I've got one of these on my bench now and it looks like it's been modded a bit to have some of the controls from the back placed on the front panel. The problem with it now is it will work fine for a little while then the signal cuts out right after the 6386. I can get it to wake up by increasing the signal level at the input momentarily or by turning it off and letting it sit for a while. I measure a plate voltage of around 47 Vdc and the heater is around 6.7 Vac even when it's not outputting anything. I can't figure out if it's one of the transformers or the tube or the socket for the tube even? Also anyone know what the "Sawtooth" control on the back does? It looks like it bias' one of the 12AU7's but I'm not sure why it's labeled "Sawtooth". For that matter what's the correct procedure for balancing that and the 6386 with the balance pot? Thanks in advance!
ITA LA-1B repair
- Thread starter vsr600
- Start date
Help Support GroupDIY Audio Forum:
So you are saying that the plate (and cathode) voltages stay the same during signal pass and no signal? I'm assuming this is using an interstage transformer (BIH-6 in the pics and wiring). And by "right after the 6386" - are you getting no signal off the plates? Behavior hints at possible heat related issue. Check usual suspects if you haven't yet- all solder joints on 6386 socket and clean sockets. Bad sockets can cause issues but I would expect there to be voltage change on plates and cathode if there were contact interruptions. Sub 6386 if you have spare.
If possible, when unit goes into no pass - inject AC signal directly to interstage primary and see if you get signal on sec or following tube's grids.
Yea the plate and cathode voltages don't change when it goes into fail mode. Is it normal for the 6386 to be running so low like that 47V as opposed to the 300+V on the other tube's plates? And yea I'm not getting any signal on the plates of the 6386. I've cleaned the socket and rewetted every connection on there. I'll try running a signal through that transformer when it's in no pass mode see if that can narrow it down. Unfortunately I don't have a spare 6386 to test with and they cost $130 
vsr600 said:Yea the plate and cathode voltages don't change when it goes into fail mode. Is it normal for the 6386 to be running so low like that 47V as opposed to the 300+V on the other tube's plates? And yea I'm not getting any signal on the plates of the 6386. I've cleaned the socket and rewetted every connection on there. I'll try running a signal through that transformer when it's in no pass mode see if that can narrow it down. Unfortunately I don't have a spare 6386 to test with and they cost $130
The GR tube plate voltage will swing high during gain reduction as bias is changed. Don't know what ITA factory value is but 47V is in the general area for that type of circuit. 300+V does sound high for the other tubes relative to other similar compressors. Did you verify that there was a reasonable cathode voltage on V2 and V3 (output driver)? Just to make sure a ground path for those tubes isn't being interrupted.
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Just re-checked the other tubes in it and the voltages i got (relative to ground) are:
12Ax7 (next in the chain after the 6386): Cathode: 1.20V and 1.14V, Plate: 140V and 147V.
12BH7A (next after that and connected to the output transformer): Cathode: 17.4V and 17.2V, Plate 363V and 364V.
12BH7A (in the feedback going to the cv control at the top in the pic): Cathode: 9.78V and 13.3V, Plate: 0.06V?? and 356.7V.
Something seems up with that other 12BH7A..
12Ax7 (next in the chain after the 6386): Cathode: 1.20V and 1.14V, Plate: 140V and 147V.
12BH7A (next after that and connected to the output transformer): Cathode: 17.4V and 17.2V, Plate 363V and 364V.
12BH7A (in the feedback going to the cv control at the top in the pic): Cathode: 9.78V and 13.3V, Plate: 0.06V?? and 356.7V.
Something seems up with that other 12BH7A..
arg looks like the UTC A18 is open between points 2/3 and B where it supplies the plate. But that has nothing to do with the input/output of the signal right? It's in the top part of the circuit in the pics. It takes it's signal input from the output of the limiter and it outputs into the 6AL5 which is on the CV line. This is possibly a completely separate problem. I have already replaced that 12BH7A and the wiring from it as it was fried when I got it.
vsr600 said:
Without schematic it's difficult. I am going by a set of drawings someone posted on this forum and am not sure if it's the same version you have.
Easy test - pull the 6AL5 and see if that cures it. If there is a timing cap that's going bad it can create issues.
An open between 2-3 and B sounds like that half of the winding is open so the side chain would only be getting half the CV signal . . i think. I'm not sure if that is causing the loss of signal but pulling the 6AL5 will remove SC and hopefully any possible effects of either bad cap or asymmetrical CV feed.
Have you drawn out a schematic of the 6386 circuit yet ?
Thanks I'll try that. A side chain isn't completely necessary on this at the moment. Would it hurt running it without the 6AL5 inserted semi-permanently? Yes I checked that thread out and it's the same though a couple resistor values are different. It's basically this http://gyraf.dk/schematics/CCA_LA1D_Limiter.gif but with an extra transformer after the 6386 and the extra 12BH7A circuit before the 6AL5.
It shouldn't be a problem. The filament voltage will rise slightly but nothing drastic. It will simply function as a line amp.
Does yours have the caps (C19 C20) and resistors strapped across the input secondary? Worth checking their connections.
Does yours have the caps (C19 C20) and resistors strapped across the input secondary? Worth checking their connections.
ah well I tried removing the 6AL5 and after about 30 minutes it still died. It doesn't have those two caps (C19 C20) but it does have the resistors and I already re-soldered those. I'm gonna try replacing the socket next and if that doesn't fix it I guess it's time to pony up for a new 6386 and maybe even that a18 transformer.
Only other things I can think of is double checking the signal trace chain at the 6386 grids/input transformer secondary - confirming signal is good there and disappears at the plate.
Check 6386 plate resistors. I suppose they could be getting spent and going open when they heat up. They'd probably read high when cold. Maybe shut off power and do quick read immediately after it goes into no pass and see if they are going open. Sometimes resistors won't show issues until under certain stress. They do take a bit of stress in that position with the current and voltage swings.
Check 6386 plate resistors. I suppose they could be getting spent and going open when they heat up. They'd probably read high when cold. Maybe shut off power and do quick read immediately after it goes into no pass and see if they are going open. Sometimes resistors won't show issues until under certain stress. They do take a bit of stress in that position with the current and voltage swings.
well I spoke to soon, it died again. Now the input transformer is giving me troubles. Driving it with a 1kHz signal from my function generator, I can get a signal on my scope from the primary but nothing on the secondary unless I crank the gain on the function generator and the transformer pop back on and act as if nothing happened. What's up with a transformer that needs to be woken up every once in a while?
