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No Bass Control

Hi Guys,
I am playing with this cute little 18w guitar amp based on a couple known design circuits.
Anyways, I have it built and actually pleased with the output and Highs & Mids, But. The Bass pot does nothing!
The Treble pot is very effective and goes from amazingly bright to as dull as a Pea Green 62 Impala.(Now Thats DULL!)

Regardless of volume levels the bass pot currently is just a decoration. Maybe somebody here has an idea on this.
I underlined the various caps to show orange drops, Black beauties & ceramics.

Schematic

Gene
PS, please dont pick on the rest of the schematic to much, I would like to fix this problem first.:xeye:
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Gene,
The bass pot is swamped out by the 10K resistor and 0.01uF cap. I wouldn't expect it to do much. Have a good look at some amp tone control circuits. You are using these as "losser elements", you may want to include them in a feedback network to provide gain when turned up.

-Chris
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> based on a couple known design circuits. ... The Bass pot does nothing!

What Chris said.

The idea looks like classic Fender, but is different in critical ways. You need three caps, but you already have 3 (including the coupling cap, not needed with the Fender Stack). Make it just like a Fender, make it work, understand what it does, before deviating from Leo's odd design.

> dont pick on the rest of the schematic to much

15K seems unlikely for the first cathode resistor, it will starve the input tube. 1.5K is common.

130Ω 5W seems small for output cathodes, even for the EL84; are you sure the tubes are not overheating?

If it hums: I'd run the screen grids no higher than the plates, i.e. tap the 100/5W from the second cap instead of the first. Also the 1K to the third cap could be 10K or 22K without hurting the input stages or driver, while losing a lot of hum.

Your 50+50uFd total plate capacitance is high by guitar standards. This is very much a matter of taste and I like your style. But another style, especially with a soft hollow rectifier (and moreso with fixed-biased output), is to go toward 8uFd total plate reservoir capacity to get a dynamic droop on pluck and sustain.

Otherwise everything looks mighty fine.
 
PRR said:
> based on a couple known design circuits. ... The Bass pot does nothing!

What Chris said.

The idea looks like classic Fender, but is different in critical ways. You need three caps, but you already have 3 (including the coupling cap, not needed with the Fender Stack). Make it just like a Fender, make it work, understand what it does, before deviating from Leo's odd design.

> dont pick on the rest of the schematic to much

15K seems unlikely for the first cathode resistor, it will starve the input tube. 1.5K is common.

130© 5W seems small for output cathodes, even for the EL84; are you sure the tubes are not overheating?

If it hums: I'd run the screen grids no higher than the plates, i.e. tap the 100/5W from the second cap instead of the first. Also the 1K to the third cap could be 10K or 22K without hurting the input stages or driver, while losing a lot of hum.

Your 50+50uFd total plate capacitance is high by guitar standards. This is very much a matter of taste and I like your style. But another style, especially with a soft hollow rectifier (and moreso with fixed-biased output), is to go toward 8uFd total plate reservoir capacity to get a dynamic droop on pluck and sustain.

Otherwise everything looks mighty fine.

OK, after further reviewing of various tone stacks, And giving the space limitations of this particular chassis (everyhing shoehorned into a hammond 6X10) I managed to alter the tone block into a more fenderish set-upCurrent Tone Block

I may still have an error, I have it wired exactly as shown, But, The bass pot sems dramatically efected by the treble pot.

When treble pot is set at maximum, Only the upper 1/3 of the bass pot has an effect and its only minimal at best.

When the treble pot is turned to its minimum setting, Then the bass pot works thru its entire travel.

Other mod I made was to change the 15K resistor to 4.7K, Possibly still a bit high, But an improvement.

130© 5W seems small for output cathodes, even for the EL84; are you sure the tubes are not overheating?

I review several EL84 amps both at The AX84 site and a few marshall schematics and the majority hover arround that value, Most say 130/5W but I have seen several @ 100/5W and only a couple @150/5W.

