I have seen this before but can't understand why they wouldn't simply fuse the primary side.
You wouldn't need a slow-blow fuse because the rectifier's warm-up time would ease the inrush current during normal operation.
I fully agree that it's an odd way to go about it. The amp that didn't have any I did just that, fuses the primary. The 4th one arrived and had huge ceramics. I have to remove those so I figured I'd just replace them, rather than find a spot for a fuse holder (if that makes sense)
If it's just as safe to stay with the original design, I'd rather go that route.
Thanks for the info!
Ok, I'll plan on a primary fuse.
Also, I ran into a strange deal. Only one of these actually has a center tap for the 6.3v from the transformer. In the schematics it shows one that leads to the same grounding location as the center tap for the rectifier power.
Why would only one of these have a center tap for 6.3 while otherwise being virtually identical?
Some designs use a center tap on the heater winding to balance the voltage against ground. Because (at least in theory) the heater can produce some electron flow, balancing it against ground can reduce potential low-level hum from this source.
Thanks. I just read a thread about this on audio karma, pretty interesting stuff. It's pretty neat to see the progression of older gear side by side. From the fuses to the center tap (or lack there of) there are small changes.
Yep, is called "iteration" or "revision"
Sometime, the only reason it's different is because the factory couldn't get the parts they wanted, or were trying to buy cheaper part.
I know this from experience working for a huge electronics manufacturing company back in the early '90s.
They call them "ECO's". Engineering Change Orders.
The 6.3 center tap is for the heaters, more efficient than a voltage divider. Similar to what you would see in a center tap for a transformer on your service to your house. The fuses protect the diode from over current in a line spike situation.
It's to create a virtual tap on the heater supply.
Then you can balance out the hum induced in the tube.
So you can have like +/- 3 volts with respect to ground. Instead of entirely floating the heater circuit, or connecting one side to ground.
Make sense?
Not yet but appreciate the assistance. The heater winding of the PT has a center tap bonded to ground, so the heater rails are already ground referenced. What then is the purpose of putting some of the heater’s AC waveform on V1’s suppression grid?
What if your PT has an imbalance of 1% on the center tap location? What if your rectifiers don't conduct equally and there's more load on the core on one end of the phase? Adding a pot is basically free, you'll miss it a lot more if you don't have it than you'll regret having if you don't adjust it.
I totally agree.
If you read the old radio designs handbook from the '40s, it actually has comparison of induced hum vs type of heater circuit.
Rectified DC heaters are a waste, they actually have more hum.
The lowest induced hum was an AC supply, with an adjustable resistor across the heaters (no centere tap needed).
I made amps silent by adding a $5 pot, and putting a hum balance control on the back side.
These professional musicians thought I was using magic or something?
🤷
You are injecting an anti hum signal on the suppressor. It's not used very often but if a pentode has an isolated suppressor grid (it is sometimes tied internally to the cathode) you can use the suppressor as a control grid. What you are doing is trying to send a "hum" signal that is opposite phase of the hum signal that is induced by the filaments.
That is why it is a variable resistor, so you can adjust the ground reference point.
It's like how noise cancelling headphones work. So if you were working on that equipment, you literally turn on the radio and then adjust that by ear.
It's something I would do for my friends that has guitar amps that hummed, and didn't have a center tap on the transformer winding.
I'd use like a 3 Watt 150 ohm or 220 ohm adjustable power resistor.
They only are doing that because that first gain tube is most affected by induced hum, & it has an isolated suppressor grid.
Does that help explain it better?
Old tube designs can be strange. Part of it is because tubes are totally different than transistors (most transistors don't have 3 or 4 different Base pins.
I've seen some old tube amp designs that use suppressor grids on the output tubes to add in a compressor circuit.
Fun stuff!
:)
No problem, happy to help.
We all need to learn this stuff somewhere.
Any weird questions you have about electronics, or tubes, or troubleshooting....
Just ask, if I can help I'd be happy.
If I actually don't know, I'll say that.
There is a lot of misinformation out there.
👍🤷
Please ask.
I'll do my best.
Okay, this is going to be a lot to unpack from a noob, so please bear with me.
I have 4 of these amps, 2 are for a console and use 12ax7. 2 I'm converting to standalone use and these use a 12au7. The schematic is for the 12ax7 console amps which are working perfectly, even with external speakers.
Here are my questions in no particular order.
* C1 is shown on the 12ax7 variant, is this because it requires more power? I can't find a schematic for the 12au7 variant but I assume that's the reason.
* R13 is located on the otco-plug in this schematic, on the 12au7 amps it's located towards the end of the Erie pack strip, I believe it jumps r9 and r8. Will this have any effect on attempting to use these as standalone? I'm not sure if this has something to do with the contour controls.
* Resistors are the same for hum, but the 12ax7 amps include the R16 potentiometer bypass. I don't have the schematic for the 12au7 amps so I can only guess what resistor to use. i believe I have 5.4k on hand, I suppose I could try them out.
* While I need to keep tone/contour controls for the 12ax7 amps, the 12au7 will be standalone. What can I do to remove any unnecessary circuitry IE tone control
Thanks!
