Deluxe Reverb voltages

[tyler8611]

Hi guys, hope this is in the right forum. I completed my Deluxe Reverb clone, powered up with no tubes and everything was fine. Did the 100k cap forming method that I always use with fine results. Voltages were normal as I checked them. Once the caps were formed, I removed the resistor from pin 8 and wired in the B+ wire. Everything was set so I fired up with the rectifier and got a 506 volt reading. Not good. Especially when I already have a zener string to pull down the overvoltaged PT. Turned off, plugged in a 5U4GB, which this PT can handle. Got exactly the same measurements.

Bad reservoir caps? PT messed up? I would say maybe the zener diodes failed but I doubt it would have even turned on if that happened. What do you guys think?

[sluckey]

You don't have a load on the power supply. No load = no current = no voltage drop.

With no tubes plugged in, adjust the bias pot for max negative voltage on pin 5 of the 6V6s. Then plug all tubes in. The voltages should drop and look more realistic.

[xtian]

Also, no need to form new caps. Vintage caps, sitting in closet, unused, maybe.

[tyler8611]

You don't have a load on the power supply. No load = no current = no voltage drop.

With no tubes plugged in, adjust the bias pot for max negative voltage on pin 5 of the 6V6s. Then plug all tubes in. The voltages should drop and look more realistic.

Ok I had the bias pot at halfway. Max negative would be the pot all the way down correct? I have a 4.7k to ground on the 10k bias pot.

[tyler8611]

Also, no need to form new caps. Vintage caps, sitting in closet, unused, maybe.

These were dated 2011 so I thought it would benefit. True though sometimes I am OCD about cap forming.

[sluckey]

With no tubes plugged in, adjust the bias pot for max negative voltage on pin 5 of the 6V6s. Then plug all tubes in. The voltages should drop and look more realistic.

Ok I had the bias pot at halfway. Max negative would be the pot all the way down correct? I have a 4.7k to ground on the 10k bias pot.

Don't care if the pot is all the way up or all the way down. Just set it exactly as I said.

[tyler8611]

With no tubes plugged in, adjust the bias pot for max negative voltage on pin 5 of the 6V6s. Then plug all tubes in. The voltages should drop and look more realistic.

Ok I had the bias pot at halfway. Max negative would be the pot all the way down correct? I have a 4.7k to ground on the 10k bias pot.

Don't care if the pot is all the way up or all the way down. Just set it exactly as I said.

Gotcha. Thanks!

[tyler8611]

So I did max out the pot and turned on and started measuring. Voltages were pretty normal, except the preamp was high- 200 on v1 plates. I assumed once it was biased correctly, things might calm down. I then went to check the cathode current, using 1 ohm resistors. None of the bias settings provided a reading for the cathode current. I changed the resistor on the pot to both 10k and 15k and it was the same result, except the amp started motorboating on startup. So I've got multiple things to sort out now.

[xtian]

Motorboating may be caused by positive gNFB, so try swapping your OT primaries (that is, the brown and blue wires connected to your power tube plates).

With NO power tubes installed, please report your maximum and minimum bias voltages when turning the bias pot (like, -50 to -35). You may have to adjust the bias voltage tail resistor before you can get it in the right range. Are we talking about a pair of 6V6GTs in this Deluxe?

[tyler8611]

Motorboating may be caused by positive gNFB, so try swapping your OT primaries (that is, the brown and blue wires connected to your power tube plates).

With NO power tubes installed, please report your maximum and minimum bias voltages when turning the bias pot (like, -50 to -35). You may have to adjust the bias voltage tail resistor before you can get it in the right range. Are we talking about a pair of 6V6GTs in this Deluxe?

The motorboating had not happened previously- only after I swapped out a tail resistor. I'll do what you described and report the findings. I just don't understand why there isn't a cathode current reading.

Also, with one of the previous tail resistors in, when there was no motorboating, I actually plugged in and got barely crackling notes only at volume 10, channel 1. Vibrato channel put out no sound maxed out. Maybe it's coupling capacitors?

[Travis_HY]

I actually plugged in and got barely crackling notes only at volume 10, channel 1. Vibrato channel put out no sound maxed out.

It sounds like the bias voltage is too negative and it needs to be closer to zero, not further away to achieve cathode current flow. The higher preamp voltages are also a give away that there isn't enough of a load on the power supply and the power section is not drawing enough current. You would know if you had your primaries swapped as you would experience an uncontrollable squealing that would only go away unless you turned the amp off. Usually, the bias voltage setting won't matter if the primaries are reversed-it will squeal no matter what.

What is the voltage on the 6V6 plates and what is the negative bias voltage you get when you read the end of the PI coupling caps feeding the power tube input grids? It should be between -25V to maybe -50V DC.

[tyler8611]

I actually plugged in and got barely crackling notes only at volume 10, channel 1. Vibrato channel put out no sound maxed out.

It sounds like the bias voltage is too negative and it needs to be closer to zero, not further away to achieve cathode current flow. The higher preamp voltages are also a give away that there isn't enough of a load on the power supply and the power section is not drawing enough current. You would know if you had your primaries swapped as you would experience an uncontrollable squealing that would only go away unless you turned the amp off. Usually, the bias voltage setting won't matter if the primaries are reversed-it will squeal no matter what.

What is the voltage on the 6V6 plates and what is the negative bias voltage you get when you read the end of the PI coupling caps feeding the power tube input grids? It should be between -25V to maybe -50V DC.

I've never had any squealing so I assume they're going to the proper tubes. Once I get the "motorboating on standby" solved, I'll go back and do the measurements you described. Literally the only change I made last was replacing a 10k bias pot tail resistor with a 4.7k in attempt to get a cathode current reading on the 6v6s. I have 1 ohm from pin 8 to ground on both and those a fine, just not getting a mA reading. Anyway, after that change, it did the motorboating on standby. Hopefully that could be as simple as bad grounding on the tail resistor, which is easy to fix. After i drained the amp measured the bias pot and resistor and they were fine.

[sluckey]

What resistance do you measure from pin 8 to chassis for each 6V6?

What voltage do you measure from pin 3 to chassis for each 6V6?

What voltage do you measure from pin 4 to chassis for each 6V6?

What voltage do you measure from pin 5 to chassis for each 6V6?

[tyler8611]

Ok so I resoldered the tail resistor ground- no change. Still motorboating. Mind you, I have a 100k/.047 across the standby, so that's why it's trickling in enough to be audible. I've used that method on all my standby switches with no pops and no problems. I double checked to make sure this one is wired and soldered correctly, so that's not an issue.

My best guess is bad filter caps, failing PT, or bad power tubes which I highly doubt since they're fairly new JJs and I tried them before in a tweed deluxe. They always light up normally, no redplating, etc.

Should I remove the tubes now and measure to see what's going on?

[pompeiisneaks]

Ok so I resoldered the tail resistor ground- no change. Still motorboating. Mind you, I have a 100k/.047 across the standby, so that's why it's trickling in enough to be audible. I've used that method on all my standby switches with no pops and no problems. I double checked to make sure this one is wired and soldered correctly, so that's not an issue.

My best guess is bad filter caps, failing PT, or bad power tubes which I highly doubt since they're fairly new JJs and I tried them before in a tweed deluxe. They always light up normally, no redplating, etc.

Should I remove the tubes now and measure to see what's going on?

You seem to be ignoring the requests of one of the best amp troubleshooters I've ever worked with, do what sluckey asked and you won't be far from a solution. IMO anyway.

~Phil