I have just finished a re-cap of my Lafayette LA-224B and have been going through the power-on procedure. A schematic is available below. Here is what I did:
- Replaced the can capacitors (only these -- the others were changed before and the amp worked fine afterwards)
- Re-wired the output stage. I removed the headphone jack, headphone switch, and phase switch from the circuit because I never used them. I realized when I was re-capping that there was a lot of "back and forth" trips through the chassis of the amp for the output signal to get just from the OPTs to the speakers, so I eliminated that and wired the OPTs to the impedence switch directly, and then the switch to the speakers. So on the schematic it basically now looks like the left channel for both channels, with the "PHO/SP" switch replaced with a wire to the speaker terminal.
- The feedback lines were also replaced with a shielded cable, grounded at the output tap end.
I'm running through a dim-bulb tester with a 100W bulb in.
When I run with only the rectifier, and only the pre-amp tubes, the amplifier comes up and is stable at ~350V (w/ preamp tubes), which sounds about right. The bulb stays off.
However, as soon as I add any power tube, in any position and any combination, the amp will come up to ~300V, but then start dropping quickly down to about ~175V before I turn it off. The bulb also starts off a bit brighter than with just the preamp tubes, gets dimmer, and then gets brighter and brighter.
I'm thinking that I introduced a short somewhere, but I'm having a tricky time finding it. It's also strange that the behaviour starts only once the power tubes start conducting. If I had introduced a short in the cans themselves, I would expect this to show up almost immediately.
So, any pointers on where I can look next? I've checked a couple times that the can capacitors are hooked up correctly, and unless there's some magic in the output stage that I accidentally removed I think I got that right.
- Replaced the can capacitors (only these -- the others were changed before and the amp worked fine afterwards)
- Re-wired the output stage. I removed the headphone jack, headphone switch, and phase switch from the circuit because I never used them. I realized when I was re-capping that there was a lot of "back and forth" trips through the chassis of the amp for the output signal to get just from the OPTs to the speakers, so I eliminated that and wired the OPTs to the impedence switch directly, and then the switch to the speakers. So on the schematic it basically now looks like the left channel for both channels, with the "PHO/SP" switch replaced with a wire to the speaker terminal.
- The feedback lines were also replaced with a shielded cable, grounded at the output tap end.
I'm running through a dim-bulb tester with a 100W bulb in.
When I run with only the rectifier, and only the pre-amp tubes, the amplifier comes up and is stable at ~350V (w/ preamp tubes), which sounds about right. The bulb stays off.
However, as soon as I add any power tube, in any position and any combination, the amp will come up to ~300V, but then start dropping quickly down to about ~175V before I turn it off. The bulb also starts off a bit brighter than with just the preamp tubes, gets dimmer, and then gets brighter and brighter.
I'm thinking that I introduced a short somewhere, but I'm having a tricky time finding it. It's also strange that the behaviour starts only once the power tubes start conducting. If I had introduced a short in the cans themselves, I would expect this to show up almost immediately.
So, any pointers on where I can look next? I've checked a couple times that the can capacitors are hooked up correctly, and unless there's some magic in the output stage that I accidentally removed I think I got that right.
Putting in a screened cable for the feedback introduces capacitance which may be enough to cause oscillations, depending on length and geometry.
You could do a quick check by changing the feedback wire back to a simple unscreened one.
Do you have a scope?
Jan
You could do a quick check by changing the feedback wire back to a simple unscreened one.
Do you have a scope?
Jan
Err, your bulb is too low wattage for a full power test... The bulb is dropping your AC supply, measure it - get a bigger bulb or a variac or go for broke.
A guestimate of the power it consumes is 85 watt. (30 for the heaters and 50 to 55 watts for the B+ = 80 to 85 watts. The 6BQ5s draw about 45 watts.)
A guestimate of the power it consumes is 85 watt. (30 for the heaters and 50 to 55 watts for the B+ = 80 to 85 watts. The 6BQ5s draw about 45 watts.)
Ah, thanks! I tried again with my variac and it seems to work ok with no magic smoke yet!
Thanks Jan. I wasn't sure about shielded vs. unshielded, but since it was crossing lots of heater wires decided to try shielded. Have removed the grounding for now -- would the recommendation be to leave it unshielded?
Any speaker output tap (4, 8, or 16) can easily drive a 2 foot shielded cable that is used to provide the signal for the negative feedback path.
But only if it Directly drives the shielded cable, and Then you connect the series negative feedback resistor At the output of the shielded cable, Not the input to the shielded cable.
Unless the negative feedback wire is Parallel to other circuit wires, or if it comes Across a High Voltage secondary wire, it should not have to be shielded.
All generalizations have exceptions.
Just my opinions.
But only if it Directly drives the shielded cable, and Then you connect the series negative feedback resistor At the output of the shielded cable, Not the input to the shielded cable.
Unless the negative feedback wire is Parallel to other circuit wires, or if it comes Across a High Voltage secondary wire, it should not have to be shielded.
All generalizations have exceptions.
Just my opinions.
Thanks 6A3sUMMER!
I've left it unshielded, and buttoned it back up. Am now running it though its paces with a wide assortment of tunes. It's good to have a project off the to-do list.
Thank you all.
I've left it unshielded, and buttoned it back up. Am now running it though its paces with a wide assortment of tunes. It's good to have a project off the to-do list.
Thank you all.
I've repaired at least 12 224's (Lafayette's, Bocama's) and a recurring problem was the bad quality impedance switch. I recommend soldering the 8 ohm secundairies directly on the output terminals as the 16 ohm impedance is seldom used. If you want the amp to sing even more decrease the negative feedback by enlarging the 7k feedback resistor using a potmeter in series with it and adjust it to your taste while listening.
Furthermore, the beautiful sound of the 224 is also determined by the paper in oil coupling caps. Unfortunately they are mostly leaking. Of course with mkp's you will still have a good sounding amp but with pio's it goes beyond that.
Furthermore, the beautiful sound of the 224 is also determined by the paper in oil coupling caps. Unfortunately they are mostly leaking. Of course with mkp's you will still have a good sounding amp but with pio's it goes beyond that.
When hunting the problem I did notice the impedance switch was a bit funny. I disassembled it and gave it a thorough clean— was quite tarnished. I will keep it in because my desk speakers are 16 ohms.
thanks for the other tips. I’ll keep them on file for when next I open it up. For now I’m just enjoying having it back together. 🙂
thanks for the other tips. I’ll keep them on file for when next I open it up. For now I’m just enjoying having it back together. 🙂