Too bright non-HRM

[Luthierwnc]

Hi guys -- been a while.

I have gotten a 100w amp to the fine tuning stage. All the settings on the first tube are great. Switch on the OD and it is piercingly bright. Turning back the tone on the guitar knob makes it sound good but I'd rather not have to. The boost and OD switches work independently. Stand-alone Dumbleator.

The amp has a bone stock 101 OD section with PS Orange Drops and RNs. I did put in a treble cut trimmer at 250k/.001 -- dialed to virtually no resistance. Tung Sol RI 12AX7. The triodes have 270p and 330p snubbers respectively. High plates with matching metal film cathode resistors, TVA 5.0uf cathode caps. The NFB loop on the second triode is switchable but the OD is too bright in either position. Neither the Drive nor Level pots have bright caps.

Plan B? Plan C is to convert it to an HRM (I did leave room for the sled) but would like to explore the possibilities first.

Thanks, Skip

[odourboy]

Is your treble bleed before or after the 150K resistor?

I've got a nonHRM build that sounds like it's identical to yours. No issue with excessive OD brightness. I have a treble bleed circuit also and the trimmer is set at 87K.

You might want to measure the grid resistors - in particular the 180K. If, for example, you installed an 18K by mistake, it would be considerably brighter. High value grid resistors work with the Miller capacitance to darken the tone.

[Structo]

I'm surprised with the Dumbleator it is still too bright.

Plus the difference between the two channels shouldn't be that much.

One change I made thanks to Scott was install a third relay that is triggered off of the OD relay.
It switches the bright cap on in clean mode but takes it off when I switch OD on.

What do you have for OD couplers and resistors feeding the pots?

Did you happen to measure your bypass caps before installing them?
You could have one that is way under spec.

[Luthierwnc]

Cap is after the pot -- but it is turned almost to a dead short.

Grids are fine. All the components are within tolerance. The OD entrance pot is pretty close to ground. I could dumb it down even more.

A couple years ago I had a similar problem in that the OD was a bit harsh. We kicked it around here but it never was able to bump the HRM out of go-to status. This time, it is too bright but everything else is glorious. I am using a .022 coupler after V1b with the bass mod. I could put that back to 80's status pretty easily with a .047 but that just lets more of everything through. Worth a try but not yet.

Truth is, my favorite noise out of these things (this is my 6th) is clean boost with the loop -- but to be an Overdrive Special, the overdrive needs to be special.

[odourboy]

Cap is after the pot -- but it is turned almost to a dead short.

What I was getting at was where did you locate the treble bleed circuit in relation to V2b coupler, 150K resistor and the ratio pot?

[LPSGME]

My amp's OD also seems way too bright.

Adding the treble bleed didn't help much as it also seemed to affect the tone in an unpleasing way.

What seemed to work better was radically upping the V2a snubber to a .001 Mallory 150.

I also increased the V2a to V2b coupler to .04 and added a cap across the PI plates (like in a BM).

[Luthierwnc]

The bleed is after the .005 V2b coupler. I have a 250k Level pot instead of the 150K resistor/100kl pot. sh

[tubedogsmith]

How high are you running the first volume, OD volume and master volumes.

[martin manning]

The bleed is after the .005 V2b coupler. I have a 250k Level pot instead of the 150K resistor/100kl pot. sh

Did you mean 250k Ratio pot? In any case you need the (150k) series resistance before the cap/trimmer shunt to ground to make a low-pass filter.

[Luthierwnc]

I'll swap out the 250k for a 100k today and report back, sh

PS Volume 4, Drive 5, Level/Ratio 5, OD entrance pot 2-ish. 462VDC on the plates. Voltages on the dropping string close to spec.

[dobbhill]

May sound silly, but I'll ask:
what speakers are you using? Do you use the bright switch? How loud are you playing the amp? The reason I ask this is because mine sounds a bit bright at lower volumes, but shines on stage with a full band.

One thing I tried and liked much is putting the treble bleed before OD1 instead of after OD2. Maybe I'm the only one that tried it, but it works well there.

Good luck.
D

[dogears]

Hmmm....

My favorite circuit has 250pf snubbers and a 68pf bright cap and it is not too bright on the OD (with loop), therefore something seems amiss.

Some thoughts

1) Make sure the drive and level pots are in spec. 100K at least. I have seen plenty of 10% under pots. Lower means brighter. So, check this. If they are low, you could raise th evalue of the series resistors slightly to compensate.

2) Make sure the OD grid resistors are correct. Something lower than a 180K on OD2 will be birghter.

3) Axe the Tung Sol reissues. Nasty shrill buggers! I like NOS, but a tested EH or JJ should be much better balanced.

4) No need to set a bleed at under 200K to ground. The more highs you dump either here or with snubbers, the less singing the amp is.

5) Try pre OD trimmer setting of 25K or so

6) Make sure pre OD network measures at least 320K to ground. Maybe it is under spec.

I try and think of more.....

[Luthierwnc]

Thanks guys. Putting the bleed between a 150k resistor and 100kl pot made the difference. I'll swap an EH from the inverter of another amp.

Cheers, Skip

[LPSGME]

How loud are you playing the amp? The reason I ask this is because mine sounds a bit bright at lower volumes, but shines on stage with a full band.

My amp's OD sounds fine even at low volume in a room, but the tone is unbearably shrill when trying to mic it. However the clean channel sounds exactly like it does in the room, maybe even nicer; just not the OD.

Even with .00133 as a V2a snubber and 270pf across the PI, I have to put a mic 3 feet away and turn it 90 degrees from the cab just to get close to a normal sound...and that is with using the neck PU; the bass volume up, treble down; no PAB; and with any speaker I try.

[Luthierwnc]

LPSGME -- are you using an active loop? They are an essential part of this sound and do take some of the bite out of the high-end.

The reference speaker in the basement is a Thiele with an EVM-12L. I like it generally but it is uncompromising when looking for flaws.

This amp won't get out of the shop until I can live with the OD so it might become an HRM anyway.