Building a JMI Vox AC30/6

[four_corners]

(Unfortunately, in a weird string of events, my last thread broke from an attachment and Doug said I have to start a new one. Funny too because it now says this is my first post again, haha)
____________________

I powered up my AC30/6 top boost build for the first time, and no sound!

I'm still waiting for one more 12AX7, but I thought I still might be able to test at least the Normal/Brilliant channel and just leave out one of the vibrato valves. Am I correct in thinking if I have the Rectifier, Power, Normal/Brilliant, and Phase Inverter valves installed, I should get something out of the Normal and Brilliant channels?

I am getting absolutely no sound/noise/anything. I'm used to getting something on builds, even just sound noise! Which makes me feel like there must be an issue with the phase inverter tube/circuit, the output transformer, or a short somewhere in the audio path?

I'm also assuming it is normal for my meter to read continuity between the 8ohm, 16ohm, and common wires out of the output transformer, meaning the secondary winding is intact? I'm actually getting a bit over 1 ohm resistance between each and chassis, so not actually a short I don't think.

All the tubes are getting decently hot, and filaments glowing on all of them besides the phase inverter, not sure if that is normal.

I did check to make sure I was getting 280V, 5V, and 6.3V from the power transformer (more like 290V, 5.3V, and 6.9V) so everything is good there I think.

I found a document that lists a bunch of voltages so I'll pull all the rest of the tubes and start checking voltages unless there is something else more obvious I should be looking at first.

Thanks for the help!

[Willabe]

Post all the voltages for all the tubes, including the heater acv's.

[pdf64]

(Unfortunately, in a weird string of events, my last thread broke from an attachment and Doug said I have to start a new one. Funny too because it now says this is my first post again, haha)
____________________

I powered up my AC30/6 top boost build for the first time, and no sound!

I'm still waiting for one more 12AX7, but I thought I still might be able to test at least the Normal/Brilliant channel and just leave out one of the vibrato valves. Am I correct in thinking if I have the Rectifier, Power, Normal/Brilliant, and Phase Inverter valves installed, I should get something out of the Normal and Brilliant channels?

Yes for the normal channel, but the brilliant channel will need the valve of top boost circuit installing.

I am getting absolutely no sound/noise/anything. I'm used to getting something on builds, even just sound noise! Which makes me feel like there must be an issue with the phase inverter tube/circuit, the output transformer, or a short somewhere in the audio path?

It seems weird. This has some likely anode voltages https://el34world.com/charts/Schematics/files/Vox/Vox_ac30volt_factory.jpg
Cathode voltages are good to check too.

I'm also assuming it is normal for my meter to read continuity between the 8ohm, 16ohm, and common wires out of the output transformer, meaning the secondary winding is intact?

Yes, normal.

I'm actually getting a bit over 1 ohm resistance between each and chassis, so not actually a short I don't think.

Short your probe tips together, what resistance do you measure, eg 1 ohm?
The point being that a winding can't measure lower than that.

All the tubes are getting decently hot, and filaments glowing on all of them besides the phase inverter, not sure if that is normal.

Getting warm or hot is what matters, there's no spec for visible light output.

I did check to make sure I was getting 280V, 5V, and 6.3V from the power transformer (more like 290V, 5.3V, and 6.9V) so everything is good there I think.
I found a document that lists a bunch of voltages so I'll pull all the rest of the tubes and start checking voltages unless there is something else more obvious I should be looking at first.

Thanks for the help!

I suggest to take your mains voltage several times during a amp test session. It can vary, sometimes quite a bit, and secondary winding / HT voltages will track it.

Voltage tables are nice, here's an example https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_lead_45w.pdf

[four_corners]

Voltage tables are nice, here's an example https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_lead_45w.pdf

Thanks for all the help again. This is the chart I found in the "A Service Engineer's Guide to the Vox AC30 Valve Amplifier" book I bought at the beginning of the project, so I'll use this as a starting place.

EDIT:
Should I be checking voltages with tubes installed? Or at least the rectifier installed?

