AB763 Combo / Princeton Chassis

[purpletele]

I am working on a Single Channel Deluxe that I laid out last year.

Everything has gone very smoothly.

I finished up wiring and double checked for unfinished components, etc.., hooked into the Light Limiter and flipped the switch with the amp loaded with tubes and  I smoked the Bias Trim Pot.

I'm comparing the wiring to my other Deluxe. 

Verified I had a 10Kl Trim Pot, verified the components are correct in that vicinity.

I am ordering a new trim pot, so that takes me out for a few days.  In the meantime, I am thinking I should pull the tubes and recheck voltages.

This is an Edcor Transformer that I had used on my other Deluxe.  I had fried the OT on the other Deluxe, but this transformer seemed to be fine.

If I remove the tubes and get standard voltages at the Rectifier and if I am get 6.3 volts at the tubes then I should assume the PT is ok correct?


I had good voltages on the tube sockets earlier in the project.

[sluckey]

You don't need any tubes plugged in to test the bias supply.

It appears the bias circuit is getting it's AC input from the rectifier socket via that violet wire. But your bias range resistor appears to only be 470Ω! If so, that's why the bias pot smoked. That 470Ω resistor is used when you have a dedicated bias tap of about 50VAC. To use that much higher AC from the high voltage winding you need to use a much larger bias range resistor, something on the order of 100K to about 220K (exact size determined experimentally to provide the bias range for the tubes you will use).

So, in addition to a new bias pot, you need a much bigger bias range resistor, and probably a new bias cap since that one was exposed to a much higher voltage than it was expecting to see. And check that 22K that's next to the bias cap.

[purpletele]

That makes sense I remember having you help me through that last time, that is what I thought was the case.

However, I checked my working Deluxe and it only had the 470 Ohm resistor with a similar layout so I don't remember or understand how we reduced the current to make it function correctly. The photo below is from my previous Deluxe.


Oh I re-read about the bias tap.  The replacement on my other Deluxe has a Mercury Magnetics Transformer and it probably has a dedicated Bias tap.

[purpletele]

Thanks Steve,

It makes sense to me.

BTW, I took my Deluxe to The Gear Page Jam in Elk Grove California today.

A lot of folks were Very impressed with your rendition of the Deluxe.  Many compliments. It sounded great so Thanks again.

I had asked for anyone attending if they could bring a 500 pF silver mica cap so I could complete the current amp and someone did, that was cool.


Here is a photo of the Deluxe under scrutiny.

 

[jamaio]

I was actually considering a similar build. I have all the parts for a Stout Reverb that I started building in a Mojo Tone PR combo cab. I lost motivation and that build has been on my bench for 2 years, untouched. I also have all the parts for a Deluxe Reverb 2 channel with a Mojo Tone DR combo cab. I have pulled my guitar out of the closet and started practicing again. Trying to decide weather to continue the Stout Reverb I started 2 years ago or move it aside and start the DR and build it as a DR. I really like the classic Fender tone.

Hijack over!!

Can't wait to hear your completed project.

[purpletele]

I was actually considering a similar build. I have all the parts for a Stout Reverb that I started building in a Mojo Tone PR combo cab. I lost motivation and that build has been on my bench for 2 years, untouched. I also have all the parts for a Deluxe Reverb 2 channel with a Mojo Tone DR combo cab. I have pulled my guitar out of the closet and started practicing again. Trying to decide weather to continue the Stout Reverb I started 2 years ago or move it aside and start the DR and build it as a DR. I really like the classic Fender tone.

Hijack over!!

Can't wait to hear your completed project.

Jamaio,


I highly recommend building the Stout Reverb first.  That is a great amp and it is very manageable and forgiving if you haven't built an amp for a while.  Definitely a better amp than the DR if you have never built an amp.

The DR is not an easy amp to start on.

Let us know what it is going to be.  The Stout in a Mojo PR would be very cool!

[jamaio]

I will finish the Stout in the PR cab, I ordered the cab with 1x12, will see how it sounds with a few speakers I have then order a good one. I did the Hoffman Blues Jr. conversion then Fender Reverb 35 to Hoffman 5f1. The 5f1 sounds awesome, need to tweak my Blues Jr some more but sounds good.

[purpletele]

Deluxe Reverb Update and Looking for advice.


I had fried my bias section due to not having the correct bias resistor to go along with the transformer. 

I received my parts and installed them.  I have a working amp and the bias is set but it gets a little muddy at higher volumes.  I have some GT 6v6 that never sounded very good installed.  It could just be those crummy tubes.



