My bad!! I completely forgot to mention that all my preamp tubes are flipped number wise; if I had wired it like it is in the schematic here, all my tubes would be wired in a confusing and reversed order. However, the pins match what they should (ie. grid leaks are on the grid, etc. etc.).
All the resistors check out at the right resistance; and like I said, initially the amp was working completely fine for weeks; sounded pretty good, loud, distorted, just like it should albeit the harshness mentioned in the first post.
I hadn't changed any of the caps or resistors prior to the output mod installation I attempted; however, after the OT was installed was when everything had decided to break. When I get a free minute, I'll check all the resistances again and double check the voltages, and make sure all the solder joints are fine.
All the resistors check out at the right resistance; and like I said, initially the amp was working completely fine for weeks; sounded pretty good, loud, distorted, just like it should albeit the harshness mentioned in the first post.
I hadn't changed any of the caps or resistors prior to the output mod installation I attempted; however, after the OT was installed was when everything had decided to break. When I get a free minute, I'll check all the resistances again and double check the voltages, and make sure all the solder joints are fine.
Sigh.
You post a schematic which is clearly NOT what you built ("other channel"??), post voltages not in accord with the posted schematic.
This isn't 40 Questions.
Post the AS-BUILT scheme with pin voltages crayoned ON the schematic. Just going from text-list is already confusing. With voltages ON the schematic, and a clear picture what you are really doing, maybe someone can help.
Well I did say in the first post that I built the preamp from this schematic; the power amp is a completely different build.
The preamp is identical to what is posted in the schematic; but the pins are flipped, so I have physically in the chassis 8 as 3, 6 as 1, 7 as 2, etc, just because the amp is laid out left from right signal path wise.
There is no schematic of exactly what I built, because its the preamp section of one thing, and the power amp of another thing. The problem is occurring in the preamp; as to why I posted the preamp schematic. The problem has also been traced to the first tube.
The 'second channel' is not a part of this circuit; its its own separate thing, having its own separate path to the power amp. I just posted the pin-out voltages as a comparison to the channel that I had built myself, and due to the fact that this separate channel has stopped functioning as well. There is no schematic for this channel, and I'm not too concerned with it as I never really used to to begin with.
The preamp is identical to what is posted in the schematic; but the pins are flipped, so I have physically in the chassis 8 as 3, 6 as 1, 7 as 2, etc, just because the amp is laid out left from right signal path wise.
There is no schematic of exactly what I built, because its the preamp section of one thing, and the power amp of another thing. The problem is occurring in the preamp; as to why I posted the preamp schematic. The problem has also been traced to the first tube.
The 'second channel' is not a part of this circuit; its its own separate thing, having its own separate path to the power amp. I just posted the pin-out voltages as a comparison to the channel that I had built myself, and due to the fact that this separate channel has stopped functioning as well. There is no schematic for this channel, and I'm not too concerned with it as I never really used to to begin with.
Jarrod, it would really help people to help you if you provide the accurate schematic of what you have built.
In fact, if you sit down and draw out the schematic from your build, it is possible you will find some of your own mistakes made during the build.
You mentioned that replacing the transformer caused more problems - that's telling you that there were either more wiring errors, or more soldering errors, made at that time.
-Gnobuddy
See that's what I thought to! But my output transformer didn't get soldered in; I have it attached via wiring clamps that connect the thick transformer cables to the lighter cables going to the rest of the circuit; all I did was screw down a few cables in order to mount this transformer, so all I had to do was swap 4 wires.
