PP EL84 tone problem

[MarcinD]

Hi,
This is my first post here so I hope I will not break any rule at the beginning:)
I'm designing and building guitar amp
2 channels with fx loop, 2 rectifiers (I wanted to make it versatile to test some ideas for next builds ;))
I based my design on JCM800 preamp as a starting point with ability to ad additional cathode bypass caps. I drawn schematic and PCB for this amp and build it. Basically the amp is working but the sound is quite dull, muddy and in general without live:) additionally I have RG550 with dimarzio fred and double whammy and new Fender american II strat HHS (with v-mod2 pickups) and Ibanez sound quite nice but Strat is from the other to bright (especially with humbacker) and this is rather unpleasant. Could someone more experience look at schematic and maybe give some ideas! I would be so much grateful :)
Additionally I'm using
OT-Hammond 1760E
PT-Amptone 2x+290V Anode 80W
choke Hammond 194A
V1 - 12AX7 EH
V2 - ECC83 JJ
PI - ECC81 JJ
Power tubes 2x Sovtek EL84

[shooter]

a 2 second glance at preamp


under all conditions?  there's like a ton of "switched" conditions

[MarcinD]

Bypass switches are not populated right now. Current configuration is basically JCM800, 2.7k bybassed with 680nF at V1 and unbypassed V2 and V3. Power stage in pentode mode and solid state rectifier. I'm wondering if Plate voltage on EL84 is not to high and I already degenerated power tubes (with diode rectifier it is about 340V on plate and 320V on suppressor grid). I finished it two months ago and I tried to make lots of modifications (voltages on preamp, bypass caps, operating points, interstages dividers, filters, coupling caps, feedback on power stage, bypassing feedback resistor) I even build cabinet and tested with Jensen C12N and Creamback. I'm starting to have ideas shortage :D I'm feeling that I'm missing something, there are so many variables so I would like to create solid baseline that's why I decided to start with simble JCM800 preamp and simple power stage unit.

[Latole]

Are EL84's tubes bias are ok ?
Schematic show 6V6's !

How many ma on each cathode ?
Plate voltage ?

[Jalmeida]

Hi,
This is my first post here so I hope I will not break any rule at the beginning:)
I'm designing and building guitar amp
2 channels with fx loop, 2 rectifiers (I wanted to make it versatile to test some ideas for next builds ;))
I based my design on JCM800 preamp as a starting point with ability to ad additional cathode bypass caps. I drawn schematic and PCB for this amp and build it. Basically the amp is working but the sound is quite dull, muddy and in general without live:) additionally I have RG550 with dimarzio fred and double whammy and new Fender american II strat HHS (with v-mod2 pickups) and Ibanez sound quite nice but Strat is from the other to bright (especially with humbacker) and this is rather unpleasant. Could someone more experience look at schematic and maybe give some ideas! I would be so much grateful :)
Additionally I'm using
OT-Hammond 1760E
PT-Amptone 2x+290V Anode 80W
choke Hammond 194A
V1 - 12AX7 EH
V2 - ECC83 JJ
PI - ECC81 JJ
Power tubes 2x Sovtek EL84

Have you double and triple checked that your layout matches the schematic? Perhaps post the layout.

[MarcinD]

Are EL84's tubes bias are ok ?
Schematic show 6V6's !

How many ma on each cathode ?
Plate voltage ?

Yes, sorry for that! I designed with possibility to bias also 6V6 and it was my initial thought to use them but I had pair of EL84 so i used them instead.
Right now J21 is shorted to ground so circuit have automatic bias (12.3V on cathode resistor) so what on my eyes it looks decent.
Regarding measuements:
Tube 1:
Plate voltage 329V
Cathode resistor voltage:12.25V
Current: 45,7mA
Grid voltage 340V

Tube 2:
Plate voltage 328V
Cathode resistor voltage:12.3V
Current: 46mA
Grid voltage 340V

I checked components few times and I didn't found any error in this area but will checked it once again. I found screenshot quite clear.
Just a note I made few stupid mistakes on pcb like (no PI input cap or "+" sign on PCB on wrong pin) but during assembly I fixed it.


All Best!

[Latole]

EL84 is a 12 watts tube

Bias tube 1 ; 329 X 45,7 ma = 15 watts
              2 ;  328 X 46. ma  = 15,09 watts

[MarcinD]

Hmmm… that’s true, I’m almost sure I calculated it already… but either way I’ve got question in ac15 there is similar situation, similar bias point. So how it is working properly? :)

[Latole]

Hmmm… that’s true, I’m almost sure I calculated it already… but either way I’ve got question in ac15 there is similar situation, similar bias point. So how it is working properly? :)

Vox style amp in "Class A" drives tubes over specs, that is the way they are designed to have their unique tone

[MarcinD]

So let me ask other way, maybe You have some experience already with such output trafo, voltages what can You guys recommend as startig point?

