I'm sorry to hear this second build has been so frustrating. Hang in there, and you will get it working eventually.Help needed please lol
I usually build a new valve guitar amp one stage at a time, starting with the power supply, then the output stage, and working my way back towards the input jack. This lets me test the circuit in stages as I go, which makes it easier to find and correct any mistakes I've made. By the time I get all the way to the input jack, there is very little left to debug, and there is a high probability that the circuit will just work.
Working with old parts (valve sockets, switches) has its own pitfalls. You need to test everything before you can trust it.
Luckily for you, valves are old technology, so just about everything in the signal chain is AC coupled. Which means that even though you've built the whole circuit in one go, you can still test one stage at a time.
Good. So at least we know all the heater wiring is okay.
(Just to double check: is there a ground reference for the heater wiring? Assuming the proper balanced heater wiring, you should read 3.15 V AC RMS at each heater pin with your DMM, with the other DMM lead grounded to the chassis.)
Looking at your circuit diagram, there is no capacitor from EL84 screen grid to ground, nor do you have a screen grid stopper resistor. I understand that C2 (2nd power supply filter cap) is acting as a screen bypass cap - but how far is it from the actual valve?
I've read a lot of horror stories about valves that oscillated violently at radio frequencies because of the lack of a screen grid stopper. If your EL84 is doing this, it can completely prevent normal operation as an audio stage. It can also interfere with radios and TVs nearby, which is illegal, so you definitely want to prevent this.
This is such a simple thing to add, that my suggestion is to wire one in before you go further. 470 ohms, 1 watt, seems to be a frequently used value.
Okay, screen grid resistor in place, on we go!
A single valve is really a very simple device. It needs the right DC and AC voltages on all terminals, and then it will work. So what you need to pin down now, is the DC and AC voltages and operating conditions (currents, impedances) at each pin.
This means you have to work with the power supply powered on and producing dangerous high voltages. As you know, this means putting yourself at some risk, so please use your HV neon-lighting training to stay safe; I wear electricians class 0 insulating gloves and safety goggles when I do this job, in addition to sitting on an insulating chair, with insulating shoes on my feet. Best to do this when other people are around, and maybe let them know to keep an ear open for any sounds that might suggest imminent death by electrocution. 😱
1) Firstly, with power off, remove the EF86 and 12AX7 from their sockets. They aren't doing any good right now, and removing them simplifies what you have to concentrate on.
2) Next, power on the circuit, and measure the DC voltages on each pin of the EL84 (except heater pins.) Measure B+ voltage at the OT primary as well.
Maybe you've done this already, in which case, please list the pins and corresponding DC voltages, along with the B+ voltage you measured.
3) Measure the DC voltage drop across the OT primary. Using Ohm's law and the primary winding resistance you measured earlier (453 ohms), you can calculate the EL84s anode current. Please report the voltage drop, and calculated anode current.
For example, if you measure 13.6 volts DC drop across the 453 ohms (DC resistance) OT primary, that tells you you have 30 mA (or 0.03 A) anode current, which should be in the ballpark for an SE EL84 stage. I don't know your B+ voltage, so this may not be exact; but it probably should be between 20 and 40 mA.
If you you get zero volts drop across the OT primary, there's a red flag: there is no anode current, which means the DC operating conditions on at least one pin of the EL84 are wrong.
4) Use the voltage you measured at the EL84 cathode, and the value of the cathode resistor, to calculate the cathode current. It should be a little larger than the anode current you calculated in step 3, because the screen grid current is also flowing out of the cathode. Please report calculated cathode current.
Subtract the anode current from the cathode current, and you get the screen current. It is typically roughly around one-fifth of the anode current for pentodes. Please report this value as well.
If you get zero volts at the cathode, this is another red flag that says you have no cathode current.
If you get this far, all DC voltages and DC currents on the EL84 will be correct. If you found and fixed any mistakes to get to this point, perhaps your circuit works now. If not, it's time to check AC conditions as well.
To avoid confusing the issue, I'll wait until you've done all these DC voltage measurements and current calculations before suggesting ways to start checking AC operating conditions.
The very first DC voltage measurement suggested in this post (measure B+ voltage) should give you a starting point. If you have a nice steady B+ voltage of about the right value, the choke is okay, and the valves should work. If the caps are bad you will have hum, but not what you're actually observing: dead silence.
Good luck!
