Mullard 5-20 new chassis problems

[PJL]

Can you:
1. measure the resistance from earth to EF86 cathode.
2. remove the ECC83 and take the readings on the EF86 again
3. measure the voltage on C4

Peter

PS: I can;t see a wiring error - you night want to check R9 270K still measures correctly

[val33vo]

Put putting sound is almost certainly motorboating probably down to some fault with the de-coupling capacitors wiring which is allowing the power stage to modulate the input stages anode voltage ( just because it worked before you cannot assume the caps will still be ok as you have rewired the amplifier, you may have strained and damaged some of the component lead out connections )

Val33vo

[Diabolical Artificer]

Your wiring is still a bit all over the shop. I cant see where your anode (pin 6) of the EF86 goes to the ECC83. It looks a bit near to your earth strap? In the Sterns plans,for the 5-10,this connection has a special note;IE keep clear of heater wiring etc. I know little compared to alot of our fellow forum members,but if you tidy your wiring up,at least you can eliminate this as a cause of potential problems. I notice you dont have a paticular colour scheme to your wiring;this may make it harder to trace any mistakes.

Advice is to remove certain valves,and treat the input stage in isolation. When I did this with my 5-10 project, the HT went up,which squews the readings. Could anyone comment on this? Andy.

[qualityten]

Originally Posted by magnetic

I notice you dont have a paticular colour scheme to your wiring;this may make it harder to trace any mistakes. ...

You're right Andy. I thought I'd help myself by using colours that corresponded as closely as possible to the originals. I've learned from this that I should not try to copy what is there, but (a) follow the designers' wiring layout and (b) use consistent wire colours.

Still laying bathroom vinyl right now, but hope to get back this later.

[PJL]

Hi Andy, if you look at the circuit there is only 15K from HT to the ECC83 and 270K for the EF86. Removing the ECC83 should not significantly change the HT to the EF86.

Until the odd voltages on the EF86 are fixed I would not be too concerned about the circuit layout.

[qualityten]

Sorry PJL, the bathroom took longer than expected so I've not got to the amp at all today.

I won't have much time tomorrow or next week, but I'll do the measurements you advise in #41 before having to park this project. Thanks to you and others for help so far. I'm looking forward to finding the fault(s).

[qualityten]

Hi Peter and others,

I'm able to return to this amp and give it some (intermittent) attention till the till the end of the Bank Hol weekend.

I checked all the wiring of the tag boards and from the boards to the valves, ground etc, taking care to follow the layout of the Mullard book too. I am confident that there is no glaring visible wiring error.

The result is that I now have audio. It is softer than it should be, but not very soft. and the oscillation is a very slow put-put. Audio disappears when the NFB is removed.

Peter, I did the measurements you advised on 14 August:
1. measure the resistance from earth to EF86 cathode. = 11.36Kohm
2. remove the ECC83 and take the readings on the EF86 again = 11.36Kohm
3. measure the voltage on C4 = 179V (i.e. a bit higher than it should be at 160V).

The other voltages are pretty much as at the start:
C15, C12, C5, C4 all within range (C4 is the exception, but not very far out).
The EL34s are spot on.
The ECC83 is out too:
1st and 2nd anode: 187V (should be 325V)
1st and 2nd Grid: 143V (should be 85V)
1st and 2nd Cathode: 146 (should be 87.5V)

[Amraduk]

Originally Posted by qualityten

1. measure the resistance from earth to EF86 cathode. = 11.36Kohm

This is definitely incorrect! It should be about 2k3.

Going by the photos you provided, the end of resistor R5 (10R) on the 4th tag from the right hand end of the lower edge of the tag board is not connected to ground, which would explain the incorrect reading you are getting.

Feedback is taken from the secondary of the output transformer and fed back to the EF86 via the parallel combination of R13 and C9, the reading you are getting is the 2K2 (R4) resistor in series with R13 (value probably 8K2) and the secondary winding of the output transformer.

The combination of R13 and R5 form an attenuator which reduces the amount of feedback fed to the EF86, because R5 is not connected to ground, the feedback is not being attenuated, so it will be excessive and probably accounts for the instability!

Regards,

Dave.

[trickie_dickie]

FROM MY PREVIOUS POST..

