Hello all, here is a piece of the heater power supply for my ARC D115 mk2. My issue, V2 and v17 shares one of 7815 regulators. V17 is a 12at7 at 12.6 volts and v2 is a 6922 at 6.3volts .My measurements are as follows ,20volts into the 7815, 15.2 vdc out, following the schematic through the 7.5 ohm resistor, the 12at7 is getting 12.3 vdc, then one side of the100ohm resistor is tied to pins 4&5 on at7 the other side goes to pin 9 on the at7, from this pin 9 to the 6922 I get 8.9 volts. I figured the 100ohm was bad,it was reading 110 ohm so I replaced with new one ,well that didn't do it. Is there something else going on here, can I replace the 100ohm with a larger resistor to drop it down to the 6.3vdc. Please advise
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Your 12AT7 pulls a "nominal" 300mA at 6.3 Volts with heaters paralleled. The 6922 is rated for slightly more current, but that's for classic Golden Age production - could be a pretty wide variation with modern production. If you've got 12.3 Volts after the dropping resistor, you're in the ballpark at least. Looks like the solution (with *that* particular pair of valves) would be to move the shunting 100 Ohm resistor to across the 6922's heater and possibly adjust.
I hate series heaters.
All good fortune,
Chris
I hate series heaters.
All good fortune,
Chris
Is this a power supply issue?
Hello all, I've attached two pics one of the actual schematics and one of how it's wired from audio research. I replaced the 100 ohm resistor with no change in voltage. The 6922 is rated for 6.3 volts at 300ma, the 12at7 is rated at 12.6 v 150ma in series and 6.3 v at 300 ma in parallel. It appears to me the at7 is wired parallel at 12.6v, am I assuming wrong? I need to get the voltage down to 6.3 or so on the 6922. Any suggestions would be appreciated. Thanks for your time.
Hello all, I've attached two pics one of the actual schematics and one of how it's wired from audio research. I replaced the 100 ohm resistor with no change in voltage. The 6922 is rated for 6.3 volts at 300ma, the 12at7 is rated at 12.6 v 150ma in series and 6.3 v at 300 ma in parallel. It appears to me the at7 is wired parallel at 12.6v, am I assuming wrong? I need to get the voltage down to 6.3 or so on the 6922. Any suggestions would be appreciated. Thanks for your time.
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12AT7 are always 6.3 wired parallel heaters or 12.6 heaters in series.
With just Vnumbers on the schematic, no way of telling which number is the 6922.
If all the heaters & B+ are hot, look at your wall plug voltage. If the amp was designed in 1965 and you're playing in 2017, wall voltage has gone up 15-20% since then. Look at putting resistance in the 120 vac line to the transformer in that case. I use cinch solder terminal strips for this sort of thing. Besure to unplug before working on mains or high voltages, and use one hand at a time and a clip lead when measuring with voltage hot. Current >24v across your heart can stop it. Wear no jewelry on hands wrists or neck, 1 v through a ring can burn your flesh to charcoal.
If only the 6922 heater is hot, a resistor in series with that is in order. The two terminal cinch terminal strips are useful. Make sure you don't buy the one where one terminal is case ground. I use elastic stop nuts on #6 screws to keep the strips in place permanently.
Cinch strips come from apexelectronic.com LA, triodeelectronics.com IL, tubesandmore.com AZ or mcmelectronics.com OH. All charge >$10 freight so look at buying something else in the box, like 600v rated wire (from the middle two) or something .
With just Vnumbers on the schematic, no way of telling which number is the 6922.
If all the heaters & B+ are hot, look at your wall plug voltage. If the amp was designed in 1965 and you're playing in 2017, wall voltage has gone up 15-20% since then. Look at putting resistance in the 120 vac line to the transformer in that case. I use cinch solder terminal strips for this sort of thing. Besure to unplug before working on mains or high voltages, and use one hand at a time and a clip lead when measuring with voltage hot. Current >24v across your heart can stop it. Wear no jewelry on hands wrists or neck, 1 v through a ring can burn your flesh to charcoal.
If only the 6922 heater is hot, a resistor in series with that is in order. The two terminal cinch terminal strips are useful. Make sure you don't buy the one where one terminal is case ground. I use elastic stop nuts on #6 screws to keep the strips in place permanently.
Cinch strips come from apexelectronic.com LA, triodeelectronics.com IL, tubesandmore.com AZ or mcmelectronics.com OH. All charge >$10 freight so look at buying something else in the box, like 600v rated wire (from the middle two) or something .
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Based on that hand drawn wiring diagram, the 12AT7's heater elements are connected in parallel, where they are spec'd to draw 300mA @ 6.3V. The 6922's heater is also spec'd to draw 300mA @ 6.3V. This identical current draw permits the two heaters to be wired in series, without any shunt resistor. Furthermore, I don't see that 100 ohm shunt resistor in the schematic, so someone appears to have erroneously added it. Maybe, they had intending to rewire the 12AT7's heater to 12.6V series mode, and so then needed to shunt aside half of the 300mA current which the 6922's heater still requires. For that matter, verify whether such a modification of the 12AT7 heater connection was performed.
The total required heater supply voltage then is 6.3V x 2 = 12.6V. The regulator supplies 15V, so that's why a voltage dropping resistor is used. It's value is calculated as: 15V - 12.6V = 2.4V. Then, 2.4V/300mA = 8 ohms. Looks like 7.5 ohms was chosen as a convenient approximation.