-Gnobuddy
Well finally figured some things out. We’ve got your standard good news, really bad news, good news again situation. Lol
First round of good news: I finally figured out what’s wrong with the amp and I didn’t electrocute myself. 😉
Very bad news: as I went back to retake old measurements and get some new ones I hadn’t taken yet I immediate came across the issue. I couldn’t get a steady reading anywhere and voltages were fluctuating all over the place as the amp warmed up until they all settled at different variations of -0.xxx Volts. I switched over to double check the AC coming into the power transformer and then back to the output readings and confirmed the worst. The power transformer is still putting out heater voltage, but the rest of it is deader than a doornail
I’m not sure where exactly but somewhere along the way between my fist tests before moding the amp and my recent troubles she gave up the ghost entirely. 🙁
Second bit of good news: I have two other identical working units to pilfer a new tranny from so at least there’s that. The next free opportunity I have to swap the bad one for a good one I will do so and then hopefully update with good news
First round of good news: I finally figured out what’s wrong with the amp and I didn’t electrocute myself. 😉
Very bad news: as I went back to retake old measurements and get some new ones I hadn’t taken yet I immediate came across the issue. I couldn’t get a steady reading anywhere and voltages were fluctuating all over the place as the amp warmed up until they all settled at different variations of -0.xxx Volts. I switched over to double check the AC coming into the power transformer and then back to the output readings and confirmed the worst. The power transformer is still putting out heater voltage, but the rest of it is deader than a doornail
I’m not sure where exactly but somewhere along the way between my fist tests before moding the amp and my recent troubles she gave up the ghost entirely. 🙁Second bit of good news: I have two other identical working units to pilfer a new tranny from so at least there’s that. The next free opportunity I have to swap the bad one for a good one I will do so and then hopefully update with good news
Sorry to hear that, but you are in a very good situation, since you have spares handy, and this one quick fix might very well bring your entire amp to life. That would be nice! 🙂...power transformer is...deader than a doornail
One thing: do a little checking to make sure that the transformer wasn't killed by a wiring error or other problem that caused excessive current draw. You don't want the replacement transformer to go the same way as the original. 😱
You're probably read about the "light bulb inline with the power cord" trick - it's a quick and easy way to tell if the amp is drawing too much current, and also to protect the expensive bits to at least some extent. Here's one implementation: Load Lamp Safely Allows Safer Electronic Testing
Here's a second implementation ( Tools of the Trade: Light Bulb Limiter – Doktor Ross Sewage ). This guy built everything into a nice little junction box, but, IMO, made two odd choices: One, he modified an AC duplex receptacle and bought a light bulb adaptor, rather than just wiring a light bulb socket into his junction box; two, he used an electrically conductive metal box, rather than an insulating plastic or wooden one.
He did ground his metal box, but I think I'd feel safer with an insulating box - safer if the ground wire ever fails or comes loose.
By the way, the bulb has to be an old-fashioned incandescent bulb, of the sort invented by Joseph Swan and copied by Thomas Edison ten years later. A modern LED bulb won't work for this purpose.
-Gnobuddy
A 5 watt tube rehearsal/studio guitar amp converted from a 1960s tape deck.
60?s tube tape recorder guitar amp demo. - YouTube
This amp was designed and constructed using the basic foundation of a 1960s (Akai M8) Roberts 770X tube reel to reel tape recorder. The amplifier circuitry has been optimized to specifically serve as a guitar amp rather than as hi-fi equipment. Where practical the original vintage components, including the tubes, have been kept.
There are 2 instruments inputs which share the common volume and tone controls. The 2 inputs however employ separate, and different, input stage tubes and therefore sound very different. The first input uses an EF86 (6267) tube. These tubes were used in 1960s vintage VOX amps, and are still used in some Dr. Z, Matchless, and other boutique amplifiers. The 2nd input uses a 12AX7 (12AD7) tube. 12AX7s are the most common preamp tubes used in guitar amplifiers such as Fender, Mesa Boogie, and Marshall. All of the tubes are the original vintage 1960s tubes. This amp is very quiet (background noise and hum) and breaks up nicely at reasonable volume levels.
The tone circuit uses a modified passive Baxandall/James circuit (Bass and Treble controls) similar to the type of tone stacks used in the original Ampeg, Orange, and some Dr. Z amps. Additionally, a boost switch increases the gain and frequency response of the amp providing additional tonal flexibility.
The output stage is a single-ended class A power amp. This amp uses an EL84 (6BQ5). Single-ended class A amps were used in many lower power 1950s-60s Fender Tweed and Blackface amps and in Vox's AC-4, which also used the EL84 tube. The amp uses a 6X4 tube rectifier. This amp works great for rehearsal, small venue use, and especially studio recording.
60?s tube tape recorder guitar amp demo. - YouTube
This amp was designed and constructed using the basic foundation of a 1960s (Akai M8) Roberts 770X tube reel to reel tape recorder. The amplifier circuitry has been optimized to specifically serve as a guitar amp rather than as hi-fi equipment. Where practical the original vintage components, including the tubes, have been kept.
There are 2 instruments inputs which share the common volume and tone controls. The 2 inputs however employ separate, and different, input stage tubes and therefore sound very different. The first input uses an EF86 (6267) tube. These tubes were used in 1960s vintage VOX amps, and are still used in some Dr. Z, Matchless, and other boutique amplifiers. The 2nd input uses a 12AX7 (12AD7) tube. 12AX7s are the most common preamp tubes used in guitar amplifiers such as Fender, Mesa Boogie, and Marshall. All of the tubes are the original vintage 1960s tubes. This amp is very quiet (background noise and hum) and breaks up nicely at reasonable volume levels.
The tone circuit uses a modified passive Baxandall/James circuit (Bass and Treble controls) similar to the type of tone stacks used in the original Ampeg, Orange, and some Dr. Z amps. Additionally, a boost switch increases the gain and frequency response of the amp providing additional tonal flexibility.
The output stage is a single-ended class A power amp. This amp uses an EL84 (6BQ5). Single-ended class A amps were used in many lower power 1950s-60s Fender Tweed and Blackface amps and in Vox's AC-4, which also used the EL84 tube. The amp uses a 6X4 tube rectifier. This amp works great for rehearsal, small venue use, and especially studio recording.
Some cabinets a friend of mine has been making for these amps. He's working on a combo cabinet now.
Dropbox - Public - Simplify your life
Dropbox - Public - Simplify your life