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Posted: Mon Dec 06, 2010 9:30 pm
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Looking at the pictures again, check the red wire, between the two big capacitors, below the bias pot, and see if that isn't the middle wire going to the output transformer. :idea:

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Posted: Wed Dec 08, 2010 9:22 am
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Here we go. I emailed Fender and got the schematic from them... does this help to determine what wire/lead to check for biasing?

http://www.imaginedesignworks.com/misc/ss22.pdf

What I found interesting is that if you look on the second page it talks about it being set at 24mV from the factory, which seems higher than the 65 DRRI, but then again, I don't know the plate voltage. It still may be set cold.


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Posted: Wed Dec 08, 2010 12:53 pm
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Awesome! I will add to my collection, thanks. It is a huge help, like having a map while driving across country. Ah yes, second page look at the two 6V6 tubes. The blue and brown wires are connected from the output tubes to the output transformer primary and looks like there are terminals, or at least connection points on the circuit board. The terminals (push on connectors) are usually marked Pxx (such as P30) on the circuit board and schematic. The test points for meter readings are marked TPxx (such as TP2) on the schematic, and you will have to locate them on the circuit, look for components to find the location of the TPs.

The B+ supply (DC voltage) to the transformer for the tubes is the red wire at terminal P30 one end of the wire, P18 the other end of same wire (maybe the red wire between the two big caps on the left?). That red wire goes to the middle tap of the output transformer from the power supply, and is approximately the Plate voltage. The schematic shows +420V here, which is at TP2. Your reading from the chassis to TP2 will be close to this number, but it's normal if there is a difference plus or minus a few volts. TP43 is the actual Plate voltage for V7 output tube. Use TP43 to get Plate volts and TP41 for the Cathode current for that V7 output tube. The schematic shows 415 volts at TP43, your reading may be a few volts different, that is OK.

You are correct, TP41 and TP40 are the two tests points to measure Cathode current with a DC voltmeter. What you are actually measuring at the TP is a voltage drop in mV across a 1 ohm resistor, which can be directly read as mA using ohm's law. Read from the chassis to a TP for the reading. The schematic does show 24 mV (therefore 24 mA) each tube for the Cathode current. The Cathode current will be a few milliamps higher than the Plate current, and the Plate current is what we are really concerned with. So just subtract 3 mA to be conservative and call it 21 mA Plate current. At 21 mA Plate current and 415 Plate volts, that is 8.7 watts of idle dissipation for the 6V6 tube. The 6V6 tube is rated for 14 watts, and you typically want to set the idle somewhere between 50% to 70% of the tubes rating, or between 7 watts and 9.8 watts idle. The conditions indicated on the schematic would give approximately 8.7 watts of idle, which is in that range of 50% to 70%, right in the middle, just on the warm side.

Since you have the TPs to check your bias, no need to check the transformer resistance, or voltage across it, unless you just want too. The actual Plate voltage will be only a few volts different than the B+, but check from the chassis to wherever the blue or brown wire connect from the transformer to get the most accurate reading of Plate volts. Be extremely careful to avoid putting your meter on the tube sockets or the solder connection points of the tube sockets, because it is too easy to slip and the terminals are very close together. This is high voltage, one slip and damage or injury can occur. Note that the output transformer wires will have one color wire for one tube and the other color wire for the other output tube. Use the corresponding TP to check the Cathode current and Plate voltage for the same tube. In other words, you don't use the Plate voltage reading on one output tube, and the Cathode mV on the other output tube. Sometimes there will be slight imbalances between the two output tubes, that's normal. You can test one side or the other, it doesn't matter the bias pot will adjust both.

Please let me know how it turns out for you. :)

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Posted: Wed Dec 08, 2010 1:42 pm
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+1 Idle rougly 10W seems warm-hot for 15Wmax (but safe)
By the way, because you substract the grille current, 12Wmax shall considered as plate dissipation (GE datasheet for instance) compare to 8W (almost 70%), otherwise consider Upk*Ik + sum(U*i), approximation ... 8) for max valve power.

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Posted: Wed Dec 08, 2010 1:55 pm
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Should be about 16-23mA


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Posted: Wed Dec 08, 2010 2:10 pm
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You people are so smart! :D

Glad I found this forum. Now I just have to worry about not electrocuting myself.


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Posted: Wed Dec 08, 2010 3:32 pm
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Tissan wrote:
...By the way, because you substract the grille current, 12Wmax shall considered as plate dissipation (GE datasheet for instance) compare to 8W (almost 70%), otherwise consider Upk*Ik + sum(U*i), approximation ... 8) for max valve power.


