Dear reader:
The power section in my home-made design is overpowering the circuit I've built; single-ended, 1ea 6V6 & 12AU7, 5k-8w OT.
The PT secondary is rated at 250vac @ 42mA and 285vac @ 0mA (exhibiting a linear voltage change between 0-42mA).
In circuit I measure 275vac before the bridge rectifier and 375vdc after the filter section (150uf/350 & 20uf/350). My circuit isn't putting a dent in the load on this transformer and I need an operating voltage of 250vac.
Please advise!
The power section in my home-made design is overpowering the circuit I've built; single-ended, 1ea 6V6 & 12AU7, 5k-8w OT.
The PT secondary is rated at 250vac @ 42mA and 285vac @ 0mA (exhibiting a linear voltage change between 0-42mA).
In circuit I measure 275vac before the bridge rectifier and 375vdc after the filter section (150uf/350 & 20uf/350). My circuit isn't putting a dent in the load on this transformer and I need an operating voltage of 250vac.
Please advise!
I'll measure when I have a chance soon.
Right now the amp is not working very properly...there is arcing in the 6v6, strange oscillation noises, no signal at all - even when the effect is bypassed...(half-true bypass...input tip is shunted to output tip, but input tip is never actually disconnected from the screen of the 12AX7. Signal does bypass properly if the unit is off however), the input tip is actually hot as well, gave me a bit of a bite.
I'll try to get that measurement soon. thank you!
I'll check out that link too, looks promising Aardvark!
Right now the amp is not working very properly...there is arcing in the 6v6, strange oscillation noises, no signal at all - even when the effect is bypassed...(half-true bypass...input tip is shunted to output tip, but input tip is never actually disconnected from the screen of the 12AX7. Signal does bypass properly if the unit is off however), the input tip is actually hot as well, gave me a bit of a bite.
I'll try to get that measurement soon. thank you!
I'll check out that link too, looks promising Aardvark!
Please please please excuse my amateur drawing skills. First schematic i've ever drawn, did this up with tinycad. Unless otherwise noted all cap values are uf/VDC & resistor values are ohm/1/4watt
Much appreciated!
Excuse the original PT 205 rating, that was what I was copying. In reality it was more like 225 because of the 115V tap and my 120-122V actual supply. My revised target (based on current stock and prices) was 250V, but there is also a 125V tap. I figured that if I hit 42mA I would achieve between 240-250 which would be ideal.
Can you tell me exactly which measurements you'd like?
Much appreciated!
Excuse the original PT 205 rating, that was what I was copying. In reality it was more like 225 because of the 115V tap and my 120-122V actual supply. My revised target (based on current stock and prices) was 250V, but there is also a 125V tap. I figured that if I hit 42mA I would achieve between 240-250 which would be ideal.
Can you tell me exactly which measurements you'd like?
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All I can say...this is not my brainchild...I copied it from an existing design. I think it is very similar to a fender champ. This is meant to be a tube head "preamp"...feeding an unbalanced tube driven Leslie speaker (which does not have a built-in pre), hence the 10ohm/5w resistor across the OT.
I feel that there are 2 problems...but I can only pinpoint one - I'm supplying the circuit with too much juice. I want the transformer to deliver under 250v so I need a full load, possibly by adding beefy resistors before the bridge?
Anyhow....let me know if your diagnosis requires any present voltages.
I feel that there are 2 problems...but I can only pinpoint one - I'm supplying the circuit with too much juice. I want the transformer to deliver under 250v so I need a full load, possibly by adding beefy resistors before the bridge?
Anyhow....let me know if your diagnosis requires any present voltages.
There clearly is not a 100K resistor in the mains feed. Not sure what you meant to put there.
Measure the resistance between 6V6 cathode and ground. Then measure the voltage at this point. This will tell us what current the 6V6 is drawing. The 6V6 might be faulty. It might be oscillating - if you copied typical guitar amp wiring practices then you also need to add typical guitar amp large grid stoppers to keep the whole thing stable.
Given that it doesn't work even in bypass then it is possible that you haven't built what you think you have built.
Measure the resistance between 6V6 cathode and ground. Then measure the voltage at this point. This will tell us what current the 6V6 is drawing. The 6V6 might be faulty. It might be oscillating - if you copied typical guitar amp wiring practices then you also need to add typical guitar amp large grid stoppers to keep the whole thing stable.
Given that it doesn't work even in bypass then it is possible that you haven't built what you think you have built.
Hi again.
Not so good news...
First, measuring the cathode to ground resistance, the measurement quickly rose off the scale.
The voltage was 20VDC. BUT, this is was measured with the "output volume" at zero. As I brought the output volume up the voltage quickly rose to over 200VDC by the time I was at the half way point in the swing...and then the output volume pot started to emit smoke so I shut everything down.
