mrgamptech

daddymack, I contacted Ampeg support and the person I spoke to said the only schematics and service manuals they have for legacy products are the ones listed on https://,ampeg.com/support/files, which doesn't even have the two schematics I already have, Rev B and Rev D. Too bad. I did find some dating info and the serial numbers from that era do not have enough info to determine the year of manufacture. I was able to determine that it is from between 1968 - 1972 based on the blue control panel graphics. At least I now know that the power transformer is not original. Oh well, the search goes on....

daddymack, This is what I tried to send from my phone, but it did not work. I assume the amp is US made and I was given it by a friend. I am not sure what year it was built because there is no date in or on the amp and it does not have a schematic anywhere. The transformer is not a modification, nor is the bias circuit since the circuit boards have not been altered. I attached the schematic showing the transformer with just the black wires on the 120V side to a previous post. The part numbers on my amp's circuit boards match the numbers on the schematic you posted, but the transformer is obviously different. I cannot find a schematic with the bias circuit in my amp, but it is original. Since I can't find the exact schematic, I am not sure about these voltages I'm measuring. I do appreciate your response and help. That is my first response to your question. I just found it here. Using your newest response as a guide, here is what I've found: Serial number - 104356 There is what's left of a paper with the tube layout and numbers and the power transformer and output transformer numbers glued to the bottom of the enclosure. The P.T. number is 8930077, which is what is on the Rev. B schematic, so it probably was replaced with the P.T. used in Rev. D. The O.T. number is 8950036, which is not on the Rev. B schematic or the Rev. D schematic. So it must be from another revision, as you suggested. As far as the P.T., I guess the tech who did the repair could not find the 8930077 or did not try. Who knows? There are a couple of mods. Someone added a fan, but it is currently disconnected. Someone also added a 47 pF mica cap between pins 7 and 8 on V1 and V2, probably to reduce high frequency noise. I've read about that in one of the London Power books and it should not affect the plate voltages in any way. Just the one 4 amp fuse, like Rev. B, but no foot switch for the reverb. It does say Linden N.J. Pat. Pending on the plate where the serial number is stamped, but there is no revision number. Thanks again for all your help. I will try to contact Ampeg as you suggested. Best wishes!

Looking more closely at the bottom right of this schematic, it seems to be from May 5th, 1977, so maybe it is a later version of the VT-40 and my amp was made sometime during a transition between versions. I don't know, unfortunately.

daddymack, I don't see the response I just sent you using my phone. I hope it shows up so I don't have to type it again. Your question makes it clear to me that I should attach the schematic I have that has the power transformer I wrote about. If my first answers to your posting do not show up, I will send them again. You will see the transformer on this schematic and if you compare the part numbers on this one to the one you posted, you will see that they do not match.

Greetings, I have a VT-40 I've been resurrecting and have run into some puzzling issues. I hope this is the right forum. First, I cannot find a schematic that completely matches its circuit. It has the older power transformer with just the single 120V input coil ( just the black wires, no blue wires), but the schematics I've found with that transformer have different part numbering from what's in my amp. The amp does not have a schematic anywhere inside the enclosure, so no help there. Second, the schematics I've found with the right numbering have the newer power transformer with both the black and blue input wires. Third, NONE of the schematics have my amp's bias circuit. They all have two 10 mfd / 100V caps connected at the negative ends by a 10K resistor. Mine has one 100 mfd / 100v cap. So if anyone has the schematic with this bias circuit, please share it with me. I have replaced all the electrolytics and some bad resistors and have it up and running, but I have some strange plate voltage issues on most of the preamp tubes. For example, on V203 all the schematics I've seen show 205V on pin1. Mine shows 148V on all my DMMs. The supply voltage is 389V vs. 354V at that point so I should be seeing a higher voltage, not lower. I've tested all the surrounding parts, which were fine, but I replaced them anyway just in case, and still no difference. The cathode is at 0.83V, so that's not the issue. On V1, V2, and V4 I'm seeing 205V on pin1, but only 86V on pin6. Again, all the cathode voltages are correct and the parts are fine. Really strange! I've repaired my B-15 and a couple of SVT3 Pros and have never seen anything like this, but this is the first VT-40. All the other tube amps I've repaired have been Fenders. So this leads to my main concern. The VT-40 starts distorting a much lower volumes that I expected. Ampeg bass amps are really clean, like my own B-15. With my Strat on full volume, the VT-40 starts distorting at about ten o'clock, unless I turn my Strat way down, and goes into full fuzz after that. Is this normal for these amps? Sorry for the length, but I wanted to give all the details and concerns.