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Old 22nd October 2018, 01:48 AM   #21
PRR is offline PRR  United States
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Would be good to have links to the items being asked-about.

First look at the PT sold with the S5 K12G and compare with what you have. About twice the iron that S5 thought was acceptable.

The XPWR025, 180VAC, will make 250V DC.

There's little detail about the S5 K12G. The 10GV8 is a 7 Watt tube. Four of them could eat 28 Watts. 240V at 28W is 0.11 Amps. If this was a fancy high power amp we might allow some dynamic current; I'm sure this is a simple amplifier of essentially steady current.

0.2 Amps seems plenty.
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Old 22nd October 2018, 02:03 AM   #22
JTCamp is offline JTCamp  United States
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Thanks PRR.
I searched quite a bit but have found different opinions. Here is a post for example I kinda sorta burned up my K12-G power transformer

that includes the following:

MODEL: K-8G
INPUT: 120VAC 60Hz (BLK-BLK)
OUTPUT: 170VAC 300mA(RED-RED)
5.9-0-5.9VAC 2A(GRN - YEL - GRN)

I just don't know enough to determine if the transformer I have will work as well as the original...my hope was for it to be better. I can't imagine that I would have picked this out without reading about it somewhere but you never know.

Attached is a schematic for reference.

Thanks
John
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File Type: png S5-Electronics-K12G-Tube-Amp-Kit-Schematic~2.png (30.5 KB, 190 views)
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Old 22nd October 2018, 02:42 AM   #23
6A3sUMMER is offline 6A3sUMMER  United States
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One thing I notice about your schematic: It shows Pentode Mode output stages. But you said you are using the 1609 transformers in Ultra Linear Mode. It is one or the other (unless you go Triode Wired).

For Ultra Linear and Triode Wired modes, I suggest using four each 100 Ohm resistors (one to each screen connection).

You should have fun listening when you wire one up for the second time.

Last edited by 6A3sUMMER; 22nd October 2018 at 02:44 AM.
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Old 22nd October 2018, 03:02 AM   #24
JTCamp is offline JTCamp  United States
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You're right! I just attached it as a reference for the original design. I will be using ultra linear. I think I had the Hammonds hooked up before but only crudely...hard to remember for sure. It sounded better to me in UL vs standard.

What do you think about my current power transformer's capacity?

Where would I add the resistors specifically? I'm green. Also, what is your reasoning for doing so?

Thanks
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Old 22nd October 2018, 03:11 AM   #25
6A3sUMMER is offline 6A3sUMMER  United States
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Some pentodes may oscillate in either Ultra Linear mode or in Triode Wired mode.
The 100 Ohm resistors would be connected directly right on the socket screen connections.

UL: The other end of the 100 Ohm resistors would go to the correct Ultra Linear taps (plate and screen of the same 1/2 of the secondary).

Triode Wired: The other end of the 100 Ohm resistors would go to the plate connection of the same tube.
This is an easy one to try if you like experimenting.

The Edcor XPWR025-120 should work OK for your amp.
And I am surprised, it has an 11.6V filament winding.

For everyone who uses a 12.6V transformer filament secondary, but uses four 10GV8 11.6V tubes:
Save your tubes from pre-mature failure
The 10GV8 filament is 11.6V @ 0.45A
Most transformers have 12.6V output, not 11.6V output.
You need 11.6V @ 1.8A (0.45A x 4 = 1.8 A).
12.6V - 11.6V = 1V Drop 0.5V per leg.
0.5V / 1.8A = 0.28 Ohms
(1.8A squared) x 0.28 Ohms = 0.9 Watts heating, use either two 3 Watt or two 5 Watt 0.28 Ohm resistors.

Last edited by 6A3sUMMER; 22nd October 2018 at 03:35 AM.
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Old 22nd October 2018, 03:17 AM   #26
JTCamp is offline JTCamp  United States
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Thank you for explaining that. I'll try your suggestions once I get to that point. Sound pretty straightforward.

