First Amp Build, the good and the bad.....

[ndross475]

So I built a Hoffman PR Head.. Fired it up today using the instructions here and Good news... no smoke... thats a good thing... After running down a few voltage checks sans tubes then with preamp, and finally power tubes I gave it a go. Pretty quiet, but for good reason... the tremolo and reverb weren't working... after doing some searching here and abroad and having done the highlighter check,, again,, I started checking components and voltages around V4, nothing at pin 1,, checked and the 220k resistor on the board tests open...

Lesson learned... check parts before installing...  Will update when I find a replacement... hopefully she is as quiet when she if fully operational...

[purpletele]

So I built a Hoffman PR Head.. Fired it up today using the instructions here and Good news... no smoke... thats a good thing... After running down a few voltage checks sans tubes then with preamp, and finally power tubes I gave it a go. Pretty quiet, but for good reason... the tremolo and reverb weren't working... after doing some searching here and abroad and having done the highlighter check,, again,, I started checking components and voltages around V4, nothing at pin 1,, checked and the 220k resistor on the board tests open...

Lesson learned... check parts before installing...  Will update when I find a replacement... hopefully she is as quiet when she if fully operational...

Sounds like a really good start! 

[ndross475]

Hopefully... It played,,just need to work out the kinks... glad it wasn't something silly I missed.. The double check with the highlighter caught a couple of those...

I am surprised it played... Still learning what all the components "do". I read that V4 is the phase inverter / tremolo osc. tube.. didn't know one would play w/o that working correctly...

Still haven't figured out how V4 is affecting Reverb, and I am hoping thats my Term problem... but no use digging anymore until I get that 220.

[sluckey]

That open 220K resistor will kill the tremolo only. It has nothing to do with the rest of the amp. Your reverb problem is a separate issue. No reason to not continue troubleshooting.

[ndross475]

Sluckey you were right... again....:-)  First the good... Reverb is working... now the bad... I am back to reverb cable building school... But it was a quick diagnosis, thankfully I had a known good set in a Deluxe and sure enough.. I now have reverb.

Now to wait til a local amp tech gets in the shop in the a.m. to pick up that 220 and see if that solves the Trem issue and this will have been pretty smooth...

[sluckey]

In the meantime, you could try anything between 100K and 470K in place of that 220K.

Are you sure the 220K is open? You've pulled it off the board and checked it? You didn't possibly forget that underboard jumper between that 220K and point B did you?

[ndross475]

Yes its open. Off the board and tested... One thing I did right was check my under-boards very well, I saw a couple of YouTubes on what's involved if that happens... didn't want to deal with that.  As this is my first build, no "extra parts" laying around, but thats ok. I am pretty happy so far... My local amp tech will have one I can pick up in the a.m.,

I know that the Trem circuit can be problematic for noise and other issues so I am hoping that this resistor will wrap it up...

I have -33.5 on pin 5 so bias looks good, thats adjusted all the way down. I have an old set of 6v6s in there now, will fine tune it when I put the keepers in it..

[ndross475]

still fighting the no Vibrato.. Wasn't the resistor.  I am fairly certain its a simple "builder error" but can't find it..

Traced the circuit
Checked continuity with all the connections to that area of the board.
Have 396v on the pot side of the 220k 0v on the other side.( so the under board is there)
The other side shows continuity to ground, not sure if thats right.
V4 pins
1 0
2 -.44v
3 0
6 186v
7 2.93v
8 50v

Clearly 1, 2, 3 are off.

Heaters are 5.6v

checked the trem foot wire and it is Not grounded (thought maybe a damaged rca during soldering)

Not sure where to look now...

On the upside, putting together the parts was a lesson in soldering and following instructions, this is where I am learning what the parts do...

Link to Hi-Rez photos..  https://nickdross.smugmug.com/PR-Build/
Please Be kind, first build..

[sluckey]

The other side shows continuity to ground, not sure if thats right.

Be specific. Are you talking zero ohms, or 100 ohms, or 1000 ohms? Don't just say 'continuity'. How many ohms do you read?

OK, there should be no continuity to ground on that side of the 220K. It should be very simple to find out why there is. Start disconnecting stuff. There is one wire to V4-1, one .002 cap, and one 1M resistor. Disconnect each one (one at a time) and see if the continuity disappears. If you still have continuity after all of those are disconnected, unscrew the board and have a look underneath. By now you should know what was causing the continuity. Let us know what you find, even if it's nothing.

[ndross475]

Meter says .6 ohms to ground... beginning disconnecting...

[ndross475]

I removed the wire V4-1 to board checked and replaced...now I have 224v @ pin 1 . Perhaps it was a bad solder joint.

Still no Vibrato though.. have reverb though... ( using a jumped old rca plug in lieu of footswitch for now)

[ndross475]

Good News and bad.... After doing everything but remove the board, some things twice, I couldn't figure out the Trem issue so I asked my amp tech to lay eyes on it before I removed the board. As soon as he plugs it in Presto... Trem works.  Now why, I am not sure other than I was trying to use a RCA jumper at times which I now realize is not necessary with the PR, missed that memo. And the foot switch I was using at times to test, has an issue. I will now be looking at it and the Trem Foot Switch RCA to make sure they both are in order.  In any event he looked it over, found two unrelated loose solder joints and gave it a clean bill of health....

On the positive side I learned more about how this amp works from troubleshooting than I did putting it together. I found a great Uncle Doug video on youtube on how the PR circuit works.

Thanks for all your help.  Looking forward to the next build... See new thread... the bug has bit...

[nandrewjackson]

Looks like a very nice first build! Congratulations