Help on Hoffman Princeton Reverb Turret Board

[Speakz]

Greetings and thank you in advance for any help.


The two things I need help with are 1. No sound and 2. how to bias the power tubes with a multi-meter.  I have the bias control pot set halfway to start. I have done highlighting/tracing to check work and checked off component values.  In the process I have found a couple things and fixed them including rewiring my input jack (which I know is upside down to prevent the resistors from touching the chassis).  Also I started out with a weber kit before switching over to the Hoffman board so I have a couple extra things in here.


I grounded the first stage directly to the buss wire vs going through the input jack ground du to space issues.  Not sure if this is a problem but I'l open to feedback.  Here an there i had to refer back to the weber for the extra jacks.


What works and have I tested with another amp builder near Boston is the power supply/6.3v winding and also the bias circuit which is slightly different but he said it would be ok to use even though its was for the 6a14HP.


For checking pin voltages I have as follows.


Cap can- 1 382v, 2 369v, 3 277v, 4 277v


V1 P1 140v, P6 139, P3 2v, P8 2v


V2 P1 365v, P6 361v, P3 3.46v, P8 3.43v
 
V3 P1 .5-1.0 (fluctuated alot), P6 .5-1.0 (fluctuated alot), P3 .001, P8 .001


V4 P1 .4-1.0 (fluctuating), P6 .2-.9 (fluctuating), P3 .562-1.0,  P8 .4-.1 (fluctuating)


Thanks for your help and any tips about reporting voltages with multi-meter


Chris

[Speakz]

I should add that I am using the hoffman standard PR OT.  And also I can put up otehr pics as needed.  I have the bias board off in the corner vs on the turret baord.

[EL34]

Do this first
http://el34world.com/Forum/index.php?topic=17701.0


Check out all the documents for the PR on the library page here
http://el34world.com/schematics.htm#Hoffman_Princeton_Reverb_

The install instructions go through how to bias
http://el34world.com/Hoffman/instructions.htm

[sluckey]

Your pic looks fine. We need more. We need to be able to clearly see the tube sockets, the board components, and wiring between the board and the sockets.

Your plate voltages for V3 and V4 are very wrong. Did your meter die on you or get switched to "AC" for those readings? Recheck.

Why do you have a jumper under that 18K/3W resistor? Talking about the one on the right in your pic.

For now set your bias pot so you have maximum negative voltage at pin 5 of both 6V6s. Leave it like that until you get the V3 and V4 voltage issues fixed. Make sure the 6V6s are not red plating.

[Speakz]

Thank you for spotting that extra jumper wire.  I will think to trace starting at the very beginning in the next build.


I biased the tubes to 25 ma and tested voltages again.


V1   P1 118, P3 1.6, P6 116, P8 1.6


V2   P1 371, P3 3.2, P6 370, P8 3.2


V3   P1 137, P3 1.2, P6 136, P8 1.2


V4   P1 1965-212(fluctuating), P3 1.9, P6 213, P7 15(?), P8 156


I happened to tap pin 7 as I went down and it came through the speaker as well as the OT connections at the power tubes.


Here are some pics


 

[Speakz]

More pics

[Speakz]

more

[p2pAmps]

You can find my pictures here of a few of Doug's PR I put together in case it helps to see more...


Cheers,


http://p2pamps.com/pr
http://p2pamps.com/kbpr

[sluckey]

Those voltages look much better. What did you do to fix?

Your first BIG pic was great! These little 648x480, not so good for troubleshooting. I did spot another jumper you need to remove. You have a green wire that connects V5 pins 1/8 to V6 pins 1/8. That needs to go because it will mess up your bias checks. I also don't see a ground wire on the 1Ω resistors on the board. Where is it?

Do you have any sound yet?

[eleventeen]

From your first pix:


[/URL]

[eleventeen]

Am I seeing this right?

[sluckey]

There should be an under board jumper as shown. Measure resistance between the two circled turrets. Should be zero ohms. If not, just put the jumper on top of the board.

[p2pAmps]

There should be an under board jumper as shown. Measure resistance between the two circled turrets. Should be zero ohms. If not, just put the jumper on top of the board.

I do not see a bias pot, am I missing it?

[sluckey]

It's on the back panel. The rest of the bias supply is on a small board near the PT and front of chassis.

