Dynaco ST70 Tube Amp Restoration: Detailed Internal Wiring #2.

[@Richard-Isley-Bigsley]

To restore this thing back to stock would have been absurd. With the know how to make huge improvements, no other course should be taken. You improved the R8 tremendously and now this ST70. I really hope people appreciate what you do because the info is invaluable. Great work!!!

[@NosEL34]

Yeah I agree. I've tried a couple ST-70 renditions but not the VTA. But it was because I was doing a comparison between ST-70's & my friend's VTA had much lower output wattage. It sounded good but I couldn't get past the wattage to move forward with building my own. After watching this video series on the ST-70 I'm definitely going to try that board & follow Skunkie's work. I like the black PCB better than the others as well. Thank you Skunkie..for all you do & share with us.

[@NosEL34]

Obviously something was wrong with my friend's VTA

[@SuperMcgenius]

Last month I sent a link and a note to the British audio file, I may not be the only one, but he did mention you in the video that I watched this morning. I hope this helps grow your channel. I really like what you’re doing.

[@SkunkieDesignsElectronics]

Thanks!!

[@Maquis1943]

Nice! Beautiful work on the wiring and cap/resistor placement, and very accessible explanations. More vintage tube amp content please! Thanks-

[@jackstanley3529]

I'm with you regarding the musical sound of the 12AU7. I've used it as the driver tube in two amplifier designs plus I have an ST70 with the 12AU7 VTA board which sounds great. The key, as you say is to run it at the right current. Speaking of operating current, info with the VTA board suggests operating the EL34s at 40mA vs the 50mA run in the stock ST70. I think that this recommendation is to preserve both the power transformer and the rectifier tube given the higher B+ current draw of the VTA board and higher present day line voltage. In both the ST70 and other P-P EL34 amps that I own, my experience is that 50mA is the sweet spot soundwise for the EL34s. To deal with this in my ST70, I'm running a reissue, 120V rated input, larger stack replacement transformer plus a second rectifier tube.

[@RobertKohut]

Anything that Roy made is amazing!! 🙂 I've had the VTA ST-70, the M-125 mono blocks, the SP-14 preamp and the PH-14 phono pre. Some of the best gear I have ever owned. VTA knows!!! 🙂

[@SkunkieDesignsElectronics]

Stay tuned for the testing :)

[@mk23hk]

Love your channel! On that star ground I see that the tab is riveted to the chassis. I have seen riveted ground tabs like that become loose, or oxidation between the chassis and tab over time. Causing ground faults. Some people solder the tabs down, but I prefer to drill out the rivet, and replace it with a screw and nut with a lock washer. Keep up the good work!! I'm always learning something here from you, and greatly enjoy it!

[@SkunkieDesignsElectronics]

If you attach everything to that one ground point tab, it honestly doesn't really matter if there is a little resistance between it and the chassis. That is the whole reason to solder every ground to one tab point! If you are trying to ground the cathodes thought the tube socket/chassis/rivets/ground tab, then it becomes a huge problem.

[@54tristin]

Best tube channel on KZbin

[@Richard-Isley-Bigsley]

On a side note, I recently bought an octet of KT77. Of the eight, 7 were good. One tube had an internal short across pins 5 and 4 that I didn't catch before placing in my X7. I powered up and noticed smoke coming out of V3 vent. Luckily I killed power immediately. Damage was only to the grid resistor between 4 and 6 (took the paint off but casing was still intact). No other resistors were affected. I ordered upgraded Vishay resistors for all power tubes and the amp is fine, in fact great. From now on I will be checking each new tube I receive for shorts using DMM. Really easy to do and will prevent a big headache. The EL34 and KT77 are great tubes for certain.

[@petegaslondon]

Oh dear - bad luck!! Shuguang's finest arent always that fine.. I'm trying to put a few protections in my own amps so a shorted tube wont kill 'em!

[@Richard-Isley-Bigsley]

@petegaslondon Oh interesting, I'd be interested in hearing how that's done. I should have mentioned it was JJ tubes. The working ones sound pretty darn good.

[@Richard-Isley-Bigsley]

Also, I was able to remedy the short between 5 and 4 by giving the tube a good whack (without breaking it of course ..lol) Then I rechecked for shorts. I will try it again soon. I've whacked other tubes into greatness before. Unpredictable creatures they are. EDIT: I gave the tube a shot and while there is no short on the pins, I am getting speaker pop on startup that was getting my solid attention. I decided to abort and will discard the tube. The seller is sending me a new one so all good.

[@petegaslondon]

"Percussive Maintenance.." :)@@Richard-Isley-Bigsley

[@Richard-Isley-Bigsley]

@petegaslondon awesome 😅

[@NebukedNezzer]

12au7 is a great audio tube

[@SkunkieDesignsElectronics]

I agree

[@bobcooke4402]

Be interested to know the -bias voltage on the tubes. The whole bias supply must float up as well… great video.

[@SuperMcgenius]

Nice work, i’m not sure why, but I’ve always found a really good preamp gave more bass drive a deeper Sound stage. A crappy preamp will have Opposite effect😊.

[@petegaslondon]

Star Ground's a must :) I too aint really into Octal Snobbery The 6SN7's nice and all - if you want to make something (unnecessarily?) Big and chunky to look good, fine i guess The 'CC88/6DJ8 and 'CC83/12AX7 are VERY linear with quality tubes, if you use em right!

[@SkunkieDesignsElectronics]

"If you use them right" is the whole key.

