Control Grid Reading On EL34.

[Gary_S]

This is another newbie question. I'm trying to work out what's at the root of this noise and problem i'm having with this 4010.

Anyway i've been taking meter readings and from what i've learnt so far there should be no VDC on the control grids of the power tubes. On V4 i have 0.3mv so virtually nothing but on V5 i have 128VDC. This is the tube that red plated before. It red plated and i've been getting terrible background interference and noise.

On these coupling caps in this amp should i be able to read any VDC on the grid side of the cap when the cap's connected up?  i know when it's disconnected it should be zero or very low, but if it's connected up and possibly being influenced by other voltages in the circuit should it show a DC voltage or not?

I'm taking it that this is the root of the noise i'm getting from this amp that 128 V from one side of the PI getting onto the grid of V5?

As far as taking meter readings is it true that you can't take a reliable reading from the grid of the preamp tubes unless you have a special type of meter? I've just been reading a bit in Aspen Pittman's book about it and someone in there said you can't take a reliable reading from those grid pins unless you have a certain kind of MM.

[HotBluePlates]

Cans of worms. Multiple...

What d.c. voltage should be present at the grids of output tubes depends on how they're being biased. If they're cathode biased, most commonly there will be 0vdc when your meter's red lead is on the grid and the black lead is at ground. If the tubes are fixed-biased, the grids are most commonly at some negative dc voltage with the same arrangement of meter leads.

Except in rare cases (and not your amp), if you have positive voltage on the output tube grids, they're redplating right then. If you ever had +128vdc on the control grid (pin 5) of your output tubes, the fuse has popped or (if the fuse is bypassed) the tubes have melted plates and/or glass and your power transformer is smoking.

Make sure the above reading is not really 0.128 vdc. Make sure you're not really measuring with the meter set on a.c. volts and seeing that number.

Noise is probably not due to d.c. voltage on output tube grids. Bias problems, smoking output tubes or power supply parts may be the result of excess positive d.c. on output tube grids due to leaky caps or a short-circuit. Rather, noise is more likely a.c. related or coming from somewhere else in the amp.

As far as taking meter readings is it true that you can't take a reliable reading from the grid of the preamp tubes unless you have a special type of meter? I've just been reading a bit in Aspen Pittman's book about it and someone in there said you can't take a reliable reading from those grid pins unless you have a certain kind of MM.

Maybe true, but incomplete.

If you attempt to measure the voltage on the grids of a long-tail pair phase inverter, or the input grid of a split-load phase inverter (also called a cathodyne or concertina inverter), then you will get misleading results with a typical multimeter.

That's because they often have a 10MΩ input impedance. For most circuits, this is high enough to not upset the circuit voltages when making a measurement. But the input impedance at the grids of these particular phase inverter circuits is very much higher than you'd expect. High enough that even a 10MΩ impedance of a common multimeter will drag down the voltage at the grids when you make a measurement.

Instead, you'd need a meter with a much, much higher input impedance, or use a different measurement technique. I have some specialized VTVMs with an input impedance of 122MΩ-200MΩ depending on the voltage range. But you could do a simple trick to understand the true grid voltage without any special meter.

Maybe you could post a quesiton about the noise you're chasing.

[Gary_S]

Thanks Hotblueplates a lot of good information there for me to think on. It's fixed bias, a Marshall 4010 50 watt combo. I'm going to double check all those readings to make sure.

This only happened after i replaced all the coupling caps in the amp so i suspected i'd burnt up the caps by putting too much heat on them and ruining the dielectric. I never used a heatsink. Anyway i bought some new caps and put them in using a heatsink but the noise still persists. Kind of at a loss as to what to do next with it as i used a heatsink and a temp controlled station set at 650 degrees and still there's problems. I unsoldered one side of the coupling caps from V1 to check if there was any DC leakage but i'm getting a very inconsistent reading from my meter, the display is jumping around all over the place and not settling in one space. Ever had that problem of not getting a clear reading like that?

