Hoffman AB763 DR - Initial power on = high B+, fried NFB resistor, low bias cur

[MonoPoly]

Hi, I just finished soldering the a Hoffman AB763 DR, double checked all the work, and went to adjust the bias. I'm curious if some of these problems might be related and if I should tackle them in any particular order.

I ran into these problems:

B+ was high. My wall voltage is 125v, but even for that I was getting around 500v on the power tube plates, around 210 on the 12ax7s, and I was getting a reading in the 400s for the 12AT7s.

Weird ticking sound from phase inverter tube that transferred to one of the power tubes. Might be a poor contact on pin 1 of the tube socket. I put in a different 12at7 (same brand) and was getting the same result. I tried Cleaning out the tube sockets (I salvaged them from a hammond chassis) and retensioning them. I have some 12AV7s that have thicker pins - should I pop one of those in to see if it fixes that problem?

Low bias current (range goes from 1-8 milliamps DC), according to the directions I need to change the one ohm resistors to something higher in value. What would be a good value to try? 3 ohms?

Last, I got magic smoke from the NFB resistor. It was a 3/5 watt 820 ohm resistor. Is this because I forgot to hook up a speaker?

Thanks!

[tubenit]

What are the specs of your power transformer?

Are you measuring voltages with tubes actually in the amp?  (sometimes people say a plate has ________ voltage but they really mean the socket pin to that plate & the tubes weren't installed)

Were voltages measured without the speaker being hooked up?

Are you using 6V6's in the DR or 6L6's/5881's?

What type of rectifier are you using? 

With respect, Tubenit

[sluckey]

B+ was high. My wall voltage is 125v, but even for that I was getting around 500v on the power tube plates, around 210 on the 12ax7s, and I was getting a reading in the 400s for the 12AT7s.

Voltages will come down once the output tubes are biased correctly. We need voltage readings for V6 pins 1,2,3 and 6,7,8. We also need voltage readings for V7 and V8 pins 3,4,5,8.

Weird ticking sound from phase inverter tube that transferred to one of the power tubes. Might be a poor contact on pin 1 of the tube socket. I put in a different 12at7 (same brand) and was getting the same result. I tried Cleaning out the tube sockets (I salvaged them from a hammond chassis) and retensioning them. I have some 12AV7s that have thicker pins - should I pop one of those in to see if it fixes that problem?

Leave the 12AT7 in the socket. Describe this weird ticking. Since you didn't have a speaker connected I must assume you are hearing the ticking sound coming directly from the tube? That's very weird. Suspect a wiring error. Could be related to burning the 820Ω resistor.

Low bias current (range goes from 1-8 milliamps DC), according to the directions I need to change the one ohm resistors to something higher in value. What would be a good value to try? 3 ohms?

Whose directions told you to change the one ohm resistors? Post a link please. Those directions are either wrong or maybe you misunderstand the intent. The 1Ω resistors simply provide a method to monitor tube current. They don't have any control over how much current is flowing. YOU NEED TO LEAVE THEM BE!

Last, I got magic smoke from the NFB resistor. It was a 3/5 watt 820 ohm resistor. Is this because I forgot to hook up a speaker?

It's very unusual to burn that 820Ω resistor. Suspect wiring error or NFB is wrong phase. You need a speaker connected to be able to hear weird sounds, but no speaker connected did not cause that resistor to burn.

In addition to the voltage readings I requested we need to see some hi-rez pics of the inside of your amp. I'm assuming you used a Hoffman AB763 board. If you used a different layout, please provide a pic of that layout.

Most problems with a new build will be wiring errors or wrong value components. Hoffman's first law states "If it were wired correctly it would be working."

[Latole]

  ran into these problems:

B+ was high. My wall voltage is 125v, but even for that I was getting around 500v on the power tube plates....
 
Low bias current (range goes from 1-8 milliamps DC), 

Thanks!

Lower is the bias higher will be outout tubes's plate voltage.

With 1-8 ma you have no load on power supply voltage= very high power supply voltage

[MonoPoly]

Thank you all a ton for the help.

I did a better job of retensioning the sockets for the 12at7s (they have thinner pins than the 12ax7s), and that seemed to have dropped the b+ a bit and fixed the ticking sound.

