Weber 18 watt tmb noise and bias

[Smitty2]

Hi all Just joined up. Like your forums..Could use some help so here I am. I did a weber 18 watt tmb and I built right to the layout weber did not send correct tweaking resistors for me to try to adjust some things.. Any how I am a at home techy that helps alot of buddies out for meager profit, Ive learned so much and still am ! But I built a weber 6m18 to spec layout no tweeks as on schemo amp is making a alot of noise on the tmb channel like specially the treble control. white noise hiss I am good witha iron all joints and components are correct.. Also my bias voltage at the bias resistor is 15v its a 100ohm 5 watt it is real hot eating tubes. The Plate v is 350, pin 9 screen grid is 335 pin 3 cathode is 15.2 I think i need to bump up the resistor to 150 or more ? this is the layout and schematic This is got me stumped and I could use some advice and input My lead dress is good and i used shieled leads where needed I got some pics of it if anyone would like to see I will send them to you I had to put a 1k in paralel at R24 to quiet it down a bit it worked real good at first but now it is back (the hiss) as for the bias, I could use some help to cool it down there is no drop across the 100 ohm 5 watt bias resistor is this right ?? Really could use some help guys Thanks i have read some of your info already but could use more specific details as I am NOT a pro but no noob either Thanks again for any input you could share to get me right....Happy New years btw

[Phil_S]

If you have a 540V secondary running through a 6CA7 (which the copper cap emulates) it should give you right around 350V B+. There should be some modest voltage drop across the output transformer making plate voltage a little less. IMO, 350V is too high for this model amp and for EL84's in general. It would be better to get plate voltage closer to 300. Are you using the 120V primary tap? If yes and you change to the 125V primary tap, B+ is likely to drop by about 15V and plate voltage, too.

Plate voltage alone could be accounting for short tube life. It won't hurt and might help quite a bit to raise the bias resistor to 150, 180 or even 220. Given your voltages, I'd skip the 150 and go right to the 220. You can experiment and see what happens. By my reckoning, you have this amp idling at about 25W static plate dissipation, which is way too high. Do this and see what happens.

You say there is no drop across the 100r bias resistor? How can this be? You should be grounding it, making the tube cathode side at 15V and the other side at 0V (ground), so you have already told us the drop is 15V. Just what are you doing to measure this? Is your meter set for VDC?

I think you need to remove that 1K parallel to R24. The TMB tone stack is configured as a cathode follower and it won't drive properly with such a low R value on the cathode.

I suggest the first order of business is to correct the bias problem and drop plate voltage if you can. After that, if there is still noise on the TMB channel, that comes next. Regarding the treble control affecting the hiss, it may simply be that cutting the treble is masking the hiss and that isn't where the problem is originating.

For the record, I'm not a pro tech. I am an amateur, but I have built a TMB18W and am familiar with this amp.

You might consider posting your problem at the Weber forum and see what you get there. You might also consider signing up at 18Watt.com, but be forewarned, due to a legal disagreement with Weber a number of years ago, they won't discuss Weber products (the moderator is likely to remove your post) and I suggest if posting there, you don't mention the brand.

[Paultergeist]

I would like to add another suggestion on top of Phil's excellent response:

It has been several years, but I built one of these amps (6M18[TMB]). The "tone stack" channel is rather high gain, and this thing is very sensitive to lead dress, etc.

The schematic shows an "optional 470K-1Meg resistor" R40 going from the tone stack to the control grid (input) of V2B. In my case, this resistor was not "optional," but rather needed to be there or the thing had hiss and too much sizzle -- I used 1meg -- the amp still has buckets of gain. Adding R40 made the hiss go away. Note that this R40 resistor is NOT documented on the layout, but rather is only present on the schematic. If you have not already incorporated this resistor, I strongly suggest that you give it a try.

Best of luck.

[M Fowler]

Good advice given already.

I would go no lower then 150R/5 to 10watt with 220uf/50v or 250uf/63v bypass cap to get the bias range right.

A lot of 18w amps run 580vct PT so if your at 540vct that should be a good HT voltage level with tube recto.

Really not sure why Weber spec's 100R bias resistor? His screen resistor choice is odd as well.