So the bit in Pittman's book about the meter is partly true. I remember it said in the book about the VTVM you mention, they said you had to use one like that or the reading would not be correct. Just using a standard meter would not work as the reading would be way off. What do you do to get round the problem with a standard MM HBP?

What about the grids on the preamp tubes? not the PI, should a standard meter work there?

Thanks HBP i appreciate the help.

[Gary_S]

I rechecked the voltage today on those power tube pins; that 128VDC was a stray reading!  on pin 5 of V5 i read -24VDC and on pin 5 of V4 i had zero, round about.

On the first cap i tested i got a DC leakage of just 11mv or 0.011V. According to my reading that cap is good. Just have to check the rest of them now! 

[HotBluePlates]

I rechecked the voltage today on those power tube pins; that 128VDC was a stray reading!  on pin 5 of V5 i read -24VDC and on pin 5 of V4 i had zero, round about.

V4 is a problem. Unsolder one leg of the coupling cap going to that grid (but make sure you truly had pin 5 first), and remeasure the voltage. You should get exactly the same measurement as you do at V5 pin 5, becuase the bias voltage is coming from the same place, through similar components and shouldn't have any voltage drop.

This only happened after i replaced all the coupling caps in the amp ...

Could even be a soldering error, or unintentional short. Always suspect that whatever you touched last is causing the problem.

I can't be sure you didn't damage the caps, but I'd think it would be unlikely. You're more likely to break the leads. I have never had a cap that I could tell I damaged from heat, even when I first started and was in the same boat you're in now.

... but the noise still persists. Kind of at a loss as to what to do next with it ...

Maybe you have another thread about this amp. But I still don't know what the noise is that you're chasing.

So the bit in Pittman's book about the meter is partly true. I remember it said in the book about the VTVM you mention, they said you had to use one like that or the reading would not be correct. Just using a standard meter would not work as the reading would be way off. What do you do to get round the problem with a standard MM HBP?

Even most VTVMs are ~10-11MΩ input impedance, just like modern multimeters. The meter I had in mind was a Hewlett-Packard 412A, which was a lab-grade DC-only meter from the early 60's.

But you don't need one of those. If you look at the cathode of a long-tail inverter, they usually have a fairly small-value bias resistor (maybe 470-2kΩ) and a large-value "tail resistor" (maybe 10k-220kΩ).

How to find long-tail grid voltage:
1. Measure the d.c. voltage from cathode to ground.
2. Measure the d.c. voltage across the junction of the bias resistor and the large-value tail resistor. Make this measurement from the junction to ground. You could also look at this as measuring the voltage across the tail resistor only.
3. Voltage across the tail resistor equals the voltage at the long-tail grid (because there should be almost zero current flowing). Difference of voltage at the cathode and across the tail resistor is the actual bias voltage the tube feels.

Because you're not measuring at the high impedance grid, you won't drag the voltage down (as much) when you measure. This is also why old Fender schematics had the voltage indicated at these points of a long-tail inverter, and not at the grids.

What about the grids on the preamp tubes? not the PI, should a standard meter work there?

Yep. They all have some form of resistor from grid to ground. Often, that's 1MΩ; your meter, having an input impedance 10 times as big, doesn't appear to change the total resistance to ground when you attach it to measure. Therefore, there's no significant loading effect.

[Gary_S]

Thanks for the great advice HBP. You know i'm starting to doubt the coupling caps are the main culprit because i really took extra care this time and soldered the caps in quick and snappy plus i put a heatsink on them just in case too. I've checked both of the coupling caps related to V1 and they check out very low for DC, like insignificant 0.010V 10mv's. So i'll go through and check them all and take your advice about checking the caps coming off the PI as something is wrong there.

One thing i've found frustrating is getting the readings; sometimes if you don't touch the probe to right on the correct bit you don't get a true voltage.

