6G3 Capacitor Question

[66Strat]

That looks good for no-load.

[dwinstonwood]

cool

[dwinstonwood]

sluckey, I started with a 10K bias resistor and thought I'd work back down from there.

With no tubes at all - no rectifier, that wouldn't make a difference, would it? - the pot gives a range of -33.97v to -25.31v.

What do you think?

I have 8.2k's. but no 12k's. I do have 3 watt 15k's.

Also, I can change the safety resistor on the pot from 27K to 22K - LTSpice shows that will make the negative voltage about 2V more positive, probably making my range around -31.7v to -23v.

FWIW, the JJ 6V6S's I bought have the same max plate diss. rating as as 6V6GT's - 14 watts.

Thanks!

[edited to correct voltages]

[sluckey]

Try it and see.

[dwinstonwood]

Thanks!

Tomorrow. Long day staring at databases & spreadsheets and the eye's are tired.

I edited the numbers in post #252:

With the 22K pot resitor it should be around -31.7v to -23v.

[dwinstonwood]

New negative bias voltage measurements:

No rectifier installed.

10K bias resistor and 22K pot safety resistor: -31.6v to -22.3v range. The middle of that range is -26.95v.

I have the pot rotated to give the maximum negative voltage of -31.6v

Should I install ALL of the tubes, hook up a speaker, and measure across the 1 Ohm cathode-to-ground resistors first?

Or, should I measure the power tube plate voltages first?

I'm sorry for being dumb about this. I've never set the bias on a PP amp before.

I want to have some idea of what I should be looking for, so I did the math for a range of plate voltages and 9.8 watts of plate dissipation (70%):

355VDC and 27.6ma
365VDC and 26.8ma
375VDC and 26.1ma
385VDC and 25.5ma
395VDC and 24.8ma

Thanks again for all the help! I'm almost there.

[sluckey]

Plug the amp straight to the wall with speaker connected. Measure plate voltage and cathode voltage. Calculate. Adjust bias as needed and repeat.

[dwinstonwood]

That had my heart racing. I didn't know what was going to happen.

Plate voltages: 357VDC for both tubes. That's within 2% of the 365 on the schematic.

So, with the bias pot turned to maximum negative voltage (-31.6v) I measure 25.6mv and 25.2mv across the 1 Ohm resistors.

That's an average of 9.07 watts of plate dissipation, or about 65%-ish.

For 9.8 watts of plate dissipation I want 27.45ma @ 357vdc. Although, I understand the plate voltage will change a bit as I adjust the bias.

One thing I noticed is that there is no noise at all. No hum, hiss or buzzing. But, that's with all knobs at zero and no guitar plugged in.

[dwinstonwood]

I think my current measurements are wrong. I think the tubes might be in cut off.

I'm getting no sound through the speaker from my guitar.

What do I check first?

[sluckey]

Power amp good. It's guitar time.

Check speaker jack wiring.

[dwinstonwood]

Does this look ok?

Thanks.

[sluckey]

It's wired correctly. Ain't lovin' the solder joints.

Monitor voltage on the 1Ω resistors while turning the bias pot. Do the voltages change?

Double check wiring with the schematic and layout. Confirm they all agree. Measure and post voltages on all tube pins. Post hi-rez pics.

[dwinstonwood]

I might have figured it out.

I'm measuring 1.4 Ohms from pin 8 to pin 1 on the power tube sockets. I need to buy some more 1 Ohm resistors and solder them better.

If I measure 25.4mv that's only 18ma.

I would need to measure 38mv to have 27ma. I think... is that math right?

[sluckey]

Those 1Ω resistors are not why you have no sound from the speakers.

[dwinstonwood]

Yes, the voltage changes as I rotate the bias pot.

