Princeton Reverb clone advice

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foreverstrung
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Re: Princeton Reverb clone advice

Post by foreverstrung »

Voltage issue. V7, Rectifier, 5U4GB,
P4 and 6, should be 340v AC, I'm getting 358.2v AC
P2 and 8, should be getting 420v DC, I'm getting 349.2 DC
This low DC voltage, I'm sure, is throwing all my other voltages off down the line. I have 2 of these 5U4GB's. Both vintage RCA's. Came from a questionable source. I'm hoping someone can point out what they think might be the issue here. Rectifier tube is my thought, but both tubes produce the same voltage, so that doesn't make sense.
Appreciate the help
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Re: Princeton Reverb clone advice

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Pics of the actual wiring might help.
The DC voltage should be higher than the AC before rectification. What is the plate voltage of V5 and 6? Did you use 1 ohm resistors from pin 8 to ground on V5 and 6? Check that value too

On the photos I notice that the bias pot is turned to zero. When the bias is loo low the plate voltage will be off spec too, remember that.

To rule out the rectifier tubes you could clip in some diodes on the empty socket and check in solid state mode
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Re: Princeton Reverb clone advice

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Pics of the actual wiring might help.
The DC voltage should be higher than the AC before rectification. What is the plate voltage of V5 and 6? Did you use 1 ohm resistors from pin 8 to ground on V5 and 6? Check that value too
Plate voltage in P5 is 340.6v. P6 is 340v.
My layout has Pin 8 and 1 to ground with a 1 ohm resistor. That's what I did. Pics show this.
On the photos I notice that the bias pot is turned to zero. When the bias is loo low the plate voltage will be off spec too, remember that.
I addressed this and set the trim pot at approx 50/50
To rule out the rectifier tubes you could clip in some diodes on the empty socket and check in solid state mode
IDK how to do this. If you could give me more details and shoot me a "how to" link, that would be awesome.

I might swap out the power tubes and see if this makes any diff, though these are a pair of NOS RCA's I got from a reputable outfit. Also, I removed and set to the side the power switch for a few of the pics to show power tube sockets better.Pics:
pic 1 today.jpg
pic 2 today.jpg
pic 3 today .PNG
pic 4 today.jpg
pic 5 today.jpg
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Re: Princeton Reverb clone advice

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Look… I will help you, but I’m going to ‘Josh’ you a little first😉
Try to figure out to swap the tube rectifier to a SS rectifier.
Ingredients: 2 HT wires, 4 UF4007 or 1N4007 diodes, 1 HT CT wire, 1 bias wire and 1 B+ wire.
Draw it out…
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Re: Princeton Reverb clone advice

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Look… I will help you, but I’m going to ‘Josh’ you a little first😉
Try to figure out to swap the tube rectifier to a SS rectifier.
Ingredients: 2 HT wires, 4 UF4007 or 1N4007 diodes, 1 HT CT wire, 1 bias wire and 1 B+ wire.
Draw it out
I appreciate all your help and the tough love stuff, but I've got no clue how to draw this up using the components you've mentioned. I've found online a couple videos of how to build my own SS tube, but if I'm going to go that far, I'll just order a new one. That being said, what's odd about the possible tube being bad is that the voltages are exactly the same on both of them. Even if they were both bad, I'd imagine there would be some variation in the voltages.
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Re: Princeton Reverb clone advice

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It’s not hard at all, but I’m trying to get you to understand how a rectifier works. Understanding what you’re doing will help you in future situations. An amp is not a simple IKEA plan you’re copying into real stuff, even though it might look like it in some way. It’s a good thing you ran into trouble now, because it helps you to understand that what I just wrote.
There are a lot of variables that could cause the discrepancy in the voltages you’re reading. Soldering quality, wrong connections, faulty part or parts, …
There are a few ways to approach this problem. - - - Experience would be the easiest path, but let’s rule that one out here.
- process of elimination: starting to look for things from the start to the finish. Start would be power supply.
- wildly starting changing parts/tubes/… NOT a good way, though sometimes effective through sheer luck
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Re: Princeton Reverb clone advice

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Pull the rectifier tube. Leave the wiring on the socket.
- measure HT voltage with amp ON. Measure between pin 4 and 6. Is het center tap soldered to ground?
- measure 5V wires between pin 2 and 8
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Re: Princeton Reverb clone advice