1. Installed new 22k resistor, new 10kl bias trimpot, new 47/100 bias cap and a 100K Bias resistor.

2. Installed 6v6 C's. With the 100K bias resistor.  I set the bias to 20mV

3. When I take the amp off of Stand By my light limiter illuminates just a bit, my heater voltages are at 6 volts,  and I am getting a reading of 420 Volts at the plate.

Questions:

Q1. If I am getting the light limiter to illuminate just slightly does that mean specifically that I have a short somewhere?



I am going to play it for a while but I don't want to take it off of the Light limiter until I know I don't have a short in a transformer.





.


BV

[sluckey]

2. Installed 6l6 GC's. With the 220K bias resistor and the 6l6GC's I can only turn the bias down to 70 mV

Need smaller resistor to decrease bias current. Try 100K. Get the bias fixed then re-evaluate. Is that PT capable of running 6L6s? Give us a link to some data for that PT.

Lot of variables with your light bulb limiter. I like to use 60W bulbs for a 2x6V6 amp, 100W for a 6L6 amp. It's OK for it to glow a bit. If it glows full bright (like you had the bulb in a real lamp socket) then you have a dead short. Partial shorts or shorts on the secondary would cause the bulb to be brighter than normal but not full bright. But you don't know what normal brightness is for that amp. Plug your other working Deluxe into the lamp limiter to get an idea.

[purpletele]

2. Installed 6l6 GC's. With the 220K bias resistor and the 6l6GC's I can only turn the bias down to 70 mV

Need smaller resistor to decrease bias current. Try 100K. Get the bias fixed then re-evaluate. Is that PT capable of running 6L6s? Give us a link to some data for that PT.

Lot of variables with your light bulb limiter. I like to use 60W bulbs for a 2x6V6 amp, 100W for a 6L6 amp. It's OK for it to glow a bit. If it glows full bright (like you had the bulb in a real lamp socket) then you have a dead short. Partial shorts or shorts on the secondary would cause the bulb to be brighter than normal but not full bright. But you don't know what normal brightness is for that amp. Plug your other working Deluxe into the lamp limiter to get an idea.

Thanks Steve,

I re-wrote my post above.

I went to 100K resistor and installed the 6 v 6 c's that I had in stock and set the bias at 20 mV, that is with a 420 V plate voltage.  I wondered if the transformer could handle the 6 l 6's so I switched real quick.

I think I just have some crummy power tubes now, but it seems to be running very stable.

I have a 200Watt bulb on the Current Limiter, the light is just lightly glowing. 

 

[sluckey]

I went to 100K resistor and installed the 6 v 6 c's that I had in stock and set the bias at 20 mV, that is with a 420 V plate voltage.

You're not using the light bulb limiter while setting the bias and checking plate voltage are you? Should be plugged straight into the wall for any voltage readings to be meaningful.

I have a 200Watt bulb on the Current Limiter, the light is just lightly glowing.

I'm surprised you can see the bulb glowing. I would rather use a 60w or 100w for a little more glow. But it's really a subjective thing. It's OK to use a big bulb. The important thing is for you to get used to what the bulb looks like for a normal healthy amp.

[purpletele]

I went to 100K resistor and installed the 6 v 6 c's that I had in stock and set the bias at 20 mV, that is with a 420 V plate voltage.

You're not using the light bulb limiter while setting the bias and checking plate voltage are you? Should be plugged straight into the wall for any voltage readings to be meaningful.   

I was using the Light limiter when checking the bias.  I wasn't comfortable with the glow.

I am trying new 6 v 6 tubes this afternoon, it was getting real flabby real quick. 

Thanks for the help.

BV

[sluckey]

The light bulb current limiter is one of those devices that you use initially when powering up an amp. As soon as you see that the bulb is not glowing abnormally bright it's time to put it away and plug the amp straight into the wall.

[purpletele]

Update on the Deluxe


Tried two new sets of power tubes and reset the bias each time without the Current Limiter.

The amp is working fine and it is stable, it sounds decent at lower volumes and then gets flabby.  I really suspect that this is just not the right transformer for this amp, so much so that I called up Patrick at Mercury and asked him to send me the same unit I used on my last DR. 

My current Deluxe is so nice and so powerful it is hard to consider building anything of lesser quality.  The Mercury Transformer is $170 so it's not untouchable fairy dusted
cork sniffer range.

I'm going to set it aside and replace the PT next week.  I am fairly confident that in my decision, we'll see next week what happens on this amp.

Thanks for the help

[purpletele]

Deluxe Reverb Combo Update


Prior to ordering a new transformer I checked all of my voltages and found that V5-7 was starving.  I wanted to change the transformer regardless, so it cam in today and I installed it and it is working fine.  The other transformer was fine as well.


