Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: smackoj on June 17, 2016, 07:24:28 pm
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hello amigos; I started a post about this amp a few weeks ago. It's a late 60's 'L &K' amp that looks to be a close copy of the Blackface Fender of that era. This amp was made in Denver, CO and uses 2-6L6 output tubes through a single 15" speaker. It has tube driven reverb and tremolo which operates off of a pulsing tiny lightbulb and that controls a light sensitive resistor. When I got this amp it had 2 bad output tubes, a blown OT and a damaged PT (one B+ winding only putting out 230 volts). I replaced the OT with a Blackface Bassman after market from Kendrick and I replaced the PT with vintage PT I had from an old Zenith tube radio/phono amp. Because I had to remove the orig. PT, I lost my separate 50 vac bias winding. I really wanted to keep the fixed bias so I installed a 1:1 ratio isolation transformer which, after connecting it through the SS full wave bridge rectifier and the already in place bias resistors and pot, I have a nice -36 vdc to bias the power tubes. All up and running the voltages look within operating ranges, and all tubes tested, the amp passes guitar sound but at a very low, fuzzy volume.
I would like any and all questions and suggestions to help me zero in on whatever is 'holding' the sound back.
Note: these pictures were from before the PT was switched and the aux. bias trans. was added. The third trans. in the picture is a large filter choke.
thanks, smacko jack
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You have to go through the exercise of measuring and then posting the tube voltages, pin by pin, so the collective brains here can evaluate. Without that, all we can do is to take blind stabs. Folks who have posted their voltages, we've fixed easily a dozen Deluxe Reverbs in the few years I've been around.
Or you can do it yourself using the Fender AB763 schematic in Doug's library. Or both.
One thing I can tell you is that it would not surprise me to to find your B+ is 100 volts low. Power transformers from radios and or test gear very seldom develop the high voltages req'd by audio amps where we are trying to squeeze most of the available power out of a pair of output bottles. *IF* that B+ is that low, your bias could be shutting off the output section to the extent that you are describing. Though it looks like yours is at least good sized.
The second thing is that all those things that were blown when the amp came into your hands speak to a major catastrophic fault. That fault could have taken out some parts that you have not yet replaced; like the resistors in between power supply nodes as one "stab" example.
If and when you get this working, I can tell you that the location you have chosen for the "bias" transformer is very likely to produce hum and noise in the form of the wires that are required to run power and unrectified low volts across your preamp area. You can produce bias, easily, from an "untapped" HV secondary on the main PT, and zillions of amps do so with no special bias tap.
But until we see that voltages spreadsheet, all we or anyone can do is to supply vague guesses.
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If all your voltages are correct, your tubes would be what I would start with.
All tubes tested, what does that mean?
I have an old simple Heathkit tube tester.
I can test my tubes and tell if they are obviously bad.
But I can't test their Gm or more advanced specs.
Tubes that tested the same on my tester, can perform significantly different in my amp circuit.
This can be easily seen when adjusting the tubes bias in a fixed bias circuit.
A tube that tests high on my tester might not do well in my amp circuit.
Or a tube that tests low or bad might still perform well in my amp.
Possibly even better than the tube that tested high on my tester.
If the voltages are good, either one or more of your tubes may not be performing well.
Or your signal is leaking or being distorted some where along the signal path.
The easiest thing to try, is swapping out your tubes with ones that you know perform well in another amp.
If swapping the tubes doesn't fix the output problem,
then start looking at all the other parts in your signal path. :icon_biggrin:
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What is your plate voltage on the power tubes? Have you replaced the preamp cathode capacitors? What are there voltages?
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Ok guys; I will record and post voltages. I am using known good tubes so I think that can be ruled out. my tube tester is old but works well so all preamp and power tubes in the good to excellent range. The PT I put into it was running 2 6v6 power tubes so low voltage is a good first guess. Note that the pictures I posted are from before I swapped out the PT, OT and added the aux. bias trans. I will post better, accurate pictures when I post the voltages. Sorry for the confusion of using out dated photos. My bad.
jack
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The PT I put into it was running 2 6v6 power tubes so low voltage is a good first guess.
And now you're asking it to power two 6L6s. Probably not a good idea.
