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Thread: 5F6A low volume

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    5F6A low volume

    Hi, I re-wiring my own 5F6A clone and now it has a very low volume. What could cause the problem?

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by vinceg View Post
    Hi, I re-wiring my own 5F6A clone and now it has a very low volume. What could cause the problem?
    Pictures and the schematic you used would be appreciated.

    nosaj

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    You are right, sorry.
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    I used Ceriatone and Robinette layout for mods
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    Quote Originally Posted by vinceg View Post
    You are right, sorry.
    Not sure, but picture 9 might show a diode shorted by a cap's lead.

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    Quote Originally Posted by Helmholtz View Post
    Not sure, but picture 9 might show a diode shorted by a cap's lead.
    Possible. Thank you for the tip. One thing to watch tomorrow with the light of day. Italy is now evening.

    This could be the cause?

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by vinceg View Post
    Possible. Thank you for the tip. One thing to watch tomorrow with the light of day. Italy is now evening.

    This could be the cause?
    Also since you used Dale resistors, we can't tell what the values are. Sometimes a wrong resistor value in certain position can choke the volume. It will be up to you to verify each one against the schematic or layout.

    nosaj

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    Quote Originally Posted by nosaj View Post
    Also since you used Dale resistors, we can't tell what the values are. Sometimes a wrong resistor value in certain position can choke the volume. It will be up to you to verify each one against the schematic or layout.

    nosaj
    I will also control the values of the resistors. Although I doubt they are wrong. Unless Mouser did not make a mistake.

    Anyway, it seems strange to me that the amp does not have a back hiss. I noticed that without guitar plug in there is not classical valve hiss, even raising volume control. Could it be a problem in V1 preamp section?

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by vinceg View Post
    I will also control the values of the resistors. Although I doubt they are wrong. Unless Mouser did not make a mistake.

    Anyway, it seems strange to me that the amp does not have a back hiss. I noticed that without guitar plug in there is not classical valve hiss, even raising volume control. Could it be a problem in V1 preamp section?
    Anywhere in the signal chain can choke the signal, whether it is in the preamp or the poweeramp.

    See this on page 47 the click test. http://theguitar-blog.com/wp-content...r-Handbook.pdf

    nosaj

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    Italy is now evening.
    Same over here in Germany.

    In winter time most of the day(s) I depend on electric lighting.

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    Old Timer J M Fahey's Avatar
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    Just curious, did it work properly before the Mods?

    Personally I have found that by far the best Mod, the one which brings smiles to users, is to rip all of them off and return amp to original circuit, go figure.

    Not kidding: amps become Classics for a reason: they provide better sound than others since forever, and peak sound and performance was reached after extensive , ear tested modding ... 50 years ago.

    Once at the peak of the mountain, all paths lead downwards

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    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.

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    Quote Originally Posted by J M Fahey View Post
    Just curious, did it work properly before the Mods?

    Personally I have found that by far the best Mod, the one which brings smiles to users, is to rip all of them off and return amp to original circuit, go figure.

    Not kidding: amps become Classics for a reason: they provide better sound than others since forever, and peak sound and performance was reached after extensive , ear tested modding ... 50 years ago.

    Once at the peak of the mountain, all paths lead downwards
    Yes, it worked well. I just decided to change the resistors and capacitors and improve the wiring. The mods concern only the bias circuit.
    However, this night I had a flash of inspiration: I forgot to solder the feedback resistor at the output. I can not wait to run into the lab to see if this is the cause of the problem.

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    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.
    these are the values I have read:

    Power transformer has 325+325 V AC than it is correct. At B+ there is 411V DC.

    V5 (6L6)
    pin3 400
    pin4 413
    pin5 -40
    pin6 404

    V4 (6L6)
    pin3 400
    pin4 400
    pin5 -40
    pin6 407

    V3 (12ax7)
    pin1 233
    pin3 41
    pin6 236
    pin8 41.5

    V2 (12ax7)
    pin1 170
    pin3 1.1
    pin6 310
    pin8 167

    V1 (12ay7)
    pin1 152
    pin3 2.3
    pin6 153
    pin8 2.3

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    Quote Originally Posted by vinceg View Post
    Yes, it worked well. I just decided to change the resistors and capacitors and improve the wiring. The mods concern only the bias circuit.
    However, this night I had a flash of inspiration: I forgot to solder the feedback resistor at the output. I can not wait to run into the lab to see if this is the cause of the problem.
    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.

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    Quote Originally Posted by alexradium View Post
    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?

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    Quote Originally Posted by vinceg View Post
    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

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    Quote Originally Posted by alexradium View Post
    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?

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    Quote Originally Posted by vinceg View Post
    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

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    Quote Originally Posted by vinceg View Post
    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:

    Quote Originally Posted by Enzo View Post
    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.

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    Quote Originally Posted by alexradium View Post
    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?

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    Bent Member Chuck H's Avatar
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    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.

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    Quote Originally Posted by Chuck H View Post
    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

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    Quote Originally Posted by Chuck H View Post
    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

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

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    Supporting Member eschertron's Avatar
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    Quote Originally Posted by vinceg View Post
    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.

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    Quote Originally Posted by eschertron View Post
    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?

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    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.

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    Master Destroyer nosaj's Avatar
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    Quote Originally Posted by vinceg View Post
    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

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    Supporting Member eschertron's Avatar
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    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.

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    Quote Originally Posted by vinceg View Post
    I realized I forgot to solder feedback resistor to out... ...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.
    Are you sure you have the correct polarity for the feedback? Having positive feedback where it should be negative could cause those symptoms.

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    Update

    I checked the heaters voltage at the output of the transformer with socket heaters wires disconnect and it is 6.25V. Right. Then, I extract all tubes and I measured the heater voltage on each socket and it is 6.10V. Then, I measured heater voltage with all tubes in and it is 5.7 -5.9 V. Doing tests, extracting preamp tubes I understood that with only the power tubes in the heater voltage is about 5.7 V. Is it right or there is a problem?

    About the low volume, I built a Audio probe and I injected a 1KHz signal at amp input. Then I checked with probe at V1 preamp out and there was a signal. Then I checked at V2 pin 8 and I heard a very strong pop and I was scared and I left. So I bet the probe at pin 3 of the power tubes and I heard an amplified noise but not the signal. I need your help to use the probe in the correct way and find the cause of the low volume.

    Update
    I think I think I found the point where the signal is interrupted. At V2 pin 1 there is signal but at pin 6 and pin 8 there is not signal. Why? I try swapped the tubes V2 and V3 but there is no change. I also checked with smartphone camera for unwanted contacts and also with tester but it solder seems right.

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    Last edited by vinceg; 12-10-2018 at 11:26 AM.

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    Bent Member Chuck H's Avatar
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    Are you using a light bulb current limiter or Variac type adjustable transformer during this testing?

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    Quote Originally Posted by Chuck H View Post
    Are you using a light bulb current limiter or Variac type adjustable transformer during this testing?
    No. Did I make any trouble?

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    Quote Originally Posted by vinceg View Post
    No. Did I make any trouble?
    No. I'm trying to figure out why your filament voltage is low. It is already below 6.3V with that transformer winding unloaded. This is a problem. It would be typical to see that unloaded voltage be higher than 6.3V to compensate for loading with tubes installed. Do you know your power transformer brand and model number?

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