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Tubelab Discussion and support of Tubelab products, prototypes and experiments

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Old 3rd January 2020, 01:53 PM   #21
PDL is offline PDL  United States
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What brand and type of rectifier are you using? I recall hearing about some unreliable Russian 5AR4/GZ34 tubes a while back. I have always had luck with the cheap Sovtek 5AR4 tubes
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Old 3rd January 2020, 05:01 PM   #22
w5jag is offline w5jag  United States
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The SSE is as close to an unconditionally stable amplifier, as there is. Oscillation is highly unusual - I can only recall one instance in a stock amp, and it was very recent:

1 channel fades after a few minutes of playing - SSE

The 12AT7 is a high gain tube, made for VHF mixing and amplification. It is more likely that if your amp is in fact oscillating, it is the front end, and not the PA tubes. The PA tubes would quickly fail if they were oscillating at any significant power level.

Good quality pictures of your board and wiring could be very helpful. By failing to post pictures, you are making it unnecessarily difficult for folks to try to help you solve this ( likely simple ) problem.
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Old 5th January 2020, 03:13 PM   #23
ejr12 is offline ejr12  United States
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I now have pictures. But, I need to get a chance to upload them to my computer for posting. Hopefully this afternoon.

In the meantime, I checked the 5v taps (yellow wires) going to the board. On is putting out over 5 volts the other less than 2.

So, I switched the yellow wires and the tube began to glow. However, without any inputs or speakers connected, I got a terrible squeal coming from the amp after a few seconds. This does not happen with the solid state rectifier.

I have decided to fix the rectifier tube issue before working on the UL taps.
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Old 5th January 2020, 11:47 PM   #24
Captn Dave is offline Captn Dave  United States
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The observation you posted re the 5 volt winding voltages isn’t clear to me. Measure the voltage between the two wires. Red lead on one wire and the black lead on the other. Please report your findings as vdc or vac as applicable so we can be sure we are on the same page. Keep one hand in your pocket when dealing with live circuits.

Last edited by Captn Dave; 5th January 2020 at 11:52 PM.
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Old 6th January 2020, 12:02 AM   #25
ejr12 is offline ejr12  United States
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I will test it this week and report back.
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Old 6th January 2020, 07:49 PM   #26
Captn Dave is offline Captn Dave  United States
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I think you said you bought a new board. I’m wondering if you noticed the addition of D3 and D4 as well as the TR1. These are necessary, your rectifier tube is not going to work without them.

Read about it here.

Starting a SSE build
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Old 6th January 2020, 09:17 PM   #27
ejr12 is offline ejr12  United States
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You are correct. The first board was bought a year ago. The second last month. I did find the information regarding D3, D4 and T1. All have been installed and best I can tell appear to be working.

Work and family obligations have prevented me from testing the 5volt taps as you suggested. What numbers should I be getting if they are working correctly?
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Old 7th January 2020, 03:27 AM   #28
ejr12 is offline ejr12  United States
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Thanks for all the suggestions along the way.

I had a chance to measure the 5volt taps this evening. I show 5vac when the rectifier tube installed and the switch open.

It appears that I also solve the squeal when using the rectifier tube. When I disconnected the inputs, I inadvertently disconnected the board’s ground. Once I grounded the board, the squeal disappeared.

This weekend, I will take another look at the UL taps.
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Old 18th January 2020, 02:41 PM   #29
ejr12 is offline ejr12  United States
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Just a quick update. I have now gotten the ul taps working. Again, not sure the problem, but they randomly worked when I connected them this time.

Thanks again
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