Please Help with Tweaking, Dogears + Co ??

['67_Plexi]

Hi,

Just finished my HRM build. It is based on s3988009 with a few minor differences. I will detail the spec below. I've used a 4PDT relay for switching channels and PAB simultaneously. The good news is there are no noises, hums, buzzes, switching noise etc and I acheived this with only 2 sheilded cables in the entire build. the not so good news is that the clean sound lacks 'sheen' and gets farty at higher 'gain' settings and the overdrive tone is rather fizzy and has upper mid honk that reminds me of a Boss HM2 pedal or possibly a half cocked wah pedal. I've checked and doulble checked component values. I'm using a Fender Esprit or a PRS Custom 22 and a 4x12 loaded with G12M70's or a 2x12 loaded with G12 Classic Lead 80's for testing. All combinations have the same characteristics meaning the amp is the common denominator !!

Here's the spec:

Chassis is from a twin reverb with an original BF Bassman PT and a Weber EL34 45W OT.
Tubes are all NOS Mullards.
Power supply is currently using the Fender style series 220uF's on the plates and then single 22uF's. The dropping resistors are similar to the schematic. I've used the BF Bassman choke.
V1 plates are around 200V, V2 around 220V and V3 around 300V. The EL34 Screens are at 438V and plates at 441V.
5.1K Grid resistors on EL34's (seems a little high to me for 34's).
All pots are as the schematic.
Tone control is skyliner with R/J and mid boost, PAB when in OD
47pF Bright on the clean master.
CL1 cathode 5uF/3.3K
CL1 Plate 220K Dale MF
1000pF/500pF treble, 0.01 mid, 0.1 bass (the mid control seems a little unresponsive ??)
CL2 Grid 220k/470pF
CL2 cathode 5uF/2.2K
CL2 Plate 150K Dale MF
Coupling Cap .047uF
No local feedback
OD Entrance 470K/47pF - 200K - 25K Trim - 4.7K Tail
OD1 Grid - 68K
OD1 Cathode 5uF/2.7K
OD1 Plate 180K Dale MF
.0022uF Coupling - 100K - 250K Pot - 150K OD2 Grid
OD2 Cathode 1uF/1.8K
OD2 Plate 120K Dale MF
No snubbers
Overdrive tone stack 500pF/0.022uF/0.022uF
PI Coupling cap 0.01uF
NFB 4.7K to 4 ohm tap, 2Kpot 390/1uF
The rest of the circuit is as per the schematic
All caps are OD PS Series and Sprague 1KV Ceramics
All plate resistors Dale RN65.

Any advice or pointers would be welcomed. I'm considering rebuilding the PS to use the series 47uFs and parallel resistors. Any thoughts ?

Thanks a ton guys !

[dogears]

Hmmm....

Firstly, the stack should be responsive..... My mid pot works just fine. I'd check that one out! Something sounds miswired in the stack.

What kind of shielded cable did you use?? Make sure it is low capacitance coax. The Beldon type shielded sucks all the shimmer.

What ground scheme did you use? I'd make sure you try and follow the way Dumble did it. Several posts address that here.

The OD should be adjusted when in PAB boost mode ONLY. The OD only sounds good in boost mode, IMHO. Try setting the HRM stack as follows. Treble up a little less than 1/3 (about 75k), mids up a little less than 1/2 (about 8k), Bass up almost half (about 420k)

Try adjust the dropping string to lower the V2 about 10 volts. Raise the V2 resistor and lower the V1 a hair to keep that at 200v.

I prefer a slightly smaller bright cap on the clean master. I use 22pf. Just experiment on that one.

Make sure your PAB is breaking the bass cap from treb pot and the bass pot wiper.

Don't bother with the 47uf caps in series yet....

Choke or no choke?? If no choke, too big can cause a bit of a lack of articulation. I switch between choke or a 390ohm resistor.

Please report back to me with your findings to the above first.

['67_Plexi]

I just realised I used the wrong forum, should have gon in discussion, sorry.

I guess it's too late to change it now.

Dogears,

Dogears, thank you for responding, it's really appreciated.

1. You have me thinking here and this could be cause of the problems. I will double check this.

2. I did use belden, but only for V2 grids, I got away without any on V1 grids.

3. The grounding scheme I used was pretty much the same as the 398009a, which is pretty much a standard Fender / Marshall type arrangement. I didn't use a ground bus on the back of the pots, but just used the back of certain pots. I have absolutely no ground loops at all. I haven't seen any of the posts about Dumble grounding schemes, but will have a look.

