Tweed Overdrive Special (continued)

[daveyajd]

I have another pair of 6L6 that I tried. They are not matched but there was no improvement when I installed those. I can't imagine that I bumped another component. The resistor still measures about 360 and the cap seemed to still be good though I switched that out just to check. I guess I'll have to start a process of checking but I'm not sure where to start.


Hey Glenn, I will try that cap test. How TEMPORARILY?
The answer to your question is simple. I was tweaking for amp feel. The scheme called for 430R and 147uf and I was just trying to see how the amp feel would change if I brought the cathode section up to the schematic specs. I changed the private setting on the clip so you should be able to listen now.

[daveyajd]

Tried pins 3 jumper cap. Did not affect anything.


Aaron

[Glennjeff]

Temporarily, like put it on test it for 10 seconds, then take it off.

Got that sound clip, that is an oscillation / motorboating problem for sure. Could be a dozen reasons for it so........

Try these as  TEMPORARY diagnostic mods (change, test, unchange). Do them one at a time.
Be especially carefull that you turn off, remove from wall socket and allow time for capacitors to discharge between each step. ( Safety First Always, no running red lights in a panic to get to work on time )

Mod 1 )  Remove the 4.7 k Ohm NFB resistor from the output transformer secondary tap. The amp may be much louder and more distorted so plan the test accordingly by lowering all volumes.

Mod 2 ) Put an alligator clip test lead to the FX return loop plug tip connection and ground the other end. The amp should make no sound of any kind (except maybe an almost inaudible little bit of hum)

( Or to achieve the same effect, remove the lead of 0.022 coupling capacitor to V4 pin 2.
Not on the tube side but on the side of the FX return send, and ground that cap lead. )

Mod 3) Do both 1 & 2 at the same time.


What happened.

[daveyajd]

Ok, I tried those diagnostics and here is what I found. The first thing I said was "What 4.7k resistor?" I looked at my schematic and my build and discovered that I had never put that in. I had a wire going from the OT secondary to the NFB.


So I put that resistor in and it helped control the noise but it was still very present.


I then grounded the FX RETURN tip and there was in fact, no sound from the amp.


My voltages are all pretty much the same although the PI voltages are at about 191v up from 176v. The other thing I noticed was that the buzz was greatly reduced when I put my probe on V4 pin 7.


Aaron

[Glennjeff]

I was a little ambiguous. I wanted you to lift the 4.7 k resistor so there was no connection between output transformer and NFB.

The changes in voltage on PI are suspicious. Have you tried changing that valve ?

Check that all components in ORANGE are secure, well soldered and don't have any stray bits of wire or solder connecting them. You may have fried the 47 ohm to ground in the PI having the output transformer connected directly to it. (GREEN)

Try the RED and BLUE mod and tell us what that results in, should be nice clean sound.

(See attached diagram, ignore the red box near output tubes cathode resistors)

Another good trick is to take a dry piece of wood or non conductive plastic, like a chop-stick or drum stick, and lightly tap around the circuit with the amp going, being very careful not to let cats or children jump into the amp. You can often find faulty connections or components that way. ALSO move any wires around with the drumstick and see if that changes the weird noise.

[daveyajd]

I was a little ambiguous. I wanted you to lift the 4.7 k resistor so there was no connection between output transformer and NFB.

The changes in voltage on PI are suspicious. Have you tried changing that valve ?

Another good trick is to take a dry piece of wood or non conductive plastic, like a chop-stick or drum stick, and lightly tap around the circuit with the amp going, being very careful not to let cats or children jump into the amp. You can often find faulty connections or components that way. ALSO move any wires around with the drumstick and see if that changes the weird noise.

Yes I also tried lifting the NFB completely. Still a very bad sound. I did try another PI tube. The sound was unchanged but I did not take measurements on it.


I also tried pushing things around with a stick. When I push down on my ground bus right around the FAT pot the buzz goes away (almost) but I still get the nasty motorboat/squeal.


I will check and try the changes highlighted in your diagram tonight and see what I can figure out. I may also remove that 47 ohm resistor and take a measurement on it. Thanks for your help. I will report back.


Aaron

[Glennjeff]

Re check and post all voltages if possible, especially around the PI. That should not have changed.
Sometimes components can measure OK but be faulty in some way, although it is rare in resistors.

