OK, I must be the most unlucky guy when it comes to trying to build 2 matching anythings.
A few weeks back I built a fender champ clone kinda amp. With a couple tweaks I ended up with an amazing sounding rendition of the 1960ish champ AA764.
So, my younger brother falls in love with this amp. Well ok, I'll help him out and build one.
I start the project by of course punching and drilling and grinding out an exact copy on another 6 X 10 Hammond chassis.
I happened to also have a second matching PT & OT.
4 hours later, All the hardware is in place, I start by wiring in my usual Power,Fliaments,PT & OT. and slowly get to the wiring in of the Resistors and Caps.
No problems encountered, Actually was sort of fun seeing as I had 1 amp done for referance.
Every part used is exactly the same as in the first amp with the exception of a tone cap, Mine has an old sprague black beauty, His has an orange drop. (.047)
So, after about 6 hours of carefully wiring and soldering in each and every part. Its Time to make noise!!
Welp, I get plenty of power, I guess a solid 4.5W. I get Plenty of great tone response from the guitar and Everything looks and sounds very sweet indeed. >>>>>UNTIL<<<<
I raise the volume to full, The treble to full, And I Cut Off? Like Flipping a switch!!
Also, unlike the first version, I get a severe squeel/ Oscillation while the guitar is not plugged in.
If I reduce the treble control, I can run at full volume, Or if I lower the volume I can run full treble. But I Can Not run both at maximum.
I have heard this type of squeel once before when I had reversed the OPT primary leads & also once when I had not grounded the speaker jack.
But in this case, Those are (according to color code) Correct and well grounded.
I have rechecked all joints,grounds, Swapped out tubes, Yada Yada, Nothing helps. (So Far)
Any ideas would be helpful, I am going to try to beat this thing tonite seeing as I have a few free hours .
Trout
A few weeks back I built a fender champ clone kinda amp. With a couple tweaks I ended up with an amazing sounding rendition of the 1960ish champ AA764.
So, my younger brother falls in love with this amp. Well ok, I'll help him out and build one.
I start the project by of course punching and drilling and grinding out an exact copy on another 6 X 10 Hammond chassis.
I happened to also have a second matching PT & OT.
4 hours later, All the hardware is in place, I start by wiring in my usual Power,Fliaments,PT & OT. and slowly get to the wiring in of the Resistors and Caps.
No problems encountered, Actually was sort of fun seeing as I had 1 amp done for referance.
Every part used is exactly the same as in the first amp with the exception of a tone cap, Mine has an old sprague black beauty, His has an orange drop. (.047)
So, after about 6 hours of carefully wiring and soldering in each and every part. Its Time to make noise!!
Welp, I get plenty of power, I guess a solid 4.5W. I get Plenty of great tone response from the guitar and Everything looks and sounds very sweet indeed. >>>>>UNTIL<<<<
I raise the volume to full, The treble to full, And I Cut Off? Like Flipping a switch!!
Also, unlike the first version, I get a severe squeel/ Oscillation while the guitar is not plugged in.
If I reduce the treble control, I can run at full volume, Or if I lower the volume I can run full treble. But I Can Not run both at maximum.
I have heard this type of squeel once before when I had reversed the OPT primary leads & also once when I had not grounded the speaker jack.
But in this case, Those are (according to color code) Correct and well grounded.
I have rechecked all joints,grounds, Swapped out tubes, Yada Yada, Nothing helps. (So Far)
Any ideas would be helpful, I am going to try to beat this thing tonite seeing as I have a few free hours .
Trout
OK,
I found the input Squeel Issue, The Input jack was not switching/making good contact to short mode with cord removed.
Whew that was an easy fix.
Now If I can get it to not reach cut-off when the pots are set at maximum I will be in great shape.
I popped a 5881 in to see if there was any direct change in the symptom Vs the 6V6gt's I have tried. " No Change"
I am pretty sure its pre-amp related, But I still dont see anything that could be the direct cause unless its bad pots? They are new from parts express. All the electrolytics are from them and everything was purchased together.
