Guyatone GA-1030 Reverb Combo

[MrPink]

Got this one locally for almost no money.
after swapping the fuse, surprisingly, it even functions. all caps are original Atlas brand, which i know usually leak a lot. But as it works, i kinda appreciate the "Originality". Tubes are original as well. Research dates it around 1971 and calls it a Princeton Reverb Copy. Not so sure about that though..

What doesnt work is:

- Tremolo
- Reverb Tank has been cut off, need to get a new one
- Original knobs have been replaced, probably been sold as fender knobs. For this price, i don't care.
- High Notes oscillate into some tinnitus style sound (Especially when playing alot of strings same time, freaking out Hendrix style etc.. / feels like it overruns itself)

What confuses me is:

- It breaks up a lot and sounds really dirty. Speaker is fine. Could this be due to the missing reverb tank? Power supply caps? Leaky caps all over? All of this?
- I've never seen this type of a Tremolo configuration before, so i need help trouble shooting and modifying speed, intensity etc.
- When attaching a footswitch to the tremolo, seems to me you can kinda bias it using the intensity knob, just no tremolo happens. A bit like a "loudness" potentiometer on a hi-fi device.
- No NFB at all? Does it make sense to add some?

I didnt even start checking values or anything. Just wanted to share this new project and gather some inside mainly on the tremolo and to which fender amp it relates, as it's an obvious copy.
Oscillating high pitch will hopefully be resolved changing one or two caps in the tone section. Maybe (probably) I will change them all in the end.
Replacing power supply caps might also reduce the dirt i guess, but it kinda sounds nice as it is as well.. Anyhow, gonna be fun.

Cheers!



Btw: Tubes in picture below are (from left to right): 6AV6 / El84 / El84 / 12AX7 / 12AU7 / 12AX7
Relating to:                                                       Tremolo/ Power Power/  PI.    / Reverb/  Preamp

[MrPink]

Front picture including avocado bread

[bmccowan]

Cool find IMO. I repaired a very similar Japanese amp several years ago for a neighbor.
I would agree that its similar to a PR but with EL84 instead of 6V6 power tubes.
If you want to keep it original, it will likely never sound great. But you are replacing the reverb tank so WTH.
The usual suspects in the dirty sound issue are tubes, filter caps, signal caps.
I think the filter caps should be replaced regardless, but you could test the signal caps and swap some tubes to test.
Are you comfortable working on amps?

[MrPink]

Thx! I Also think it very cool, and even like the dirty sound..
Yep, I've worked some amps before and I feel comfortable swapping components and modding for different values.

You're right about WTH I'm gonna change components anyhow...
I really don't want it to get too clean by bringing in new caps. I like the dirt, I just aim for some more flexibility and a working tremolo...

[Willabe]

I really don't want it to get too clean by bringing in new caps.

Changing the B+ filter e-caps is not about getting the amp to sound clean. It's to protect the amps transformers. If/when any of those e-caps dies it can kill the PT. Built in '71, those e-caps are ~ 53 years old. Too old.

I'd change those e-caps as soon as possible.

[astronomicum]

Those Atlas Caps are almost certainly why the Trem is not working. I typically replace them all and test them for leakage after removal and find that on average, 50-75% are leaking DC. They also may be throwing the bias off on the EL84s. The main E-caps (Mitsushitas?), on the other hand, I have found are typically OK, although I replace them anyway.

[MrPink]

Thx everyone replying

Gonna change the Filter Caps Like willabe suggested
Also noticed the pt getting really hot.. Not yet smelly But to hot to keep your fingers on.

Gonna change old Atlas caps at least in the trem section, probably also in the tone stack. This will then change the sound But probably is still a good idea

Funny though, no hiss at all, no weird effects or crackling either, Both channels working fine
Its Why i thought of keeping it original cap wise

Just way more dirt then i‘d expect

I will Check the area around the speed pot first before changing coupling caps and keep you updated
Maybe its just some dead solder as the intensity pot seems to interact with general Sound/ Whats your opinion regarding this Tremolo?

[bmccowan]

Also noticed the pt getting really hot.. Not yet smelly But to hot to keep your fingers on.

