LA2A trouble shooting... help needed

[johnheath]

I have built a LA2A clone with toroid transformers with 12vdc for heaters for three of the tubes and 6vdc for the 6AQ5A.

I hvae three distinctive problems and I have tried to solve these but without any luck .

Problem number one is that it has quite a lot of hum... that disapears when the volume knob is turned upp to very high settings.

Number two... the voltage around v3 is way off and that is even though I have resoldered and changed all components for that specific tube. I have changed the tubes three times and I cannot see that I have three tubes with the exact same fault.

Number three is that the vu-meter is not moving unless the gain knob is set to max... which is not a practical setting.

The V3 tube and the meter are both directly connected so I think that solving the voltage in V3 should solve the meter problem aswell.

There is a slight mod in the circuit and that is that V1 is a 12AY7 and resistor R9 and R13 are both 100k. (CJ mod I suppose) I have tried this on another LA2A I built some time ago and it works perfect.

Voltages on all tubes are as follows:

V1: 12AY7A
pin 1 - 99v
pin 6 - 104v
pin 3 - 1,44v
pin 8 - 1,77v
pin 2 - 0v
pin 7 - 0v

V2: 12BH7A
pin 1 -100v
pin 6 - 222v
pin 3 - 3,53v
pin 8 - 104,8v
pin 2 - 0v
pin 7 - 74,1v

V3: ECC83 (12AX7A)
pin 1 & 6 - vary between 85 - 260v
pin 2 & 7 - 0v
pin 3 & 8 - vary between 0 and 0,67v

V4: 6AQ5A
pin 1 - 0v
pin 2 - 5,9v
pin 5 - 147v
pin 6 - 111v
pin 7 - 0v

V3 have reasonable values right after start up but after 20 - 30 seconds cathode values drops to 0v and both plates rise to 260v.

Voltage right after rectification is 281v and is steady. B+ is steady at 265 - 268v.

Voltage at neon lamp is little low at 55v instead of given 65v.

This is what I have =)

I add scheme and pictures for all to see.

All tips are velcome.

Thanks

/John

 

[johnheath]

And...

 

[johnheath]

And...

 

[johnheath]

I have more pictures if needed but this one might give some info?

Thanks

/John

 

[PRR]

> V3 have reasonable values right after start up but after 20 - 30 seconds cathode values drops to 0v and both plates rise to 260v.

If the grid is staying at zero, I would suspect the heater goes cold.

As you don't show your as-built heater scheme for 12v and 6V, I don't know where V3 is getting heater power from. I can't figure your 12 Ohm resistor either. However it does heat at first, so I would be looking for a loose heater connection.

How hot does your 12V regulator get? The actual device (21V should be safe to touch), not just the heatsink it is supposed to be intimate with.

Very unlikely possiblity. I have only seen this once. A 6SJ7 in a 200AB which I recovered from a Dumpster. It would heat for a few seconds and then cut out. Since this is a metal tube, it was not clear from the glow (or no-glow). Drove me nuts for a while. My ass*umption is that the heater was cracked, would touch when cold, but pulled apart when hot.

 

[johnheath]

Hi

Well the V1 - V3 gets 12vdc for their heaters (series) and the 12ohm resistor brings the regulated 12vdc down to 6,2 vdc for the heaters  in the 6AQ5A. I have a few details to fix in the scheme but basically it is all there.

Heatsink or not? Well the circuit is basically copied from another circuit  near the same as this one and the  copied version required  a heatsink... all just for safety. Until now I haven't felt it more hot than I can grab it for any prolonged time I want.

Speaking of the heaters they are all checked and are  fine... I haven't seen any drop in voltage in the heaters... honestly I have not been checking them so often either. I have noticed that the 78S12 regulator stays at 11.86volts and if you mean that the heaters  goes cold maybe that could be the reason, but on the other hand I use that 12vdc heater arrangment in other stuff and it works just fine.

What I cannot figure out is why the voltages change. I mean the R33 220k resistor is fed from B+ with 268 volts and it only drops the voltage 6 volts. I have changed it twice already just to be "sure". I am at work right now but I'll check for loose heater connections later but they seemed to be just fine when I sat down with it before going to work.

Just of intrest... what voltages would you like to see on the grid in the V3?

