Dynaco ST70 Tube Amp Restoration: Rewiring the Amp!

[@frankgeeraerts6243]

HI steph, ..Using 10 Ohm resistors on the cathode ( fixed bias ) is good practice...and as a little benefit those unbypassed rsistors linearise also a small amount of the tubes . A trick for biasing with individual bias pots for each tube of the push pull , is to set the neg voltage equal for both tubes , the difference will show one tyube hotter than the other, than adjust the tube with the lower current up to the right current , next put the voltmeter across the two cathodes of the tubes ( just on the upper side of the 10 Ohm resistors ) and adjust the hotter tube until the reading is O mV......retouch if necessary after tubes are fully bumed up and are stabilized ............correct and adjust when the tubes are burned in .... after the necessry hours of listening . NICE clean up , remarquable job....

[@Michael-bj4sz]

Dynaco sold ST-70’s as a complete product; they also offered a service to build kit amps, like your phono amp. I had a new factory built Dynakit ST-70 and wished I had kept it. It was new in box, my dad bought it, then passed away and mom lost it in a closet.

[@fredflintstone8048]

I'm enjoying the series on this amp. Thanks for sharing. I'm in the process of reworking a PAS preamp.

[@adamphillips1263]

I'm a pack rat so I would have kept those stands but I'm glad you found a use for them. You may want to consider getting some cork sheet (think Michael's craft supply - FelPro is too expensive) and use 3M contact cement to attach it. It will provide a slight non-skiddish quality as well as prevent scuffs and scratches on the unit being worked on. Thanks again for all you do.

[@ywsx6489]

7:35 cardas ccbp binding posts. $172 for 2 pairs. Used the gold plated version in several of my projects built 10yrs ago before crazy inflations.

[@SkunkieDesignsElectronics]

Yeah several people pointed that out, insane...

[@lambrosratchis3062]

Yes I would like to see the other amp the way you think it should be

[@SuperMcgenius]

I’ve been watching you for a long time and really enjoy your open mind and progress that you’ve been making in audio .keep going . The lovely thing about the amp is that there were so many made. greetings from Montreal, temperature 25° so tube amp helps to heat My house😅

[@SkunkieDesignsElectronics]

I’m glad you are enjoying the journey!

[@MojoMedicineMan]

🎉

[@bootstrapjesse]

Oh WOW. I love those amps. I have two that Kenny Russell built for me and two that I did myself. They sound amazing! Really enjoyed the video (s). Thanks for all you do!

[@SkunkieDesignsElectronics]

You're welcome! Glad you like them.

[@NickP333]

The idea to use all those Solen film caps. Looks good on paper, just not in real life. Those old speaker binding posts are very good quality and made by Cardas. They’re also $$$. I’d personally hold onto em. You never know when you might need some longer ones like that. 🤷‍♂️ Good find with those 10K Bourns 10 turn pots. I’ve bought em too, and yup, they’re expensive. The chassis looks fantastic. It looks brand new. I’ve used that Blue Magic for years, and by the looks of yours, so have you. lol I think you’re gonna really dig this amp. If you haven’t heard one, they really do sound like a nice tube amp, unlike a lot of new tube amps that sound more like SS.

[@stephenkeever6029]

I was always suspicious of using the chassis as a ground in these amps. That was a good analogy about a car chassis ground. It just seems like it invites hum and noise. It would be interesting to find out if your grounding mod made the signal to noise better. Really enjoying this series!

[@SkunkieDesignsElectronics]

Yeah, it makes no sense, especially the higher current cathodes.

[@johnnytheg]

What a shame to see a legendary amp bastardized so badly, but I am glad it's in good hands and being made back into a great, real ST-70 again.

[@bobcooke4402]

Smart moves on all your power supply and grounding updates. Check to see if the board mounting holes are grounded. It’s another potential ground return. Especially with the 3 wire power plug. The og versions were 2 wire I think.

[@SkunkieDesignsElectronics]

Right and the board mounting holes appear to be grounded.... Might have to find a way to insulate it from the chassis. No way would I operate ANY tube amp with no safety ground/2 wire plug.