[dwinstonwood]

V1 Pin:
1 - 160.6v
2 - 0v
3 - 1.632v
6 - 160.3v
7 - 0v
8 - 1.632v

V2 Pin:
1 - 171.2v
2 - 0v
3 - 1.415v
6 - wild fluctuations and speaker static/scratching
7 - 0v
8 - wild fluctuations and speaker static/scratching

V3 Pin:
1 - 329.7v and speaker static/scratching
2 - 0v
3 - 5.15v
6 - 328.9v and speaker static/scratching
7 - 0v
8 - 5.15v

V4 Pin:
3 - 350.5v
4 - 349.5v
5 - -28.3v and speaker static/scratching

V5 Pin:
3 - 349.5v
4 - 349v
5 - -28.3v and speaker static/scratching

[shooter]

6 - wild fluctuations and speaker static/scratching

is that the trem tube?  If so you need to ground the trem FS to turn off
static scratch sounds like bad solder

[dwinstonwood]

Thanks shooter. I wondered about the trem. So, I can just take a male RCA plug and connect shield to pin and plug it into the footswitch jack?

The static only happens when my multimeter test lead rubs on the socket pins. My hand might have been shaking a bit. There's no noise when the test lead is making firm contact with the pins.

V3 plate voltages seem high, but I don't know.

[pdf64]

Using a regular DMM, resistance readings below a few ohms must be regarded as ‘indication only’.
Try imagining that you’re using an analog meter.
Accurate measurements require a dedicated, specialist ’low ohm meter’.

[shooter]

V3 plate voltages seem high, but I don't know.

is V3 the PI?
here's the fender numbers, does look like you have something in the PI wrong (IF V3 is PI).  double check the cathode config and values

[sluckey]

V3 pins 2, 3, 7, 8 are way wrong. Probably the cause of pins 1,6 being high. Start looking there. Check pin 3,8 resistance to chassis, meter probe directly on socket and chassis. What have you?

[dwinstonwood]

Long Post...

Thanks shooter.

OK, it's working. Tremolo, tone knobs, and all.

I lifted out the board and removed all but one wire from underneath. I transferred them to the top of the board.

I didn't find any errors in my components or connection points.

But, I think I know what was going on. That upper left corner that was under my skin from the beginning was the problem, I'm pretty sure. When I pushed the wires down into the eyelets, some of the wires went in too far, and I think something in that corner was shorting to the chassis. I removed the whole cluster (f...) and rewired it in a simpler, more logical way (see photo).

Now, the anodes of V3 - yes, shooter, that's the PI - measure: pin 1 - 224.5v and pin 6 - 226.8v. Or, the other way around, my mind is spinning. Anyway, that's spot on with the schematic which says 225v and 230v!

There is work to do. It is dead quiet with no guitar plugged in. I get some hum with the guitar. It is completely controlled by the volume knob. It sounds just like single coil pickup noise to me.

Also, I don't have the plastic knobs attached and I hear a slight sound when I touch the metal pot shafts. That could be me and the carpet, I don't know. Also, the normal volume pot is a bit scratchy.

Power tube plates measure 359.7vdc for the right and 360.3vdc for the left.

Power tube cathode voltage readings are 24.8mv for right tube and 23.7mv for the left one. I can juice it up to 26ma if I want.

So, it's a working amp. It's a loud amp. I still need help smoothing out some noise issues, but some of that might be my soldering.

I have all you all to thank. Especially sluckey who put up with my questions, etc.

I need a break. Then, I'll tackle what's left to do.

Thanks!
David

[dwinstonwood]

V3 pins 2, 3, 7, 8 are way wrong. Probably the cause of pins 1,6 being high. Start looking there. Check pin 3,8 resistance to chassis, meter probe directly on socket and chassis. What have you?

sluckey, I was writing while you posted that.

I will measure 3 and 8 resistance to chassis as you state. Probably later tonight. I just got off the amp build rollercoaster. But, something I did knocked that anode voltage down from 329v to 225v!

BTW, the Tremolo is great! It is strong and has a wide range.
The 6V6S's are running at about 8.75 watts dissipation. That's like 62.5%. I'll see if I like that before messing with the bias.

[dwinstonwood]

...Check pin 3,8 resistance to chassis, meter probe directly on socket and chassis. What have you?

Wow, for one thing the pin 8 wire had broken loose. Probably from when I was rewiring the heaters.
Resistance from V3 pin 3 to ground and pin 8 to ground are both 9.12K ohms. Is that good or bad?
V3 cathode voltage measures 18.72v. The schematic indicates 20v, so that's fine, yes?

