No bias reading on V5, Marshall 1987 clone.

[kawi10r998cc]

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...

 

[J M Fahey]

I finished up a 1987 clone (using MetroAmp's design) about a week ago

Since you were savvy enough to build an amp, go the extra mile (although it's actually the extra inch ) and add the proper 1 ohm cathode resistors, and measure properly.

Bias probes are just a convenience to avoid opening an amp chassis or soldering ... you are way beyond that

EDIT: reread your post, you are confusing us too, you contradict yourself or are inconsistent, please rewrite your question:

No open connections.

Please state the value you read.
Neither 1 ohm nor 1 M are "open" yet mean very different things.

All showed no impedance.

Meaningless, everything has some value of impedance/resistance, post the numbers.

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.

Junk that unreliable probe and measure straight at the tube sockets.

the tube in V4 would light up very brightly, not red plating, but it would brighten up quite a bit.

What colour does it "light up"?
Orange? .... blue? ..... white? .....

V5 stayed the same as far as appearance.

Probably not getting signal or power or both.
Bent open socket contacts?

I find this very odd as V4 was acting as though it was being driven hard but the volume was low.

It may be working alone, without V5 help.
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.

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

Then you ARE getting bias voltage to the tube socket.

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.

Ouch !!!!!!

There's some comments out there about JJ having different pin diameter than other tubes, so they might make poor contact on worn/stretched sockets or ruin them for other tubes.
Your BIG suspect (in my view) is V5 socket, I'd put a new one there, then add necessary 1 ohm cathode resistors and measure straight inside the amp.

It will probably work now, post results either way.

probably not an issue if first tubes in a new socket

 

[Rogue]

You can use this method to check your bias current in each tube without a bias probe....

http://www.aikenamps.com/index.php/the-last-word-on-biasing

• The plate current can also be measured by first measuring the resistance across each side of the output transformer primary (it will usually be different on each side) with the power off. Make a note of the resistance on each side, and then, with the amplifier on, measure the DC voltage drop across each side of the output transformer. Divide this number by the previously measured resistance, and you end up with the plate current for the tubes on that side. Again, if there is more than one tube on each side, you must divide the total current by the number of tubes. This method is extremely accurate, and much safer than the shunt current measurement method, because a slip of the probe won't short anything out due to the high resistance of the voltage measurement setting on the meter compared to the very low resistance of the current measurement setting. You can also make a safer measurement by clipping the negative side of the voltmeter on ground, and measuring the center-tap voltage of the output transformer and the voltage at the plate of each output tube. Subtract the plate voltage from the center-tap voltage and you have the voltage drop across each side, and can then use this to calculate the current in each tube, again dividing by the number of tubes on each side.

For the feedback.....I'm just throwing this out there, hopefully someone else will chime in.....you may be getting positive instead of negative feedback. I'm not sure how to measure this, but you can find out by swapping the wires to the grid on the power tubes and firing it up. If it's worse...undo. Probably easiest to do on the board at the 220k resistors.

 

[kawi10r998cc]

You can use this method to check your bias current in each tube without a bias probe....

http://www.aikenamps.com/index.php/the-last-word-on-biasing




For the feedback.....I'm just throwing this out there, hopefully someone else will chime in.....you may be getting positive instead of negative feedback. I'm not sure how to measure this, but you can find out by swapping the wires to the grid on the power tubes and firing it up. If it's worse...undo. Probably easiest to do on the board at the 220k resistors.

Thanks for the info! I actually found that exact page before...I've just used the bias probe b/c of the convenience but I'll be investing in some 1 ohm resistors to do it the "right" way.

As far as the positive vs negative feedback...this is the first I'm hearing of it...but I'm still a newbie despite building four amps, so I will definitely look into it. And you're right, it will be a minor affair to switch the wires around and the 220k resistors. I may just do it and see if it makes a difference. I have noticed that on amps that are biased super hot that they feedback really easy, and given my weird bias issue I kind of assumed that whatever is causing my readings to be off is likely the culprit in the feedback as well b/c it appears to be driving one tube super hard and the other far less, if at all. I'll switch them out though and post the results soon.