[@bobcooke4402]

@@SkunkieDesignsElectronics I’ve used everything from nylon screws/nuts to plastic standoffs. The 4-40 nylons screws are a good fit…

[@SkunkieDesignsElectronics]

Turns out it's silent so this isn't going to cause an issue. Likely because it's so low current + not having the higher current output tube ground going through the chassis too.

[@bobcooke4402]

@@SkunkieDesignsElectronics that’s good to know. Thank you!!!

[@sopasadena1499]

Skunkie. Love the work you do on amps. Will you provide detailed written instructions on deviation from tubes for hifi instructions. Greatly appreciated. Note ASR review of a later Dynaco build. Can’t wait to see your measurements of the final build and listening impressions. Thanks again

[@SkunkieDesignsElectronics]

I never do written instructions. I'll be showing in video detail the changes.

[@chrislj2890]

Could you paint the transformers, possibly with one of those hammer finished paints?

[@SkunkieDesignsElectronics]

Folks can paint them any color they like :) I'm leaving the patina as it fits their age.

[@nbramaputra]

We love you sciunky,in modern amps i wold avoid rectifier tube,if you not can get nos.or used old stock.fender made print amps with modern rectifier tubes,and it was not a good idea.i think.better to instal standby svitsh.by the way i found out how to put on a choce on an amp without, just put it beside the ac input on outside of amp.

[@SkunkieDesignsElectronics]

I love rectifier tubes :)

[@nbramaputra]

@ i have a bunch of old ez and az tubes, dont need them,

[@SkunkieDesignsElectronics]

[e-mail] :)

[@1mctous]

Speaking of OCD, Tubes4HiFi is on v. 23 of the driver board. As you said, it should be a good candidate for a sub-$1.5K tube power amp.

[@SkunkieDesignsElectronics]

Pretty sure it will be!!!

[@rickg8015]

Hi Stephe.. Do try the diode (two 1N4007 or UF4007) protection for the 5AR4.. Those MIT caps on the board are quite good on some circuits, and are pricey these days, please don’t include them with the Solens in the bin..

[@Pksparty2112]

The amp was definitely sold as a factory built or a kit. I was always wondering if there was any real advantage between the 2.

[@chatrs1025]

I saw a good deal come up locally on a pair of NOS Tango U708 trans but I don't understand that they got two seconday winding tabs: one for Zs=8 Ohms impedance and the other for Zs=6 Ohms. The table printed on top of the trans said when Zs is 8 Ohms,, the primary impedance Zp could be either 7k or 5k and when Zp is 6 Ohms, the primary impedannce Zp could be either 5k or 3.5k Ohms. How could that be since there's only one set of primary winding hook up taps. You are the fan of Tango trans. I guess I might be able tp explain. I planned to use these U708 with a 300b SE amp.

[@SkunkieDesignsElectronics]

They are just backing into the primary impedance by the load placed on the secondary. There are two sets of secondary windings that provide different turns ratios. If you put 8 ohms on one winding, you end up with 7K, if 8 ohms is placed on the other you get 5K. Lower the impedance to 6 ohms, and it will reflect 5K and 3.5K respectively. Their current "universal" transformers do the same thing having multiple secondary windings designed to operate this way. FYI these ones you are looking at are no longer in production. www.tubebooks.org/file_downloads/tango_tamura/Tango_Transformer.pdf All that said, those transformers are designed/rated to use for a SE EL34 and I don't think they would be a great fit for a 300B amp. They are gapped for 70ma, and almost any normal 300B operating point will exceed this and saturate the transformer (turn it into an electromagnet).

[@chatrs1025]

@SkunkieDesignsElectronics Thanks very much for the info.

[@Michael_k_DK]

As I recall it is stainless steel chassis’s they used correct me if I am wrong

[@SkunkieDesignsElectronics]

I've seen originals very rusted so I think they may be nickel plated steel and then polished? I know the new ones are polished stainless.

[@RobertKohut]

Roy sold tubes4hifi and I believe Bob retired. It seems the new owner keeps the tradition going.... 🙂

[@SkunkieDesignsElectronics]

Yeah, I have been talking with Dan :)

[@tryintobesoquiet]

Don’t throw those Eminent Tech solid copper binding posts out!

[@ericlane5949]

Please send me those binding posts instead of throwing them out!

[@SkunkieDesignsElectronics]

I really meant I'm not using them on this ST70.