New Build (hooray) needs compression

[Harris]

I've been mostly lurking here and reading almost every day since I found the site about 3 months ago. Have learned a lot and you guys have been very helpful.


While it still has some build issues, I have fired up my first and new Dumble. Sounds great, but still far from the grail. However, I'm unsure where to start tweaking to get more compression. Any help to get me closer to the grail will be most appreciated.

Cl1 is 220K 5uf/3.3K
CL2 is 150K 1uf/2.2K
w/local feedback loop with 22m

OD1 is 180K 5uf/3.3k
OD2 is 120K 1uf/1.8K
w/270pf caps on each side of OD2


HRM amp

Voltages

1a 209 2.07
1b 223 2.22

2a 247 2.19
2b 239 1.93

3
1) 287
2) 37
3/8) 60
6) 284
7) 39

4

3) 438
4) 436
5 -51

5

3) 438
4) 436
5) -54

[pedro]

Well done,

HRM amp.....

do you have PAB in place ? ...needed for HRM ....... and suggest you drop the feedback on CL2 plate-grid - I was advised to do so and its definitely better.

more compression comes for me when power stage is working a bit harder - not noticeable at very low room vols.

I's sure others will dive in with sugestions

Rgds
Pedro

[dogears]

Hi Harris,

Based on your wacky voltages and use of the 3.3k cathode resistor, it looks like you copied that 97 schematic. Good starting point but really off.....

Here is what to do. DO all I suggest and then evaluate. It is a system.

1) Reduce the V2a cathode resistor to 2.7k to 3k. I use a 2.87k in mine.

2) Axe the local feedback on V1b.

3) Use a full 500pf bypass cap on the V2b grid resistor

4) Use a 150k-180k resistor to ground from the V1 dropping resistor. This simulates the FET and will lower the plate voltages of your preamp tubes. They are way high. Shoot for 200v on V1 and 220v on V2

5) The pre OD network must be 470k with 47pf bypass into 200k into 25k trim with 4.7k tail. THis is a lowgain network. Crank the trimmer and ONY ONLY ONLY use the amp in PAB boost mode. Adjust stack when in boost mode. Set the mids about 40% up, the bass about 45% up, and the treble up at about 30%

6) No snubbers on V2

7) HRM treble cap must be a 500pf (not the 330pf in the schematic)

8 ) Put a 5uf bypass cap on V1b. You need to slam the OD with bass and filter at the end.

9) Definately measure your playtes and cathodes. Try and pick tight tolerence stuff. RN65 resistors btw..... CHange your V1 tube. It looks bad.....

Try this stuff and get back to me!

[Harris]

Thanks guys!!!!!

With all the homework that Dogears has given me, I'll see you in a couple days!

[dogears]

Hi Harris,

I had a typo on #8 I meant to that you should have a 5uf on V1b. V2b gets the thinner 1uf.

Thanks guys!!!!!

With all the homework that Dogears has given me, I'll see you in a couple days!

[Harris]

Well, all the changes are done. They sounded/looked like a lot more work than they really were. Instead of bleeding off B+ to ground on v1, I just put in a bigger dropping resistor in the power section. I tried it both ways and if there was a differnence in tone, it wasn't much.

Scott, I can't thank you enough. The change is substantial...for the better. You saved me weeks..possibly months of tweaking with your very good advice.

Anything else you have for me? The tone is much improved, but I'd say I've got another 10% to go before I approach the tone that you get on some of your clips...and Ford and Carlton and some of the others here.

[dogears]

Hi Harris,

Maybe some stack tweaking?? How are the trimmers set??

Please measure the drive pots. See if they are way off.....

Also, measure the HRM treble trim. Put a resistor in front of it to bring it up to 250k if it is low.

I would use the dropping resistor I suggested. That is the exact way Dumble did it... Maybe it effects things, maybe not... But I would use it.

What is the size of your OD2 grid??? I use 180k

Do you have a .0022uf on OD1 coupler??

No bright cap on the OD master IMO.....

Well, all the changes are done. They sounded/looked like a lot more work than they really were. Instead of bleeding off B+ to ground on v1, I just put in a bigger dropping resistor in the power section. I tried it both ways and if there was a differnence in tone, it wasn't much.

Scott, I can't thank you enough. The change is substantial...for the better. You saved me weeks..possibly months of tweaking with your very good advice.

Anything else you have for me? The tone is much improved, but I'd say I've got another 10% to go before I approach the tone that you get on some of your clips...and Ford and Carlton and some of the others here.

[Robert]

Scott,

What is the dropping resistor on V1 you are referring to?

Thanks!

[dogears]

Just hanging a resistor to ground from the end of the string. It simulates the load of the FET. For those of us that do not have a FET...... If you follow the values on the schematics, you may wind up 20-30v hot....

Scott,

What is the dropping resistor on V1 you are referring to?

Thanks!