 

[Blue Strat]

Sorry I skipped over some of the long posts, but are pins 1 and 8 on both power tube sockets connected to ground...solidly?

 

[kawi10r998cc]

1.) Since you were savvy enough to build an amp, go the extra mile (although it's actually the extra inch ) and add the proper 1 ohm cathode resistors, and measure properly.

Bias probes are just a convenience to avoid opening an amp chassis or soldering ... you are way beyond that

EDIT: reread your post, you are confusing us too, you contradict yourself or are inconsistent, please rewrite your question:


2.) Please state the value you read.
Neither 1 ohm nor 1 M are "open" yet mean very different things.

Meaningless, everything has some value of impedance/resistance, post the numbers.


3.) Junk that unreliable probe and measure straight at the tube sockets.

4.) What colour does it "light up"?
Orange? .... blue? ..... white? .....

5.) Probably not getting signal or power or both.
Bent open socket contacts?

6.) It may be working alone, without V5 help.
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.


7.)Then you ARE getting bias voltage to the tube socket.


8.) Ouch !!!!!!

There's some comments out there about JJ having different pin diameter than other tubes, so they might make poor contact on worn/stretched sockets or ruin them for other tubes.
Your BIG suspect (in my view) is V5 socket, I'd put a new one there, then add necessary 1 ohm cathode resistors and measure straight inside the amp.

It will probably work now, post results either way.

probably not an issue if first tubes in a new socket

Sorry it's taken me so long to respond, I got married the day after I posted the question and am on the honeymoon now. Poor timing? Yes...unfortunately that didn't occur to me at the time ha ha! With that said, I'm replying on my phone so formatting isn't the easiest, so I'm just going to number my responses to your statements/questions.

1.) I decided to put in the 1 ohm resistors before I posted this just to take any possibility that the bias probe is part of the problem with taking readings on V5, but no one locally had them, so I'll have to order them online. With that said, I've tested it on three other amps and it worked flawlessly on every socket except V5 on this amp, it's likely not the issue, but I'll be adding the resistor regardless asap.

2.) Sorry if that statement wasn't clear. It made sense to me but you know how that goes! Essentially what I was saying was that I had continuity on all pins when measuring from the inside of the amplifier on the tube socket solder tabs, to the bias probe pins on the side that you plug the tube into after plugging the probe into the amp. I did that so I'd know that I have good connections when I plug a tube into the socket. All connections in tube socket where solid. All impedance readings were 0 or 0.1 ohms, as they should be since it's a short run between the two sides I was testing.

3.) Will do!

4.) Tube would have the standard orange glow in "standby" that is caused by the heaters. This is not what I am referring to here as odd. The glow I'm speaking of only occurred when the amp was "on" and an input was being put through the amp. Essentially when I took the amp off "standby" and turned it "on" the tubes would glow a faint blue (I've seen many tubes do this that were fully functional, many of them were new tubes in fact, always Octals though, so I know these tubes are fine). After playing a guitar through the amp, the faint blue glow would get much much brighter, but only on V4, and it would only glow brighter when an input was present. Idling in "on" position it was a steady glow. I've seen tons of tubes that fluctuate in this blue glow when the amp is being played on, but it normally needs to be dark to see, or the fluctuations are faint. This was glowing very brightly when it was being played through. I tried six different tubes and all glowed like this when playing almost equally so I know it's not the tubes themselves causing this. It must be in the amp. I assumed this means that only that tube is getting signal, which would explain why the other tube isn't changing at all in appearance. Unfortunately I don't have an oscope at my disposal so I cannot check for signal.

