hard to play amp - cure ? Clip added

[ayan]

Unless I've missed something, I believe no one has mentioned the fact that you don't seem to use shielded cable anywhere... or perhaps your pictures are dated and you've already changed that. At any rate, using shilded cable in strategic places (as per the real thing) will gradually subtract some of the high end such that you may end up where you want.

Cheers,

Gil

[mat]

Ceramic is fine Mat.

Ok!

Make sure your lead dress around the sockets is neat. Check around V2. Move the V2b plate wire next to the cathode wire and the lead going to the back panel too.

Already as mentioned

Also, do you have a 47pf on the clean master? You must have one there with HRM amps. Try 33ma-34ma on the bias.

47pF on the master. I'll try hotter bias.

There is about 2ma of screen current so your real bias point is lower.

Ok, good to know (always learning).

.02uf output grid couplers will soften the attack. They are essential with the Marshall output section.

.02's in there with BM PI.

Lastly, add a 33uf 450V filter cap to further decouple your loop circuit.

I already have filtercaps on power board for both sides of the d-lator as in: http://www.pichotel.com/pic/16028B31e/148313.jpg

Reduce your loop dropping resistor by a little and add another resistor.

I have 40k and 39k in there.

Change the Dumbleator recovery amp cathode bypass to 10uf. Change the bootstrap on the cathode follower to 27K and 1.8K from 10K and 1.5K.

Ok, will do that.

If you have reverb, make sure the transformer and recovery are also decoupled with their own caps. This is all important.

No reverb (yet). Big thanks for the tips

That sounds a tad cold to me but I don't have a bias calculator here at work. The "barking" may be the crossover distortion jumping in. Make sure to get the bias up to about 70%.

Ok Bob-I, will readjust the bias.

If you have 488 on the plates you should be biased around 35mA, if you're running the JJ E34L than you should be closer to 40mA.

Yes funkalicousgroove, JJ's in there. I'll try 38-39.

If Certain notes are really Jumpy you might consider going back to the 8ohm tap for your NF and installing a trimmer in series with the NF resistor and adjust it until it's liveley but not disproportionate.

Would You suggest doing this with BM style PI ? I have 100k + 25k trim (dialed to zero) going to the 4ohm tap now.

Unless I've missed something, I believe no one has mentioned the fact that you don't seem to use shielded cable anywhere... or perhaps your pictures are dated and you've already changed that. At any rate, using shilded cable in strategic places (as per the real thing) will gradually subtract some of the high end such that you may end up where you want.

Sorry Gil, older pictures. I have coaxes on 6 spots in there. It really helped the amp to sound better when I did put them in.

Thanks everyone !

Btw. I bought today 3 (high quality) noval sockets. I'll put them in as soon as I have the time..

[dogears]

Mat,

I meant that you should add another 33uf cap to the loop to feed the caps you have now. Maybe lower the 40K to 30K, then add a 10K in series. At the junction of the 2 resistors, add the 33uf. That will isolate the loop from the rest of the B+. It seemed to help my tone. Kind of like powering the loop with a seperate power supply. It is easy to try! Just tack something in temporarily.

I have 225V on my recovery plate and 260V on my cathode plate. You should feed the loop from the screen supply. Seems you are not based on your dropping resistor sizes....

[kleinm]

Make sure your lead dress around the sockets is neat. Check around V2. Move the V2b plate wire next to the cathode wire and the lead going to the back panel too.

I gave this a try on my amp like you recommended. Sure enough, a subtle but noticeable reduction in the noise floor AND a gently smoother tone. Worth the 5 seconds, for sure!

[makrisp]

A) Drop the voltages @ the power tubes to about 400 - 420 volts

B) Increase the negative feedback resistor. (Tune it with an adjustble pot until the amp sings.

C) What are the voltages in the PI section? (Should be almost perfectly balanced, volts @ 300V each side)

[mat]

A) Drop the voltages @ the power tubes to about 400 - 420 volts

Interesting, I've been wondering how the amp would react with lower voltages on the power tubes. Btw. how can I do that ?

B) Increase the negative feedback resistor. (Tune it with an adjustble pot until the amp sings.

I have 100k in series with 25k trimmer. I change the trimmer to bigger value.

C) What are the voltages in the PI section? (Should be almost perfectly balanced, volts @ 300V each side)

I'll post the new measurements tomorrow (at work right now).

Thanks makrisp,

[makrisp]

A) Drop or increase the voltages to the plates of the power tubes by increasing or decreasing the plates (meant power resistors) resistor values. (Employ top notch resistors with high wattage ratings)
A') Do not forget to rebias every time there is an adjustment, as it will be off.

Ohm's Law V=IR
V=Voltage
I=Current
R= Resistance

B) The same applies to the preamp tubes, increase voltage to the preamp tubes will increase gain, decrease voltage to the plates of the preamp tubes will make the stage sound browner and distort earlier. Same goes for the power tubes.
If you are looking for a Robben Ford type sound, the amp will need to run on lower plate voltages (180-190), where as if looking for a more 80's dumble sound higher plate voltages are needed (200-210volts).

C) Do not be fooled by the circuits posted, they are a good starting points. The voltage & current are what really matter. Adjust section by section in the amp w/ your ears. Stick w/ the same guitar and speaker setup when perfecting the tone.

D) The PI and NFB loop are critical spots to spend time customizing.
Balance that PI perfectly, or as best you can.
D') When adjusting the PI , rebias the power tubes everytime - do not forget!

E) Employ top notch tubes. (The TAD 6L6's are awesome, if you can't get NOS tubes).

