Super Reverb Attempt to Clone with ugly distortion

[Lee_ranaldo]

Hi folks!

i've received this amp a long time ago and i rebuild it completely.
first it had wrong values everything. Coupling caps in the tonestack were wrong

I've replaced all the coupling caps, grid resistor and 1.5 Resistor. The reverb transformer went bad (had internal arcs) so u could see those sparks inside the reverb driver tube. Replaced all the pre-amp tubes.

Which is the symptom?..

It suffers from blocking distortion that means if u go above 4 the amp start to farting on even though u lower the bass (it happens less on the high impedance input on the vibrato channel). Also i can't get rid of the vibrato ticking even though i've installed a cap that filters it.

Changed the cathode bypass caps and the resistors (were wrong).

I redid the whole ground wiring (using ground bus method). this took off a lot of ground noise (almost zero)

The only difference is this amp uses a solid state rectifier and looks more like a blackface twin reverb circuit with the tonestack of a super reverb.


It keeps farting, before i couldn't use the bright control and turn the volume up. I've disconnected the feedback and the amp seem to had more headroom but more noise..(hum but when u have a faulty instrument cable)

gonna mod the schematic and post it here.

Any suggestions?

[tedmich]

A fender with ugly distortion? Congratu....oh not the good kind...

Others will know more but if it had a tube recto the B+ may be +20v higher than it should be, check the anode resistors to be sure they are the stock values. A zener (or zener + MOSFET like R.G. mentions) trick could be used to pull the B+ down to stock values.

[Lee_ranaldo]

Originally Posted by tedmich

A fender with ugly distortion? Congratu....oh not the good kind...

Others will know more but if it had a tube recto the B+ may be +20v higher than it should be, check the anode resistors to be sure they are the stock values. A zener (or zener + MOSFET like R.G. mentions) trick could be used to pull the B+ down to stock values.

I'm getting like 425-430 Volts DC on the plates so it's not that issue i guess. This amp is a clone. This was made by a known "amp builder" here. He used to clone amps in the 70's.

So this chassis is made of thick aluminum, a brownish hard board, eyelets and everything. I had to replace the heather wires since it had a very thin one. The ground paths were completely messed up.

The transformers are also handmade so dunno about interleaving and stuff. I'm replacing the 220K resistors that are connected to the bias circuit since they're carbon resistos and are drifting in value (i've measured like 260K instead of 220K).

This is close to a Super Reverb AB763 with a SS rectifier. This Rectifier has 4 diodes instead of 6 like any normal SS rectifier found on most fender's so dunno if this has a impact in the final performance or also the lack of ceramic caps in parallel with the diodes.

anyway i'm re-wiring this , shorting the leads. but the thing that amazes me that the amp sounded better without the feedback connected to the Output transformer. Is this "normal"?

Thanks for your reply mate!

[tedmich]

Originally Posted by Lee_ranaldo

anyway i'm re-wiring this , shorting the leads. but the thing that amazes me that the amp sounded better without the feedback connected to the Output transformer. Is this "normal"?

Little seems normal about this amp! A shot in the dark but a backwards OT primary will give positive feedback (PFB) and sound horrid, switching the primaries could then give a real NFB which might sound AOK.

[Chuck H]

What?!? An amp that's had all kinds of problems, may yet have more, and it has ugly distortion

Just kidding. Seriously, you can't partially rebuild an amp when you don't know the entire circuit unless you understand enough about electronics to design unique circuits that will work with the rest of the amp. In other words, restoring the PI components to stock Super Reverb (or whatever) values is only a proper fix for a Super Reverb and could cause new problems in an amp that is not a Super Reverb.

I think you should either rebuild the whole thing into one known circuit and layout or turn the amp over to a skilled tech for repair. Otherwise you run the risk of fixing parts of the circuit only to find new problems were created and then fixing those... etc. I've seen guys here chase problems in odd builds and conversions for months only to decide they don't really like the amp. Don't let this happen to you.

IMHO an amp with a bad ground scheme, oscillations, drifted values, incorrect value components and ugly distortion is a good candidate for a new build platform. You can't polish a turd, but you can use for fertilizer to grow something new.

