[_Knighthawk_]

Hi all,

I'm experiencing a challenge with a current build. The two primary symptoms I have identified thus far:

1) Channel 2 (Vibrato channel) is acting erratically. When I apply any volume on the pot, I almost immediately get wild hum at fairly high volume. Channel 1 seems pretty good - although there is some high frequency ringing when I apply instrument signal. Both channels are extremely quiet until signal is applied.

2) My voltage reading on Pin 1 of V5 seems to be way out of whack. I believe I should be expecting ~210V but I'm seeing 460V. It is possible that the original high definition drawing is calling for ~270V - can't tell if that is a one or a seven after the 2 - but either way, I don't think I should be seeing 460V. As you can see from the image, it is receiving power B+2 power from the board via the 220K resistor.

My guesses thus far are that there may be a grounding problem so I have double- and triple-checked all ground connections. No issues there. I also tried swapping out tubes (I've gotten as far as V3). I have tested the sweep in the 1meg volume pot and swapped out the shielded cable from the channel 2 jack. Anyway, this is a bit of a mystery to me. Anyone have any ideas?

 

[Paul Running]

~210V but I'm seeing 460V

My first thought is no plate current is flowing in that tube.
What do you measure on the cathode (pin 3) of that tube? Should be about 2VDC.

 

[Always12AM]

Schematic

https://cdn.shopify.com/s/files/1/2325/2729/files/Deluxe-Reverb-AB763-schematic.pdf?5926397210715640943

Layout

https://cdn.shopify.com/s/files/1/2325/2729/files/Deluxe-Reverb-AB763-layout.pdf?5926397210715640943

Make sure the lead from the doghouse is on the left side of the 220k. Also make sure the 220K is correct. Could be a 22k and not doing it’s job.

Also, make sure the lead from pin 7 is going to the right point with a continuity test.

Actually, make sure everything on the vibrato section is in the right place. Make sure both the left and middle tab of the speed pot are grounded by the 100-120k resistor.

 

[_Knighthawk_]

My first thought is no plate current is flowing in that tube.
What do you measure on the cathode (pin 3) of that tube? Should be about 2VDC.

The voltage really jumps around quite a bit on pin 3. It ranges from 1.6 VDC to 2.8VDC.

 

[Alan Small]

 

[_Knighthawk_]

Schematic

https://cdn.shopify.com/s/files/1/2325/2729/files/Deluxe-Reverb-AB763-schematic.pdf?5926397210715640943

Layout

https://cdn.shopify.com/s/files/1/2325/2729/files/Deluxe-Reverb-AB763-layout.pdf?5926397210715640943

Make sure the lead from the doghouse is on the left side of the 220k. Also make sure the 220K is correct. Could be a 22k and not doing it’s job.

Also, make sure the lead from pin 7 is going to the right point with a continuity test.

Actually, make sure everything on the vibrato section is in the right place. Make sure both the left and middle tab of the speed pot are grounded by the 100-120k resistor.

View attachment 471316

I have verified the 220K resistor more than once (I measure each resistor before it goes on the board but this doesn't mean I haven't made this mistake!).

I can verify that the lead from pin 7 originates from the correct location (continuity tested on completion and again now - I dot these as in the image below.

I can verify that there there is a 100k resistor connected to ground bus from lugs 2 and 3 of the (3M RA) speed pot.

Thanks for the drawings! They are bit clearer than the Fender ones I was using (pin 1 should be 270 V).

 

[Paul Running]

It ranges from 1.6 VDC to 2.8VDC.

It should be a steady reading if the cathode by-pass cap is functional; have you verified the by-pass cap? If the by-pass cap was open, the meter would jump around from the low frequency oscillator effect.

 

[Paul Running]

Oddities will occur when plate voltages exceed 350VDC on 12AX7s...460VDC on an active 12AX7 would do it.
How much voltage is dropped across the 220KΩ plate resistor?

 

[Paul Running]

I am curious about the coiled, red-insulated wire in the image.

 

[_Knighthawk_]

Oddities will occur when plate voltages exceed 350VDC on 12AX7s...460VDC on an active 12AX7 would do it.
How much voltage is dropped across the 220KΩ plate resistor?

The voltage drops from 475V to 382V. That said, the voltage really jumps around a lot on

I am curious about the coiled, red-insulated wire in the image.
View attachment 471327

The coiled red wire is not connected to the circuit. It is merely extra shielding around the three (twisted) wires coming out of the doghouse. The idea (according to my research - this wasn't my idea) was to shield those wires from the wiring that goes up to the pots in the vibrato channel preamp section. I cannot say to what extent this may or may not be effective since this is the first time I have done it and I don't yet have a fully functional circuit. That said, a primary goal is to make these amps as quiet as possible (which is very challenging in channel 2 of the DR).

 

[_Knighthawk_]

Update: I swapped out some tubes. Moved a (known working) 7025 into V4 and put in a different (known working) 12AX7 into V5. No discernible difference.

 

[_Knighthawk_]

Here's a link to video/audio of what i am hearing and seeing:

IMG_0749.MOV

drive.google.com

As an aside, when I go to drain the caps at the B+1 junction, I am seeing a bit of an arc (wee spark). That can't be good, of course, but I am concerned that it might mean something that I haven't the experience to ascertain.

 

[_Knighthawk_]

It should be a steady reading if the cathode by-pass cap is functional; have you verified the by-pass cap? If the by-pass cap was open, the meter would jump around from the low frequency oscillator effect.

I verified each of the by-pass caps before assembly (they are good quality caps). Should I pull them and measure them again? If my memory is correct, they cannot be checked in-circuit.

 

[Alan Small]

very nice work.
perhaps new 2.7k and 25/25 on cathode

 

[Paul Running]

As an aside, when I go to drain the caps at the B+1 junction,

What value of discharge resistance are you using?
Reservoir or filter caps in most deluxe-sized tube amps, store immense amount of electrical energy. If the resistance to ground is low, she's going to spark, at those high voltages.

 

[Paul Running]

Should I pull them and measure them again? If my memory is correct, they cannot be checked in-circuit.

I wouldn't right now. I believe that the prime issue is the operational voltages.

It is merely extra shielding around the three (twisted) wires coming out of the doghouse.

It's also an inductor. Without knowing the exact content of the wires that it surrounds, it has a potential to interfere with the fidelity of your signal.
Edit: Is the red wire terminated at both ends?

 

[Alan Small]

What value of discharge resistance are you using?
Reservoir or filter caps in most deluxe-sized tube amps, store immense amount of electrical energy. If the resistance to ground is low, she's going to spark, at those high voltages.

bleeding from a plate of the first preamp tube is what I was taught to release the stored energy in a slow and controlled way...no pop or spark

 

[_Knighthawk_]

I wouldn't right now. I believe that the prime issue is the operational voltages.

It's also an inductor. Without knowing the exact content of the wires that it surrounds, it has a potential to interfere with the fidelity of your signal.
Edit: Is the red wire terminated at both ends?

It is not terminated at either end. By 'terminated', I assume you mean run to ground?

 

[_Knighthawk_]

bleeding from a plate of the first preamp tube is what I was taught to release the stored energy in a slow and controlled way...no pop or spark

Got it! Thanks.

 

[_Knighthawk_]

What value of discharge resistance are you using?
Reservoir or filter caps in most deluxe-sized tube amps, store immense amount of electrical energy. If the resistance to ground is low, she's going to spark, at those high voltages.

It's a 1500 ohm, 5 Watt resistor.