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here are a few updated pictures and the voltages I recorded.
All heaters recorded at 6.65 vac
6L6 power tubes recorded pin 3 335vdc both tubes; pin 5 neg. 41 vdc both tubes; pin 6 334 vdc both tubes
Preamp:
V1 pin 1 240 vdc pin 3 2.0 vdc pin 6 235 vdc pin 8 2.0 vdc
V2 pin 1 332 pin 3 3.2 pin 6 332 pin 8 3.2
V3 pin 1 204 pin 3 1.6 pin 6 334 pin 8 3.2
V4 pin 1 238 pin 3 2.0 pin 6 240 pin 8 2.0
V5 pin 1 329 pin 3 0.0 pin 6 273 pin 8 0.0
V6 pin 1 148 pin 3 1.3 pin 6 137 pin 8 1.3
Trans. lineup: left to right PT, aux. bias trans, power filter choke, OT
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Are there any new thoughts, ideas or suggestions re: this amps very low output? I could really use a little help. thanks amigos
:icon_biggrin:
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You need to put some black tape on that photo cell while testing that amp because ambient room light is causing it's resistance to be very low and that will almost kill your guitar signal. Or you could just make the room totally dark and see if the amp volume increases.
I would substitute some known good tubes, especially power tubes. I don't trust tube testers. Then I would put a proper PT in the amp. That tiny radio PT is not enough for a 6L6 amp.
Rather than use a separate transformer for bias I would just connect to a PT red lead. Very easy to do on that amp. Just put a jumper between a red wire on the rectifier board to that blue resistor at the point where the old PT blue wire was. Then replace that blue resistor with a 100K/3W resistor. May have to raise or lower the value of that 100K a bit until the bias pot can swing between approx. -40v to -55v.
The zero volt readings for V5 pins 3 and pins 8 are wrong, but if this is a true Blackface clone V5 should be the tremolo tube and should not be affecting the sound.
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Build yourself an audio probe and see if you can determine where you are loosing the signal. http://www.diystompboxes.com/pedals/debug.html (http://www.diystompboxes.com/pedals/debug.html)
Or the 'Listening amp' on the EL34 World site. Both work about the same. Last time I had this happen it was that a whisker from the shield of a shielded cable in the tone stack was grounding out part of the signal. The Listening Amp can be found here
http://el34world.com/Hoffman/tools.htm (http://el34world.com/Hoffman/tools.htm)
I plug the audio probe into the out put of a cd player and use it to inject the signal where I want to in the circuit...The cap needs to be rated for 600 volt.
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Uhhh, what the heck is that extra transformer between the PT and the choke? The one labeled "Triode 1:1 isolation"?? An "old style" interstage transformer?
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So one must assume that your V2 is the reverb driver because a) same volts, both halves, and 2) BIG B+ ("same" as your highest B+ on the 6L6 tubes--- 3 volts is "nothing" once we are up in the 300's) I guess this is possible because on your amp, judging by the pots, the leftmost channel (as viewed from the outside of the amp) is the rev channel with more pots. So the layout of this amp could be quite "rearranged" relative to a vanilla AB763. OK.
On a normally laid-out 2 channel amp we would not expect to see big B+ on V2, we' would expect such on V3, normal position of the reverb driver = 12AT7 with BIG volts on its plate(s).
V5 is probably not working. We expect to see 1-2 volts on the cathode of a 12A_7 that is working and passing current. This is IMO the BEST SIGN of a working tube---provided the cathodes sit on top of any sort of resistor, which they usually do, but do not have to. Still...2 halves of a 12A_7 with zero on the cathodes suggests neither tube-half is conducting.
I'm also not that happy with the progression of lower-lower-lower volts starting from the highest power supply node (connected to the plates of the 6_6, followed by the rev driver, followed by the PI, followed by all the preamp tubes.
And yes...I was going on your first pictures. That (what's in there now) looks to my eye like a dual 6V6 power transformer and not a dual 6L6 sized one.
And speaking of reverb transformers...WHERE IS your reverb tranny?? Unless senor "1:1" is it?? Does that tranny have an output lead that goes to the reverb can via one of the back-panel RCA jacks?
