Flakey Tremolo channel on Guyatone Reverb Jazz (Fender Pro reverb copy)

[Sansteeth]

Hello everyone,
I'm giving up, tried to fix it myself I just can't figure what is wrong with it, also I just don't have the experience yet.
So, I have this very nice diode-rectified copy of a Fender Pro reverb by Guyatone (Reverb Jazz / GA1500) but the tremolo channel is a little flakey.
- its volume is a little lower than the normal channel, whereas it should be the opposite (3 gain stages for the Tremolo channel, 2 for the Normal)
- the tremolo acts up: it works but takes some time to get started, on my oscillscope I don't see a linear SPEED or INTENSITY pot course
Or I will max out the INTENSITY knob and bring up the SPEED knob but it will take some time for the oscillation to catch up. It feels like the amp has a very sluggish response to speed knob changes.
Sometimes a lower SPEED rate will lower the INTENSITY as well (less oscillation amplitude), even if the INTENSITY pot is still on 10

1) I swapped my brand new V5 and V6 for even more brand new tubes. No changes
2) changed the roach for a new one
3) measured the phase shifting network between the GRID and the PLATE:
.022uF = .03uF           
.01uF = .009uF            1M = 1.14M
.01uF = .014uF            1M = 1.06M

The 220K resistor on B+ measures 230K
The 330K resistor between SPEED pot and ground measures 321K
The 2.2M resistor that goes to the bias circuit (?) measures 2.33M

Plate voltage on V5.1 is 276VDC (Fender PR schematic shows 280VDC)
The strange thing is the SPEED pot, it's a B2000K (should be a 3M linear as per schematics) it measures 24.9M. Looks original though.
I obviously dont have a linear 3M pot lying around, so I hooked up my decade box in place of the pot and flipped a couple of switch and see if I get at least a more stable response from the amp, but on my oscilloscope it's still all over the place really.
I'm out of ideas here. I don't know if the values of the phase shift stage are outrageously out of whack, but at the same time, even if they were I would have the wrong speed, or it just wouldn't work. Instead I'm getting a tremolo that does it's own thing, respond very sluggishly to SPEED changes, etc.
If you folks wanna chip in, I would appreciate greatly!

(I added a bunch of schematics in the attachment, the GA1100 is the 100w version of the same amp, file is maybe easier to read)

Sansteeth

[sluckey]

The speed pot should be 3M-RA (reverse audio taper). The Intensity pot should be 50K-RA (reverse audio taper). The foot switch has to be plugged in and the switch closed to enable the tremolo.

[Sansteeth]

Hey,
thank you for your reply.
Both Guyatone schematics calls for a 3M (B) pot for speed and 50K (B) for intensity though
The footswitch does not have to be plugged in on the Guyatone, it's a jack connection (not RCA) and the tremolo works when you start turning the speed or intensity knobs regardless of whether a footswitch has been plugged or not.

[Sansteeth]

No one has any idea?
Measured components seem more or less within specs, but tremolo can be fiddly and they probably need to be stricter tolerance for the phase to shift like it supposed to, but seems it's my first tremolo repair, I'm not sure what "within specs" mean in this context.
Again, the tremolo "works", it just doesn't work properly. If I turn the intensity knob to the max and then bring up the speed knob, it takes some time for the tremolo to get "up to speed", and still on the max intensity setting, the speed rate will affect the intensity when it shouldn't.
Maybe I'm trying to fix something to big for me at this point 

[PRR]

> takes some time for the tremolo to get "up to speed"

That doesn't make sense. Amplitude/intensity goofiness does make sense; maybe?

Why haven't you replaced that Speed pot yet? The taper-direction is electrically unimportant but the range of values is. 

Also try another tube. And the cathode cap under it.

[Sansteeth]

Hey, thanks for your reply

> takes some time for the tremolo to get "up to speed"

That doesn't make sense.