I know for sure that my transformer is 11.6 on the heater winding at 2A, so I think I'm good there. I think this may be why I picked this transformer originally because it had the spec'd tube heater voltage available. My concern was with the 180V winding at 200 mA capacity. I had read that the original was closer to 300 mA. PRR mentioned that it'd probably be fine but I'm just trying to be sure. I didn't want to move forward with the wiring only to find out that the transformer can't supply the necessary amperage.

John

Last edited by JTCamp; 22nd October 2018 at 03:38 AM.
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Old 22nd October 2018, 08:48 PM   #27
PRR is offline PRR  United States
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Pentode/triode will not be a significant difference to the power transformer.

_I_ think that PT is nearly twice as big as necessary (OTOH the S5 PT may have been marginally small). It is "wrong" in that you maybe could have spent less. However a PT in the hand is always better than any "just-right" PT which has to be custom-wound and shipped heavy-mail.
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Old 22nd October 2018, 11:16 PM   #28
JTCamp is offline JTCamp  United States
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Ok guys, I'm going to take your word for it. I really appreciate your help so far.

I have attached a paint modified schematic.

Can someone explain why the red circled line does not have a coupling capacitor? I would think that the B+ would be feeding into the grid on tube 2A. I would think it would need to be similar to the yellow circle.

I understand that V2A and V4A are being used a phase splitters. Also that V2B, V1B, V4B, and V3B are amplifying. I only think I know this from watching Uncle Doug and Blueglow Electronics on youtube. What I don't know is the function of V1A and V3A.

What I'm getting at is where should I use shielded wire? I'm trying to determine the audio path but I also want to understand where high voltage might exist and prevent the use of sheided wire. I have highlighted in blue what I think it the audio path...for one channel.

Thank you,
John
Attached Images
File Type: png S5-Electronics-K12G-Tube-Amp-Kit-Schematic~2.png (119.8 KB, 143 views)

Last edited by JTCamp; 22nd October 2018 at 11:21 PM.
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Old 23rd October 2018, 12:05 AM   #29
PRR is offline PRR  United States
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You can figure it out.

What is the likely DC voltage at the plate of the first stage? What is a good DC voltage for the grid of the second stage?

What is your line-in signal level? What signal is needed at the grids to the power tubes?

Where is signal at or below line level, and might need shielding? Where is it amplified-up above line level and likely does not need shielding?
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Old 23rd October 2018, 07:19 PM   #30
6A3sUMMER is offline 6A3sUMMER  United States
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JTCamp,

Be careful of shielding.

With a very low impedance signal source driving the input, then when the 100k pot wiper is turned to 50k (worst case condition), the pot wiper output impedance is 25k. This impedance has to drive the capacitance of the shield wire, the grid to cathode capacitance, and the 1+ Miller capacitance of the input tube. This might decrease the high frequency response. This effect could be even worse if the signal source is a high impedance, depending on where the pot wiper is set.

Similar effects are true for the other stages; where plate impedance in parallel with plate load 150k or 47k (and cathode impedance in parallel with 47k) have to drive the shield wire capacitance and grid capacitances.

Short shielded wires that are low capacitance per foot will probably not be a problem; but that requires attention to the distance and shielded wire specifications.

You have a series of 3 areas where the shield wires might have an effect. Suppose the capacitance of each of those 3 areas are -1 dB at 40 kHz. That would be a total of approximately -3 dB at 40kHz, and -1 dB at 20kHz.

I do not use shielded wires on my amplifiers, and I am able to get the output ripple to be less than 500uV, with several amps with less than 100uV. That requires good layout; wiring placement; transformer, choke and OPT orientation and spacing; and good attention to ground loops . . . especially the loop of the first and second cap of the B+; as well as the ground loops of both the input and output connectors. I do not use negative feedback (which sometimes amp designers use when attempting to reduce hum).

Last edited by 6A3sUMMER; 23rd October 2018 at 07:28 PM.
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