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Hi All,
My ST35 has been working well for months but now has developed a "sound" issue.
During bass passages-it can make a real "fizzy" sound and sometimes it gets a 'static pop sound" it can make my speakers really move the cone out.
Seems the amp is ok about the first minute and then gets worse as it gets hotter.
I have already tried different pre-amp/phase inverter tubes. Bias seems good. I haven't open the chassis up yet-looking for some pointers. Sounds kinda like a capacitor going bad and I was going to look at the electrolytic ones first to see if I could see in issues but I don't know were to go from there.
Looking for advice.
Thanks,
Mark

I'd look for underrated coupling caps, too

Those can multi-caps are notorious for failing over time. Time to get a new one.

"Pop, fizzzz.... phhhhht, pop,.....channel in... channel out..... phhhhtttt...fizzzzz, pop..."

Corroded pins and/or socket contacts. Sometimes very difficult to get cleaned well enough to last more than a few weeks between plays before the corrosion returns. If it happens with known clean pins then you have socket issues.

20

Is it one channel, or both?

Shannon

One Channel-and this is DIYtube kit not original.
Thanks
Mark

MacL wrote:One Channel-and this is DIYtube kit not original.
Thanks
Mark

Good info. Let's us know it isn't a power supply issue.

1) First just swap the power tubes from one channel to the other. Does the problem follow an EL84/6BQ5? If so, easy fix.
2) If the problem says in the same channel, I'd look at the two coupling capacitors (i.e. C1 & C2 or C3 & C4). Maybe a simple DMM impedance check across all four of these will show a leaky one.
3) But if you have a failed cap, it's probably one of the bypass caps. Look at C through C8. These getting wonky with bass content seems like a big clue. If Xicon branded, switch to Nichicons from the current parts list.

Shannon

Hi All,
Three months later and I found the time to test again.
What should the the impedance (resistance?) be across c1-c4? I checked all with capacity meter and they were within spec.
Changed out C5 & C6... original caps "looked" fine. I really appreciated the high quality board-hard to get caps off without toasting the foil.

Still have same "fizzy" sound with occasional pop in left channel. Sounds better during first 30 sec past warm up then degrades- it also goes away when I power down as sound fades away.

I would greatly appreciate any troubleshooting advice on the next move.
Thanks,
Mark

So with C5 & C6 replaced, the prime suspects are still C1 & C2, the sockets, and maybe a flaky C20 (see if it looks damaged). You could monitor V3-2 and V4-2 for a strange DC leakage bias from a faulty coupling cap with the amp powered up.

Did you swap the power tubes from side to side? Can you change V1 and V2?

Shannon

Hi All,
Thanks for the continued support and advice.
replaced c1 and c2 today and it really didn't help... frustrated I started playing around swapping tubes... I first I thought I had bad tubes but I am now pretty sure that I have bad sockets. They are Beltons with a good tight fit so I think it must be the solder joints to PCB board. I can actually put a little pressure on the V1 Tube with my finger and get the one channel to completely cut out. I tried other tubes with the same results. Later this week I will try to re solder the socket and test again. Are tube sockets on a PCB problematic because of the heat cycle?

MacL wrote:Hi All,
Thanks for the continued support and advice.
replaced c1 and c2 today and it really didn't help... frustrated I started playing around swapping tubes... I first I thought I had bad tubes but I am now pretty sure that I have bad sockets. They are Beltons with a good tight fit so I think it must be the solder joints to PCB board. I can actually put a little pressure on the V1 Tube with my finger and get the one channel to completely cut out. I tried other tubes with the same results. Later this week I will try to re solder the socket and test again. Are tube sockets on a PCB problematic because of the heat cycle?

More likely stress cracks from tight socket getting wiggled to pull the tubes. The solder joints need to be well mounded and smooth, silvery, top and bottom of the hole.

20

I know one tube amp manufacturer who says power tubes should never be mounted to PCBs as the higher temps will degrade the boards near the sockets over time. It is the bane of Audio Research who otherwise make excellent products. That said the belton sockets are very tight and probably aren't the best choice for PCB mounting.

I used belton in my amp but mounted them to the chassis.

Any progress?