495V on pin3/4 of output tubes, new Triode ST-70 build

[gregk]

Hi all,

I just finished trying to bias the amp with a 3A slo blo fuse, I got one side (the side opposite the rectifier) up to .65V and the rectifier side was just over .51V when the fuse blew. I was going extremely slowly, increasing by something like .05V each turn and then waiting 10 seconds before turning again. So the fuse is definitely not the issue.

I will wait to see what the Triode tech has to say and let you all know what I find out. If anyone has ideas on here, I would be interested.

Thanks all,
Greg

[gregk]

Also, I dont know if this is significant, but there is a slight but noticeable physical vibration coming from the amp. I can feel it in the screw driver when I am trying to bias. I thought I read somewhere that this was normal as long as it isn't excessive, but wanted to post here just in case.


thanks,
Greg

[EWBrown]

The physical vibration comes from the power transformer, the 60Hz line voltage and current flowing through the power transformer, will generate some 60 Hz mechanical vibration, and some may also be caused from the transformer's magnetic field coupling onto the steel chassis itself, this is more of an issue with the horizontal mount power trannies, especially those which don't have the bottom end bell.

If you have individual 10 ohm cathode resistors for each EL34, then the voltage reading through each one should be no greater than 500 mV, which represents 50 mA. 400mV / 40 mA will be easier on teh EL34s and the rectifier tube, and teh power trannie, and C354 choke, and the difference in sound quality would be negligible.

If the cathode resistors are shared for each pair, then double the voltage (and current), as the pairs will consume twice the current as individual tubes.

J/J 5AR4s can be truly cheesy, their QA has fallen through the factory floor in recent years , though I've also I've had very good reliable ones, as well as some which ranked in weakness with those cheap "Made in Chian" 5AR4s which barely make the grade as a 5Y3GT. The Sovtek 5AR4/GZ34s are rugged and reliable, like an old T-54 tank.

If you suspect a bad power trannie, the quick and easy test is to simply pull out the rectifier tube, and leave all the other tubes in - they will draw only filament current - and see that the fuse doesn't blow, and there is very little heat coming from the power trannie, as the High Voltage B+ won't be generated without the rectifier in place.

/ed B in NC

[Writer Frog]

Hi Greg,

I am sorry to hear that a 3A slow blow fuse did not make the problem go away.
I am guessing from the Triode's parts list, a pair of EL34s share a common 10-Ohm cathode resistor.
Can you measure the voltages at Pins 1, 3, 4, and 8 for each of the EL34s when the bias is set at 0.5V?

This is what you can expect to see:

Pin 1: G3 = 0.5V
Pin 3: Plate = 415 ~ 440V
Pin 4: G2 = 418 ~ 440V
Pin 5: G1 = -35 ~ -45V
Pin 8: K = 0.5V

If your measured voltages are reasonably close, then the most likely culprits would be the rectifier tube and the power transformer.

I've had good luck with Sovtek 5AR4s.

[gregk]

I thought I would do the test you suggested real quick before work. So I put in a new 3A fuse. I turned the amp on and first there was a very slight crackle from the speakers for a half second, then there were 2 flashes from the rectifier tube, but the amp kept on running. I turned it off. I have not seen it do this before, a flash in the recto tube only happens when the fuse is blowing up. I did adjust the bias a bit below where I had it yesterday before turning it on.

I would think that at this point the most likely problem is the recto tube. Even if that doesnt mean its the problem, its worth trying a new recto tube. I have to place an order for some speaker hardware anyhow, so I'll order a new recto tube too.

thanks all,
Greg

[gregk]

I just ordered the Sovtek 5AR4 fron Antique Electronic Supply.

In case anyone was curious, I don't intend to use the ST-70 primarily for music listening, it is for my Super Stereo Guitar Rig! (SSRG!), as a power amp. I am building some full range speakers that go along with it. I will also build a wooden case that matches the speakers for the ST-70 with metal corners and a handle to make it more portable, it will completely surround the ST-70, cage and all but with the front and back exposed.

I need the full range speakers for my guitar rig because it needs to sound good with an acoustic as well as an electric. I know its totally unorthodox, but I have spent years trying my guitar through various speakers and this should give me the best sound with out spending a thousands of dollars more, and still be reasonably portable. I use a guitar processor thats gives me basically my entire guitar sound other than what comes from the guitar itself, so I dont need a regular guitar amp thats designed to shape the sound significantly.

Anyhow, if I ever get it all finished I will show you all how it looks.

Will let you know when I get the new 5AR4 and have had a chance to try it.

thanks all,
Greg

[Writer Frog]

Hi Greg,

I am looking at the SDS cap board parts list to see what sort of input capacitance (the cap before the choke) it has:

C1 = 82uF 400V Snap in Capacitor
C2 = 82uF 400V Snap in Capacitor
C3 = 390uF 250V Snap in Capacitor
C4 = 390uF 250V Snap in Capacitor
C5 = 270uF 250V Snap in Capacitor
C6 = 270uF 250V Snap in Capacitor
C7 = 270uF 250V Snap in Capacitor
C8 = 270uF 250V Snap in Capacitor
C9 = 100uF 100V Radial Capacitor
C10 = 100uF 100V Radial Capacitor

I am guessing that it uses two series-connected 82uFs for input capacitance of 41uF. With a typical electrolytic capacitor tolerance of +- 20%, the worst case capacitance of ~50uF may be a little too much for a marginal rectifier tube.