I found out something strange as well tonight after unsoldering the grid side of one of these caps to test it i turned on the mains power with standby still off and even though standby was off i was getting a bit of noise coming through the speaker!  Anyway as i poked around checking voltages i noticed the noise disappeared when i touched the probe to the grid of V2 the 2nd pin. That's just something that came into my head it may not be related.

Yeah HBP i had another thread up about this amp describing the noise, it's like static and kind of crunchy sounding, not a whine no popping or anything like that just like general background interference.

Cheers HBP.

[HotBluePlates]

...  Anyway as i poked around checking voltages i noticed the noise disappeared when i touched the probe to the grid of V2 the 2nd pin. ...

This is a clue. 

When you measure voltage, your meter looks like a resistor between the two meter leads. So when you connected your meter lead to V2 pin 2, you probably grounded that grid, and shunted any noise present at that point to ground. This tells me the noise is probably coming from a point before V2 pin 2.

Do you hear the noise when playing through the low input? Do you only hear the noise when the amp volume is turned way up?

I'm wondering if this is just typical background hiss, which is a combination of the type of resistors used, the fact they absorb moisture over a period of years, and the way the preamp tubes are biased.

Another possible aggravating point is all the bright caps in the preamp bypassing various resistors and the preamp volume control. These resistors form voltage dividers and reduce the overall signal level, while the bright caps allow treble to pass without signal loss. They also allow high frequency noise (hiss) to pass without reduction.

If you could tolerate an overall darker tone, I'd experiment with remove those bright caps. They are C4, C5 and C8 on the schematic and the p.c. board. You might get a noise reduction right there...

[Gary_S]

No it's present this noise even with nothing plugged into any input on the amp and with all the controls zeroed.

Did some more measuring tonight around the PI, after reading your advice. So V5 is getting a negative voltage on the grid of -24V and on the grid of V4; nothing. So i measure the voltage at the plates of the PI, both showing the same figure roughly. So the signal go's out from the plate and hits the coupling cap that i fitted. This is the cap that's feeding V4; i measure the voltage on the grid side of that cap and there's nothing but is that not as it should be anyway because the cap is blocking DC. I thought that there should be no DC showing on that measurement anyway? Anyway on to the other plate of the PI and measuring the cap feeding V5, taking the measurement from the grid side there's that .24V that getting onto the grid of V5. I can't understand why there's the discrepancy between those two signals coming from the plates of the PI.

I can't be sure but i'm thinking this is the problem here but i'm not sure 100%.

I'm going to measure the rest of the caps probably by taking the grid sides out the circuit and see what i get. It's a pain you can't just get a measurement without unsoldering those!!!!

Thanks HBP.

[HotBluePlates]

Did some more measuring tonight around the PI, after reading your advice. So V5 is getting a negative voltage on the grid of -24V and on the grid of V4; nothing.

Is V4 redplating? If no, your measurement is lying to you. If V4 had 0vdc on it, the tube would have cherry red plates and be trying to pop power supply fuses.

... i measure the voltage on the grid side of that cap and there's nothing but is that not as it should be anyway because the cap is blocking DC. I thought that there should be no DC showing on that measurement anyway? ...

Not exactly. Coupling caps block the d.c. on Side A from getting to Side B.

You have plate voltage (couple-hundred volts) on Side A. But output tubes need bias, and your amp has a fixed negative voltage applied to the control grid. That's Side B.

You have d.c. on both sides, you just don't want the big positive voltage on one side mucking with the smaller negative voltage on the other side. Some day I'll post a picture of a 6V6 I have with a hole melted in the plate. That's what will happen if you don't have good fusing and get a massive failure of that coupling cap.

So 0vdc on V4 is a major problem, and you should be seeing major impacts resulting from it. If you don't you're not getting valid measurements.

... Anyway on to the other plate of the PI and measuring the cap feeding V5, taking the measurement from the grid side there's that .24V that getting onto the grid of V5. I can't understand why there's the discrepancy between those two signals coming from the plates of the PI. ...

WAIT... 0.24v???