4. The PAB is automatically on when I'm in overdrive. It switch with 4PDT relay doing both duties at the same time.

5. Will change the Clean Master bright cap, I thought it was a bit heavy duty to.

6. PAB appears to be working correctly, but I wiill of course take a look with a fresh set of eyes.

7. OK

8. Yeah it has an original BF Fender Bassman choke (don't ask, I just had one). It only measure 65 ohms DC resistance, which seems a little suspect. Sould I just dump it and use a 390ohm power resuistor ?

Again, thanks and sorry for using the wrong forum!!

['67_Plexi]

One more thing:

Apart from on the preamp plates, is there anywhere else that MF resistors are best suited ?

[dogears]

Two suggestions.... Use coax instead of the horrid high cap Beldon. It is really bad stuff. Try straight wire to A/B! Bet you hear it.....

Also, try the choke replacement resistor. I love it!

['67_Plexi]

I'll take your word, just ordered a 390 ohm 25W !
I ordered the parts to upgrade the PS as well, might as well do it whilst I'll already be working on the PS.

Any thoughts on other locations for MF apart from the plates ?

[dogears]

A 5 watt resistor is fine for choke replacing. You can try a 470ohm screen resistor as a test! The 25 watter is really big. I advise against it..... Unless you have plenty of space!!

I use MF for cathodes (1% tolerence is good), PI stuff, and the slope.

I'll take your word, just ordered a 390 ohm 25W !
I ordered the parts to upgrade the PS as well, might as well do it whilst I'll already be working on the PS.

Any thoughts on other locations for MF apart from the plates ?

['67_Plexi]

Duh, I actually meant 5W !! 25W......I think I was having flashbacks to modifying power supplies after removing field coil speakers in 1950's class A amps !!

Yeah I tried Dales on the slopes, but couldn't hear any difference...however as we know my amp isn't really the best test bed at the moment. I guess you could liken it to polishing a turd.

[bluefireamps]

NFB 4.7K to 4 ohm tap, 2Kpot 390/1uF

Are you really using a 1uf cap there or a .1 (100nf)? It's easy to goof up. I've done it..............

[v00d00blues79]

I would bet he's using a 1uF there. The schems, if i recall correctly, call for a .68uF, so it wouldn't be that much more. And from what i hear, it sounds a bit better with a 1uF.


Andy

[dogears]

Hi Andy,

Every 90s Dumble I have seen had a 1uf. A couple of 70s era amps had the .68uf. Even the 80s stuff seemed to generally be 1uf. This is an area for trial and error IMO. There is a noticeable difference in upper mids from dropping a 1uf to a .68uf

I would bet he's using a 1uF there. The schems, if i recall correctly, call for a .68uF, so it wouldn't be that much more. And from what i hear, it sounds a bit better with a 1uF.


Andy

[bluefireamps]

I guess I should have asked if he used a .1uf in place of the 1uf accidentally. That's what I meant. Really.

[dogears]

Yeah... I know....

I don't think the accent cap would matter to the syptoms described. With the accent off, cap size is moot. On, and the .1 would add only the very very high end and certainly not cause a cocked wah sound.

I am leaning on bad HRM stack adjustment and incorrectly wired front stack.

I guess I should have asked if he used a .1uf in place of the 1uf accidentally. That's what I meant. Really.

[jaysg]

The OD only sounds good in boost mode, IMHO.

Hmm...that's an interesting comment unto itself. I realize the thread is about an HRM amp. Is that how you feel about the non-HRM also?

So you use Clean, Clean + PAB, and OD + PAB?

[dogears]

Exactly!!! Which is corroborated by Gil's personal observations from the stage one night while on the same bill as Mr Ford. Robben only used those 3 tones. If you listen to Jing Chi Live, you can clearly hear HRM artifacts...IMO. Also, Handful of Blues. Or maybe Up the Line from Blue Moon.

Non HRM is much different. It sounds great with both regular and PAB modes. Once again, Gill recently observed Robbin and his recently re-done Dumble and it most likely was Non HRM as Robben used regular OD much of the night while occasionaly turning the PAB on.

The OD only sounds good in boost mode, IMHO.

Hmm...that's an interesting comment unto itself. I realize the thread is about an HRM amp. Is that how you feel about the non-HRM also?

So you use Clean, Clean + PAB, and OD + PAB?