Paralleled 6SL7 Stage Driving TMB - Too Bassy

[W5FH]

Seeking a little guidance on how to reduce the bass response in a recent build. I have a paralleled 6SL7 (33K Rp, each cathode identical 1% 1.5K Rk, 15 ufd 2% polycarbonate film Ck) driving the standard Fender TMB as shown on Duncan calculator. B+ running 275 vdc. Treble pot wiper is tied to 1 meg volume control driving grid of 6SJ7. The 6SJ7 is center biased and driving a 6SL7 LTP phase inverter to P-P 5881's.
The amp sounds really good but too much on bassy side, with bass control at bottom end still a little too bassy. I realize that the output impedance of this parallel 6SL7 stage with 33K Rp is pretty low, somewhere near 12K. When I reduce the driving impedance on Duncan calculator from 38K to 12K I see the low-end increase some on graph. I am wondering if this is the main cause of my excess-bass problem and what should I do to correct it?
I realize that when paralleling the 6SL7 I am creating significant input capacitance. I have a measured voltage gain of 33. Taking Miller effect into account, I should have approximately 220 pF input capacitance. I have a single input jack with 1 meg to ground and a 10K input grid stopper. During play testing of amp this situation was taken into account and volume control on guitar kept at 100%. I mention this as I don't want to rule it out as playing a role in my excessive bass sound. We did rotate through many 6SL7 tubes, all NOS and of many mfgr's. Also did same with some 6SJ7's. Noticed slight differences but the excessive bass was always present.
Thanks

[sluckey]

Generally speaking, reducing the size of cathode bypass caps and coupling caps will reduce the bassy sound. Post an as built schematic for more specific suggestions.

[johnnyreece]

I would like to second what Steve said. For a quick test, you could try disconnecting the cathode bypass cap. If that doesn't do enough, you'll probably want to look at coupling caps next. Hard to say without seeing the whole circuit (again, like Steve said...).

[jbrew73]

Are you using 10% audio taper pots?

If you turn the bass and mids all the way down and treble all the way up is it still too much bass?

Could it be treble shy instead bass heavy?

[Helmholtz]

For a quick test, you could try disconnecting the cathode bypass cap.

Removing the cathode cap will lower the gain above the corner frequency but increase the bass response.

[Stevem]

To me the 6S family of tubes are on the fat sounding side for guitar use all on there own, don’t get me wrong I love my 66 reverb rocket, but I need to ask why are you running it in parallel?

[TUBEDUDE]

What Sluckey said. If that's not enough relief, possibly a treble peaker could be added. But as Sluckey said, post the schematic for more specific answers.

[W5FH]

Thanks, great replies-
Sorry about no schematic, I would be ashamed to publish my handwritten notations. This amp is a test platform and my first venture into 6SL7 driving a 6SJ7. I paralleled the 6SL7 input stage to get a more textured sound. Very similar to Clubman concept. Will try a 6SH7 next, in place of 6SJ7. Was a little set back at the strong bass as that tone stack has worked well in many builds, but all 12AX7 types. This amp has spring reverb with a 6SN7 configured in SRPP driving 800 ohm input tank. Have two stages of 6SL7 reverb recovery. Using a 330K resistor between 6SJ7 and phase inverter to take reverb across. Upstream side of 330K feeds 1 meg dwell pot to 6SN7 SRPP driver. 250K pot after second recovery stage 6SL7 adjust reverb level back in through 470k resistor. Very similar to ProSonic reverb circuit. My first build with transformerless reverb and it sounds pretty good.
I believe some things may point to that parallel input stage involving the bass. The lower output impedance of stage increases bass level in tone stack slightly. Possibly nature of 6SL7 tone, as mentioned in reply. I do have adequate treble as judged by players. I am going to replace the 15 ufd Ck for each triode with 2 ufd film cap and will report back.

[johnnyreece]

For a quick test, you could try disconnecting the cathode bypass cap.

Removing the cathode cap will lower the gain above the corner frequency but increase the bass response.

Maybe there's a terminology difference here. Proportionally, yes, I suppose losing the gain in the higher frequencies might be perceived as increasing bass response, but the actual gain of the bass frequencies will drop, correct? I don't know exactly what the OP is looking for. Of course a smaller bypass cap would be ideal; I was merely suggesting a quick firing up of the iron to remove a leg of the cap and see if that takes any flub out (if that's even the issue). If OP has smaller caps to try as bypass, by all means give it a shot.

Of course, if my understanding is flawed, I never mind being corrected!

[Helmholtz]

but the actual gain of the bass frequencies will drop, correct?

The gain below the lower corner frequency is not affected by the cap.
Without the cap the frequency response is flat down to zero Hz (DC).
So without cap the frequency response is the same as with an infinite value cap, but the gain is lower by 5 to 6dB.

[W5FH]

Reporting back-
First I lowered the cathode bypass cap on each triode in parallel stage from 15 ufd to 2 ufd each triode. This helped but did not get the bass down enough.
Next I lowered the cathode bypass on 6SJ7 second stage from 20 ufd to 10 ufd and at same time lowered value of screen grid bypass from 0.1 ufd to 0.047 ufd. These changes made the biggest difference in getting bass down. Now the TMB stack settings are not at extremes and have very usable ranges plus the amp really sounds good. Speakers used in evaluating were an ET65/ET10 pair in a 12", 10" combo, an ET10/Retro10 in 2x10 combo and a pair of Vintage 30's in a 2x12 open back cab. Am excited to now experiment with getting that sweet octal tone. I will say that this amp is just about dead quiet turned up fully. Have AC on filaments. However, I had to hand pick the 6SL7 and 6SJ7 tubes. I have quite a few of these, most brands represented, all USA old stock going back into the 1940's. Maybe 1/3 of the ones I tried were acceptable for use in the amp, being both quiet and with minimal microphonics. I did try some of the mid-80's ECG-Sylvania JAN 6188's in place of 6SL7. Of course these tubes were very quiet and they sounded good. I started out with them knowing the amp would be more than likely quiet. Now I have old stock USA - the metal 6SJ7Y (GE, some RCA) seems to be one of the quietest of 6SJ7's. Of course a red 5693 was tried and found acceptable. After culling out the noisy ones had fun tube rolling and comparing the USA old stock TungSol, KenRad, National Union, RCA, GE and Sylvania 6SL7's. There were differences that were noticeable, and a few seemed to stand out, but all sounded good.