[pdf64]

Voltage tables are nice, here's an example https://el34world.com/charts/Schematics/files/Marshall/Marshall_jtm45_lead_45w.pdf

Thanks for all the help again. This is the chart I found in the "A Service Engineer's Guide to the Vox AC30 Valve Amplifier" book I bought at the beginning of the project, so I'll use this as a starting place.

EDIT:
Should I be checking voltages with tubes installed? Or at least the rectifier installed?

Excellent!
To get those circuit voltages, valves will need to be fitted and working in the relevant circuits. The voltages won't be valid for the circuits around any missing valves.
Hope that makes sense?

[Willabe]

EDIT:
Should I be checking voltages with tubes installed?

Yes, that's the point.

You want to know what the tubes are doing in the circuit.

[four_corners]

EDIT:
Should I be checking voltages with tubes installed?

Yes, that's the point.

You want to know what the tubes are doing in the circuit.

Okay I thought so I just wanted to check. Like I mentioned I'm waiting for one last 12AX7, but I'll at least put in the rectifier, power tubes, Normal/Brilliant tube, Phase Inverter, and Top Boost tube so I should have what is necessary for at least the Normal and Brilliant channels.

[four_corners]

here are some V readings and some notes. All readings in the attached chart are with all knobs fully CCW (at 0).
___________

When I turn the Brilliant pot up, pin 1 on all tubes have a decent voltage swing. This only happens with the Brilliant knob and not the Normal knob.

TB pin 1 - Brilliant fully CCW starts around 218V, will drop down to around 205V as I turn to noon, then full CW is about 275V.

V2 pin 1 - Brilliant fully CCW it starts at 300V, then goes down to 260V around noon, then full CW is 295V.

V1 pin 1 - Brilliant fully CCW it starts at 170V, fully CW is 295V.

With the Brilliant pot turned up some/all, the Treble, Cut, and Normal Volume pots will change the voltage a handful of volts, depending on how high the Brilliant pot is.

Brilliant/Treble/Cut/Normal do the same for pin 6, which makes sense (both of the cathode pins per tube).

This also sort of happens with power tubes.

Pin 1-2 on V3,V4,V5,V6 start at 0V with Brilliant at CCW, and go up to 36V at noon, then back down to 2.5V at CW.

So it seems something is going on. Not sure why I'm not getting any sound on the output. It makes sense the Vib/Trem channel isn't doing anything since those 3 tubes are not installed currently. Not really sure why the Normal volume knob isn't changing anything unless the Brilliant knob is turned up.


The voltages I'm getting are all a good bit higher than the documentation, but at the bottom it mentions the AVO multimeter vs modern multimeter thing, so seems like the numbers I'm sharing aren't super off (some more than others).

[Willabe]

No. You don't need to turn the knobs for anything. Just leave them at 0.   

Where are the dcv's for the power tubes?

The heaters, you measure across from pins 4/5 and 9 on the 12A_7 tubes for the total acv. That's all you need to know. You can check pins 4/5 to ground and pin 9 to ground if you want. You show 6.8acv at pin 9. Did you run a twisted pair for the heater wires?

[four_corners]

No. You don't need to turn the knobs for anything. Just leave them at 0.   

Where are the dcv's for the power tubes?

The heaters, you measure across from pins 4/5 and 9 on the 12A_7 tubes for the total acv. That's all you need to know. You can check pins 4/5 to ground and pin 9 to ground if you want. You show 6.8acv at pin 9. Did you run a twisted pair for the heater wires?

Sorry for the confusing voltage chart on the 12AX7s.

All heater wires are twisted. All 290V to rectifier are twisted. All mains are twisted.


12A_7:
The 6.8V AC is from pin 9 to ground (or pin 9 to 4/5 as you mentioned since 4/5 are tied to the chassis).


POWER:
Pin 1/2 to chassis is 0.001V DC
Pin 3 to chassis is 31V DC
Pin 7 to chassis is 410V DC
Pin 9 to chassis is 410V DC

[Willabe]

12A_7:
The 6.8V AC is from pin 9 to ground (or pin 9 to 4/5 as you mentioned since 4/5 are tied to the chassis).

