New Build Mystery

[jazjamr]

6V6 JTM build in Hoffman Stout Chassis. Without Power tubes all voltages seem normal. Power tubes installed 330 volts B+, 370 on pin 3 of Power tubes, seems impossible? -70 bias on pin 5, 50 millivolts at bias check point. With 52 volts bias supply doesn't seem right . Built to Hoffman 6V6 Plexi schematic. Classictone Vibroluxe 325-0-325 power tran. Classictone Trainwreck style Output. 39 and 35 vohms to center tap of output tran. It's a mystery to me!

[labb]

I would double check my meter.

[sluckey]

None of these voltages make sense. I'd expect that 325-0-325 PT to have a B+ of 425 or more. Pull the 6V6s and check the B+ and plate voltages. What have you?

A 52VAC bias tap can make over -70VDC, but if you put -70VDC on the grids of a 6V6 I would expect the tubes to be in cutoff (or very near). It should be impossible to get 50mV across a 1Ω cathode resistor.

I suspect a wiring error(s). Carefully check your amp against the layout and schematic. Post some hi-rez gut shots.

[jazjamr]

My sincere apologies. 430 B+, 470 on Plates pin3. I have 2 meters . The first meter wouldn't hold a reading on pin 3 with power tubes in. The second meter gave the 470 reading. I put an old pair of 6L6GC in for the time being, I was afraid of harming the new 6V6s. Meters read voltages fine with no power tubes. Preamp voltages loog good.

[jazjamr]

Power tubes removed. 474 B+, 474 pin 3 plates

[jazjamr]

Well, one of my meters is bad on high DC voltage. Started again Brought - bias voltage down to -45volt range, Bias in 40 millivolt range ,6L6GC tubes, it held for a couple of minutes . 434 volts B+ 434 volts pin 3, then voltage started climbing again. It isnt holding bias voltage steady. I'm going to go back over the circuit again after a break. Thanks!

[sluckey]

It's possible that you have an oscillation going on in the power amp. Disconnect the feedback wire and leave it disconnected until the amp is behaving. Does the plate voltage look normal now? And the grid bias voltage? And the cathode mV reading?

Also, you say you it's built IAW Hoffman's Plexi-6V6 schematic. Did you use a PT red HT wire (325vac) and 220K/3W bias range resistor as shown on the schematic? Or did you use the Classictone 50vac bias tap? If the latter, did you use a much smaller bias range resistor, more likely in the range of 470Ω to 1K?

Hi-rez pics are very helpful troubleshooting over the net, so I ask again.

[jazjamr]

I have tried to download pictures through my I phone but didn't succeed. 50 volt bias tap-470 ohm 3 watt-1N4007 reversed- 100uf 100volt pos. to ground. 50 k Trimpot 1/2 watt.- Bias to 1, 3 to ground through 47k,2 to junction of 100k resistors as per schem. I'll try disconnecting NFB after I go through the circuit again. I've had to reverse Brown and Blue before on builds, maybe after NFB removal I'll try that. Just seems that after a couple minutes the - bias starts ramping up on its own. Might try substituting Trimpot with fixed resistor at value pot last held bias. I really enjoy building all my equipment. Guitars pedals amps. I also enjoy reading the build threads!

[jazjamr]

Disconnecting the NFB seems to have helped. Bias is now fairly steady at around 40 millivolts 430 volts + and pin 3. I plugged in guitar, bias rises to 60-70 millivolts when strumming chord with vol. up. However there is very little output to speakers. I've followed signal through amp with signal tracer to output of PI caps. you can hear the increase in gain at each stage. Tomorrow is another day. More double checking component values and circuit accuracy. Thanks

[sluckey]

Disconnecting the NFB seems to have helped.

So now you should reconnect the NFB and swap the OT plate leads. Still stable?