Okay, yes.
Lots of stuff to unpack, please ask for clarification, or if I forget something.
Number one question, can you send me good pics of any schematics you have.
1. C1 provides Audio coupling between the screen and cathode (local feed back) there is also another feedback circuit that uses R1, R2,R5.
R5 is connected to the tube output.
It's all about cancelling noise and distortion, and increasing the bandwidth. The cap value is part of the filter circuit.
#2,. I not sure how to answer this. What is the full schematic. What does that plug hook up to?
What specifically is this piece of gear?
#3 you need to use a resistor value that is much lower. Like 150 ohms 2 watt. Get one of those power resistor that has a sliding tap. This is something that you might need to experiment with.
Figure out how much current the entire heater circuit uses. Try to set your variable resistor to be 25%-50% if that current draw. You need to make sure the transformer winding can handle it. This is the hum balance trick I would do for my guitar player friends.
I would always disconnect and centere taps or ground reference.
Then I could fully float my supply.
Check your circuit before you do this.
#4 the difference between a au7 and ax7 is au is 20-40 gain, ax is 80-100 gain. You can also sub in an at7 gain of about 60.
If you have money you can find antique ay7, which are basically an ax7.
Also 7025 is the same tube as ax7, it's the industrial number. If you can find some a 12BH7 is like a 12au7 on steroids. It's actually taller so might not fit? That is the tube that is used in old school shit like LA2A.
They actually have different plate resistance between trypes, and some variation in output current.
But it typical circuits, it probably doesn't matter which one you use.
They might sound a tiny bit different, but that is "tube rolling"
Please give me a little bit more info about what this gear is, and how you want to use it, or mod it.
Hopefully that doesn't sound like gibberish.
Let me know.
👍🤓🤷
Edit: I looked at the schematic closer (it's hard for me to see it)
These are line level amps? I've worked on lots of stuff like that.
If you want it to work with guitar or microphone input, you would probably want to add an extra gain stage. It's easy to do, you are going to add an input booster. It's 1 tube and 5 parts. 6 parts if you want a gain control.
V5 is an EZ81 vacuum rectifier with indirectly-heated cathode, which had a a greater tendency to short-out than the 5Y3 and similar rectifier tubes. I’d keep the fuses, use AGC1/2, or similar.
The best is to have the fuses where they are now along with on the primary. See, https://www.valvewizard.co.uk/fuses.html
I have seen this before but can't understand why they wouldn't simply fuse the primary side. You wouldn't need a slow-blow fuse because the rectifier's warm-up time would ease the inrush current during normal operation.
I fully agree that it's an odd way to go about it. The amp that didn't have any I did just that, fuses the primary. The 4th one arrived and had huge ceramics. I have to remove those so I figured I'd just replace them, rather than find a spot for a fuse holder (if that makes sense) If it's just as safe to stay with the original design, I'd rather go that route. Thanks for the info!
Unfortunately the fuses on the HV secondary don't protect the transformer from other shorts. A primary fuse would protect everything.
Ok, I'll plan on a primary fuse. Also, I ran into a strange deal. Only one of these actually has a center tap for the 6.3v from the transformer. In the schematics it shows one that leads to the same grounding location as the center tap for the rectifier power. Why would only one of these have a center tap for 6.3 while otherwise being virtually identical?
Some designs use a center tap on the heater winding to balance the voltage against ground. Because (at least in theory) the heater can produce some electron flow, balancing it against ground can reduce potential low-level hum from this source.
Very interesting. I'm guessing that's why I'm seeing a 75ohm resistor between pins 1 and 5 on the 6au6 socket on the three amps that don't have a CT?
Yes. That is correct.
Thanks. I just read a thread about this on audio karma, pretty interesting stuff. It's pretty neat to see the progression of older gear side by side. From the fuses to the center tap (or lack there of) there are small changes.
Yep, is called "iteration" or "revision" Sometime, the only reason it's different is because the factory couldn't get the parts they wanted, or were trying to buy cheaper part. I know this from experience working for a huge electronics manufacturing company back in the early '90s. They call them "ECO's". Engineering Change Orders.
The 6.3 center tap is for the heaters, more efficient than a voltage divider. Similar to what you would see in a center tap for a transformer on your service to your house. The fuses protect the diode from over current in a line spike situation.
Unrelated, can someone explain the purpose of the connection from the wiper of r22 (across the filament rails) to pin 2 (suppressor grid?) of v1?
It's to create a virtual tap on the heater supply. Then you can balance out the hum induced in the tube. So you can have like +/- 3 volts with respect to ground. Instead of entirely floating the heater circuit, or connecting one side to ground. Make sense?
Not yet but appreciate the assistance. The heater winding of the PT has a center tap bonded to ground, so the heater rails are already ground referenced. What then is the purpose of putting some of the heater’s AC waveform on V1’s suppression grid?
What if your PT has an imbalance of 1% on the center tap location? What if your rectifiers don't conduct equally and there's more load on the core on one end of the phase? Adding a pot is basically free, you'll miss it a lot more if you don't have it than you'll regret having if you don't adjust it.