I guess I'm messing with the bias because it's a new trick.  I adjusted the current up to 26ma (same as schematic). Plates are at 362v (365 on schematic) and dissipation is 9.4 watts, or 67%.

I'm glad I went with the Hammond 290HAX Harvard PT. All of the voltages I've measured have been very close to the schematic's numbers. I know that doesn't necessarily mean a lot in the real world, but I at least feel like I succeeded in something.

[shooter]

Resistance from V3 pin 3 to ground and pin 8 to ground are both 9.12K ohms

add 6800+820+1500 

[dwinstonwood]

Resistance from V3 pin 3 to ground and pin 8 to ground are both 9.12K ohms

add 6800+820+1500 

Well, I'll be!

[66Strat]

Have you tried it with a guitar?

[bmccowan]

Glad to see you have it up and working. Reason to celebrate.

Wow, for one thing the pin 8 wire had broken loose.

I had noticed in some of your photos that some of the solder connections looked a bit dodgy. You may want to read up a bit on technique and go over connections before you consider it a wrap.
BTW you said that a few connections may have grounded under the eyelet board. I use turrets, but does you eyelet board have a blank board under it. From the Fenders I've worked on that is pretty standard and the blank board prevents unintended grounding. Maybe they are not all done that way?

[dwinstonwood]

Have you tried it with a guitar?

Yes! It sounds really good. The Tremolo works great and it's loud. The tone controls have a wide effect; the normal channel does not seem dark at all. But, I'm testing with a cheap 8" speaker sitting on its magnet on the bench. So I'm not getting the best results and that is probably making it sound brighter than it will in a cabinet.  Tomorrow I'll run it through the AC4 cabinet and speaker.

I'm spreading out the spending on this project. Now that I know it works I'm buying a combo cabinet and a 12" speaker (that's a rabbit hole in itself). I think the single coil-type noise will abate when it's in a solid cabinet with shielding under the top. Right now, it's open to every electrical thing in the room. But, it's pretty quiet.

[dwinstonwood]

Glad to see you have it up and working. Reason to celebrate.

Wow, for one thing the pin 8 wire had broken loose.

I had noticed in some of your photos that some of the solder connections looked a bit dodgy. You may want to read up a bit on technique and go over connections before you consider it a wrap.
BTW you said that a few connections may have grounded under the eyelet board. I use turrets, but does you eyelet board have a blank board under it. From the Fenders I've worked on that is pretty standard and the blank board prevents unintended grounding. Maybe they are not all done that way?

After using a turret board I found the eyelet board more of a challenge. It's hard to seal up those things without solder running down. And, yes, I intend to go over the entire amp and touch up the joints.
I used a Hoffman board - very thick and rigid - and mounted it on 1/4" nylon standoffs. But, a second piece of insulation might have been a good idea.

Thanks for all of your great advice and observations bmccowan!

[shooter]

buying a combo cabinet

since your heads in the hole, leave it a head unit and get a 2 X 12 

[bmccowan]

I used a Hoffman board - very thick and rigid - and mounted it on 1/4" nylon standoffs

That sounds good and different and better than the fender eyelet boards which seem to be some kind of cardboard soaked in some magical waxy concoction. You won't need a blank board under that board. After a while those fender boards look like the rolling hills of Oklahoma.

[dwinstonwood]

since your heads in the hole, leave it a head unit and get a 2 X 12 

Great, then I can multiply exponentially the number of options by mixing two different speakers! It could take me months to decide.

This thread has 2230 views. So, I guess a number of people have seen me stumble through all my trials and errors. But, it's a great collection of a lot of expert advice and technical knowledge from you all.

[shooter]

unless you added your voltage readings to the schematic already, you're NOT done 

play 1st though 

[dwinstonwood]

unless you added your voltage readings to the schematic already, you're NOT done 

Good point! And, I definitely intend to share my schematic, layout and BoM.

[sluckey]

I guess I'm messing with the bias because it's a new trick.  I adjusted the current up to 26ma (same as schematic).

Get yer terms right. There is no mention of "current of 26ma" on the schematic. There is mention of -26V of negative bias voltage. That -26V bias will determine how much current flows through the tube, so the two numbers are related but not the same. It's pure coincidence that there is -26V on the schematic and you set your tube current to be 26mA.