I honestly didn't make any changes to the circuit other than screwing in the new transformer; honestly! Which is why this is confusing me so much
However, I will try to draw out the circuit as close as I can once I get more free time. Maybe something will be obvious to ya'll that isn't to me. Also I should finaly get some pictures... I keep forgetting
I honestly didn't make any changes to the circuit other than screwing in the new transformer; honestly! Which is why this is confusing me so much
However, I will try to draw out the circuit as close as I can once I get more free time. Maybe something will be obvious to ya'll that isn't to me. Also I should finaly get some pictures... I keep forgetting
Well, but something has changed in that moment. I would not suppose that some resistors mysteriously changed places... As you never were really satisfied with the results this may mean that some fault or wiring error has been around all the time.I honestly didn't make any changes to the circuit other than screwing in the new transformer; honestly! Which is why this is confusing me so much
I really recommend to get a little bit into theory. With some knowledge what is going on in the triode stages you could have found the fault as soon as days ago. The voltages you measured around the CF stage speak for themselves and deliver clear hints on what to do next. No need to mess around with signal injectors or strange semiconductor tubes
I can only speak for myself, but I would never test a whole amplifier before not having captured all relevant DC values and not having verified they make sense or point to a problem. I am serious about that. Tube amps are not rocket science but with voltages of some 100 volts present it's no picnic either.
And finally, although you heard it before, please be clear with the schematics. Would have saved quite some time until now.
If you cannot find the problem, draw a schematic of the CF stage with its two triodes (the second and third) and write the respective voltages to every connection, then post it here. A hand- drawing will do nicely. You may even take additional readings with the tube taken out, might also reveal something.
Happy X-mas and maybe you get a working amp today!
Last edited:
When I first began to build electronics, my soldering technique was poor, and simply giving my circuit board a little whack with a finger would often make it either start working, or stop working.I honestly didn't make any changes to the circuit other than screwing in the new transformer; honestly! Which is why this is confusing me so much
I don't know exactly what happened during your transformer swap, but as George said, something did happen...we know this for sure, because the amp began to behave differently after that.
-Gnobuddy
ALL FIXED!!!
Your hunch on the cathode of the first preamp tube was correct; for whatever reason, the resistor was giving the correct resistance unloaded and under load as well, however when I was poking around with my ohm-meter, touching either side of the resistor let LOUD GUITAR pass through the amp unyielded!
I unsoldered and resoldered the resistor and got a small bit of signal.... so for an experiment I put another resistor in parallel with it and low and behold, signal! The amp sounds great now!
The second channel had this same issue; the cathode resistor of the second half of the tube was fried. Really odd that both those resistors died at the same time.
The amp sounds great now! Second channel needs to be fixed as well, but I need some parts to come in. I think I'll end up turning it into a separate cascaded or parralleld gain stage, as it was pretty useless as it was.
I'll post some pictures soon; the cabinet is all finished as well!!
Thanks for all the help guys!
and before I forget; what is the easiest/best way to lower the voltage going to the 6v6; now that everything is all settled, I want to make sure the tubes dont fry!!
Your hunch on the cathode of the first preamp tube was correct; for whatever reason, the resistor was giving the correct resistance unloaded and under load as well, however when I was poking around with my ohm-meter, touching either side of the resistor let LOUD GUITAR pass through the amp unyielded!
I unsoldered and resoldered the resistor and got a small bit of signal.... so for an experiment I put another resistor in parallel with it and low and behold, signal! The amp sounds great now!
The second channel had this same issue; the cathode resistor of the second half of the tube was fried. Really odd that both those resistors died at the same time.
The amp sounds great now! Second channel needs to be fixed as well, but I need some parts to come in. I think I'll end up turning it into a separate cascaded or parralleld gain stage, as it was pretty useless as it was.
I'll post some pictures soon; the cabinet is all finished as well!!
Thanks for all the help guys!
and before I forget; what is the easiest/best way to lower the voltage going to the 6v6; now that everything is all settled, I want to make sure the tubes dont fry!!
Congrats! 
You've been told it would work eventually
Looking forward to seeing some pics.
But seriously: Yes, plate voltage is quite high. The original 6V6 specs read 315V maximum. What bothers me more is that you measured a screen voltage higher than plate voltage, which would not be good.
You've been told it would work eventually Looking forward to seeing some pics.
Get a different transformerand before I forget; what is the easiest/best way to lower the voltage going to the 6v6; now that everything is all settled, I want to make sure the tubes dont fry!!
But seriously: Yes, plate voltage is quite high. The original 6V6 specs read 315V maximum. What bothers me more is that you measured a screen voltage higher than plate voltage, which would not be good.
