Dynaco PAS tube preamp bench test procedure following repair using oscilloscope + audio generator

[@andrefiset3569]

I owned one of these in the past. I put new boards with new parts and it sounded a lot better after that, also added an output cap which was added later in the production. One of the treble pots was not of the correct value on my factory build. Sold it after building a 4S universal preamplifier.

[@sipet214]

I am confused with grounding issues. I found on the KZbin that some people say that the oscilloscope should be grounded and an amplifier which has been tested should be ‘floating’ or not-grounded in order to avoid grounding loop and damage the oscilloscope. Please confirm what is the right way of grounding.

[@d-labelectronics]

The Dynaco was not designed to be grounded. I cannot answer your question in reference to this video

[@sipet214]

Thank you for the quick response. I am following your KZbin Chanel for years and I really appreciate your work. May I ask you please to make a video regarding the grounding in relation of using an oscilloscope for testing an amplifier.

[@RobsFixitShop]

I love the groovy opening!

[@EJP286CRSKW]

Not much point in re-testing the tone controls after the first test via phono. They haven't changed. But when you test the phono stage you could check the RIAA. Just check that the response at 20Hz is +19.911dB and at 20kHz is -19.911dB, both referred to 0dB at 1kHz. If these three points are correct, the whole curve has to be correct. At least if you find a huge anomaly at LF or HF you know there is something wrong. You have to crank the level down to avoid clipping at LF. Wondering why this preamp runs on such a high voltage!

[@brentnearhood8874]

Thanks! Terry 🔌

[@vintagetubeamplifiers]

When you say not to ground the oscilloscope and audio generator are you plugging them into your variable isolation transformer and the pre-amp into a different power source?

[@d-labelectronics]

Nope, just using a ground lift adapter on the scope. The Audio gen and PAS already have 2 cond cords

[@NosEL34]

Right on 👍

[@Davesintexas]

I can use your help. I have a Peavey classic 30 II. There is a part missing. D1. Looks like a diode spot. A diode installed on its side. Can you help? Please

[@robertdiffin9136]

Beautiful restoration on an elegant unit.

[@alanblumenstock7863]

Why would you add bleeder resistors on you replacement PS board?

[@d-labelectronics]

The board is configured as the PAS was originally designed, just updated to a PCB. Bleeders could be added if desired

[@RaymondLau-u7x]

Seems like a bleeder would be a good idea to let the power supply caps drain out after the dynaco is turned off. Maybe 220k or 330k from B+ to ground.

[@vfrfoxtrott6430]

👍