Sorry I'm not sure I understand, what does not make sense? What I am saying? Or what the amp does?
If it's the first, I can try to explain better:
Intensity is at 10, Speed is at 0. If I bring up the speed knob to, say, 5 or 6, nothing happens for a couple of seconds, then I get a slight throbbing and then the speed accelerates (was what I meant by "gets up to speed") until it stabilizes.
If I have the speed knob full on, tremolo stops working (which makes sense since that pot is 20M instead of 3M)
If I lower the speed, I do get a lower speed but the wrong intensity (very shallow effect even at 10 on the intensity knob) until it stops.

Amplitude/intensity goofiness does make sense; maybe?

Not sure what you mean

Why haven't you replaced that Speed pot yet? The taper-direction is electrically unimportant but the range of values is. 

Also try another tube. And the cathode cap under it.

I haven't replaced the pot because I don't have any, I made an order but I'll probably get it in a couple of weeks since everyone's home having nothing else to do than order and solder a bunch of parts together to make noise and electronic webshop are saturated 

EDIT: since I didn't have a pot that was remotely close to 3M, I bypassed the speed knob with my decade box and  flipped a few switch to give me different values between 0 and 3M, the results I got where the same as with the stock pot.

I already tried another tube... and another one. I'll measure the cathode caps, thanks for the tip!

[sluckey]

I bet your flakeyness goes away when you put a 3M-RA pot in the circuit. But, while waiting, put a 3.3M or 4.7M resistor across the pot. If you had ordered parts from Hoffman you would have them already. 

[Sansteeth]

Just edited my previous post: I bypassed the pot with my decade box with values between 0 and 3M, I got the same result as with the 20M pot.

If you had ordered parts from Hoffman you would have them already. 

  I doubt it, the United States is a big place but the world is even bigger, and that's where most people actually live, but I take it as a compliment: that means my English is not that bad 

[sluckey]

Sorry. I don't even know if Doug will ship to Nijmegen. Even if he does, it's likely to be delayed at Customs or your port of entry.

[Sansteeth]

Yep, tried to ship stuff from the US to Europe... not fun, especially lately.
I tried your trick with the resistor across the pot, on the scope screen, the tremolo does act in a more predictable. Gotta do the real test with a guitar tomorrow and will report back.
Funny thing though... If I run the signal generator through the tremolo channel "something" squeals and puts out that same frequency, if I change the frequency, the squealing change accordingly (the amp is hooked up to a dummy load, no speaker). I cannot figure out where the sound comes from, maybe the reverb transformer... No squealing on the Normal channel.
At this point I'm pretty much expecting you folks to tell me: yeah, it's supposed to do that, won't be the first or last time I feel like an idiot 

[finkaudio]

Good evening/afternoon from Germany!

I have worked on several Guyatone amps in the past. In fact, I'm just working on a GA-1050 now.
The first thing to do is changing ALL of the grey Atlas Oil caps. They are leaky and lossy like crazy - you don't see that on your standard measurement device. Use Orange Drops or Mallory 150 or whatever you have sitting around. The electrolytic capacitors should go as well....but that's pretty normal for a > 40-year-old device.

Regarding the Tremolo footswitch, the GA-1050 normally works like a standard Fender - no switch, no Tremolo. A shorted 6.3mm connector should do the job. Maybe your unit was modified with a socket that got a switch to ground the signal.
Most of the 6.3mm sockets are broken anyway....I replaced them with Switchcraft versions.

I need to check the value of my Tremolo potentiometer in my unit....they never failed, so I only cleaned them and that's it. In case, you need to change, you can buy the Fender type, but you should order a Fender knob as well, as the Guyatone got curled 6mm shafts and the original knob does not fit the new 6.3mm potentiometer. But I'm pretty sure, if you change the capacitors, the tremolo will come back again and the original potentiometer should do the job.

Regarding buying parts, you can do that in Germany without problems. Best would be TubeTown in Bremen or - if you accept a higher price - TubeAmpDoctor - TAD. Both ship within one or two days and Netherland is around the corner.