The only possible variance I can see would be the resistors used seeing as they are all NOS ohmites Lil Devil carbons. I have built several other amps with these and not had any issues thus far.
Right now as it stands, The cut-off is very sharp, Like flipping a switch at around 95% setting on the tone or volume controls. The
Schematic Is correct to both my working and non-working units.
Trout
I found the input Squeel Issue, The Input jack was not switching/making good contact to short mode with cord removed.
Whew that was an easy fix.
Now If I can get it to not reach cut-off when the pots are set at maximum I will be in great shape.
I popped a 5881 in to see if there was any direct change in the symptom Vs the 6V6gt's I have tried. " No Change"
I am pretty sure its pre-amp related, But I still dont see anything that could be the direct cause unless its bad pots? They are new from parts express. All the electrolytics are from them and everything was purchased together.
The only possible variance I can see would be the resistors used seeing as they are all NOS ohmites Lil Devil carbons. I have built several other amps with these and not had any issues thus far.
Right now as it stands, The cut-off is very sharp, Like flipping a switch at around 95% setting on the tone or volume controls. The
Schematic Is correct to both my working and non-working units.
Trout
When the amp is operated in the "cutoff" condition can the signal be traced to find the point of loss? Have you tried another 12AX7? If I had to make a wild guess, I would look for an open condition at one of the grids. A bad pot could cause this if the ground connection went open. Try temporarily bridging a 100K resistor from each grid to ground to see if this helps.
Another possibility is a strong oscillation (above the audio range) that causes the cutoff. Check to see if there is any voltage that changes from the working to the cutoff conditions.
Another possibility is a strong oscillation (above the audio range) that causes the cutoff. Check to see if there is any voltage that changes from the working to the cutoff conditions.
tubelab.com said:
First Off,,
WELCOME BACK!
I Hope some normality is coming back to you and your family!
I Put a 12AU7 in, and I can not get it to cut off and output was conciderably lower. I put in a 12AT7, Pooof, Same cut-off condition. 3 Different brand 12AX7's Same.
I left my meter at my brothers so I will have to pull voltages off tomorrow afternoon.
My piece of junk simpson is out of Battery again so I can't check much as far as ohms.
So looks like tonites a wash.
Gene
Joel said:
I am inclined to fully agree that theres an oscillation issue, The part I am not understanding is simply this. I have 2 units I personally built Both using the same PT, OPT,Pots Caps,and Chassis layout. Amp "A" sings sweetly with no issues or apparent oscillation, Amp "B" clearly has an issue.
This is a really simple amp and I am sure the units are true to the original schematic. I do not understand how fender could have produced (guessing) thousands of these amps with a 50% error possibility?
The decoupling is set to exactly as shown on schematic.
Could you expand a bit on lowering the power supply impedance?
The leads(signal) are very short & shielded.
gene
BTW, Today I replaced EVERY cap in the amp with different brand NEW caps, I replaced and routed differently the signal leads on the pots,
Other than sockets and transformers & chassis I litterly rebuilt this thing only to arrive at the same point.
Hi Gene,
At least you are consistant!
Sometimes the position of the parts is all it takes to cause oscillation. It could be the design is borderline stable, and with the other transformer, it's not stable. Try bypassing the plate resistor in the preamp stages one at a time with a small capacitor. Maybe 50 pF to start. That should introduce a high frequency cutoff.
-Chris
At least you are consistant!
Sometimes the position of the parts is all it takes to cause oscillation. It could be the design is borderline stable, and with the other transformer, it's not stable. Try bypassing the plate resistor in the preamp stages one at a time with a small capacitor. Maybe 50 pF to start. That should introduce a high frequency cutoff.
-Chris
anatech said:
Oh My Gosh Yes, I should have started designing oscillators!! Or Hey Wait, I know, Valve dog whistles!! Yeah Thats It, Come to think of it, No wonder our pooche hides at the other end of the house when he see's me headed for the garage!