"Looking at your pics....you definitely have a wiring error. Look at the busbar on the FOURTH tag, under the yellow cap C2. It should be on the SECOND tag, the ground end of the 100 ohm resistor R5."

Originally Posted by Amraduk

This is definitely incorrect! It should be about 2k3.

Going by the photos you provided, the end of resistor R5 (10R) on the 4th tag from the right hand end of the lower edge of the tag board is not connected to ground, which would explain the incorrect reading you are getting.

Feedback is taken from the secondary of the output transformer and fed back to the EF86 via the parallel combination of R13 and C9, the reading you are getting is the 2K2 (R4) resistor in series with R13 (value probably 8K2) and the secondary winding of the output transformer.

The combination of R13 and R5 form an attenuator which reduces the amount of feedback fed to the EF86, because R5 is not connected to ground, the feedback is not being attenuated, so it will be excessive and probably accounts for the instability!

Regards,

Dave.

Well, that's two of us have seen the fault.

[qualityten]

Thanks Dave. R5 was connected to ground (underneath the tagboard), but when I disconnected it to check I couldn't believe my eyes that it was measuring about 880k, not 100R as marked!

Replacing it with a good 100R resistor has restored the correct resistance of 2k3 and restored loud audio. However this is distorted, and unaffected by disconnecting the feedback.

Voltages on the EF86 are now quite different. Before replacing R5 they were double what they should be, but now are they are two-thirds of what they should be:
Anode: 51V (should be 85V)
Screen grid: 60V (100V)
Cathode: 1.4V (should be 2.1V)

[qualityten]

Thanks too Richard for post, just seen. As I said, I did double check the boards and connections last night before posting and tidied up. I made sure that the EL34s were connected to board and o/p transformer exactly as required.

As I say R5 was connected correctly, but the resistor was faulty, probably from being moved.

I have to go to work now, but have posted a few pictures to show the current state of affairs.

Thanks again for all the help!

[qualityten]

Other points: I should have added that the voltage on C4 is now also down to 94V.

The apparent wiring error mentioned by Richard (Busbar on fourth tag, etc) is because the busbar bends under the tagboard at that point. There is no connection to the fourth tag and the busbar connection to the second tag is done from below.

I've replaced R13, which was 9k9, instead of 8k2 as marked, but this does not affect the distortion.

[qualityten]

Apologies for further post, but I've replaced some other other old resistors that had drifted a lot. The result is that the EF86 voltages are getting closer to where they should be: Anode 71V, Grid 84V, Cathode 2.1V.

The distortion is still there and there is now a bit of very high frequency oscillation that disappears when feedback is disconnected.

I would appreciate any further pointers forum members can offer. Thanks.

[Amraduk]

Originally Posted by qualityten

Thanks Dave. R5 was connected to ground (underneath the tagboard),...

A pity you didn't mention that instead of just saying that it was connected correctly! It isn't apparent from the photos and does look as though it isn't connected. I must apologise, when I said: "...the end of resistor R5 (10R) on the 4th tag...", that should, of course, have been the 2nd tag! Why I wrote the 4th, I don't know!

...but when I disconnected it to check I couldn't believe my eyes that it was measuring about 880k, not 100R as marked!

That only added to the impression that it wasn't connected to ground! However, it does explain why the reading was wrong.

Replacing it with a good 100R resistor has restored the correct resistance of 2k3 and restored loud audio.

That's progress!

I've replaced R13, which was 9k9, instead of 8k2 as marked...

That explains why the reading was a bit higher than I would have expected.

However this is distorted, and unaffected by disconnecting the feedback.

Voltages on the EF86 are now quite different. Before replacing R5 they were double what they should be, but now are they are two-thirds of what they should be:
Anode: 51V (should be 85V)
Screen grid: 60V (100V)
Cathode: 1.4V (should be 2.1V)

Obviously, there must be another fault somewhere, that's what needs finding next! I was looking at the photo and noticed that the component from the grid (pin 9) of the EF86 to the input socket, looks like a tubular ceramic capacitor. It should be a 4k7 resistor! That could account for the incorrect voltages. I would suggest measuring it to see if it's a capacitor, or a resistor, also check the value marked on it. Where does the anode (pin 6) go to? It should go to R6 (100k) and R3 (4k7). See if that gets you anywhere.

