My bench is a mess!!! lol
4 amps in 3 weeks is not a good thing, Especially with the oscilliscope now sitting there open and getting new tubes and cleaned.
I Checked the voltages again and Its as listed on the schematic.
Also checked and replaced the resistor, The original measured 476 ohm which was within tolerance.
The Bypass cap is 50uf/50V. Perhaps to large !
The original champs shows 25UF, Alot of hot rodded schematics show 50-100UF.
The Signal path cap value omitted on my drawing to the grid of the EL84 is .1/600V
The Thing that puzzels me a bit is the voltages to the plate of the EL84 seems high for that tube.
I am not sure exactly what to expect from that transformer, But it is labeled as shown on the schematic.
All voltages listed on the schematic are with tubes installed.
I actually ordered about $60.00 worth of new production 1W resistors yesterday even. I will no longer be using my huge supply of ohmite lil devils carbons.
Also ordered a big supply of pots, Both types.
The tiny bypass caps I have been using will now be all larger 100V. The specs sheet does show them as a bit more accurate.
Welp,, back to my mess.
Gene
4 amps in 3 weeks is not a good thing, Especially with the oscilliscope now sitting there open and getting new tubes and cleaned.
I Checked the voltages again and Its as listed on the schematic.
Also checked and replaced the resistor, The original measured 476 ohm which was within tolerance.
The Bypass cap is 50uf/50V. Perhaps to large !
The original champs shows 25UF, Alot of hot rodded schematics show 50-100UF.
The Signal path cap value omitted on my drawing to the grid of the EL84 is .1/600V
The Thing that puzzels me a bit is the voltages to the plate of the EL84 seems high for that tube.
I am not sure exactly what to expect from that transformer, But it is labeled as shown on the schematic.
All voltages listed on the schematic are with tubes installed.
I actually ordered about $60.00 worth of new production 1W resistors yesterday even. I will no longer be using my huge supply of ohmite lil devils carbons.
Also ordered a big supply of pots, Both types.
The tiny bypass caps I have been using will now be all larger 100V. The specs sheet does show them as a bit more accurate.
Welp,, back to my mess.
Gene
poobah said:
Yep, The pot is original 1 meg value. The only thing I modded from the original AA764 champ schematic was as follows, Swapped 6V6GT out to an EL84, Then Used a 6CA4 rect because the PT I used has no 5V taps.
It sounds great, My tin ear says about 3-4W tops on output before it pukes.
My Standard AA746 Champ, (1 of 2) that actually does work is clearly louder.
The Build is for this 13yr old prodigy who lives a few houses away. He seems to not make enough on his paper route to buy a cheap amp. It was a stretch for him just to get a Fender Squire Strat for Xmas as his family just bought a new home here by me, Then Dad's health took a bad turn.
So,
This kid is a rather exceptional student so I thought I would surprise him for his Birthday. This is all stuff I had laying around. Even got a nice Vintage 60's Jensen 12" to put in the pine combo cab
I am going to try to convince him to do something really cool.
Raw Pine Cab, As he learns a new song, I want him to wood burn the title and artist and year into the pine,Then if he sticks to it, In 10 years the thing will look awesome!!
Gene
I know Tubelab was lurking around here yesterday. Give him a PING. I don't know the specifics of changing from 6v6 to el34. Might need to biasing or??? How much faith do you have in your output tranny???
Also are you sure you have the 2700 Ohm feedback resistor hooked to the right end of the secondary?
Also are you sure you have the 2700 Ohm feedback resistor hooked to the right end of the secondary?
poobah said:
ooopps 6v6 to el34
6V6 to EL84/6BQ5
Yeppers, The resistors correct.
The OPT is a know champ drop replacement in from Triode Elec. I am certain its good and rated for this duty.
I am honestly not certain the correct bias setting for 6BQ5 But I suspect its close. Over in the 18W.com site I have seen guys trying to stay near or under 250 plate voltage
Tube Data
Gene
Did a bit more with this amp today, mostly on the cab.
I did however re-test voltages at the 6BQ5 and during the test at the plate,As soon as I touch the probe to the socket pin, I get a very sharp rather loud squeel.
Cathode voltage is 10V with a 270 ohm resistor bypassed with a 50uf cap. That works out to about 37MA, just under the transformer rating.
Was 14V/470 ohm = about 30MA
I am thinking of increasing the 2700 ohm feedback resistor to reduce its gain effect. Eh worth a try.
Interestingly, When The amp is displaying great tone up to about 8-9 on the volume pot, Plate voltage is 301V, When I go past that point, It drops sharply to 267. So somethings clearly not right, But, Its closer since I lovered the current a bit.