Mark

[Smitty2]

Thanks for your help guys. I will make some changes see if i can reign in the bias and try your suggestions. The 18 watt site isnt letting no one sign up i guess ?? I tried.. Ive researched alot of weber builds and they all look older and different. That is how I ended up here reading one of your posts not just pertaining to mine but i like to soak up what i can on all different levels I did notice that while checking voltages I seem to notice that when i checked the 2 470k voltage divider? from V2 pins 2 & 7 with my meter it qwelled some of the niose specially on pin 2 pin 7 made some funny noises but also quieted the hiss and there voltages were about 42vdc and 68 vdc on 820ohm to pin 3 The "optional" resistor for the tone stack didnt come in the kit but would I put that in series with the shielded lead to the middle treble lug ? I know stupid question.. but i dont read schematics real well and that is the way I see it. Am i wrong? I am going to go buys some parts guys Thanks so far for the help very much Ill post my finding As to the voltage across the 100 ohm having no drop I mean before it gets to the tube. From the cap can to the resistor to pin 9 on the El84 resistor does not drop the voltage at all it is 335vdc from the cap can through resistor to the el84..I am getting the 350vdc from the output xformer to the El84 plates.. and also my kit did not come with a HT fuse at all either. Hope I didnt confuse you to much on these details sorry if i did As I said I am not real savy with some of the info that is why i posted the layout an schemo if you could tell me where on them i will better understand... Thank you guys a ALOT so far i do understand I think except for the 1 meg optional location I did notice I am getting some AC leakage into the tmb tone stack about 4.5VAC an thought that is what the issue was but when I added the parell resistor on r39 it got rid of it but it has come back now. It was quiet and sounded great, vicious but not much head room at all even on channel 1.. Peace guys thanks

[Phil_S]

I wondering if the copper cap is functioning as one might expect. Looking at Weber's spec, it says it will drop 28V at Imax. Imax = 150mA. The 18W amp probably draws somewhere around 110mA in actual operation, suggesting to me that it will not drop the full 28V. If you figure this on a straight line basis, it might drop 21V, but I'm thinking it is not a linear device, though I have no way of knowing one way or the other. It may be a modest factor in the resulting plate voltage.

I see the recommendation for R40. The other thing it says is to try a 12AT7. I'd also try a 5751 and a 12AY7. These are all "freebies" if you happen to have the tubes

Ordinarily, for hiss, I'd suggest installing 1W or 2W rated plate load resistors. Unfortunately, for a cathode follower, there is no plate load resistor. Maybe a 1W or 2W rated cathode resistor will help? It might be worth a try to experiment with 47K and 68K in place of 56K, I'm not sure. That assumes the cathode follower is the source of the hiss. There are several other tubes with 100K plate load resistors that are also possible targets.

In any case, the problems described seem like they can be addressed in pretty short order.

[Phil_S]

Smitty: you and I were posting at the same time. I just saw yours.

The 18W site recently emerged from a site software management problem and I understand they are admitting or will soon admit new members.

The pair of 470K on pins 2 & 7... you are referring to R8 and R7? These are grid leak resistors. They provide a ground reference for the phase inverter. You say that probing those killed the hiss? I'm not sure what to make of that, but maybe you have a cold solder somewhere near there resulting in a faulty ground?

The optional R40. This goes directly on the wiper (middle) lug of the treble pot. Connect the other end to C15 (.022uf cap) You shouldn't need shielded cable here.

There is no drop between the copper cap rectifier and B+1 because that is the direct connection of the rectifier to the first filter capacitor, C8. C8 connects at point A, the center tap of the output transformer. The OT should cause some modest drop in voltage at the EL84 plate (pin 7.) Maybe it is so little you don't see it.

4.5VAC at the TMB stack could be B+ ripple or it could be leakage. Specifically, at which component? R39? Figuring that out will take some poking around with the meter to see where it originates. Sometimes new filter caps take a brief amount of time to begin functioning as they should. Give this a rest for a while and work on the other stuff.

[M Fowler]

Yeah might need to run a real 6ca4 rectifier tube for some voltage dropping.

[ChrisM]

If your B+ is about 350V you want a 150R 5W bias resistor.

Screen dropping resistor should be 1.5K, PI dropping resistor should be 8K2 and the preamp dropping resistor should be 2K2.