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Reverb tranny sits near V1 and V2. V2 is the reverb driver. The extra transformer is for his bias mod as stated in the original post. Physical layout is a bit different from the normal Fender blackface.
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Thanks amigos. I will cover the light sensitive resistor and see if that helps.
Yes, V2 is the reverb tube. It is wired through the chassis to the reverb transformer.
And yes I think the physical layout has been moved around like chess pieces when compared to blackface Leo.
I will definitely replace the PT with a more appropriate piece asap. Lost my wifi so it takes longer to find the right item.
I have a listening probe made up somewhere around here. I have a good listening amp so I will give that a try
I will report back asap with what I find out :icon_biggrin:
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I tried covering the LSR with no noticeable change. Amp passes sound but almost no volume.
I will try a different (higher power) PT as soon as I can find a decent one and get it shipped.
I will update the condition of the amp after the new PT is installed.
thanks mucho amigos.
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You're getting good volts on your power tubes. So, maybe your PT cannot fully drive 6L6's. That should not prevent it from getting 5 watts or so out of those tubes which is maybe 1/8th or 1/10th of what we think of out of a pair of 6L6; and depending upon your speaker, that can be loud but in any evcent it should be perfectly audible. Are you settled on using 6L6 output tubes or would you be happy with 6V6 tubes?
Have we finished the exercise of working through your voltages?
I myself think you have a non-working tube, by that, I don't mean (necessarily) a BAD tube, but a tube that is not set up to work, to draw current. If your signal passes through such a tube, it will be weak or nonexistent. That condition will not be affected much by changing out your PT to one of greater ampacity.
In your situation, I would be going back to what I thought was V5 with zero volts on the cathodes. The only way that can occur is if the cathodes are BOTH (independently, remember we are talking about 2 triode tubes in one bottle) connected directly to ground. There are circuits that do this but I would posit that there are fewer of those in amps than circuits where the cathodes *DO* sit on top of resistors. That is a very suspect condition, to me, unless I know very well that both cathodes are grounded.
How about turning power off and letting the amp sit for a couple of minutes. Measure ohms from those 12A_7 cathodes to ground, pin 3 and pin 8. If those readings are zero, meaning if those cathodes are directly connected to circuit ground, then in operation, you can expect those cathodes to be at zero volts. But if those readings are some nonzero number of ohms and we are talking the usual 820 or 1K or 1.5K or 2.2K or maybe 4.7K, something like that, then zero volts on those pins means that tube isn't working. And if it isn't working then it is not passing signal. Or making few-Hz tremolo wiggles.
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... I really wanted to keep the fixed bias so I installed a 1:1 ratio isolation transformer ...
Why go to that trouble? You can make a bias supply by tapping off one side of the PT's HT winding with a voltage divider and a reverse biased diode (like several fender amps do).
here are a few updated pictures and the voltages I recorded.
All heaters recorded at 6.65 vac
6L6 power tubes recorded pin 3 335vdc both tubes; pin 5 neg. 41 vdc both tubes; pin 6 334 vdc both tubes
Preamp:
ON your re-build, have you got screen resistors installed between Pin 6 and pin 4 on each output tube? What resistance are they?
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Latest update on this amp's low volume condition. I rec'd and installed the more powerful PT and now showing 390 vdc on the plates and screens of the output tubes. No improvement in volume. I am going to make another listening probe (last one is lost somewhere) and put it to use trying to find where the signal is not amplifying.
ans. to question about screen resistors. Yes they are present. I think they are 470 ohms.
thanks amigos. :icon_biggrin:
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are you sure the speaker is good?
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yes the speaker is good. thanks
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Maybe a leaky coupling cap somewhere throwing off the bias in one of the preamp stages. (That can reduce signal level.) Test each coupling cap for DC.
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V5 pin 1 329 pin 3 0.0 pin 6 273 pin 8 0.0
Did you ever fix this issue? Zero volts on pins 3 and 8 indicate V5 is not conducting. What is the purpose of V5 in that amp?
Post some new tube voltages now that you have a proper PT.
EDIT... Also, none of your tube voltages fit for a LTP PI or cathodyne circuit. Both of those circuits would have a large voltage on a cathode. V6 is probably the phase inverter, but which circuit? Maybe a paraphase?
