I acquired an Oliver G-300 amp head, quite a few months back, and stored it away. A couple of weeks ago, when I had some days off, I rode over to Durham (NC) to Church, Tuesday night (19Aug) for a pot-luck supper. On Sundays, I carry show-and-tell to my friend Don, who is our music leader and loves to see my amp projects.
Anyway, I pulled this amp out of storage to carry for him to see that Tuesday evening. Of course, after the meal, he sat with this very heavy amp head on his knees, just turning it around while both of us studied it closely. I had not powered it up, so I could tell him anything about it.
Told you all that, to tell you this. It didn't go back into storage. The next day (Wednesday), I pulled the tubes and tested them. It had two modern JJ 7027A's, so that tells me that someone has used, or attempted to use it in the recent past. One of the three 6EU7's was bad, so I replace it with a known good one out of my stash. The 6AN8A was bad, so I found one in an old tube caddy, that I had not inventoried. The 6DR7 was very good.
Warmed it up on the lamp limiter, going through several stages of progressively higher wattage bulbs, until I reached 100W. That's when I switch over to full wall voltage/current. Not having any literature on this amp, but armed with the schematics of the other models offered, I was testing it with an 8 ohm impedance speaker.
It sounded great. Beautiful Reverb and Tremolo, testing with a little Supro guitar. So, I shut it down, put my bias test adapters in. Note here: I had already removed the chassis from the cabinet, but kept them close together, to be able to connect the Reverb tank. Not only did I want to check the bias, but I wanted to get some voltage readings and create a chart for my records. So, with the chassis inverted, and everything connected. I made another quick check to verify that the amp was still working ok. It was, so I began the process of taking measurements. This lasted for about 30 minutes. When I finished, I shut it down. By the way. The bias looked good. With a plate voltage of 518vDC, a desired bias would be 47+mA. One tube as right at that, and the other was something like 44mA. I was satisfied.
This is where the fun stopped. After taking my readings and shutting it down to disconnect the bias tester, I fired it up one more time to strum a few chords through it, before putting it back in the cab. No sound. I shut it down and checked the speaker. A bit over 6 ohms DC. So, I put it back and fired it up. Checked all my voltages and the bias. Everything checked the same as it was when it worked.
I made a quick listener with a powered headphones and a .022uf cap as my probe (rubber gloves on my hands). I had a good strong signal (guitar in a chair where I could pluck a string) after the first gain stage on each channel. A bit weaker where all combines at the grid of the 6AN8A. Nothing past that.
By the way, I have an untested PA-100X that I've had quite a while. I borrowed this tube from that one, tested it, and installed. Now, I have a weak sound out of the speaker. Very weak at full volume, from either channel. So for now, I'm thinking I'm losing it at this tube.
That was Wednesday (20Aug). I've spent the last week hand-tracing the circuit and making a schematic, as well as a crude layout of both boards. Finished that Wednesday of this week (27Aug). Both are 99% accurate. I will say 100% when I am able to troubleshoot the amp properly, and verify my work, as I go through it.
Unfortunately, I am working long hours, starting today (Fri), so troubleshooting is on hold. I am attaching my work, so far. When I am at a point (maybe the end of next week) where I can troubleshoot again, I will post my progress.
In the meantime, please look it over and comment. Note corrections for me to make, if you see errors. Thanks.
Jack
Anyway, I pulled this amp out of storage to carry for him to see that Tuesday evening. Of course, after the meal, he sat with this very heavy amp head on his knees, just turning it around while both of us studied it closely. I had not powered it up, so I could tell him anything about it.
Told you all that, to tell you this. It didn't go back into storage. The next day (Wednesday), I pulled the tubes and tested them. It had two modern JJ 7027A's, so that tells me that someone has used, or attempted to use it in the recent past. One of the three 6EU7's was bad, so I replace it with a known good one out of my stash. The 6AN8A was bad, so I found one in an old tube caddy, that I had not inventoried. The 6DR7 was very good.
Warmed it up on the lamp limiter, going through several stages of progressively higher wattage bulbs, until I reached 100W. That's when I switch over to full wall voltage/current. Not having any literature on this amp, but armed with the schematics of the other models offered, I was testing it with an 8 ohm impedance speaker.
It sounded great. Beautiful Reverb and Tremolo, testing with a little Supro guitar. So, I shut it down, put my bias test adapters in. Note here: I had already removed the chassis from the cabinet, but kept them close together, to be able to connect the Reverb tank. Not only did I want to check the bias, but I wanted to get some voltage readings and create a chart for my records. So, with the chassis inverted, and everything connected. I made another quick check to verify that the amp was still working ok. It was, so I began the process of taking measurements. This lasted for about 30 minutes. When I finished, I shut it down. By the way. The bias looked good. With a plate voltage of 518vDC, a desired bias would be 47+mA. One tube as right at that, and the other was something like 44mA. I was satisfied.
This is where the fun stopped. After taking my readings and shutting it down to disconnect the bias tester, I fired it up one more time to strum a few chords through it, before putting it back in the cab. No sound. I shut it down and checked the speaker. A bit over 6 ohms DC. So, I put it back and fired it up. Checked all my voltages and the bias. Everything checked the same as it was when it worked.
I made a quick listener with a powered headphones and a .022uf cap as my probe (rubber gloves on my hands). I had a good strong signal (guitar in a chair where I could pluck a string) after the first gain stage on each channel. A bit weaker where all combines at the grid of the 6AN8A. Nothing past that.
By the way, I have an untested PA-100X that I've had quite a while. I borrowed this tube from that one, tested it, and installed. Now, I have a weak sound out of the speaker. Very weak at full volume, from either channel. So for now, I'm thinking I'm losing it at this tube.
That was Wednesday (20Aug). I've spent the last week hand-tracing the circuit and making a schematic, as well as a crude layout of both boards. Finished that Wednesday of this week (27Aug). Both are 99% accurate. I will say 100% when I am able to troubleshoot the amp properly, and verify my work, as I go through it.
Unfortunately, I am working long hours, starting today (Fri), so troubleshooting is on hold. I am attaching my work, so far. When I am at a point (maybe the end of next week) where I can troubleshoot again, I will post my progress.
In the meantime, please look it over and comment. Note corrections for me to make, if you see errors. Thanks.
Jack