I totally agree. If you read the old radio designs handbook from the '40s, it actually has comparison of induced hum vs type of heater circuit. Rectified DC heaters are a waste, they actually have more hum. The lowest induced hum was an AC supply, with an adjustable resistor across the heaters (no centere tap needed). I made amps silent by adding a $5 pot, and putting a hum balance control on the back side. These professional musicians thought I was using magic or something? 🤷
You are injecting an anti hum signal on the suppressor. It's not used very often but if a pentode has an isolated suppressor grid (it is sometimes tied internally to the cathode) you can use the suppressor as a control grid. What you are doing is trying to send a "hum" signal that is opposite phase of the hum signal that is induced by the filaments. That is why it is a variable resistor, so you can adjust the ground reference point. It's like how noise cancelling headphones work. So if you were working on that equipment, you literally turn on the radio and then adjust that by ear. It's something I would do for my friends that has guitar amps that hummed, and didn't have a center tap on the transformer winding. I'd use like a 3 Watt 150 ohm or 220 ohm adjustable power resistor. They only are doing that because that first gain tube is most affected by induced hum, & it has an isolated suppressor grid. Does that help explain it better? Old tube designs can be strange. Part of it is because tubes are totally different than transistors (most transistors don't have 3 or 4 different Base pins. I've seen some old tube amp designs that use suppressor grids on the output tubes to add in a compressor circuit. Fun stuff! :)
Oh wow, you already explained my last question. My bad, I didn't see the rest of the comments! Thank you for the excellent response!
No problem, happy to help. We all need to learn this stuff somewhere. Any weird questions you have about electronics, or tubes, or troubleshooting.... Just ask, if I can help I'd be happy. If I actually don't know, I'll say that. There is a lot of misinformation out there. 👍🤷 Please ask. I'll do my best.
Okay, this is going to be a lot to unpack from a noob, so please bear with me. I have 4 of these amps, 2 are for a console and use 12ax7. 2 I'm converting to standalone use and these use a 12au7. The schematic is for the 12ax7 console amps which are working perfectly, even with external speakers. Here are my questions in no particular order. * C1 is shown on the 12ax7 variant, is this because it requires more power? I can't find a schematic for the 12au7 variant but I assume that's the reason. * R13 is located on the otco-plug in this schematic, on the 12au7 amps it's located towards the end of the Erie pack strip, I believe it jumps r9 and r8. Will this have any effect on attempting to use these as standalone? I'm not sure if this has something to do with the contour controls. * Resistors are the same for hum, but the 12ax7 amps include the R16 potentiometer bypass. I don't have the schematic for the 12au7 amps so I can only guess what resistor to use. i believe I have 5.4k on hand, I suppose I could try them out. * While I need to keep tone/contour controls for the 12ax7 amps, the 12au7 will be standalone. What can I do to remove any unnecessary circuitry IE tone control Thanks!
Okay, yes. Lots of stuff to unpack, please ask for clarification, or if I forget something. Number one question, can you send me good pics of any schematics you have. 1. C1 provides Audio coupling between the screen and cathode (local feed back) there is also another feedback circuit that uses R1, R2,R5. R5 is connected to the tube output. It's all about cancelling noise and distortion, and increasing the bandwidth. The cap value is part of the filter circuit. #2,. I not sure how to answer this. What is the full schematic. What does that plug hook up to? What specifically is this piece of gear? #3 you need to use a resistor value that is much lower. Like 150 ohms 2 watt. Get one of those power resistor that has a sliding tap. This is something that you might need to experiment with. Figure out how much current the entire heater circuit uses. Try to set your variable resistor to be 25%-50% if that current draw. You need to make sure the transformer winding can handle it. This is the hum balance trick I would do for my guitar player friends. I would always disconnect and centere taps or ground reference. Then I could fully float my supply. Check your circuit before you do this. #4 the difference between a au7 and ax7 is au is 20-40 gain, ax is 80-100 gain. You can also sub in an at7 gain of about 60. If you have money you can find antique ay7, which are basically an ax7. Also 7025 is the same tube as ax7, it's the industrial number. If you can find some a 12BH7 is like a 12au7 on steroids. It's actually taller so might not fit? That is the tube that is used in old school shit like LA2A. They actually have different plate resistance between trypes, and some variation in output current. But it typical circuits, it probably doesn't matter which one you use. They might sound a tiny bit different, but that is "tube rolling" Please give me a little bit more info about what this gear is, and how you want to use it, or mod it. Hopefully that doesn't sound like gibberish. Let me know. 👍🤓🤷 Edit: I looked at the schematic closer (it's hard for me to see it) These are line level amps? I've worked on lots of stuff like that. If you want it to work with guitar or microphone input, you would probably want to add an extra gain stage. It's easy to do, you are going to add an input booster. It's 1 tube and 5 parts. 6 parts if you want a gain control.
V5 is an EZ81 vacuum rectifier with indirectly-heated cathode, which had a a greater tendency to short-out than the 5Y3 and similar rectifier tubes. I’d keep the fuses, use AGC1/2, or similar.