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Regarding testing the coupling caps: Do I need to have one end disconnected and the amp on to test them ?
Sluckey; I have not fixed V5 yet. It is for the tremolo. V6 is the phase inverter but I am not sure what type it is?
Now that I have the higher voltage PT installed, I will re-record all the voltages and post them so we can sort of re-assess that part/.
Thanks for all you amigos hanging in with this project. I don't have the net at home now so checking in on the forum takes a bit longer.
Muchas gracias. :icon_biggrin:
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Regarding testing the coupling caps: Do I need to have one end disconnected and the amp on to test them ?
Yes. Leave the end that connects to the plate of the previous stage connected. Turn the amp on and measure voltage between chassis and dangling end of cap.
V6 is the phase inverter but I am not sure what type it is?
Since this is a blackface clone the PI should be a 12AT7 in a LTP circuit. But your cathode voltages don't indicate that. A LTP should have a pretty big positive voltage on the cathodes, not 1.3v. Either your voltage readings are wrong or this is a problem ***IF*** this is a LTP. Post some hi rez pics showing the circuit board and tube sockets and maybe we can figure it out.
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I did quite a bit of checking and testing today. I think the 6.3vac heater winding may be too lite on this PT. With all tubes in it shows 5.7vac on all the heaters. It does much better with power tubes out (6.5vac).
I don't know if I can get better pics of the phase inverter cicuit....but I can draw it and post a copy.
Here are the new voltages after a new (diff) PT installed:
Note: V2 is used for the reverb and V5 is used for the tremolo effect.
Heaters 5.7 vac
Power Tubes
Pin 3 Pin 4 Pin5
V7 363 v 359 v -36 v
V8 361 v 357 v -36 v
Preamp :
Pin 1 Pin 2 Pin 3 Pin 6 Pin 7 Pin 8
V1 270 v 0 v 2.2 v 270 v 0 v 2.0 v
V2 360 v 0 v 2.0 v 360 v 0 v 2.0 v
V3 228 v 0 v 1.6 v 223 v 0 v 1.6 v
V4 251 v 0 v 2.0 261 v 0 v 2.0 v
V5 352 v -28 v 0 v 300 v -28 v 0 v
V6 171 v 0 v 1.2 v 158 v 0 v 1.2 v
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Trace the cathode circuit under the PI.
If it is a classic Long Tail, there will be 10K-47K common cathode resistor and 20V-100V at the commoned cathodes.
Smells like someone shorted this out.
Then your power stage gets Push but no Pull, which will be significantly less than half of proper power output.
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thanks PRR. I will give it a try tonight when I get home from work. :icon_biggrin:
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I don't know if I can get better pics of the phase inverter cicuit....but I can draw it and post a copy
Why not? Just turn the chassis 180° from the angle of your first gut shot so we can see the tube sockets, board, and connecting wires. Just looking at your pic of the board I'd say the PI is definitely not a Fender variety LTP. Being able to see what connects to each tube pin would probably let us know which kind of PI you actually have. An accurate schematic would be even better.
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I will try to get a better photo and post it. I don't think I can get much of a 'close-up' but I can easily turn it around and shoot the PI section for you.
I don't have a schemo of this one to share. sorry.
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Sluckey- I am adding a couple pics of the PI area of the amp. I hope this helps sort this out. Thanks.
PRR- There is not a common resistor for the cathodes of the PI tube. Both pin 3 and 8 have separate 1.5K resistors. Pin 3 has a .01 cap across the 1.5K resistor and pin 8 has no cap across it. Both of these cathode resistors meet at an apex along with two other leads. I did not turn on the amp and check the voltage at that apex. If I need to do that I will gladly and post the results. Thanks
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I've drawn the schematic for your PI. It's a paraphase inverter. I have two questions. See the attached pic...
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The loose wire/resistor goes to the speaker out jack for negative feedback (I disconnected it when I started searching for answers). I mean without a schemo I am believing it is for neg. feedback?
I don't know about the other question (connected to ground) but when I get home I will check and respond whether it does or not.
thanks
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I don't know about the other question (connected to ground) but when I get home I will check and respond whether it does or not.