Apart from that, something is wrong here, according to your measurements screen voltage is even higher than its supply voltage B+2 which I find hard to believe. Would you measure again and include cathode voltage? Then it would be possible to tell whether the 6v6 is working in a safe range or if you could possibly make bias lower by making the cathode R higher.
Last edited:
My congratulations as well. The best kind of Christmas present!
It was more than a hunch, you know. It was a logical deduction based on the voltage you measured, and knowing how the circuit works. That's why GeorgeK and I both spotted the same problem once you posted your measured voltages.
Hmm. When set to an ohms range, a DMM itself acts as a resistor of sorts. In fact, the normal rule is that you should never connect an ohm-meter to a circuit when the power is on, because (a) you won't get correct resistance readings, (b) you may damage the meter, and (c) you may damage the circuit!
In this case, paralleling the DMM's resistance with the existing cathode resistance gave you a combined lower resistance, and that let more cathode current flow, and that seems to have been what your amp needed to work. Conclusion: the original cathode resistance was too high in value.
(This could have been a mistake in circuit design, a mistake in reading the colour code, a faulty resistor, a resistor damaged during soldering, or dry solder joints.)
It is! Sometimes low-probability events do happen (someone wins the lottery), or maybe it has to do with something else, for example, excessive heat while soldering, or physical damage while installing the resistor prior to soldering.
A lot of Internet photos show amp builds where the resistor leads are drum-tight, perfectly straight and tight between the turrets or eyelets or tag-boards or whatever. This is actually a very bad idea. Temperature changes can shrink the resistor, and drum-tight leads will pull the resistor apart under it's coat of paint, causing the circuit to fail! (Usually one metal end-cap pulls off from the actual resistive element inside.)
We never saw photos of your build, so I don't know what the issue might be. Just some things to consider.
Traditional "old school" way is to use a whole new transformer (expensive!). An alternative old-school way is to add a second "bucking transformer" which subtracts off some of the 120V AC from the mains before it ever enters the main transformer.
A newer method is to use a big power MOSFET to drop some B+ voltage (basically, Dana Hall's "VVR" circuit, which Google will turn up for you.) It is cheaper and lighter and smaller than adding a bucking transformer.
But the voltages you posted, while outside the ratings of a 6V6, are not unusual in guitar amps. Leonidas Fender was an accountant, and he always tried to squeeze more power out of (cheaper) smaller valves rather than spend a few more pennies for the next bigger output valve. So he routinely built amps that exceeded the 6V6 voltage ratings. (I have a 65 Princeton Reverb reissue amp - the Fender official schematic shows 440 volts on the anodes of the 6V6's
)440V is insane, but 360 volts was not unusual at all in 6V6 guitar amps. So I don't think you have any reason to worry with 340 - 350 volts on a 6V6 anode, as long as you're not exceeding maximum power dissipation limits. (We haven't seen your output stage schematic or measurements, have we?)
-Gnobuddy
jarrodthebobo:
to put it clear, it's not the voltage on the plate that is worrying me, I rather think that you made some measuring mistakes (B2 and screen!), so repeat it to make sure you're not running into problems.
I think in in AB push-pull stages you can go higher as you can at least keep idle current and power dissipation at a reasonable value. You can't go that way in a SE power stage. At some point, you either get a wrong operating point or you fry the tube.I have a 65 Princeton Reverb reissue amp - the Fender official schematic shows 440 volts on the anodes of the 6V6's
Oops, I completely missed that. The amp was on? I can only emphasize what Gnobuddy said:
Rule #1 for ohmmeters: Measurements in a powered circuit are useless.
The results will be plain nonsense. And even when powered off, you have to consider that other parallel impedances, that even may not be obvious at a first glance, will tamper with your readings, just as residual voltages from not completely discharged capacitors. The latter being especially true in tube amps.
Last edited:
Two questions, if I may:
1) Did your measurements show all the resistors the proper values (matching the colour code and schematic) when you measured them? Nothing was wrong with the values of either of the 12AX7 cathode resistors?
2) When you measured a resistor, did you measure right across the body of the resistor (from one wire lead to another), or across the solder joints at each end of the resistor?