[p2pAmps]

It's on the back panel. The rest of the bias supply is on a small board near the PT and front of chassis.

Now I see it Steve, little dark on my old Mac...   

[Speakz]

Sluckey, I did remove that jumper under the 18/3w resistor and that may have corrected those readings?  I think this is the same one you are referring to eleventeen. 


Also, I touched up those to solder joints eleventeen.  Question on that? Even though it runs down into the turret should I fill it further until it bubbles at the top?


Sluckey,
With regard to the V5 1/8 and V6 1/8 should I wire them individually and use two seperate wires running to the 1 ohm resistors for each set or just cut it? The wiring diagram shows them the way I have it unless I'm misreading it?


I tested for that under board jumper and it reads 0 ohms.  Also, I can see the reflection in the chassis underneath as well so I was able to visually double check.


msscaggs, that is some great wiring!  I will refer to that for my next build.  Color choice is nice as well makes muc heasier to follow everything around the board.


Thanks for all the help.   Hope fully I haven't missed any of your questions. 


Still no sound.  I am adding better pics

[Speakz]

I upped the size of the pics and added a 100w full spectrum bulb to illuminate things a bit better.  Let me know if you need a close up of anything but I think you should be able to zoom in with better resolution now.

[Speakz]

More

[Speakz]

I forgot to answer the 1 ohm resistor ground question.   I believe I do have those lugs threaded to hit ground? Are you able to see with the new pics?

[Speakz]

more

[sluckey]

These pics look much better.

With regard to the V5 1/8 and V6 1/8 should I wire them individually and use two seperate wires running to the 1 ohm resistors for each set or just cut it? The wiring diagram shows them the way I have it unless I'm misreading it?

Cut is quick. Unsolder is preferred.

Look at the layout again. Hoffman's layout shows pin 1 and 8 connected together with a black wire that runs to the 1Ω resistor on the board. There is no wire running from one socket to the other socket.

I believe I do have those lugs threaded to hit ground? Are you able to see with the new pics?

I see the 5 turrets strung together but I don't see a wire connecting those turrets to ground. Is it under the board? See my attached pic...

[Speakz]

You rock! 


I went back and looked again and realized what you were asking even before your recent post and it is now working.  Thank you for clarifying the pin 1/8 question.  I don't know why those were connected but I seperated them. 


Feels like Christmas morning     My kids are going to be thrilled that its up and running.


Now that those two sockets are seperated would it make sense to re-bias?  The specs say the tube can handle 80 ma at the very top.  Is there a sweet spot with these EHX 6v6's or just personal taste up to about 70% of the max specified?


Also, only 1 input jack is working so I will go ahead and take those out and carefully re-solder the one that isn't working. 


I realized after working on this thing very slowly for a year that I forgot most of what I read in the very beginning once EL34 referred back to those beginning pages.


Woohoo!

[sluckey]

Where was the ground wire?????

Now that those two sockets are seperated would it make sense to re-bias?  The specs say the tube can handle 80 ma at the very top.  Is there a sweet spot with these EHX 6v6's or just personal taste up to about 70% of the max specified?

I'd set the bias at 70%. Play around that area. You will not be anywhere near 80mA. 6V6 max plate dissipation is 14W. 70% would be 9.8W. So, for example, if your plate voltage is 400v then your bias will be set for 9.8/400 = 24.5mA. So, adjust the bias for 24.5mV at pin 8 of a 6V6.

I think Doug has detailed instructions for setting the bias in the library link at the bottom of this page.
 

[sluckey]

Wire your jacks EXACTLY like this and they will work as designed...

[EL34]

Do this first
http://el34world.com/Forum/index.php?topic=17701.0

That is why you do my tracing tip after you finish the build

I am glad Steve likes to help you but the amp is at your house and it's better if you do all the tracing and double checking first
Then if you get stumped, post your issues here

[p2pAmps]

Do this firsthttp://el34world.com/Forum/index.php?topic=17701.0

That is why you do my tracing tip after you finish the build

I am glad Steve likes to help you but the amp is at your house and it's better if you do all the tracing and double checking first
Then if you get stumped, post your issues here

I couldn't agree more with Doug.  I have learned that you go slow, double check yourself each and every move then check it again before you move on.  Do very very neat work so if you have to troubleshoot an issue you don't have to dig through a bunch of spaghetti to find something.  Then and only after you have exhausted all efforts come to the experts here on this forum.  Aren't we all lucky to have you guys...

