Supro inspired amp to build soon just finished 1st schematic

[swingarm]

Hey Dave, no worries bro.

[Chuck H]

Not to take any sort of controlling position in the thread, so, swingarm, submitted for your consideration...

My favorite bias arrangement for EL84's is actually a hybrid using a zener to limit the voltage rise (and therefor some of the self adjusting nature of cathode bias). So it's cathode bias to a point and it stops self adjusting as it nears square wave clipping. The cathode bias is set at or close to class A (that is, 90 to 100% dissipation). I use a scope to determine the point where the tubes begin to clip, not just compress, but actually start to flatten the signal. That's the zener voltage. So the tubes behave as if cathode biased when clean and then tighten up at a slightly cooler bias point when clipping. This gives all that cool EL84 chime and avoids the buzzy/fizzy crossover distortion associated with cathode bias shift when you're really banging on them. I've experimented with fixed and cathode bias at many current levels with EL84's and I've settled on this arrangement for every EL84 amp I build. Take this with all the salt you need.

I would never expect anyone to bend to the opinion of the forum as a matter of conformity to the community. That would only serve to promote mediocrity and discourage experimentation. But these are good guys here. WRT most members you can always take their word as a genuine effort to assist based on experience and not just an arbitrary regurgitation of web lore. So it's pretty valuable stuff whether you use it or not. So...

Just bend to my will

No, but seriously, I think you'd be doing yourself a disservice if you don't at least try the bias arrangement I suggested because it's REALLY easy to implement and it sounds better than everything else.

JM2C

[swingarm]

I have another davis 106 Chuck that I will try your arrangement in next. Just a quick question...What circuit should I put in it? I have only ever built a Marshall 18 watt using el84. I also have a 6gw8 amp waiting for a circuit. I do value your opinions and knowledge. Thanks for your help guys. I will likely fire this up tomorrow. Wish me luck!

[Dave H]

You will soon be assimilated and become part of the collective. Resistance is futile

For that other Davis, what about building something with bass and treble controls this time like an AC15 Top Boost for example?

[Chuck H]

You will soon be assimilated and become part of the collective. Resistance is futile

Landru! Guide us!

[swingarm]

I refused to join the collective! I must admit we are all collectively slaves though. I want hair like that dude Chuck!

OK I fired chucky up and it was weak like a strong radio. Actually that was after I realized (from the glowing red plates) that there was an error in the bias. I had the 220k resistors on the pots incorrectly wired to 3 instead of 1. the tone control effects the volume also. So I'm going to have to look closer later. got go to sleep (night shift) Maybe when I get up you all will have it all solved.

Dave I was thinking ac15 too.

Ni night

[Chuck H]

When building one offs it's VERY common to have wiring errors. Troubleshooting it is the sort of thing that will give you a headache from all the intense and prolonged focus. It's so "normal" for me now I sort of like it Then there's that nauseated feeling that comes with long term frustration. I wouldn't even feel normal if I didn't feel like that for at least a third of my day. This is why it's not a group of old men at Denny's sitting around a table talking about their dead wives We (men) are wired for stress. So strap in, buck up and be sure to scowl while you do it.

As to that guys hair... I use to have tre' bitchin' hair. Now I'd be happy just to have more than I do. Even that guys.

Your bias circuit (grid loads, pots, etc.) does have some redundancies as mentioned earlier. I don't see how miswiring the pots should cause a bias failure. No matter what lugs you have attached there should always be the pots resistance or less between the grids and ground or bias. I will say that the circuit you posted confused me as to how it's switching could function properly. I didn't see any good way for bias voltage to get into the circuit at all. The bias and it's switching circuit could be trimmed down to something more like the circuit I posted. It would be equally effective, MUCH easier to troubleshoot and less prone to errors with fewer parts to fail. It's just a matter of practicality to me though.

Can you post a schem of your actual bias circuit and report on measured bias voltages or continuity to ground (depending on switch position) at the grids?

I'm about to start a night shift job. Not permanent though. I'm a painting contractor and I'll be working on a business that's, well, open during business hours. Small sympathy pains.

[swingarm]

Ha ha hilarious post Chuck! Ya I took a break from that amp for a bit but I will report back very soon.

Something is wrong in the Bias for sure.Im not getting any variance from the bias pots. Hey Chuck as the Buddhists say
"attachment leads to suffering" LOL

I'm getting -61 at the diodes.

Here are some basic voltage measurements below.

voltages.pdf

[swingarm]

Finally success! I abandoned the fix bias switch for now. Put in a 240 ohm resistor and a 33uf bypass and it sounded pretty good. I had miswired my inputs too. The amp sounded ok but farty at high volumes. I changed the .047 caps off pin 1 and 6 of v1 to .022 and wow! this amp sounds really good, fantastic, just as I had hoped! I also changed the resitors at the input to 1m and the caps to .047 from 100k and .005. Those are the only changes.