All Best!

[labb]

Those EL84 should be red plating if those numbers are correct. I just had a set of JJ’s red plate at 11.9 watts.

[Latole]

Those EL84 should be red plating if those numbers are correct. I just had a set of JJ’s red plate at 11.9 watts.

Wich amp ?

[labb]

18 watt JTM 45. It had a shared cathode resistor of 150 ohm. Plate voltage of 344 VDC. Your grid voltages don’t look right. Normally plate higher than grid

[PRR]

328 X 46. ma  = 15,09 watts

Is that cathode current? In EL84, screen current is at least 10% of cathode current, so plate only gets 42mA, which is still 13.6 Watts...

But this can easily be trimmed on smoke-test?

[MarcinD]

Yeap, that’s true I measured od cathode, I will redo measurements on anode side then.

[Bieworm]

On our 18 watt amps over at 18watt.com we always use a 180R cathode resistor on PP EL84 amps with about 340-345V on the plates. We also advise using a cathode bypass cap of at least 1000uf, preferably 2200uf/63V. This keeps the low end in a better spot.

[MarcinD]

Hmmm…
I understood 180 ohms common rasistor for both cathodes right? And what about suppressor screen resistor?:) and additional question regarding OT (1760e) which im using I think it is quite small, do you think that changing it to something bigger for example 1750pa will be beneficial for tone?

Btw I connected external bias and biased for 330V/33mA and it is already much better!

[Bieworm]

Hmmm…
I understood 180 ohms common rasistor for both cathodes right? And what about suppressor screen resistor?:) and additional question regarding OT (1760e) which im using I think it is quite small, do you think that changing it to something bigger for example 1750pa will be beneficial for tone?

Btw I connected external bias and biased for 330V/33mA and it is already much better!

On the screens a 1k for each tube and yes, I generally use the 1750 PA. Never tried another OT in those amps

[nworbetan]

18 watt JTM 45. It had a shared cathode resistor of 150 ohm. Plate voltage of 344 VDC. Your grid voltages don’t look right. Normally plate higher than grid

Quoting this because I think it's worth looking into.

Using your measurements it looks like the resistance of the OT primary might be approx 240 ohms.  Is there a chance that you put R59 in the wrong place and it's affecting the plate voltages but not the screen grids?

Also, to get it to sound a little closer to a JTM45 you might want to consider adding a ~1K resistor between the B+ and the two 470 ohm screen resistors (R62 & R63).  This will also lower the screen grid voltages a bit, but you should verify the plate supply is correct before adjusting this, imo.

[MarcinD]

So OT is wired directly to tube socket but
1. I changed screen resistors in pentode mode to 1.8k (I didn't had any 1k resistor unfortunately) but results are:
- B+=352V
- V screen = 332V
- V Plate = 335V
- I anode = 32mA
- P = 10,72W (disipation on tubes)

Amp started to sound warmer and tighter it is much better now than at the beginning. Tomorrow new OT should arrive so I will check also the new one and I need to order 1k resistors!

BTW: OT primary winding dc resistance (one half) = 160 Ohms

[labb]

17 volt drop across the OT and 160 ohm resistance would give you I=v/r   17/160= .106 amps. How did you get the 32 ma

[MarcinD]

Measured to cathode not to ground :)

[MarcinD]

Hi!
So I tested amp this weekend during small jam session with new resistor on cathode and screen and damn that sound good! Warm, fat crunchy sound with lots of "chugs" and with tube screemer amazing very focused rhytm tone! Thank You guys for all Your support! Old transformer will be used for some matchless clone ;)
BTW i have few additional questions:
1. What is the best way to attenuate signal on Master pot. Right now basically 20% is starting to overdrive output stage
2. What is the best place to put effect loop. RIght now I have it post Master pot and hmm... with FXloop on master starting to be a bit useless

All Best
Marcin Dudek

[Bieworm]

That is a little typical on these amps. There is a lot of volume soon on the dial. Hard to use as a quiet amp, but at a good/kinda loud volume they sound like heaven. I would move the MV after the PI. The FX loop right before the PI. My 2cts

[Latole]

To have better control on volume pots, use log or A pots.
If your pots are linear or B pot, this is the issue

[Bieworm]

To have better control on volume pots, use log or A pots.
If your pots are linear or B pot, this is the issue