I think this one might be fixable. If I put in a 12AU7, The reduced gain allows the volume to be increased to full "10" but its not very loud and very dry and lifeless.
Gene
I did however re-test voltages at the 6BQ5 and during the test at the plate,As soon as I touch the probe to the socket pin, I get a very sharp rather loud squeel.
Cathode voltage is 10V with a 270 ohm resistor bypassed with a 50uf cap. That works out to about 37MA, just under the transformer rating.
Was 14V/470 ohm = about 30MA
I am thinking of increasing the 2700 ohm feedback resistor to reduce its gain effect. Eh worth a try.
Interestingly, When The amp is displaying great tone up to about 8-9 on the volume pot, Plate voltage is 301V, When I go past that point, It drops sharply to 267. So somethings clearly not right, But, Its closer since I lovered the current a bit.
I think this one might be fixable. If I put in a 12AU7, The reduced gain allows the volume to be increased to full "10" but its not very loud and very dry and lifeless.
Gene
anatech said:
Chris, I'll try that.
This is the first time I have ever built anything with a eyelet board. And I just found out quick how critical moving a simple wire a 1/2 inch can cause problems. I am getting some kind of effect moving the input lead that runs to the input jack. I move it left It actually whistles. Might need to be replaced with a shielded lead.
I had followed the cathode voltage and there was no change showing on my meter when it suddenly drops at the plate.
This is not really making sense.
More tinkering will be required. I beleive theres some high freq's involved also because bass notes tend to ring in an unusual fashion.
Gene
Yes, I was reading this thread, but I was at work. Too many bosses around to be seen on the forums. Computer time at home is being spent on web site updates.
The sudden drop in plate voltage means a sudden increase in tube current. Unfortunately the usual cause for a sudden jump in current is oscillation. Possibly at a frequency above the audio range.
I would also suggest trying a lower valued grid resistor. I would also try tacking a .1uF 600 volt cap in parallel with each of the 450 volt electrolytic caps. This is to see if they may be allowing some coupling between the input and output stages. The layout may be providing a coupling path.
301 volts on the plate, 10 to 14 on the cathode puts about 290 volts across the tube. With 37 mA that works out to 10.7 watts of plate dissipation. Max spec is 10 watts. I would increase the cathode resistor to avoid a tube meltdown. 30 mA puts you just below 9 watts. I have found a great variation in 6BQ5/EL84's. Some seem to handle tons of abuse, while others spark out the first time you dime the volume knob, and stomp on the distortion pedal. 7189A's are the best for this use, but they are getting rare and expensive.
Time to figure that scope thingy out! When you have nothing better to do, just hook your guitar up where the probe goes, and turn the knobs until you get some cool pictures. No amp, no voltages, no smoke! You will learn something about the scope, and about why the guitar sounds different depending on how you play it.
OK,
I am OHHhhhhh so close now. I can now acheive full volume without cut-off.
I feel I am right on the edge but this is clearly the closest so far.
Heres the updated as it sits Schematic
I played it about 25 minutes this morning and as far as audible defects I can detect on certain extended low notes a bit of what sounds like distortion generated by HF oscillation.
It is easily 95% better than the original and thats at full volume Treble and Bass settings.
This amp is extremely Bright! Crisp! ( Pick A Verb )and can sting the ole ear drums. At full volume is breaks up nicely, nothing excessive, Just nice.
I wonder if lowering the value of the ceramic 220pf a bit more might help?
I am also thinking a grid stopper on the EL84 might be a good idea now, Perhaps 6.8K ?
Gene
I am OHHhhhhh so close now. I can now acheive full volume without cut-off.
I feel I am right on the edge but this is clearly the closest so far.
Heres the updated as it sits Schematic
I played it about 25 minutes this morning and as far as audible defects I can detect on certain extended low notes a bit of what sounds like distortion generated by HF oscillation.
It is easily 95% better than the original and thats at full volume Treble and Bass settings.
This amp is extremely Bright! Crisp! ( Pick A Verb )and can sting the ole ear drums. At full volume is breaks up nicely, nothing excessive, Just nice.
I wonder if lowering the value of the ceramic 220pf a bit more might help?
I am also thinking a grid stopper on the EL84 might be a good idea now, Perhaps 6.8K ?
Gene
Hi Guys, Getting back to the original post on my Bandmaster Plus Clone Thing. 1st posts in this tread.
Hints of whistle with full presence setting.
We had basically felt the Presence pot was the culpruit.
I removed it and put in a 4.7K resistor and the fixed resistor seemed to clear it up. Though still just on the edge very tolerable.
