Vox Super Beatle

rockinrob86 · Sep 29, 2020

I’ve been working on a vox super beatle, and I think I am on the right track, but wanted to run some measurements by someone who actually knows what they’re talking about, especially with solid state amps!

Anyways, this is a garage find I have been restoring. All of these tests were done with a 130mv ac 1K sine wave signal from my signal generator (also measured 306 mv dc, is that normal??)

Plugging the signal into the preamp, I can turn the volume to 4ish before the wave starts to flatten. I set it to right before it flattened, and I measure 600MV AC and 1.185vdc out from the preamp.

Plugging the preamp into the power amp, I measure at the output 1.77vac and .024vdc and a clean wave on my scope.

If I plug the generator directly into the power amp, I measure 2.6 VAC and 0.48 VDC and a clean wave.

If I have no input and am not viewing the power amp on my scope, I see 400mv dc. With a speaker attached, I measure 164mv dc.

My questions:
do I have an issue with DC offset? I don’t want to fry a speaker!

Why does the preamp amplify the signal generator signal, but result in a lower VAC output from the power amp than if I put the same signal generator signal into the power amp directly?

Do these voltages seem reasonable for both the output of the power amp and the preamp? I am using it into an 8 ohm cab, which according to specs should result in 30w output from the head. It didn’t seem very loud, but I didn’t test it with a guitar for long because I was concerned with the possible DC offset issue.

Gus · Sep 29, 2020

Have you looked around the following site?
http://www.geofex.com/

rockinrob86 · Sep 29, 2020

yep, I actually purchased and installed his preamp pcb for the amp. He doesn't really cover much on what the operating specs of the power amp are supposed to be, and I was trying not to annoy him with a more general question.

I have no idea what a normal AC voltage would be for a solid state amp, and I am not sure about the high DC I am seeing on the output with no load, so I don't feel comfortable testing further without some confirmation. I usually work on tube stuff, and solid state still seems like black magic!

CJ · Sep 29, 2020

which preamp channel are you using?

mid boost on? distortion boost on?

what are your tone controls set at?

those 60's transistors are gonna drift after this long, the schematic says 0 to 450mv offset, which is pretty loose compared to todays standards, but there is a transformer between the driver and output devices so hopefully that will prevent a speaker melt down,

what kind of pwr transistors?
original might have been germanium,

there is a compressor/limit circuit on the original, do not know if you have it with the new board, but there is an adjustment pot for limiting, which is probably why you generator and preamp are outputting different voltages on the power amp.

do you have a schematic for the original amp?

i have a pdf, let me know if i should post it,

i added extra heat sinks to the Beatle i worked on as they had a rep for overheating then failure. would not want to cook some Bulldogs if you have them,

you might try swapping a pair of the output transistors to see if offset is reduced,

i would install new 220 10W resistors, they have probably drifted,

and i would check the .22 ohm resistors, or just change them,

do you have a DC pwr supply with ammeter?

determine 0.22 ohm DCR by running say 500 ma thru the resistors, measure voltage across resistors,

DCR= V/I, so for 500 ma you should read .5 x .22 = 110 mv DC.

or just put 100 mv across resistor and see what I is, divide it into V and you have DCR. normal leads on DMM will skew these low DCR measurements, and hooking up a 4 wire meter takes longer than it is worth and more cerebral than using Ohms Law.

rockinrob86 · Oct 1, 2020

CJ said:
which preamp channel are you using?

mid boost on? distortion boost on?

what are your tone controls set at?

those 60's transistors are gonna drift after this long, the schematic says 0 to 450mv offset, which is pretty loose compared to todays standards, but there is a transformer between the driver and output devices so hopefully that will prevent a speaker melt down,

what kind of pwr transistors?
original might have been germanium,

there is a compressor/limit circuit on the original, do not know if you have it with the new board, but there is an adjustment pot for limiting, which is probably why you generator and preamp are outputting different voltages on the power amp.

do you have a schematic for the original amp?

i have a pdf, let me know if i should post it,

i added extra heat sinks to the Beatle i worked on as they had a rep for overheating then failure. would not want to cook some Bulldogs if you have them,

you might try swapping a pair of the output transistors to see if offset is reduced,

i would install new 220 10W resistors, they have probably drifted,

and i would check the .22 ohm resistors, or just change them,

do you have a DC pwr supply with ammeter?

determine 0.22 ohm DCR by running say 500 ma thru the resistors, measure voltage across resistors,

DCR= V/I, so for 500 ma you should read .5 x .22 = 110 mv DC.

or just put 100 mv across resistor and see what I is, divide it into V and you have DCR. normal leads on DMM will skew these low DCR measurements, and hooking up a 4 wire meter takes longer than it is worth and more cerebral than using Ohms Law.

Thanks CJ,

I used the normal and bass with similar results, tone controls set to flat. It has the original germanium, the new board still has the limiter (I forgot about that!), I have a pdf schematic I found through google, the resistors measured good, and I do not have a bench power supply.

I ordered some new transistors RG recommended online, but didn't want to replace them unless needed, as they will need some finagling and I don't want to create more problems guessing.