I know, but that’s not the issue. I always use log pots for volume

[MarcinD]

Yeap, I'm also using log pots for volume ;) just gain in general is very big, I will probably just attenuate signal with resistor (470k res and 500k/A Master pot) at least for drive channel. Anyway amp sound sweet so if someone is interesting to build it, I can share schematics and gerber files for PCBs (as soon I will update PCB for front panel because right all pots are turning left ) Few photos of build in attachment:)

[Latole]

Replace a 12AX7 ( 110 % gain) by a 12AY7 ( 45% gain)

[MarcinD]

You mean on phase inverter? That's actually quite a good idea, next time I will order something I will add this tube to order and will give a try.

[Latole]

You mean on phase inverter? That's actually quite a good idea, next time I will order something I will add this tube to order and will give a try.

Most of the time it work very well as V1

[MarcinD]

Hi There!
I thought the problem is solved but unfortunately I'm still thinking that this not how it supposed to work. Especially that I finally was able to change panel PCB and now I'm having also some stability issues on high volume with tube screamer in front of amp. Can I guys ask You one more time to help :) I can do measurements if You will need something else:)

Main problems:
1. I think output power is to small (1kHz signal is giving max 9,59V undistorted signal over 8ohms resisitor (11,5W)
2. Master is usable up to 3 out of 10. Then tone starting to be really unpleasant and stability become a problem.
3. In general I would expect rather creamy tone and I'm getting shrilling and unpleasant especially on higher volume.

My guesses
1. Choke can be to small hammond 194A (however this replacement for 2x6V6 design)
2. Maybe I should try to connect common tap of OT before choke as in soldano?
3. PI is starting to have strange "spike" in waveform (on one side of wave form) at higher gain and in general maybe there is to much gain in PI for EL84?

Schematics is in attachment with measured voltages!

additionally below You may find all measurements I've done:
Stage / B+ / A / K // input AC (RMS) / out AC(RMS) / gain
V1 / 254V / 195V / 1,5V // 50mV / 2,72V / 34,7dB
V2 / 254V / 230V / 2,2V // 50mV / 325mV / 16,3dB
V3 / 261V / 147V / 0,83V // 49mV / 2,2V / 33,4dB
V4 / 260V / 260V / 148V // 2,26V / 2,35V / ~0dB
Inv1 / 280V / 148V / 36,8V // 50mV / 415mV / 18,3dB
Inv2 / 280V / 138V / 36,8V // 50mV / 382mV / 17,7dB

Output tubes - autobias (130ohms cathode resistor), Raa = 8400ohms
Tube / B+ / A / Sg / K
PV1 / 357V / 346V / 347V / 13,5V
PV2 / 357V / 346V / 348V / 13,5V

Caluclated output power on 8ohm resistor
Vo = 9,59V (measured just before clipping on output tubes)
Po ~ 11,5W which is rather low for 360V B+ (Am I right?)

I'm using OT (Hammond 1750PA)

I will really appreciate all Your hints!

[sluckey]

Swap the OT primary plate leads at the tube sockets. Better? Worse?

[MarcinD]

Hi! Thanks for answer! I will definitely check this :) why did You thought about it because of low output power or overall tone problem? I’m asking because I will be able to check tone tomorrow on lower volume and unfortunately I will have chance to really crank it up on proper cabinet in friday and why do You think it should help?

[kagliostro]

Can you post a schematic ?

Franco

[MarcinD]

Can you post a schematic ?

Franco

Hi! I did in attachment of my last post (pdf file) :)

All Best!

[sluckey]

Swap the OT primary plate leads at the tube sockets. Better? Worse?

why do You think it should help?

It may not help. I suggest this because you have not mentioned that the NFB phase has been verified. Incorrect phase does not always result in obvious scream or howl. Sometimes the symptoms are more subtle. Anytime you build an amp that has a NFB loop you need to test to be sure the phase is correct, so swap the PT primary plate leads to be sure. Better? Worse?

[MarcinD]

Ohhh, sure:) I checked NFB and I know it very well because I did it wrong with connected speaker (my neighbors also are aware :D) but anyway I just measured it and it looks ok:)

And a little update cathode resistor is 180 not 130:)

ALl Best!

[PRR]

Can you post a schematic ?