The total required heater supply voltage then is 6.3V x 2 = 12.6V. The regulator supplies 15V, so that's why a voltage dropping resistor is used. It's value is calculated as: 15V - 12.6V = 2.4V. Then, 2.4V/300mA = 8 ohms. Looks like 7.5 ohms was chosen as a convenient approximation.
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Hello guys, this might help...I've purchased this amp about a month ago, it works perfectly, it came with3 Russian type 6H23n in V1, V2, V3 positions. I noticed V2 was burning very brightly compared to the other 2. I assumed higher voltage on V2. The MK2 owners manual says (3)6DJ8's in those positions. I was told it was the MK2 model, from what I read on the net the mk1 has 6922's in those positions, the amps look identical except for different tubes .Some people use the 6922 in the MK2. Would the 6dj8 solve this problem?
@Dlav - Starting multiple threads on the same topic in different sub-forums is against the Forum Rules, your threads have been merged.
I do not. Power OpAmps what for are made?
I can even pull half of tubes out of my amp; the rest will see the same 6.3V (a half of 12.6).
Just one opamp and a pair of 1% resistors.
Filament troubles
Hello all, I purchased this ARC D115 mk2 amplifier used, about 2 months ago, from a person I trust, he was the original owner, its never been in the shop for repairs, its all original. Here's my issue. There are 3 6DJ8's, V1,V2,V3. V1&V3 share the same filament supply, there filament's glow at the same intensity which I consider normal. V2=6DJ8 shares the filament supply with V17=12AT7, the problem is V2 burns so much brighter than V1&V3. and V17 you can not see the filament burn at all total darkness. Here's what I've done so far, replaced V2 with a new tube, I've checked filament voltages with a varic set at 120VAC. At pins 4&5 of the 12at7 =12.27vdc, now pins 4&5 are tied together with a 100ohm resistor to pin 9, at pin 9 the voltage is 8.89vdc then it jumps to pin 4 of the 6dj8, pin 5 of the 6dj8 goes to center pin of the 7815 regulator. I assumed the 100ohm res was bad,it was reading 110ohms, installed a new 100ohm with the same results. I'm not a tech by any means, I just don't know where to go from here, should I be concerned with the voltage of 8.89v on a 6dj8, could the at7 be bad? The 12at7 is a driver for the 6550 regulator for the screens. I've attached a schematic of the power supply and a photo of the actual make up. Thanks for your time
David L.
Hello all, I purchased this ARC D115 mk2 amplifier used, about 2 months ago, from a person I trust, he was the original owner, its never been in the shop for repairs, its all original. Here's my issue. There are 3 6DJ8's, V1,V2,V3. V1&V3 share the same filament supply, there filament's glow at the same intensity which I consider normal. V2=6DJ8 shares the filament supply with V17=12AT7, the problem is V2 burns so much brighter than V1&V3. and V17 you can not see the filament burn at all total darkness. Here's what I've done so far, replaced V2 with a new tube, I've checked filament voltages with a varic set at 120VAC. At pins 4&5 of the 12at7 =12.27vdc, now pins 4&5 are tied together with a 100ohm resistor to pin 9, at pin 9 the voltage is 8.89vdc then it jumps to pin 4 of the 6dj8, pin 5 of the 6dj8 goes to center pin of the 7815 regulator. I assumed the 100ohm res was bad,it was reading 110ohms, installed a new 100ohm with the same results. I'm not a tech by any means, I just don't know where to go from here, should I be concerned with the voltage of 8.89v on a 6dj8, could the at7 be bad? The 12at7 is a driver for the 6550 regulator for the screens. I've attached a schematic of the power supply and a photo of the actual make up. Thanks for your time
David L.
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The 12at7 heaters are in parallel, so they need 2x150ma = 300ma.
The 6dj8 draws 365ma
the 100ohm resistor is there to compensate for the extra 65ma that the 6dj8 draws.
if all where in order then both tubes in series should draw 12.6volts at 365ma.
and the connection between the 6dj8 and the 12at7 should be at 6.3 volts.
ok, lets speculate:
so yes either the 100ohm resistor is bad (not in your case) or the 12at7 somehow lowered in resistance! (approximately 10ohms)
try this:
replace the 12at7 with a 22ohms resistor (this is the approximate resistance of the 12at7 heater) and see if the 6dj8 glows normally.
The 6dj8 draws 365ma
the 100ohm resistor is there to compensate for the extra 65ma that the 6dj8 draws.
if all where in order then both tubes in series should draw 12.6volts at 365ma.
and the connection between the 6dj8 and the 12at7 should be at 6.3 volts.
ok, lets speculate:
so yes either the 100ohm resistor is bad (not in your case) or the 12at7 somehow lowered in resistance! (approximately 10ohms)
try this:
replace the 12at7 with a 22ohms resistor (this is the approximate resistance of the 12at7 heater) and see if the 6dj8 glows normally.
Hello captain archer, I did check the cold resistance across the 12at7, per side they were both 4.1ohms each for a total of 8.3ohms or so. I don't have any extra 12at7 at this time to try in its place. The tube that came in the V2 is a Russian 6n23p I've read on line that's its the same as a 6dj8 I would assume it has the same heater current draw. The getter flashing on the at7 is a transparent silver. To install the 22ohm resistor, where does it tie into? I've not done any tube rolling these are the tubes that came with the amp. V2 always glowed much brighter than the rest. I didn't think it was normal that's why I started checking voltages. Captain on the at7, reading voltage from pin 9 to 4&5 across the 100ohm I get 3.82 CDC or so if this helps. Thanks for your advise...
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