I can attest that there is no inconsistency between the amp and schematic - i've gone over it several times and what you see is what I have in front of me. except for the smoke. Would it be beneficial to post the tinycad document file instead of a bmp file so that you guys can play around with it?
thanks so much for the help.
adam
p.s. regarding the 18K in series with the primary - this IS a mistake (look who's foot is in his mouth now...), the resistor is actually connected straight to the lamp, and both the lamp and resistor are in parallel with the primary. Simply a shunt for the bulb.
Not so good news...
First, measuring the cathode to ground resistance, the measurement quickly rose off the scale.
The voltage was 20VDC. BUT, this is was measured with the "output volume" at zero. As I brought the output volume up the voltage quickly rose to over 200VDC by the time I was at the half way point in the swing...and then the output volume pot started to emit smoke so I shut everything down.
I can attest that there is no inconsistency between the amp and schematic - i've gone over it several times and what you see is what I have in front of me. except for the smoke. Would it be beneficial to post the tinycad document file instead of a bmp file so that you guys can play around with it?
thanks so much for the help.
adam
p.s. regarding the 18K in series with the primary - this IS a mistake (look who's foot is in his mouth now...), the resistor is actually connected straight to the lamp, and both the lamp and resistor are in parallel with the primary. Simply a shunt for the bulb.
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EDIT: Never mind 
What leadbelly said. You're schematic doesn't show the common side of the output jack being grounded, but I'm guessing that it is and that you are effectively grounding the entire amp through the output jack (which shorts when nothing is plugged in). When the pot is turned down it has almost zero resistance, but as you turn it up it's resistance increases, which means more heat dissipated since you are pulling the current load of the entire amp through it. Eventually you exceed it's wattage rating and the result is smoke.
Replace the pot and ground the "B" bus, then try it again.
EDIT #2: The more I look at it the more I see stuff wrong. As it sits, with the effects relay not energized you are tying the "D" bus to ground, which is shunting the signal off of the plate of the 1st 12AX7 half to ground, which means no signal and no sound. It is effectively a mute switch in that position. I see that the basic idea is for "A" to be connected directly to the output jack via "C" and ground the plate through the cap, this bypassing the signal and muting the amp. The main problem here is that the feed-through signal is going to be partially to completely shunted to ground depending on the position of the output pot. It will be shunted the least with the pot in the center position, but as you rotate it toward one extreme or the other more of the signal will be shunted to ground either directly or through the OPT secondary.
So, I take it this is supposed to be an amp emulator circuit which is fed into another guitar amp or a power amp?
What leadbelly said. You're schematic doesn't show the common side of the output jack being grounded, but I'm guessing that it is and that you are effectively grounding the entire amp through the output jack (which shorts when nothing is plugged in). When the pot is turned down it has almost zero resistance, but as you turn it up it's resistance increases, which means more heat dissipated since you are pulling the current load of the entire amp through it. Eventually you exceed it's wattage rating and the result is smoke.
Replace the pot and ground the "B" bus, then try it again.
EDIT #2: The more I look at it the more I see stuff wrong. As it sits, with the effects relay not energized you are tying the "D" bus to ground, which is shunting the signal off of the plate of the 1st 12AX7 half to ground, which means no signal and no sound. It is effectively a mute switch in that position. I see that the basic idea is for "A" to be connected directly to the output jack via "C" and ground the plate through the cap, this bypassing the signal and muting the amp. The main problem here is that the feed-through signal is going to be partially to completely shunted to ground depending on the position of the output pot. It will be shunted the least with the pot in the center position, but as you rotate it toward one extreme or the other more of the signal will be shunted to ground either directly or through the OPT secondary.
So, I take it this is supposed to be an amp emulator circuit which is fed into another guitar amp or a power amp?
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One-two punch!
Thanks LB, bus B was indeed not grounded.
A friend of mine suggested I re-wire the relay to be more direct. I've also cleaned up the schematic a little, take a look at the revised sketch. Will the circuit be happy when bypassed?
I happen to have one 1M audio pot on hand - would this serve better in place of either 500K In or Out vol pot? (more headroom?)
Tube Leslie speakers are only equipped with power amps (12AU7 inverter, 6550 finals) and require a hot signal to run. My best results have been achieved by running a low level clone organ into a 15W (x2 EL84) amp wide open before the Leslie. Pure bliss. and in general I like the idea of having a preamp which can be clean or (really) dirty.
Thanks LB, bus B was indeed not grounded.
A friend of mine suggested I re-wire the relay to be more direct. I've also cleaned up the schematic a little, take a look at the revised sketch. Will the circuit be happy when bypassed?
I happen to have one 1M audio pot on hand - would this serve better in place of either 500K In or Out vol pot? (more headroom?)
Tube Leslie speakers are only equipped with power amps (12AU7 inverter, 6550 finals) and require a hot signal to run. My best results have been achieved by running a low level clone organ into a 15W (x2 EL84) amp wide open before the Leslie. Pure bliss. and in general I like the idea of having a preamp which can be clean or (really) dirty.