If it hums: I'd run the screen grids no higher than the plates, i.e. tap the 100/5W from the second cap instead of the first. Also the 1K to the third cap could be 10K or 22K without hurting the input stages or driver, while losing a lot of hum.

Resistor to third cap is indeed already 22K. I hadnt noticed the mistake in the first schematic, But It was corrected in the assembly and my bench notes.

So, all in all, The Bass pot does something other than fill a chassis hole, But it still is questionably incorrect.

Gene
 
Eh,, Somebody call Darwin please?

I get Dumb(XXX) award for pulling a 680 ohm resistor intead of a 6800 ohm in the new tone block. :whazzat: Now its corrected and functional.

Now, A new concern has possibly surfaced(I didnt notice in prior tests at lower volumes)

Im using a few difernt sets of NOS Tubes, I have a pair of amperex's, 3 pairs of mullards, And a couple silvertones (relabed GE's?)

There is a certain amount of glow that emits from inside the tube,Medium / Low orange color. At Idle with guitar plugged in This is a constant. At Low to low/medium volume settings while playing this remains about the same low/ medium glow.

BUT, With Volume and gain cranked up into the upper regions, The intensity of the glow from inside the tube goes up

You can see this noticable change with each single note or cord plucked.
Is this a normal phenomena or am I about to get a lesson in 101 ways to fry valves ?

I dont recall seeing this type of thing in my seymour duncan amp running el34's. And the only oddity I ever see in my 6V6gta amp is a few cuties purple flashes against the glass occasionally.
(1948 Clear bottle RCA's)

I can see this possibly contributing to heating thats doesnt seem to be an issue at lower levels.

Any insight on this guys?
Gene
 
Yvesm said:


Looks like :xeye:

Try to feed the EL84 screens from the point labelled "B" in your power supply :eek:


Yves.

Ok, I moved the connection from point A to point B/C and its trashed. I get A Pulsating ourput.

Now, I only moved where the 100 ohm resistor conects from A to B/C
Meaning I left the 100 ohm still inline to both pin 9s

Anything over a minimal level setting ocillates and Pulses like a motoboat on 1 cylinder. So, I suppose this is not going to work in its current location.

I am going to return it to its original location and see if it reverts back to its previous preformance.

Also, Prior to this change, I restored the 15K resistor on the 1st input stage. Reason: Way to much Overdrive/Distortion. I preffer A cleaner sound, Then let the pedals cover any other voices I may want. I Supose It might be better to just remove the 100uf cap (1st prestage) but I was hoping to get a bit of overdrive/distortioin at just upper levels.

gene
 
Hmmm,

Ok, I have returned the Connection of the 100 ohm resistor to its original location at Point A. Back to original condition.

This baby really cranks as is, But that weird glowing is still a concern.

I do however feel perhaps the EL84's are indeed getting fairly hot compared to the rectifier tube which is very similar in size.

After looking over several marshall and AX84 schematics, The 130/5W resistor is middle of the pack and should have been well inside specs. So, Now I guess I bump it up to 150/5w? or 180/5w?

The Highest I have seen on any guitar amp circuit is 150.
Seems at this point like I am so close yet so far off.

Gene
 
Hi,
I'm new to the forum, but guitar amps have been a hobby of mine.

First on your screen supply you should have a 100 ohm resister connected to each screen. These should be connected to the lug on the tube socket. You can go as high as 1000 ohms to get rid of motorboating. You should also get your grid resistors as close to the tube socket as possible.

Your pre-amp has VERY high gain with a 12ax7. Try plate resistors between 100k and 220k, and cathode resistors between 1000 and 2700 ohms. Bypass caps are optional, 25uF is plenty, 100uF won't hurt. You might like the sound of a .68uF on the second stage. You'll get a boost in your mids and highs with out muddying the bottom. You see this in many Marshal amps.

I would also decouple the phase inverter from the pre-amp. you can take your plate supply and screen supply from the same point, then the phase inverter from the next highest voltage and finally the pre-amp at the lowest point.