[@mikeh2520]

12AU7 tubes are indeed very good sounding. Audio Note uses them in their gear including the DAC that I have. Any tube can be made to sound poor in a badly designed circuit.

[@SkunkieDesignsElectronics]

Absolutely, drag the load line down into the heel of the curve... of course it will be nonlinear! Honestly it seems like someone at some point said these are non-linear and now everyone online parrots that nonsense.

[@Richard-Isley-Bigsley]

Bottlehead built its' legendary Crack around the 12AU7. I've never come across anyone who said the Crack sounds terrible. Most are blown away especially when pairing with any Sennheiser HD-6XX variant.

[@SkunkieDesignsElectronics]

Right?

[@mikeh2520]

@@Richard-Isley-Bigsley I'll have to try a Sennheiser HD-6XX at some point. I'm using headphones completely assembled from a variety of 3rd party Grado upgrade parts. I suppose I could call them FrankenGrados.

[@GiguereSylvain]

Great work on a classic! Question: since the power tranny was meant for lower a/c voltahe (117v) and it's now runningva bit high, 'could' choosing another rectifier, or going for 'choke input' power supply instead of attacking a capacitor as the first component bring the high voltage to a more desirable, lower value? Just asking myself..

[@SkunkieDesignsElectronics]

Choke input would radically lower the voltage and you need a choke designed for that use. But yes, a different rectifier could lower it, but be careful, a lot of the other rectifier tubes have 3A heaters!

[@insurrectionindustries1706]

Actually it’s been tested thoroughly and scientifically and it isn’t the tubes or the board design that was the problem, but that board did have significantly higher distortion, but appeared to be mainly related to a significant decrease in negative feedback compared to the original design and not a problem with the board or tubes. If you increase feedback to the original level the distortion decreases accordingly. Just wanted to clarify that in case there are some misconceptions

[@NosEL34]

I came across a Miller Audio website that has made some negative feedback boards for improving the issues. They are supposed to work on the VTA boards also. I've never tried one of their NFB mods though.

[@SkunkieDesignsElectronics]

Stay tuned for my testing... Not sure who tested the results or how, I didn't hear or see anything to worry about.

[@bobcooke4402]

Nfb resistor does affect a few things. You can run anything from 3-10k ohms.

[@SkunkieDesignsElectronics]

If a tube amp has under 1% THD, not sure how anyone could complain. If you are wanting 0.01% THD, get a SS amp.

[@richardrahn3486]

Hi Stephe, I’m just curious why you chose the vta board vs the original dynaco board?

[@SkunkieDesignsElectronics]

I don't have an original board, the tubes used are NLA/expensive NOS, and several people said this board works well. No, I'm not a ST70 expert that has spent decades working on these nor have I tested 17 different driver boards. The next video shows the performance testing, this looks like a good option to me as a reasonable price but I'm not gonna say this amp I did is the best ST70 ever done. Please just enjoy the video series for what it is, rebuilding a dead ST70 using a commonly used upgrade board into a nice sounding amp.

[@matytinman]

ASR: Review and Measurements of Dynaco ST 70 Look at the first measurment: FFT graph

[@SkunkieDesignsElectronics]

Not sure I trust his reviews on tube gear. I respect his measurements on SS stuff, but his review of the leaf audio tube preamp was completely whacked from what I measured.

[@jjsant3250]

I see a SM cap near the input pot. Psionic Audio has flagged the ones that say "SM" on them as being prone to eventual DC leakage.

[@SkunkieDesignsElectronics]

Link? And yeah this is the cap that came with the board :(

[@TheDIMONART]

Wiring procedure and style is not that simple like it looks, actually. You need to know a lot about EMI interference and how to avoid problems with EMI/RF. Twisted signal wires, distance between signal and power lines, perpendicular style of wiring (signal and power lines crossed perpendicular, not parallel!). All those wiring techniques will help!!

[@SkunkieDesignsElectronics]

Which is what I was doing...

[@TheDIMONART]

@SkunkieDesignsElectronics Yup!! All is beautifully made in your case🙂

[@matytinman]

GR-Research / Danny Richie: Miller Audio Brought Me Something Special... and its only $2400!!! kzbin.info/www/bejne/g3-Qn6x7nb6Ko80 I wrote: [ The BIG problem in the old Dynaco is the quality of the electrical supply. From Miller web: * No Rectifier Tube Eliminates a weak point in the original ST-70 design, the single heavily loaded 5AR4/GZ34 Rectifier Tube. * Solid-State On-Board Rectifier Some Audiophiles claim to prefer a tube rectified sound that can “distort” or "sag" when under load (very desirable in a Guitar amp). With high fidelity sound reproduction the last thing you want is sag induced distortion from your amplifier’s power supply. NOTE: Most new tube amps designed today do not use a Rectifier tube. ]

[@SkunkieDesignsElectronics]

So I guess since all of my amps use a tube rectifier they are trash?? Thanks..

[@matytinman]

@@SkunkieDesignsElectronics WEAK point != trash

[@petegaslondon]

Well, different strokes for different follks! I'm into PP amps and Lowest impedance power rails - my main gripe against Tube Rects is the reliability, having seen* em blow up a few times as a kid *Maybe I'm (self) biased, but these were name brand Mullards and stuff, and when they went it wasnt a pretty sight Todays guys from China I pers wouldnt [push to max ratings, and make sure theres a fuse or two present - if you really like the extra glow, go for it I guess..;)

[@SkunkieDesignsElectronics]

Yeah I want to start playing with super low DCR power supplies.