Natima
New Member
Here's the low down... I picked up this amp because I wanted a conversion project as a learning experience, and after watching a couple of YT videos and looking at the schematic of a few old PA amps it looked like a simple enough project to dip my toes into.
Below are the original schematic and an altered one with all my changes marked in red (Well... nearly).
A few things I am really thinking about, and I'm not really sure where to start, are as follows.
1. Converting to passive tone controls.
- Pros and cons?
- Complexity of the mod not worth it?
- Trying to find D-shaft pots to fit the original knobs in the values I need seems difficult.
2. Converting cathodyne to Long Tail PI.
- Much the same... is it worth it?
3. The one change not listed in my modified schematic is the replacement of the 30/30 C19A/B to a 50/50... which caused some arcing on start up of the 5U4GB due to exceeding the spec sheets capacitance values.
Now I have two options, reverse pins 2 & 8 and swap to a GZ34, or buy a 32/32 cap, and replace that.
4. Some small questions.
- Why no bypass cap in V2B cathode? What if I put a 25uf in there?
- Ways of making use of the (now disconnected) Phono pot?
- Mic1 and Mic 2 are identical... seems a little pointless having to identical inputs... is there a good way of putting V1B to better use and differentiating the two channels?
- Is V3A just driving the PI? Providing 0 actual gain?
https://photos.google.com/share/AF1...?key=aHJPS0dRMk9PWWhIalRPSDVNU0tManV2azgxZ19n
Below are the original schematic and an altered one with all my changes marked in red (Well... nearly).
A few things I am really thinking about, and I'm not really sure where to start, are as follows.
1. Converting to passive tone controls.
- Pros and cons?
- Complexity of the mod not worth it?
- Trying to find D-shaft pots to fit the original knobs in the values I need seems difficult.
2. Converting cathodyne to Long Tail PI.
- Much the same... is it worth it?
3. The one change not listed in my modified schematic is the replacement of the 30/30 C19A/B to a 50/50... which caused some arcing on start up of the 5U4GB due to exceeding the spec sheets capacitance values.
Now I have two options, reverse pins 2 & 8 and swap to a GZ34, or buy a 32/32 cap, and replace that.
4. Some small questions.
- Why no bypass cap in V2B cathode? What if I put a 25uf in there?
- Ways of making use of the (now disconnected) Phono pot?
- Mic1 and Mic 2 are identical... seems a little pointless having to identical inputs... is there a good way of putting V1B to better use and differentiating the two channels?
- Is V3A just driving the PI? Providing 0 actual gain?
https://photos.google.com/share/AF1...?key=aHJPS0dRMk9PWWhIalRPSDVNU0tManV2azgxZ19n