Ampeg Reverberocket HUMMMMMMM

[alange5]

This forum has helped me knock out projects left and right.  I am hugely appreciative!  On to the next one...


I recently acquired a 60's Ampeg Reverberocket (top facing controls, model R12R B with 7591's and diode rectifier).  Here's a hard-to-read schematic that matches up with the schematic on the amps' back panel:
http://tubeamplifierparts.com/schematics/ampeg_r12rb_schematic.gif


At some point in its life, it had a 3-prong cord installed, and at least one electrolytic cap was replaced.  Repair sticker inside indicates it was last serviced by Central Jersey Music Service (owned by Dennis Kager - an Ampeg employee from yesteryear.  Cool!)


Now to the problem: As soon as the amp warms up, a nasty low-frequency hum is introduced.  The hum is pretty much consistent and independent of the volume control, but the reverb knob changes the volume and frequency of the hum. Reverb still works.  Hum is consistent whether the reverb is switched on or off, and it's still present even when disconnecting the reverb tank. I think I've isolated it to the V3 reverb send/return circuit.  Swapping tubes doesn't change anything.  Removing V3 altogether kills the hum.  All other amp functions work as they should.  Here are my voltages at V3:


Pin 2 (plate) - 105V (schematic calls for 130)
Pin 3 (cathode) - 1.5V
Pin 5 (plate) - fluctuates up and down from 93V-120V (schematic calls for 100V)
Pin 5 (cathode) - 1.15V


Any ideas?
FYI, the amp is populated with mostly old blue "Kondur" capacitors, and a few of those red/pink Tiny Chiefs.

[eleventeen]

You probably need to replace the full lineup of e-caps. All of them. If the amp hums like you say and those caps are the originals, you really can't say you've unholstered your weapons and attacked the problem unless and until those have been replaced. Those voltages you posted do not seem very "off" to me. Tube amps can be +/- 20%; at least that's what any Fender schematic says. A triode with a volt or two on the cathode sitting on top of a 500 ohm > 2.2K resistor is just about right.....shows the tube is conducting current and can be presumed working, at least on first glance.


Remember, a diode rectifier turns on instantly (versus a tube rectifier that has to warm up) and so those caps get hit hard on startup. There's a big voltage surge because the power section of the amp has not yet begun to conduct because the heaters aren't yet lit up. So those caps get hit with very high volts on startup since there is no current draw on the power supply to speak of. They're considered well past their expiration date if they are original 60's caps. Gotta change 'em.

[Willabe]

Gotta change 'em.

Agree. 

[alange5]

Looks like antique electronic supply has a (close to) drop-in replacement for the can cap.  I'll replace it and report back

[Jack_Hester]

I agree with replacing the eCaps.  But......  I've got a B-12-X and an early 60's Reverberocket that both suffer from the same malady, other than the caps.  The brown fiber tagboard conducts, on each of these amps.  On both, you can actually measure voltage from ground to non-metal points on the board.  I made a layout drawing of the Reverberocket, but have not really started the process for the B-12-X. 

Hopefully, yours is not the case.  Worth the check, though, when you get into it for the cap replacement. 

Jack

[alange5]

I know it's been a while, but I got the Ampeg sorted out. Here's what happened in my case:


I replaced all the filter caps.  Still hum.
I replaced any signal cap that looked visually deformed (swollen, dry, etc).  Still hum
I removed the reverb tank and noticed one of the springs had come unattached, pulling away its contact pins at both in & out transformers.  Replaced it with a MOD type C tank.  Still hum.
I cleaned all the contacts, reflowed some joints, double checked the grounding.  No dice.
I replaced the 7591's with a matched pair of Tung Sol reissues.  Nada.


In the end, the culprit was a single 6SL7.  It's a Magnavox-branded tube, stamped "Made in England".  The internal structure looks very similar to this one here:
http://www.upscaleaudio.com/brimar-cv1985-6sl7gt/


I never suspected the 6SL7's, because rearranging them (V1, V3, V4) produced no audible difference in the hum.


So when all is said and done, I potentially spent a lot more time, money, and effort than I needed to, but what I'm left with is a beautiful refurbished amp, that operates quiet as a church mouse.


P.S. - can't speak highly enough about the Mod type C tank (ordered from fliptops.net).  It's the most lush reverb I've ever heard.  It's absolutely a worthwhile replacement.  Full up, it sounds like you're playing underwater.  Also very impressed with the Tung Sol 6SL7's and 7591's.  They are dead-quiet and sound really nice.


As always, thanks for lending your ears and wisdom.

[PeaceMaker]

Hi All,


Not sure this will help but just incase... About a year or so ago I purchased a Brand-New Ampeg GVT15 and it came with a terrible hum straight from the factory- come to find out the connections on the reverb box where switched/mismarked... I just switched the connections on the reverb box (I had to put the red wire on the black terminal and the black wire on the red terminal) and no more hum... She sure sounds pretty now.


Take care,
PeaceMaker

[Tadarainia]

Hum is consistent whether the reverb is switched on or off, and it's still present even when disconnecting the reverb tank. I think I've isolated it to the V3 reverb send/return circuit.  Swapping tubes doesn't change anything.  Removing V3 altogether kills the hum.  All other amp functions work as they should.  Here are my voltages at V3:


Pin 2 (plate) - 105V (schematic calls for 130)
Pin 3 (cathode) - 1.5V
Pin 5 (plate) - fluctuates up and down from 93V-120V (schematic calls for 100V)
Pin 5 (cathode) - 1.15V


Any ideas?
FYI, the amp is populated with mostly old blue "Kondur" capacitors, and a few of those red/pink Tiny Chiefs.

Hello. Happy to hear you found your hum. The reverb send is plate 5 of V2 (6SN7)...... return is 1/2 of V3 (6SL7). The schematic is confusing. A lot of work going on in that side of V2 to drive that high impedance tank...5W plate resistor and 3W cathode resistor. I was almost sure it would have been there.
I'm trying to build a clone of this amp but can't find the voltages anywhere online. Or even just the Voltage at the reservoir cap would get me going.

The reverb in these amps might be my favorite ever! Happy to hear there's another one of these back in service.