according to the schematic that I have the balance POT goes from the + input of the RCA connector to the POT then to pin 2 of the 12AX7
The phono channels are simply piggy backed in on the input lines from the 7025 to the 12ax7s
I removed C25 and R4 leaving C24 in the connection
This is exactly how Channel 1 is wired
since doing this both the balance and sound quality have improved considerably
Balance is almost perfect and it could just be my left ear being worse than my right that is actually causing the balance issue
The phono channels are simply piggy backed in on the input lines from the 7025 to the 12ax7s
I removed C25 and R4 leaving C24 in the connection
This is exactly how Channel 1 is wired
since doing this both the balance and sound quality have improved considerably
Balance is almost perfect and it could just be my left ear being worse than my right that is actually causing the balance issue
Here is the original schematic and wiring:
http://afterlifedallas.com/modules/coppermine/albums/userpics/10002/185aa_sams_6.jpg
and here are the edits i made in red:
http://afterlifedallas.com/modules/coppermine/albums/userpics/10002/185aa_sams_6_phono_delete.jpg
in fact the phono tube, the 7025 is sitting on my desk and is no longer in the amp
These are the same mods that people on the DIY Tube website as well as audio karma have done to the exact same amplifier
http://afterlifedallas.com/modules/coppermine/albums/userpics/10002/185aa_sams_6.jpg
and here are the edits i made in red:
http://afterlifedallas.com/modules/coppermine/albums/userpics/10002/185aa_sams_6_phono_delete.jpg
in fact the phono tube, the 7025 is sitting on my desk and is no longer in the amp
These are the same mods that people on the DIY Tube website as well as audio karma have done to the exact same amplifier
according to the schematic that I have the balance POT goes from the + input of the RCA connector to the POT then to pin 2 of the 12AX7
The phono channels are simply piggy backed in on the input lines from the 7025 to the 12ax7s
I removed C25 and R4 leaving C24 in the connection
This is exactly how Channel 1 is wired
since doing this both the balance and sound quality have improved considerably
Balance is almost perfect and it could just be my left ear being worse than my right that is actually causing the balance issue
I have the diagram for the 185aa version. If you have R14, 470K, on channel 1, to ground in your circuit, that is the non-adjustable equvalent of the balance pot that sets the input impedance to the 12AX7 for channel 1. You have to duplicate that or remove R14, too. However, removing R14 is not a good idea. You need the 470K input impedance.
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ah, i will have to see if I have removed R14 then
that could solve the balance issue
thank you
I went back to edit my post about removing R14.
R14 provides bias for the grid, as does the balance pot in the other channel. It must be present. You could swap the balance pot for a fixed resistor, so both channels are the same but why? Unless your room is perfectly symmetrical you might need to adjust the balance. And you have left a coupling capacitor in place to isolate the valve from any DC at the input? It may be that channel #2 is now getting its bias from the source i.e. two mistakes have cancelled each other out (no capacitor, no resistor).
This might be obvious, but just in case anyone is confused, the phono inputs (now removed) look like they were intended for ceramic cartridge, not MM.
This might be obvious, but just in case anyone is confused, the phono inputs (now removed) look like they were intended for ceramic cartridge, not MM.
ok thanks for the advice, I will more than likely add a 470k resistor to Ch 2 then to keep them even when i get home in the morning
I will also double check the caps when i get home, I do recall that I used the same size caps on both channels and left out the smaller one, I may have made a mistake on my phono delete schematic and will double check it in the morning
I will also double check the caps when i get home, I do recall that I used the same size caps on both channels and left out the smaller one, I may have made a mistake on my phono delete schematic and will double check it in the morning
on the subject of power iron I was wondering what would be a good direct replacement for the power transformer?
The power transformer isnt giving me any problems but I would like to recreate this amp with all new parts following the original schematic but I cant seem to find a power supply with exactly the same specs as the one it has now
If I were to at some point need a replacement power transformer what would be a good replacement?
would I need to change any resistors or the design of the circuit if I used a different power supply?
Ive been doing alot of research on how to design tube amp circuits and im still not clear on it all, answers to these questions would help considerably
Im still reading the NEETS books and doing alot of research on the webernet
The power transformer isnt giving me any problems but I would like to recreate this amp with all new parts following the original schematic but I cant seem to find a power supply with exactly the same specs as the one it has now
If I were to at some point need a replacement power transformer what would be a good replacement?
would I need to change any resistors or the design of the circuit if I used a different power supply?