No, you wired the heaters the old way. You should wire them 1 heater wire to pin 9, the other wire to pins 4/5 for all the 12A_7's. The EL84 heaters should be wired up with 1 heater wire to pin 4, the other to pin 5.

And you ground the heater CT lead to the chassis or if the PT's heater secondary wind doesn't have a CT, you make an artificial CT with 2 > 100 ohm resistors. 1 R goes to each heater leg to ground.

For the power tubes for DC, what should each pin be measured to, chassis ground?

Yes, you always measure from pin to ground with a tube.

[Willabe]

TB pin 1 - Brilliant fully CCW starts around 218V, will drop down to around 205V as I turn to noon, then full CW is about 275V.

V2 pin 1 - Brilliant fully CCW it starts at 300V, then goes down to 260V around noon, then full CW is 295V.

V1 pin 1 - Brilliant fully CCW it starts at 170V, fully CW is 295V.

Pin 1-2 on V3,V4,V5,V6 start at 0V with Brilliant at CCW, and go up to 36V at noon, then back down to 2.5V at CW.

When you post a voltage you need to tell us if its ac or dc.

Where you injecting a signal into the amp when you took these readings?

Those plate voltages, if there dc, should not be changing by turning the volume pots. Somethings wrong with this. 

[four_corners]

12A_7:
The 6.8V AC is from pin 9 to ground (or pin 9 to 4/5 as you mentioned since 4/5 are tied to the chassis).

No, you wired the heaters the old way. You should wire them 1 heater wire to pin 9, the other wire to pins 4/5 for all the 12A_7's. The EL84 heaters should be wired up with 1 heater wire to pin 4, the other to pin 5.

And you ground the heater CT lead to the chassis or if the PT's heater secondary wind doesn't have a CT, you make an artificial CT with 2 > 100 ohm resistors. 1 R goes to each heater leg to ground.

For the power tubes for DC, what should each pin be measured to, chassis ground?

Yes, you always measure from pin to ground with a tube.

Thanks for all your help Willabe, I really appreciate it and all your patience with me!

As you can tell I'm a newbie at this :) I've built tons and tons of complex synths and studio rack gear, but I managed to pick one of the more complex projects for my first build!

https://www.mojotone.com/VOX-Style-AC-30-Power-Transformer

This is my PT, so it looks like I need to make an artificial center tap:

- 6.3V wire to a 100 ohm resistor to ground
- the other 6.3V wire to a 100 ohm resistor to ground

No, you wired the heaters the old way. You should wire them 1 heater wire to pin 9, the other wire to pins 4/5 for all the 12A_7's. The EL84 heaters should be wired up with 1 heater wire to pin 4, the other to pin 5.

I feel like I was saying the same thing, but maybe I'm misunderstanding. Not a schematic, but I've attached a screenshot of the layout drawing to show how I have one heater wire to pin 9, the other to pin 4/5. I guess the difference is I have 4/5 tied to the chassis, should I cut this?

The second screenshot shows how I have the EL84's wired, one heater to 4 and one heater to 5.

This layout drawing that I'm using along side the original 1963 JMI schematic shows a center pin on the EL84's tied to pin 3 and then to the 50 ohm cathode resistor. I have not done this on mine, do I need to add some sort of lug to the middle of the EL84 socket to connect to the cathode?

[Willabe]

This is my PT, so it looks like I need to make an artificial center tap:

- 6.3V wire to a 100 ohm resistor to ground
- the other 6.3V wire to a 100 ohm resistor to ground

I guess the difference is I have 4/5 tied to the chassis, should I cut this?

Yes.

I knew something was wired up wrong because you were getting the full 6.3acv at a single pin on the preamp tubes. You'll know you have them wired up correctly when you only get 1/2 the 6.3acv, so ~3.15acv at pin 9 to ground and at pins 4/5 to ground.

