Interesting findings re: caps and HRM

[dogears]

Agreed. On/off occurs around 50K. But, you do hear and feel a difference up to 500K to 600K.

We get the same result...Bass comes in quickly on a 1m or 500k trimmer..With the 1M after 500k 1/2 way up pretty much loses it's range...Depending on the amp a 56K slope in the od can help with it being a bit on the edgie side if you are using a bright speaker...



Tony VVT

[talbany]

You can dump highs to ground at the input of the overdrive master volume, which is a 1K pot. Try anywhere between 100pF and 250pF... The sound will get sqashed if the cap is too large for your amp, so you need to experiment. However, with the right size cap, you will be able to open the treble trimmer on the HRM and still get a good sound with EVM speakers. THAT's when the entire amp will begin to make more sense. Of course, if you use other than EVM speakers in the first place, you may save yourself this additional step.

Gil

I thought about doing some sort of treble bleed there makes sense to me... My treble pot is usually set between 100-125k very rarely past half..I find this is a balance in how people like to set the presence on the clean channel..If they like more NFB on the clean side it usually in general works ok... If they run less NFB a little treble bleed there might be the ticket...Since all of our off the shelf Overdrives are mainly PC board amps they seem to have quite a bit more gain associated with the more efficient layout..Along with this you get a brighter more sizzly tone as well...Many people might tell you that when they make the leap to PC boards and tell you the tone doesn't change much they are lying to you.. ESPECIALLY!!! when it comes to high gain topology...To me it's not better or worse just different!!!.. So in that comes a whole new mindset on how to tweak.. I find that the older type grail preamp tubes work better in a PC style amp than the hand made boards since the more efficient layout helps keep the low end tighter longer as well as a pronounced mid range and a top end that can get away from you real quick... The older tubes help this noticeably...The Older tubes for me in a non PC amp can make the low end a little loose for my taste as well as lacking a bit on the mid range..It's even worse when it comes to the NON-HRM here..although does apply to a few HRM's... I have ranted about this before so I'll leave now!!!


Tony VVT

[Bob-I]

Agreed. On/off occurs around 50K. But, you do hear and feel a difference up to 500K to 600K.

Yes, but to my ears, the sweet spot is at about 130-150K. The 1M didn't allow me to find that spot, too sensitive.

That said, the real surprise to me was the PI caps, the 3 .1uF's and one .022uF. The Xicon really opened the amp up.

[Structo]

I have the same values in orange drops.

So just the switch to the Xicons made an audible difference?

[Bob-I]

I have the same values in orange drops.

So just the switch to the Xicons made an audible difference?

Yes, the 6PS sounded a tad muffled, the Xicon's opened it up.

as already mentioned, it may be a bad batch of 6PS caps but I'm not taking chances on buying another bunch.

[Structo]

Yes, I am starting to wonder about the 6PS caps I have in my amp as well.

No matter what I have tried there is this fuzzy high end hash that seems to ride on the notes.

Even with the treble bleed circuit and a 47pf cap across the PI plates it's there.

[talbany]

No matter what I have tried there is this fuzzy high end hash that seems to ride on the notes.

Even with the treble bleed circuit and a 47pf cap across the PI plates it's there.

Tom

Is that a 6V6 amp that has that fizz on the note issue




Tony VVT

[Structo]

No it's a D'Lite 44 with Winged=C= 6L6GC's.

I have just about every change suggested here to bring it closer to a true Dumble. Of course it doesn't have the Rock/Jazz switch.

High plates, 110K/120K PI, 330R Choke, OD Entrance mod, etc.

I installed the Prosonic OT from Angela which is much beefier.

I even changed out the power supply filter caps and bias cap to the F & T ones last week to try better filtering, but I didn't really notice any change.

This problem has plaqued me from the beginning.
I love this amp and would love it more if I could get rid of the hash.

[talbany]

Tom do you have a good picture...I'll try to help if I can
Also som voltages specifically the PI..
Also do you notice it on the clean channel with the Volume dimed and the master pushing the output section.. Or is it mostly in the OD?






Tony

[Structo]

This forum software is screwed up. I'm having trouble up loading a picture.

Anyway I have some voltages I recorded a few months back and they should be the same.
The only difference from then to now is the change to F & T filter and bias caps.

I have the FET resistor at 150K.

I also changed the Ayan treble bleed to a daughter board that is mounted above the 180K on OD2.

V1 pin 1 190.6v, pin 3 1.74v, pin 6 195.6v, pin 8 1.65v

V2 pin 1 204.5v, pin 3 1.64v, pin 6 205.8v, pin 8 1.61v

V3 pin 1 294.6v, pin 3 57v, pin 6 291.3v, pin 7 35.2v, pin 8 56.8v

V4 pin 1 36v, pin 3 434.6v, pin 4 431.8v, pin 5 -47.3v, pin 8 36v

V5 pin 1 35.3v, pin 3 435v, pin 4 431v, pin 5 -46.7v, pin 8 35.3v

Do you think the preamp plate voltages are too high?

Yes I notice the hash/fizz on the clean channel as well.
Really at most volumes even cranked up I hear it as well on both channels.

I am running this into 2 EVM 12L Classics at 8 ohms.

