spongy feel in preamp

[12AX7]

In the past i have gotten a nice spongy feel in cascaded preamps at times but i never really noticed what particularly caused it. For example, one thing i HAVE noticed is that using bigger cathode bypass caps at V1A and smaller after that seem to add to that feel, tho only to a small degree. It seems to me i get a lot more of it w/o using treble peakers or other things that accentuate top end. But when i do that it sounds too wooly and lose definition. Anyways i was just wonder if anyone has found something that helps to get a spongy feeling attach within the preamp. The power amp  is already running out of headroom at band levels so i don't want to add a sag resistor or anything in the power amp. I want to get it in the preamp. Ideas?

[joesatch]

i increase the plate resistor for V1 to achieve that. Ex. swap a 100k for a 220k.  Does add more gain though (overdrive gain not volume gain)

[12AX7]

I tried that and didn't like the tone at all. It's funny tho....whenever i write out a question like that often times i find potential answers to my own question and i had an idea from writing that post i will try later on. As i said, i get a squishier feel when i lose highs in the treble peakers. I find with this amp if i lose the bleed cap on the gain pot it gets squishier and fatter but loses too much definition and gets wooly. It hit me after i wrote the initial post to try it like that, IE: remove the bleed cap but replace the first .022 coupling cap with a .002. Gonna try it once it gets later, as at 8am if i try it the neighbors will be at my door with shotguns.

[Lectroid]

I'm still new to building amps, but I've had good luck on Fender designs by lowering the preamp bypass caps down to the 1uf -4.7uf range.  Tightens up a flabby bass sound and brightens the tone.  Kind of like the opposite of what 'spongy' sounds like to me.  Is that what you mean?  Some builders recommend going as low as .68uf, as on Marshall often did on first stage.

i increase the plate resistor for V1 to achieve that. Ex. swap a 100k for a 220k.  Does add more gain though (overdrive gain not volume gain)

@joesatch,
I don't understand that.  Raising the plate resistor would lower the B+ to that plate.  That seems to me like it would result in lower output current and lower amplification.  But you say it results in more gain.  What am I missing?  Thanks.

[joesatch]

@joesatch,
I don't understand that.  Raising the plate resistor would lower the B+ to that plate.  That seems to me like it would result in lower output current and lower amplification.  But you say it results in more gain.  What am I missing?  Thanks.
[/quote]

To me spongy means compression. lowering the plate voltage will do that. Sometimes i lower all the gain stages plates slightly instead of the first one by a great amount. I wouldnt change the v1 100k to a 220k without lowering the coupling cap to .0022 or so. Leaving it at .022 will create mud with lower plate voltage

[JPK]

Here's a gain calculator for varying plate and cathode resistors. 220k/1.5k will be 79 gain. 100k/1.5k will be  62 gain.


https://www.ampbooks.com/mobile/amplifier-calculators/output-impedance/calculator/

[echuta13]

I haven't messed with voltages in the pre-amp section that much, so this is just another direction you could consider but you may want to drop/raise your supply voltages to see if you can find your "sweet" spot.   I did do a little messing around in a Champ clone once upon a time and I did notice a difference when I bumped up voltages (a "bolder" sound), but it was a long time ago.  Just something to consider.  Link to a thread for some relevance: https://el34world.com/Forum/index.php?topic=18468.0

[12AX7]

The voltages were lower to begin with but i replaced the droppers with lower values to increase V because it was just too much and dynamics went out the window. I know you can get that feel w/o losing dynamics and focus because i've had that before. So i want to do it more along the lines of EQ changes in the right places. I just tried what i said above in the third post and it may have worked, but again i have to wait till later in the day because very low volumes are spongy by nature. But it DID seem to work in that i was able to lose the bleed cap and the .002 couple kept the mud away. Hopefully that'll work. But keep the idea coming...

[joesatch]

What circuit is it? A schematic would help. There are some tried and true mods if you're working with a known circuit (slo, 2204, etc)

[12AX7]

Well it's fairly close to a 2204 w/o a cold bias stage.

[uki]

Got any audio sample of that "spongy" tone !?

[12AX7]

No, it's a feel thing.

[joesatch]

If you're looking for spongy (sag, compression ?) in the preamp section there's not alot you can do besides playing with plate voltage unless someone knows something i don't.

[sluckey]

A good pair of deck shoes will give you that spongy feel regardless of plate voltage. 

[Lectroid]

Or dry rot in your floor joists...

[sluckey]

Or dry rot in your floor joists...

That's why I quit playing guitar on the deck. I hate that spongy feeling!   

[echuta13]

If you're looking for spongy (sag, compression ?) in the preamp section there's not alot you can do besides playing with plate voltage unless someone knows something i don't.

No doubt!  Getting the right combo of voltage, bias, and voicing is probably what is needed.  Sounds like a rabbit-hole of doom! 

[12AX7]

I have the voltages at V1 and 2 as low as i feel i can go now w/o getting too raggedy and sloppy sounding. And i'm not saying it has no sag, but it's just a touch stiffer than i like in the highs. I've already tried lowering cap values like the treble cap in the stack and peakers to 250pf. That helps too a bit by reducing those high mids 500pf and 1000pf bring and putting the top in a higher place. Hi mids are where i hear the stiffness.

[PRR]

> Raising the plate resistor would lower the B+ to that plate.  That seems to me like it would result in lower output current and lower amplification.  But you say it results in more gain.

Tube gain has very little to do with voltage. (Anyway raising the cathode resistor similarly may put the DC voltage back.) The plate resistor is also an audio load so higher R is less lost gain.

Refer to the tube-manual RC tables. "***" is the Fender-customary Rp=100k Rg=220k rk=1.5k and about 300V supply. Gain is 52. Drop supply to 180V, gain is 47, a hardly audible change; 90V supply gain is 0.67 which is less but not a whole bunch less. (Yes, the maximum output drops about as fast as supply.) OTOH, double the Rp to 220k and gain rises from 52 to 59: again, hardly audible.

Also note that RCA suggests you change Rk for every diddle of Rp or Rg. The junior engineer worked late to experimentally find these "good" values. Anything close is probably close enough for jazz.

[12AX7]

Well youse guys were right. I had 4.7k droppers i tried after seeing it in another schematic and i had i think 8.2k before. In my search i tried a 27k after the PI for the V1 and2 nodes then a 10k. (leaving the 4,7k on the V1 node) The 27k was just ratty and the 10k did nothing much. So after posting this and seeing so many suggest that i started experimenting with larger and larger values a bit at a time stringing resistors to get certain values. Then i found a 18k that i believe i may have used as a dropper years ago since it was a 3w. Bingo ! All the sag i want w/o sounding ratty and even better dynamics. I guess i really had to nail the perfect voltages. So thanks for pushing me. I have messed with PSU dropping resistors a good bit but this was the first time it really hit the nail on the head. Really improved the amp a lot probably way more than any other single weak.

[JPK]

Can you post a schematic or sketch which one you changed to 18k?

[tubeswell]

'Spongey' could mean the filter cap at the power supply node for the preamp is not working/not connected properly. Measure the DC voltages of all the filter caps on the power supply rail. If the pre-amp filter cap and the filter cap before that, are at the same VDC, then you'll know that the pre-amp filter cap is not doing its job. (This will cause a 'spongey' sound when you play loud notes/chords, because the signal is feeding back through the power supply resistor to the next filter cap stage).