HRD Preamp Plate Voltages - SOLVED

[bbmade]

SOLVED - Apparently you can get away with not using a 1kV cap for the treble control on a Blues Junior but cannot on a Hot Rod Deluxe.  I think it was ultrasonic oscillation because I used a 500V silver mica for the treble capacitor. 

Holy buckets.

Working on a Hot Rod Deluxe and have low volume.  Speaker is connected to the correct jack but, I’m wondering about plate voltages.

V1A = 211V
V1B = 163V
V2A = 74V
V2B = 205V
V3A = 256V
V3B = 259V

So, V2A seems way off and I have tried other tubes.  The plate resistor for V1B (R11) is way off in value and the meter was jumping around in value so I’ll be replacing that next.  I just don’t know what a bad plate resistor would cause.  R11 should be 100K and when I first noticed the issue it was measuring 45K.  Then just before I quit today I couldn’t get a consistent reading on it.

I should have mentioned, I have already done all new filter caps and when plugging into the power amp input the volume is louder than when plugged into the guitar inputs.  It’s an amp from 2003.

Any thoughts?

[nandrewjackson]

How old is it? Heavily used?
Seems to me those have been around for a long time. Maybe time for full recap and new plate resistors throughout.

[bbmade]

How old is it? Heavily used?
Seems to me those have been around for a long time. Maybe time for full recap and new plate resistors throughout.

Apologies, I should have mentioned I did install new filter caps.

[nandrewjackson]

If it has been used for a kajillion or so hours, personally, I'd get new plate Rs and new Rs in the power supply and power tube screen Rs too, while it's "in the shop".


Another few dollars in parts, a couple more hours under the soldering iron.


I'm not directly addressing the low voltage issue, I'm not super familiar with the insides of that amp.


I've seen those HRDs that look like they've been through the ringer, and I guess lots of players really dig 'em
 

[bbmade]

If it has been used for a kajillion or so hours, personally, I'd get new plate Rs and new Rs in the power supply and power tube screen Rs too, while it's "in the shop".


Another few dollars in parts, a couple more hours under the soldering iron.


I'm not directly addressing the low voltage issue, I'm not super familiar with the insides of that amp.


I've seen those HRDs that look like they've been through the ringer, and I guess lots of players really dig 'em

I pulled R11 and out of circuit it measured 98K.  I installed a new 100K that I measured before installing and it was also fine.  Once installed, it measure around 80K and was slowly drifting as if the cap was charging.  No idea.  After I powered it up then turned it back off the resistor now 255K.  All the other preamp plate resistors measure fine in the PCB.  R11 is a mystery to me.  Something seems to be causing the value to go nuts.

[sluckey]

R11 is a mystery to me.  Something seems to be causing the value to go nuts.

Many times a resistor will read strangely while in circuit. If you disconnect one lead I bet it will read 100K. Chasing R11 will likely lead you down a rabbit hole.

[bbmade]

R11 is a mystery to me.  Something seems to be causing the value to go nuts.

Many times a resistor will read strangely while in circuit. If you disconnect one lead I bet it will read 100K. Chasing R11 will likely lead you down a rabbit hole.

Well, R11 is a plate load resistor.  I don’t understand why all the other ones measure fine in circuit and this one doesn’t.  I realize some resistors won’t measure their stated value when in circuit and maybe I’m wasting time on this one.  I can’t find any info on what the plate voltage should be for V2A in a HRD but, it’s being fed plate voltage from the same source as V1 and V3 but seems quite different than the others.

Once again I find myself at the limits of my ability. 😤

[bbmade]

R11 is a mystery to me.  Something seems to be causing the value to go nuts.

Many times a resistor will read strangely while in circuit. If you disconnect one lead I bet it will read 100K. Chasing R11 will likely lead you down a rabbit hole.

One other observation is R16 which is the plate resistor for V2A.  One side is 323V and other side of R16 is 74V matching the plate reading.  That seems like excessive drop but I just swapped it for another 100K and it’s the same thing.  Maybe that’s the design and I’m wasting time on these.  The amps is just so quiet!  Tried another speaker too just in case but it was the same.