[Smitty2]

Thanks guys ...Yes alot to process will check for bad joint on the 470's Ok dumb question but I am referring to R11 100 ohm 5 watt as the Bias resistor am I correct here ? not the 500ohm cement resistor (I used a 500 ohm instead of the 2 250ohms they sent me) tied to the 150v 100uf cathode cap to ground that I am reading 15vdc bias current on.. The amp has been built for about a month and I used all sozo caps save for one that was the wrong one sent in the order *eber isnt very good at filling orders R24 is the resistor I had to add a paralel 1k to and that made the hiss dissapear and all was well until now but the 15vdc bias was still there when I built it and i new it was high just not how bad it was really but Ive played the heck out of it and then it started acting up. I realize you guys are much sharper than me, thank you for your help but if you could refer to the specific locations on the schematic I will learn more as I am still not up to snuff on some of your sugestions but will take some time and step back and learn I do get the 1 meg on the trebel wiper but all i have on hand is 1/4 watt will that work for that ? and yes I think you may be right about the copper rectifier got a tube coming soon. Seriously thinking of buying all carbon comps and sprague caps for it Wife is bitchin at me got to go LOL thanks a bunch all of you Here is another pic should it be any help

[ChrisM]

R9 and R10 are the bias resistor.

They put the two 5W resistors in parallel to get a 10W resistor of the correct value.

[Richie]

First they show a .1uf cap on the 18 marshall channel off the tone pot on the layout, but show .01uf in the schem. the .01 would be the more correct one for that.

The 150ohm cathode resistor should be used to see what that gives you.
Also these amps do run the crap out of the EL84s, Most try to use the Sovtek 6pn14 or more 7189 type.. JJ etc..

Also if the amp seems to have too much bass, or bass control doesn't do much change the second .022 cap to a .01uf cap. the one going to the middle pot.
Also this amp doesn't have any resistors going into the cathode follower from the volume pot. They give you an optional 470k over in the PI section to reduce drive into that tube.
But also since this amp does not have a pre amp gain control. just volume, there should be a couple resistors where the wire feeds back into the TMB channel tube. One resistor will be inline, and another will be to ground.
Its a voltage divider. And you can play with values to get what you like. But to start try using 2 470k. and go from there. This should help.
Also Be sure if only using one channel, to keep the other channel volume control turned down.
The 50uf bypass caps, could probably be 25uf, and cut down some hiss.
The tone slope resistor is showing 56k.. you can try 47k 39k 33k
And the treble cap.. from 250pf to 330,390,470 or 500pf. Whatever you like.
Sometimes depends on what speakers you use how you might want to tune that section.
And i guess you could use another 470k or resistor as the optional one they list. Then too, it could be different values depending on what your looking for,or how it effects to sound. As others mentioned, lead dress, down to trying different tubes in the preamp, some may be more quiet,less noise.

[Smitty2]

Thanks Ritchie and Chris ...and every one else I do have a grasp on what i need to do now. Hopefully I can figure it out think I got a good shot and will give her a go But one last dumb question What is the purpose of the 100 omh resistor R11 100 ohm 5w... if it is not the bias resistor and Ive been seeing mentioned that I need at least a 150ohm bias resistor, if R9 and 10 are the bias resistor(s) it is already 500 ohms? the 100 ohm 5 watt in R11 drops no voltage at all ?? what does it do? I guess is my question LOL Thanks guys I know I trying to learn some stuff and thanks yous for being patient and dumbing it down for me.

[Paultergeist]

R11 is the screen resistor for the power tubes. The purpose of that resistor is to drop the voltage from the (B+) power rail -- you want the screens to be "positive," but a bit less positive than the plates (anodes).

[Smitty2]

Ok thanks ...It is 335vdc on the screen grids. 350 on the plates. If I have a 500ohm 10 watt bias resistor will lowering the value, lower the voltage on the plate? Everyone is saying to use a 150 to 250 ohm it is currently a 500 ohm resistor here, does it need to be 10 watt My main goal is to get the B+ down from 350vdc as this will in effect drop bias voltage also..... correct now Im confusing myself lol I need to go back through and re read some of the posting you all sent me.... thanks Paulr