kawi10r998cc
Member
- Messages
- 41
Good evening! I'm writing as I have a crazy problem and I cannot find answers anywhere. I finished up a 1987 clone (using MetroAmp's design) about a week ago and when I went to test it out, the voltages all looked good but I could only get bias readings on V4. On V5 I couldn't read plate voltage or bias with the probe (I can read plate voltage if I measure internally on the pins but don't have a 1 ohm resistor on pin 1 so I can't take bias reading without the probe). FYI I use the bias probe that plugs into the tube socket and hooks to a multimeter.
I assumed that there may be a pin connection in the socket that was open so I inserted the bias probe into V5's socket, and began checking for continuity from the socket on the inside of the amp, to the side of the bias probe that the tube plugs into. No open connections. All showed no impedance.
I then verified that all wires were soldered to the proper pins by comparing them to V4 and then to the schematic/layout, but everything was perfect.
I biased V4 to 28 mA (I had 510 VDC on pin 3) and started testing it with an input and it sounded fine so I turned it up to volume level 2 and it fed back quite easily, and if I struck a power cord the tube in V4 would light up very brightly, not red plating, but it would brighten up quite a bit. V5 stayed the same as far as appearance. I find this very odd as V4 was acting as though it was being driven hard but the volume was low. I turned the bias down to about 20 mA and I could crank it up to 10 and no red plating or variations in brightness. The amp sounded fine but would still feedback a bit easier than it should.
Obviously something is up, but I've reached the end of my rope as far as troubleshooting is concerned. The bias voltages seem a bit off to me, but everything tests ok except for V5 with the bias probe.
Here are the voltages I'm getting:
V4 pin 2/7 6.9 VAC across the pins
pin 3 510 VDC
pin 4 509 VDC
pin 5 -47.2 VDC
pin 6 509 VDC
V5 pin 2/7 6.9 VAC across the pins
pin 3 510 VDC
pin 4 509 VDC
pin 5 -47.7 VDC
pin 6 509 VDC
*Wall voltage 125 VAC
*Power transformer 752 VAC across both leads, 378 VAC each lead to ground
*Bridge rectifier out 492 VDC in standby, 508 VDC when on.
*-69.7 VDC just after the diode in the bias circuit, 378 VAC before.
The only issue I had was when I first started it up the HT fuse popped immediately. I replaced it and powered it up again as I couldn't find any issues (was using new tubes). I went to check bias, and I had crazy readings. I couldn't get them to hold steady but V4 red plated almost immediately, so bad that the plate has a visible dark place on it. I immediately shut it down and discharged all the caps and found that I had accidentally wired the bias voltage input to the DC out of the rectifier instead of one of the AC legs.
After fixing the problem all the voltages appeared basically normal except for VDC in standby seems higher than it should. I'm used to seeing more like 350-400 there in standby and jumping up to 450-500 when on.
The tubes I am using are JJ's 6CA7's as I'd never used them before and wanted to try them out. After V4 tube red plated I removed the tubes and put them in a JTM 45 I built and they played fine. Sounded great, I think that I like them more than the EL34's in the amp normally, but that's another topic.
After putting the 6CA7's in the JTM 45 I decided to try out the EL34's in the 1987 since I know those tubes are similar but different, and it may make a difference. It made a difference alright, the MAINS fuse blew a second after I took it out of standby and into "on". I checked for shorts, put in a new fuse, tried again. Same thing. Put the 6CA7's back in, no MAINS fuse blowing, amp played fine but still exhibited the symptoms I listed at the beginning of the post.
I ordered in brand new 6CA7's just in case, but the same story with them as far as operation and not reading bias voltage on V5 goes. All my voltages and bias readings remain steady...no fluctuations to be found.
Any help would be greatly appreciated, this is crazy, I'm completely lost here...
I assumed that there may be a pin connection in the socket that was open so I inserted the bias probe into V5's socket, and began checking for continuity from the socket on the inside of the amp, to the side of the bias probe that the tube plugs into. No open connections. All showed no impedance.
I then verified that all wires were soldered to the proper pins by comparing them to V4 and then to the schematic/layout, but everything was perfect.
I biased V4 to 28 mA (I had 510 VDC on pin 3) and started testing it with an input and it sounded fine so I turned it up to volume level 2 and it fed back quite easily, and if I struck a power cord the tube in V4 would light up very brightly, not red plating, but it would brighten up quite a bit. V5 stayed the same as far as appearance. I find this very odd as V4 was acting as though it was being driven hard but the volume was low. I turned the bias down to about 20 mA and I could crank it up to 10 and no red plating or variations in brightness. The amp sounded fine but would still feedback a bit easier than it should.
Obviously something is up, but I've reached the end of my rope as far as troubleshooting is concerned. The bias voltages seem a bit off to me, but everything tests ok except for V5 with the bias probe.
Here are the voltages I'm getting:
V4 pin 2/7 6.9 VAC across the pins
pin 3 510 VDC
pin 4 509 VDC
pin 5 -47.2 VDC
pin 6 509 VDC
V5 pin 2/7 6.9 VAC across the pins
pin 3 510 VDC
pin 4 509 VDC
pin 5 -47.7 VDC
pin 6 509 VDC
*Wall voltage 125 VAC
*Power transformer 752 VAC across both leads, 378 VAC each lead to ground
*Bridge rectifier out 492 VDC in standby, 508 VDC when on.
*-69.7 VDC just after the diode in the bias circuit, 378 VAC before.
The only issue I had was when I first started it up the HT fuse popped immediately. I replaced it and powered it up again as I couldn't find any issues (was using new tubes). I went to check bias, and I had crazy readings. I couldn't get them to hold steady but V4 red plated almost immediately, so bad that the plate has a visible dark place on it. I immediately shut it down and discharged all the caps and found that I had accidentally wired the bias voltage input to the DC out of the rectifier instead of one of the AC legs.
After fixing the problem all the voltages appeared basically normal except for VDC in standby seems higher than it should. I'm used to seeing more like 350-400 there in standby and jumping up to 450-500 when on.
The tubes I am using are JJ's 6CA7's as I'd never used them before and wanted to try them out. After V4 tube red plated I removed the tubes and put them in a JTM 45 I built and they played fine. Sounded great, I think that I like them more than the EL34's in the amp normally, but that's another topic.
After putting the 6CA7's in the JTM 45 I decided to try out the EL34's in the 1987 since I know those tubes are similar but different, and it may make a difference. It made a difference alright, the MAINS fuse blew a second after I took it out of standby and into "on". I checked for shorts, put in a new fuse, tried again. Same thing. Put the 6CA7's back in, no MAINS fuse blowing, amp played fine but still exhibited the symptoms I listed at the beginning of the post.
I ordered in brand new 6CA7's just in case, but the same story with them as far as operation and not reading bias voltage on V5 goes. All my voltages and bias readings remain steady...no fluctuations to be found.
Any help would be greatly appreciated, this is crazy, I'm completely lost here...



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