[bmccowan]

But, it's a great collection of a lot of expert advice and technical knowledge from you all.

And that's what makes this board - and the internet for that matter - so great. Sometimes you need to weed out the chaff, but the advice has often been a savior for my projects and no need to wait for a book to be published. Need advice? It is less that 5 minutes away! What a world we are experiencing!

[dwinstonwood]

Get yer terms right. There is no mention of "current of 26ma" on the schematic. There is mention of -26V of negative bias voltage. That -26V bias will determine how much current flows through the tube, so the two numbers are related but not the same. It's pure coincidence that there is -26V on the schematic and you set your tube current to be 26mA.

Sorry! That's like the third time I've mixed those two numbers up. It's hard for me to keep straight because of that weird coincidence.

But, if I understand it correctly, it's getting the current right that really matters, yes? I mean, the negative grid voltage - the thing on the schematic and the thing I adjust with the bias pot - is what regulates the current flowing through the tube at idle. So, the current is what I'm ultimately setting when I bias the tubes, as I understand it, right?

Since I set the bias by measuring across the 1 Ohm resistor and adjusted the bias pot until I read 26mv, I guess I should also measure the negative voltage from pin 5 to ground to see what negative voltage I ended up with. But, whether that turns out to be -26v or -30v on the grid doesn't really matter does it?; it's getting 26ma of current at idle that does, yes?
That's how I grasp the concept of biasing the amp. I just seem to keep getting those two "26's" mixed up.

[dwinstonwood]

I've been noodling for an hour, or so. I'm playing it through my AC4 Celestion speaker.
It really does sound great. Very full sound using the Normal channel. The Tremolo is crazy good. It has excellent speed and Intensity ranges.
Using the center position on my guitar - which is wired like a humbucker - it is honestly dead quiet. I hear no hum, or hiss at all. Thanks sluckey and Hoffman for your grounding wisdom.
But, in single coil settings I get some buzz depending on how I turn the guitar. I can make it go away if I turn the guitar away. (I was sitting about two feet in front of the amp, facing it.) I think a lot of that will diminish when the amp is in a cabinet and I'm standing farther away. But, I don't think I need to fiddle with wiring or grounding. It's great like it is.
After an hour the PT is warm, but I can keep my hand on it; it's not hot.
I have the 6V6S's set at 26.4ma and 25.7ma. I'm guessing that's pretty close between tubes (I'm guessing because I don't know what's normal matching is).

Here's a photo of the rig. Next thing is to order a cabinet and choose a 12" speaker. This amp deserves a good investment.

And, yes, I built the whole thing on that little table in my bedroom. I don't have a workshop where I currently live. But, I did have a window to open to let out the soldering smoke!

[sluckey]

You set a goal, asked a lot of questions, learned a lot, took a lot of ribbing, stayed the course, and ended up with a good looking amp! You should be proud!   

[dwinstonwood]

Thank you sluckey.
This has been an awesome project and every minute I've spent on this forum has been great - even the reprimands.

This tube amp stuff is mysterious and deadly for beginners like me. And, that's a lot of the allure. And, owning a handmade vintage-design amp that I could never afford to buy is totally unbelievable.

I hope there's somebody else here who wants to build an amp and saw the support and direction I got and decides to dive in. ...and, buy your parts from Hoffman!

[bmccowan]

Well done! Your attitude is perfect. I've repaired/rebuilt about 60 amps and built maybe 30 from scratch or from organ or PA chassis, and I still learn stuff every week from this forum. It's the best. I love the North Face motto: "Never Stop Exploring" It applies to the stuff we all do. Congratulations on finishing your amp!

[dwinstonwood]

Thanks bmccowan! There were one or two points when I was almost ready to just box it all up. Your encouragement helped me stick to it.

I'm still finishing up my layout, schematic and BoM, but here are the last measurements I took. It's weird, but they're all within about 5% of the schematic (except the negative grid voltage). If that's what one wants. I'm sure this design sounds great with the BF AB763 DR PT and its higher voltages; most new ones are probably built that way. But, as I said, I'm glad I went with the Hammond 290HAX Harvard PT.