Chuck

[Lee_ranaldo]

I actually did that..

Rebuilded..

and i'm electronic engineer and also a sound engineer so i guess i could handle this kind of problem :P

i hope

[MWJB]

Can you post some pics.

4x1N4007 diodes for the rectifier are fine, Fenders typically did not have caps bypassing the diodes until their very last days.

[Alex R]

Negative feedback cuts noise and oscillation, makes the amp more linear, and also cuts gain. Without it the amp will be gainier and kind of 'looser'; most guitarists like the non-nfb sound better but it does cause problems sometimes.

Guessing, but from all you've said it seems likely that disconnecting the nfb loop has given rise to the problems you describe, by allowing the amp to oscillate when you turn it up. You might experiment with the resistor value in the nfb loop, increasing it as far as you can till the problems start.

I'm not an electronics engineer, but plenty of them bring their amps to me to be fixed. There again, I couldn't design you a switch-mode power supply. Horses for courses.

[Lee_ranaldo]

Thanks for ur reply mate

the problem is this was built as a twin reverb with the tonestack of a super reverb (AB763) except this differences

"on the twin reverb design there is a 100K and 82K resistors after the Phase Splitter tube." Same as on this amp..

Super reverb AB763 has 2 100K resistor in the same place (i've replaced them).

The other minor difference is the vibrato tube, Twin has a 100K cathode resistor with a 25uf cap, i've replaced it with a 22uf cap and this 100K resistor but found out that the 56K in the super reverb will make this stage have an little bit less gain maybe?. that will cut the annoying thicking on the vibrato that i've already tried to fix with a cap (0,02uf accros the 10M resistor or directly to ground).
every cap has been replaced except the filter caps. the resistors also has been replaced mostly the 220K ones found in the 70uf caps and the ones in the bias splitter section.

i'll attach the schematic of the circuit that is close to this amp.

BTW it has Mesa Boogie Tubes.

[Alex R]

- suspect you're looking in the wrong place. Try and see if it farts out with the nfb loop back in. The changes to the tone stack etc don't sound like they'd make an awful lot of difference to anything, to me.

Ticking vibrato is a whole nother thing.

[Lee_ranaldo]

Originally Posted by Alex R

- suspect you're looking in the wrong place. Try and see if it farts out with the nfb loop back in. The changes to the tone stack etc don't sound like they'd make an awful lot of difference to anything, to me.

Ticking vibrato is a whole nother thing.

it farts with the NFB
without it (but has a cleaner sound) = more headroom.

i'm gonna test it when i reach home. since i've been busy with other amps.

[Chuck H]

You might try reversing the OT leads on the power tube sockets and reconnecting the NFB loop. The amp should have LESS gain with the loop in place. If there is more gain with the loop in place you must have PFB (positive feedback) and this can make an amp unstable in it's worst (and usual) case.

BTW I didn't mean any disrespect by my last post. I tend to be glib or droll and I didn't know your chops.

Chuck

[Lee_ranaldo]

Good news

i've fixed almost everything except.

The only channel with problems is the vibrato channel

When i pass till 4 it gets that ugly "broken speaker" distortion
but on the normal channel i have no problems

got rid of the ticking on the tremolo section
no hum noise on the background
performs like a twin reverb.

i've also installed 2000pf 630V film caps from 1,5K resistor to ground as shown in the superreverb AB270 model (helps with ultrasonic oscillations).

anyone has a clue of what's happening here?

[MWJB]

Please post dc voltages for all tubes, please post a pic of the circuit board, pots, tube socket wiring.

With a good layout those 2000pf caps are not necessary, Fender only started using them when they revised layout in the SF days...by '78 they appear to have been employing chimps to wire up their amps.

What's your idle plate current?

[Lee_ranaldo]

i have the bias voltage set very low for testing purposes.

i've fixed the problem

was a leaky cap... replaced that one with a brand new poly film 0,1 cap
and the farting problem on bass frecuencies in the vibrato channel is gone

gonna test the amp a few hours and then remove those 2000pf caps. Before thius the amp had ultrasonic distortion problems.. the sound was cutting off pass 4 on the vibrato channel.

Everything works in order now.

