HUM! Need some help...

MHProd · Post by **MHProd** » Tue Jan 20, 2015 12:34 am

So I removed a VVR and PPIMV from a JTM45, reconnected the bias circuit and power stage and now all I get is a nasty hum when I turn on the Standby switch (with that switch off, the hum is gone).

Triple checked all connections, re-soldered everything, nothing has changed...

I tried the amp on a variac and the hum makes the B+ jump up and down between 300 and 370(?)VDC (that's with the Variac at 85V). The regular B+ for this amp is around 440V.

Checked the ground, continuity, OT center tap and choke are on the mains 32uf... I can't find where it's coming from...

Any ideas?

neikeel · Post by **neikeel** » Tue Jan 20, 2015 8:26 am

Need pics and a voltage chart

danman · Post by **danman** » Tue Jan 20, 2015 6:40 pm

Are you seeing any signs of redplating on the power tubes?

MHProd · Post by **MHProd** » Tue Jan 20, 2015 9:19 pm

No redplating but I haven't tried the amp at 120V, kept the variac at 85V max and that's when the B+ starts to oscillate.

I have to double check the OT wires under the board to make sure I have the CT in its right place. I can't think of anything else...

I get the hum with all knobs on 0. Might try to remove the power tubes and see if it's still there.

MHProd · Post by **MHProd** » Tue Jan 20, 2015 11:48 pm

Tried to get some reading with the tubes in. Brought the variac to approx 115V, turned the standby switch on and blew the mains fuse instantly.

...

Lbrown3743 · Post by **Lbrown3743** » Wed Jan 21, 2015 12:00 am

What is the VVR?

MHProd · Post by **MHProd** » Wed Jan 21, 2015 12:26 am

VVR is an internal variac of sorts from Hall Amps. The bias supply and B+ was connected to it.

Re-soldered one side of the choke and OT CT to the mains 32uF and added a wire that goes from there to the standby switch. The other side of the standby switch goes to the rectifier's pin8.

I also reconnected the junction of the bias resistors to the bias supply and reinstalled the connection to pin5 of the power tubes.

MHProd · Post by **MHProd** » Wed Jan 21, 2015 12:48 am

Turns out I blew the HT fuse too...

Been looking for something that could be shorting but can't find it. All connections are solid...

neikeel · Post by **neikeel** » Wed Jan 21, 2015 12:37 pm

neikeel wrote:

Need pics and a voltage chart

Pics/numbers?

danman · Post by **danman** » Wed Jan 21, 2015 6:05 pm

You mentioned working on the bias circuit. Have you checked to make sure that the bias caps and diode are orientated properly? Also be sure to check under the turret board for anything that may be touching the chassis such as a solder blob or bare wire. Does it blow the HT fuse if you turn it on without power tubes? Like Neikeel mentioned, pics and voltage readings will be helpful if you can get them before the fuse blows.

MHProd · Post by **MHProd** » Sat Jan 24, 2015 3:41 pm

The bias circuit was already there and working. I just removed the PPIMV and added the bias resistors + reconnected those to the bias circuit (15K and 10uF).

I can not get a voltage reading on the power tubes since the fuses blow. Without the power tubes however, I can bring the amp to 120V and the fuses are fine. I tried with a different set of tubes but the same thing happen... So it's not the tubes either.

MHProd · Post by **MHProd** » Sat Jan 24, 2015 6:26 pm

Acutally, power tubes are fine as long as the PI tube isn't in. Tried a different tube: same result.

Strange since I haven't changed anything to the PI.

MHProd · Post by **MHProd** » Sat Jan 24, 2015 7:21 pm

B+ 435V on the plates. KT66 are biased at 37ma. No red plating. Bias circuit works fine. -48V on Pin5.

I looked under the board and all is good. Checked all the grounds for continuity. All good.

If I add a tube in the PI, that's when the issue start. Any measurements I should take on the PI without the tube ?

Tried to upload a picture but I get "Sorry, the board attachment quota has been reached."

neikeel · Post by **neikeel** » Sun Jan 25, 2015 5:18 am

Do you think that the PI output couplers could have been damaged with the soldering/desoldering? The hum could be dc passing and then red plating with tubes in.

The PPIMV is located around the PI and you have demonstrated that area to be where the problem is.

Try a free host like Photobucket. The board has only limited capacity for images

MHProd · Post by **MHProd** » Sun Jan 25, 2015 2:48 pm

neikeel wrote:Do you think that the PI output couplers could have been damaged with the soldering/desoldering? The hum could be dc passing and then red plating with tubes in.

The PPIMV is located around the PI and you have demonstrated that area to be where the problem is.

Try a free host like Photobucket. The board has only limited capacity for images

Thanks Neil.

How would I go about testing this on the output couplers?

Will try photobucket.

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