5F6A low volume

[vinceg]

feedback resistor just lowers the overall gain,assuming it is the right value and the phase is correct,hence negative.Resistor and cap change has a slight effect,for good or worse,only if your building technique is the best possible required,from what i see it's not,sorry,most probably you have some bad soldering or miswire,although voltages are looking fine,which is a good thing.

I checked all the way but I not able to find the cause of the problem. About feedback resistor, before I had 27K with 16Ohm output. Leave it or change with 75K?

[alexradium]

I checked all the way but I not able to find the cause of the problem. About feedback resistor, before I had 27K with 16Ohm output. Leave it or change with 75K?

27k is the original referred to a 2 ohm output,no wonder it oscillates at 16 ohms,try 75k at 8 ohm it should be ok

[vinceg]

27k is the original referred to a 2 ohm output,no wonder it oscillates at 16 ohms,try 75k at 8 ohm it should be ok

I have two 8 Ohm Jensen speakers connected in series, so I have to use the 16 Ohm output, right?

[alexradium]

I have two 8 Ohm Jensen speakers connected in series, so I have to use the 16 Ohm output, right?

you can use whatever output you want for the feedback,just use the correct value

[Chuck H]

Update
I realized I forgot to solder feedback resistor to out. Unfortunately I do not think was the problem. Now the amp has right volume but sometimes it disappears. It also seems to produce some sort of oscillation when I increase the volume control and it has background noise.

Enzo said:

If an amp was working, then you do something to it, and it no longer works right, it is a safe bet the work you did was the cause. SO look especially at every part you changed, every joint you soldered.

The volume disappearing could be a consequence of an intermittent fault or short. It could be a crossed component lead like Helmholtz spotted, a bad solder joint or a host of other things. Since the amp worked before the problem is almost certainly a result of the work you performed. You say that the work was ONLY done on the bias supply. But you "forgot to solder feedback resistor". The feedback resistor is not part of the bias supply. So you must have done work on circuits other than the bias supply. Like Enzo said, check everything you worked on.

JM2C but if you ran the amp with a 27k resistor from the 16 ohm tap before the work then leave it that way for now. Once the amp is working correctly you might consider changing some things, but you should get it back to working like it was first.

[vinceg]

you can use whatever output you want for the feedback,just use the correct value

Sorry but I do not understand. Before re-wiring I had a 27K feedback resistor, two 8 Ohm speakers in series but I do not remember which OT output I used and the amp worked well, it did not oscillate. Now, I leave everything as before or I put the 75K resistor that corresponds according to a 16 Ohm output?

[Chuck H]

If you had the output transformer or phase inverter leads lifted and there is any possibility that you reconnected them differently then this may cause the amp to oscillate with the feedback resistor reconnected.

As was mentioned before... The problem is almost surely a mistake in your work. Narrow your search to the work you performed.

[vinceg]

Enzo said:



The volume disappearing could be a consequence of an intermittent fault or short. It could be a crossed component lead like Helmholtz spotted, a bad solder joint or a host of other things. Since the amp worked before the problem is almost certainly a result of the work you performed. You say that the work was ONLY done on the bias supply. But you "forgot to solder feedback resistor". The feedback resistor is not part of the bias supply. So you must have done work on circuits other than the bias supply. Like Enzo said, check everything you worked on.

JM2C but if you ran the amp with a 27k resistor from the 16 ohm tap before the work then leave it that way for now. Once the amp is working correctly you might consider changing some things, but you should get it back to working like it was first.

Thank's Chuck, the problem is that I do not know what checked

[vinceg]

The problem is almost surely a mistake in your work. Narrow your search to the work you performed.

Yes, I agree. Unfortunately I re wired the entire circuit and mod the bias circuit. So, I would need a starting point. I have already checked all the connection with tester, the value tension at the pin seems right... I do not know what to do

[vinceg]

Checking heater voltage I discovered that it is 5,20V. Could it be the cause of the problem?

[eschertron]

Checking heater voltage I discovered that it is 5,20V. Could it be the cause of the problem?

That is definitely a problem. Heater voltage less than 90% of 6.3vac will cause issues.

I'd suspect the heater voltage is a symptom rather than a cause, though, based simply on the thread discussions so far.

[vinceg]

That is definitely a problem. Heater voltage less than 90% of 6.3vac will cause issues.

I'd suspect the heater voltage is a symptom rather than a cause, though, based simply on the thread discussions so far.

Why a symptom? The voltage of the heaters depends only on the power transformer. Maybe you mean that I also have a problem with the transformer as well as the circuit?

[vinceg]

One thing I forgot to say before. When I checked the voltages at the plates, in the V5 (6L6) the tester did not read the voltage immediately. In the tester's display I saw the value go up to about 400V and at the same time I heard a whistle that stopped when the tester finished the measurement. While in the V4 (6L6) the reading of about 400V was instantaneous and silent.

[nosaj]

Why a symptom? The voltage of the heaters depends only on the power transformer. Maybe you mean that I also have a problem with the transformer as well as the circuit?

A symptom because something may be pulling it down. Remove all tubes and check filament voltage at each socket. Should be seeing around 6.7VAC unloaded give or take.
nosaj

[eschertron]

that ^^^
Plus, what else may be accidentally shorted to the PT windings? A wire whisker, or blob of solder, and now two circuits have become one - to the detriment of both. If a lot of the build was redone during your mods, then a lot of the build needs to be checked and debugged. I know my heart sinks when I contemplate tearing a bunch of stuff out just to start again, but patience and focus is really the foundation of any troubleshooting.