Hi Tissan,

I always counted the tubes rating as the Plate dissipation only, not the whole tubes dissipation. Do I have that wrong?

Upk*Ik + sum(U*i) I think I understand this, is it: [(voltage from Plate to Cathode) x (Cathode current)] + ( sum of grid currents) ? (I don't know if I got the last part right). Can you translate? :lol:

I know that the Plate current + the grid current(s) = Cathode current, but are you saying that we should count the grid currents as part of the dissipation rating of the tube?

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Posted: Wed Dec 08, 2010 9:39 pm
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I tested the voltage @ TP43 and it was spot on with the schematic (415VDC). Now I know this may be a stupid question, but if I want to check TP41 (24mV) what do I set my multi-meter dial to?

Also, do you still keep the black probe on the chassis (ground) or do you measure TP41 by putting the probes on the annode/cathode of the diode?


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Posted: Wed Dec 08, 2010 10:08 pm
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Not at all. TP41 is also a DC voltage, the reading will be in mV DC (0.024 Volts DC is 24 millivolts DC, or 24 mV DC), so a low DC volts scale on your meter (maybe you have a 2 VDC or 200 mV DC scale?). Check your schematic notes, second page at the bottom - "All voltages measured with respect to ground..." Meaning put the negative lead to the metal chassis. You can clip the negative lead to the chassis, or put it in a screw hole on the chassis and leave it there, then use your positive lead as a probe, moving from point to point for your readings. So, yes keep your black lead on the chassis for test point readings. Make sure to follow the schematic notes when applicable, such as the settings for your knobs and switches (for example reverb off, etc). Please keep me updated, sounds like you are off to a great start. :)

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Posted: Thu Dec 09, 2010 12:49 am
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For shimmilou

What you are doing is very appreciated, in the right way, and pedagogic.
My remark is only for challenging electronics backgroung (Ph.D. drawback) As it is, the complete power is defined as sum (V*I).
If you consider only the plate-kathode power, the power is defined as (Vp-Vk)*Ip to be compared to "Plate power", Ip is estimated by substracting Ig of Ik in that sens your method is a good approximation but you have to compare to plate power (for a 6V6GT GE it is 12W not 15W).
If you consider the valve power, the power is defined as ((Vp-Vk)*Ip) + ((Vg-Vk)*Ig) + ... to be compared to full power of the valve.

(Physically power have no sens, only energy is real it is what you pay, power is used to compare such as % is used to compare 2 data that have note the same range)

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Posted: Thu Dec 09, 2010 7:23 am
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Thanks Tissan,

I think I understand, thank you. :) The letter designations are different for us sometimes. Same science, different languages, cool. 8)

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Posted: Thu Dec 09, 2010 10:45 am
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Tissan wrote:
For shimmilou

What you are doing is very appreciated, in the right way, and pedagogic.
My remark is only for challenging electronics backgroung (Ph.D. drawback) As it is, the complete power is defined as sum (V*I).
If you consider only the plate-kathode power, the power is defined as (Vp-Vk)*Ip to be compared to "Plate power", Ip is estimated by substracting Ig of Ik in that sens your method is a good approximation but you have to compare to plate power (for a 6V6GT GE it is 12W not 15W).
If you consider the valve power, the power is defined as ((Vp-Vk)*Ip) + ((Vg-Vk)*Ig) + ... to be compared to full power of the valve.

(Physically power have no sens, only energy is real it is what you pay, power is used to compare such as % is used to compare 2 data that have note the same range)


The Weber Bias tables seem to take this all into account pretty well:

http://www.webervst.com/tubes1/calcbias.htm

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Posted: Mon Dec 13, 2010 6:05 pm
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Thanks for everyone's help. I finally got it figured out and dialed in.

There is one more question I have though. There is about a 1mv difference between V1 and V2 (23.4 - 24.4)... I'm assuming that is normal, but just wanted to double check.


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Posted: Mon Dec 13, 2010 6:29 pm
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Such variations are common within the cascaded gain stages of a pre-amp section.

Arjay

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Posted: Mon Dec 13, 2010 7:34 pm
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Yes, thanks you guys ! Just being a bystander on this one, I found those schems to be very cool. The power amp is real basic old school stuff. glad to see that. Tissan and Shimmilou, it's always a pleasure to read your posts. Good info for every level of amp enthusiasts ! Art

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