Then add those 100 ohm R's, 1 to each side of the heater wind to ground. It's to kill hum.

Look at the Fender schematic attached. It shows how to wire them up.

https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab568_schem.pdf

[four_corners]

This is my PT, so it looks like I need to make an artificial center tap:

- 6.3V wire to a 100 ohm resistor to ground
- the other 6.3V wire to a 100 ohm resistor to ground

I guess the difference is I have 4/5 tied to the chassis, should I cut this?

Yes.

I knew something was wired up wrong because you were getting the full 6.3acv at a single pin on the preamp tubes. You'll know you have them wired up correctly when you only get 1/2 the 6.3acv, so ~3.15acv at pin 9 to ground and at pins 4/5 to ground.

Then add those 100 ohm R's, 1 to each side of the heater wind to ground. It's to kill hum.

Look at the Fender schematic attached. It shows how to wire them up.

https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab568_schem.pdf

Awesome, thanks for the help there.

Looking back at the Top Boost mod document, I have a feeling I did something wrong (as you suspected, haha), as it is the Brilliant pot that gets modded too, and its the one messing with the voltages.

I can see where I got confused now with the mod, so I'll fix this too.

EDIT: maybe i did it correct, haha. I'll triple check...

[four_corners]

This is my PT, so it looks like I need to make an artificial center tap:

- 6.3V wire to a 100 ohm resistor to ground
- the other 6.3V wire to a 100 ohm resistor to ground

I guess the difference is I have 4/5 tied to the chassis, should I cut this?

Yes.

I knew something was wired up wrong because you were getting the full 6.3acv at a single pin on the preamp tubes. You'll know you have them wired up correctly when you only get 1/2 the 6.3acv, so ~3.15acv at pin 9 to ground and at pins 4/5 to ground.

Then add those 100 ohm R's, 1 to each side of the heater wind to ground. It's to kill hum.

Look at the Fender schematic attached. It shows how to wire them up.

https://el34world.com/charts/Schematics/files/Fender/Fender_super_reverb_ab568_schem.pdf

Question:

On the TB mod, it says "Isolate this line and earth to tag at V1 only". Since this comes from the same documents that have the old way of grounding pin 4/5 to the chassis (like you mentioned), should I just have the other Bass pot connect to the 10k, 56k, 1k5 resistors, and 25uF cap, and NOT then attach it to the chassis ground tag by V1?

Also says "Connect heaters to V2" right below that. I assumed that meant just attach it to all the other 12A_7 heaters (which is what I did) and isn't in some way a continuation of the previous sentence about grounding?

[Willabe]

On the TB mod, it says "Isolate this line and earth to tag at V1 only". Since this comes from the same documents that have the old way of grounding pin 4/5 to the chassis (like you mentioned), should I just have the other Bass pot connect to the 10k, 56k, 1k5 resistors, and 25uF cap, and NOT then attach it to the chassis ground tag by V1?

No.

This has nothing to do with the heaters ground. Certain parts of a circuit need to be grounded.

Also says "Connect heaters to V2" right below that. I assumed that meant just attach it to all the other 12A_7 heaters (which is what I did) and isn't in some way a continuation of the previous sentence about grounding?

It's just telling you the heaters for the top boost tube need to be wired up.

[Willabe]

.... the original 1963 JMI schematic shows a center pin on the EL84's tied to pin 3 and then to the 50 ohm cathode resistor. I have not done this on mine, do I need to add some sort of lug to the middle of the EL84 socket to connect to the cathode?

No.

[Willabe]

As you can tell I'm a newbie at this :) I've built tons and tons of complex synths and studio rack gear, but I managed to pick one of the more complex projects for my first build!

This is a pretty big/complicated build and it has a mod that's separate. This is not a good 1st build to take on. It can be done but there's a lot of very basic things you are missing.

On top of that, lead dress and layout are going to be very important as well as grounding.   

[four_corners]

As you can tell I'm a newbie at this :) I've built tons and tons of complex synths and studio rack gear, but I managed to pick one of the more complex projects for my first build!