I wonder<ponders> a 5640, a few calculater batteries,a el-cheapo opt made of fine wire wrapped around a old iron barn nail, a piezeo speaker,and the body of a marks-a-lot magic marker.
I'll try the 50pf's tonite, Got my VM back a bit ago so I'll pull a few referance voltages also.
Gene
anatech said:
Hey,, I am a very light hearted guy! Honestly even if they dont always work, Its still a lot of fun trying to solve the mystery & learning more and more about this great hobby!
Nope, not old fender transformers. They came from some old Devry Technical Intutute class project kits.
I think years ago, before sillycone, & Before Compfusers they had radio and TV repair classes. You know, Earn Big money in the rewarding technical field ads! haha
Anyway, I aquired several of the original class project power supplies (built) I have built several low power amps with 6AQ5's, EL84's and 6V6gt's with these. of course results have varied.
Gene
I tryd bypassing the plate resistors as suggested but they had no effect.
I did get some voltages , These were with tubes in place, Power on, Volume at 90% Treble & bass set same.
OK Here We Go, Voltages.
These are very low based on original schematic. Mind you the output is very good I am guessing easily 4-5 W. And amazing quality as well as great break-up/Distortion in upper output regions.
Power On, Guitar plugged in, No playing, just idle.
12AX7 Plate #1- 147V (pin 1)
12AX7 Plate # 2 - 152V (Pin 6)
6V6 Plate voltage 236V (pin 3)
Power supply readings @ first stage of cap 249
Then 1K 5w resistor
Second stage 245V
then 10K resistor
Third stage 221V
I measured the voltage at (pin 8) of the 6V6 15V
Gene
I did get some voltages , These were with tubes in place, Power on, Volume at 90% Treble & bass set same.
OK Here We Go, Voltages.
These are very low based on original schematic. Mind you the output is very good I am guessing easily 4-5 W. And amazing quality as well as great break-up/Distortion in upper output regions.
Power On, Guitar plugged in, No playing, just idle.
12AX7 Plate #1- 147V (pin 1)
12AX7 Plate # 2 - 152V (Pin 6)
6V6 Plate voltage 236V (pin 3)
Power supply readings @ first stage of cap 249
Then 1K 5w resistor
Second stage 245V
then 10K resistor
Third stage 221V
I measured the voltage at (pin 8) of the 6V6 15V
Gene
Trout said:
It just dawned on me perhaps the voltages were suposed to be read without the preamp and output tube, so I checked them without the tubes. (Rectifer tube only)
Voltages are still not as high at some points and higher at others.
12AX7 Plate #1- 322V (pin 1) (Schematic says 200V)
12AX7 Plate # 2 - 323V (Pin 6)(Schematic says 205V)
6V6 Plate voltage 328V (pin 3)(Schematic says 350V)
Power supply readings @ first stage of cap
327V (Schematic says 360V)
Then 1K 5w resistor
Second stage 328V(Schematic says 350V)
then 10K resistor
Third stage 329V(Schematic says 330V)
Interestingly, It only fails at about 95% volume and only with treble at full. Or, Treble at 95% and volume at full. Weird interaction.
gene
'Scope anyone?
Here's where you need a Oscilly-scope so you can make better oscillators, oopps I mean amplifiers! You don't need a fancy one, just one good enough to SEE what's going on! A signal generator would help also. Most 'scopes have a 1kHz 1V p-p test/calibration osc.
Try backing off on the feedback resistor; increase the value to see if it stops. Sometimes placing a small cap 10-150pf across the feedback resistor (phase advance) will help. Another trick is an R/C network across the OPT primary.
I'll lay odds it's the OPT, everything else being identical. General use as opposed to OEM OPT's have a somewhat broader tolerance range. Doesn't necessarily mean the OPT is bad, just has different parameters. You could swap the OPT's and if it follows...
pain-in-rear I know!
Example I have a stereo (negative feedback)
amp where the OPT's on each side behave differently from each other in the HF spectrum sooo... just a guess!
