

These resistor parts suggestions that follow are not just theoretical. They HAVE been tested on refined High Efficiency ALTEC VOTT and GPA 604 MLTL systems. I would, and do, use these same resistor brands in amps I build, for my own use. ( See - separate Jeff's High Performance Amp thread. )

The typical resistor cost will be 25 cents each, for ½ Watt rated Roederstein brand MK3s. Realize, one could easily spend 7 to 15 dollars each, exotic ½ Watt audiophile resistors, but the entry-level ....Get*SET* Go... circuitry is, IMHO, not quite truly worthy in topology, of such expense. Twenty five cents each in a MK3 is peachy good enough !!  A balance, and maximum bang-for-the-buck, is thoughtfully being offered.

Applied to three sensitive circuit locations, is an experienced builder's technique of using TWO closely “matched resistors in parallel ", ( started commercially in the early MFA Luminescense tube preamp ) - each paralleled resistor at twice the desired Ohmic value. This SOUNDS slightly better, when the two Rs are closely matched to each other, than just one resistor ( of the desired value ). Now, ..... you know this.

High sound-quality resistors are VERY important to use in a simple SET amp. There are so few parts, and - we hear them all !!

I would subjectively say, these following resistor changes ( versus the Build Manual's suggestions ) will make the music presentation, and someon's overall listening enjoyment improve by 5 to 12 percent. It will sound better, play better timbre-wise, harmonically, and transparency-wise,..more the real thing !!

In certain Ohmic values, Percy does not have any stock of Roederstein MK3s, and he substitutes PRP brand resistors. I have NOT tested PRPs, and so, I can not, and do not, suggest using PRP resistors at all. We will instead, go to the nearest Roederstein MK3 value we can find, and the Ohmic value difference will not be of consequence.

In TWO instances, ( grid stoppers and bias resistors ) I am specifically suggesting a CONSERVATIVE Ohmic value change, that I know, ( from 6B4G amp building ), WILL sound better, AND will improve reliability.


Here we go :


(4) R1, R2, R9, R10.......Stock 475K...... .use (4) 499K MK3s.............$1.00

(2) R3, R4.......................Stock 1 K...........use (2) 243 Ohm MK3s.....$0.50

(2) R5, R6.......................Stock 3.01K..... .use (4) 6.04K MK3s...........$1.00

(2) R7, R8 .......................Stock 270K..... .use (4) 549K MK3s........... $1.00


NOTE : R5 to R8 above, for the resistance of the Driver tubes' Anodes and Cathodes. This might be the most performance-effective $2.00 one could EVER spend on a resistor implementation !! In both instances, MK3 Rs are paralleled.


(6) R15a-c, R16a-c.....Stock 2.7K 3 Watt.....use (4) 2.21K 5 Watt Mills MRA-5s, .... 1%

....................................... $4.75 each,..........or ...........$19.00

NOTE : Two 5 Watt Mills are used, in parallel, where three 3 Watt Rs existed in parallel.

This Ohmic change ADVANTAGEOUSLY raises the 6B4G's Rk value ( from 900 Ohms to 1,105 Ohms ), and it eliminates the 6B4G from operating at 97% of its rated plate dissipation.

WE MAY really want to go higher ( to 1,245 Ohms in the future, but this initial suggestion of mine is a CONSERVATIVE 205 Ohm Rk increase, a very small, appropriate change “ in the RIGHT direction for sure “ simply a decent first step . )


(1) R 19.................... Stock 10K 1 Watt........use (2) 19.1K ½ Watt MK3s ...................$0.50


TOTAL resistor cost : $23.00 plus about $8.00 shipping.

Permanent amplifier enjoyment / system gain from these resistor changes : ...............estimated at 5 to 12 %.

- - - - - - - - - - - - - - -

Of course, as I have written elsewhere, the Power Supply is mis-designed - needlessly OVER -filtered.

A re-do of the B+ filtering, ( to the 6B4G Finals stage ) is NOT part of this post, at all.

Simple power supply B+ filter implementations / optimizations ( to the 6B4Gs' Final stage ) will, however, easily provide us with an additional 20 to 30 % subjective gain over the stock Get*SET*Go amp's overall performance and enjoyment, ( as it uniquely does, with all triode amps ).

This power supply re-do / improvement, gets covered, later - on its own, in a separate set of posts..

Also, a post on improved Rk all-film cap bypassing, relating to the Get*SET*Go amp, and MOST other triode SETS, is also in the works, needed !!!

Have lotsa fun.


Dowto1000
 
Last edited:
Top