Birthday amp gone goofy

[lego4040]

Good morning folks,it's been awhile but I have a issue with a amp that worked on the bench before I put in the cabinet. So I built my son the Stout TMB for his Birthday, I repurposed a beautiful Poplar Cab and put in a brand new WGS 12" Green Beret. I'm sure my problem is my error but I have a legit question. I shortened all long leads to neaten chassis up because it's housed in a Allan Amp 5F1+ chassis", it has the second preamp tube socket. When I was putting it back I decided to use a empty socket for another output speaker jack. I was only using the 8ohm lead but decided to also use the 16 ohm for the other jack. When I fired it up it made loud hum then a staticky loud sound sound, powered it down immediately and I haven't had a chance to trouble shoot. Either I made a error on wiring when I cleaned inside or it is my mistake of using both transformer outputs to individual jacks( they're not on selector switch) or both. This amp kicked ass on the bench and I can't wait to give it to my son, I gave him my 1980's Danelectro Jimmy Page 59 U2 for xmas.

[sluckey]

I doubt the extra speaker jack is the culprit. Examine all the extra work you did after the amp was working. Compare to the layout and schematic. If you still don't find the mistake(s) then measure voltages on all tube pins. Post the voltages here. High rez pics may help also.

[lego4040]

Before/after

[sluckey]

Before/after

Huh?

[lego4040]

pictures didnt load up, sorry

[sluckey]

Can't tell anything from your pics. Read reply #1 again.

[lego4040]

I really was just showing the area at main caps where I had shortened the lead lengths. I'll pull apart tomorrow and get cracking at it. Thanks for the help, I was on your site today catching up with your work, my coworker got the chance to witness a real TrainWreck in action. He never heard of them and said it was the most interesting amp he ever heard

[lego4040]

I was able to get the amp onto bench and open her up, I am looking at the PDF for Stout build online, have there been changed made? I don't remember that feedback resistor. This past year has been a nightmare both at work and home so I haven't been 100% there. I walked of my job last year after 11 years and went back to my original operations plant with some to work with good old friends. This past ten months have been awesome until yesterday's Enginners meeting, three of unofficially gave notice, my boss who's a great friend and fellow guitar player walked off the job and called it a day after this meeting and had to be physically held back during this heated argument with Mgmnt right before the engineers meeting. Sorry for the ramble but they literally threw the entire operations plant under the bus. II'll fire up amp tomorrow Steve

[sluckey]

I am looking at the PDF for Stout build online, have there been changed made? I don't remember that feedback resistor.

Looks like it. You may need to look at the older drawings. Didn't you keep a copy of the docs that you built your amp with? That would be a good idea. Hope you got the docs for your AC-15 also. Nothing lasts forever.   

[lego4040]

Oh I have hard copies of everything, notes and all. Worst case scenario, My son gets the Sluckey AC-15

[lego4040]

I got the Stout out and on the bench and I found one definite noise maker. I forgot to re-solder the Pilots 100 ohms back to ground  . I also have a question, I cant remember why I wired up the bridge rectifier this way but I didn't ground it cause I have the center tap? I have the two AC's going in and The B+  DC coming outgoing to standby. Yes the Power Tranny has a center tap for high voltage out and I think that's why I did it, Should I ground the bridge rec tab anyway and use both? or am I good. I read here on the info page that with the bridge rectifier grounded you don't need to have a center tap.

[sluckey]

We need full disclosure. Doug's Stout does not have a bridge rectifier. The Stout has 5 tubes. I only see 4 sockets on your amp. What have you built? Show us your schematic.

Has this amp ever worked?
 

[lego4040]

Instead of using V5(5Y3) tube rectifier I installed a bridge rectifier, I am reusing a chassis I had and I was short on space. I know Marshalls made both Solid state rectifier and tube rectifier amps. Thats why you only see four

[sluckey]

Has this amp ever worked?

You cannot simply replace a 5Y3 with a bridge rectifier without making some other major changes, like different PT. Tell us exactly what you have done. Which PT. Schematic would be very helpful.

[sluckey]

Is it possible that you are only using half of the bridge? If so, that will work.

It's very important to know if this amp has ever worked.

[lego4040]

The PT is the Hammond 290CAX and the schematic is the original Hoffman stout build that was up[IMG[URL=http://s77.photobucket.com/user/legomaniac4040/media/Mobile%20Uploads/3F3D57BE-2DC1-40ED-BCE2-BF274E868A90_zpsvistvtkc.jpg.html]http://]http://i77.photobucket.com/albums/j72/legomaniac4040/Mobile%20Uploads/3FA55EEF-6E56-4B1D-9471-2900959B10F1_zpsiyqq7icy.jpg[/IMG][/URL]

[lego4040]

It did on my bench, I even have the Voltage readings. Ill look for them tonight at home when I get off work. No Im not building amps when I work solo shift

[sluckey]

Examine all the extra work you did after the amp was working. Compare to the layout and schematic. If you still don't find the mistake(s) then measure voltages on all tube pins. Post the voltages here. High rez pics may help also.

[lego4040]

the one problem I found immediately was I didn't re-solder the Pilot lamp 100 ohm resistor's.