This is a pretty big/complicated build and it has a mod that's separate. This is not a good 1st build to take on. It can be done but there's a lot of very basic things you are missing.

On top of that, lead dress and layout are going to be very important as well as grounding.

I understand this isn't a good 1st build to take on, but here I am 

I know from my questions it sounds like I may have never touched a soldering iron in my life or read a schematic, but I really have built a lot of gear. I've built things like a P2P Teletronix LA-2A to original specs, a full Buchla Music Easel, 8-voice polysynth with over 2,000 components, including hundreds of 0605 smd parts by hand. Soldered many 128 pin QFP IC's under a microscope by hand without a hot air machine, rebuilt tons of 80's synths, etc...

You are correct though that amps are a whole other beast, but I'll get there (hopefully!).

Again, I really appreciate all your help. I'm going to fix the grounding issues you mentioned, and I'll go from there.

[pdf64]

If there's still absolutely no hum /hiss, are you sure that the OT secondary is properly connected to the speaker?

[pdf64]

A useful procedure here may to isolate and deenergise the amp, then measure the resistance to the chassis of each valve socket lug.
Then repeat for the resistance to the rectifier output lug8.
The resulting table of those results may be enlightening :)

[Willabe]

This will answer a lot of your questions;

https://robrobinette.com/Amp_Stuff.htm

[four_corners]

If there's still absolutely no hum /hiss, are you sure that the OT secondary is properly connected to the speaker?

I have also been finding it very odd that there is absolutely zero hum or hiss too...
Here is the OT i'm using, and the way I have the secondary wired (black to speaker, green to speaker, with the black grounded to the chassis).

On the primary side, I have the 2 red wires are tied together, connected to C39 and pin 8 of the rectifier, with the white to pin 7 of V3/V5 and pin 7 of V4/V6 (the EL84s).

I should be able to check at pin 7 of V3/V5, and pin 7 of V4/V6 to confirm there is indeed audio leaving the power tubes, basically to check if there is something screwed up with the OT? Plug in a guitar, play something into the amp, and measure the DC voltage to see if it moves with the signal?

[mresistor]

Four_Corners   so this is the 36w version of the AC30     https://el34world.com/charts/Schematics/files/Vox/Vox_AC36_Schematics.pdf

[Willabe]

Plug in a guitar, play something into the amp, and measure the DC voltage to see if it moves with the signal?

NO!    Don't do that!    You'll blow up the meter. 

You measure at the speaker/OT secondaries for AC voltage. You don't measure to see if the plate dcv moves.

With a tube power amp, the signal goes out from the power tubes plate, into the OT primary, out the OT's secondary, to the speaker, then back into the OT secondary, into the OT primary back to the power tube. But it's now 2 to 3 times the tubes dc plate voltage.

[four_corners]

Plug in a guitar, play something into the amp, and measure the DC voltage to see if it moves with the signal?

NO!    Don't do that!    You'll blow up the meter. 

You measure at the speaker/OT secondaries for AC voltage. You don't measure to see if the plate dcv moves.

With a tube power amp, the signal goes out from the power tubes plate, into the OT primary, out the OT's secondary, to the speaker, then back into the OT secondary, into the OT primary back to the power tube. But it's now 2 to 3 times the tubes dc plate voltage.

I got bad info then from the rest of the internet (, haha, note to self: only listen to people on the Hoffman forum  . I was pretty sure it would blow the meter like you mentioned, thats why I asked here first after reading that elsewhere.

[Willabe]

Your switch for the OT secondary is wired as a direct short. No wonder you don't hear anything. Where did you get that switch layout drawing? Did you make it?

You don't need a DPDT (double pole/double throw) you only need a SPDT (single pole/double throw). You don't need the ground on the switch.

In your layout drawing, you have the OT secondary ground wired to the center 2 lugs/poles of the DPDT switch. And the 8ohm/4ohm leads wired to the outside lugs. So which ever way you throw the switch the secondary blue or green lead is shorted to ground.

