Really boomy bass on 5F1 clone... any suggestions?

PonchoGreg

Hello! I built up a 4W 5F1 kit from Ampmaker (http://www.ampmaker.com/store/WF-55-4w-tweed-style-amp.html) a few years ago and the issue has to an extent always been there, but I guess I'm noticing it more since I play HB-equipped guitars more regularly...

I love the general tone of the amp and the fact that it is warm and "woody" for lack of a better word, but bass notes, specifically between E2 and E3 (so 80hz to 165hz, roughly) are boomy as all hell. As in I can physically feel the bass booming out of control. At the moment the head is driving a 10" Warehouse Speakers Veteran in an open-back 10" cab. I have tried various different placement options and it doesn't do much. I even ran the head through the speaker of my Deluxe Reverb (so a larger cab and 12" Celestion G12V-70 speaker, in a different room altogether) - helps a bit but still pretty boomy.

Has anyone ever had the same issue with this type of amp? Any suggestions towards a cure (components? valves?)? To be clear I'm not complaining of a lack of treble and looking for ways to boost it. It's more about taming the bass!

Thanks y'all!

ICBM

Does it have negative feedback? I can't quite tell from the layout. If not, it definitely needs it. If it does, it may need more.

PonchoGreg

Thanks @ICBM ;. Yep, it does:

"Just like the vintage Champ, this circuit uses negative feedback: a small amount of the power amplifier's output signal is fed back into the preamp. This negative feedback is taken from the 4-ohm tap on the output transformer and runs (via connection point D on the diagram) to the cathode of the second amplification stage. It's in the opposite phase to the signal that comes from the amplifier's Volume control and that acts to lower distortion in the output stage."

That's the schematic: http://www.ampmaker.com/images/ak00kit/ak00sc2.jpg

How would you go about introducing more negative feedback?

Having looked into it, I might try playing around with filter caps as well (especially the preamp one) as it's a 22uf as opposed to the traditional 8-10uf, which apparently can contribute to more bass.

PonchoGreg

Nevermind, found the answer to my question re: negative feedback!

http://www.ampmaker.com/infocentre/thread-3.html

"* Lowering R7 (perhaps to 4.7k) increases negative feedback
* Increasing R7 (perhaps to 100k) decreases it
* Adding a pot and a capacitor turns it into a Presence control (beyond the scope of this page, but have a look at typical Marshall 50W and 100W schematics to see how it's done). "

ICBM

You can increase the negative feedback by moving the connection D to either the 8-ohm or the 16-ohm tap on the OT (probably the easiest to try) or reducing the value of R7 (easiest done by paralleling it with another resistor, start by trying another 22K).

Filter caps don't actually affect the bass response much in my opinion.

I also see it has no valve rectifier - that might make quite a difference to the low end. Try replacing D5 with a resistor, something like 100 ohm 10W.

PonchoGreg

Cheers @ICBM ;, quite a few options to tinker with - god bless turret boards

In your experience, is it worth playing around with the coupling caps (lowering values)?

ICBM

PonchoGreg said:

Cheers @ICBM ;, quite a few options to tinker with - god bless turret boards

In your experience, is it worth playing around with the coupling caps (lowering values)?

Possibly, but try the NFB first. If you're going to replace a cap, try C1 first - reducing the value of C2 will also reduce the effectiveness of the NFB at bass frequencies since it's within the loop.

PonchoGreg

Excellent, cheers - will order a bunch of cap/resistors and play around a bit (when everything is safely discharged of course!)

ICBM

The good news is that R13 is a built-in drain resistor, so simply unplugging the amp and leaving it for about a minute (probably much less) guarantees it's safe.

GreggHermetech

I built the WF-55 a few months ago and love it, but am also finding the low/sub bass a little overpowering. I run it into an open back cab with a Weber 12A125A 12" speaker, and I changed the 12AX7 for a NOS RCA 12AY7 for slightly lower preamp gain (4w is still ridiculous for at home playing!) I am thinking of doing these mods in this order: 1) Changing C1 (0.022uF) for a lower value. I read this should act like a high pass filter, with lower value caps increasing the cutoff point of the filter, therefore filtering out more of the sub bass. I've ordered a load of caps to try it: 0.015µf, 0.01µf, 0.0047µf, 0.0022µf or 0.001µf That should give me a good idea. I DO like the warm sound of the amp so don't want to rip out too much bass. Hoping one of these values should be just right. 2) Once the overall amp's bass is voiced to my taste, I want to add a negative feedback pot on the front panel. I am gonna do what Uncle Doug suggests in this video (link below), namely replace the fixed 22k resistor for a 10k resistor in series with a 50k pot, That should give me the ability to go from around double the amount of NFB I currently have with the new pot at minimum (10k), for even quieter/cleaner/tighter bass, to 60k of resistance with the pot at max, giving me way less NFB for a raunchier/raw/louder/noisier sound for easier overdriving. At least I think I got that straight, please correct me if wrong! I will measure where 22k is on the pot's travel with my DMM so I know where "standard" is. 3) Adding a "Framus" Mid Scoop control , as detailed on Rob Robinette's page here: https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Framus_Mid_Control I'm hoping this will allow the WF55 to get more into "Blackface" territory with scooped mids, also reduce the bass a bit when needed, and be more versatile for things like Funk strumming where I want a really clean sound with rolled off lows and mids. When I put the values into the online TSC it looks like it'll do just what I want, even rolling off 6dB from the bass too at the min setting: https://www.guitarscience.net/tsc/framus.htm#RIN=1k&R1=68k&RM=1M&RL=1M&C1=0.001u&C2=0.001u&RM_pot=LogB Each mod is a little more involved than the one before it, which is why I'm gonna do them in that order. But I'm imagining it should turn it into a much more versatile amp when all are done. Gonna label them "GRUNT" (NFB) and "FUNK" (Mid Scoop). I now have the parts ready to go. I was just wondering if anyone else more experienced may have tried some of these or might like to comment on my proposals/let me know any "gotchas" etc. Thanks in advance for any advice! Gregg

GreggHermetech

Is there a way to edit posts? I see it somehow stripped all the formatting from my post above. Anyway, I replaced the 0.022uF coupling cap with a 0.0022uF one, and can report that it has tightened up the low end nicely, without losing the regular warmth in the bass notes. It's just a lot of the sub flub has now gone. Win! That was a very easy mod, will attempt adding the NFB pot later this week, a little bit more involved (replace a resistor and add a pot in series, fit pot to front panel). Wish me luck!