Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: silat on August 27, 2022, 12:08:51 pm
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Does anyone have a layout and schem on a 5f1 Champ with a master Volume? Or at least where I can get a hold of one? I would be very grateful.
Thank you in advance.
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You really don't need a layout for this simple mod, right?
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It just looks like 2 connections from the grid of the 6v6 to a 250k pot to the cap .
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Thank you Sluckey, I appreciate it.
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I've been told putting a MV pot right there isn't good because it changes the output tube bias throughout the swing of the pot.
That statement seems reasonable, but my tube theory is LEGO style lol
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I've been told putting a MV pot right there isn't good because it changes the output tube bias throughout the swing of the pot.
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If the valve is good then grid current should be negligible. So the control grid stays at 0VDC whatever the pot setting.
However, a failsafe backup resistor, to maintain a grid leak path in case the pot becomes open circuit, would be a good idea.
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Thank you both for more input!
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I added the MV and it works but I should have more volume. I did not have enough in the first place. So I I always wanted to add a master volume to one of my Amps and thought the is would be the best for the experiment. Maybe someone else can see see why I do not have
much volume and the pot cuts out at at intervals. At the beginning about two I have a fair amount but Not a whole bunch and then it gets lower until about 7 or and then cuts off after that and there is a minimal amount.
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Seems you have a bad pot or connection or an alternate interrupt cable
About How To may be you can be interested to read this old thread
https://el34world.com/Forum/index.php?topic=20883.msg224791#mlsg224791 (https://el34world.com/Forum/index.php?topic=20883.msg224791#msg224791)
Franco
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Put a 25uF cathode bypass cap on the first preamp stage (just the first, not the second). That will significantly increase your gain and maybe wake it up a bit.
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I added the MV and it works but I should have more volume. I did not have enough in the first place.
You do realize that adding a MV will not increase the volume, right?
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Thanks for all the comments from danhei, kaligstro,nandrew jackson,pdf64,Sluckey.
I am checking in as I am on the road right now. When I get back home I will try the suggestions. I thought for sure I had a bad pot. So i put it to the board for help.
Sluckey, no I did not know, but I do now. It gives me excuses to try knew things, experiment and learn something new.
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An MV is like any other gain control pot, it allows maximum signal to the next stage when the pot is at full rotation, and progressively lower signal amplitude as to wind back the pot rotation.
But ‘maximum signal’ itself depends on the amount of signal gain in the circuit before the pot. (i.e. The pot’s only function is to cut existing gain.)
If you want more gain, you need to either add another gain stage to the preamp before the pot, or change the tube circuit to higher gain tubes (like small-signal pentodes), or mess around with increasing the existing B+ voltage, preamp tube load resistance, bias-point, and cathode bypass caps.
Increasing B+ gives you more headroom but you need about 50 extra volts to notice this. Increasing preamp tube plate resistor size will increase gain (up to a point) - if you’ve got 100k, try 150k, or 220k. Decreasing preamp tube cathode resistance (somewhat) changes the bias point, which can help increase unbypassed gain. If you’ve got 1k5 at V1a, try 1k (or 820R) if you want more gain from an unbypassed cathode resistor. Adding cathode bypass caps to any cathode-biased stage) will boost gain. 22uF will give maximum frequency boost on V1a. You can also do that on V1b if you implement a vibrochamp NFB circuit.
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Thanks Tubeswell for all of the info, I will try that next.
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Thanks Tubeswell for all of the info, I will try that next.
You might or might not like the result of trying everything. So maybe try one tweak at a time, so you can assess the results for yourself.
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I can't try anything now it stopped working.... Everything lights up and it was playing .... So I decided to try some different tubes, I put some NOS tubes in then the 12Ax7 lights up in a flash and everything stopped....No sound after changing the tubes, no sound putting the same tubes back in. I am on the road again so I cannot try anything until I get back.
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Did you only replace the 12AX7? or did you also do some soldering/tinkering?
Check the wiring around the 12AX7 socket to make sure there are no shorts between different tube socket pins (and look for signs of charring from arcing between the socket pins - in case it was a bad tube, and if there is any charring clean it thoroughly off, or maybe replace the socket). In fact, check all the circuitry for any unwanted shorting - use your R-meter to check for unwanted DC-continuity where you know there shouldn't be any.
Once you've established there are no shorts, pull all the tubes out, check (and replace) your mains fuse and fire it up to see if it works without tubes. If the mains fuse still blows, you'll know there is still a short. Disconnect all the PT secondaries and repeat until the PT stays on. If it don't stay on with all the secondaries disconnected, you'll know its the PT
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I replaced the 1/4 speaker jack and the 1m volume pot. I replaced all the tubes and then put the originals back after the light show.. I will try it when I get home.
Thanks again for answering so quickly.
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Did you only replace the 12AX7? or did you also do some soldering/tinkering?
Check the wiring around the 12AX7 socket to make sure there are no shorts between different tube socket pins (and look for signs of charring from arcing between the socket pins - in case it was a bad tube, and if there is any charring clean it thoroughly off, or maybe replace the socket). In fact, check all the circuitry for any unwanted shorting - use your R-meter to check for unwanted DC-continuity where you know there shouldn't be any.
Once you've established there are no shorts, pull all the tubes out, check (and replace) your mains fuse and fire it up to see if it works without tubes. If the mains fuse still blows, you'll know there is still a short. Disconnect all the PT secondaries and repeat until the PT stays on. If it don't stay on with all the secondaries disconnected, you'll know its the PT
I replaced the 12ax7 socket..I have lights on the amp but no sound. Strange it just stopped working.
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Renew PIN solderings
Franco
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I resoldered the pins... Tried the amp with the tubes in place, I then took out all the tubes, the power light was lit and did not shut down. I don't know what to try now.
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Measure some DC voltages. All of them. Proof is in the pudding.
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I Have too much or too little DC voltage where it is needed. I am supposed to have at least 165 or so DCV on the plates of the two halves of the 12Ax7 Pins 2 and 6 but I have 269 DCV on pin 6 and nothing on pin 2. pin 1- 213, pin 3- 1.27,pin 8-2.28
6v6 pin 1-20VDC which is about right. pin 3-387, pin 4-346, pin 8-20 VDC
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I Have too much or too little DC voltage where it is needed. I am supposed to have at least 165 or so DCV on the plates of the two halves of the 12Ax7 Pins 2 and 6 but I have 269 DCV on pin 6 and nothing on pin 2. pin 1- 213, pin 3- 1.27,pin 8-2.28
Plates are pins 1 and 6. Pin 2 igs grid and should be zero.
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I Have too much or too little DC voltage where it is needed. I am supposed to have at least 165 or so DCV on the plates of the two halves of the 12Ax7 Pins 2 and 6 but I have 269 DCV on pin 6 and nothing on pin 2. pin 1- 213, pin 3- 1.27,pin 8-2.28
Plates are pins 1 and 6. Pin 2 igs grid and should be zero.
I have got 213 on pin 1 and Pin 6-269... Seems really high on both but especially pin 6