Fender Hotrod Delux

[Hambly]

Hi

I have a Fender HotRod Delux and find that I can create a good balance in volume levels between the clean and drive chanel. However, when I kick in "More drive", the output increases excessively. Is this normal and if so, is there a mod that I can do, like fitting an additional pot somewhere in circuit, to reduce the "More drive" output to more manageable levels.

Hope someone can help.

[Robert M. Martinelli]

Hi, and welcome to the forum!
To modify your HRD you have to understand how the "more drive" circuit works first. If you look to the attachment ( the part of the schematics relevant to the "more drive" feature ) you'll see there are two JFETs ( J111 ) which are operated by the "more drive" switching circuitry ( not shown ). The purpose of these two JFETS is to alter the "cathode bypass" for V2a and V2B.

As you might know already, a "cathode bypass" cap ( a capacitor placed between cathode and GND ) is there to stabilize the cathode bias voltage and to allow a stage to release its maximum designed gain.

When the two FETs are not conducting, their RDS (off ) is to be considered infinite, so, since they've their Drain and Source // with the two 100 K resistors, it's just like the FETs are not there, so you have 100 K between the capacitors and GND, and the cathodes are NOT bypassed ( lower gain ).

When the two FETs are on, their RDS ( on ) is about 30 Ohm, so the // between each FET and the 100 K resistor is 30 Ohms, the caps are now to GND ( 30 Ohms are negligible ) this means the cathodes of V2a and V2b are now bypassed, so V2a and V2b are now releasing more gain.

Now, for the possible mods:

1 - You could add a switch to operate the two FETs separately ( this way you would have three options "normal gain" - "a little more gain" - a lot more gain" )

2 - You could limit the amount of bypass for one ( or both ) stages by adding a resistor ( or even better, a trimmer to suit the amount of gain when the "more drive" is on to your needs or taste ) in series between the FETs sources and GND.

3 - You could implement both of the above mods

As you're new to the forum, and I don't know your level of expertise, I'd like to add a word of caution : Be aware that the voltages inside a tube amp are lethal, so, if you're not familiar with tube amp repair techniques and procedures, I'd suggest to gather some information and knowledge before starting.

Hope this helps

Best regards

Bob

[Hambly]

Fender Hot Rod Delux

Hi Bob

Thanks for the reply. Much appriciated.

I have had a look at the circuit diagram that you sent me and assume the pot would need to be installed in location "A" as shown on the attached page 1. However, when I look at the PC board in the amp, the ciruiting is as shown on page page "B". My knwowledge of electronics is not great, but the PC board seems to differ to the circuit diagram. CR16 is also not present and appears to have been replaced by R106.

Could you please assist me by indicating where the pot should be installed relative to the various components, as I am a bit stummped. Also, any suggestion on the resistive value of the pot I should use.

Thanks again for your help.

Regards

Hambly

[Robert M. Martinelli]

Nope, the trimmers ( used as variable resistors in this configuration ) go in series with the FETs source ( or drain, it actually doesn't make much of a difference, as they're only suppposed to add their value to the FETs RDS(on) ).

I have exemplified both mods in the attached schematic, the switch allows you to operate the two FETs independently or together, thus bypassing only one cathode or both, while the trimmers allow you to fine tune the amount of bypass when the relevant FET is on. I would start with trimmers in the 5 to 10 K range. With the "more drive" feature ( and the new switch ) on, rotating the trimmers fully CW shorts the FETs source to GND, so you get the same gain you were getting before. Rotating the trimmers CCW reduces the gain, while opening the new switch leaves only the cathode bypass on V2a, reducing the gain on V2b, this way you' ll have three options - "normal gain" - " a little more gain" ( "more gain" on and new switch off ) - "a lot more gain" ( "more gain" and new switch both on ). The second and third option gain will have to be fine-tuned to your taste via the two trimmers.
Be aware that, by adding this series resistor(s), the bypass cutoff frequency will change, so you might want to "fine tune" the cutoff frequency by altering the bypass capacitors ( C8 and C9 ).

Sorry for the low quality of the drawing, bitmaps ain't the best thing when it comes to technical drawings.

Hope this helps

Best regards

Bob

[Hambly]

Hi Bob

Thanks very much for your help. Most appreciated.

Regards

Hambly