[HotBluePlates]

... I cant remember why I wired up the bridge rectifier this way but I didn't ground it cause I have the center tap? I have the two AC's going in and The B+  DC coming outgoing to standby. Yes the Power Tranny has a center tap for high voltage out and I think that's why I did it, Should I ground the bridge rec tab anyway and use both? or am I good. ...

No, do not ground the bridge rectifier negative (-) lead.

You probably asked about the limited space at some past time, at which point folks noted that if you only use the "~" (a.c.) and "+" (positive output) pins of a bridge, it is exactly the same as having a regular solid-state full-wave rectifier instead of using the 5Y3.

Your PT is sized voltage-wise for a full-wave rectifier, which you now have. If you were to unground the PT high voltage center-tap and ground the bridge's "-" lead, you would get double the voltage output, probably killing all your filter caps. If you use the bridge's "-" lead without un-grounded the PT center-tap, you would short half of your PT high voltage winding, popping fuses at a minimum.

So you have that wired correctly as-is.

[lego4040]

I'll keep you posted, hopefully I can finish tomorrow and fire it up again on Wednesday. Thanks for that info, I had the amp in Limbo, I had it running before I left my last engineering plant last June

[lego4040]

Haven't had a chance to do much, I have been going over wiring looking for possible shorts, etc...... Today I watched a new Uncle Doug video posted and I think I found my problem. I changed my output jacks to a new set I had and yep they were the shorting kind that Uncle Doug had mentioned in his video not to use on your OT, ITwill fry your OT. Time to check that out, my bad if it is the problem

[sluckey]

I don't see anything connected to the switch terminal on your jacks. If so, it it perfectly safe to use those jacks for multiple impedance taps.

Today I watched a new Uncle Doug video posted and I think I found my problem. I changed my output jacks to a new set I had and yep they were the shorting kind that Uncle Doug had mentioned in his video not to use on your OT, ITwill fry your OT.

Can you provide a link to that video?

[lego4040]

Here's the link

At around 12 minutes is the part about grounded jacks

[Paul1453]

I also heard Uncle Doug make that statement.

I wasn't sure what to make of it though.

Which state is likely to hurt the OT?

Jack is shorted to Gnd, or jack is Open.

It seemed to make sense that shorted to Gnd could be bad, but I was unsure. 

Are the normally open jacks better as output jacks?

[lego4040]

Well I just checked my OT and it seems Fine. I have 298.2ohms red-pin7 and 320ohms red-pin 7 on the input side off the OT. I lifted the shorting tab like Doug did so it doesn't touch anything. It's now the usual output lead to tip tab and ground to ground tab

[sluckey]

I don't think uncle doug put much thought into that statement. 

[Paul1453]

I don't think uncle doug put much thought into that statement. 

So you would say the switched jacks are good for both inputs and outputs?

That is what I have been using lately.
Some schematics do seem to indicate switched jacks on both the inputs and outputs.

[Ed_Chambley]

If using a multi-tap transformer and have a jack for each tap instead of a switch, then a shorting jack should not be used.  You would not want to ground any of the windings I would think.  Therefore it would be cheaper and better use of non-shorting jacks.


I prefer to use Stereo Switchcraft jacks for inputs and use the shorter as an additional sleeve connection.  Seems to make for a more positive connection and a solid feel.

[lego4040]

If using a multi-tap transformer and have a jack for each tap instead of a switch, then a shorting jack should not be used.  You would not want to ground any of the windings I would think.  Therefore it would be cheaper and better use of non-shorting jacks.

That is where my mistake was Steve, I have since made the correction and the readings I posted a reply back is what I found on my OT. I'll be firing up the amp tomorrow to test agin

[lego4040]

Wiring finished, chassis back in cab, poor man variac ready to go and making sure all tubes are seated good. Two things the only incandescent bulb I have crapped out so I can't go any further today. Bad news on the 6BQ5's I think, what I thought I heard is now visual proof. I have no tube tester but is there a way to test with a ohm meter from pin to pin?

[Paul1453]

You could check the heater filament with your meter.   

And check for shorts between the other pins.

That's about all you could check with only a DMM.

[lego4040]

Will do, I only have one other set and I'm not ruining them till I'm sure I have my issue solved. I sware it has to do with my error on the OT. I had the amp running in my shop, played the guitar through it and it sounded great.  I have the Voltages on a sheet then made the OT change after I put chassis in cab, I'll post my readings from first start up when I get home tonight

[sluckey]

Bad news on the 6BQ5's I think, what I thought I heard is now visual proof

What do you think you see?

The best way to check a tube is to substitute a known good tube. If you don't have spares, I know you have two of those in that AC-15.

[lego4040]

   Those were out of the AC-15, the bottom of tube around the pins are all dark brown, I dont remember them being there before. Both tubes test 1.5ohms on the heaters. Gotta go witness a Fire Damper now with electricians,BrB.

[lego4040]

Well the tubes doesn't show any thing shorted out, only the internal connection between P1-P2 and off course the heaters which have a ohm resistance of 1.6 each tube. Ill slap them back in, test connections and fire it up again tomorrow.