Attachments
One-two punch!
Thanks LB, bus B was indeed not grounded.
A friend of mine suggested I re-wire the relay to be more direct. I've also cleaned up the schematic a little, take a look at the revised sketch. Will the circuit be happy when bypassed?
I happen to have one 1M audio pot on hand - would this serve better in place of either 500K In or Out vol pot? (more headroom?)
Tube Leslie speakers are only equipped with power amps (12AU7 inverter, 6550 finals) and require a hot signal to run. My best results have been achieved by running a low level clone organ into a 15W (x2 EL84) amp wide open before the Leslie. Pure bliss. and in general I like the idea of having a preamp which can be clean or (really) dirty.
Thanks LB, bus B was indeed not grounded.
A friend of mine suggested I re-wire the relay to be more direct. I've also cleaned up the schematic a little, take a look at the revised sketch. Will the circuit be happy when bypassed?
I happen to have one 1M audio pot on hand - would this serve better in place of either 500K In or Out vol pot? (more headroom?)
Tube Leslie speakers are only equipped with power amps (12AU7 inverter, 6550 finals) and require a hot signal to run. My best results have been achieved by running a low level clone organ into a 15W (x2 EL84) amp wide open before the Leslie. Pure bliss. and in general I like the idea of having a preamp which can be clean or (really) dirty.
Hi Cancon,
I think there are a few problems with your schematic- a fender champ it certainly isn't.
As you have identified, you had no ground connected on the audio portion of your schematic.
As you found out that is potentially very dangerous, especially if you hold a guitar connected to it and ground your other hand.
The strange oscillation you are experiencing is down to the fact that your input stages have huge low frequency gain, the 150 uF cathode bypass caps are massively excessive and the lack of damping between the plate loads (100k, and 220k) means that you will experience motorboating on your power supply lines, I reckon. Also, the amount of gain between these 2 stages means that you will probably be throwing an excessive amount of volts into you the grid of your 6v6.
A champ has a lossy tone network between the input stages and the driver, and then the driver usually has some nfb associated with the output. Changing the pots to 1 M will add more gain- lifting the signal higher above ground and raising the grid bias. You may even cut off one side of you signal by overloading the grid and slowing down the release of charge from the grid with a 1M value.
To lower the ht, the easiest thing to do in my experience is to connect the centre tap via a high power zener. Cheaper than a choke, and you will still get some power 'sag' effect ie. power compression. A choke will stiffen the ht. See this thread http://www.diyaudio.com/forums/tubes-valves/79854-lowering-ht.html
Can't understand the side chain to the output- this will take away some of the simplicity that makes these amps so playable.
I would suggest that you build something that you know works- follow a champ schematic, not something that a source of unknown reliability posts online.
Double check you schematic- I am guessing that the symptoms that you are descibing re. smoking output pots means that you are putting excessive power through this. Most of the leslies loose power and impedance match through the umbilical, or at the input of their amps with a filter of power resistors IIRC. I haven't been in one for a year or too so don't remember.
Good luck.
Bernie.
I think there are a few problems with your schematic- a fender champ it certainly isn't.
As you have identified, you had no ground connected on the audio portion of your schematic.
As you found out that is potentially very dangerous, especially if you hold a guitar connected to it and ground your other hand.
The strange oscillation you are experiencing is down to the fact that your input stages have huge low frequency gain, the 150 uF cathode bypass caps are massively excessive and the lack of damping between the plate loads (100k, and 220k) means that you will experience motorboating on your power supply lines, I reckon. Also, the amount of gain between these 2 stages means that you will probably be throwing an excessive amount of volts into you the grid of your 6v6.
A champ has a lossy tone network between the input stages and the driver, and then the driver usually has some nfb associated with the output. Changing the pots to 1 M will add more gain- lifting the signal higher above ground and raising the grid bias. You may even cut off one side of you signal by overloading the grid and slowing down the release of charge from the grid with a 1M value.
To lower the ht, the easiest thing to do in my experience is to connect the centre tap via a high power zener. Cheaper than a choke, and you will still get some power 'sag' effect ie. power compression. A choke will stiffen the ht. See this thread http://www.diyaudio.com/forums/tubes-valves/79854-lowering-ht.html
Can't understand the side chain to the output- this will take away some of the simplicity that makes these amps so playable.
I would suggest that you build something that you know works- follow a champ schematic, not something that a source of unknown reliability posts online.
Double check you schematic- I am guessing that the symptoms that you are descibing re. smoking output pots means that you are putting excessive power through this. Most of the leslies loose power and impedance match through the umbilical, or at the input of their amps with a filter of power resistors IIRC. I haven't been in one for a year or too so don't remember.
Good luck.
Bernie.
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