I'm asking because we still have a mystery - we never found out why your amp wasn't working. The resistance values in the SEplex schematic seem okay, yet your amp showed an abnormal cathode voltage, and you had to change the cathode resistor before it worked. Those two facts contradict each other, and your measurements should have shown why.
-Gnobuddy
I was measuring right against the body of the resistor actually; was just physically easier for me since I have shaky hands so being able to brace the probes against the body of the resistor helped to get a reading.
And yes, all the resistors ohmed out to the right value; the 'broken' 820ohm resistor on the cathode was actually EXACTLY 820ohm, funnily enough. However, this part is definatly the broken item as bridging it fixed the problem (I actually still have another resistor in parallel with it currently as I'm waiting for the replacement part to come in). Maybe I'll check the 'broken' part again just to see what its saying now on the meter.
Very strange build this has been, haha!
And yes, all the resistors ohmed out to the right value; the 'broken' 820ohm resistor on the cathode was actually EXACTLY 820ohm, funnily enough. However, this part is definatly the broken item as bridging it fixed the problem (I actually still have another resistor in parallel with it currently as I'm waiting for the replacement part to come in). Maybe I'll check the 'broken' part again just to see what its saying now on the meter.
Very strange build this has been, haha!
And I think we may now have our answer!
Your measurements tell us that the resistor itself was the correct value. But the resistance from 12AX7 cathode to ground was the wrong value (too high).
Therefore it was one (or both) of the connections at the ends of the resistor that was the problem.
So you had excessive resistance either between the resistor end and valve socket, or other resistor end and ground. Either way, it was almost certainly a bad solder joint.
This could be caused by a variety of reasons, ranging from using the wrong soldering iron to lack of cleanliness of the parts to poor soldering technique. Using old valve sockets or old vintage resistors can often cause this sort of problem because of all the corrosion on those parts.
That certainly won't hurt!
I know it's been frustrating, but on the plus side, you now have a working amp, and if we can also learn exactly what went wrong, then all the headaches will have been worth it. You will be better armed for your next build / debug / repair.
-Gnobuddy
Actually the tube won't care too much about what you measure at the resistor body, the relevant value is what the cathode sees. In this case, the better way to go would have been to measure over GND and the cathode connection on the tube socket, just as you would do when measuring cathode voltage. Are you sure it is not something trivial as a bad soldering joint?
I wonder what value you really have there at the moment...What resistor did you put there and why isn't it the correct part?
Its a 470ohm resistor in parallel with the 820ohm resistor and ODDLY enough, going from ground to the cathode of the tube shows 820ohms on the tube... very strange... should be a much lower value, or at least in the high 200s or so...
well, that part will be coming out soon anyways. Shipment of my new parts came in so I'll be pulling that weird part out and putting in a brand new one that's hopefully a bit less on the weird side.
I ordered quite a few extra parts since I was originally going to rebuild the preamp from scratch again, but now since I don't have to, I guess its time to start redesigning the 2nd channel.
I currently have it as only half a triode that runs via a switch either directly into the power amp (for a very quiet clean tone for practicing late at night), or into v1b for a higher gain crunch tone. However, it honestly just sounds like the first channel so there is no real need for it to exist.
Tube buffered effects loop, here I come!
well, that part will be coming out soon anyways. Shipment of my new parts came in so I'll be pulling that weird part out and putting in a brand new one that's hopefully a bit less on the weird side.
I ordered quite a few extra parts since I was originally going to rebuild the preamp from scratch again, but now since I don't have to, I guess its time to start redesigning the 2nd channel.
I currently have it as only half a triode that runs via a switch either directly into the power amp (for a very quiet clean tone for practicing late at night), or into v1b for a higher gain crunch tone. However, it honestly just sounds like the first channel so there is no real need for it to exist.
Tube buffered effects loop, here I come!
The 470 ohm resistor has a bad solder joint at one end, and is not making contact. No current is flowing through it, that's why you're not reading about 300 ohms.