6 v 6 GT's
Plate Voltage 414 V
Bias set at 25 mV

The amp is stable from a power standpoint.  I went through and measured new voltages and compared them to my Deluxe from Last year.  All of the voltages are good with exception to V5.

I have reviewed the components in the PI section but I haven't found a glaring error, but it seems to be there.

The amp is flabby and doesn't chord very well, but it is not disastrous.

Any ideas of where I should focus?


Thanks for any ideas anyone might have.


BV

[tubenit]

The amp is flabby and doesn't chord very well, but it is not disastrous.

I would try .047 instead of .1 coupling caps between LTPI and power tubes.

I would lower all the cathode caps on the first 3 gain stages to 5uf.   Or the 1st one being 10uf and the 2nd & 3rd being 5uf.

With respect, Tubenit

[ac427v]

Before you blame the power transformer or crappy tubes, compare your voltages to a fender schematic. The phase inverter is not working right. You have 195v and 403v on the plates. That is a must do repair before trying other things.

[sluckey]

Remove C23 (.1/250V) from board. Is pin 7 voltage now about the same as pin 2 voltage?

[purpletele]

Remove C23 (.1/250V) from board. Is pin 7 voltage now about the same as pin 2 voltage?

Thank you,I was curious about that cap connecting those circuits well, but when I lifted a leg and took it out of circuit it did not change the voltage at V5-7 which is still in the mV range.


I tested the 100 ohm resister that shares a ground with the bias section, I thought that might have been compromised when my bias system smoked.  That resister is reading 99 ohms, so that seems fine.

The reverb and tremolo curcuits are working fine.

I checked my jumpers as well and they are indeed in place.  Refloated any joints that didn't look good.  Tacked back the .1/250 cap.


Thanks for the input.

BV

[purpletele]

The amp is flabby and doesn't chord very well, but it is not disastrous.

I would try .047 instead of .1 coupling caps between LTPI and power tubes.

I would lower all the cathode caps on the first 3 gain stages to 5uf.   Or the 1st one being 10uf and the 2nd & 3rd being 5uf.

With respect, Tubenit

Thanks Jeff, I want to eliminate any component or wiring issues before I play with changing values.  I have a big voltage bust on v5-7.

[purpletele]

Before you blame the power transformer or crappy tubes, compare your voltages to a fender schematic. The phase inverter is not working right. You have 195v and 403v on the plates. That is a must do repair before trying other things.

ac427v,

Thanks, I know I have an issue in the PI and I am searching for the issue now.  The original transformer has been questionable and I wanted to change it out for confidence in for the future, but the original transformer is fine and may be used in another project.   

[sluckey]

With power off, measure resistance from V5 pin 7 to chassis. Put one meter lead directly on the tube socket. What have you?

[purpletele]

With power off, measure resistance from V5 pin 7 to chassis. Put one meter lead directly on the tube socket. What have you?

OL!!!

[sluckey]

Wrong answer. It should be about 1M. Check V5 pin 2 to chassis. What have you?

[purpletele]

Wrong answer. It should be about 1M. Check V5 pin 2 to chassis. What have you?

1M

[sluckey]

Stay online with me and well have this fixed in a couple minutes. Back on pin 7... Follow the wire to the board and check resistance from that 1M resistor to chassis.

[purpletele]

Stay online with me and well have this fixed in a couple minutes. Back on pin 7... Follow the wire to the board and check resistance from that 1M resistor to chassis.

I am getting an OL on the 1 M resistor to the chassis

[sluckey]

Check the other end of the 1M resistor. Should be about 22K

[purpletele]

Check the other end of the 1M resistor. Should be about 22K

I get 22K on the common rail and OL on the resistor

[sluckey]

Either the 1M resistor is bad or it's lost it's connection to the turret or the turret has lost it's connection to your lacing jumper.

[purpletele]

Either the 1M resistor is bad or it's lost it's connection to the turret or the turret has lost it's connection to your lacing jumper.

Great session Steve!  Thank you


That was really weird, I pulled the resistor and it measured at 99M, de-soldered the holes and reinstalled the resistor.  It measured fine, then I soldered it up and it cut of the flow of electrons?  I floated it again and drew som solder down the lead and all is now good. 

The new transformer will allow me to run 6 L 6's, so that will be fun. 

I'll re-bias and play the heck out of it today.

Another successful troubleshooting session for today!  you are on a roll.

BV

[sluckey]

Throw that resistor away and put a good resistor on the board. Otherwise you will revisit this problem. I've see too many of those carbon film resistors where a lead would lose connection inside the body of the resistor. Sometime you could actually pull the lead out of the body. I don't use them because of this.