B+ was high. My wall voltage is 125v, but even for that I was getting around 500v on the power tube plates, around 210 on the 12ax7s, and I was getting a reading in the 400s for the 12AT7s.

Voltages will come down once the output tubes are biased correctly. We need voltage readings for V6 pins 1,2,3 and 6,7,8. We also need voltage readings for V7 and V8 pins 3,4,5,8.

V7/8 : 1/8) 153-158v depending on bias pot position 3/4) 484v 5/6) 54v

V6: 1) 295v 2/7) 164v 3/8) 202v 6) 324v

Weird ticking sound from phase inverter tube that transferred to one of the power tubes. Might be a poor contact on pin 1 of the tube socket. I put in a different 12at7 (same brand) and was getting the same result. I tried Cleaning out the tube sockets (I salvaged them from a hammond chassis) and retensioning them. I have some 12AV7s that have thicker pins - should I pop one of those in to see if it fixes that problem?

Leave the 12AT7 in the socket. Describe this weird ticking. Since you didn't have a speaker connected I must assume you are hearing the ticking sound coming directly from the tube? That's very weird. Suspect a wiring error. Could be related to burning the 820Ω resistor.

Low bias current (range goes from 1-8 milliamps DC), according to the directions I need to change the one ohm resistors to something higher in value. What would be a good value to try? 3 ohms?

Whose directions told you to change the one ohm resistors? Post a link please. Those directions are either wrong or maybe you misunderstand the intent. The 1Ω resistors simply provide a method to monitor tube current. They don't have any control over how much current is flowing. YOU NEED TO LEAVE THEM BE!

Thanks - I misread. it mentioned raising the value of the 470 ohm, 3 watt resistor. That makes a lot more sense.

Last, I got magic smoke from the NFB resistor. It was a 3/5 watt 820 ohm resistor. Is this because I forgot to hook up a speaker?

It's very unusual to burn that 820Ω resistor. Suspect wiring error or NFB is wrong phase. You need a speaker connected to be able to hear weird sounds, but no speaker connected did not cause that resistor to burn.

NFB phase? Is that just making sure the wire is going to the signal of the output jack and not the ground? The OT is from a p/p El84 hammond organ amp and came with the chassis, so I'm not sure if that had something to do with the NFB resistor blowing. The OT also has some ultralinear NFB taps that I bundled up and wrapped in heat shrink. (I drilled holes for a typical DR OT in case the el84 OT doesn't work) I'll double check the wiring in that part of the circuit.

In addition to the voltage readings I requested we need to see some hi-rez pics of the inside of your amp. I'm assuming you used a Hoffman AB763 board. If you used a different layout, please provide a pic of that layout.

Most problems with a new build will be wiring errors or wrong value components. Hoffman's first law states "If it were wired correctly it would be working."

I'll get some more pictures up of the rat's nest shortly (this is my first amp, so now I know a lot of reasons why I should have just used a standard chassis/layout when it comes to something like a deluxe reverb). It's the hoffman ab763 turret board.

The B+ might make sense now. The PT is rated @ 660 ct., but mine might be a bit higher than rated (I'm reading 356vac from rectifier pins 6 and 4 - also need to factor in my wall voltage is putting out 125v). I'm using a 5ar4 rectifier. The 5v supply from the PT is 3 amps, so I'm thinking there are options if the best solution is to change the rectifier.

[Latole]

I understand building a first amp is like a rocket science and it is not easy.

About the rat nest, with this wiring mess, the amp will be very very noisy

[MonoPoly]

I understand building a first amp is like a rocket science and it is not easy.

About the rat nest, with this wiring mess, the amp will be very very noisy

Yea, I went into the project as a learning project largely using salvaged parts I had laying around or picked up super cheap. I figure I won't be making very usable amps until I get some experience. Once I get it working, I'll eventually disassemble and put into a chassis that is much more fitting, so I can actually get the wiring much neater and clean.

[shooter]

The OT also has some ultralinear NFB taps

UL taps are typically primary side OT going to G2 (screen grid)think of it as the best parts of triode/pentode in one wire 

[sluckey]

I don't see any filter caps.

[MonoPoly]

I don't see any filter caps.