It should be a SPDT switch with the speakers positive wire hooked up to the center switch pole, which is constant. And the 8ohm and 4ohm secondary leads go to the 2 outside lugs, 1 to each lug. It should be a large body switch to handle the output current. Never flip that switch with the amp on.

Look at the DPDT switch wiring below. The left side of the switch is 1 pole, the right side is the 2nd pole.

[four_corners]

Your switch for the OT secondary is wired as a direct short. No wonder you don't hear anything. Where did you get that switch layout drawing? Did you make it?

You don't need a DPDT (double pole/double throw) you only need a SPDT (single pole/double through). You don't need the ground on the switch.

In your layout drawing, you have the OT secondary ground wired to the center 2 lugs of the DPDT switch. And the 8ohm/4ohm leads wired to the outside lugs. So which ever way you throw the switch the secondary blue or green lead is shorted to ground.

It should be a SPDT switch with the speakers positive wire hooked up to the center switch pole, which is constant. And the 8ohm and 4ohm secondary leads go to the 2 outside lugs, 1 to each lug. Never flip that switch with the amp on.

Look at the DPDT switch wiring below.

That isn't a switch, its a terminal block like this...

[Willabe]

It's drawn like a switch.

Why did you use a terminal block instead of running the OT secondary fly leads to the speaker jacks? I wouldn't cut those short.

[mresistor]

What diagram - instructions - schematic -layout or data   are you using as a guide to build this amplifier?????

[four_corners]

These are the documents I'm working from.

A Service Engineer's Guide to the Vox AC30 Valve Amplifier
- Schematic for normal 63' AC30/6
- Layout for TB in panel version (what I'm building)
- Layout for power section.

That drawing is from this PDF that I'm also referencing

[four_corners]

It's drawn like a switch.

Why did you use a terminal block instead of running the OT secondary fly leads to the speaker jacks? I wouldn't cut those short.

I'm not at home right now so I didn't actually have a photo of the terminal block, that is just one I found from a reference photo. In my build I looped them and tied the excess with a zip tie. I'll skip the terminal block for now and go straight to the jack.

[Willabe]

The book you bought might be good, might not be so good, we don't know if there's any mistakes in it. 

The speaker jacks can be done a few ways, with an Z (impedance) switch or with 2 separate speaker jacks. If you use the 2 jacks you can not use shorting jacks, or just don't wire up the shorting switch contact.

I look at all the info you posted so more, I see his terminal speaker block. And he used a PT that had a CT on the heater winding.

Your going to want to look at this from our friend Sluckey from his AC15 build, look at the trem/vibrato mods for speed and depth pot instead of switches. Makes the trem much more usable.

His schematics, layouts, mods are at the very bottom.   

https://www.sluckeyamps.com/VAC15/ac15.htm

[Willabe]

And your going to need a light bulb limiter. Read the whole thread.

Reply #8 has a picture of what LBL could/should look like. 

https://el34world.com/Forum/index.php?topic=32424.0

[four_corners]

The book you bought might be good, might not be so good, we don't know if there's any mistakes in it. 

The speaker jacks can be done a few ways, with an Z (impedance) switch or with 2 separate speaker jacks. If you use the 2 jacks you can not use shorting jacks, or just don't wire up the shorting switch contact.

I look at all the info you posted so more, I see his terminal speaker block. And he used a PT that had a CT on the heater winding.

Your going to want to look at this from our friend Sluckey from his AC15 build, look at the trem/vibrato mods for speed and depth pot instead of switches. Makes the trem much more usable.

His schematics, layouts, mods are at the very bottom.   

https://www.sluckeyamps.com/VAC15/ac15.htm

I have a dim bulb limiter I've been using. Any time I mess/change anything, I first fire up the amp with the LBL, if it lights up bright then dims down, I'll unplug then plug the amp directly into the mains for checking voltage, etc...

On my old (dead) thread, a bunch of helpful info from Slucky's site had been shared that I referenced, but I lost a decent amount of the stuff I didn't download or bookmark, so I appreciate you sending over that link.

