How to repair Dynaco PAS tube Preamp D-Lab Tek Tips Power supply fix

Here I am, the Fix, At your service! This Dyna PAS came in for a Cap board, well as usual, that turned into more. Lucky, this is a very nice clean unit, so the additional efforts were minimum. I love Dynaco. I would put them up against a Marantz Preamp any day. Yes, I have covered the installation of the SDS Labs Cap board in the past, so I will post the link to that video for your review. This video is full of valuable Tek Tips when you are tackling this task. If you do not feel comfortable performing this job, let me know, send it to my shop. I am developing the D-lab PAS Plus cap plus high current switching PCB. Soon I will demo this new system, plus offer it for sale. The offering will be limited, unless it goes crazy!
Thank you all again for being by my side. Here is the link to the original step by step procedure. • How to Replace filter ...

[@richsherman3673]

I always connect all PAS Preamps to a Dual Channel Scope and look for channel to channel balance. Often these preamps have Volume and Balance controls that are gone, and the sonic improvements by upgrading the Volume Control is amazing. The Balance Control is difficult to find used, but re-soldering 50 year old connections makes a Huge Improvement. SDS Labs Board is a must have. Love the 12X4.

[@bobrowlette4143]

About the the potential for the Dynaco PAS to sound very good... Several years ago, a friend had one of these that was heavily modded by a shop that specialized in such things. At first I suspected snake oil until I actually heard the thing. Along with an also modded Dynaco Stereo 70 and a pair of Chartwell LS-3/5a speakers, it made a very remarkable sounding system indeed. Much higher sound quality than many larger / higher dollar systems I've heard.

[@waynewayne3709]

Good to see a step by step instructions on working with test equipment and valve audio equipment.

[@superhet7281]

An excellent tutorial. Thanks, Terry! And I like the Fab Four shirt! Your videos are always very good, but they’re even better when they’re about a piece of equipment that I own.

[@dirtydon8661]

Great video and tech tips. You the man Terry!

[@garysmith8455]

Oh man!, I have been through the changing of components in my 1965 PAS-2 preamp. Changed out to a modern power supply and ended up with MORE hum then the original. Selenium/cap assy. replaced with same type you have there Terry. Ended up putting the ORIGINAL powersupply back in operation, had to read the assembly directions carefully and make it all look like original. The only thing done now, was a complete cap job. The FACTORY ASSEMBLED preamp remains UN-changed and it sound SO good with a pair of Mark 3 amps with safety changes made, all built on the same date and consecutively numbered.... Klipschorns UN-altered from 1968 making music............FUN STUFF! Thanks for the stemless wine glass (O;

[@RCALivingStereo]

Nice work indeed, and that wine is very good to I had that a few times

[@lcarliner]

There is one mod that David Berning did with the PAS 2 that made a dramatic improvement in sound quality. It was to replace the Centralab integrated resistor/capacitor pack with discrete components and to replace any ceramic capacitors that pass audio through it with high quality non-electrolytic types.

[@fabinhoosmar]

Está ai, um aparelho que merece meu respeito. Bom trabalho colega. abssss

[@swinde]

Based on the front panel this should be a PAS-3 or possibly a PAS-3x. The PAS-2 did not have this panel, and the PAM-1 was a monophonic preamp designed to mate with their Mark III and Mark IV power amps. The PAM-1 had no power supply and drew their power from the power amp supply through the octal connector provided.

[@ordinaryaverageguy76]

Leaving a tube rectifier in has an advantage (assuming the xfmr isn't overworked). With it, the HV comes up slow, no filter cap inrush, so it's easy to slip in a low-current fast-blow pigtail fuse to help protect the transformer winding.

[@EJP286CRSKW]

Also the HT comes up to a lower value, because the rectifier drops tens of volts, which means the HT current is lower as well.

[@mikelove9832]

Thanks Straight Ahead !

[@decsnspace]

This video couldn't have came out at a better time for me. Just got my Blueglow/Kegger KT88 SE amp built a few weeks ago and I'm currently using a Dynaco PAS3X to drive it as a preamp (BTW, the KT88 SE amp sounds incredible!!!). I have already updated the power supply board like in your previous video, but I'm currently using the rectifier tube. I'm thinking I might try changing to solid state per your advise in this video to reduce the load on the transformer. I also am having some intermittent rustling in the left channel so either have a bad connection, dirty socket, or perhaps a bad tube that I need to track down. Thanks Terry! Great video once again! Keep it up!

[@decsnspace]

@@goodun2974 thanks for the tip. I'll keep that in mind if some deoxit doesn't fix the issue.

[@oldschoolelectronics8573]

Very nice!🎛

[@lincolnmicrophonellc]

Nice work! Wow they let that power transformer go to the field with the filament winding working @ > 100% headroom 😬

[@LiveMusicOntario]

Sold my PAS 2 to a hobbyist just this past June 2019. My father built it, Similar, but with the gold front and brown knobs - no where as pretty as this one, for sure.

[@ec8107]

The other option for the rectifier is to make a diode rectifier out of an old tube base and diodes. You get a solid state rectifier without modifying the circuit.

[@TheGuitologist]

I cringed when you said he wanted to leave the rectifier tube. Nice video, Terry!

[@ctbcubed]

The owner probably doesn't understand that the rectifier has nothing to do with the audio chain and didn't want to lose that "warm" tube sound.