[purpletele]

Throw that resistor away and put a good resistor on the board. Otherwise you will revisit this problem. I've see too many of those carbon film resistors where a lead would lose connection inside the body of the resistor. Sometime you could actually pull the lead out of the body. I don't use them because of this.

Thanks for the tip.  I thought they were an upgrade from carbon composite resistors that drift with age.

What do you recommend for my two 50 watt projects.  I have the chassis for the Dual 50 ready and I am ready to load components on the Fowler HRP #39.

The amp sounds great !

[sluckey]

I've become very fond of metal film. Tight tolerance, cheap, and dependable.

[purpletele]

I've become very fond of metal film. Tight tolerance, cheap, and dependable.

I am disheartened to have another issue come up that may possibly be a component again.

I installed 6 L 6's and reset the bias without issue, let it idle for a couple of hours while I was working.

I settled in to real dig into the amp and I am pleasantly surprised at how nice and rich it sounds in the combo,

I hit a hard low note and the amp jumps into reverb feedback, I have to pu it on standby and then it plays silky again.  If I hit a quick hard low note it will definately jump into a reverb feedback mode.

Any thoughts on what might be going on and How I look for a culprit?


BV

[ac427v]

My first thought is that you have acoustical (not electrical) interaction between the speaker and the tank. If the feedback goes away when you plug the amp into a separate speaker cabinet then you can focus on fixing by isolating the tank better.

[sluckey]

Disconnect the violet feedback wire from the board. Any better?

[purpletele]

My first thought is that you have acoustical (not electrical) interaction between the speaker and the tank. If the feedback goes away when you plug the amp into a separate speaker cabinet then you can focus on fixing by isolating the tank better.

Thanks ac,


I tried with a separate cab last night and got into the feedback thing.  I'll try disconnecting the feedback connection.


BV

[purpletele]

Disconnect the violet feedback wire from the board. Any better?

Well that seemed to work, can you explain what led you to that and what you think is going on?

I'm grinding it now to see if I can get it to fail, but so far it has been stable.

BV

[sluckey]

Anytime someone mentions feedback on a new build and that amp has a NFB loop I always suspect wrong phase on that NFB loop. Disconnecting that wire is a way of testing the NFB. But that's not the fix. To fix it, reconnect that feedback wire and swap the OT plate leads (blue and brown leads) on pins 3 of the output tube sockets. Or, if the OT leads are too short you can swap the white wires that connect the board to pins 6 of the output tube sockets.

[purpletele]

Anytime someone mentions feedback on a new build and that amp has a NFB loop I always suspect wrong phase on that NFB loop. Disconnecting that wire is a way of testing the NFB. But that's not the fix. To fix it, reconnect that feedback wire and swap the OT plate leads (blue and brown leads) on pins 3 of the output tube sockets. Or, if the OT leads are too short you can swap the white wires that connect the board to pins 6 of the output tube sockets.

Great info, I'll make those changes.


The amp sounds great.



Thank you,

BV

[purpletele]

Anytime someone mentions feedback on a new build and that amp has a NFB loop I always suspect wrong phase on that NFB loop. Disconnecting that wire is a way of testing the NFB. But that's not the fix. To fix it, reconnect that feedback wire and swap the OT plate leads (blue and brown leads) on pins 3 of the output tube sockets. Or, if the OT leads are too short you can swap the white wires that connect the board to pins 6 of the output tube sockets.

I made the recommended changes and the amp screamed at me like a cornered racoon.  Ironically the OT wires being changed didn't make a difference as long as the NFB was disconnected.

I played it for a while with the original OT configuration and the NFB disconnected and it sounded nice, and then I dug into a low F and it that note got really distorted.

I think I'll make a video.

[sluckey]

OK, so NFB was correct all along. Put everything back like it was. But now you know. This is a test you should do on every amp with NFB that you build or replace an OT.

If you turn the reverb pot to zero does the feedback occur?

[purpletele]

Here is a quick video that highlights the low frequency distortion issue.

https://www.youtube.com/watch?v=IqRhCH_uA-w

[purpletele]

OK, so NFB was correct all along. Put everything back like it was. But now you know. This is a test you should do on every amp with NFB that you build or replace an OT.

If you turn the reverb pot to zero does the feedback occur?

I shot a quick video of the low frequency distortion, you can view the link below.  I had the NFB disconnected at the time of the video.  The distortion occurs when the Reverb is off or slightly on.


UPDATE @ 1:00


Found another bad CC 100K resistor on my Speed Pot.  Changed that out and hooked up the NFB and it appears to have solved the issue.  The amp is noticeably more quiet now.  When I grind a low note it contains the chaos, so no more distortion, at least so far.  The amp is purring nicely now.


We'll see in the daylight hours when I can get a little volume going.


BV