Are you measuring a.c. voltage? If so, you'll see a.c. volts, cause you can hear noise in the speaker. That's a waste of your time. Measure d.c. only.

No it's present this noise even with nothing plugged into any input on the amp and with all the controls zeroed.

Put the pieces together.

All controls 0 = noise.
Probe V2 pin 2 = no noise

The source of the noise is between the volume control and V2 pin 2.

If the noise were occurring before the volume control, turning the volume off would kill all noise. As I mentioned, when you probed V2 pin 2, you grounded that point through the meter. Grounding that point kills noise, so the noise is happening before it.

The schematic doesn't tell what point in the circuit is V2 pin 2. Is this the cathode follower section, where the output goes to the tone stack, or is this the half that is connected to the 470k/470k voltage divider?

[Gary_S]

One of the power tubes was red plating like crazy, i'm sure it was V5 not V4, i'll put in the old tubes and check again, i just don't want to put my new tubes in until i've sorted this out.

The -24V is DC on V5, i'm not measuring AC there.

Meant to say about the noise that was coming off the amp that went away when i probed the grid of V2: that happened when i had one side of one of the coupling caps from V1 unsoldered to check it for leakage. After i checked it,  i soldered it back in and that noise is gone now.

You have plate voltage (couple-hundred volts) on Side A. But output tubes need bias, and your amp has a fixed negative voltage applied to the control grid. That's Side B.

Ok, so on the B side of all these caps should  i be able to measure a negative voltage or should it  be zero?  because i always thought that the caps blocking the DC getting through the other side would mean you would get a zero reading on the B side?  These are all the little things it's hard to pick up. 

I'm learning a lot of good stuff from you HBP, cheers.

[Willabe]

Ok, so on the B side of all these caps should  i be able to measure a negative voltage or should it  be zero?  because i always thought that the caps blocking the DC getting through the other side would mean you would get a zero reading on the B side?

Not exactly. Coupling caps block the d.c. on Side A from getting to Side B.

You have plate voltage (couple-hundred volts) on Side A. But output tubes need bias, and your amp has a fixed negative voltage applied to the control grid. That's Side B.

You have d.c. on both sides, you just don't want the big positive voltage on one side mucking with the smaller negative voltage on the other side.

Yes the coupling caps between the PI and the power tubes grids block the PI plate dcv.

But because your amp is grid bias (not cathode bias) you still have to have a negitive voltage for the bias on the power tubes grids.

Look in here to read up on biasing it will help you. It's from Dougs library of tube amp information.

http://www.el34world.com/charts/fenderservice5.htm

            
                          Brad      

[alerich]

One of the power tubes was red plating like crazy, i'm sure it was V5 not V4, i'll put in the old tubes and check again

If you are absolutely certain you have no negative bias voltage at pin 5 of V4 I would not install any output tubes in the amp. Negative bias voltage should be present at pin 5 of each of your output tube sockets even without tubes installed. Under normal conditions the same negative voltage should be present at pin 5 of both sockets (V4 and V5). If you don't have that negative bias voltage at pin 5 of both sockets you should stop and troubleshoot and find out what is wrong with the bias circuit. Until you correct the bias circuit and have negative bias voltage at pin 5 of both sockets with no tubes installed it is not advised to install the tubes. Find out why you have no negative bias voltage on pin 5 of V4.

[Gary_S]

Thanks willabe and alerich i appreciate the advice. No i'm not installing the EL's until i get the proper bias reading as they are brand new JJ's and they would be ruined within minutes with that reading being way out.

I was measuring on the board and those two bias feed resistors, the 220k one's, they appear to help set the negative voltage or have some influence on it. I checked the voltage on those and the one that feeds the power tube grid that has the -24VDC appears to have the negative figure on it whereas the one that leads to the tube that has no negative bias on it's grid, that resistor has no negative reading on it's far side although it has the negative reading on it's far side. I'm wondering if this bias feed resistor feeding V4 is toast and that's why the negative voltage is not getting through and on to V4.