[jazjamr]

I've been busy. Rehearsal for Huntington Blues Society competition and building a deck at home in the morning . Brown and Blue OT Plate leads reversed, NFB reconnected.  B+ 437v. V4 432v. V5 436v. V1 162 v. Bias V4 40.4  V5 44.2   Bias junction -67 v.  Better.  By the way, I bought a new meter today.  Output to speaker is extremely low. Screen Grids are I-2 volts  below plates. I have installed a Rectifier tube in this build so I can regulate B+. Got a GZ34 in now. Signal trace still shows signal coming through preamp to P.I. caps .

[John]

Output to speaker is extremely low.

Somewhere signal is going to ground instead of where it should be... maybe a strand of solder off a socket pin, anything like that?

[shooter]

Signal trace still shows signal coming through preamp to P.I. caps .

and then........

[sluckey]

...Bias junction -67 v.  Better.

What exactly is the bias junction? 6V6 tubes will pass very little signal with -67v on pins 5. 6V6s will normally need only about -35v on pins 5. When you get the pin 5 voltage down to about -35v you amp will be much happier and much louder.

[jazjamr]

photo 1

[jazjamr]

photo 2

[jazjamr]

photo 3

[jazjamr]

photo 4

[jazjamr]

I have an older pair of 6L6GC installed now. Amp was so unstable initially, I was afraid the new pair of 6V6s would be damaged. Bias junction is the point that  -bias voltage is inserted at junction of 2 100K resistors at P.I. caps feeding Power Tube grids. Now that I was able to download photos you can see it is between the .1uf caps. - bias voltage comes from 50k Trimpot on rear panel. 6l6 GC bias is around 40 millivolts, -67 bias voltage at junction. When I get the bugs worked out I will install the 6V6s and rebias. Volume is extremely low. Tomorrow I will take resistance readings at all points of the O.T.

[jazjamr]

With The 6L6GCs -67 still seems too high with bias at 40 millivolts. Should be in 45 range? I set up all my Tube Rectifier amps to run 6l6 or 6v6. Most venues we play 6V6 is enough volume. I used the 6.6k Trainwreck style O.T. ala the Germino35 which can run multiple tube types.

[shooter]

seems too high

if your plate is still 432vdc * .04A = 17.3W, don't know if the 40mA is shared, do believe that's kinda hot for the V tube

[Willabe]

Bias junction is the point that  -bias voltage is inserted at junction of 2 100K resistors at P.I. caps feeding Power Tube grids.

....the pin 5 voltage.......

Measure the -bias at pin 5.

You only need to measure for -bias dcv at the 'junction' if your not getting -bias dcv at pins 5.

Use alligator clips on the meters leads to measure at pins 5. 

[Willabe]

He has 2 speaker jacks, maybe their wired wrong? Shorting the output to ground.

And the speaker impedance switch might be wired wrong.

Edit; Looking at the 2nd speaker jack, looks like a shorting jack.

 

[John]

photo 2

I doubt it's your problem, but just to be sure I'd bend out that little Xicon cap on your pot. The legs look mighty close to the housing.


And like Willabe says, check bias voltage on pin 5 of each tube, and double check your speaker jacks. Maybe even disconnect the shorting jack entirely?

[sluckey]

I think I see a problem with the speaker jack wiring. Try this test to see if volume improves...

Leave the speaker plug connected as is in the pics. Plug another bare phone plug into the extra speaker jack. Just a dummy plug with nothing connected to it. Does that help?

I noticed that your bias supply is quite different from the Hoffman schematic. I suggest you add the missing 15K resistor and 10µF filter cap and wire the bias pot just like Hoffman's schematic shows.

There's still something that does not make sense about your -67V bias voltage and your 40mV at the cathode 1Ω resistor bias test points. One of those readings is very unlikely. Bias voltage should be in the range of -45v to -50v to produce 40mV cathode voltage. This condition may be related to the possible speaker jack problem that I think I see.

[Willabe]

I think I see a problem with the speaker jack wiring.