Also, while you were soldering in the 470 ohm resistor, you accidentally fixed the bad solder joint you previously had on the 820 ohm resistor. So now you have 820 ohms, just as you should have had, all along!
Why not revisit your soldering technique, and spend some time improving it? It will make all future builds and repairs go much better.
Sometimes I wonder how many different sounds can be squeezed out of a 12AX7. We may have already found all of them. After all, valve guitar amps have now been around for maybe 90 years (there were early electric guitars in the late 1920's)!
If you feel like it, why not post the schematic? A lot of valve effects loop circuits get things wrong, most often ending up with too much source impedance. Sometimes way too much!Tube buffered effects loop, here I come!
-Gnobuddy
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I was thinking about doing one of these! The second one I didn't really feel would be fully necessary; I don't really think I'll need an input buffer for the effects if I'm already going to be have an 'effects send' knob; can just knock the signal down to where the pedals/rack gear are comfortable and then use the recovery stage (or schematic one for that matter) to bring the signal back up to unity.
Or maybe my thinking on this matter is wrong! Hahah. I'm not exactly opposed to doing a 'full tube' effects loop vs. the '1 triode' effects loop; I just don't know if the extra work is truly neccesary to get a functioning circuit, hahah
I have some bad news - both of those circuits have some serious problems. Unfortunately, this seems to be common with FX loop designs.
Let's start with the first circuit. There are two big problems with it:
1) The first is the 1 mega ohm "Effects Send Level" pot. The source impedance (from the pot wiper) can be as high as 250 kilo ohms when set to half resistance.
This huge source resistance will almost certainly cause huge amounts of "tone suck" because the capacitance of the cable going to the FX pedal input will cause drastic treble roll-off.
The very high impedance also makes the circuit prone to pick up hum and electrical interference.
For reference, it is best to keep this source impedance below 10 kilo ohms, i.e., twenty five times lower than circuit #1!
Unfortunately, you can't just replace the 1 meg pot with a 10k pot to solve this problem - that would load the preceding stage so heavily that it won't work at all.
2) There is nothing to limit the maximum voltage that might be fed to the external effects pedal. If the 1 Meg pot is being driven by a 12AX7 or similar, you might have up to maybe 200 volts peak to peak of signal at the top end of the 1 meg pot. Most FX pedals will be fried if you feed more than a few volts to their input.
So if the preamp is being overdriven and you turn up the "FX Send" pot too far, you might damage your FX pedals!
As you can see, this first circuit "design" is terribly flawed, and should be consigned immediately to the nearest recycling bin. The person who came up with it did not understand basic electronics concepts, and so made major mistakes with the design.
The buffer is intended to solve the problem of excessively high source impedance in the "FX Send" signal that we've just been discussing. The cathode follower will have a low output impedance, so you can safely run a cable to your FX pedal without suffering "tone suck".
There is still the problem that if you turn up the FX Level pot too far, you might fry the input of your effects pedals. But this time the problem is easy to solve - replace the 1M pot with something with lower resistance (maybe 10k), and insert a fixed resistor in series with it to cut down the voltage. Choose the fixed resistor to cut the maximum signal voltage at the top of the Send Level pot to a safe amount - say two or three volts peak to peak for normal guitar pedals.
The value of the fixed resistor depends on how much signal you get out of the master volume control (preceding this FX loop) when it is set to maximum, and the preamp is heavily overdriven.
Suppose you can get 50 volts peak to peak out of it. You need to cut it down to 3 volts peak to peak, which is 6% of 50 volts.
That means the 10k pot is going to be 6% of the total resistance, so 1% is 1.666k, and therefore 94% will be (94 x 1.666) or 156.6k. Either 150k or 120k ohms will probably be a good starting point for the value of the fixed resistor.
I would suggest also inserting a 1k resistor in series with the 0.68 uF capacitor. This is called a "build out resistor", and it keeps the 12AU7 stable even when driving a capacitive load (guitar cable).
The good news is the recovery stage looks good (it's almost identical in both schematics, except for the cathode bypass cap in circuit #1).
So, if you make the two changes I suggested above, the second FX loop circuit looks like it should work.
-Gnobuddy
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.