It's all very wordy trying to describe it on the net, if you could see it you'd understand in a heartbeat.

What i think may have happened here is this: after taking out the old caps a lot of the solder pads were overheated so i had to use a patch wire as a connection. I think maybe there's a missed connection in there as, through using a wire and dispensing with the trace, there may have been an unnecessary connection made somewhere and that's what's screwing up the bias figure.

Thanks for your ideas guys. 

[Willabe]

I was measuring on the board and those two bias feed resistors, the 220k one's, they appear to help set the negative voltage or have some influence on it.

No, they have no influence on the negative bias voltage, because the grid draws no bias current. They (220K) are the grid return R's and are referenced to ground through the bias supply. On a cathode bias amp they would go straight to ground. On a grid bias amp their junction is where you apply the negative grid bias.

I checked the voltage on those and the one that feeds the power tube grid that has the -24VDC appears to have the negative figure on it whereas the one that leads to the tube that has no negative bias on it's grid, that resistor has no negative reading on it's far side although it has the negative reading on it's far side. I'm wondering if this bias feed resistor feeding V4 is toast and that's why the negative voltage is not getting through and on to V4.

The R that goes to the power tube grid from the junction of the 220K R and the PI coupling cap is the grid stopper R.

I think you just found your problem.    

Please read the link I posted from Doug's library on bias, it will help you greatly with understanding bias in the power amp.

              
                        Brad      

[HotBluePlates]

So it sounds like your noise issue is gone, and now you just need to fix the bias issue. Yes, that 220k where you get no negative voltage is likely a problem; either the resistor itself or the connections through traces/wires.

And see the pic below to see what we're trying to describe regarding phase inverter coupling caps.

There is (blue) B+ voltage getting dropped by load resistors to plate voltage on one side of the coupling caps, and (red) bias voltage being applied to the output tube grids. So there is d.c. on both sides, and the cap just makes sure one d.c. voltage doesn't upset the other d.c. voltage. because they're widely different voltages.

[Gary_S]

That's great stuff HBP, and on my 4010 schematic as well!  it makes it all the more understandable, the bias supply. I took a reading with the meter on the resistor that has the negative voltage on it and it shows up at nearly bang on the 200k but on testing the other problem one i'm having a difficulty getting anything from it at all, reading wise. I don't want to be too quick to jump to conclusions though as, through measuring other resistors in the circuit, there's a lot of them that are hard to get a reading from. I don't know why that is?  maybe some oxidation built up on the leads blocking a reading or something like that? Most of the resistors on the board when i check them it's difficult to get a reading from them hence why i'm not jumping the gun in assuming the bias one is knackered. I think it might be the problem though. I'll be working on it on Sunday and i'll see if i can get any further. I might pull that resistor from the circuit and see if i can measure it when it's out of there.

Yeah the noise issue that was happening and disappeared when i probed the grid of V2 is gone. As i said it was happening when i had one side of the coupling cap from V1 unsoldered to check for DC leaking. After i soldered it back in the noise just went away.

Thanks for the diagram with the highlighting, it really makes things clear.

[Gary_S]

Ok, hopefully i might be getting to the root of the problem here: i measured the bias feed resistor going to V4 and i finally got a reliable reading from it, it read only 52k instead of the 220k it should read, the other feed resistor is almost bang on the 220k. Would that be the cause of the negative voltage not being correct on the grid of V4?

What would cause that resistor to degrade like that? could it have been anything to do with soldering the cap in connected to it and maybe it's had too much heat or maybe just wear and tear? it's a JCM800 from 1982 and the resistors have never been touched before now.

[sluckey]

The fact that the resistor only measures 52K is not the problem, but it may be a clue to what is the problem. That resistor could be 1Ω or 10MΩ or anywhere in between and you would still measure the same negative voltage on each side of it "if" there was no other problem.

BTW, the schematic shows those resistors to be 220K, not 200K. Which meter are you using (make and model)? With power off, how much resistance do you measure from pin 5 of each EL34 to chassis?

