I have learned to always check upstream for possible sources of hum.
1) A faulty interconnect cable, or connection (had this one, twice)
2) a low level cable close to, and parallel to, an unshielded power cable.
3) my own problem was long RCA interconnects (problem solved with XLR cables + transformer XLR->RCA box)
4) hum worse with high efficiency speakers
Do pay attention to the discussions here (several discussions here on DIYAudio) about:
1) power transformer position, mounting, rotation, possible shielding
2) grounding schemes - star grounding is my favorite
3) avoidance of open loops
4) twisting of all AC wiring (keeping short, and hiding under metal bottom plate also)
5) Mu Metal shielding of input transformer (and keep these as far from power transformer as possible)
1) A faulty interconnect cable, or connection (had this one, twice)
2) a low level cable close to, and parallel to, an unshielded power cable.
3) my own problem was long RCA interconnects (problem solved with XLR cables + transformer XLR->RCA box)
4) hum worse with high efficiency speakers
Do pay attention to the discussions here (several discussions here on DIYAudio) about:
1) power transformer position, mounting, rotation, possible shielding
2) grounding schemes - star grounding is my favorite
3) avoidance of open loops
4) twisting of all AC wiring (keeping short, and hiding under metal bottom plate also)
5) Mu Metal shielding of input transformer (and keep these as far from power transformer as possible)
With the inputs shorted and speakers connected there is no hum, it's amazingly quiet. With the inner connects plugged into the amp, no hum or noise, when the amp is connected to the preamp I use in the workshop, I get hum.
I'm not going to worry about it since this preamp will not be used except in the workshop.
I am worried that after a couple of hours of playing music, the heatsinks are 61'C. I'll try the new feet when they arrive, and if that doesn't lower the temperature, then I'll have to build a "babysitter".
I'm not going to worry about it since this preamp will not be used except in the workshop.
I am worried that after a couple of hours of playing music, the heatsinks are 61'C. I'll try the new feet when they arrive, and if that doesn't lower the temperature, then I'll have to build a "babysitter".
After wondering where the THF 51 I had ordered for my buzzing amp might be I got a note from pras saying the package had been returned to her. It had been shipped in early December.
She asked if if the US was accepting parcels as if I would know. I told her I figured we were accepting numerous cargo ships every day.
She said she would try sending it again.
She asked if if the US was accepting parcels as if I would know. I told her I figured we were accepting numerous cargo ships every day.
She said she would try sending it again.
Off subject - has anyone noticed the new WESTERN ELECTRIC 91 amplifier with a single 300B with a specified output of 18 watts per channel which seems physically impossible without some help.
Was wondering if this 300B might be within a mu follower circuit like the SB? This is why I made the post here.
The looks of the thing are obviously highly inspired by the WAVAC amplifiers.
It is built, I presume, in the Rossville, Georgia facility.
Any interest here? Not in buying one - though the price seems reasonable for this kind of thing - $13,500.
Was wondering if this 300B might be within a mu follower circuit like the SB? This is why I made the post here.
The looks of the thing are obviously highly inspired by the WAVAC amplifiers.
It is built, I presume, in the Rossville, Georgia facility.
Any interest here? Not in buying one - though the price seems reasonable for this kind of thing - $13,500.
Which would not allow double the expected output power.
Maybe WE is re-introducing the EIA power rating?
Maybe WE is re-introducing the EIA power rating?
From: https://www.westernelectric.com/91e#spec
https://www.tubecad.com/2019/05/blo...t source is,amplification component decreases....
Apologies for being off-topic. No more on this subject from me.
- Steered Current Source Technology (SCS)
The 91E uses Western Electric’s patented steered current source technology. The unique parallel feed topology contributes half of AC current to the plate (anode) of the 300B by modulating quiescent current. As a result, half of the power dissipation occurs in the tube, allowing for a greater output power of 20 watts per channel.
https://www.tubecad.com/2019/05/blo...t source is,amplification component decreases....
Apologies for being off-topic. No more on this subject from me.
Finally got around to building channel number two. I went to fire up prior to hooking up the SIT. LED’s lit then a few seconds later a small pop and little wisp of magic smoke. Going over the Gain PCB I realized I reversed the polarity on C3. I reinstalled a new cap with the correct polarity and fired back up. LED’s no longer light up. I couldn’t find any obviously burnt components. I’ll have another look tomorrow. I suspect IRF510 might be toast? Anything else I should look for?
sorry, my bad
I've read C3 but somehow was fixated on C1 (all ref to schm in post #2, SIT pcb)
in case that C1 was rotated, 9K1 are only parts being in danger of reversed/dead short C1
but, if C3 - that means IRF510 is directly stressed with reversed/dead short C3, and when IRF510 goes Dodo, there is also possibility that one of 9K1 is gone too
so, - replace IRF510 and both 9K1 resistors; even if one of resistors is measuring OK, bad history dictates replacement
anyway, take that episode as Schooling - next time you'll take more care of cap orientation, especially having all those additional "+" signs on pcb, exactly for checking purpose
me - done that rotation too many times, so I'm always putting "+" sign for triple check ......
I've read C3 but somehow was fixated on C1 (all ref to schm in post #2, SIT pcb)
in case that C1 was rotated, 9K1 are only parts being in danger of reversed/dead short C1
but, if C3 - that means IRF510 is directly stressed with reversed/dead short C3, and when IRF510 goes Dodo, there is also possibility that one of 9K1 is gone too
so, - replace IRF510 and both 9K1 resistors; even if one of resistors is measuring OK, bad history dictates replacement
anyway, take that episode as Schooling - next time you'll take more care of cap orientation, especially having all those additional "+" signs on pcb, exactly for checking purpose
me - done that rotation too many times, so I'm always putting "+" sign for triple check ......
Yep, it was C3. In process of replacing IRF510 and related resistors and will report back. Not to blame anyone, but my 6 year old son was helping stuff that board. I should have paid more attention and quadruple checked. Good lesson for me and my apprentice. 🤓
Would reversed C3 stress jfet's at all? Hoping I don't need to match a new set.
Would reversed C3 stress jfet's at all? Hoping I don't need to match a new set.
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Phew! Glad JFets are safe.
So I replaced R2, R3, R4, IRF510, and C3. LED’s are lit. No smoking parts. But I can’t adjust P1 more than 11mv.
Other measurements around the amp.
Input voltage 68.5V
JFet1 drain 7.7V
SIT Gate -3.5V
So I replaced R2, R3, R4, IRF510, and C3. LED’s are lit. No smoking parts. But I can’t adjust P1 more than 11mv.
Other measurements around the amp.
Input voltage 68.5V
JFet1 drain 7.7V
SIT Gate -3.5V
well, bummer
it seems you have one or two of JFets (most likely in upper trio) got voltage surge, even if I didn't expected that
possible explanation ( which I expected least likely) is that one of 9K1 gone Dodo, reversed cap gone open and then IRF gone to short
there is no other explanation for decreased max Iq than it is prescribed .......
just in case - powered off, check resistance across R8 (resistor in parallel to P1) ; we are expecting 0R if P1 is working properly, so for max Iq, it must be close to 0R
it seems you have one or two of JFets (most likely in upper trio) got voltage surge, even if I didn't expected that
possible explanation ( which I expected least likely) is that one of 9K1 gone Dodo, reversed cap gone open and then IRF gone to short
there is no other explanation for decreased max Iq than it is prescribed .......
just in case - powered off, check resistance across R8 (resistor in parallel to P1) ; we are expecting 0R if P1 is working properly, so for max Iq, it must be close to 0R