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|11th March 2008, 11:21 AM||#1|
Join Date: Mar 2008
Tube Amp reverb noise, no vibrato intensity, unknown model. Help please
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)? I may need to change it to a 50k Reverse audio one (mainly used to photoresistor driven reverb circuits) or a 250k linear one (used in old bias vary fender circuits). What do you think?
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- it was really weak. 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?
My opinion is that the problem is the impedance mismatch since it's a reverb transformer driven reverb circuit (the hum may be caused because I connect the tank the wrong way). I already ordered a 4AB3C1B tank.
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
PS. When I will fix the problem I will post some picture of the chassis and the amp.
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