Shame I can't play guitar... lol. I'm learning slowly though. So thanks for all your help. Now I have two switches on the front to use. Any ideas? Thanks for all the help fellas. And of course. thank you to my friend Chuck

[Chuck H]

It's worth noting that things went better as you paired down

Modern guitar amps are complicated beasts. That's why REALLY GOOD players like the old ones

The goals are simple. Make the amp so it produces acceptable clean power for it's tube compliment and voltage. Then give it acceptable EQ capability or a balance you prefer. Make it so the power tubes and preamp tubes clip in a pleasant way. Nuances like attack envelope and "sag" make themselves apparent for adjustment along the way. You know it when you hear it. Trouble shoot the problems. Which is MUCH more easily done with straight forward designs. When the amp sounds good you've cooked up a recipe that works. No need for a bundle of switches and knobs. Some experience will get your designing to a point where parallels present themselves. Then you can more effectively implement this/that switches. With no known info about "this" or "that" it's just arbitrary. Just make stuff sound good. Then, later, when you've learned what works you can this/that or and/or without the problems. If you never get beyond just "this" or just "that" it's alright because you've made them sound good and know how. Too many guys try to tackle complicated projects with no idea what the outcome should be and end up with either junk or nothing.

JM2C on amp design.

[Dave H]

Finally success!

Congratulations!

Is your PI/output stage now like the the schematic in post 83? That looks good (no 'extra' components) except for one small point, it has 6V6s with a 250R cathode resistor but you have EL84s. EL84s with 240R will be biased very cold and have crossover distortion (see first plot below). I'd try it with 135R (2 x 270R in parallel) or 150R (see second plot with 135R).

For one of your unused switches why not try Chuck's zener trick across the cathode resistor for a simple fixed/cathode bias switch? A 15V 5W zener would be about right.

[swingarm]

Is your PI/output stage now like the the schematic in post 83? That looks good (no 'extra' components) except for one small point, it has 6V6s with a 250R cathode resistor but you have EL84s. EL84s with 240R will be biased very cold and have crossover distortion (see first plot below). I'd try it with 135R (2 x 270R in parallel) or 150R (see second plot with 135R).

I removed everything else i had prior. I just unhooked one leg of the cap to block the grid and removed the diodes (buzz fix). The fixed bias circuit is still there just unhooked. The amp is dead quiet at high volume so Im pretty happy about that too. I will pick up some more 10 watt resistors in the range you mentioned. I don't have any at the moment. Thanks. Oh! I also used a 250ka pot for the treble. I read different places that the tone control was unresponsive (may have been feedback from a clone manufacturer of this style amp) I find the tone to be very responsive and has very good range.

For one of your unused switches why not try Chuck's zener trick across the cathode resistor for a simple fixed/cathode bias switch? A 15V 5W zener would be about right.

Yeah I need to get some of those too.

[swingarm]

It's worth noting that things went better as you paired down

Indeed.

Modern guitar amps are complicated beasts. That's why REALLY GOOD players like the old ones

So logic says old players are the really good one too?

I'm an older player and playing an older design so I'm well on my way

The goals are simple. Make the amp so it produces acceptable clean power for it's tube compliment and voltage. Then give it acceptable EQ capability or a balance you prefer. Make it so the power tubes and preamp tubes clip in a pleasant way. Nuances like attack envelope and "sag" make themselves apparent for adjustment along the way. You know it when you hear it. Trouble shoot the problems. Which is MUCH more easily done with straight forward designs. When the amp sounds good you've cooked up a recipe that works. No need for a bundle of switches and knobs. Some experience will get your designing to a point where parallels present themselves. Then you can more effectively implement this/that switches. With no known info about "this" or "that" it's just arbitrary. Just make stuff sound good. Then, later, when you've learned what works you can this/that or and/or without the problems. If you never get beyond just "this" or just "that" it's alright because you've made them sound good and know how. Too many guys try to tackle complicated projects with no idea what the outcome should be and end up with either junk or nothing.

I think you are right Chuck good advice. I would add that complicated projects are ok too though, but you need the motivation to finish the job. Its been my experience that the most important part is to get moving and do it. No? I built a timber frame in 2002 I had never done that before but read the book ordered the timber and away I went. I finished it and then my ex left me LOL. I had to sell it. Its like that joke from the movie (cant remember the name) What do hurricanes and women have in common? They both come in wet and wild and when they leave they take your home and car etc. LOL I digress..for sure though simple designs are great. I think my next project will be the super scaler like in tut 5. But I want to put power scaling in it. Its a very simple design and such a versatile power amp if power scaling is installed.

[g1]

Finally success!... Put in a 240 ohm resistor...

You have been assimilated.
Welcome to the collective.

[swingarm]

Noooooooooooo!!!!!