Let the magic smoke out of R30

[Steven-H]

All,
I completed the build of my Tubelab SE this morning. Powered it up w/o any tubes and things checked out. I then put my rectifier in (GZ37) and once it started to heat up, R30 gave up the ghost. Any thoughts as to where I should look ? Perhaps Im shorted somewhere ?

Thanks,
Steve

 

[Steven-H]

Also I should add that once I powered the unit down - I still have a residual 400vdc or so in the caps - not sure if that helps track down how far down the circuit I need to look. Also voltage at R16/R27 - so assume that means It wasn't U2/U3 ?

 

[Steven-H]

Well at least got the R30 situation figured out - had a bad ground connection....Trying to test for bias, etc now.

 

[Steven-H]

Looks like my diodes fell victim as well...getting 45mV out of the bias at most =(

Also right now Im getting about 460v B+ (unloaded) - not sure what I should be shooting for ?

 

[Steven-H]

Issues persist.

The diodes are fine, getting -318vdc out of them. However post R5, I only have 45mV - so assume at the minimum R5 is bad ?
Also I plugged the 300b's in, and the fils light for 15 seconds, then go out. Assume the regulator is getting too hot ?

 

[zimmer64]

Hi,

can you post pictures? that might enable some of us to help.

Cheers

Michael

 

[Steven-H]

Michael,
Absolutely, sorry I should have yesterday:

Im using a Hammond 273cz transformer, 193 Choke, motor run cap in parallel w/C5, bias meters, and electraprint outputs.

 

[timhowell]

It does sound like your regulator is getting too hot. I had the same problem and ended up switching to 45s instead of 300bs as I couldn't fit a large enough heat sink in the chassis I was using.

 

[Steven-H]

Thanks Tim. Kind of disappointing -as these are the ones specified in the BOM. I have the space for larger heatsinks...so will try that...if not my only option then will be to use an external supply, as I need the extra power of the 300b.

 

[JP]

The BOM heat sink is not sufficient for 300B. This is covered in the build docs on tubelab.com.

 

[Steven-H]

I ordered a much larger heat sink, should be here Monday. Hopefully by then I can track down this bias issue.

 

[Steven-H]

R5 is bad. Should be 270 ohms, it is measuring about 380k. Will replace and report back.

 

[Steven-H]

Update - I now have -370v on the grids. R5 was bad, so I replaced w/what I had on hand (330ohm 5w). Should be close enough for now - will have to wait and see once the new heat sink arrives and I get the tubes back in

 

[Steven-H]

Update 2 - One step forward, two back. I do not have any plate voltage on the front-end of the amp. The main power tubes are OK - but looks like U2/U3 may have fried. I may as well order another Q1/Q2 while Im there.
Surprising the damage R30 caused......

 

[Steven-H]

Ok guys - here's where I am:

When I do not have the 5842's in the amplifier, I get the following voltages on U2/U3:

G: 122v
A: 477v
K: 477v

My unloaded B+ is 477v. Does that appear correct ? Im assuming no - because when I put the tubes back into the sockets (only the drivers and the rectifier are in) I get:

G: 1.02v
A: 477v
K: 268mV

Here's the data sheet for the devices:
http://ixdev.ixys.com/DataSheet/98704.pdf

I purchased replacements, but want to run this past you all before I go ahead and replace.

Thanks,
Steve

 

[Steven-H]

Well I got anxious and installed the replacements for U2/U3 - same result =(

 

[Steven-H]

Bump for some assistance please

 

[Evenharmonics]

Did you write down the voltages during your "check out" process? If you did, can you post it like the way shown here?

 

[Steven-H]

1) w/ no tubes in,
B+ = 0v
B- = 0v. R6 is bad
Output tube filament supply left =4.96v , right =4.96v

2) w/ rectifier tube in,
B+ =474v
B- = 0v. R6 is bad
Output tube grid to ground left = 0v , right =0v

3) w/ rectifier tube and input tubes in,
B+ =474v
B- =0
Input tube plate to ground left =1.2v , right =1.2v

4) w/ all tubes in and set the output tube bias,
B+ =474v
B- =0
Input tube plate to ground left = 1.2v, right = 1.2v

 

[canersahin]

Steven,

Which part list have you used? If you used the part list that I post, please beaware I found a mistake in the part list.

I have updated the list.

R13 R24 R30 and R16 R27 values were faulty (Wrong digikey part numbers).

Might be the cause of your problem?