This looks like a fun project. Good luck.

Fred
 
Whew what a day.

Its about 97 degrees in my garage workshop are and needless to say, Mods especially soldering gets a bit slow as you move in slow motion so as to not overheat the cranial matter..

OK, SO here we go,
I think what I currently have is about as close to a Twangin Fender Tele sound that heats up to a really nice SVR Strat sound.
In fact, My strat has never ever come this close to a Tele type Crispness.

Flip of the pickup selector and a minor tweak on the gain pot, And Mean SRV Blues sound with lots of bite.
I am 95% pleased at this point. (*see Below)

I updated the
Schematic to exactly as it sits.

Also, heres a few pics
Top Front
Top Back
The Scarey Side

Tight Fit in a 6X10 Hammond chassis. Mods are worse than a roomier chassis, but hey less wire too.
*My 5% dislike is still bass weakness. I thought I would see just a bit more fullness from the low freqs. It is tolerable but more is always better.

Gene
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> a certain amount of glow that emits from inside the tube,Medium / Low orange color. ... BUT, With Volume and gain cranked up into the upper regions, The intensity of the glow from inside the tube goes up

Where is "inside the tube"? The cathode glows orange, of course. The plate (the main metal object inside the glass) really should not glow orange, ever. G2 sits inside the plate: if it glows orange so bad that you can see the glow coming out the ends of the plates, your tubes will be ruined very quickly. Orange glow in "empty space" is gas, and very bad, but unlikely that four different pairs are gassy.

What is your supply voltage? No clue on the schematic, except 500V caps suggest a frighteningly high voltage for the EL84 (300V design centre max). That sure would explain "glow", even with "normal" cathode resistor.

> I moved the connection from point A to point B/C and its trashed. I get A Pulsating ourput.
Anything over a minimal level setting ocillates and Pulses like a motoboat on 1 cylinder.


Classic "motorboating". I did wonder why you didn't already have that, with all that 3X12AX7 voltage gain pretty much on one decoupling cap. I didn't analyze the push-pull and pentode effects because it is too humid to think.

You got away with it because G2 had two decoupling stages from the input stage. Moving G2 put you only one decoupling stage away from the input stage. Although an ideal push-pull stage presents a balanced load to the decoupler, real tubes never balance exactly.

Yet it is very unusual to have G2 fed from a "dirtier" point than the plates.

The conventional way would be: Plate and maybe G2 fed from first or second filter cap. You may be OK feeding plates from the first filter cap. Then another cap for the driver, and yet another cap for the input stage. In this case I would run the first two input stages from one decoupler. The driver gets its own. Ideally the G2 screens get their own decoupler, but they can often be run from the plate decoupler. The decoupler for the two low-current input stages should have a Large resistor to improve decoupling: 47K or 100K, or even more.

More bass? Increase the input cap to at least 0.02uFd. Yes, guitar can't go below 80Hz, but 0.01uFd+1Meg will shave a part-dB off the balls of the axe, and that can be enough to notice. And the 0.01 on the dead grid of the driver should probably be more like 0.1uFd: this has to really hold down the dead grid or the output stage goes out of balance and gutless in the bass.

> Way to much Overdrive/Distortion.

Of course. You have way too much gain. Change the driver to 12AT7 or even 12AU7. Fender uses 12AT7 here even for output tubes with MUCH higher drive requirements than the hot little EL84. Also lower value preamp plate resistors are a better way to cut gain than mis-biasing the cathode with a too-big resistor. And a Fender with a similar tube lineup will usually have a lossy mixing stage for a second channel or reverb; you may have to put some fixed loss in there somewhere.
 
Hers More Info:
More bass? Increase the input cap to at least 0.02uFd. Yes, guitar can't go below 80Hz, but 0.01uFd+1Meg will shave a part-dB off the balls of the axe, and that can be enough to notice. And the 0.01 on the dead grid of the driver should probably be more like 0.1uFd: this has to really hold down the dead grid or the output stage goes out of balance and gutless in the bass.

