Dynaco VTA-ST70 Tube replace and bias set

New tubes in and it blows fuse.
Bad new rectifier tube. Bias set with old rectifier.

[infinitecanadian]

One of the best things about tubes besides their amazing audio output is the fact that if one fails all you have to do is replace it with another.

[christopher]

I finally found and bought a Fluke 12 meter after watching you. I had been using a cheap Chinese capacitance meter that worked horrible, along with using a regular multimeter, it is nice to have both things combined. Also, I have always found that the best endorsement any tool can have is seeing a professional like yourself using it religiously.

And because I watch all your videos, the first thing I did was change that battery out for a fresh one so I don't get erroneous readings. Let's hope my buttons and screen hold out.

[srtamplification]

I don't know where you are getting your tubes from, but Antique Electronics Supply will replace any tube that is bad within 10 days of purchase. I have done it a couple of times. I buy all of my tubes for my customers and myself, from AES for that reason. They also have the best selection of tubes IMO.

[Electrondivize]

The tungsol & JJ GZ34s have been known to arc over new from the box if the input caps value exceeds the maximum value given in the RCA receiver tube manual.

[robertseely]

Same amp same problem. It don't like modern line voltage 124VAC. Installed variac, set to 117VAC, no problem!

[EldaLuna]

know tubes sound good when the music sample even sounded amazing through my crappy monitor speakers through display port. this is why i love tubes so much plus fun to have something about glass bulb things that get warm/hot and glow while making sound just feels more.. natural aha.

[vibra]

I LOVE tube circuits!!! Thanks for this video.

[frankscarano]

The 5AR4 was notorious for this problem mainly because of todays line voltage. Back when this amp was made and this tube was made the line voltage was at least 5-10V lower and this tube in this ST-70 and even the MKIII was driven to nearly its max and now with higher ac feeding it, it is going beyond its threshold and thats why it is arcing and blowing the fuse. Its a smaller design with a cathode that was able to deliver much higher voltages than a 5U4 and does work well under the right conditions. Sadly The new russian tubes designs are even worse being able to handle these amps high AC being fed. I myself have ordered these very russian tubes and ran into the exact same issue you are having! however All is not lost...There is a modification to the circuit to help correct this that involved moving some of the wiring and adding two 1N4007 diodes that will help buffer this tube some against the higher AC this tube has to endure. Look into this.

[joemartin]

I Sue know tons about JJ an Gold Lion KT 77’s 2 were bad out of the boxes. The rectifier tubes are the hardest tune to get right for Russia or China . I use vintage old stock RCA rectifier. My one is in use for 15 years

[giorgostexnikos]

best video ever for tubes.thenks.more like this video please.

[JuanLega]

I have the same Amp. In my opinion with a solid state rectifier and the Kt77 tubes, it sounds amazing.

[johnsimms]

Just another reason why I prefer a solid state rectifier.

[tonnihaugebilde]

Had a similar problem with a Reflector 5AR4 a couple of years ago. Also arching. Why wont they make good tubes anymore? Oh well, money talks! One definately need to protect the new 5AR4's with uN4007 or better!

[frankgeeraerts]

Do not use original nos 6L6G tubes ! ( check original datasheet)
EL34 and 6CA7 ( KT77) the latter one is a beam power tube, the first a penthode..pin out of these tubes /screen is not internal connected to the cathode as is custom for other tubes.
Best sounding and reliable GZ34 was made by the Philips expensive.

[zxztv]

Dam shame about the rectifier tube :-(, i think the makers don't care because you have very little choice other than buy a new off them again.
Sounds like a quad op amp circuit and ref pot to auto adjust the bias would have been a nice option.

[jeffmoe]

Add diode mod to 5AR4, common problem with these tubes arcing, probably higher line voltage?

[CrashPilot]

Can't this tube be replaced with a diode rectifier (invisible, below the metal) without harming the Tube-Sound?

[billwilliams]

12voltvids, older circuits use to "Positive bias" the tubes so that the tubes shielding had very high voltage on the shielding of the tube, any reasons why to "positive bias" the tube? Why did they switch over from using AC +6vac to +12vac on the tubes heaters to using a DC voltage on the tubes heaters? If AC ripples percentage AC ripples voltage will increase higher if the tubes would draw more current any reasons why?

[briancampbell]

I'm confused when setting the bias...can i play the amp for one hour then remove the inputs to check bias...?
but then again i heard it's not good to run the amp without a load connected...🧐

[briancampbell]

I couldn't find anything on the internet if i'm using a dynaco that has its transformer changed for 230v electric, if the bias setting would differ from what the amp recommends 1.56v on 120 v america...my common sense says it should still be the 1.56v setting as if it were on 120v...because the transformer makes the amp work on 230v and I would think that has nothing to do with the bias setting...❓