I got my pots friday Fed-Ex,
This morning I installed Them, But now I am getting just a hint of whistle again at full setting.
Now, I was Thinking ( UT OHHH )
If I increase the value of the resistor Feeding back from the speaker ( 68K) in the circuit is it possible to improve the situation a bit?
Schematic
Gene
Hints of whistle with full presence setting.
We had basically felt the Presence pot was the culpruit.
I removed it and put in a 4.7K resistor and the fixed resistor seemed to clear it up. Though still just on the edge very tolerable.
I got my pots friday Fed-Ex,
This morning I installed Them, But now I am getting just a hint of whistle again at full setting.
Now, I was Thinking ( UT OHHH
)If I increase the value of the resistor Feeding back from the speaker ( 68K) in the circuit is it possible to improve the situation a bit?
Schematic
Gene
Hey Trout,
Yeah that would be worth a try... you could also stick a 20 - 27K resistor in parallel with the pot... one end to the wiper and one end to the "tube end" (lower) of the pot. This would turn the pot into a 4k or so... little less gain (effect) out of the presence control.
Play with both... you might get more crunch and power by just upping the feedback resistor.
Yeah that would be worth a try... you could also stick a 20 - 27K resistor in parallel with the pot... one end to the wiper and one end to the "tube end" (lower) of the pot. This would turn the pot into a 4k or so... little less gain (effect) out of the presence control.
Play with both... you might get more crunch and power by just upping the feedback resistor.
I suspect that the whistling sound happens when the presence pot is set at the minimum resistance extreme. In this position the feedback is shunted to ground and the amp operates at maximum gain. If this is the case the 56K or 68K resistor should have no effect. If you like the sound otherwise, and just want to kill the squeal, add a small resistor between the pot and the .1 uF cap.
If the whistling occurs at the maximum resistance extreme of the pot, then the amp is going unstable when the feedback is increased (minimum gain). If this is the case, raising the resistor value rould reduce the total feedback.
The feedback should reduce the gain, if it causes instability then the feedback is out of phase. This could be as simple as swapping the plate leads (I assume that you have tried that), or it could be a phase shift in the output transformer that is different that the one Fender optimized the circuit for. If this turns out to be the case there are a few possible cures. Try putting a small cap 100 to 500pf in parallel with the 56 or 68K resistor.
If the whistling occurs at the maximum resistance extreme of the pot, then the amp is going unstable when the feedback is increased (minimum gain). If this is the case, raising the resistor value rould reduce the total feedback.
The feedback should reduce the gain, if it causes instability then the feedback is out of phase. This could be as simple as swapping the plate leads (I assume that you have tried that), or it could be a phase shift in the output transformer that is different that the one Fender optimized the circuit for. If this turns out to be the case there are a few possible cures. Try putting a small cap 100 to 500pf in parallel with the 56 or 68K resistor.
OK,, I think I can grasp most of this.
I have a pair of these amps, 1 does it, The other is cured. Only difference being the OPT used. I have Killer gain in the subject amp on the High gain channel, The Clean channel has slightly less and far less distortion.
My brother borrowed this one friday for his bands weekly tune up practice, ( His Randall took a dump) He Felt this amp at about 25-30W made his 100W Randall look like a walkman in fullness of tone & output level.
So, now his randall is getting boxed up and sent for warranty service LOL
The Whistle exposed itself, but just lowering the level on the presence pot got it by fine. I just hate leaving it not perfect
Gene
I have a pair of these amps, 1 does it, The other is cured. Only difference being the OPT used. I have Killer gain in the subject amp on the High gain channel, The Clean channel has slightly less and far less distortion.
My brother borrowed this one friday for his bands weekly tune up practice, ( His Randall took a dump) He Felt this amp at about 25-30W made his 100W Randall look like a walkman in fullness of tone & output level.
So, now his randall is getting boxed up and sent for warranty service LOL
The Whistle exposed itself, but just lowering the level on the presence pot got it by fine. I just hate leaving it not perfect
Gene
Tubelab,
I am confused as hell. Presence is about boosting highs right? Which way does the pot wiper go (up or down) to increase presence?
By the way Gene... trash everything I said... How is the pot wired? Would an arrow meaning clockwise go up or down?
I gotta redraw this thing... not making sense to me here.
I am confused as hell. Presence is about boosting highs right? Which way does the pot wiper go (up or down) to increase presence?
By the way Gene... trash everything I said... How is the pot wired? Would an arrow meaning clockwise go up or down?
I gotta redraw this thing... not making sense to me here.
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