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Cheat and look at documents in the downloads section with SS rectifiers. 😉
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Re: Princeton Reverb clone advice

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It’s not hard at all, but I’m trying to get you to understand how a rectifier works. Understanding what you’re doing will help you in future situations. An amp is not a simple IKEA plan you’re copying into real stuff, even though it might look like it in some way. It’s a good thing you ran into trouble now, because it helps you to understand that what I just wrote.
There are a lot of variables that could cause the discrepancy in the voltages you’re reading. Soldering quality, wrong connections, faulty part or parts, …
There are a few ways to approach this problem. - - - Experience would be the easiest path, but let’s rule that one out here.
- process of elimination: starting to look for things from the start to the finish. Start would be power supply.
- wildly starting changing parts/tubes/… NOT a good way, though sometimes effective through sheer luck
I understand and I am very grateful your spending any time with me, helping me to understand how it works. I do hope you'll continue.
It's how I learned to work on Harley Davidsons. I had a very experienced friend that would guide me through it. He's make me do all the actual work, but he kept an eye out and with a little nudge when needed.

HT voltage at P4, (with rectifier removed) is 358.2v. P6 is 358.2v. AC
P2 is 4.8v and P8 is 5.0 AC
Center tap of the HT is soldered to ground
Cheat and look at documents in the downloads section with SS rectifiers. 😉
I couldn't find a drawing in the downloads. I'll look again tomorrow. As well, I'll watch the videos again that actually fabricate the SS rectifier to help get a better understanding. They actually were very interesting.
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Re: Princeton Reverb clone advice

Post by JMPGuitars »

Superlite TMB, Tremolo TMB, etc. all have SS rec.

Schematic is all you get 😉😉:
ss rec.jpg
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Re: Princeton Reverb clone advice

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JMPGuitars wrote:
Thu 08/03/23 11:17 pm
Superlite TMB, Tremolo TMB, etc. all have SS rec.

Schematic is all you get 😉😉:
ss rec.jpg
Correct Josh… but his amp has also got fixed bias. I am leaning towards drawing this one out to solder to an octal socket, which would seem to me like a good idea.
In fact, I would also prep the socket that way to have safety diodes too when using a tube rectifier
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Re: Princeton Reverb clone advice

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Correct Josh… but his amp has also got fixed bias. I am leaning towards drawing this one out to solder to an octal socket, which would seem to me like a good idea.
In fact, I would also prep the socket that way to have safety diodes too when using a tube rectifier
This sounds like a video I watched yesterday of a guy who built a relatively crude rectifier in the philippines. His reasoning was no access to a replacement so he built a solid state rectifier. I thought it was pretty interesting.
I get the schematic Josh sent and even remember building it in the past. However I had no idea of it's function at that time.
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Re: Princeton Reverb clone advice

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Don’t forget the bias wire tap
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Re: Princeton Reverb clone advice

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Don’t forget the bias wire tap
Continuing forward,
I get how to run the diodes and resistor to create the SS rectifier, but I don't understand this bias wire tap. Is this something else I should add from the drawing Josh posted? I'd hate for something to go wrong. You guys know the number, right? 911
While waiting to move fwd with this, I went thru and checked all the readings of all my resistors and all my caps. I found 1 incorrect value resistor and replaced it with the correct one. It was supposed to be 2.7k form V3B to the output. I had a 47R in there. IDK why. Obviously this made no diff to the voltages.
I'm concerned, slightly, with the 4 JJ's (20uF's). There supposed to run in tolerances of +30% or -10%......... 3 of them are slightly more than 10% under value. I've got replacements caps for these spots if needed. But I'm not going to just start replacing parts until running it by you guys.
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Re: Princeton Reverb clone advice

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foreverstrung wrote:
Sat 08/05/23 7:38 pm
Don’t forget the bias wire tap
Continuing forward,
I get how to run the diodes and resistor to create the SS rectifier, but I don't understand this bias wire tap. Is this something else I should add from the drawing Josh posted? I'd hate for something to go wrong. You guys know the number, right? 911
While waiting to move fwd with this, I went thru and checked all the readings of all my resistors and all my caps. I found 1 incorrect value resistor and replaced it with the correct one. It was supposed to be 2.7k form V3B to the output. I had a 47R in there. IDK why. Obviously this made no diff to the voltages.
I'm concerned, slightly, with the 4 JJ's (20uF's). There supposed to run in tolerances of +30% or -10%......... 3 of them are slightly more than 10% under value. I've got replacements caps for these spots if needed. But I'm not going to just start replacing parts until running it by you guys.
Dude.. the only 27k resistor in that schematic is in the bias circuit. That resistor sets the bias.. that’s the one you need to look at and change when biasing the amp. That one is where people wire in a 10k pot to make the biasing easier. Tis is perhaps the most important resistor in the amp! Now from the pics I see you have a bias pot. Assuming it’s a 10k pot that resistor would be better at a 22k value to have some range.
Although it is very important to know what you’re doing at this point you must he sure to read 20 to 23mA at the pins 8 of the 6V6’s. Princetons like to he biased cold because that will make the tremolo run better/stronger