Try shorting out that resistor, 0.1uF cap of pin 7, other side to 47 Ohm, instead just ground other side (PURPLE).
Swap that 0.1uF cap if possible also.

The fact that touching pin 7 with probe reduced buzz, and that voltages have changed in the PI, suggests that region has problems.

[daveyajd]

So I started poking around my grounds and found that the ground from the relay was not cutting it. Resoldered it and a couple in that area and that made a big difference. It is actually playable again now. There still seems to be a bit more noise and a few weird oscillation type sounds as I fiddle with the knobs. I won't be able to give it full out test run until tomorrow and then I will take all the measurements and look closer at the PI section because I think there is still something going on there. Thanks for your help Glenn and I will keep you posted.


Aaron

[tubenit]

Aaron,

Glenn has been giving you excellent advice in trouble shooting!  Keep working with his suggestions if you're not satisfied the issue is resolved yet.  Truly sorry to hear your fine amp has developed a glitch in the tone.  You will get it fixed! 

With respect, Tubenit

[daveyajd]

Alright, here's where I'm at.


Checked all components and solder joints in the circle and replaced the .1 cap. Voltages came down a bit to 180v and the hum reduction when I touch pin 7 is gone. I do have a few more things so I will address them one at a time.


There is  a hiss when the PPMIV is rotated to a few spots. I attached a sound clip for a listen.
https://soundcloud.com/daveyajd/clean-channel-hiss


The PPIMV is just backed off from full. (all the way up and there a hiss unless I have the clean channel level all the way up)


I play a little something, then I turn the PPIMV down until you can hear the hiss come on, then I put my meter probe on lug 1 and the hiss goes away but the guitar sound is unchanged.


I did not use shielded cable from here to the .047 caps. Would rewiring this help?


One other strange thing I noticed is that there is a lot more noise when I use a 4.7k resistor on the NFB versus just connecting it with a wire only.


Aaron

[Glennjeff]

Try once again. disconnecting the feedback, note what happens, reconnect feedback 4.7k and place .001 - 0.01 across anodes of output tubes, note any changes. Sorry, but these diagnosis things can be a bit circular.

It is definitely WRONG (nonsensical) to have output transformer connected directly to NFB without a reduction resistor (4.7k)

It does sound like a NFB oscillation problem but I'm not there with a CRO so I can't do much more than make suggestions based upon unfounded assumptions. These amps are as quiet as the dead when working correctly. Sorry for the somewhat macabre reference but they are so quiet, I as a technician often panic when I first switch them on coz there is nothing.

The THING is, you just have to keep trying different solutions. It is a learning experience, and when you finally beat a tough one , you feel really good, "empowered" to use a modern terminology.

I happen to have a thousand yards of lightweight shielded cable, I use it for everything. My amps are noise free. Try shielding or twisting the cables that connect PI to PA. (I don't think it's that coz you get no changes poking lead dress with a chopstick)

Did you try shorting the 47 Ohm resistor to ground to see what effect it had.

Was this recording done with the suggested RED/BLUE MOD which bypasses all switching and connects clean preamp directly to PI.

What is the voltage on the PI cathodes (42 V whatever, what is it)

My strategy here is to try and isolate the faulty LOGICAL BLOCK (Preamp Clean, Preamp Dist, PI, PA, NFB). Unfortunately with Oscillation Instabilities they can be occurring in the Radio spectrum, way above audible frequencies, and, just like radio they go everywhere and are difficult to isolate without the correct test equipment.

I am willing to stick with it if you are Aaron, (Speaking of sticking, nice picking by the way)
Glenn.

[tubenit]

Again,  follow Glenn's advice!  I think he is heading you the right direction.  Stick with it, you will get this resolved.

With respect, Tubenit

[Glennjeff]

I just had an idea.

With the amp wired as per diagram you should be able to plug your guitar directly into the effects return and get a "bedroom comfortable" sound. I just did it with my very similar amp and it is quite a nice clean sound. Like a 2 watt amp. What happens with your amp when you do this.

[tubenit]

I just had an idea.