Yes, you see it too?

When you enlarge the pic you can just barely see the shorting tab on that 2nd jack. But I can't see if its wired up.

[sluckey]

The switch lug definitely has a solder blob and what looks to be a bare wire headed over to the ground lug. Sticking a blank phone plug in the jack would tell. The more I look at it, the stronger I suspect this is the problem.

[jazjamr]

First ! You guys are the greatest. It is refreshing to find people of like mind ,willing to share their knowledge and passion, in this self centered world in which we live. Thank You All. 100 % correct on the speaker jacks. I have disconnected the extension jack. I need to order a long thread non shorting jack. We have volume now. Amp is making spaceship LOL noises with NFB connected and disconnected. I checked the O.T. resistances. 35 and 37 ohms to center tap primary side. .2,.3,.4 ohms secondary side. Tran. is 4,8,16 0hms secondary. Classictone 6.6K Trainwreck style O.T. Should I swap Brown and Blue leads again even though I checked it with NFB connected and disconnected? The 32x32 uf can cap is several years old, never used before. It checks 32uf +- on my meter. Aluminum chassis w/ isolating washers on all jacks. No continuity anywhere to chassis. The Bias circuit is the Fender style . P.T. has 52 volt Bias tap. Thanks Again!

[Willabe]

Glad you got it going.

You can use that jack, just don't wire up the switch connection.

[jazjamr]

I got the amp up and running today. In my concern about board layout for the small stout chassis. I overlooked 2 basic things. The output extension jack wiring and I forgot to add a chassis ground. The aluminum chassis is not used for circuit grounding. I used a board mounted ground buss grounding each cathode as close to its preamp tube as possible. The input and output jacks are isolated from chassis and wired to the buss. Amp seems very quiet on the bench. Circuit is very bassy through the cabinet it was plugged into. I will try some different speakers for comparison. My sincere thanks to all who contributed their knowledge. I will try to post a video later. Thanks again to all!!!!!

[Willabe]

...... I forgot to add a chassis ground. The aluminum chassis is not used for circuit grounding.

I used a board mounted ground buss grounding each cathode as close to its preamp tube as possible. The input and output jacks are isolated from chassis and wired to the buss.

The chassis has to be grounded for safety too. Somewhere, anywhere. And to act as a noise shield for the internal wiring and circuit.

You out foxed yourself.

[jazjamr]

Played the new amp at our weekly Maple Lawn Brewery gig this week. I've always played Fender Black and Brownface amps so this is a little different animal. The gig is pretty low volume, so I was running the volume at 3-3 1/2. Couldn't really open it up. Our drummer is pretty controlled and the volume is great for the patrons. The amp has a lot of sustain on single notes even at 3. I play strat style guitars, and I prefer the cleans of a JTM 45 or 1986 Bass style Marshall. All the band members complimented the tones. V1 has shared cathode cap 4.7uf, 820 ohm resistor. The 25uf cap was a little too much bass. I tried a 1uf cathode bypass cap on V2a, but couldn't tell much difference. 470K Mixer resistors with a 500pf bypass cap. .1uf caps on phase inverter output. JJ 6v6 power tubes. Sylvania 5U4G rectifier tube puts out about 340 volts to plates. The amp sounds fantastic! I want to thank all the members of the forum for their help.

[jazjamr]

I am trying to post pics of the amp in its cabinet via I phone, but the forum says the attachments are too large. I've posted chassis pics before I don't know what I'm doing wrong. I'll keep trying. I built a cherry Trainwreck style cab. Thanks

[PRR]

...via I phone, but the forum says the attachments are too large.

I dunno who is the boss, you or the iPhone. General tips on image size:

https://el34world.com/Forum/index.php?topic=464.0

[pompeiisneaks]

Doug does limit the sizes of the images.  Sometimes it's best to use an image sharing site like imgur.com.  Setup a free account, upload and create an album and share the links.

~Phil