Bottom of Post #30.
Or:   https://el34world.com/Forum/index.php?action=dlattach;topic=30899.0;attach=110624

[kagliostro]

Try to add a Grid Stopper to V1B and V2A ( try with 10K)

Connect the Stopper directly to the pin of the tube

Franco

[MarcinD]

Tomorrow I will have possibility to crank up and check so I will definitely try it:) One think that is curious is why I'm getting only undistorted 10-11W out of amp. I changed bias a little and now I have B+ = 366V, currents in tube 1 = 28mA, tube 2 = 26mA, anode voltages = 355V on both, I also changed screen resistors to 4,7k and now I have 345V of screen voltages and on cathode resistor 12,87V (bias is mixed resistor + external voltage).

[kagliostro]

...One think that is curious is why I'm getting only undistorted 10-11W out of amp...

Unwanted HF oscillations ??

I missed the world undistorted

Franco

[shooter]

undistorted 10-11W out of amp.

is that based on a scope of the speaker n math or just tube dissipation?


I would expect about 11/12W clean at the speaker in a Class A amp.

[kagliostro]

To fix excess in gain try with Plate Split Load Resistor


Franco

[MarcinD]

undistorted 10-11W out of amp.

is that based on a scope of the speaker n math or just tube dissipation?


I would expect about 11/12W clean at the speaker in a Class A amp.

Measured on resistor connected to output and ram measurement in scope. Then simple math. On speaker there will be phase shift and without measuring current also it will be difficult to calculate power directly.
Kagliostro plate split load?:) could You elaborate?

[shooter]

for audio phase angles are in the imaginary number area, not to relevant to rock n roll amplifiers.


I've build a dozen, mostly SE Class A's, couple PP, clean is ~~~75% of MAX audio, tube power/dissipation is whatever gets me the biggest cleanest Audio power without melting the glass.
here's how I do Max clean output.  I even do some math for voltage gain and audio power, based on the values the scope provides.

[kagliostro]

.... plate split load?:) could You elaborate?

See the attached schematic

in the example I used a 120K plus a 100K resistor

this will result in near 50% signal feeding the following stage

but you must find the right values for your gain stage, say 150K + 68K that will feed the following stage with a higher signal respect the previous example, you must try different values remembering that if your original Plate Resistor is 220K the sum of resistors values must be 220K, this will maintain the character of the original stage and feed the following stage with less signal level

Franco

[MarcinD]

I guess that this is to improve master volume usability, basically I lower NFB resistor to 47k and it lowered overall gain what basically fixed the problem, additionally I did some changes in power stage and the tone is much better but there are still some problems that I would to ask You to support:)

1. Stability issues - on high volume, high gain especially with some boost (like tube screemer). It looks like this is problem in preamp especially that I recently changed front panel (on first PCB I did it a little bit too fast and potentiometer was on wrong side and I did some change in channel switching:D) those ares basically the biggest changes. In attachement You may find schematic comparison of new and old panel (on new tone stack LDR is not installed and R33 is 33k.
           What I tried
           1. I tried to put 100pF capacitor on first stage (between anode and cathode)
           2. I untwisted grid from anode and cathode wires of each step.
           3. I replace 68k V1 grid stopper with 33k and put it directly on tube socket
           4. I added 10k grid stoppers resistor on V2 and V3
Nothing really help, the only way to play with higher gain is on relatively low volume.
2. How to properly connect input jacks (hi and low) - especially shields of those shielded input cables. I guess using shild as gnd connection is not the best option. Maybe I should use two shilded wires for GND and signal and both shields I should tied to GND on PCB? What do you think?
3. I noticed that stage 2 and 3 are particulary sensitive if I will put my finger close to elements for stage 2 and 3 there is very well audible hum in speaker, what is interesting there is no such effect near stage 1 where theoreticaly gain is the highest.
4. I think I have some ground loop problem in fx loop and in general it looks like recovery circuit in fx loop have to low gain (even with fx volume on 10 guitar is quiter than without fx loop)
5. In general there on higher volume there is quite a lot of noise (not hum), I assume this is mostly resistors thermal noise. Could You give a hint how to start investigation or where to look?:)

All Best guys and thank You very much for all of Your support!

[Bieworm]

3. I noticed that stage 2 and 3 are particulary sensitive if I will put my finger close to elements for stage 2 and 3 there is very well audible hum in speaker, what is interesting there is no such effect near stage 1 where theoreticaly gain is the highest.

You’re interpreting this wrong. Although the gain in stage 2 may be less than it is in stage 1, the signal is larger to begin with. IOW, stage 2 takes off where stage 1 ends

[AlNewman]

You should only need shielded wire for signal lines, and they can only be grounded at one end, otherwise you could end up with all sorts of issues.

Normally the really critical connections for shielded wire are at the input jack to 1st grid, or any long runs, (like between pots, etc.)