Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: MMWDMP on February 14, 2021, 05:56:28 pm
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Hey everyone,
Newcomer to the forum and pretty new to building amps. I am currently working on a Sunn Model T clone and all works well except a case of the infamous presence squeal. While the sound is the same as the classic presence squeal, it doesn't quite behave exactly the same and does not seem to be from any of the normal causes.
As far as I can tell it seems like it is coming from the power section rather than the preamp. The squeal is present with or without an instrument plugged in and is even present when the master volume (pre phase inverter) is turned all the way down. With all the preamp tubes removed the squeal also remains although does get worse. The worsening is a result of the phase inverter tube (V3) being removed while removing V2 and V1 has no impact on the behavior of the squeal. The master volume does have a small impact on the squeal but as stated it persists even when set to zero. Similarly, while the squeal is present with no guitar signal, having a signal can also impact the squeal. With a signal, the treble and mid range tone control also will impact the squeal. Here's a video showing the issue and going through some of the controls: https://photos.app.goo.gl/QDNyBWBfXmqdXaKV8 (https://photos.app.goo.gl/QDNyBWBfXmqdXaKV8). Here's a schematic for reference as well: https://1.bp.blogspot.com/-bBUTbP4o--w/XVQa4FoCJdI/AAAAAAAAJBg/euDrlVU7vTUJF_jx6RlgtIgQzKCuy281QCLcBGAs/s1600/sunn_model_t.jpg (https://1.bp.blogspot.com/-bBUTbP4o--w/XVQa4FoCJdI/AAAAAAAAJBg/euDrlVU7vTUJF_jx6RlgtIgQzKCuy281QCLcBGAs/s1600/sunn_model_t.jpg)
Here's everything I've tried so far:
- Shielded the bottom of the amp with aluminum
- Replaced nearly all signal carrying leads with shielded wire
- Chopsticking the amp to check for changes
- Tapping components checking for microphonics (tubes/caps all good)
- Reseating tubes in socket
- Adding more power filtering caps
- Swapping OT plate leads for positive/negative feedback check (became howl/rumble so returned to original)
- Removing NFB entirely (made issue worse)
- Removing presence control pot (made issue worse)
- Checked for resistance and voltage in ground points (all looked good)
- Raising swamper resistors from 1.5k to 4.5k
- Plugging amp into different wall socket
These all had no effect unless noted.
Really struggling to figure out what the issue could be. Lead dress isn't the best but also not the worst plus chopsticking wires doesn't produce any effect so I doubt that is the culprit. There is a single "star" ground that the PT center taps, filtering cap negatives, speaker jacks, OT, and then two ground busses (one for bias circuit and one for preamp) connect to. Front panel controls are grounded direct to their case which grounds on the chassis. Any help or input would be really appreciated!
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reverse the OT plate wires - did the UL tap wires follow?
be sure the UL taps are wired correctly.
how are C9, C10 R20, R22, R26 wired?
--pete
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So it does that without a valve in V3, the LTP?
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Bingo!
Sometimes we lead ourselves to believe the causes are super complex when they're often very simple. Like looking at the color of 2 wires...
I double checked the wiring of the UL taps and they were reversed. Fixing this has remedied the issue. The amp still gets a slight squeal when maxed out but it now behaves as expected and adding shielding removes the issue entirely.
Thanks so much for your help!