No Bass Control

[Trout]

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.

[anatech]

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]

> 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.

[Yvesm]

Hi Gene,

Have a look at :

http://www.duncanamps.com/tsc/index.html

Yves.

[Trout]

Originally posted by PRR
> 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

[Trout]

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. 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]

Is this a normal phenomena or am I about to get a lesson in 101 ways to fry valves ?

Looks like

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


Yves.

[Trout]

Originally posted by Yvesm


Looks like

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


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

[Trout]

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

[fredl]

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