THanks for all your replies mates! i'll post the idle current measurements soon since i'm gonna bias this pair of tired mesa powertubes

all the pre-amp tubes are brand new JJ/TESLAS

[Lee_ranaldo]

im still getting blocking distortion on vibrato channel with this settings

max bass, middle at 6 , trebble at 10 and volume at 3-4 (with bright switch up)

the amp sounds cleaner though

i still get some low but nasty distortion on when i set the intensity all way up
and a strong ticking

since this amp doesn't have an original fender photocell and neon lamp unit
i'll suspect from this. However the bias since it's low (just for testing purposes) may cause this.

Although, this amp has a 3.3M resistor in series with the 1Mohm resistor on the input of the first triode of the tremolo circuit

dunno how this affects the overall performance

[MWJB]

That's a lot of bass...?

Cold bias (low current/high negative voltage) won't cause blocking distortion...too much current can make the amp rather garbled. You say you have low current for testing...what is it?

Some pics & dc voltages might really help to pin down a suspicious looking part or triode, especially if there is no exact schem for the amp.

Try and isolate the area in which the blocking distortion is taking place, pull reverb driver & tremolo tubes to eliminate those parts of the circuit (though I seriously doubt the vibrato tube is screwing up your tone), then look for low plate voltages/undersized cathod resistors, oversized bypass cap & coupling caps. Make sure you have a 12AT7 in the PI position.

[Lee_ranaldo]

i don't have a camera right here to take pics.

will do that now.. i'll post the values here

[Lee_ranaldo]

I've did the measurement again
Bias Voltage (just for tests) = - 56 Volts DC

It's an Excel Spreadsheet

Thanks in Advance.

[MWJB]

-56v tells me nothing unfortunately, you can't set or read bias by negative voltage alone, we really need to know the plate current in mA. Either fit 1ohm cathode resistors to the power tube sockets & read dc voltage accross these, or shunt the OT (use clips/wire grabbers on the meter leads, make connections with amp unplugged from the wall).

From your voltage chart, plate & cathode voltages at V1 & V2, pin 8 are strangely high...what are the value of the cathode & plate resistors feeding the second halves of V1 & V2?

[MWJB]

Heater voltages 5.86VAC? That's no good, low heater voltages will sound mushy with poor fidelity. Any PT serial numbers/ratings available?

Bias the power tubes to 30mA per tube, pull any unused preamp tubes, avoid using a variac or similar in line with the power suply, now what are your heater voltages?

[MWJB]

V5 voltages look high, but -dc to grid looks low at -37v. Cathode voltage at the 2nd triode seems low.

Given your B+ is only in the early 400's check that you have the appropriate plate resistor & cathode values. There doesn't seem to be the expected voltage drop accross the plate resistors.

[Lee_ranaldo]

all the resistors are stock ones, double checked values. ( Super reverb AB763 Circuit ). THe only 2 different resistors are a 22K resistor in the bias splitter section (instead 27 K shown on schematic) and a 100K cathode resistor on the vibrato section.

So Cathode Resistors are the stock ones, Those are brand new and also the caps.

The B+ without tubes it's almost 445VDC.
The resistors in the cap sections (1K and 4,7K) are ok.


Maybe it's because it's a little bit high since it doesn't use the recfifier tube or since the PT is homemade.

[MWJB]

Then you must have a poor ground connection at V1/V2 pin 8s. Voltage at pin 6 of V1 & V2 should be close to pin 1 (which looks OK). Yours are 50v higher which is suspect.

B+ isn't the issue, you might see up to 525vdc on a Super Reverb. It's the discrepancy at pin 6 plates of V1 V2 that's the problem. Your B+ is low for a Super Reverb.

Are the pin 8cathodes for the first 2 tubes seperated, or do both tubes share 1 resistor. If shared, should be 820ohms. If seperated should be 1500.

[Lee_ranaldo]

it's all ok all the resistors. the ground connections too
i did the bus wire ground scheme on this and everything is properly connected EXCEPT

that cathodes from V1 and V2 were connected to the 2.2K resistor and the V3 cathode was connected to the 820 ohms resistor. the farting on the vibrato channel stopped

thanks mate for your help seriously!

[MWJB]

Good news! You're welcome.