[bmccowan]

Its good to be in the voltage range intended for the design IMO, but not go nuts trying to hit some ideal figure. My 6G3 is in the same range, as you know. With the AB763 amps, Fender was trying to get loud/clean, and succeeded. That sound is great too, but different. When using adjustable bias on this model, the bias setting affects the tremolo. So for me, once I arrived at the resistor values that allowed the bias pot to be useful, I adjusted the bias with the tremolo on until it sounded good to me. Get it too hot and it sounds like the choppers in Apocolypse Now.
I wish I had documented my previous builds like you are doing. Now if someone asks a question, or if I want to repeat something in a new build, I need to open the darn thing up! I just started a new project and will document as i think there will be twists and turns. It's an all octal 6V6 take on a Matchless Clubman circuit with iron from a Hammond AO-43. Sluckey's website has really helped me build amps from retired Hammond equipment.
Enjoy the amp!

[dwinstonwood]

When using adjustable bias on this model, the bias setting affects the tremolo. So for me, once I arrived at the resistor values that allowed the bias pot to be useful, I adjusted the bias with the tremolo on until it sounded good to me. Get it too hot and it sounds like the choppers in Apocolypse Now.

That's interesting. I understood how the DC bias voltage passes through the Intensity pot along with the Tremolo signal en route to the power tube grids, but I didn't really connect the dots to think about how the bias voltage affects the Tremolo. Way back in this thread someone - probably you - stated that too hot of a bias would mess with the Tremolo in a bad way. As it is, I think my Tremolo sounds great, that is, it sounds the way I think I want it to.

It's an all octal 6V6 take on a Matchless Clubman circuit with iron from a Hammond AO-43.

Well, please document the build! It will be an opportunity for me to learn a bunch of stuff.

I'd like to build something next with slightly uncommon tubes... maybe a cathode biased 7591A amp with Reverb.

[bmccowan]

Well, please document the build! It will be an opportunity for me to learn a bunch of stuff.

Probably me too. I'm headed out for a long weekend, but when I get back I'll pick that project back up - I've populated the board and laid out the locations of iron and tube sockets so far.
There are good threads here for using unusual tubes. My project will use 2 6SL7s and a 6SH7 or 6SJ7 in the preamp. fairly unusual today, but pretty common years back.

[dwinstonwood]

I've ordered the cabinet from Peter Mather. He's just down I-64 from me. I went with the combo. His cabinets look really nice.
I think I've also decided on a WGS G12C/S. It's based a the vintage Jensen, but the "/S" version has a softer top end. Buying a speaker is tough, but I haven't heard a bad recording of this one, and I haven't read a bad review.

[dwinstonwood]

Well, a lot more playing and a bit more tweaking.

I went back to the 27K resistor (from 22K) on the bias pot. This allowed me to lower the bias current a little more. I now have it set to about 22ma and 23ma. This is cooler - closer to 60% - but, it really sounds better to my ears. It may not be for everyone.

Also, yesterday I had a chance to crank it a bit. I noticed that when using the Tremolo, if I muted the strings I could hear a faint thumping. Not surprisingly, I came across a fix proposed by sluckey. I wired in a 1N4007 on the Intensity pot and the amp is now totally quiet with the guitar muted, even with the Intensity pot up full.

So, my question is this: is the thumping due to a more negative grid voltage? In order to bias my amp at around 23ma I need about -31V on the grids. Could this be why Fender called for -26V on the schematic?

Anyway, I knew there would be a fine-tuning stage involved in this build. So far it hasn't been frustrating, just methodical.

[shooter]

why Fender called for

I suspect fender did what you did, adjusted for current, then measured bias voltage and wrote it on the schematic.
NONE of the systems I every worked on were = for bias, just "within excepted range"
think of it like an "indicator" that something might be amiss if it's way off, 5 is not way of

[dwinstonwood]

Thanks shooter. And, I guess some tubes have a wider range of useable/practical bias current settings than others. The 6V6 seems to have a somewhat narrow range of about 7ma  - 8ma from being too hot to too cold. At least with my plate voltages.