There is a change I am totally just screwing up the speaker wiring even though it seems very straight forward (would be hilarious after all this if its just something as stupid as that). I don't have an AC30 sized speaker cabinet right now, I'm just using the 1x12" speaker from a Blues Jr for the time being. I also tried the amp through a Fractal X-Load LB-2 Load Box with no luck.

I think the disconnect from my old post and this one created a lot of confusion for everyone trying to help me, so I appreciate your patience.

[four_corners]

Tiny bit more "data" to share, may be nothing though.

If I plug into the Normal jack and crank the Normal pot up pretty high with my meter set to AC, my meter reads 0.001 V AC, and if I strum my guitar pretty hard, it will move up to 0.020 V AC, which I know is not nearly enough to be audible or correct for an amp if I actually plugged into a speaker, but it does show that something is happening between the amp input and the output.

I'm going to go through and check voltages again with every tube installed and try to be as thorough as possible.

[pdf64]

Thinking about it, EL84 anode current looks to be completely cut off. ie so no output is possible.
The results of the tests noted in post 22 would be helpful.

[four_corners]

Here are the most up to date voltages:

GZ34
between pin 2 and 9 -      5.3V AC
between pin 4/6 and gnd -      310V AC


EL84 (all)
between pin 4 and 5         6.6V AC

between pin 3 and gnd      31.3V DC
between pin 7 and gnd      416V DC
between pin 9 and gnd      410V DC
Pin 1/2 and gnd                Kinda hovers around 0V AC or DC


V1 (Norm/Brill)
between pin 4/5 and 9      6.6V AC

between pin 1/6 and gnd   200V DC
between pin 3/8 and gnd   2.3V DC


V2 (PI)
between pin 4/5 and 9      6.6V AC

between pin 1/6 and gnd   293V DC
between pin 3/8 and gnd   75V DC
between pin 7 and gnd      51V DC


V7 (Vib pre and gain)
between pin 4/5 and 9      6.6V AC

Not getting anything else really... need to figure out what is up here. I tried swapping tubes with no luck either.


V9 (osc.)
between pin 4/5 and 9      6.6V AC

between pin 1/6 and gnd   170V DC
between pin 3 and gnd      2V DC
between pin 8 and gnd      28V DC


V8 (mod.)
between pin 4/5 and 9      6.6V AC

between pin 1 and gnd      104V DC
between pin 6 and gnd      105V DC
between pin 3/8 and gnd   4.5V DC


Top Boost
between pin 4/5 and 9      6.6V AC

between pin 1/7/8 and gnd 230V DC
between pin 3 and gnd      1.3V DC
between pin 6 and gnd      360V DC


C11 - 305V DC  (voltage doc. says "10 volts quiescent. 12.5 volts at 30 watts output.")

[four_corners]

Thinking about it, EL84 anode current looks to be completely cut off. ie so no output is possible.
The results of the tests noted in post 22 would be helpful.

Just posted some other voltages as you were posting this, but I'll go back and try out your suggestion from earlier. Thanks!

[pdf64]

...
C11 - 305V DC  (voltage doc. says "10 volts quiescent. 12.5 volts at 30 watts output.")

That's the pin3 cathode, yours is 31V, I suspect the cathode resistor is open circuit.

[four_corners]

...
C11 - 305V DC  (voltage doc. says "10 volts quiescent. 12.5 volts at 30 watts output.")

That's the pin3 cathode, yours is 31V, I suspect the cathode resistor is open circuit.

BINGO!!!

Just noticed the wire connecting pin 3 of the EL84s to R24 (cathode resistor) had somehow broken off, but was lodged under a tiny piece of wire insulation and wasnt connect! Time to go solder it back.

[pdf64]

Great!
Please continue with the tests, there's a few other suspects :)

[shooter]

while you're out rabbit hunting what's the new cathode to ground VDC read.

[four_corners]

while you're out rabbit hunting what's the new cathode to ground VDC read.

