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Help with voltages on a JE Labs 2A3 SE amp?
Help with voltages on a JE Labs 2A3 SE amp?
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Old 2nd October 2018, 09:36 PM   #1
KG Salvage is offline KG Salvage  United States
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Default Help with voltages on a JE Labs 2A3 SE amp?

I've been building the JE Labs 2A3 SE amp. I have a test circuit complete on breadboards and my voltages aren't quite right.

You can see in the photos what the voltages should be in the square blocks, ignore the voltages I have beside them. I've gone over everything over and over. My resistances are all correct. I've tested the tubes again. Not sure what to do. I know slight changes in the bias can have a lot of affect. Also, this is my first build.

Should I adjust the resistance to get the correct voltage? I'm not sure how it would affect my plate loads and load lines.

Also, when you have direct coupling like this, how would the load lines be done for the second stage. The grid would be 70v , and the plate is at 210v. It's bias is 76v. Would you go by the -6v for your load line. Would the plate be 210v - 76v = 134v. I'm just not sure how that works when direct coupled.

I didn't show in the schematic. The bias voltage on the first stage should be 1.5v , and the second 76v.

Any help is appreciate , Kennyg
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Last edited by KG Salvage; 3rd October 2018 at 01:13 PM.
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Old 2nd October 2018, 10:50 PM   #2
rayma is online now rayma  United States
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Quote:
Originally Posted by KG Salvage View Post
I've been building the JE Labs 2A3 SE amp. I have a test circuit complete on breadboards and my voltages aren't quite right.

Should I adjust the resistance to get the correct voltage?

The grid would be 70v , and the plate is at 210v. It's bias is 76v. Would you go by the -6v for your load line. Would the plate be 210v - 76v = 134v.
This is well within expected tolerances for tubes, another tube will differ.

For DC yes, but the AC load line (at the DC bias point) deletes the bypassed resistance.

Last edited by rayma; 2nd October 2018 at 11:07 PM.
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Old 2nd October 2018, 11:04 PM   #3
R Kneck is offline R Kneck  United States
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Which schematic of JE Labs are you using? JE Labs Stereo SE 2A3 amp schematic dated 9/01 uses the Angela Universal transformer. The secondary of the power transformer has taps at 320-0-320 and 380-0-380 volts.
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Old 2nd October 2018, 11:36 PM   #4
KG Salvage is offline KG Salvage  United States
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Well, it's not that simple. I had a large power transformer that I've been wanting to use. It's much larger than needed, but will work fine. It is 375-0-375. As for the schematic. I think the one in my photos is a sun audio. I originally was looking at a JE Labs schematic. There are so many that use the exact same 2 stage 6SN7 setup. For 2A3s, 45s, 300b, ect. Some use 2.5k, 3.5k and 5k Opts. Another thing I'm dealing with is I'm using 5k output transformers. All the 2A3 schematics have 2.5k-3.5k OPT. I had the OPTs, power transformer, two NOS RCA 2A3s, and two RCA red base 5692s. So this is what I'm working with. I've purchased some cheap tubes till I get it close. I'm to the point that all my voltages are good except what I stated at the beginning of the thread. I think I may try my good tubes now. I've had no issues other than the voltage differs.

I'm understanding load lines fairly good except for when directly coupled as I asked earlier. Hopefully someone can help me with that.

Kennyg
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Old 3rd October 2018, 03:55 AM   #5
famousmockingbird is offline famousmockingbird  United States
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You will just get less power but also less distortion, so no biggie. Do you have multiple taps on secondary? If so use the 16 ohm tap with 8 ohm speakers.
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Old 3rd October 2018, 06:54 AM   #6
6A3sUMMER is offline 6A3sUMMER  United States
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I have used a 5k transformer with a 2A3.

A problem with using 8 Ohm speakers on the 16 Ohm tap is that you will double the insertion loss of the transformer (loss that is due to the primary DCR and secondary DCR).

Suppose the primary is 5k with a DCR of 250 Ohms, and the secondary is 16 Ohms with a DCR of 0.8 Ohms.
If the load on the 16 Ohm tap is 16 Ohms, the transformer insertion loss will be 1 dB.
If the load on the 16 Ohm tap is 8 Ohms, the transformer insertion loss will be 2 dB.
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Old 3rd October 2018, 12:26 PM   #7
Ketje is offline Ketje  Belgium
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There is something wrong with the resistors or the measurements

Mona
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Old 3rd October 2018, 01:10 PM   #8
KG Salvage is offline KG Salvage  United States
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My op transformers only have 8 ohm taps. So that solves that issue. I hooked it up to some speakers and it sounds really good. I think I can get a little more power out of it though. I hooked it up to the scope with a 1vpp tone, and I'm only getting around 11vpp out. One good thing is there is no hum.

Kennyg
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Old 3rd October 2018, 02:51 PM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by KG Salvage
I know slight changes in the bias can have a lot of affect.
No. Small changes in bias usually have no effect. Bigger changes in bias have some effect. You have small changes.

Assume up to 20% 'error' in most voltages is fine for valves.
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Old 3rd October 2018, 06:57 PM   #10
KG Salvage is offline KG Salvage  United States
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Very good. Thanks.
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