Regards,

Dave.

[qualityten]

Sorry about not making things clear the first time Dave.

The green component from EF86 pin 9 to input is a 4k7 resistor. I've replaced it with a new one, just to be sure. EF86 anode is connected to R6 and R3. I've just rechecked continuity.

I've also replaced the 390k R7.

EF86 still measures: An 75V, Grid 86V, C 2.1V. Grid should be 153.5V.
The ECC83 voltages are a bit low: a1,a2 316V and 309V, g1,g2, 75V and 68V, k1,k2 78V and 78V.

Cheers, David

[Amraduk]

Hello David,

Originally Posted by qualityten

Sorry about not making things clear the first time Dave.

I'll let you off this time!

The green component from EF86 pin 9 to input is a 4k7 resistor. I've replaced it with a new one, just to be sure.

OK. It's a rather unusual resistor if it's a hollow tube! I have seen something similar, if not the same, a long time ago and I might still have some somewhere.

EF86 anode is connected to R6 and R3. I've just rechecked continuity.

What are their values? I can't again locate the circuit I found on the net, so I can't check! Some I found had different values and component reference numbers! I have found another but I can't remember if it's the same. Where are they on the tag board?

I also replaced the 390k R7.

IIRC, that's the screen grid (g3) feed resistor, isn't it? You might also care to check the screen grid (g3) decoupling capacitor (C3, 0.05uF? Possibly 0.047uF) in case it has gone leaky.

EF86 still measures: An 75V, Grid 86V, C 2.1V. Grid should be 153.5V.

I take it that is the screen grid, g3? In a previous post, you wrote: "Screen grid: 60V (100V)", was that not correct?

The ECC83 voltages are a bit low: a1,a2 316V and 309V, g1,g2, 75V and 68V, k1,k2 78V and 78V.

Assuming the HT supply to the EF86 is the 170V, and the resistor values are correct, as shown on the circuit I'm using, that gives these currents:

C: 0.9mA
A: 0.95mA
G3: 0.2mA

The cathode current should be the sum of the anode and g3 currents = 1.15mA. Where the additional current is coming from, I'm not sure, however, the values are quite close, so could be measurement errors, so may not be significant. The grid (g1) is at ground potential as far as DC is concerned and as the cathode is at 2.1V it makes the grid negatively biased with respect to the cathode, so there shouldn't be any grid current flowing, though I'm not certain if that amount of negative bias is sufficient top prevent it.

Regards,

Dave.

[PJL]

It had to be that resistor! now we are getting closer. The EF86 volts are a bit low but not too bad. The ECC83 anodes look good as they are roughly the same (well matched).

1. Check the EF86 grid voltage, it should be near 0
2. Check the voltage on C4
3. Check the cathode voltages on the EL34's
4. Is the disortion better or worse with the -ve feedback?

Don;t leave it running for too long...

[qualityten]

Thanks Dave and Peter. Dave, the best link to this amp is: http://www.r-type.org/static/5-20.htm This particular one is a home-built one I bought abroad last year; it must have been built in the 50s or 60s, hence vintage parts such as the green, tubular 4k7 resistor. R6 is 100k and R3 is 4k7. Yes, it is the same screen grid. I've just seen that the DC conditions in their book are different from those on the link above. In the book it says this voltage should be 153.5V, while the link says 100V!

I have to go to work now, but will get onto the measurements later.

David

[qualityten]

Dave, C2 (0.05uf) is good; my tester can't go up to C3's 50uF, but it's a new cap so should be okay.

Peter, the measurements are:
EF86 grid (pin 9) is 0V
C4 voltage is 152V
Cathodes of EL34s both 30V
The distortion is worse with NFB. (I should say that the feedback circuit values are C9 220pf and R13 8k2.)

Still wondering if using old Dubilier PIO caps for C12 and C15, rather than a new 8+8 can would affect distortion?

Hope that helps with the diagnosis!

David

p.s.

[PJL]

DC conditions are within tolerances. Output valves look OK.

Try swapping C10 and C11 to the opposite valves and test the effect of the NFB again.

Can you describe the distortion. Does it get worse with volume?