Tube Amp reverb noise, no vibrato intensity. Help!

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Tube Amp reverb noise, no vibrato intensity. Help!

Postby tedsorvino » Sun Mar 09, 2008 8:20 pm

Hi guys, I'm Ted and I'm from Greece. I'm new to the forum.
Some weeks ago I started restoring a very old tube amp (I believe early 70s , late 60s) with the help of a competent radio technician. The cabinet has the brand ACOUSTA and the model KING REVERB 100 W / with EQ. It looks like a silverface clone without the normal channel (even if the chassis is similar to the Twin, the normal channel has become a 5 band parametric EQ) So I plug only through the vibrato channel. The cabinet has 1 15'' inch speaker (it was empty-I replaced it with an eminence beta) and two piezo horns in parallel (I put new ones in).
So far we replaced all the electrolytic capacitors (the FIVE large filter ones, three 47uf 100v, and six 22uf100V), we changed the 4 power tubes 6L6 (original were some great silvania and Philips JAN ones) with new electro Harmonix ones (I also changed the Bias a bit). And all the pre amp tubes (some great mullard and philips and general electric ones) with 3
12ax7s and 2 12at7s (all electro harmonix) The pre amp tubes (and their sequence) are like a twin's (or anything similar) ones (minus the normal channel driver tube).
So even if I haven't mounted the chassis back in the cabinet it sounds quite clean (there is some little interference in loud volumes (haven't replaced the ac cord yet) but nothing annoying.
I also bought a good fender vintage switch with rca plugs for switching the reverb and the vibrato on and off.
So I switched on the vibrato and nothing really happens (I can hear a ticking becoming faster or slower according to the movement of the speed pot) because the intensity pot doesn't react (It's a newly installed 50k plastic one) . I guess my vibrato circuit is a bias vary one since I cannot see any roach in the chassis. I have swapped tubes but nothing changes. I have no idea if it's one of the round brown caps .01,.02, .01 etc. or the blue mallory .1 600v ones or any resistor. Could you please help me with it (I'm not good with such things and my tech is not familiar with vibrato circuits)? Is there any problem with my new intensity pot (the soldering connections are really good)?
And then a second problem. I attached a reverb tank (I didn't know the exact specifications for the appropriate reverb tank) made by accutronics (model 4BB2B3C) to the input, output rca sockets of my amp (the tank input to the reverb input, adn the tank out to the reverb out of the amp) with a simple rca
cable (it's an ok standard one). I realised as soon as I was increasing the reverb pot value, a horrible hum was increasing as well. I changed the reverb tubes to no change. I used other tanks (an old gibbs one for hammond and a smaller accutronics 8BB2A1B) BUT NOTHING CHANGED. I swapped the rca cable jacks (input to output, output to input) and even if the hum was not apparent (only a hiss) the reverb was gone. I have to note that the tanks are not mounted yet and they were quite far away from the speaker so no feedback was happening just noise like a ground loop or something. I don't know if my tanks are not appropriate for this amp (they work fine to a different amp- a carlsbro transistor one) but I cannot determine which tank would be appropriate. I don't think that the lead is a major problem or that the amp is out of the chassis And I cannot also say if there is a problem with my reverb circuit since I don't know what to check (I only replaced the tubes - 12at7 driver, 12ax7 recovery). Any ideas please?
As you realize I cannot provide any schematic about the amp since I cannot draw one and there is no available (I can't find any info regarding the amp) on the internet. So any help or info would be much appreciated (here in Greece it's pretty difficult to find a trustworthy tech or info provider).
Thank you in advance for your time
Yellow_Light_Colorz_PDT_13
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Postby erichayes » Mon Mar 10, 2008 1:19 am

Hi Ted,

First off, welcome to the forum. I have enough faith in the denizens here that, somehow, a solution to your problems will be forthcoming. It might take a little time, though.

If I infer correctly, this is an amp that was either scratch-built on a Fender chassis, or a heavily modified Fender circuit. There are no schematics available. The basic amp is working, but there's no reverb or vibrato/tremelo function. You can hear the vibrato generator doing it's thing, but only peripherally--it's not getting coupled to the signal path.

I'm going to peruse my library of Fender circuits (fortunately, guitar amp copycats aren't terribly inventive) and I'm sure Mike and Ian, among others, will chime in as well.

Bottom line is: Do you want to get the existing circuitry to work, or do you want the functions to work, even if it means doing significant rework to the circuitry?

Finally, what test equipment is at your (or your tech's) disposal?
Eric in the Jefferson State
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Postby tedsorvino » Mon Mar 10, 2008 2:05 am

Thank you for your kind words and warm welcome Eric.
Well as far as your bottom line question is concerned I would prefer to make the existing circuits work and function, but as long as I am a musician I would
say that function is my main concern.
After some research in fender circuits I have to add some things. I think my new 50kA intensity pot doesn't work because fender amps use either 50kRA or 250kL pots as intensity pots.
As far as the reverb circuit issue is concerned, i think my tank is not up to the right specs for a reverb transformer driven circuit (like the one my amp has) but more for an op amp driven one. So I think I will try one similar to the fender twin ones (with much lower input impedance)
What do you think? Will it make any difference?
The testing equipment me and my tech have are just basic multimeters (he may have something better as well but not anything hi tech- he's just a much better tech- radio enthusiast than me. I just play music and like vintage gear). :)
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Postby cartoonweirdo » Fri Mar 14, 2008 8:02 pm

Hi ted,

As to your reverb tank, Fender used an Acutronics model number 4AB3C1B. If memory serves they were 4 ohms on the input with the output jack grounded to the reverb tank casing and the input isolated. Acutronics still makes them and other replacement versions are also available.

The inpedence will make a big difference as to the sound of the reverb, but my guess is that the hum is from a ground loop caused by the input or output jack of the tank interacting with the in or out jack of the amp.

Your vibrato circuit is interesting to me. Fender made several models with a bias modulation circuit, but all were short lived and most of the Fender vibratos in the world are black face opto-isolator style. In your case we should start with the basics.

All vibratos are made with an oscilator (look for a tube with 3 caps connecting the plate back to the grid) doing the work of swinging a voltage. The speed knob sets how fast the swing happens. After this the intensity or depth knob is wired as a voltage divider (basicly a volume knob) and the swinging voltage is sent off to interact with the audio.

If you have no opto-isolator (that's the little bug you were talking about earlier right?) odds are good that it wants to oscillate the gain (usualy by altering the bias) of either your driver or power tubes.

Figure this out the easy way by doing the following: On the plate of the oscilator tube read the voltage with a VOM. Set the speed as slow as possible. You should see a changing voltage. Go to the next place the voltage is sent. Check there. Keep going until you get to a tube. If there is little or no voltage somewhere the trouble is between the last place there was voltage and the place there is no voltage. Make sure to turn the depth knob all the way up while you do this. Also make sure that anything going to ground in theis part of the circuit is connected properly. If you want this fixed quickly just replace all the involved parts. There are usualy not many of them and resistors and caps are inexpensive.

Good Luck,
Carl
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Postby tedsorvino » Sat Mar 15, 2008 3:40 am

Thank you very much Carl. As far as the reverb goes I already ordered the new tank and I think that it will make a difference because of the grounding and impedance scheme. The vibrato intensity on the other hand seems quite more complicated. As I mentioned the pot I replaced might be the problem. I already ordered a 50K reverse audio (used in all opto isolator fenders) and a 250k linear (used in all bias vary systems) just to check out the differences. The one I will replace is a 50k log (not found in any fender schematics). If there is no change I will try testing and changing the part of the circuit you mentioned.
Thank you very much for your advice.
Ted
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