Wayne
Here's where you need a Oscilly-scope so you can make better oscillators, oopps I mean amplifiers! You don't need a fancy one, just one good enough to SEE what's going on! A signal generator would help also. Most 'scopes have a 1kHz 1V p-p test/calibration osc.
Try backing off on the feedback resistor; increase the value to see if it stops. Sometimes placing a small cap 10-150pf across the feedback resistor (phase advance) will help. Another trick is an R/C network across the OPT primary.
I'll lay odds it's the OPT, everything else being identical. General use as opposed to OEM OPT's have a somewhat broader tolerance range. Doesn't necessarily mean the OPT is bad, just has different parameters. You could swap the OPT's and if it follows...
pain-in-rear I know!Example I have a stereo (negative feedback)
amp where the OPT's on each side behave differently from each other in the HF spectrum sooo... just a guess!
Wayne
I suggest that you swap all the tubes in from the working unit, just as a sanity check.
Then since it's not likely the simple Rs and Cs, it's probably the output transformer, or layout, or grounding.
I thought your OT was rather close to the input jack, just a feeling, probably not an issue. I'd agree that it's probably tolerances in the OT.
The arcing that you were getting before is an indication of serious inductive kickback, it's the only way to get voltages like that and I again suggest that you add the RC network that I mentioned.
It would also help to measure voltages in the good amp as a comparison.
Pete B.
Then since it's not likely the simple Rs and Cs, it's probably the output transformer, or layout, or grounding.
I thought your OT was rather close to the input jack, just a feeling, probably not an issue. I'd agree that it's probably tolerances in the OT.
The arcing that you were getting before is an indication of serious inductive kickback, it's the only way to get voltages like that and I again suggest that you add the RC network that I mentioned.
It would also help to measure voltages in the good amp as a comparison.
Pete B.
Took another look at the schematic and there is no explicit feedback around the volume/tone section and therefore it must be a radiated, induced, or sneak path.
The shared plate supply cap to the 12AX7 is a possibility, could bypass it with a .1 or .47 400V.
A grid stopper from the volume control wiper to the grid might help, say 10K.
The output transformer turns ratio might be different making the output section on the edge of stability, you could double the feedback resistor to reduce the amount of feedback and raise the gain margin.
Keep high voltage swing signals down close to the ground plane.
Pete B.
The shared plate supply cap to the 12AX7 is a possibility, could bypass it with a .1 or .47 400V.
A grid stopper from the volume control wiper to the grid might help, say 10K.
The output transformer turns ratio might be different making the output section on the edge of stability, you could double the feedback resistor to reduce the amount of feedback and raise the gain margin.
Keep high voltage swing signals down close to the ground plane.
Pete B.
Hi Gene,
The other "good" unit is probably oscillating too, ultrasonically - but not enough to cut off the amplifier.
With the high gain, multiple cascaded stages, marginal decoupling, el-cheapo transformers, and rat's nest layouts of the typical guitar amp - it's a wonder this sort of thing doesn't happen more often.
My guess is that your layout sucks. This is not a component issue - don't bother replacing caps or resistors. And I bet your output tranny is up to the task.
Try this:
1) Double the size of your grid stopper resistors on the power tubes, and make sure that they are up next to the tube pins.
2) try to shorten any leads in the chassis that have slack, if possible.
3) make sure that signal leads aren't touching any other leads, and that they are elevated above the chassis.
4) When possible, use the component leads themselves to make a connection.
5) if none of that helps, consider removing the global feedback loop. It's completely out of place in a guitar amp anyway, IMO.
As much as musicians may worship Fender amps, I don't think anyone can claim these were paragons of engineering. They were usually "designed" at the edge of stability and/or operability.
my 2c.
Joel
The other "good" unit is probably oscillating too, ultrasonically - but not enough to cut off the amplifier.
With the high gain, multiple cascaded stages, marginal decoupling, el-cheapo transformers, and rat's nest layouts of the typical guitar amp - it's a wonder this sort of thing doesn't happen more often.