[ayan]

This forum software is screwed up. I'm having trouble up loading a picture.

Anyway I have some voltages I recorded a few months back and they should be the same.
The only difference from then to now is the change to F & T filter and bias caps.

I have the FET resistor at 150K.

I also changed the Ayan treble bleed to a daughter board that is mounted above the 180K on OD2.

V1 pin 1 190.6v, pin 3 1.74v, pin 6 195.6v, pin 8 1.65v

V2 pin 1 204.5v, pin 3 1.64v, pin 6 205.8v, pin 8 1.61v

V3 pin 1 294.6v, pin 3 57v, pin 6 291.3v, pin 7 35.2v, pin 8 56.8v

V4 pin 1 36v, pin 3 434.6v, pin 4 431.8v, pin 5 -47.3v, pin 8 36v

V5 pin 1 35.3v, pin 3 435v, pin 4 431v, pin 5 -46.7v, pin 8 35.3v

Do you think the preamp plate voltages are too high?

Yes I notice the hash/fizz on the clean channel as well.
Really at most volumes even cranked up I hear it as well on both channels.

I am running this into 2 EVM 12L Classics at 8 ohms.

Your volatges are nominally fine. You say there is a high end fizz even on the clean channel.... Do you mean the high end is unpleasant sounding, or there is like alka-seltzer fizzing crap going on? If the former, you just need to voice that until thing sound right to you... Maybe try a 100K slope resistor? It appears you have a 150K AND a classic tone stack with a .047uF middle cap, 330 pF treble acp, etc.

However, if you have some actual fizzing going on, you have an oscillation somewhere in there and the only "easy" method to find it is tracing the signal with a scope and determine where it begins.

Gil

[talbany]

Tom

The voltages on you PI look a little high.. 250 should be where you should be looking for there... A fizz on the note in my experiences is usually a PI issue or the first place I look ...Then the output section... Try this trick.. The .047 coming off V2B lift the wire feeding the relay then lift the wire coming from the effects loop or master where ever from the entrance to the PI.. run a wire from the .047 down to the input to the PI.. This will bypass the relays master volume and effects loop... If it's still there after that test then then we can narrow it down somewhat...If it still does it I would lower the PI voltage to around 250...Also try a 12 at there that will lower the voltage to around 250...see if it still does it...A few ideas...Cool!!!




Tony VVT

[Structo]

Gil,
It is a hard thing to describe what I am hearing.
For the most part the amp sounds good.
What I am hearing is a kind of high frequency distortion thing on top of the note.
I know what filament rattle sounds like and it is sort of like that but not.

I tried the Skyline, mid .01uf but didn't like it so reverted back to .047uf but kept the 250K mid pot.
Yes the slope is the stock 150K.
It seems like I tried the 330pf treble cap but went back to the 270pf.

It may be an oscillation but I don't have a scope soooo.....

Tony,

I will try that bypass trick and see what it gives me.

What do you mean by this?

"Also try a 12 at there that will lower the voltage to around 250"

Thanks guys, keep 'em coming.

[talbany]

Sorry when I wrote this I was on the way out and hurried through it..
I was thinking Fender Twin when I wrote it... 12 AT's draw more current than an AX so when you switch between an AT and an AX your plate voltage will jump 10 and in some cases 20 volts going to an AX.. What my post should have read was with an AT the sweet spot is 250.. With an AX a bit higher 260-270... 294 seems a bit high for an AX... Normally I wouldn't mention this however too much voltage on the PI can certainly cause the anomaly you describe.. If your PI is setup around 294 with a 12 AX and you swap it with a 12 AT it will drop 10-20 volts putting it around 265-270 well within an operating point of the AT hopefully eliminating the anomaly.....(Max plate voltage on an AX 300v)....A simple test and worth a try!!



Tony VVT

[ayan]

Gil,
It is a hard thing to describe what I am hearing.
For the most part the amp sounds good.
What I am hearing is a kind of high frequency distortion thing on top of the note.
I know what filament rattle sounds like and it is sort of like that but not.

I tried the Skyline, mid .01uf but didn't like it so reverted back to .047uf but kept the 250K mid pot.
Yes the slope is the stock 150K.
It seems like I tried the 330pf treble cap but went back to the 270pf.

It may be an oscillation but I don't have a scope soooo.....

Tony,

I will try that bypass trick and see what it gives me.

What do you mean by this?

"Also try a 12 at there that will lower the voltage to around 250"

Thanks guys, keep 'em coming.

Tom, it kind of sounds like you may have an oscillation going on inside the amp. If that is the case, you should try to chase it down and correct it before you change parts, IMHO. No chance of borrowing a scope, at least? Question for you: if you plug into the return of the FX loop, i.e., into the PI, do you hear that or is it gone that way? If it's still there, you know the oscillation is generated at the PI or afterwards, so you can check for cold solder joint, move wires, etc., between the PI and the end of the chain (i.e., the OT).

If plugging into the PI sounds good and clean of the fizz, then you know the gremlins are introduced in the first stage, since you hear the fizz on the clean channel as well. So, you could concentrate on that. The only difficult thing is that the oscillation can generate pretty much anywhere, but the culprits are usually located on the high impedance portions, so, on the networks that lead to the grids.

Hope this helps a bit,

Gil