Super Reverb Attempt to Clone with ugly distortion

[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]

Test Chart

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.