OK. And check the value of that negative feedback resistor too.
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The NFB resistor is 27k.
I had to travel so won't be able to confirm the ground issue for a day or two. It is going to take a little unsoldering to get a clear look at that connection.
:icon_biggrin:
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Answer to Sluckey question "does this go to ground?" No, there is a jumper underneath that goes to the bottom left 27k resistor and the red wire to pin 6 of the phase inverter socket.
thanks amigo
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Where are you at with this amp? Did you ever try simply unhooking the LDR (to rule out a bad part)?
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PRR; I covered the LDR with no difference. I took out the tube (V5) with no difference. The amp powers up and seems stable but does not have any volume. I can hear some mild hiss thru the speaker but very little guitar signal. I have been meaning to do a 'listening' amp test but got side-tracked with two young grand children staying over.
thanks again.
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... I covered the LDR with no difference. ...
This assume a properly-functioning LDR.
What occurred to me is, "What happens if the LDR is malfunctioning, and stays in a low-resistance state even in absolute darkness?" Seems quick & easy to simply unsolder the LDR and see if the amp functions normally (assuming this is an LDR between signal & ground).
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I will give the "unsolder" a try and report back. I will check the path also "signal to ground"
thanks mucho.
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I tested the amp with the light resistor removed. No difference. Very low guitar sound and somewhat distorted.
I am wondering if I wouldn't be better off building up one of EL34's blackface fender boards and putting that into the chassis?
I probably could have built 2 of the AB763 brds in the amount of time I have into trying to fix this thing.
I would like to hear other member's opinions if anyone wants to throw one in?
thanks amigos, Jack
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I probably could have built 2 of the AB763 brds in the amount of time I have into trying to fix this thing.
If you're just interested in how many amps per hour you can make, that works, If you're interested in * why*, then fixing broke things gives you way more understanding and a pretty good "skill set"
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I would not put a Hoffman AB763 board in this amp. The physical layout is different and will cause a lot of grief wiring it up.
I wonder if maybe a problem snuck in when you added that bias transformer? I'd just remove that and put the amp back to stock. Ideally, put a proper PT with a bias tap in the amp. The PT you are using now does not have adequate filament current capacity. If you just have to use that inadequate PT, still remove the extra bias transformer and do the simple mod I suggested for the bias supply...
Rather than use a separate transformer for bias I would just connect to a PT red lead. Very easy to do on that amp. Just put a jumper between a red wire on the rectifier board to that blue resistor at the point where the old PT blue wire was. Then replace that blue resistor with a 100K/3W resistor. May have to raise or lower the value of that 100K a bit until the bias pot can swing between approx. -40v to -55v.
And draw a schematic. Post it here and we'll help debug it in case there are some errors.
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Good suggestions. thanks to both friends.
I don't have the bias trans in it anymore SL I have the tap wired as you suggested and it's working fine. I see your point that the heater current is too much for this PT. I draws down to 5.8 vac with all bottles in and 'on'. I will seek out a better PT and post when I have a new one installed.
thanks so much amigos. JD
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The plate/screen voltages you posted on July 4th are too low for a typical fixed bias 6L6 amp. I would expect something between 400 and 450. The bias is also too low. I would expect an adjustable range between about -40 to -55. You need to just go ahead and set the bias to maximum voltage on pin 5 for now. We'll set it correctly once the amp is alive. And the 5.7v filament string. Those are all signs of a wrong PT. Exactly which PT did you use and where did you get it? I want to look up the specs to determine if it's just not a good choice for this amp or if there is maybe something wrong with it. Either way, you need to put a better PT in it. Maybe a PT for a Super Reverb.
But, the PT is not likely the cause for your low volume. You say your tubes are good, but have you actually substituted known good tubes in this amp? Substitution is the most reliable way to check a tube. I have never relied on a tube tester as a means of troubleshooting.
I have a few questions for you and based on the answers I'll suggest a troubleshooting procedure for you.
1. What is your level of electronics knowledge and experience?
2. What test equipment pieces do you have and are you proficient with them?
3. Is the volume low for both channels?
OVER...