[Gary_S]

The fact that the resistor only measures 52K is not the problem, but it may be a clue to what is the problem. That resistor could be 1Ω or 10MΩ or anywhere in between and you would still measure the same negative voltage on each side of it "if" there was no other problem.

BTW, the schematic shows those resistors to be 220K, not 200K. Which meter are you using (make and model)? With power off, how much resistance do you measure from pin 5 of each EL34 to chassis?

I meant to say 220k, i altered the post.

Extech 330 meter.

What else would influence that problem?

V4 5.58k

v5 278k

[sluckey]

V4 5.58k

That's the real problem. There is probably a dead short  to ground on the side of the 220K resistor that has no negative voltage. The 5.58k reading is your 5K6 grid stopper resistor. Should be easy to spot, but may be a little challenge since you're dealing with a PCB.

[Gary_S]

V4 5.58k

That's the real problem. There is probably a dead short  to ground on the side of the 220K resistor that has no negative voltage. The 5.58k reading is your 5K6 grid stopper resistor. Should be easy to spot, but may be a little challenge since you're dealing with a PCB.

Thanks sluckey. What would you do whipe the resistor out and replace with a new one or is there something else i'd need to have a look at?

I don't have any spares i'll have to order some in. Well i have a load of spare resistors but they're too small because they're all 1/4W and i think those 220k's are 1/2W?

Thank you.

[sluckey]

Well, you'll need to eventually replace the resistor, but you really need to find that short to fix the problem. If you can't find it, take some hi rez pics. If you have a PCB, take pics of both sides. Maybe we can spot something.

The short is at the junction of the 220KΩ and the 5K6Ω. Check resistance from the bad resistor to chassis to confirm. There's also the coupling cap and a tiny snubber cap at that junction. I don't suspect any of those components. I suspect something like a solder blob or bridge.

This is a PCB circuit right?

[alerich]

What i think may have happened here is this: after taking out the old caps a lot of the solder pads were overheated so i had to use a patch wire as a connection. I think maybe there's a missed connection in there as, through using a wire and dispensing with the trace, there may have been an unnecessary connection made somewhere and that's what's screwing up the bias figure.

You may have created a short in the circuit when you repaired the solder pad/trace. Double check any repairs you made to the traces.

[Gary_S]

Well, you'll need to eventually replace the resistor, but you really need to find that short to fix the problem. If you can't find it, take some hi rez pics. If you have a PCB, take pics of both sides. Maybe we can spot something.

The short is at the junction of the 220KΩ and the 5K6Ω. Check resistance from the bad resistor to chassis to confirm. There's also the coupling cap and a tiny snubber cap at that junction. I don't suspect any of those components. I suspect something like a solder blob or bridge.

This is a PCB circuit right?

Yeah it's a pc board sluckey. I thought if everything checked out continuity wise then it was good to go, but obviously not. Because i checked all the connections for continuity and was getting it. I think with the missing solder pads and having to use patch wire to make some of the connections that's the problem. Round about that area the signal comes out the cap and is going towards the grid connection of the next stage but as far as the actual trace go's; at that point there is also the connection to the 220k resistor. There i actually bypassed that trace and linked the three connections via patch wires. I think that's the problem area there.

You may have created a short in the circuit when you repaired the solder pad/trace. Double check any repairs you made to the traces.

That's definitely the problem right there at that junction i mentioned above. This came about because i was using an unregulated weller iron that was too hot for the board and i lifted a few of the solder pads which then gave me the headache of trying to link the connections via bus wire 

Thanks for helping guys.

[Gary_S]

So i got into the PCB of the 4010 this afternoon and looked at that junction area of the coupling cap and the 220k bias resistor. Although i had connected everything up and put a patch wire between the cap output and the wire to go to the grid of V4 the patch wire that went from the cap to link with the bias resistor didn't seem to be soldered into position properly. I had manually made sure i got a connection but the joint didn't look as though it was soldered or it was a very tenuous connection.