They are in a doghouse - they each read around 17uf and were rated for 500v, when I think the schematic was 16uf and I think 450/475. I figured within a 10% tolerance was fine.

[Latole]

Why don't you follow AB763 DR layout ? Or you did not know it ?

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_aa763_layout.pdf

[sluckey]

Why don't you follow AB763 DR layout ?

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_aa763_layout.pdf

Can't use that layout with a Hoffman board.

[MonoPoly]

Why don't you follow AB763 DR layout ?

https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_aa763_layout.pdf

If you are referring to how the turret board, tubes, and transformers sit in the chassis, that was the original plan, but...

The DR cabinet I had sitting around turned out to be 1.5" narrower than a DR cabinet so a DR chassis would not fit - but I did have that hammond chassis sitting around that would fit in it, so I figured, "I've got a lot free time for the duration of the pandemic, why not?" Had I not already ordered the turret board, I would have opted for something designed for a 16" chassis. I know/knew the learning curve was going to be very steep, so I didn't want to spend a ton of money assuming this would be more about learning that getting a great amp. (I only spent about $250 on this - would have probably been more like $700-800 if I bought cabinet, chassis, faceplate)

[Latole]

I now understand your approach better.
On the other hand, you should know that the preamp section (the 12XX7 tubes) must be as close as possible to the components.
You must absolutely avoid long wires which otherwise will act as an antenna and make the amplifier very noisy. Maybe even unusable.

Look at Fender layout how are the wires with 12XX7 tubes

This chassis was not intended for this type of assembly.

It will be a first experience.

[MonoPoly]

I found that the ground buss for the bias circuit/nfb circuit wasn't connected to anything (thought I connected it underneath the board), so I think that NFB resistor fried because that would have been the path of least resistance to ground (through resistor -> OT secondary winding -> ground). Probably also caused certain biasing issues. Now I'm running into a slightly different biasing issue. When I go past the 10ma point, it automatically jumps into a zone where it rapidly fluctuates from 40-80ma. Same problem with different tubes. I don't think I have a 10k pot sitting around, but probably have a 5k.

[MonoPoly]

I now understand your approach better.
On the other hand, you should know that the preamp section (the 12XX7 tubes) must be as close as possible to the components.
You must absolutely avoid long wires which otherwise will act as an antenna and make the amplifier very noisy. Maybe even unusable.

Look at Fender layout how are the wires with 12XX7 tubes

This chassis was not intended for this type of assembly.

Ha- tell me about it.

I was also reading that the preamp tubes should be a distance away from the OT and PT. Is that less of a concern than the length of the wires? Would it be better to move v4-6 closer to the OT and PT? Or should I continue my current plan of waiting to find a deal on a donor chassis/cabinet?

[Latole]

You will find that there is a lot to know before you build an amplifier.
The arrangement of everything is not due to chance.

This type of chassis seems to have been made for point to point without a turret board. The old way.
With this way of building we can avoid the problems that I mentioned

[sluckey]

When I go past the 10ma point, it automatically jumps into a zone where it rapidly fluctuates from 40-80ma. Same problem with different tubes. I don't think I have a 10k pot sitting around, but probably have a 5k.

Sounds like oscillation. Do you have a speaker connected? Do you hear a loud squeal when the current jumps up? Assuming there are no other wiring errors, the NFB phase is possibly wrong. Swap the OT primary plate leads to correct this.

[MonoPoly]

When I go past the 10ma point, it automatically jumps into a zone where it rapidly fluctuates from 40-80ma. Same problem with different tubes. I don't think I have a 10k pot sitting around, but probably have a 5k.

Sounds like oscillation. Do you have a speaker connected? Do you hear a loud squeal when the current jumps up? Assuming there are no other wiring errors, the NFB phase is possibly wrong. Swap the OT primary plate leads to correct this.

Thanks!
Yep, it was totally oscillation. It was coming from the rat's nest to v5 rather than the NFB being out of phase. I could get it to mostly go away by moving the wires around, but I'm now in the process of moving V6 over and moving V5 where V6 is currently. I'm hoping that is a more permanent solution and the proximity of v6 to the OT doesn't cause issues.

[bnwitt]

Looking at the photo of the loaded and wired turret board it doesn't look like your laced bus wires are soldered at every turret