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The PT I put in is a pull from a vintage source. I believe it was in an oscilliscope? I bought it off ebay and don't have it in front of me to give any make or model for you. I use the wifi at the library until I get it at home again. Because I just moved I need to find my box of known good tubes but I will find them and give better known tubes a try.
1. I have a 1 yr. training cert. for eled. tech and would call myself a novice at repair. That's why I like building on known good designs because my success rate is higher.
2. I have basic equip.... current limiter, listening probe, signal generator, variac (no oscilliscope)
3. yes, volume is low and garbled on both channels.
thanks SL
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Your choice of PT is poor again!
Since you have low volume in both channels the problem is likely in the PI, power amp, and/or power supply. It could even be a faulty speaker jack or speaker. Please note...
*** YOU MUST CONNECT THE SPEAKER TO THE MAIN SPEAKER JACK!!! see pic
Set your sig gen for a 500Hz 1Vrms sine wave signal and connect it to the junction of those two 220K resistors in the top right corner of the attached pic. That is the input to the PI. How is the speaker volume?
Also, take another hi rez pic that clearly shows that end of the board, the PT, and the tube sockets. You'll need to take the pic from the front side of the amp to show what I want to see.
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One thing I will chime in with is: An scrounged oscilloscope transformer, even a fairly good sized one can end up being a very poor choice for a dual-6L6 tube amp. Such an amp we want to see a strong 100 ma minimum ampacity and some folks think 100 ma is what you need for dual 6V6. If it's from a 45-tube tube Tektronix scope, OK. But a used Heathkit or EICO or RCA or Sylvania "TV repairman" quality or "school quality" scope, these might have as few as half a dozen tubes most of which are 12AU7s which need 1-2-3 ma per tube and you just do not have the gas in that transformer to make the amp work. Indeed, one of the most remarkable things about scrounging a dead oscilloscope is how very, very few usable (for amps) parts you get out of them. I would say, the knobs are going to be the most valuable parts.
This is also true of MOST (not all) test equipment and is ALWAYS true of tube video equipment. Tube video stuff is designed to produce a 1 volt signal. End of story. All you'll be able to use is the knobs and chassis. I have some tube video gear that makes 90 volts B+. Most test gear makes +250 volts and runs 2-3-4 little tubes. So if you wish to build a low power 6G-series Princeton, OK. Otherwise, there is astonishingly little value in most low-to-mid-end test gear unless of course you're wanting to use it as test gear.
(http://i172.photobucket.com/albums/w32/ttm4/scopes_zpsy9urdg4i.png) (http://s172.photobucket.com/user/ttm4/media/scopes_zpsy9urdg4i.png.html)
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OK, poor choices = poor guitar amp. I will try the signal generator idea asap. I believe that I have the speaker out jacks in reverse order to what your picture shows SL? I will double check to make sure I am plugging into spkr out and not the ext. jack. I will test the spkr out jack also. I am certain the spkr is in good working order.
I bought the wrong PT and I understand the issue of not enough usable current to run 2 big bottles. Thanks for taking time to offer your experience.
JD
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Just to be clear... Your o'scope PT is wrong for that amp, but I don't believe it is holding the sound back. Speaker jack certainly could be if you're using the wrong one. Use the MAIN. Try that before doing the sig gen thing. And post that pic I requested next time you log on.
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I appreciate your taking it in just that way. There's a recycling joint next town over that has 3 old scopes, 1 an EICO, 2 actually a semi-nice Dumont, and a really old 120 lb Tektronix. I could get them for a song but I tell you, they are loaded with sub-useful 12AU7 tubes and I have done these before, you end up with a pile of weird parts you'll never use and I already have a few truckloads of those. I didn't bite on that 5-scope package either, because I looked up the schemos and it was the same story. It's too bad but you can't wish them into having good parts in them. They are what they are.
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12AU7's kind of get a bum rap - but they are good for reverb driver tubes ,,..
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No argument, but after you take apart your first CONN (and most other) organ and have 36 of them, I think you have the application covered.
If you find a Schober organ for free or close, it is my strong suspicion that those use 12AX7 like most other organs use 12AU7.