Ive been doing alot of research on how to design tube amp circuits and im still not clear on it all, answers to these questions would help considerably
Im still reading the NEETS books and doing alot of research on the webernet
hmmm, i may be interested
PM me with some details on it
I added a 470k resistor to channel 2 and now im getting no sound from channel 2, even after removing the resistor...
I swapped the tubes around and channel 1 works with both tubes so i didnt hurt the tubes
now im going to have to track down what went bad
if I remove both inputs from the RCA connectors I get a hum from both speakers...
its goes away when I have both RCAs plugged in
Im probably just going to rewire the whole thing, its a mess of wires under there anyway
I will leave the power wires alone but the majority of the signal wires im going to remove and replace, probably replace the caps while im in there too
PM me with some details on it
I added a 470k resistor to channel 2 and now im getting no sound from channel 2, even after removing the resistor...
I swapped the tubes around and channel 1 works with both tubes so i didnt hurt the tubes
now im going to have to track down what went bad
if I remove both inputs from the RCA connectors I get a hum from both speakers...
its goes away when I have both RCAs plugged in
Im probably just going to rewire the whole thing, its a mess of wires under there anyway
I will leave the power wires alone but the majority of the signal wires im going to remove and replace, probably replace the caps while im in there too
If swapping a resistor creates problems, how much more will a rebuild do? Changing a circuit can create faults as well as fix them.
If it worked, and now it doesn't, then it is most likely that you have introduced a fault. It is just about possible that it is merely a coincidence, but unlikely. Take a deep breath, and think things through before heating up the soldering iron. What is the 470K supposed to do? Where is it supposed to go? Given that it should be present, how come it seemed to work without it? You need to have answers to all these questions (hint - see previous posts by me and others).
If it worked, and now it doesn't, then it is most likely that you have introduced a fault. It is just about possible that it is merely a coincidence, but unlikely. Take a deep breath, and think things through before heating up the soldering iron. What is the 470K supposed to do? Where is it supposed to go? Given that it should be present, how come it seemed to work without it? You need to have answers to all these questions (hint - see previous posts by me and others).
ok after replacing the cap on the dead channel and replacing the resistor I now have full sound back
I replaced the cap and resistor on the other channel, Ch 1 and the sound is perfectly even now
I replaced the ceramic disc caps with mica caps as well
much better sound quality now
can someone tell me what the 470k resistors function are in the input lines? I dont have them connected and am having no issues at present
Im about to take a nap before i have to head out to work, I work graveyard, so the amp isnt going to get used til i get home in the morning
what purpose do those resistors serve and is it necessary for them to be installed?
I replaced the cap and resistor on the other channel, Ch 1 and the sound is perfectly even now
I replaced the ceramic disc caps with mica caps as well
much better sound quality now
can someone tell me what the 470k resistors function are in the input lines? I dont have them connected and am having no issues at present
Im about to take a nap before i have to head out to work, I work graveyard, so the amp isnt going to get used til i get home in the morning
what purpose do those resistors serve and is it necessary for them to be installed?
Use the meter to compare a good channel to a misbehaving channel. If you make a change in a circuit and something doesn't respond as you expect, test like points between good and bad channels to find mistakes or failure points and go from there. And if it ain't broke, don't fix it. The engineers wanted those 470k resistors to ground, in there, the circuit will deliver sound with out them because they fine tune the performance of the input signal to the grid and help keep the grid at (-) DC potential to the cathode. But without them the tube operating perameters will be different. Poor performance in different modes and probably shorter tube life from higher current through the tube.
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ok I will put the 470k resistors in there and see what it sounds like
so far as I played with it before i had to head out to work it sounds great and I discovered that one of the tweeters in the speakers i bought has gone out, this would also explain the off balance
Im going to buy a new 500k POT and put it back in line as per original specs and see if I can use it to adjust the balance, with the original POT in place it made no difference and at around halfway would just cut out the channel completely... could just be a bad or dirty POT
so far as I played with it before i had to head out to work it sounds great and I discovered that one of the tweeters in the speakers i bought has gone out, this would also explain the off balance
Im going to buy a new 500k POT and put it back in line as per original specs and see if I can use it to adjust the balance, with the original POT in place it made no difference and at around halfway would just cut out the channel completely... could just be a bad or dirty POT
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