Just a thought.

[Ty_Bower]

I turned the amp on... then there were 2 flashes from the rectifier tube, but the amp kept on running. I turned it off. I have not seen it do this before, a flash in the recto tube only happens when the fuse is blowing up.

The flashes you see are caused by arc'ing in the rectifier tube. The high voltage between the cathode and anode finds some way to leap through the vacuum and it creates an arc. Sometimes it is due to gas in the tube, or maybe the plate is too close to the cathode at one spot, or maybe there is a carbon track across the mica spacer. Whatever the reason, once a tube arcs the first time it is extremely likely for it to arc again.

Arc'ing in the rectifier tube is usually immediately followed by a blown fuse. JJ rectifier tubes have a bad reputation. The Stereo 70 is especially hard on rectifiers. As you increase the bias your power tubes will draw more current, making an arc in the rectifier likely to happen. Increasing the capacitance immediately after the rectifier tube makes the problem worse. The original Dynaco design only used 30uF at this position, while your SDS board has 33% more.

I'm going to say your blown fuse problems have been cause by a dodgy rectifier all along. Consider springing for a good quality 5AR4 or GZ34 tube, or consider a solid state substitute like the Weber Copper Cap.

[gregk]

I ordered a sovtek this morning. I figured the arcing wasn't generally good for it.

Hopefully the new recto is last piece I need to get this thing working.

Greg

[gregk]

how about Groove Tubes GZ-34? Anyone have an opinion on the modern ones for use in an ST-70? Made in the motherland! Just curious, because its the only one I can find locally.

Thanks all!
Greg

[Ty_Bower]

There's tons of threads on "which is the best GZ34". Just search around this forum, AudioKarma, diyAudio, and Audio Asylum.

Groove Tubes are all re-labels. Who knows where they bought them. Could be Russian, could be Chinese. They're not American made - as far as I know, no US manufacturer still produces the 5AR4.

Older (1995~2005?) Chinese rectifier tubes have a bad reputation, even worse than the JJ. They're known as tiny fireworks displays in a bottle. On the other hand, recent reports suggest the latest batches of Chinese GZ34 are actually quite good. I have a pair of "Ruby" labeled Chinese rectifiers, and they work great in my Mark III.

Sovtek seems to make a consistent 5AR4, but they're seldom noted to be outstanding. I have a Sovtek 5AR4 which I've used in light duty applications (amps which drive a single pair of 6L6 types, or a quad of EL84) but I don't know how well it would hold up in a Stereo 70. Keep the first cap small, and maybe it'll be OK.

If you want a really good rectifier, you probably need to find an old (but strong) Blackburn Mullard. They're identified by hole in the key pin and the distinctive Philips codes printed in grey paint near the base of the tube. The Japanese versions (Matsushita? Hitachi?) with the cross shaped seams in the top are good too. Just be warned they will cost you $50~$100, and you'll probably have to hunt one down on eBay. Occasionally you'll get lucky and grab one for cheap, usually when the seller doesn't really know what he's selling.

The old US rectifiers (GE, Sylvania) aren't bad either. Not quite as good as the Mullards, but usually better than today's stuff.

Here's a tip that will make any vacuum tube rectifier have a better chance of lasting longer: Install a pair of solid state diodes between the plates of the rectifier tube and the leads of the power transformer. You can use the spare pins on the rectifier socket as tie points. There is a good thread around here which mentions the idea, and it has a nice photo in it which clearly illustrates the wiring.

edit: Here's a good thread:
http://www.diyaudio.com/forums/tubes-va ... stion.html

Look at Trout's diagram in post #12, and note Kevin's comment in post #13.

[gregk]

Awesome, thanks for the info. I might try that diode trick, I have some 1n4007's I can use. Also, I am considering the Webber Copper Cap Rectifier, that seems like a great idea long term wise. The only concern I have is that it would look weird, and this is a major problem for listening to music.... I will do some research on them to see how people like them.

The Groove tube I saw at Guitar Center said made in Russia on it, maybe its worth getting and trying?

I have read mixed reviews about how much difference the rectifier actually makes in a tube amp, but I get the impression that as long as it works its fine and there isn't much difference in sound. I dont have enough experience with tubes to know the difference yet myself.

The JJ that I have in the ST-70 now was supplied by Triode with the kit (I paid the extra $100 for the tubes included).

Thanks for all the info,
Greg

[Ty_Bower]

The Groove tube I saw at Guitar Center said made in Russia on it, maybe its worth getting and trying?

If it said "Made in Russia", then it must be a Sovtek. Compare to the photos of the Sovtek 5AR4 sold at The Tube Store and Tube Depot:

http://thetubestore.com/sovtek5ar4.html
http://www.tubedepot.com/so-5ar4.html

Or, just look at the photos at Guitar Center:
http://www.guitarcenter.com/Search/Defa ... &Search=Go

The GT is upside down, but even in these tiny photos you can see the plate structure is the same. Why they charge 2.5x as much for the GT, I have no idea.

[gregk]

Yea thats the one I saw. Maybe I will get it (and get ripped off on it) just so I can try this ST-70 this weekend with the series diode mod you suggested. What is funny is that guitar center online sells the original Sovtek version for 12.99. I guess the GT logo costs 20 dollars to add...

Thanks,
Greg

[Ty_Bower]

I guess the GT logo costs 20 dollars to add...

They're also giving you a 90 day warranty. It's got the replacement cost already factored in.