My 2 cents...(worth nothing :))

Scaggs

[sluckey]

My 2 cents...(worth nothing :))

Your money's good here anytime. 

[Speakz]

Thank you guys.  Unfortunately I kept overlooking that the ground wire on my tracing sheet.  I had everything marked but that.  I only work late in the evening after kiddos go to bed and I miss things because of it.


I'll have to ask Bob when I see him what his thoughts for having the wire between V5 and V6 were.  Is that something that people have done on previous designs of this amp type?


mscaggs, I thought your pics and work are something to aspire too and I will look at again as I go along in the next build.  I had a longer post answering questions and thanking people but it never took and after rewriting it again some things got lost.   


Now that I have a better understanding of the building process I can see where testing each section as you go along in addition to tracing makes things go well.  I never even thought to trace the buss wire going between lugs but I will next time around in addition to paying more attention to the grounds wiring.  Go slow, trace everything, test with the meter as I go along, and reread all the install instructions again. Thanks for the reminders.


I will work on the input jacks some more tonight.

[EL34]

Unfortunately I kept overlooking that the ground wire on my tracing sheet.  I had everything marked but that

Nothing wrong there, that's exactly how the tracing deal works

When you are done tracing, everything should be high lighted

I do the tracing system to double check all my work, it works like a charm

[sluckey]

I'll have to ask Bob when I see him what his thoughts for having the wire between V5 and V6 were.  Is that something that people have done on previous designs of this amp type?

No, people don't do that. Connecting the two cathodes together will not keep the amp from working. But, now you can no longer check the bias of each tube separately. The two 1Ω bias resistors are now in parallel, making them look like a single 0.5Ω resistor. This will mess up your bias readings, especially if you don't think about what you've done, or if you don't understand what I just said.

[Speakz]

It makes sense to me but only recently.  I have been watching/reading some of Gerald Weber's educational materials over the past month prior to that and this build I've had no experience with electrical work or circuits.   


My training is in psychology/clinical social work and Combat Trauma Treatment/Child therapy/Forensic work and teaching graduate students in those areas.  Learning about circuits has been a nice change of pace. 


The toughest thing which I'm beginning to understand particularly when checking voltages is where different stages begin and end and knowing what series or loop your looking for when testing. I'm learning about it now on the video actually.  I'm starting to see it.  The power section,, filament winding/pilot light, bias board are making sense but the stages of the preamp with the OT and reverb are still coming together.


Thanks you guys.  I'll check back later

[EL34]

The toughest thing which I'm beginning to understand particularly when checking voltages is where different stages begin and end and knowing what series or loop your looking for when testing.

You should go to my library and start looking at all the free documents

Have you been here and looked at everything?

http://el34world.com/schematics.htm

One of the documents is this current flow document
http://el34world.com/charts/currentflow.htm

[Speakz]

This is extremely helpful.  I haven't seen anything detailing things in this fashion.  I found a signal flow diagram on a 5E3 fan page.  I have mostly stuck to the kits page and related materials but now I will certainly look more around in the schematics.  The physics of it and manipulating electricity and polarity in this way is really something to behold.  To many it's probably simply taken for granted but I find it extraordinary.


So I can basically separate these sections as I build unpowered and use an ohm setting to make sure the a particular signal path is complete. Then with the info on voltages which are common in a preamp design developed for a 12ax7 triode tube type (?) and test with the corresponding current type setting at various pins or location to make sure the component values are all working together properly as designed.  This visual aid really helps.  I'll keep dissecting it and poke around in the library some more.  Thanks EL34 

[EL34]

There's a bunch of handy links at the bottom of all the forum pages
Makes it quick to get to the library page and other pages

[Speakz]

So I want to make sure I get this right.  When I wire these inputs should I bend T and S and wire them all together with 6.8 and 1m all in the one spot then attach the ground only coming from G in input 1?


 

[sluckey]

This picture came from the Tube Amp Library of Information link at the bottom of this page. Take a look. There's a world of useful info on that link.

Red dots indicate solder connections...

[Willabe]

with 6.8 and 1m all in......

That's a 68K resistor = 68,000 ohms, not 6.8 ohms. And 1meg ohm = 1M, not 1m. Lower case represents milli.