With the amp wired as per diagram you should be able to plug your guitar directly into the effects return and get a "bedroom comfortable" sound. I just did it with my very similar amp and it is quite a nice clean sound. Like a 2 watt amp. What happens with your amp when you do this.

Good idea, that can further define where the problem is.   

As a side note, playing an acoustic guitar with a very slight delay thru the effects return can actually sound pretty good with an amp like this.

With respect, Tubenit

[daveyajd]

Again,  follow Glenn's advice!  I think he is heading you the right direction.  Stick with it, you will get this resolved.With respect, Tubenit

Oh, I plan on sticking with this all the way. I'm hooked now!    And everyone here has been awesome throughout this adventure.


It seems like this is almost a different amp today. Very playable and no real oscillations. Changes her attitude on a dime. I guess I should give her a name!   


So I disconnected and recounted the 4.7k NFB. Much less noise with the resistor in. (As it should be) So now that is good. I then put  a .001 cap jumper the across the tube anodes. I didn't notice much of a difference but if there was any it seemed to be a bit noisier.

With the amp wired as per diagram you should be able to plug your guitar directly into the effects return and get a "bedroom comfortable" sound. I just did it with my very similar amp and it is quite a nice clean sound. Like a 2 watt amp. What happens with your amp when you do this.

I did plug in to the FX return and that is a very cool trick. The amp is sooooo quiet that way. That is the noise level I want to achieve through the main input.


The recordings last time were done with the switching relay connected not using the red/blue mod.


I attached a new voltage chart and a couple of recordings of the clean and OD. On the recordings you hear me talk, then play a bit, then I turn each knob up full then back down to half one at a time so you can hear the hiss added by each pot.


Since everything sounds much better I would love to know how to go about eliminating that hiss?


https://soundcloud.com/daveyajd/clean-channel


https://soundcloud.com/daveyajd/od-channel

[Glennjeff]

OK, you have made progress.

The fact that there are no problems when you plug into the FX return suggests that the PA, PI and NFB systems are all OK.
The amp will not sound that quiet once the preamp is in and the guitar is plugged in and turned up.

What happened to the horrible wailing, moaning and farting sounds we were listening to a couple of days ago ?
I thought they maybe had the makings of the latest trend in modern music
I hope they stay away forever personally, I'm a bit old fashioned.


Noise ( normal  hum and hiss) assessments are only realistic if the PPIMV is set to full volume.

When the guitar is NOT plugged in there should be a noticeable hum and hiss on the CLEAN channel when all controls are set to full throttle. What do I mean by "noticeable" , much louder than a cats purr-r-r-r-r-r-r-r-r, but nowhere near as loud as a cat's meow.
(That is the only way I can express it in somewhat realistic everyday terms)

The CLEAN channel should be basically inaudible when all controls are set to 2 o'clock with the guitar not plugged in.

Just to clarify, where is the PPIMV set during these recordings. I would suggest turning all you knobs to about 2-3 o'clock with the guitar plugged in but turned down to like 1/8 volume (so that it doesn't howl and feedback) and then adjusting the PPIMV to make sure it is stable at all settings. Then turn all controls to full and adjust PPIMV making sure there are no signs of moaning / motor boating. Moaning and Hissing are very different things. Moaning can be very high pitched and sound like hiss but it is very different, it has a definite note ( fixed pitch or pitch bending up and down) buried in it.

There should be no hint of oscillation (moaning / wailing sounds), and no put put putter fart motor boating at any gain settings really, but at everything on spinal tapping 11 some amps may never be absolutely stable.

All the noise I am hearing on those recordings is coming from the clean channel, they are just made worse by the extra gain when overdrive is engaged. I am not sure if they are terribly abnormal although they do sound a bit too load to me. Depends once again on where the PPIMV is set.

Could the wailing / motor boating  have been due to the lack of 4.7k NFB resistor and we got side-tracked / knocked of our rails inadvertently chasing ghosts. Possibly.

What type of resistors are you using around the first valve.

Now that was very long winded wasn't it. The actual sound of the amp is GREAT. What loudspeaker are you using.

If you could do those assessments and get back to us it would be helpful. We all love to hear others play on the amps they build so record some more with PPIMV at full tilt. If it's anything like my D'Mars you may need to put earplugs in and warn / bribe the neighbours.


