ST70 issue

Hi,
About 2 weeks ago, an issue started with my system with the first symptom being a constant hiss in the right speaker.
After checking everything that could be checked (including cables, preamp, switching speakers, speaker rheostat (AR2AX) etc...), I noticed today that my ST70 tubes (all having very close values and being quite new) do not hold bias anymore on both channels: whenever I check the bias on each channel, values keep moving up and down quite rapidly. Yet, sound is great on the left speaker but the right speaker stills makes hiss and 'pops'.
My ST70 is a stock one, with only the PC-3 caps having been replaced with orange Sprague by the previous owner who also changed the output RCA connectors. Regarding the latter, touching them when plugged in the amp sometimes creates some kind of interferences in the right speaker as well.
Does the selenium rectifier have to be removed at first? What key areas should I pay attention to?
Should anyone have an idea of diagnosis or any area to be looked at would be welcome.

Thank you very much in advance.
Best.

alyosha wrote:Hi,
About 2 weeks ago, an issue started with my system with the first symptom being a constant hiss in the right speaker.
After checking everything that could be checked (including cables, preamp, switching speakers, speaker rheostat (AR2AX) etc...), I noticed today that my ST70 tubes (all having very close values and being quite new) do not hold bias anymore on both channels: whenever I check the bias on each channel, values keep moving up and down quite rapidly. Yet, sound is great on the left speaker but the right speaker stills makes hiss and 'pops'.
My ST70 is a stock one, with only the PC-3 caps having been replaced with orange Sprague by the previous owner who also changed the output RCA connectors. Regarding the latter, touching them when plugged in the amp sometimes creates some kind of interferences in the right speaker as well.
Does the selenium rectifier have to be removed at first? What key areas should I pay attention to?
Should anyone have an idea of diagnosis or any area to be looked at would be welcome.

Thank you very much in advance.
Best.

Is the hiss there even if you disconnect the preamp ( and shorts the inputs ) ?

About bias wandering, it could be the rectifier tube , it could be the selenium ( which should be
replaced anyway), and it could be an aging resistor taking it's last breath. It also could be
a tubesocket ( rocking the tubes one by one might "cure this" ), it could be a tube that's
reached end-of life.

Sorry about beeing unspecific. But the amp should be taken out of service and given to
a repair shop.

Hi, thank you for your reply.
The hiss is only present when the amp is connected to the preamp, and only on one channel.
I switched to another fully working tube preamp and the issue remains the same.
Also, everything being connected, the same strong hiss is present when I switch speakers.
You mention a potentially problematic resistor, having very limited knowledge in electronics, where should I be looking considering it's the right channel involved? Thanks.
Regarding the way to replace the selenium rectifier, any diagram available? I will check on the forum now.
Again, thanks very much for your response !
Best.

Hi, this is the inside of my amp, anyone seeing anything unusual regarding the issue I mentioned please? Selenium rectifier has been replaced. Thanks.

[img:903d]https://servimg.com/view/17462961/10][/url][/img]

I don't know what your long term plan is with this ST70, but a few things to consider.
First and foremost, consider upgrading your driver board to the VTA, the improvements will be amazing!
If you do go with this upgrade, some major changes in the chassis wiring will need to be done.
BIASing will now be for EACH power tube, which will require the addition of BIAS resistors to each output tube. You can remove the existing BIAS pots and may even consider the Pentode/Triode switch upgrade.
Get rid of the selenium and replace with 1N4007 diodes.
Also get rid of the Stereo/Mono switch, the VTA driver board is not really designed to be used with that.

As to your current issue, as has already been mentioned, replace the selenium with 1N4007's.
Check your BIAS resistors, check tube sockets.
How old are the output tubes and rectifier tubes, do you have any spares?

But really, I still recommend that upgrade, and use that opportunity to give everything a good cleanup, especially the tube sockets.
By the looks of this amp, I think it has reached the end of its usable life in its current condition, doing 'band aid' style of repairs will only give you a short term solution.

Thanks for your reply.
My plan is not to do a full upgrade but, rather, keep the amp as stock as possible. In that case, what would you suggest are the priorities to check, change etc. ? Resistors? Caps?
Selenium has already been replaced, output and rectifier are NOS mullard in very good condition and very close values.
Thanks.

I would probably replace the BIAS resistors, clean the tube sockets and even 'crimp' the contacts in the tube sockets up a bit so there is a better connection to the tube pins.
Also check for dry solder joints, especially on your problem channel area, thats a start anyway.

MontanaWay wrote:I would probably replace the BIAS resistors, clean the tube sockets and even 'crimp' the contacts in the tube sockets up a bit so there is a better connection to the tube pins.
Also check for dry solder joints, especially on your problem channel area, thats a start anyway.

You could try : swap the two 7199 tubes ( turn off, gently remove them and install ).
If the noise follows the tube you have found the culprit. Get another 7199,
If noise stays at the same place the problem is on the board.

It's unlikely that the EL34 is the problem.

Thank you all for your help.
Here is a pic of my pc3:
[img:0b0d]https://servimg.com/view/17462961/11][/url][/img]

What I noticed is that some values seem different from the ones on this forum posted layout:
[img:0b0d]https://servimg.com/view/17462961/12][/url][/img]




Which caps / resistors are crucial to check in this case?
Many thanks again.
Best.

oh and I forgot to mention, 7199 tubes are not the issue. thanks.

I also have some old Dynaco MK3s.... all stock .....an thay sound great!...With NOS 6AN8 that most lov to hate...hehe....But have fun with tubes ....Hoho ....in 2014,gofig...




Thanks for the site Bob......

You said that some of the components - caps, I assume - on your board are different than the pic posted with the driver cap/resister values. The pic corresponds to the parts list on the original ST70 manual. So if you are trying to duplicate the original, the pic's values are what you want.

FWIW, Dynakit can supply new boards with original type carbon resistors/current type caps or boards with film resistors. The fact that the oem carbon resister boards are often out of stock seems to indicate that they are quite popular for whatever reason.

Thank you all so very much for your help.
Funny enough, when I replugged the amp this afternoon, everything worked absolutely fine and I have no idea why, except that:
- I used lower quality and value tubes,
- I have also noted that some wires next to the input terminal were very close to each other and I just made some space between them.
Now, strangely, bias setting is much more accurate.

Anyhow, I will consider the options you guys provided me with, thanks again to you all!
Best regards.

Make a mental note of which wires you moved to "fix" it ... if the problem re-develops (and I suspect it will) that would be the first place to check for cold or cracked solder.

Replacing the tubes and getting better results could also indicate dirty or loose sockets. I'd at least give those a good cleaning and retension them as well, then try your original tubes and see what happens.

Thanks, that I of course did, along with tightening tube socket pins.
Thanks again for your reply.