Don't wait for it to get smelly. You are going to want to figure this out. Could be those filter caps and/or their associated resistors - could be the power tubes - could be the bias.
Got multimeter? Spare tubes?

[MrPink]

Guess your right…

Sure, multimeter pliers tubes cable and some caps flying round
Also tons of resistors of different kinds
Sadly no Power supply style caps though

Power tubes do no redplating, Sound is stable.. still curious on intensity knobs effect on bias… Its there? Am i hallucinating?

[SEL49]

still curious on intensity knobs effect on bias… Its there? Am i hallucinating?

If the voltage on the wipe of the intensity pot is really changing when turning the pot, a likely suspect would be a leaky cap. Possibly the .047 cap from the trem tube or the two .022 caps between the PI and the output tubes.

[Willabe]

Also noticed the pt getting really hot.. Not yet smelly But to hot to keep your fingers on.

I wouldn't run that amp anymore until I changed those B+ filer e-caps. 1 or more of them could be leaking voltage to ground, drawing more current from the PT, trying to keep those e-caps charged up.

After that, check the bias like bmccowan said. And listen to SEL49 on the trem. 

[MrPink]

Appreciated, I'll do as you told
Gonna keep you updated when filter caps come in and i swapped some parts...

thx

[MrPink]

Update:

Filter Caps came in, I swapped the whole filter section + every electrolytic cap elsewhere in the amp. also changed all coupling caps in the amp. the only ones i left original are in the tone area around the first preamp tube and some of the small ones in the reverb circuit.

What can I say? It sounds delicious, the tremolo is beautiful as can be and the amp runs quiet as anything. PT doesnt get hot anymore, overlapping/distortion on high notes is completely gone. Gonna find me a fitting reverb tank and get this working to, even though i don't even miss it right now.

Didn't change any tube btw, and they are all original matsuhitas.

For the moment I have a new favorite amp. It's incredibly good sounding.
Thx for all the help

[bmccowan]

Hey - that's great!! What could be better than finding an unknown amp, fixing it up, and having it exceed expectations.
Good luck finding the right tank. What info do you have to go on?

[MrPink]

Good luck finding the right tank. What info do you have to go on?

Not much at all. Just starting this. Figure i should measure the reverb transformers impedance and refer to it as input impedance of the tank, than choose some fender standard value as output impedance and considering it is a smallish amp probably take a type 8 reverb tank. Could be horribly wrong here though as I clearly lack knowledge. 
Any quick explanations are welcome, as it's hard/impossible for me to find out what tank was in there originally. cant even find pictures of this amp involving the reverb tank...


Edit: figured I'm on the wrong track here. now in the middle of research

[bmccowan]

Like a lot of the circuit, the reverb circuit is pretty PRish, despite the 12AU7 and the somewhat different driver tranny. Of course that is a math-avoidance comment. But I'd guess you will end up with a PR compatible tank.
I've read Merlin's take on tanks that there is a fair amount of leeway.

[MrPink]

Been trying to find fitting input and output impedances of a replacement tank now for a while, reading myself into the topic but still not getting half of it. Also I feel like I cannot replace with a fender standard, as to my understanding the PR uses both triodes of a At7 to drive and half an Ax7 to recover, while this one just uses one triode of an AU7 to drive and the other half to recover; also my reverb tranny is 10k primary 8ohm secondary, not the fender value. Can anyone suggest good (and free) literature on how to determine the values based on the schematic/measurements?

Like a lot of the circuit, the reverb circuit is pretty PRish, despite the 12AU7 and the somewhat different driver tranny.

[SEL49]

You are over thinking this. According to the transformer on the schematic, the tank should have an 8Ω (or 10Ω) input. Doesn't matter what kind of tube. The tank output should be 2250Ω (or 2575Ω). The typical Fender tube amp tank (4AB3C1B) is perfect for this. If you need a short tank then get an 8AB3C1B.

Both of those tanks are meant to be mounted horizontally with open side facing down. If you need to mount upside down or on a side wall of the cab, or just want more info on reverb tanks, read this.