Anyway i redid the manual connection of the parts and soldered it nicely in place. I checked the readings and things seemed ok. I took a voltage reading from the grids of both power tubes and got -31.04DC on V4, and -31.23DC on V5. So at least they're matched and things seem ok there.

I haven't replaced the power tubes and tried it yet! kinda afraid i'll be disappointed and there will still be a noise issue there somewhere, even after fixing this!

Would the bias being totally off on one tube like that cause all that terrible background noise? i know it can cause the red plating issue but i don't know if it also affects general bad noise coming from the amp?

[HotBluePlates]

Anyway i redid the manual connection of the parts and soldered it nicely in place. I checked the readings and things seemed ok. I took a voltage reading from the grids of both power tubes and got -31.04DC on V4, and -31.23DC on V5.

Congrats! Way to hang in there to the end.

I haven't replaced the power tubes and tried it yet! kinda afraid i'll be disappointed and there will still be a noise issue there somewhere, even after fixing this!

Would the bias being totally off on one tube like that cause all that terrible background noise? i know it can cause the red plating issue but i don't know if it also affects general bad noise coming from the amp?

Not usually "general bad noise".

If a tube loses bias, it trys to draw massive current, and the audible sign is usually hum coming through the speaker that keeps getting louder as the tubes redplate. There's a couple of possible causes for that hum, and when it happens to you and you see those tubes redplate, you'll know how it sounds different than normal background hum from other causes.

Now that you know you have proper bias voltage available, just dive in and replace those output tubes. If you have any remaining noises, you just have to chase them down one at a time. Eventually, you'll have it resolved.

[Willabe]

I took a voltage reading from the grids of both power tubes and got -31.04DC on V4, and -31.23DC on V5.

Nice.  


           Brad      

[Gary_S]

Many thanks to you guys for helping this relative newb out.

I've learnt quite a bit already listening to the advice you gave out regarding my amp.

It was a joyous moment when i checked the bias on the grid of V4 and i got that nice reading

Cheers again for all your help.

[Gary_S]

Put the output tubes in and switched the amp on today. I flicked the power switch to on and left it on standby and i thought "i hope this is it!"  flipped the standby and then.... lovely! none of that hideous noise and scratchy interference going on. Joy pure joy isn't it when you get moments like that?

So i just have to set the bias, as these are brand new EL34's and put everything back in place and that's it!

One question i have is about the space under these coupling caps; i left a space there to fit a heatsink (alligator clip)  and someone advised me in that type of situation; if there's a gap between cap and board, to put some clear silicone sealant underneath the cap to prevent any possible stress on the leads that could lead to breakage down the road. I don't know about that what do you guys think? would you do that?  The thinking behind it is when the amp heats up and the PCB expands and contracts slightly with the heat if there's that potential for movement it might lead to breakage 

Anyway i'd like to thank everyone who gave me advice on this thread; you taught me a lot of good stuff that really helped me out. not only for this issue but in general with amps.

[HotBluePlates]

... when the amp heats up and the PCB expands and contracts slightly with the heat if there's that potential for movement it might lead to breakage   ...

Maybe. But that p.c. board is epoxy and fiberglass, so it is a somewhat similar material (although much thinner) than Hoffman board material.

If it makes you feel good, go for it. Caps generally don't need heatsinks, because they dissipate an exceedingly small amount of power compared to resistors.

I've also never put the silicone goop on anything, and have an amp I built ~15 yrs ago still plugging away.

[Gary_S]

Thanks HBP, i might leave it with the silicone. Annoying thing about is was maybe the caps i put in before were ok and i ditched them in the mistaken belief they were the problem instead of where i eventually found it!!!! Funny how you can get suckered into going down the wrong path!!

Anyway i biased the amp tonight and set it around 42ma as i like it a bit hotter than what Marshall recommends, they say about 39ma.

Thanks i appreciate it, this is a great board!!