A Hammond S-6 chord organ has I think ten 12AU7 and 2 12AX7 and 3 qty 12BH7 which are useful in some cases. There's one for free next town over and it's at least not a toneweeel organ so I would not feel bad dismantling it. Still, although they (used organs of various types) are unquestionably the BEST source of used parts especially tubes and/or complete working amplifier chassis you will find in 2016, I kind of don't like taking apart working organs. They also generate a tremendous amount of crud you have to haul to a dump.
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eleventeen is one of my junk gear guru heros. :worthy1:
I try to buy cheap junk tube gear and make great guitar amps from the parts.
PTs and OTs are the most critical parts of a tube amplifier.
I now understand that many tube junk PTs just aren't up to the job of many guitar amp circuits.
You might be able to use an oscope PT on a low voltage (+250 - +275V) low current (less than 80mA) circuit.
So there is still hope to put your PT to good use on one of those circuits.
Some other low voltage 6V6/6AQ5 circuits might fit the bill. :dontknow:
Trying to power a big bottle/bigger wattage amp on a PT that isn't up to the job will leave you. :BangHead:
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"can't wish them to have good parts in them" amen eleventeen. exactly what the doctor tells me nowadays!
here is the pics you asked for SL. I am sure I am using the right spkr jack FYI.
thanks mucho amigos.
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... here is the pics you asked for SL. I am sure I am using the right spkr jack FYI. ...
Does this mean you're plugging the speaker into the jack furthest from the power tubes (as Sluckey showed in his pic)?
The jack which has the shorting switch wired is the one away from the power tubes. If you did not use that one, you will get the extremely weak, distorted sound you're describing, because the speaker output would be mostly shorted to ground.
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I am using the spkr jack closest to the power tubes! I will try the other jack and report back.... could it be something this simple?
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I am using the spkr jack closest to the power tubes! I will try the other jack and report back.... could it be something this simple?
Yes.
One of the jacks has a shorting switch (a la Fender's "Ext Speaker" jack). If you plug your speaker into the jack without the shorting jack, the unused jack is shorting most-all of the speaker output to ground.
This happens to folks all the time who copy a Fender layout, but don't realize there is a correct jack to use if only 1 of the 2 jacks have an actual speaker connected.
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Pardon me but isn't the shorting jack (12A) usually closest to the power tubes? And isn't the main jack the one that is shorted? Why is it different on this amp?
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Pardon me but isn't the shorting jack (12A) usually closest to the power tubes?
Maybe with some amps. I've never seen any standard for this, nor any reason for a standard. As long as the jacks are labeled why would you need a standard?
And isn't the main jack the one that is shorted?
yes
Why is it different on this amp?
It's not different. But the jacks are not labeled on the chassis and that can make you think it doesn't matter if you don't know how to look at the wiring to determine which is which.
What I don't understand is this was brought up 6 days ago and he's just now getting around to saying he's connected to the wrong jack. Should only take 5 seconds to just move the plug to the correct jack. I even posted a pic. :dontknow:
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I took one of his pics and blew it up and the jack closest to the power tubes (left as viewed from the back) is a shorting jack type 12A. It is possible that when used it could still be shorting somehow. I blew up a view that shows the other jack (right as viewed from the rear) and it is hard to tell if it is a shorting jack type 12A or not. It is possible that he has 2 type 12A shorting jacks installed. And if that is the case then the OT is going to be shunted no matter what jack he uses, and would only work with two speaker plugs inserted into both jacks.
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Both of the jacks are 12A switching jacks, but the one I labeled as MAIN is the only jack that has a jumper between the switch and ground. The jack closer to the output tubes has nothing wired to the switch lug so it may as well be a type 12. Look at the pic in reply #30. You can clearly see this.
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Oh Yeah! I see that in pic 30. Both are 12A but only the one farthest away from power tubes is shorting. Thanks Sluckey.
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sorry for not writing back sooner friends. I have had to put the amp away for a week because I have my 2 toddler grand children at the condo and couldn't work on it. they go back to Nebraska on Sat. so I will try plugging it in then. I'm sorry I frustrate you SL. I absolutely do not question your knowledge senor.
jack d
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no frustration here.
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OK, after many a day away from the forum I can gladly report that using the 'correct' spkr jack does make the amp work with much more volume. However the hum and somewhat distorted guitar notes make it unusable in it's present state.
thanks amigos, jack d