Help with noisy Ampeg type Reverb

Discussion in 'Amp Tech Center' started by bparnell57, May 17, 2015.

  1. bparnell57

    bparnell57 Poster Extraordinaire

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    Hey guys! I have a 1962 Guild 98RT, which is known to be a very close clone, even down to the cabinet and control panel, to original first generation Ampeg Reverberockets.

    I'm not very familiar with how the capacitance driven reverb works at all, and I would love some help, as I'm trying to see what individual components could be suspect with my issue. A high quality image of the original schematic from inside the amp is included in this post.

    My main issue is noisy reverb. It seems to be a white noise or hiss or static or something of the like. No hum. It's only present when the reverb is switched on, and it's volume varies with the reverb control, but not with the volume control, and it is unaffected by the tone control. It is at a pretty unacceptable level of noise and I'd like to see if any of you could help me. I suspect it to be one of the 3.3k resistors or the .01 capacitor on the output of the reverb tank. Do you think this is a good place to start?

    On another note, I'm getting a scratch type sound when switching on the tremolo. I suspect there may be DC on the switch or perhaps it needs to be cleaned. Any tips?

    Thanks for the help everyone!

    Edit: another question: Why does it indicate that the 6SN7 used for send and return on the reverb had to be a selected tube. Could this be for noise reasons?

    ImageUploadedByTDPRI1431913844.509914.jpg
     
  2. bparnell57

    bparnell57 Poster Extraordinaire

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    Not sure why my quality was limited on that large pic. Here it is in a lower quality but split up so it may appear better. ImageUploadedByTDPRI1431914040.875542.jpg ImageUploadedByTDPRI1431914050.907086.jpg
     
  3. muchxs

    muchxs Doctor of Teleocity

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    Guess I gotta get my head in Ampeg mode. :rolleyes:

    6SN7s, 6SL7s and the like are wonderful fat sounding tubes unless they're noisy and microphonic.

    You might have a tube issue. When they say, "Use a selected 6SN7" that means start with a bunch of them and find one that plays nice with your amp.

    The crackle on the trem pot might be a worn out 50+ year old pot.
     
  4. Jack Hester

    Jack Hester TDPRI Member

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    My suspicion would be the plate resistor on the recovery end (the triode just downstream of the spring tank). Replace that resistor with a metal film. Plate resistors can induce hiss into the signal.

    I have better schematics of your amp, but the file is too large to upload here. So, I've provided a link to download:

    http://www.mediafire.com/view/c40qduun2s2zvvd/Guild_98RT.pdf

    Note: There are three schematics in this file. The last is a redrawn, corrected version that someone made. When troubleshooting, always compare component values with the schematics, but don't be surprised if they don't match the drawings. Have an ohm meter handy to measure the resistance values when the color bands are hard to read. They get that way.

    Jack
     
  5. BobbyZ

    BobbyZ Doctor of Teleocity

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    All the electrolytics new ? That's step one. (can't rember if you did that) Those tubes are a royal ***** ! You just keep swapping them till you find a good set.

    One of my RRs had an awful sound with the verb turned out to be a bad connection on the tank, RCA jack wasn't grounding.
     
  6. bparnell57

    bparnell57 Poster Extraordinaire

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    Thanks for the tips guys. I guess I'm going to have to get a stash of 6SN7's to try. I tried reseating the RCA jacks on the tank and it didn't do anything. I also was suspecting the cathode resistor. I'll have to see if I have that particular value in my small metal film resistor stash. Otherwise, I never got around to recapping it but I will probably order the caps in this next week or so. I hate cap cans... :rolleyes:

    Today I'm taking it outside to get some of the spray paint off. I did the lower back panel last night (I'm using a spray on graffiti remover that's not very tough) and it ended up taking off the outmost layer of black and leaving an odd pattern of red in the valley's and the original blue/gray on the bumps. Looks better than it did. Much shinier and not as crusty.
     
  7. BobbyZ

    BobbyZ Doctor of Teleocity

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    Trust me do the recap first. Cap cans only suck if you don't have a big hot iron like my 200 watt Burtha.

    Well they do sorta still suck on a Reverberocket because there's so much going on inside a small space.
     
  8. Cleeve

    Cleeve Tele-Holic

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    If the capacitor in parallel with the cathode resistor wears out and dies shorted instead of open in the recovery triode, the operating point of the triode will shift making more noise, distortion, and gain as the poor thing has to go to grid leak mode to survive. (which will be tough with a reverb pan coil between the grid and ground!)

    See if the recovery cathode voltage is a volt or two above chassis ground, and even if that's not the problem, I'd replace the plate resistor and cathode capacitor anyway.

    The schematic calls for a selected 6SN7 most likely to find tubes that are only microphonic on the triode doing the reverb driving, and not microphonic on the recovery side triode.
     
  9. bparnell57

    bparnell57 Poster Extraordinaire

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    First off, I swapped the tube (which was a 6SL7 :eek:) for a NOS 6SN7GTB I had laying around and the hiss went down quite a bit, but it's still somewhat unacceptably loud. I guess I'll have to order the parts to recap it. Otherwise I could probably go get my $300 bucks back from Sam Ash as it's still under warranty and has been in twice with a replaced output transformer and a replaced V1, which didn't even fix the fact that I clearly said it wasn't grounded and was shocking me as well as how the reverb was noisy. I've since added a 3 prong cord with the appropriate death cap snipped out.

    I took off the black spray paint yesterday. Now it's down to red and brown hahaha. Maybe I'll get that layer off in the future, or recover the thing. This amp is truly a mess.

    Should I fix it or take the money and run if I can?
     
  10. Cleeve

    Cleeve Tele-Holic

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  11. bparnell57

    bparnell57 Poster Extraordinaire

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  12. BobbyZ

    BobbyZ Doctor of Teleocity

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    Get it working 300 is cheap for an amp like that. I'm on my way home from my brother's in Indiana where I brought both 62 Revererockets plan was for him to pick out the one he wanted. He kept both.
    Supposedly he'll bring one back in June but I may get the old "oh crap I forgot that amp". :p
     
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