PS: Ken Fischer of Trainwreck fame had his reasons I'm sure.

[tubenit]

Overall, your amp sounds good to me & I like the tone of it.  OD channel tone is very nice.

My opinion is the Dumblish type builds, the Tweed BluezMeister, the Tweed Overdrive Special and the D'Mars are not designed to have all the potentiometers dimed. 

And some of the real Dumble and Dumble clone videos I've watched left me thinking that some of those amps have waaayyy more hiss then I'd be comfortable with personally.

I don't play my TBM or the D'Mars with the volume knob (clean channel) past 5. It's usually closer to 4. My D'Mars is the quietest amp at idle that I've ever owned.  It is considerably quieter at idle then the original Princeton Reverb or the Carvin amp that I owned even with the D'Mars at a louder volume then the PR or Carvin.

The TBM and the D'Mars is quiet enough at idle (at the volumes I normally play at) that one might enter the room and not realize the amp is on.  (on clean channel)

Having said that,  I have no doubt that IF I cranked pots to 10 that I'd have unwanted noise/hiss.

I use a 12AY7 in V1 in both of my amps.  And a 5751 in the OD channel.  My current D'Mars has a 5879 in the OD also.

I use Vishay Dale RN65 resistors on the plates for the gain stages.  I think those may add to helping eliminate noise/hiss. Note those on 2, 4, & 6 turret rows in picture.

I'd recommend using a 12AY7 (Doug carries EH 12AY7's that I like) or a 5751 in V1.   You can also put a 220p cap from pin 6 to pin 8 on V1b to lower hiss.

With respect, Tubenit

[tubenit]

I would also like you to note that I did the cap foil orientation on my turret board for the D'Mars.  I am convinced that greatly helps reducing noise. 

http://el34world.com/Forum/index.php?topic=11427.0

The "enhance cap" across the LTPI entrance plate resistor helps ALOT also in reducing unwanted noise in my experience.

I have plenty of sustain for my playing in my amps with just the clean channel using a 12AY7.  I don't think one needs a 12AX7 in V1.  If you don't want a 12AY7 then use a old stock or NOS 5751.

This sound clip is the D'Mars with just the clean channel only using a 12AY7.  Notice that there is no noticeable hiss at all.

http://www.soundclick.com/player/single_player.cfm?songid=12595832&q=hi&newref=1

This sound clip is  repetitively obnoxious but shows the sound differences & contrasts between clean and OD with the Tweed Overdrive Special I had. It starts with clean to slight overdrive to more overdrive.  IIRC,  this had either a 12AY7 or 5751 in V1 and then a 5751 in V2?  The amp was not noisey at idle.

http://www.soundclick.com/player/single_player.cfm?songid=11332809&q=hi&newref=1

With respect, Tubenit

[daveyajd]

Yes the recordings were done with the PPIMV all the way up with the exception of the last bit on the OD channel recording where I turned it down to reduce hiss and check for any oscillations. The cab I'm running through is a two twelve with two vintage Jensen P12Q or P12N (can't recall which) installed.

My tube line up right now is RCA 7025s in both V1 and V2, JJ ECC81 in PI and JJ 6L6GC PTs.

Could the wailing / motor boating  have been due to the lack of 4.7k NFB resistor and we got side-tracked / knocked of our rails inadvertently chasing ghosts. Possibly.

What type of resistors are you using around the first valve.

All of my resistors are metal film and did research and test all of my Mallory 150s for outer foil and orient them correctly. I also have the 220 enhance cap across the LTPI as per Tubenits suggestion. I may put one on V1b pins 6-8 .

It's interesting to me that I had finished my build and played with it for a week or so and it was dead quiet. I mean REALLY quiet on both clean and overdrive with the knobs dimed. And that was with the NFB connected without the resistor. All of this hiss really started after I tried to mess with the PT cathodes. I will try setting knobs as you suggest and get a recording of each channel like that.

I know that this can be quieter so I am working towards that. But it is playable again now and that is a great thing.

Aaron

[tubenit]

It's interesting to me that I had finished my build and played with it for a week or so and it was dead quiet

Yeah, if you had it dead quiet ........ I would definitely invest the energy into getting it back to that.  Just remember what you originally did was successful so you can recreate that.

with respect, Tubenit