5.) It is getting power. I don't know if it is getting signal unfortunately as I don't have an oscope. Is there another way to check for signal? I considered pulling V4 tube out to see if V5 was outputting the signal, but I know that causes an issue with impedance mismatch in the output transformer, so I wasn't sure if I should try this...of course if I did it it would be for a very brief time and at a very low volume. Would this be ok? Although signal or no signal that still wouldn't explain why I can't get any bias readings on V5...

6.) Agreed. This was addressed in question 5 so for the sake of time I'll just refer back to that.

7.) Yes sir! That is what is so confusing. I can read all the proper voltages on both V4 and V5 on the inside of the amp, so I "should" be getting my bias reading on both power tubes...but I'm only able to read it on V4 for some reason. I'm almost convinced that the same issue that is preventing me from reading bias mA on the tube socket is the same thing causing the other issues I'm having.

8.) I agree with replacing V5 to at least eliminate that as an issue...I'll do that asap, even though it seems to be perfectly fine...wouldn't be the first time I thought something was ok when it wasn't!

It is indeed a new tube socket and a new tube.

 

[kawi10r998cc]

Sorry I skipped over some of the long posts, but are pins 1 and 8 on both power tube sockets connected to ground...solidly?

Sadly, yes.

 

[kawi10r998cc]

Update: I pulled V5 and played an input through the amp (very very low volume and very briefly), and it sounded fine.

I then replaced V5, and removed V4 and got no output in any channel. So it appears I'm getting no signal on V5.

I do not have an oscope...any other way to check for signal to see where I am losing it?

Also, would having no signal input into a tube cause the issue I'm having with getting NO bias reading on V5?

 

[zenas]

Check the voltage at each power tube pin. Also a signal tracer would be handy here. Pretty much just a capacitor on a stick hooked to a small amp. I use some cheap old powered computer speakers and a cheap radio for the input signal.

Got a couple of ocilloscopes and signal generators I should start using.

 

[kawi10r998cc]

Update #2: Well, I changed V5's socket and that was it. Plays great now! Internal socket connection for the cathode was jacked up, which would explain all the funny issues above with no bias reading...all the voltages were there, but there was no reference (ground) on pin 8 INSIDE the socket itself, it checked ok when taking readings inside the amp, but the ground never made it into the tube from the socket apparently.

For future reference for anyone that runs across this post I'll summarize briefly...if you try to take bias readings and have absolutely no mA readings (mA will show up as mV when measuring across the 1 ohm resistor but are still "technically" mA of current), but you have your negative bias voltage on pin 5, and you have your positive voltages on pins 3, 4, and 6...then your ground to pin 8 is not connected somewhere! Be it at the actual solder tab, the place that the ground wire terminates at the chassis, or inside the socket. The only way to have no bias readings if you have all your voltages is the lack of ground to the cathode (pin 8).

 

[J M Fahey]

Glad you found it

By the way, it was suggested in the very first answer (post #2):

V5 stayed the same as far as appearance.
Probably not getting signal or power or both.
Bent open socket contacts?
.............................
Your BIG suspect (in my view) is V5 socket, I'd put a new one there, then add necessary 1 ohm cathode resistors and measure straight inside the amp.

 

[kawi10r998cc]

Glad you found it

By the way, it was suggested in the very first answer (post #2):

You are correct sir! You hit the nail on the head! You and Blue Strat were both right actually, just different kinds of right ha ha! Thank you both for your help!

I would've replaced it from the get go but didn't have a spare. Odd way it was malfunctioning though...the ground wire at the solder tabs checked out to be solidly connected, and if I used my meters probes to verify that there was continuity from pin 8 on the socket internally to externally that also checked ok. It just didn't make proper contact when a tube was inserted apparently. Strange, but glad it's done!

Now my bright channel sounds a bit thin and tinny, normal channel sounds great though, so I suppose I've still got some work ahead of me. Im going to recheck all the voltages and change tubes to see if I can narrow down or eliminate the issue like that...if not I'll be back for more opinions ha ha!

Thank you again for your help!