[@thomasmiccolis9051]

You evidently know nothing about surge currents & cathode stripping. The rectifier tube has some unique benefits in this application. Best choice is to put in an NOS 12X4

[@tasmith1969]

That appears to have the 3X tone controls so you will need two 1uf capacitors that go between the controls and the board. Those prevent DC from getting to the output jacks.

[@vuch9208]

I enjoyed watching your video! Would it be possible to replace the old transformer with a larger one so it wouldn't be as stressed as it is per your explanation?

[@cowasakiElectronics]

I'm really liking your videos and would like a valve amp myself as I really love the sound. All the ones I have tried require too much volume before you get the distinctive sound. Do you have any suggestions for a decent valve amp that gives that decent valve sound without blasting the windows out? I love the vintage AC sound so something with that TYPE of sound but quieter? Thanks.

[@torchandhammer]

I don't have a Dynaco but I had just poured a glass of red wine so this seemed like a good choice.

[@rciancia]

Nice fix and explanation Terry. One has to wonder why that switch was bypassed... Did you consider that maybe the switch is bad and not working... or perhaps dirty and someone didn't know about DeOxit ? Great video and explanation of how to approach troubleshooting.... thanks man ! Ron

[@EngineeringVignettes]

Looks good. Pad 14 looked a little funny though... Cold solder joint? Hard to tell from the video... Cheers,

[@TheRadioShop]

You sure take on some interesting projects Terry. Nice work.

[@RobertKohut]

Nice!!

[@intothevoid9831]

Judging by how that diode terminal stack was wired, it looks like they tried to make a full wave diode bridge. Unfortunately it didn't seem they knew what they were doing.

[@preiter20]

Great troubleshooting video Terry. What type of hook-up wire do you routinely use? Solid core or stranded? 18, 20, or 22 gauge? I m looking for solid core wire for repairs and I am having trouble finding a source. I find solid core much cleaner to solder than stranded. For tube equipment it needs to handle 400 or 600 volts I believe.

[@roberthurless4615]

Try Amp Parts Direct in Altamonte Springs, FL. They sell great wire for amp work. It is not solid core, but it stranded with a top coat so it works like solid core. It is rated at 600V.

[@neccron9956]

It looks like the science project was an attempt to regulate the power supply with some Zeners.

[@timthompson468]

I’m curious. Would it be useful to add a small filament transformer for the rectifier tube to reduce the load on the main transformer?

[@intothevoid9831]

You'd be better off just using a beefier power transformer period.

[@d-labelectronics]

Triode electronics sells a Heavy duty PAS Transformer, The model PA522, upgraded for $119.99

[@sergeorge75]

Where are you located, are you in Napa Valley and have shop there?

[@cesarrivera5871]

How do I buy this part?

[@stokessd]

For anybody who would like to get the circuit board directly, I have redrawn the board and put it on OSHPark for anyone to buy directly. You'll have to order parts, but the board schematic is the same. OSH Park sells boards in groups of three, so get two friends and split the board cost: oshpark.com/shared_projects/yKbz8cQ9

[@cornelishummel7987]

47k as bleeder for the capcitors then in vieuw second voltage gone

[@douro20]

Why did they use a 12v filament supply anyway?

[@douro20]

@@electrofrying1685 These things weren't cheap to begin with...why they felt they had to make that kind of concession I can't understand...

[@thomasmiccolis9051]

They actually didn’t. Sadly Terry misrepresented what was actually implemented in this design. The rectifier tube 12.6VAC. However, the 12AX7 filaments are connected in series pairs & fed 22VDC. Why? This runs the tubes at lower filament current, lowers noise & lengthens tube life. [This is actually done in some tube testers as a “life test”]. The Selenium rectifier has inherent voltage drop in it as well so when replacing it with diodes & new, lower ESR, modern electrolytic capacitors a power dropping resistor is usually needed to get 22-23VDC. This is why the Telefunken 12AX7s sound great & lasted so long.

[@tubeDude48]

Get rid of all those Bumble-bee caps!

[@tubeDude48]

@@electrofrying1685 - Tomato - Tamato

[@bebo5558]

Should rename that thing P.O.S.!

[@daveanderson5680]

That preamp was modded to reduce preamp noise floor by removing balance pot and other un-needed front panel switches. The quietest preamps have the fewest switches and pots as possible.....my personal stereo Waga preamp only has 1 volume pot and 1 function switch

[@intothevoid9831]

If they wanted to improve noise floor they would have replaced all that wiring on the selector switch to shielded coax, it was probably done by a purist to remove all the tone/EQ circuitry. Not an unpopular option either, new faceplates to accommodate said option are available.

[@thomasmiccolis9051]

Exactly! The difficulty is at what point is “the juice not worth the squeeze” with the prices of these preamps nowadays.

[@richardbrobeck2384]

yep what mess to fix

[@Robert-xn3jb]

Best way to fix a Dynaco PAS preamp is with a heavy sledgehammer. Never could get these to sound decent and I have owned all versions.

[@Robert-xn3jb]

I bought into some of the online hype. These are miserable preamps.

[@jukingeo]

7:10 So what's the best way to avoid spending $90 for a new power transformer? Go to Triodeelectronics and spend $99 for that cap board there. That board costs more than the transformer, you got to be kidding me, right? Go buy Axial caps and put them on a turret board for less than half that. Great video though!

[@josephmartinez3851]

First

[@douro20]

"Leave the (12X4) tube in"...snob

[@kevinbeckenham3872]

Looks like a birds nest inside