F) Keep the amp on the bench for a few weeks, come back listen and make adjustments, do not rush, walk away and come back the next day if you get frustrated. Change one component at a time, so you can understand its impact.

G) You will not be dissapointed.

[heisthl]

A) Drop or increase the voltages to the plates of the power tubes by increasing or decreasing the plates resistor values. (Employ top notch resistors with high wattage ratings)

I've like to see what schematic has Plate resistors on the output tubes????

[dogears]

Mat, don't do anything until you post the latest measurements. Also, decoupling the loop.....

[mat]

Mat, don't do anything until you post the latest measurements. Also, decoupling the loop.....

Ok, here they are:

B1 476
B2 473
B3a 372
B3b ?
B4 452
B5 340
B6 325

V1 1=197,8 3=1,875 6=201 8=1,814
V2 1=205,5 3=2,035 6=212 8=1,878
V3 (d-lator) 1=317 2=21 3=41 6=248 8=1,861
V4 (PI) 1=252 2=35,3 3&8=55,1 6=241.5 7=37,4

V5 3=479 4=474 5=-40
V6 3=479 4=474 5=-40

choke-2k2-22k-2k2-150k(to fet)

After putting the 22uF/450V decoupler in I noticed the sound to be more clear which is a good thing but the basic problem still remains. Also the mid pot still does nothing while PAB off

[dogears]

You have much too much voltage on the recovery amp of the loop. Shoot for about 220V.

Obviously, you have an issue with your tonestack. I bet when you fix it, the amp improves

[makrisp]

A) Drop or increase the voltages to the plates of the power tubes by increasing or decreasing the plates resistor values. (Employ top notch resistors with high wattage ratings)

I've like to see what schematic has Plate resistors on the output tubes????

Let me re-phrase and thank you for the correction as that is what happens when you rush w/ statements:

The power resistors related to the intial power caps, or initail dropping resistors.

[makrisp]

Mat, don't do anything until you post the latest measurements. Also, decoupling the loop.....

Ok, here they are:

B1 476
B2 473
B3a 372
B3b ?
B4 452
B5 340
B6 325

V1 1=197,8 3=1,875 6=201 8=1,814
V2 1=105,5 3=2,035 6=212 8=1,878
V3 (d-lator) 1=317 2=21 3=41 6=248 8=1,861
V4 (PI) 1=252 2=35,3 3&8=55,1 6=211.5 7=37,4

V5 3=475 4=468 5=-40
V6 3=475 4=468 5=-40

choke-2k2-22k-2k2-150k(to fet)

This layout shows the values of the build:

http://www.pichotel.com/pic/16028B31e/148308.jpg

After putting the 22uF/450V decoupler in I noticed the sound to be more clear which is a good thing but the basic problem still remains. Also the mid pot still does nothing while PAB off

Mat,

Look at the values in ( ) Get them as close as possible.

V1 1=197,8 3=1,875 6=201 8=1,814 (1 =185, 6 =185)
V2 1=105,5 3=2,035 6=212 8=1,878 ( 1=200, 6=200)
V3 (d-lator) 1=317 2=21 3=41 6=248 8=1,861 (1 =255, 6= 255)
V4 (PI) 1=252 2=35,3 3&8=55,1 6=211.5 7=37,4 (1 = 300, 6 = 300)

V5 3=475 4=468 5=-40 (Aleady answered this)
V6 3=475 4=468 5=-40 (Aleady answered this)

[dogears]

Paul, The Dumbleator runs about 230V on the recovery amp and not 255V. I have lots of research on that matter. You are correct about the cathode voltage. 255V is about right. However, some Dumbleators dramatically changed the bootstrap values to double the cathode voltage to over 30V. Sounds better to me. I have lots of pictures of several different Dumbleators with accompanying measurements. I think the dropping string needs some mods. Up the 10K to something like 15K or more. Then up the dropping resistor to the pin 1 cathode follower side a little too, maybe to 33K or so.

Also, Mat's V1 and V2 voltages are in the ballpark (assuming that V2A is really 205 and not 105). This is an HRM and not an early 80s like yours. Of course, trying something a tad lower is a good idea but IMHO not the source of his issue. I personally have lots of time on a real one that I have been in.

The PI voltages are different because Mat is using the Bluesmaster PI that Dumble switched to in his later years. Same as in Carlton's amp, among many others. Still his voltages are whacked here. He should have another 40V on his PI pin 6. This PI has lower voltages and when working right sounds amazing.

I am confused about why there is a 7V difference between pics 3 and 4 on the outputs. Maybe there should be a 1 to 2 volt differential. Something seems wrong here?? Maybe the 1K choke replacement resistor is WAY too big. I'd go to a 330 ohm. I have tried 1K and it did not work at all for me. Bad tone......

Look at the values in ( ) Get them as close as possible.

V1 1=197,8 3=1,875 6=201 8=1,814 (1 =185, 6 =185)
V2 1=105,5 3=2,035 6=212 8=1,878 ( 1=200, 6=200)
V3 (d-lator) 1=317 2=21 3=41 6=248 8=1,861 (1 =255, 6= 255)
V4 (PI) 1=252 2=35,3 3&8=55,1 6=211.5 7=37,4 (1 = 300, 6 = 300)

V5 3=475 4=468 5=-40 (Aleady answered this)
V6 3=475 4=468 5=-40 (Aleady answered this)

[dogears]

I have no idea what you refer to. The initial dropping resistor occurs after the screen caps which is after the main caps. Do you mean the balance resistors? Dumble used mainly 270K, although I have seen 220K there.

Let me re-phrase and thank you for the correction as that is what happens when you rush w/ statements:

The power resistors related to the intial power caps, or initail dropping resistors.