The .01 Is actually already a .1 in the output stage(mistake was on drawing) but not in amp.

I noticed on a lot of amps that there is no cap on the input. There was a .01 on a couple, Others had nothing. I have moved it up as far as .1 without noticeable differences.

I have just tested a Mullard 12AT7 long plate in the driver location and It clearly lessens to viciousness of the amp (Much Nicer) But, alas, the bass is still a little weak. The Amperex EL84/6BQ5's do edge out the Mullards in sound quality in some ways on this amp. The Amperex's are a bit harsher but really lend to the twang factor in a clean sound. You can actually get very close to the old Rickenbacher/Vox combo in brightness with a Strat no less. The Mullards, (both sets) I own are much less defined and in fact seem muddy and vague on detail.
Perhaps the OPT just doesn't render itself well in this application.
The Power Trans is 300-0-300 so it should be just about right as it was a pull from a 60's PP EL84 amp. The OPT was also pull from an old PP EL84 mono block Lafayette amp.
The mysterious pulsing orange glow inside the tube as virtually disappeared. Now the run at a constant glow anyway. They seem less hot, But then again I am not sure how hot is considered to hot.

All in all, it looks like This ones about a wrap, As I am a sucker for punishment , Now I am going to try my hand at a pair of 7868's in an amp.

I Truly appreciate everyone's help on this project. If I could bottle 10% of the knowledge in this groups forum, I would make a fortune on Ebay!

Gene
 
Yvesm said:


... and your OPT somewhat small :xeye:

Yves.

I found an issue, My Work area is a 3 car garage with 10 ft ceilings. I had the amp connected to a 20 yr old celestion speaker that has seen its fair share of action & abuse. The Cabinet was place at least 15 ft in every direction from the walls and of course 10Ft from ceiling.

I took the amp over to my brothers home studio and hooked it up to a couple fresh greenbacks and discovered an entirely different animal.
Its actually way to Loud in a 15X17 room.

I built him a really nice computer recording system a year ago, Dual Monitors, AMD 3200 with huge serial ATA drives really nice studio sound card and we loaded her up with XP pro and Cubase SX.
I am putting together a couple sound clips to post because as it turns out, The End product is rather impressive. On the new speakers the Bass pot had to be reduced to about a "3" out of 10.
Treble was even stronger. Plus, Not having the cabinet setting in the middle of the workshop floor, And moved closer to the wall made a huge difference.

What I really need now is a schematic for any easy to build output attenuator. In order to get some of the meaner crunch sounds levels need to be to high to be in the same room while recording. Personally, I find that rather awkward.
Gene
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
> easy to build output attenuator

Put a BIG resistor in the B+ line.

Try 5K 20Watt between the rectifier and the first filter cap.

This keeps essentially the same conditions in the amp, including the relationship between the output tubes and the speaker (which an audio power brake will upset), but will punk-out (or fuzz-up) at a lower power level.

Also your tubes will last forever........
 
Dang, Thats Sounds way to easy!!

No you have me thinking, A multi position switch to vary the degrees of attenuation, Perhaps low, Medium And High.

That way when we play at our customary Bday Parties or New Years parties at out homes we can set more or less as needed.
We have 3 different recording locations all of which are different sizes. Eh, Maybe maybe just 2 steps would do, But none the less,Great Idea.

I had no idea that was even possible, Though I do I know of a few amps that have power soak devices already built in, I just havent gotten my hands on any schematics for them. Other than my Seymour Duncan Convertable 100 which has a type of power attenuation system allowing 5-100W adjustment. I had never found that one particularly effective.

Thanks TONS!!
Gene
 

anatech

Administrator
Paid Member
2004-06-06 8:31 pm
Georgetown, On
Hi Gene,
Keep in mind those switch contacts will have full B+ on them. Most switches are not rated that high for voltage breakdown. And you don't want the thing to arc to the chassis and kill the power xformer. Of course it wouldn't do this right away, it may take from months to years to happen.
-Chris