Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: dickjonesify on February 26, 2026, 10:03:30 pm

Title: Mixing Resistors
Post by: dickjonesify on February 26, 2026, 10:03:30 pm

I have this amp...

(https://lh3.googleusercontent.com/pw/AP1GczM-b-Vud-zY1HbyePgqAH6iu3UP7elfo86cr4LjLgYTofoPUeBfCOPOpnfVBUOlSlIec4tva4xg7v6S3a58PKf_hguH2M3rC5iOt9lUou2Khsa8BzayTaSXj2_NmK6lAabOyV_mqfBvH3bj98jkkKV1DQ=w1170-h903-s-no?authuser=0)

And I'm gutting the 2nd channel to put the normal channel of this in instead (with a switched bright cap)...

(https://lh3.googleusercontent.com/pw/AP1GczOHn6j-RHksqkj1i78Rx3Ut5c6QsHf7DzWOIgz4rtwfrwGAsqFM6UfVgqltfB_aFFihDqoBDhl4i8bAAQyBSrPUPzcnC-6VEn24a5jqSsmQuaYtICia_PKpf85maS1TgaROPJwc6sMEUe35s1woaPOB4A=w1170-h678-s-no?authuser=0)

I plan on leaving the mixing resistors from the Gibson (R17, R18) SO: should I delete the 270K mixing resistor from the 5F6A channel? I'm sure there is math associated with this.

Side note, the Gibson's tone stack is just... awful haha so I will most likely change that out to something else as well. One problem at a time.

Thanks

Title: Re: Mixing Resistors
Post by: tubeswell on February 27, 2026, 07:21:03 am

 there are ‘normally’ 2 equal value mixing resistors in a passive resistor mixer. As to whether you keep the 470k in series with another resistor or not in the circuit you are planning depends on whether you like the result or not I guess.

Title: Re: Mixing Resistors
Post by: dickjonesify on February 27, 2026, 08:10:53 am

Quote from: tubeswell on February 27, 2026, 07:21:03 am

there are ‘normally’ 2 equal value mixing resistors in a passive resistor mixer. As to whether you keep the 470k in series with another resistor or not in the circuit you are planning depends on whether you like the result or not I guess.

Ok, cool. I guess I’m just making sure it’s more of a matter of taste and less a matter of blowing something up  :laugh:

Thanks

Title: Re: Mixing Resistors
Post by: pdf64 on February 27, 2026, 09:11:30 am

Sorry but I find your circuit description somewhat ambiguous. A schematic of what you're planning would be much better, just sketch it out on paper, take a photo and upload it.
Or photoshop those Gibson and Fender schematics together somehow.

The concern being that the mixing resistors in the Gibson are at anode V DC, whereas all likely channel output points in the Fender are at 0V DC.
Hence a blocking cap will be needed.

Title: Re: Mixing Resistors
Post by: dickjonesify on February 27, 2026, 11:32:00 am

Quote from: pdf64 on February 27, 2026, 09:11:30 am

Sorry but I find your circuit description somewhat ambiguous. A schematic of what you're planning would be much better, just sketch it out on paper, take a photo and upload it.
Or photoshop those Gibson and Fender schematics together somehow.

The concern being that the mixing resistors in the Gibson are at anode V DC, whereas all likely channel output points in the Fender are at 0V DC.
Hence a blocking cap will be needed.

I will make that drawing! Good idea.

To simplify with a quick question: If you were building a 5F6A but only used one channel, would you omit the 270K resistor right before the grid of V2A?

Title: Re: Mixing Resistors
Post by: sluckey on February 27, 2026, 11:56:57 am

Quote from: dickjonesify on February 27, 2026, 11:32:00 am

If you were building a 5F6A but only used one channel, would you omit the 270K resistor right before the grid of V2A?

yes