Thanks

/John

 

[PRR]

> 12ohm resistor ...12vdc down to 6,2 vdc for the heaters in the 6AQ5A

Ah. I overlooked that. 0.45A*12r= pretty near 6V. Fine.

> 78S12 regulator stays at 11.86volts

11V, 13V, I'm not fussy. Your symptoms suggest the heater is going COLD, not just a little cool.

> what voltages would you like to see on the grid in the V3?

Well, zero, like most tubes.

> R33 220k resistor is fed from B+ with 268 volts and it only drops the voltage 6 volts.

You used a meter to check that. If we assume a 10Meg meter, on a 220K resistor, and no other path, you really read 10Meg/10.220Meg or 0.9784 of the rail, or 262V, which is indeed 6V shy.

So the tube is passing NO (none, nada, zero) current to the Plate.

At the cathode you say it decays to "0"(?). This indicates NO current to cathode.

If the grid had gone highly negative.... but you say 0.

If the tube pins lost contact in the socket.... but this would be abrupt, would change if you glared hard at the tube, and anyway you got two sections in parallel so a single bad plate or cathode pin would just cause a drop, not a complete nothing.

And the way you say it decays in 20-30 seconds is VERY suggestive of a 12AX7, worked at low current, starting hot then heater fails. Tubes passing big current will "quit" when the cathode cools a few hundred degrees. But 12AX7 often works full-capacity at 11V heat or less.... it has tons of reserve capacity, and can probably cool for many seconds before it "quits".

So.... bad heater pin joint, bad socket contact at heater pin, or fracture inside the tube. The first can be checked with a bright light, a sharp eye, and careful poking or re-soldering. The last can be checked with another 12AX7 (swap V1 and V3, see if the fault moves with the bottle).

 

[johnheath]

Yes, exactly… 11.5v or a bit more than 12 is no problem for the heaters as far as I know and would not affect the tube in such way as it is affected now.

About the grid values I expected it to be 0v but you said that if the grid was zero the heater was cold… I just got curious here since I am no pro what so ever =)

Yep, no current to the plates and I do not know what comes first if it is the cathode that drops or the plates rising in voltage? I have checked with different tubes like some old 7025, ecc83 and some Mullard 12ax7's… same problem so maybe a lost contact in the socket (rare).

I will resolder the heater connections in this afternoon because right now I need to sleep.

I will also check the grounding both for the hum and perhaps to see if it is the problem with the malfunctioning cathode?

Thanks again

/John

 

[johnheath]

Aaaarrrgh - changed tube socket, changed the tube several times, resoldered all components, checking all grounds… again and again but still the same problem.

Anyone? Please =)

/John

 

[CJ]

nice build!

smaller pic attached,

could be a ground problem, or an oscillation thing,

some folks used to put grid stoppers on that cathode follower with good results,

let me see if i can dig up an AC voltage chart that might help,

 

[Gert]

Hi,
Seems like you shorted "heater bias" and "0V heaters" when I look at the schematic!
Disconnect "0V heaters" if yo want that 54V level!
Greatings
Gert

 

[CJ]

voltage charts>

http://groupdiy.com/index.php?topic=36666.msg450421#msg450421

 

[johnheath]

Thanks CJ

It seems like I got the tube back in the game last night. Actually I desoldered everything concerning V# and now the voltages are just fine. But when looking at your voltage charts I see that my clone has values that matches all three different comps. But I am in the ballpark.

Now I just have to figure out why the meter don't move as wanted. It starts to make small wiggles when the gain knob is sett to near maximum… not good.

Is this meter failure or… something else?

Thanks

/John

 

[johnheath]

And thanks you CJ for your comment on the build =)

 

[johnheath]

Also… looking at the "difference between…" chart I can see that R38 is not use in the two previous versions but in the latter they add a 220k resistor…. in the scheme I followed it says that R38 should be a 22k.

I used the same scheme on my first LA2A build and that unit works great so I guess that the value of the R38 is not all that crucial.

/John

 

[CJ]

check the wiring on the meter switch, it is pretty confusing, took me a week to get it wired right,

you can also disconnect the meter from the circuit and inject a small signal via a series resistor and see if you are getting the right deflection,

 

[johnheath]

Ok - problem with voltage and meter problems… it is a bit embarrassing to tell you guys that I aciidently soldered wrong leader to wrong contact in the input xor….well, well… fixed as mentioned =)

Thanks though =)

/John