[lego4040]

Alright, those Nos Baldwin 6BQ5's held up, I put them in my stereo amp and it played nicely. So here are my numbers

[Paul1453]

Most of those voltages look OK.   

V2 with 75V on it's cathodes doesn't seem quite right to me.   

Something must be wrong with your final Wattage calculation.

I'm pretty sure you can't get 32W out of a pair of 6BQ5's. 

[DummyLoad]

75V @ 3,8 (k pins) is normal for a LTPI. study more paul. 

idle dissipation is a bit high (16W/tube) but not of range of normal for this type of amp.

everything else looks OK.

--pete

[sluckey]

Your voltages look fine. Plate voltage is about 20 to 25 volts higher than any of my cathode biased EL84 amps. You missed two steps in your power calculation. First, the plate voltage is actually the voltage between plate and cathode, so you must subtract the measured cathode voltage from the measured plate voltage. Use this value for the actual plate voltage. Second, the power you calculated is for TWO tubes. So, in a perfect world, each tube will only dissipate half that value. Even after you correct your power calculation, your tubes are running very hot. That's typical for EL84s to be abused.

There are a few things you can do to lower the power dissipation... lower B+, lower screen voltage, or increase the value of the cathode resistor. The cathode resistor is what I'd change since it's easy. I use 150Ω (rather than 130Ω) cathode resistors with my EL84s and my dissipation is still hot even with my lower B+. I would try a 200Ω or 250Ω cathode resistor in your amp and recalculate the dissipation. Or not worry about it and just replace tubes as needed.

Now, back to your problem. If the amp was working before you shortened the leads (and whatever else you did, either purposefully or unintentionally), then the problem has to be what you did. Reexamine your work. It will probably be helpful to click on the link in my signature line at the bottom of this reply.

[lego4040]

Yep, back to basics. We fired up amp, started playing and 5 min later I got a farting sound out of speaker. I powered it of and I'm back to square one. I'll change that cathode resistor,

[lego4040]

Thanks for that tip Steve, Ill print out layout in grey-scale but I don't think there is a link to the original layout. I did test the 12AX7's and EL84's in my stereo amp and worked fine. I tested the OT for shorts and that was I had no shorts to OT case or Primary into secondary. Ill keep chipping away looking for my problem

[Paul1453]

75V @ 3,8 (k pins) is normal for a LTPI. study more paul. 

--pete

I'm studying some, but I can be a bit dense at times.   

I just didn't recall any of the circuits I had worked on having that high a voltage there.

That doesn't mean there weren't any, just none my pea brain could recall.

I'm sure my Plexi is different, but it only had 27V there.

Most of the cathode bypass caps I remember were cool with a 25V ecap.

I know I need a lot more studying to get even close to your guys understanding of these circuits.   

[lego4040]

I'm no Guru at All, This site has helped me immensely. I printed out the updated version and except for two or three changes I can follow, I will make those updated changes tho. I may pull board circuit board up slightly so I can peak underneath to see if solder slag is causing a issue

[Ed_Chambley]

As Stuckey mentions, EL84's get abused and to me the little extra you get from them running them hot is not worth it.  I run them at a maximum of 310VDC and I know they can run hotter, but if using a 130 Ohm Cathode resistor, usually 310 VDC will get your where the tubes last.


Got a friend with a DR Z 18 watt MAZ I think.  Eats tubes like crazy as he gigs the amp frequently and it is run hard.  Checked and the plate voltage was about 400 VDC.  I had a difficult time getting the correct size cathode resistor to work well enough.  I even tried using 5 watt wire wound pots to get the bias under 90%.


Finally I simply installed a 10 watt dropping resistor prior to the first node and brought it down to 325VDC.  There is a funky CF tonestack in the MAZ and it has a 100K resistor to ground prior to the stack and I had to change this to a 68k to get the tonestack back functioning well.


He has been running the same tubes for quite a while.  EL84's are fairly cheap and some claim the JJ's hold up best, but everyone I know that uses low watt EL84 amps and runs them wide open toasts a lot of tubes.

[lego4040]

I spent the last few days bringing the amp to the updated version, I also threw in a 10W 250 ohm bias resistor. I'm about to add the feedback resistor and the schematic shows 82k and the layout shows a 120k. Thoughts on which one I should start with

[sluckey]

Yes, I would use one. 

[lego4040]

Ok I threw in a 82k, I went over wires, blow out chassis and got preamp tubes in. Gotta get kids from bus stop and take to docs, will put rest of tubes in and fire up later tonight

[lego4040]

Alright I'm stumped. I fired amp up, looked good at first until I took readings on V2 pin2 & 3. Firstly pin1 gave me a motor boat sound, p2 gave me 110 Vdc and p3 over 100vdc. My B+ went up to 359 too. I gotta go back to beginning and pull it all apart

[lego4040]

I said was going back to square one and I did. I followed the link above and went over board/layout with a highlighter, all checks out good. I lifted one side of the leads so I can check under the boards for any shorts, all good. I got Reid off the bridge rectifier and put the 5Y3 GT tube in, I still need to wire it in. Then I need to place the can cap where it is shown in the picture.