My guess is that your layout sucks. This is not a component issue - don't bother replacing caps or resistors. And I bet your output tranny is up to the task.
Try this:
1) Double the size of your grid stopper resistors on the power tubes, and make sure that they are up next to the tube pins.
2) try to shorten any leads in the chassis that have slack, if possible.
3) make sure that signal leads aren't touching any other leads, and that they are elevated above the chassis.
4) When possible, use the component leads themselves to make a connection.
5) if none of that helps, consider removing the global feedback loop. It's completely out of place in a guitar amp anyway, IMO.
As much as musicians may worship Fender amps, I don't think anyone can claim these were paragons of engineering. They were usually "designed" at the edge of stability and/or operability.
my 2c.
Joel
OK,
No grid stopper on power tube in schematic or amp (yet)
I replaced a couple solid wires with shielded leads (green ones in photo)
Removed but no change in symptom.
Heres a few pictures just after the feedback was removed.
Top View
Bottom View
Bottom View at Angle
Larger Bottom View
So, lets recap,
Tryd bypassing the plate resistors on 12AX7's with 47 pf (NC)
Replaced every Cap (Electrolytics & Signal)
Added shielded leads.
Tryd 3 different sets of Tubes all NOS all different brands.
Unbolted the OPT and rotated it into several positions with no effect on symptom.
Went around all grounds with a jumper each one to earth connection, (a single know point). No Change.
I guess this OPT is Total Junk.
I wrote triode electronics to see if they have had any information that might lead me to a cure.
I am guessing that I will be needing to replace the OPT of which I only have more of the same on hand.
Perhaps a hammond 125SE?
Gene
No grid stopper on power tube in schematic or amp (yet)
I replaced a couple solid wires with shielded leads (green ones in photo)
&
I think I have that covered as well as possible givin the chassis size and layout.
Removed but no change in symptom.
Heres a few pictures just after the feedback was removed.
Top View
Bottom View
Bottom View at Angle
Larger Bottom View
So, lets recap,
Tryd bypassing the plate resistors on 12AX7's with 47 pf (NC)
Replaced every Cap (Electrolytics & Signal)
Added shielded leads.
Tryd 3 different sets of Tubes all NOS all different brands.
Unbolted the OPT and rotated it into several positions with no effect on symptom.
Went around all grounds with a jumper each one to earth connection, (a single know point). No Change.
I guess this OPT is Total Junk.
I wrote triode electronics to see if they have had any information that might lead me to a cure.
In the first amp I had a PT that was dumping over 100ma into a transformer that should only see 40ish ma. Once I replaced it with the one exactly like the one I am using on this amp. It was cured, or lets say no more arcing. There May still be a oscillation iuues, But it is not effecting the amp at all.
I am guessing that I will be needing to replace the OPT of which I only have more of the same on hand.
Perhaps a hammond 125SE?
Gene
where are your grounds??
Gene,
Look closely at your inside layout picture again. I see five or six seperate "ground" points, which are merely the lugs on the tube sockets, and/or capacitor cans. They don't even seem to be tied to the chassis in some cases. You're assuming these are all good COMMON grounds for the signal. I wouldn't assume that at all. Especially not on a painted chassis like that Hammond box.
All your circuit "grounds" should be attached to a single heavy bus wire of at least 16ga, and then attached to the chassis at a single point (usually one of the power transformer bolts). That point should be scraped down to bare metal on a painted chassis.
Joel
Gene,
Look closely at your inside layout picture again. I see five or six seperate "ground" points, which are merely the lugs on the tube sockets, and/or capacitor cans. They don't even seem to be tied to the chassis in some cases. You're assuming these are all good COMMON grounds for the signal. I wouldn't assume that at all. Especially not on a painted chassis like that Hammond box.
All your circuit "grounds" should be attached to a single heavy bus wire of at least 16ga, and then attached to the chassis at a single point (usually one of the power transformer bolts). That point should be scraped down to bare metal on a painted chassis.
Joel
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