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Hello all. I am in the process of working on my1974 (maybe a 1975) 100 w Twin Silverface with orange frame JBL D120’s. The amp was pretty much functional as is with some minor (I hope) symptoms I’ve read about in other posts that where found to be related to old caps. All my caps appear to be original, the amp was in storage in my home for many years.I was the original owner way back when but sold it and then re purchased it some years ago. I don’t know what the 2nd owner did with it, other than adding the 3 prong ac cord (which was left in the 3 way ground switch circuit) so I don’t know the total history.
My original goal was a recap, tubes and black face conversion. I have a tech background in the printing industry but am a novice with regards to tube equipment. After reading many posts on this site and the ampage archeives, (information overload perhaps) I am no longer so gung ho on doing everything at once. I have decided to go the more cautious route of first doing all the electrolytic caps, tubes and installing a new 3 prong ac cord (busted off ground prong) I plan on taking the ground switch out of the circuit. While I am in there I will check for obvious bad resistors as well (it seems that the ones in the dog house are known to fail due to high temps). I have a good Digital Meter and an old O scope that may or may not work.
After the above work is done, I will test the amp and go from there. Any suggestions are appreciated. I do have several questions regarding the next steps, any input is greatly appreciated:
1. The amp has the original blue non polarized Mallory caps (coupling I believe). I may attempt to leave them in for now. I will research past threads on this subject and also try to find some basic cap tests I can do with a DMM. I don’t have a cap tester. Any thoughts on the old blue Mallorys?
2. After reading many posts on the subject, I am worried about several issues( i.e. parasitic oscillation, poor grounding etc) with the black face conversion. I read some previous posts where guys have gone to great measures to resolve these issues. How common are these problems? Are there varying levels to doing the mod? Maybe something less than reproducing the entire blackface circuit?
3. I have found a blackface kit offered by Kendrick Amps for about $35. Any opinions on this kit? I spoke to a tech there and he indicates that I will be able to properly bias my power tubes after installing the kit. From looking at the kit it looks like it is just some resistors and maybe one cap.I did order the book “A Desktop Reference of Hip Vintage Guitar Amps” which has a chapter on this conversion. I found out later that Kendrick includes this chapter with the kit. Oh well, I’m sure the rest of the book is interesting as well.
4. Any recommendations for where to purchase tubes that are well matched? I am considering Hoffman amps for this. They claim to burn them in (or something like that) for 24 hrs before matching. What brands are good matches for my Twin without breaking the bank?
I would like to again thank all the great contributors on this site. It is uncommon to see such generosity in sharing knowedge that had to be hard to come by. Any thoughts are greatly appreciated.
Thanks again, Pete
My original goal was a recap, tubes and black face conversion. I have a tech background in the printing industry but am a novice with regards to tube equipment. After reading many posts on this site and the ampage archeives, (information overload perhaps) I am no longer so gung ho on doing everything at once. I have decided to go the more cautious route of first doing all the electrolytic caps, tubes and installing a new 3 prong ac cord (busted off ground prong) I plan on taking the ground switch out of the circuit. While I am in there I will check for obvious bad resistors as well (it seems that the ones in the dog house are known to fail due to high temps). I have a good Digital Meter and an old O scope that may or may not work.
After the above work is done, I will test the amp and go from there. Any suggestions are appreciated. I do have several questions regarding the next steps, any input is greatly appreciated:
1. The amp has the original blue non polarized Mallory caps (coupling I believe). I may attempt to leave them in for now. I will research past threads on this subject and also try to find some basic cap tests I can do with a DMM. I don’t have a cap tester. Any thoughts on the old blue Mallorys?
2. After reading many posts on the subject, I am worried about several issues( i.e. parasitic oscillation, poor grounding etc) with the black face conversion. I read some previous posts where guys have gone to great measures to resolve these issues. How common are these problems? Are there varying levels to doing the mod? Maybe something less than reproducing the entire blackface circuit?
3. I have found a blackface kit offered by Kendrick Amps for about $35. Any opinions on this kit? I spoke to a tech there and he indicates that I will be able to properly bias my power tubes after installing the kit. From looking at the kit it looks like it is just some resistors and maybe one cap.I did order the book “A Desktop Reference of Hip Vintage Guitar Amps” which has a chapter on this conversion. I found out later that Kendrick includes this chapter with the kit. Oh well, I’m sure the rest of the book is interesting as well.
4. Any recommendations for where to purchase tubes that are well matched? I am considering Hoffman amps for this. They claim to burn them in (or something like that) for 24 hrs before matching. What brands are good matches for my Twin without breaking the bank?
I would like to again thank all the great contributors on this site. It is uncommon to see such generosity in sharing knowedge that had to be hard to come by. Any thoughts are greatly appreciated.
Thanks again, Pete
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