booooomy sound, anything looks odd in this tone stack pic?

[fkitch]

New filter caps but still super super boomy sound and bass control does not appear to do anything (testing with a guitar and 12 inch speaker)
Anything obviously wrong, or not guitar amp compatible, here please? No schematics available, appears to be a take on an AC30.36 Bass, more pics here https://el34world.com/Forum/index.php?topic=31154.msg343339#msg343339 
All tubes glow, I just have one spare EL84 and I want to rule out as much as possible before investing in new tubes. Very basic, I know, but it's where I am! Thanks

[Ronquest]

What do you want from this amp?  Restore it or work it into a guitar amp?  Forget the AC30 schematic and draw out what you have, if you care enough to have a schematic of it, as it sits.  A schematic will get you more help here too.  Hard to tell what we're looking at, as half the wires disappear and there's no good layout to follow.

[fkitch]

What do you want from this amp?  Restore it or work it into a guitar amp?  Forget the AC30 schematic and draw out what you have, if you care enough to have a schematic of it, as it sits.  A schematic will get you more help here too.  Hard to tell what we're looking at, as half the wires disappear and there's no good layout to follow.

I understand, thank you, yes the idea is to turn it into a guitar amp. I started noting down the differences with the AC30 but you are right , will be better to have a complete schematic so that you guys can actually help. I will be back! (may take months LOL).
BTW last night I found out that one of the PTs 6.3V outputs is giving less than 1V, missed that in my initial checks, need to be more patient…
Thank you

[Bash]

Nice old amp.
That 2n2 capacitor looks a bit bloated, but that could just be the picture.

You could try to desolder one leg of it, so you can measure whether it is still Ok and hear whether it made any difference at al.

[fkitch]

Nice old amp.
That 2n2 capacitor looks a bit bloated, but that could just be the picture.

You could try to desolder one leg of it, so you can measure whether it is still Ok and hear whether it made any difference at al.

TY! Will check that
Ok, this PT is not something I understand, the 6,3V secondary outputs have, each one, a 340 Ohm resistor to ground (and as I said before one reads just 0,6V)
The EZ81s filaments , pins 4 & 5,  if I am not wrong, are connected to a different PT secondary that reads (always without tubes) 24V in one pin and 19V in the other. No resistors connected to this secondary, I understand EZ81s need 6.3V on their filaments?
The high voltage secondary appears ok 325V steady , it’s CT is connected to ground.
Looks I have a few things to do before actually worrying about the tone stack!

[sluckey]

The correct way to measure filament voltage is to connect one probe directly to a PT heater lead and connect the other probe to the other PT heater lead. Don't connect one meter probe to chassis because you may get some strange or misleading readings.

[fkitch]

The correct way to measure filament voltage is ...

6.8V AC without tubes, 6.5 AC with them
Thank you Sluckey

[AlNewman]

I replied to your linked thread, but reducing your coupling cap at the output tubes could help.

[WimWalther]

Just FYI, those gray painted resistors are notoriously unreliable. I'd consider replacing them on sight.

So is this thing European? It sort of has that look..

[fkitch]

Just FYI, those gray painted resistors are notoriously unreliable. I'd consider replacing them on sight.

So is this thing European? It sort of has that look..

Correct, made in Spain in the early 60s
Ok, will swap that resistor, thanks!!
Are the other darker gray, much bigger, ones also of that prone to failure type?

[fkitch]

draw out what you have, if you care enough to have a schematic of it, as it sits.

You were so right suggesting the 'reverse engineering' route, thanks
I figured out the schematic (first time I do something like this, took hours and massive use of continuity tester, there may be mistakes) and found out somebody had been messing in the power section , R30 was missing and R26 was 2,2k instead of 150R.  Sorted that and the red plating problem was gone.

reducing your coupling cap at the output tubes could help.

I swapped them for .047uf ones, C6 and C9 in the schematic, better sound, less boominess, thanks
I also change the two cathode bypass caps in V1, by mistake I installed 0.047uf ones, I am leaving them for now.
Removing the three big yellow .047  caps in series in the OT also helped with the sound

Please, I still find the amp sound too dark, would appreciate if you guys take a look at the schematic and suggest what else I could try.
Reminder, this was originally a 'contrabass' amplifier, I am trying to turn it into an AC30ish guitar amp
Also, vol control (a new 500k log pot) has no much useful range, vol gets too high too soon
About bias I get ca. 110% plate dissipation. Also, heater voltage is 6.42V, center OT voltage 345v
need to figure out the speaker impedance somehow!

[fkitch]

Checked again and found 2 extra connections that I missed initially,
Among the many things I do not understand as a novice I just can't id the high pass and low pass filters... when I look for tone stacks schemes online I find things much simpler than what I see here.
I attach the updated scheme with the 2 additional connections I just found in blue. Also i highlight in green everything I installed new so far. I removed the 1st diagram I attached to avoid misunderstandings
Please have mercy with this newbie and help sort dark tone (too much bass, not enough treble) and the loud volume thing maybe a MV or gain control?

Thank you!

[tubenit]

Download and use the Duncan Tone Stack Calculator.  Look at the James tone stack and try different values to lessen bass.


TSC (duncanamps.com) 


Another option is to change the post phase invertor coupling caps from .047 to .02.  That is what I would do given the amp uses EL84 tubes.  That change would be an "easy" fix, I think.


With respect, Tubenit

[fkitch]

Look at the James tone stack

Wow, no idea about that stack , I was looking at FMV ones only, the James - Baxandall indeed is similar to what this amp has, will investigate further thank you!!!
BTW I found the calculator online https://www.guitarscience.net/tsc/baxandall_passive.htm#RIN=0&R1=680k&R2=47k&R3=0.1&R4=1500k&R5=2.2k&RB=1000k&RT=1000k&RL=1M&C1=2000p&C2=10000p&C3=1p&RB_pot=LogA&RT_pot=LogA, no need to install software
I changed the coupling caps from .1 to .047 with a significant improvement, yes, worth trying .022 thanks again

[fkitch]

Look at the James tone stack

Tubenit ,  beers on me if you visit Madrid , or send me a PayPal account for virtual beers

After thorough investigation of James tone stack and which amps used it I replicated the tone stack of an 1973 Orange Graphics mkii  (preamp has just 2*12AX7 as mine)

Total absolutely radical change wow, sounds great!! Still maybe a bit of bass fine tuning but great tone!

So, project now has turned into an Orange graphics 1973 ‘cousin’ but tube rectified and with 4xEL84s , I will add a F.A.C. Switch and couple of things more
TY!!