Weber 18 watt tmb help trebel and bass very scratchy mid ok, random pops static

[Smitty02]

Hi guys I could use some help with my build its a weber 18 watt tmb kit, wired up good works and sounds good except Ive been trying for 2 days to debug this. Sorry Its late and I dont have any numbers to give, but will post tomorrow. My main issue is that the treble and bass controls are scratchy and noisey through out there travel and it goes into random popping and static that is very annoying but playable ... heaters are good v2 pin 3 i believe is 82v dc, v1 an v2 pin 3 are like 17v dc, B+ seems good, it sounds great except for this I just replaced the caps with sozo's and now it is doing the treble and bass thing before the switch it was prone to these ramdom popping and static but worked ok other than this, I wish weber would give better instructions and voltages on their schemos I am totally baffled could use some of you guy's help. Its my first build went well seemed simple enough except for what they dont tell u like how to trouble shoot and debug issues. I also just replaced all the tube sockets yesterday that went well also but still the popping and static, I made a ground buss wire its not very noisey I dont think its a ground issue I believe its got todo with the tmb channel tone stack as channel 1 works great. I am fried on this I will post some better info in the morning, but any input I would be gratefull for thanks ahead of time scowered the net only found 1 issue like this with no solution

[Jazz P Bass]

Try 'jumping' across the tone control circuit.
Or lifting the ground reference to it.
Either way should eliminate the tone control circuit from the equation.

[Smitty02]

I got it right, re wired the controls today. Trebel control is way noisy channl 1 is fine. channel 2 cntrols all work , now more popping and random fits of static just controls mainly treble noisy as heck I put a 12AU7 in V3 and its useable but may need to tweek the 820 gain resistor to get it unless it somthing else I would think it would work just fine as its right to the schematic and leAD dress is good grounds are good I am befuddled lol any other guys have this problem ?? any input welcome thanks

[Smitty02]

I meant no more random fits of popping and static just real noisy treble control mainly, should I use shielded leads for the treble ? I tried one it didnt help

[olddawg]

I meant no more random fits of popping and static just real noisy treble control mainly, should I use shielded leads for the treble ? I tried one it didnt help

Do you mean noisy as in hiss as you turn it up or scratchy as you turn the control? If it is scratchy, you have DC on the pot. That is interesting since you said you just replaced some caps. Did you get all of the polarities right? If you are complaining about hiss, I find that in these 18 watt clones that putting the best NOS tube you can find in V1 makes all of the difference in the world.

[Smitty02]

It hisses mainly but you can hear the pot travel just a bit as well with the bass control but that is quiet, I replace all the small cap none of the big litics but was just looking at voltages and the 250pf has 92vdc on the tube side and 2vdc going to the pot where as the other 2 022's have 0 vdc going to pots is this normal, I do got crappy tubes in it right now but there not microphonic but I have a 12au7 in the PH as a AX7 is way to much hiss is it possible I have a bad silver mica 250pf ? kit came with sino el84's as well as the pre"s I sold off the pre's useing my backups sovteks wa's tried a vintage rca 12ax7 in V1 but still hiss other than this all seems well Wonder if I got a bad cap some where thnx ol'dawg

[Jag]

The way I read it, you didn't have the scratchy pot(s) before you changed the caps in the tone stack with Sozo's? By the measurements in your last post, I'd say ry another 250pf in the tone stack.

[loudthud]

Sticking a 12AU7 into a circuit that is intended for a 12AX7 can cause problems. The plate voltage of the 12AU7 is too low and that causes grid current to flow, thus the scratchy pots. Check that the plate to cathode voltage of any triode is at least 100V.

[Smitty02]

Shoot will change to a at7 thanks a heap Loudthud. I did not realize this I finished it off after getting it off the bench it did get quieter less interfearance but I thnk some good RIGHT tubes will cure most of it. if not can I change a resistor to drop the voltage to the PH and lower the gain?

[loudthud]

I would change the cathode resistor. With 300V B+ and 100K plate resistor the cathode resistor needs to be about 3.9K ohms. If you want to change the plate resistor instead, make it 47K with a 1.5K cathode resistor. Adjust as needed but try to get the plate voltage with no signal up to about 60% of the B+ voltage. That will give you maximum headroom.

[Smitty02]

Thanks Loud I put a At7 in it and it is just so noisy (treble control) I need to lower the gain for sure. I am not real good with schematics If i post a link to the layout could you show me what I need to do I do way better with layouts or even the schematic is numbered but I can follow it as it is understandable for me mostly heres a link if you could possibly walk me through your changes I would greatly appreciate it http://taweber.powweb.com/store/6m18tmb_layout.jpg and the schemo http://taweber.powweb.com/store/6m18tmb_schem.jpg I dont want to fry anything...my email is bobsmith5566@gmail.com if you want to send it to me.... yes Im a newb I would be most grateful, I can send you a gut shot pic if you would like Im kind happy with it. LOL just wish i was smart enough to understand what you explained Man it sounds great even with the noise on the treble control very harmonic and the dynamics are unbelievable it does seem like it is got a ripple or oscillation with the at7 it rings when cranked Thanks a lot Loud, and any one else that may be able to help this newbe tame his beast. I hope im not being to forward or doing anything Im not sapose to do on this forum, if so it is unintentional.. I read alot of posts here and am learning a ton

[Smitty02]

I think I do have dc on the tone pots ? will 2 volts dc make it act this way? The treble control is way to hissy and the bass is scratchy just a bit as is the treble the mid is dead quiet. I have not changed a thing on the schematic I built as is with their values . all wiring is correct I put a .0022 cap in place of the 250pf just for now to knock the hissy treble down and noticed pots are now more scratchy but less hiss ??? Loudthud which resistors are you refering to on the schematic Power tubes or pre's ? thanks you guys

[Smitty02]

Major break through

Ok Ive racked myself on this build trying to find the reason for the way to hissy treble and a bit of dc on the pots . Here is what I found and would like to know if you guys think ? if it is not a good solution yet it worked perfect I was exploring the tmb channel tone stack the 250pf C14 had 5vdc on it the resistor right next to it goes to ground it a 1 watt 56k R24 I pulled it and checked it it was not up to snuff, it gets hot and when I pulled it and measured it it was not 56k it measured like 48k hot after it cooled it came up to about 50k, I added a 5 watt 10k resistor as I didnt have anything bigger at 56k that would take heat better it had/has 78vdc on to ground. Any how the amp settled down no more random ticks and pops which was a issue but not a real bad one as it was mild, and it solved the tone control issues. It is now quiet, well acceptably quiet considering the gain it has, it wont embarase me as it was nasty before. Can anyone check the schemo for me and tell me if this will be a problem http://taweber.powweb.com/store/6m18tmb_schem.jpg heres a link I added a 10k 5 watt to R24. Thanks guys and please post if you think it is not a good idea Thanks again