My favorite topic: Hi pass filters (now in E-100/300 -series)

[T-200]

Hi guys,

When I modded my T's I managed to find all the hi pass and low pass filters in them with a lot of help from SimonB and Brendon (some of them I had to put back after I got a real Leslie 147). When I got my E-333 I have been studying a little bit of the tube circuits. As the sound of my E sounds a little thin even after changing all the electrolytic caps I started searching possible "bass killers" in the amps.

I read a few pages by Kon Zissis and Frederic Somerville where they both said that changing the feedback resistors and caps you can add bass or treble/midrange frequencies. I found out that in both preamps there is a feedback circuit (see the schematics enclosed). If I'm on the right track here increasing the value of R104 in the preamp gives more bass and decreasing the value of C102 increases treble. You can also add a resistor in series with the C102 to even shape the sound more. Removing the feedback circuit would be too radical, I think.

Anyone with some knowledge about tube circuits please comment...

E-100 preamp.jpg

Jussi

[T-200]

OK, no comments for two days... Guess I'll have to try it when I have time and let you know the results when the smoke has cleared...

Jussi

[Tubejack]

C101 also appears to be a response modifierin that schematic, although it's difficult to know for sure. Small signal pentodes like this don't normally employ P-G feedback. I'll bet that if you sneeze on it, you'll get ultrasonic oscillations. C101 might be intended to prevent that. Hammond uses frequency shaping seemingly everywhere, and I think you'd have to reconstruct the entire organ to know all the whys and wherefors. After playing with the values in my M-103, I've decided that external tone controls will be much more versatile in terms of massaging the sound.

[T-200]

OK Tubejack, I will calm down and study more...

Jussi

[T-200]

I checked my archives and found out that Kon Zissis has modified the pentode preamps (6AU6) in his C3 by changing the feedback caps and resistors with good results. I don't know how the EF86 based preamps in my E differ from those in AO28 or M series preamps. The feedback circuit looks the same but the cap and resistor values are different...

Jussi

[indianajo]

I'm off the Internet 4-5 days a week battling vines trying to eat my power pole, trailer, fences, gate -----.
I would start off by increasing the value of the coupler cap from the EF86 to the next state. what was that C103?
You did replace the 100 uf @ 3 v bass boost caps in the cathode circuit, didn't you?
EF86 is the most linear of all tube audio amplifiers, going for about $30 apiece for real GEC or Mullard versions, which Hammond E's and H's have several of.
Inflection frequency is proportional to 1/sqrt(RC) so series caps block bass and parallel caps to ground eat it up. Hammond uses e-caps in the cathode circuit to boost bass response. I couldn't find 3 V rated caps so I used 10 V rated ones. My organ also has a bass cut pot in the pedal/percussion amp, which was turned down on all my H's. Maybe yours is, or the pot wiper is dirty? Turning the bass up makes the EF86 rattle in the socket on the reverb input, but who needs reverb? If I want the bass turned down I push in the sliders.
Also, how is your 5AR4 rectifiers? Old tired ones will cut the bass. It would show B+ voltage sag at max volume. Warning use clip leads on the negative of the meter to only use one hand at a time measuring B+ voltage. I've had to replace the 5AR4 3 times in my ST70 power amp to keep the output power up to 35 W/ch, in addition to the e-caps. A tip from the MacIntosh amp salesman put me on to it- when I bought the amp it was putting out 7 W/ch. But then I ran that amp for about 1000-2000 hours a year for a couple of decades before output tubes became unavailable (1980-2000). Oh, tired output tubes on the bass channel can cut power, too. That is the 7591's. It looks like 7591's are good for about 30 W push pull, what are you getting at maximum volume? P=(V^2)/R where Hammond speakers on the H are 8 ohms. (measure the speaker).

[T-200]

Friends, I did it! I totally removed the feedback resistor and cap from the vibrato pre-amp just to try it. Tons off bass and keyclick and volume!!! In fact a bit too much now, but I'm sure with the right values it'll be a great improvement to get closer to the almighty B3. It even boosts the percussion sound (which was a bit thin) when "Add vibrato" tab is selected.

Next I'm gonna try both preamps with same values as in B3 feedback circuit (10 M and 39 pF) and adjust from there...



Jussi

[T-200]

Now it's the way I like it ... so far. I tried the 10M / 39 pF combination, but the pleasant growl/overdrive started too early when I pushed the pedal down. I tried a few combinations and settled with 5,6K / 39 pF in both non-vibrato and vibrato amp. I got more bass and more keyclick, but the growl starts in the last third of the expression pedal and it's really cool tube kind of overdrive. The attack, percussion and keyclick are much
thicker and B3'ish now. This is an easy fix and adjustable to your taste. If you want more bass raise the value of the resistor and if you want more treble lower the value of the capacitor.

It was worth trying

I will send some pictures soon so you know what parts to change, if anyone's interested. I recommend that you try...

Jussi

[T-200]

Here's a picture where the parts to be changed are marked. It's interesting that you can also get a real tube overdrive because the mod increases the gain of the pentode preamp and overdrives the next preamp stage with a half of 12AU7 if you don't turn down the organ volume pot. In E-300 the other half is not used because in E-100's it's for brush/cymbal. So it's possible to modify the input circuit of that half with the volume pot and put a switch to select the inputs to get an overdrive switch with adjustable growl...

E-100 pre w.jpg

...and then some...

Jussi