No those caps are just fine
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Re: Princeton Reverb clone advice

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Dude.. the only 27k resistor in that schematic is in the bias circuit. That resistor sets the bias.. that’s the one you need to look at and change when biasing the amp. That one is where people wire in a 10k pot to make the biasing easier. Tis is perhaps the most important resistor in the amp! Now from the pics I see you have a bias pot. Assuming it’s a 10k pot that resistor would be better at a 22k value to have some range.
Although it is very important to know what you’re doing at this point you must he sure to read 20 to 23mA at the pins 8 of the 6V6’s. Princetons like to he biased cold because that will make the tremolo run better/stronger
I changed out the 2.7k resistor yesterday and after, there was no significant change with the voltages anywhere. The schematic for the rectifier shows that I should be getting 420v DC off pins 8 and 2 and I'm still only getting 352-ish V DC.......The AC side, pins 6 and 4 are fine at 348v

My trem pot is 25k, not 10k. That's what the layout on this build calls for. 25k.

I checked mA @ pin's 8 and I'm not getting proper the readings you described. The readings I got off my DMM set on DCmV was like .2. I'm not sure what to make of that
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Re: Princeton Reverb clone advice

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Your amp is biased way too hot. 350V on the plates means there is a lot of current being drawn. Do the tubes redplate?
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Re: Princeton Reverb clone advice

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Your amp is biased way too hot. 350V on the plates means there is a lot of current being drawn. Do the tubes redplate?
I apologise. I think I'm doing a poor job of communicating. I'll try to be more clear. Specific.
Tubes are not red plating
According to these schematics, my plate voltages, V5P3 and V6P3 should be 410v. I'm getting 348v
My last post was V7 Voltages. The rectifier. The DC side is still like 70v lite. (352v when I should be 420v).
AC side, Pins 4 and 6 are spot on at 345v. (schematics call for 340v)
I'm having a hard time getting the plate "currents" of V5 and V6 because the B+ side of this rectifier is all wonky......
I've been reviewing tons of videos and links explaining details, mods and ideas...Not sure where to go just yet. I was waiting to hear back from you before doing anything drastic
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Re: Princeton Reverb clone advice

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I found a mod to add a switch @ the rectifier that switches between SS and rectifier tube, but there were a couple things I wasn't clear on. The SS w/diodes thing I've used in the past has a 100 ohm 10w resistor. The mod I saw didn't show one (though it could have been further inline). Josh's drawings of the SS resistor show a 100ohm 10w resistor too.
As well, with this layout I'm using on this build, if you look at this layout, (I'll attach), it's got a HT wire running V7P6 to the PCB board. I'm unsure how to implement that into this SS rectifier modification.
I could draw up a SS rectifier considering this layout (because it differs from the original AA1164 schematic a bit) and maybe you could check it before I do anything
LAYOUT_princeton_reverb (1).pdf
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Re: Princeton Reverb clone advice

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Hold on... I suggest that you first find out if it's the rectifier tube that's flakey or that the problem is somewhere else.
1. Pull out the rectifier tube and start the amp up
2. measure the voltage between pin 8 and 2 (should be +- 5V AC)
3. measure the voltage between pin 4 and 6 (should be +- 640V AC)

If this is all ok switch off the amp. Put the rectifier tube back in and pull the power tubes and preamp tubes.
1. measure the voltage between ground and pin 8 on the rectifier (should be around 400V DC)
2. if it's again around 355V DC then your rectifier tube is the suspect, if the voltage is rather up to spec then the problem is probably in the bias circuit or the power stage.

My guess would be the bias setting. OTOH, I think my Princetons have about 370V DC on the plates and the tubes draw about 22mA.
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