guild T1-RVT noisy reverb

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hi everyone, new here but so glad this forum exists!!

I love the Thunder 1 Reverb amp, but ever since i got it (about a year ago), the reverb has been relatively noisy. when i first bought the amp i had it generally serviced by an amp tech but that didn’t seem to clean up the reverb noise, so i lived with it. lately i’ve been wanting to mic up the amp and record and i simply can’t do it with that reverb hum, so now i’m interested in seeing if there’s any way to make that noise go away.

details:
when the reverb knob is turned completely to the left (off) or footswotch is turned off, the noise is gone. the noise gradually gets louder as reverb pot is turned up. footswitch seems to work normally when turning reverb on/off — not glitchy or scratchy and wiring in it looks good.

the reverb itself works normally (as far as i can tell), but of course this background noise is ever present.

all soldering, wiring and grounding in the amp looks good to me, and when the reverb is off, the amp is surprisingly quiet for a 1967-68 amp. i pulled out the reverb tank and checked it and wiring/grounding, moved it around in different orientations and none of this had any effect on the hum noise.

i’m really not much of an amp expert, and would appreciate any ideas/steps on how to troubleshoot this! best bet is probably to bring it to an amp tech, but let’s see if we can diy this thing.. i’m up for it if you guys are

if more information or pictures/audio clips are needed, please let me know...
 

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You can try cleaning the RCA jacks and the pins on the 6gw8 with a brass brush. Have you had the filter caps changed?
 
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ok which rca jacks are u speaking of? i haven’t removed the chassis yet, so possibly in there? i need to find the work order from when the tech serviced it—
 
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ok found the work order from the amp tech... no caps replaced. what exactly are the filter caps and where do i find them and test if they are the culprits? thanks!!
 

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Filter capacitors are part of the high voltage circuit in the amp. The reverb tank in the bottom of the amp has a couple of wires plugged into it. It would be something you would have your tech look at.
 

Nuuska

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Hello

I do not have schematics for this amp - so I will put in one wild guess. If someone would send me the schematics, I might be able to help more. Someone on this forum must have them...

Anyway - that wild guess - the original message does not tell whether the footswitch is connected all of the time - and while I have no pictures or schematics, I assume it is wired in all the time.

In some tube-amps the reverb kill-switch is actually a connection between signal and ground at a relatively high impedance point. Therefore it should be screened cable to fight the 50/60Hz omniprecent electric field. While in "reverb-ON" position one lead is connected to signal path - therefore acting as antenna. With shielded footswitc-cable the shield is permanently grounded to amp ground and thus preventing MOST of the hum. If that cable was replaced with a "lamp cord" or similar, the reason is there.

But like I said - pictures and schematics - please.


EDIT

I found one schematic in the www - might be right one or not - but that does not have a footswitch or a place for it - only a switch. It seems this amp has two speakers - each with its own amp - two tubes for main - one tube for reverb. Triode-pentodes. The tube, that is between tank output and reverb volume is the only one, that gets its anode voltage from point 6 - which is the last anode-voltage point in a chain of series resistors + filter caps. Point 5 before that is the anode voltage for reverb power-amp tube.

So please clarify three things.

Does the combo have two speakers - each with their own amp?

When playing with reverb OFF - is there sound through second speaker?

Is the footswitch original?
 
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hi thx default and nuuska.

the amp has two speakers, one main and one for reverb.

when reverb is off or the reverb speaker is unplugged from the board, there is no sound from it but the main speaker still works and sounds totally normal.

the footswitch is original and hardwired with a thick cable

the noise/hum i’m speaking of increases in volume as the reverb pot tone knob is turned up. so basically the more reverb the more noisy. but the reverb sound itself does increase as well and sounds good. when i listen near the reverb speaker, it sounds like the noise/hum is being produced there at the speaker
 
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Nuuska

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Hello

OK - so I make some vague assumptions.

In the schematics I found, the switch altered the input for second amp between dry sound and wet sound. So I have different schematics - but probably close enough in other respects.

As I wrote, the second amp has its own filter capacitors - so the problem might be in those. But that does not quite make sense, while the volume does not affect the power tube, which gets less filtered anode juice than the preamp-tube.


Interesting dilemma - please let us know how this ends - others are welcome to chime in, too.
 

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Yes, that's it. Guild had an add-on on reverb amp that had it's imput clipped to the speaker terminals of your main amp. They transplanted the circuit of that amp into the chassis of the T-RVT and the mid-sixties Thunderbird. Kind of hard to diagnose issues over the internet. Where are you located?
 

Nuuska

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Hello

That was the schematic I found, too. I was assuming, that the switch is in the amp - but it seems to be a footswitch.
According to this there should be dry sound from the small speaker with REVERB OFF - signal "A" from anode V3A
In this position the tremolo affects main speaker only.
In REVERB ON - reverb level controls amount of sound from small speaker - tremolo comes through, too.
The footswitch cable should be three wires + shield. Very potential hum-antenna.

Filter capacitors. The power-tube V6 for the small amp gets its anode voltage from point 5 and is obviously clean, because there is no hum without reverb.
The driver tube V5 gets its voltage from point 6 which is derived from point 5 - therefore that must be ok, too.

Next the tank. Many tanks have "floating" connections - that is the signal wires do not make contact with the body of the tank. In that case the body must be connected to amp chassis with a wire to provide shield ground.

Finally - does valve V5 have shield? Or any other valve ?
 
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thanks so much for he info!

1. footswitch - how would i troubleshoot to see if it is the issue?

2. i will take a picture of the tank and wiring tomorrow and see what u think

3. valves - i really don’t know how to answer your question about shields.. can u guide me on how to check this please?

thanks!
 
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thx for the local help... i’ll give him a call or the other amp tech if i can’t figure it out.

busy day, not able to take picture of reverb tank today but will get to it and post a link here when i do... thx for everyone’s help so far. keep it comin please if you have any info/ideas! thx
 
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Hey there, found a mail order supplier of tubes for the Thunder 1 T1-RVT in the US-West Coast, he lives by me: https://nesstone.com/
The guild uses some less common tubes. I don’t currently have any of the ECL82. I do hve the 6GW8 and the 12AX7. The 6GW8 I have are very high grade Holland made vintage BugleBoy brand at $20 each. They are not listed on the site but I can provide those if you’d like. Since my Dad & brother & I are first owners , going with the best
 

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20 a piece is not a bad price for those particular tubes. ECL82/6BM8's are more common and still in production.
 
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