Gain reduction

sjhusting · Message non lu par **sjhusting** » 05 août 2006, 21:12

I took my G5 to an open-air jam session for the first time last night, and determined that I'm just getting too much distortion. Below 10:00 on the volume, I have a nice, warm clean tone. Around 12:00 there is significant distortion, actually about the amount I would like to be the maximum. By 2:00 there is really too much. Unfortunately when playing with a drummer the amp gets turned up close to all the way just to be heard. So I want to reduce the gain.

Right now I am experimenting with lower-mu preamp tubes (ecc81 at the moment, will also try ecc82 and 12ay7) but if this doesn't get me where I want, how can I reduce the gain? Would reducing the value of the cathode bypass capacitors help, or even completely removing them? If it's power stage distortion, other than increasing plate voltage, how can I reduce distortion there?

Comments from the other musicians were that the clean tone sounded good, the distorted tone got lost in the mix. I was using an Ibanez archtop with low-output Hauessel humbuckers.

steven

Message non lu par **charpy** » 06 août 2006, 0:25

First of all : a reduced gain will produce a less distorted sound but also a lower volume. so it will be hard to play next to a battery if you want to keep a clean sound even if you use a powerfull 4x12 /105dB stack.

Anyway an ECC82 will probably be the fastest solution.

you can remove C1 / 22µF for a little bit less gain and to prevent input blocking distortion if your humbukers are driving the input stage too hard.

you can also reduce the gain right before the power stage if you add a 1M resistor between C6 (22nF) and R17 and if you change R17 (1M) to 100k or less for less gain.
Or, another way to do the same is to add a 10k résistor between the power supply for the preamp and R16 (the 100k plate resistor of the second gain stage) then connect C6 at the juction of 10kand 100k résistors instead of the plate-100k R16 point.

It is not a big deal, you should have fixed it soon!

sjhusting · Message non lu par **sjhusting** » 06 août 2006, 10:04

Thanks, I'll give it a try this week.

steven

Message non lu par **McColson** » 06 août 2006, 14:35

The ECC82 will be a good solution, i think too.
And a switch to bypass the cathode capacitor of V1a, the 22µF one.
The result will be less gain for sure, but less bass too.

sjhusting · Message non lu par **sjhusting** » 08 août 2006, 21:20

McColson a écrit :The ECC82 will be a good solution, i think too.
And a switch to bypass the cathode capacitor of V1a, the 22µF one.
The result will be less gain for sure, but less bass too.

Well, you were right about the ecc82 - distortion down, volume way down, bass down. Next I'll try the switch on the bypass cap.

thx again

Message non lu par **charpy** » 08 août 2006, 22:30

I think McColson said wrong, you must not bypass the 22µF capacitor but unplug it with the switch as you prefer at the cathode or ground side.

bypassing this capacitor will set the cathode voltage to ground so it will "missbias" the triode because it will bypass the cathode resistor too.

Message non lu par **McColson** » 08 août 2006, 23:10

I'm not said wrong, I just wrote like I can !

Listen charpy now

I've said "bypass" because it's the AC bypass capacitor for the cathode. But don't short the cathode to ground, it's sure !

Message non lu par **vitriol82** » 08 août 2006, 23:29

Hey!!
One question, using an ECC82, it's not the same consumption?
We are really far of the 1 mA of the ECC83, no?

Don't we must reduce the value of the 68k to increase anode voltage?

I think that the ECC83 has 1.2 mA in plate intensity and the ECC 82 has 10 mA !!

It's wright , no?

Sorry, english isn't my native language,.....con!

Message non lu par **McColson** » 09 août 2006, 0:11

Yes for sure, but with no change, generally, the ECC82 give less gain than an ECC83, it's something like plug and play (not really, but sometimes it's sound good)

sjhusting · Message non lu par **sjhusting** » 09 août 2006, 10:01

It was clear about the bypass cap. I'm just looking for a way to do it without drilling a new hole in the chassis. I'm looking for a push-pull pot.

My first impressions about the ecc82, in addition to the vol/bass/gain reduction, is that "something important" is missing in the sound. I don't know how to describe it, it is there with ecc81 and ecc83, but not with the ecc82. The amp sounds lifeless with the ecc82.

I am going to try to find a 12dw7 (ecc823) which will give me a high/low mu pair. JJ makes one with the triodes reversed (ecc832); if I can I will try both tubes.

thx again

Message non lu par **McColson** » 09 août 2006, 10:17

Try also a good 5751, I have one in my G5, the gain is 30% less than a 12AX7 and maybe more than 30% compare to the EH12AX7 who have a big gain. And this tube is very sensitive, I don't lost the character of the amp.
You touch some good GE Nos 5751 today, 19.50€ in the TT shop. Maybe less on eBay.

sjhusting · Message non lu par **sjhusting** » 09 août 2006, 11:34

I'm a big fan of the 5751, I use it in V1 of my Fender, it brought the amp to life. I've been meaning to buy more anyway.

slajeune · Message non lu par **slajeune** » 09 août 2006, 14:54

Hi All,

I am not very familiar with the G5 but here are my thoughts. From my understanding, past a certain point on the gain knob, you have both sources of distortion preamp and power tube distortion. To tame the preamp distortion, remove the second bypass cap (cap C5 on the V2 schematic).

As for the power tube distortion, I have no information on the the actual voltages involved, but I think that 120ohm as a cathode resistor for an EL34 makes it biased hot and therefore, generate quite a lot of distortion. I would try increasing it to about 200ohm to see if it gives you a bit more headroom. As a side note, on a similar AX84 project they use a 330ohm cathode resistor for the EL34 but at a higher plate voltage.

Two simple modifications that don't involve any drilling!

Cheers,
Stephane.

Message non lu par **bilbo_moria** » 09 août 2006, 17:06

slajeune a écrit :Hi All,

I am not very familiar with the G5 but here are my thoughts. From my understanding, past a certain point on the gain knob, you have both sources of distortion preamp and power tube distortion. To tame the preamp distortion, remove the second bypass cap (cap C5 on the V2 schematic).

Interesting analysis, to be verified by experimentation ...

slajeune a écrit : As for the power tube distortion, I have no information on the the actual voltages involved, but I think that 120ohm as a cathode resistor for an EL34 makes it biased hot and therefore, generate quite a lot of distortion. I would try increasing it to about 200ohm to see if it gives you a bit more headroom. As a side note, on a similar AX84 project they use a 330ohm cathode resistor for the EL34 but at a higher plate voltage.

The actual bias (cathode) current for the EL34 is ~70mA so we can't say it's hot ... as the typical operation would be 100mA for the anode current and 15mA for G2 current (and then 115mA for cathode current) ...

Am I somewhere wrong ?

sjhusting · Message non lu par **sjhusting** » 09 août 2006, 21:08

I'm not really detecting any serious power amp distortion, except what may have been due to the driver stage overdriving the grid. With an ecc82, it stays clean all the way up.

Oh, and mine is based on the V1 schematic (with the addition of the very important electrolytic on the screen of the EL34), so the cathode resistor is still 150R. I was planning on changing it to 120R so I could switch EL34 and 6L6. I measured 80mA over the 1R resistor.

steven