[EL34]

So I want to make sure I get this right.  When I wire these inputs should I bend T and S and wire them all together with 6.8 and 1m all in the one spot then attach the ground only coming from G in input 1?

Again,
This info is all in the library 


The library is your friend
The link is at the bottom of all forum pages.

[Speakz]

Greetings,


I have rewired my inputs but am still getting no sound except for a very light hum which makes me believe the speaker is receiving some signal from the amp. 
Previously, with your help the ground issue was fixed and the old Weber input wiring was working one input.  I rewired as you posted but now have now sound coming through. I also tried using new input jacks in case the old ones were not good.  I've tested the ground connection from the input jack all the way to th te PT lug and it shows a connection.


Here is a picture of my input and V1.  Any thoughts or things I should try would be much appreciated.

[Speakz]

V1 Pic

[sluckey]

Check resistance from V1 pin 2 to ground as follows...

Set meter to measure resistance. Connect one probe to ground. Connect the other probe to V1 pin 2.

1. With nothing plugged into either jack, the resistance should be 34K.
2. Insert a blank phone plug into the LO jack. Resistance should be 68K.
3. Move plug to HI jack. Resistance should be 1.034M.
4. Insert a second plug into the LO jack. Resistance should be 1.068M

[eleventeen]

In your V1 pix, 2 posts above this one....I see a black heater feed wire going to pin 9 and I see a brown one going to pin 4.....and while I see there is solder filling up the hole in the lug  on pin 5...I don't see a connection, a jumper if you will, between pin 4 and pin 5. It's perfectly possible that I am not seeing the jumper as it could be hiding under brown heater-feed wire. It just does not look like it's there. It's absolutely possible that if you are a newb, you may have wired the heater only to one lug. Or you wired it to 5, thought you made an error, then moved it to pin 4. Has to go to BOTH pins 4 & 5. Or there has to be a jumper between pins 4 & 5, however you wish to state it in words. Pins 4 & 5 are shorted together AND fed that brown wire.


If that jumper is not there, that tube will LOOK LIKE it is lit up, just quickly looking at the powered-up tube, but only half of the dual-triode will be lit up (you'll only see ONE dot of light atop the innards of the tube when looking at the tube powered up) but the unlit half will be dead as a doornail. I mean, DEAD, shut off, not working. As if it was pulled out of the amp.

My bet is that this is the problem. Turn the amp on and look at that V1 tube. No meters. Do you see two dots of light or just one? Compare to the other dudes.

So these 12A_7 triodes are two identical triodes or are supposed to be. Therefore, it does not matter if the very first triode your guitar signal hits is 1-2-3 or 6-7-8. The Hoffman board *probably* follows Fender but may not, I have not checked. It would not matter. But it *COULD* be that XYZ amp co and Hoffman designate the tube-halves differently.


All that is meaningless, you need not care. But you MUST care that 4 and 5 are to be jumpered together for the tube to be fully lit up if you are powering the heaters w/6.3 VAC, which you clearly are. If that tube has a dead (unlit) half, you better believe it could 100% kill your guitar signal.

[rosincore]

I agree with eleventeen, pin#4 looks to be soldered, but pin #5 looks NOT to be soldered or attached to pin #4, as eleventeen said, right there is where the signal will stop, within the tube--this side of the tube is not operating and there will no output to the coupling cap to pass signal to the output section.

[Speakz]

First, thanks guys for all the help...


To answer your questions first I will give the readings I got and the setting I had it on.


Reading V1-P2 and Ground


No plug reading on 200k setting read 00.3


Plug in #2 input 200k setting read 00.2


Plug in #1 input 200k setting got 3.0, then 1.8, switched to 2M setting and got .005


To answer your other questions guys 4 and 5 are soldered together its just a very small wire with no shielding and its not showing at that angle of the pic, if you want more pics I'm happy to provide.  There are more above earlier in the thread as well.


Also I tapped the plug while it was plugged in and the tapping came through the speaker very slightly.. plugged in guitar no sound, did it with reverb tank connected and not.


Question for you: If some solder had leaked down on V1 P2 into the area of the pin what effect would that have?

[Speakz]

Eleventeen and Rosincor I went back and looked for the second part of the tube to be lit and found that there was only 1 side lit-up.  I replaced that tube with another and also re-soldered for good measure.  I now have clacky clack from slapping the strings and loud buzzing but no musicality coming through..?  Once step closer?