Well it works! Sounds like a guitar amp (sort of, haha)

I was playing with it for about 5 minutes, everything working fine (it seemed), but I wasn't looking at the tubes. About 10 minutes later, I started playing it, and noticed one of the EL84 seemed wayyy too bright/hot. My thermal gun was saying something like 350-400 degrees F... so I can't really run it for more than a minute without getting worried. When I put it through the LBL, the bulb starts bright, dims down, then sure enough, starts glowing brighter and brighter.

It seems like if V6 was getting super hot, V4 should as well? I can try swapping that specific tube with another one to see if something is bad in it.

Also, still an issue with V7 not heating up, need to track that down.

[four_corners]

while you're out rabbit hunting what's the new cathode to ground VDC read.

I don't have a variac, but maybe I should pick one up, that way I can try to get some attenuated voltage readings without feeling like I'm going to blow up that EL84. I know they can get pretty darn hot and be okay, but this seems a bit excessive.

[Willabe]

Attenuated voltage readings wont tell you anything, there not real world operating conditions.

The more the current, the more the heat. Tubes are red plating.

Could be a bad tube, could be thermal run away. Might need to use a larger K R for the power tubes to get their output under control.

Should use a 1 ohm R on each EL84 tube K. It goes from the tubes K to the single gang K R. Then you can measure for dcv across each 1 ohm R, 1dcv = 1mA of current. 

 

[Willabe]

Sluckey's web site;

Amp projects and more...

And like I posted before, you need to go through Rob Robinette's web site;

https://robrobinette.com/Amp_Stuff.htm

And Merlin's web site;

https://www.valvewizard.co.uk/

[shooter]

I was playing with it for about 5 minutes,

put this on your bench;
"NEVER EVER add signal to a new amp BEFORE you know what the tubes are BIASED at"


measuring that cathode Resistor, 50 ohms by your schematic and math will tell you IF the amp is BIASED correctly
math;
I=E/R  so measure VDC across the resistor, DIVIDE that by 50
since you have 4 tubes, divide your answer by 4
now P=E*I  so take your plate vdc and multiply it by the answer you have once you divided by 4.


your old numbers for example;
31.3VDC / 50 ohms = .626 divided by 4 (tubes) = 160mA per tube!!
you had about 400VDC plate so 400 * .16 = 64WATTS from a 14W tube, boom, send in the new quads!!!

[four_corners]

I was playing with it for about 5 minutes,

put this on your bench;
"NEVER EVER add signal to a new amp BEFORE you know what the tubes are BIASED at"


measuring that cathode Resistor, 50 ohms by your schematic and math will tell you IF the amp is BIASED correctly
math;
I=E/R  so measure VDC across the resistor, DIVIDE that by 50
since you have 4 tubes, divide your answer by 4
now P=E*I  so take your plate vdc and multiply it by the answer you have once you divided by 4.


your old numbers for example;
31.3VDC / 50 ohms = .626 divided by 4 (tubes) = 160mA per tube!!
you had about 400VDC plate so 400 * .16 = 64WATTS from a 14W tube, boom, send in the new quads!!!

All very good info!

New voltages for the power tubes

pin 3 to gnd      14V  DC
pin 7 to gnd      325V DC
pin 9 to gnd      315V DC


So new cathode to ground is 14V DC, and its a 47 ohm resistor, so:

14VDC / 47 ohms = 0.298 / 4 = 74.5mA per tube

Probably a bit less than 74.5mA per tube though since the plate and screen are a little different?

325*.0745= 24.21W, still too hot for a 10-14W tube.

Basically it seems like I should replace the 47ohm with something like a 90ohm resistor.

EDIT:
Actually, I'm realizing that changing the cathode resistor value can effect the plate/screen/cathode voltages, so I can maybe alligator clip a 100 ohm resistor, and see what voltages I get + the wattage of the tubes, and go from there.

If that still leaves that one tube way hotter than the rest, I'll possibly just use Willabe's suggestion of individual cathode resistors (even though these were supposedly matched by Apex Tube Matching). If I used individual cathode resistors, would they all need individual bypass caps too, or can they still share the same big bypass cap?