Dynaco-Doctor Forum

Welcome! Need support, you got it. Or share you ideas and experiences.

Return to Dynaco-Doctor Forum

ST70 Triode Electronics upgrade boards... voltage problem on driver section

195.240.163.192
Posted on September 13, 2009 at 06:02:00
telewatt@telfort.nl
Audiophile

Posts: 3
Joined: September 13, 2009
 My first message on this forum, so Hi to you all!

The last few days I searched this forum for an answer but
did not found anything that matched close enough.

2,5 years ago I bought a stock Dynaco ST-70, build long ago
with some modifications on the power supply capacity (done badly).

The bias was never stable (Sometimes the EL34s red-plated) and the power transformer was vibrating excessivly. Never the less, I was happy the way it sounded, no humm or audible distortions.

A month ago I thought it to be time to do something about it myself.
So I orderd some stuff at Triode Electronics in Chicago: the new driver board and new capacitor board with parts. Also a 120/240v PA060 power transformer and new 10k bias pots.

I stripped the old amplifier, but keeping the choke, the 2 output transformers, it's chassis with the power switch and RCA input jacks and the 5 octals.
This to make it look like building a ST70 kit from triode electronics.

Within a few days, taking some time to populate the boards with their parts, building and wiring everything toghether I was done... although that's what I thought. I chose to use two EF86 (Russian 6J32P) and a ECC99 to plug in on the driver board. Before I started building I checked all reisitors with the ohm meter and they proved to be alright.
Here are some results:

No hissing or smelling/burning parts, no redplating... the amp was doing it's job, only at one channel. So I figured out to connect J1 with J2 on the driver board to function properly with a ECC99 as a phase inverter. This is for the filament on this tube.

Then I started measuring the plate and filament voltages with no tubes (except the rectifier tube) and the bias pots turned to their minimum. I probably should have done this earlier, but luckily I left the amp on for a minute or three the first time: all the plate voltages on the driver board are about 2 times too high. Pin 1 on the EF86s measures 160v instead of the 55-80v stated in the manual.
I always use the chassis as the negative side for my volt meter.
No problems with the filament voltage, these are okay for all tubes. Also the output tubes have their plate voltages of about 485v so that's fine too.

My attention drifted from the driver board back to the capacitor board and vice versa... I don't know much about tube technology so I got puzzled. After checking all the connections I made there seems to be nothing wrong. The boards provided by Triodeel are very thick and sturdy... with the holes plated all the way through... I'm impressed about that.

Back to the capacitor board I measured (only with the recifier tube in):
Pin 8 (rectifier) to eyelet B+: 485v
E19: 390v
E20: 460v
bias R: -44v
bias L: -29v

Are there some cases I shouldn't use the chassis to measure plate voltages? But I don't think so... anyway I'd be happy to read some reactions on this one, thanks ahead!
RE: ST70 Triode Electronics upgrade boards... voltage problem on driver section, posted on September 13, 2009 at 10:50:25
Bill Thomas
Industry Professional

Posts: 1330
Location: Southern USA
Joined: January 3, 2004
 First, the original tube sockets are ALWAYS a potential problem. They should be replaced. This is a probable reason why your bias would be unstable.

The "crackling" noise could be due to LOTS of things, but pull the driver tubes and run the amplifier. (Make SURE you set the bias correctly - even with the driver tubes pulled).

If the crackling noise is gone, the problem is on the driver board. If it is still there, the Output tubes and coupling capacitors are possible problems. Also, if the bias supply capacitors are not good, hum will be introduced to the grids of the Output tubes.

This is a good starting point. Let us know what you find.

Hope this helps.

Bill

RE: ST70 Triode Electronics upgrade boards... voltage problem on driver section, posted on September 13, 2009 at 11:54:52
telewatt@telfort.nl
Audiophile

Posts: 3
Joined: September 13, 2009
 Hi Bill,

I'm sorry if I wasn't clear enough but...
The crackling noise was due to grounding with my voltmeter-pen on the input tubes, no crackling noise on the amp whatsoever! The bias ain't a problem either, it's stable after building the amp with new boards and power supply.
The only drawback was how to measure the plate voltage on the input tubes. And now the only problem that remains is a bit of hum on both channels, just audible at about 1,5 meter from the speaker.
RE: ST70 Triode Electronics upgrade boards... voltage problem on driver section, posted on September 13, 2009 at 06:32:04
truep
Audiophile

Posts: 247
Location: Texas
Joined: March 28, 2004
 try the measurements with the tubes pluged in. With no load they will read high.

RE: ST70 Triode Electronics upgrade boards... voltage problem on driver section, posted on September 13, 2009 at 07:05:37
telewatt@telfort.nl
Audiophile

Posts: 3
Joined: September 13, 2009
 Thanks! I didn't use any tubes at the measuring because that's what is stated in the driver board manual, I should measure without any tubes except for the rectifier.

I've done what you suggested but I also connected a set of speakers... so I nearly jumped off my seat when I put the voltmeter on the first pin of the EF86... lot's of crackling noise! So I disconnected the speakers for a while to measure and now the values make sense... on pin 1 I measured 60 volts. So not putting in any driver and output tubes seemed to be the problem.

But I have hum on both channels... and it became a bit louder when I slowly cranked up the bias... even at 0,7v (1v max for this configuration) it was too loud.

(The power transformer I mentioned before is not a PA-060 but a PA-521, over here it's 230v AC main.)

Page processed in 0.057 seconds.

[ Top Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2009, Audio Asylum, all rights reserved ] [ Go Green ] [ Take5 ]
[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]