Hello,
I have a Magnatone guitar amp that has no power transformer that I would like to make safe to use. I've attached the schematic. Could you modify it so I can make this amp safe? I'm not familiar with adding an isolation transformer. Also, there is a 50/50 150V electrolytic. Can I use the 100/50 150V I have on hand or does it need to be a closer match? Thank you
I have a Magnatone guitar amp that has no power transformer that I would like to make safe to use. I've attached the schematic. Could you modify it so I can make this amp safe? I'm not familiar with adding an isolation transformer. Also, there is a 50/50 150V electrolytic. Can I use the 100/50 150V I have on hand or does it need to be a closer match? Thank you
Last edited by a moderator:
This would work as an isolation transformer.
https://www.amazon.com/Isolation-Transformer-Quick-Pack-115V/dp/B005T7TDKW
https://www.amazon.com/Isolation-Transformer-Quick-Pack-115V/dp/B005T7TDKW
So after surfing some forums I think I have the idea. I would connect the power cord leads to the primary of the iso transformer and the secondary would be connected to where the original power cord was and as I am going to ad a grounded plug, connect the ground to the chassis. I found this Triad isolation transformer for a few bucks less. Is this correct? Thank you
Okay, thanks. My amp is a little different from the schematic upon further inspection. It's using a 35W4 instead of a 35Z5. I noticed there are 7 pins when checking for the cathode and checked the tube complement. Additionally, there is a 3.9K resistor across the cap and not a 380R. Should I leave 3.9K or change it to 380R. It all looked factory fresh before I started working on it. Thanks.
The 35W4 costs about 1/3 of a 35Z5, so that is probably why it was substituted. It appears the heater current draw is similar, so the heater string should be fine. However, the 35W4 has an inverse voltage maximum of 360V vs 600V for the 35Z5. 360 V is barely enough for your application, so to increase the safety margin I suggest that you add a IN4007 diode in series and right before the 35W4 rectifier anode. (This would be similar to the well known “yellow-sheet mod” (https://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-yellow-sheet-diode-mod)
I am not familiar with this amplifier, but my guess is someone misread the resistor color bands and installed a 3900 Ohm resister in stead of the specified 380 Ohm. This will cause large drop in the high voltage supply of ~100 V or more. Unless this is a known “correction“ of the amp’s schematic, I would change it back to the specified 380 Ohm, 1 watt.
I am not familiar with this amplifier, but my guess is someone misread the resistor color bands and installed a 3900 Ohm resister in stead of the specified 380 Ohm. This will cause large drop in the high voltage supply of ~100 V or more. Unless this is a known “correction“ of the amp’s schematic, I would change it back to the specified 380 Ohm, 1 watt.
Last edited:
Relocated to the instruments & amps forum per forum policy.
I think that power supply resistor was actually meant to be 3800 ohms based on what I see on the schematic, so a 3.9K seems appropriate. That resistor is providing B+ to the screen grid of the 50L6 and plates of the 12AX7A so unlikely more than 5mA or so full tilt? The plate supply for the output tube is coming off of the first section of the pi-filter. Likely the 3.9K is required to get the ripple down to an acceptable level with that 50uF cap.
The 1N4007 diode add is an excellent idea.
The 1N4007 diode add is an excellent idea.
Thanks Kevin. Kevin is correct regarding the resistor. I did not notice that the transformer HV is tapped before the resistor and upon closer inspection it looks like the schematic actually shows 3800 rather than 380 O(hm).
I was getting ready to solder this and I thought if current flows through the tube- cathode to anode and through the diode- anode to cathode, could you explain why I don't connect it anode to anode to keep the current flowing? Thank you
Good to think things through before mistakes are made! But in this case all is well.
The banded (marked) end of the In4007 is the cathode, which should be connected to the rectifier anode at pin 5. So if you use electron current flow convention (as opposed to conventional current flow which is in the opposite direction) you have electrons flowing from tube rectifier cathode to anode and from tube anode to the cathode of your SS diode towards the anode and out to the (new isolation) transformer on the pull cycle of AC. An anode to anode connection would block electron flow in both directions (if I understood the question correctly)
The banded (marked) end of the In4007 is the cathode, which should be connected to the rectifier anode at pin 5. So if you use electron current flow convention (as opposed to conventional current flow which is in the opposite direction) you have electrons flowing from tube rectifier cathode to anode and from tube anode to the cathode of your SS diode towards the anode and out to the (new isolation) transformer on the pull cycle of AC. An anode to anode connection would block electron flow in both directions (if I understood the question correctly)
Okay. Thanks for clarifying. If this was other than a tube amp, would we connect it positive to positive? Thank you