Hope this helps

Best regards

Karl-Heinz

[Sansteeth]

Hey!
Wow! thanks a lot for the reply, it does seem like there is a lot of those in germany (bought mine close to Köln actually).

I have worked on several Guyatone amps in the past. In fact, I'm just working on a GA-1050 now.
The first thing to do is changing ALL of the grey Atlas Oil caps. They are leaky and lossy like crazy - you don't see that on your standard measurement device. Use Orange Drops or Mallory 150 or whatever you have sitting around. The electrolytic capacitors should go as well....but that's pretty normal for a > 40-year-old device.

Mine does not have the Atlas caps anymore, it does have every other possible cap though, WIMA, Ducati (made in Italy ah), something that looks like Roederstein, some glass ones too I think? (the dark red, square ones)
I tested most of them for capacitance but I'm not equipped for ESR or leaky cap measurements.
I changed the electrolytics in the power supply a year ago or so.

Regarding the Tremolo footswitch, the GA-1050 normally works like a standard Fender - no switch, no Tremolo. A shorted 6.3mm connector should do the job. Maybe your unit was modified with a socket that got a switch to ground the signal.
Most of the 6.3mm sockets are broken anyway....I replaced them with Switchcraft versions.

The Tremolo jack connector grounds the signal when no plug is inserted, I tried a shunted jack connector and got the same result as no connector plugged at all.
I find it really strange that the schematics call for a 3M speed pot but my amp comes with a 20M pot, made in Japan, that looks pretty original.

I've definitely considered changing all the capacitors but I feel like it's me giving up on the troubleshooting part of the repair, plate voltages on preamp tubes seem to be fine per Fender's schematics, the second triode of the tremolo 12AX7 is being fed a 404VDC which I find a little high maybe but I think you're right and I should just change all the caps, it definitely won't hurt.
In the back of my mind there is also the problem that my tremolo channel is quieter than my normal channel (which might or might not be related to how my tremolo is acting up), I compared AC voltages at the grids and plates of each gain stages of the two channels and while I get approximately the same readings on the two first stages of both channels, I get a massive loss of voltage between the second and third gain stage of the tremolo channel (where the reverb is) even when no reverb tank is connected.
There's a lot to it and it most likely is all connected 

[shooter]

but I feel like it's me giving up on the troubleshooting part

You'll find "climbing inside" an oscillator usually doesn't play by the same troubleshooting rules. 

[Willabe]

... I compared AC voltages at the grids and plates of each gain stages of the two channels and while I get approximately the same readings on the two first stages of both channels, ....

Always check the cathode (K) voltages too.

[finkaudio]

Looks like your amp was heavily modified before.....but honestly, the capacitors I can see are looking like somebody grabbed together what he could find in the drawer for old radio parts  . The roach also does not look original.... more like a Fender one. But I know, it's more or less the same.

On the photo, I can also see the switch on the socket for the Tremolo....OK,  so you are right.

There are still 3 original 33uF (or 25uF) Bypass caps in the unit. Two on the far left side and one connected to the ground of one of the reverb sockets. They should also be swapped.

I can see from the switches that your unit is a bit different to mine. Your power switches are looking completely different (much bigger). 

Good luck with you repair!

ATB KH

[Sansteeth]

You'll find "climbing inside" an oscillator usually doesn't play by the same troubleshooting rules. 

Ahah yes, I guess an oscillator as a first real troubleshooting experience is a bit too ambitious 
My brain was starting to self-oscillate.... 

Always check the cathode (K) voltages too.

Good call, I checked every other possible voltages on this amp except cathode voltages ahah. Cathode-to-ground or cathode-to-grid though?

Looks like your amp was heavily modified before.....but honestly, the capacitors I can see are looking like somebody grabbed together what he could find in the drawer for old radio parts  . The roach also does not look original.... more like a Fender one. But I know, it's more or less the same.

There are still 3 original 33uF (or 25uF) Bypass caps in the unit. Two on the far left side and one connected to the ground of one of the reverb sockets. They should also be swapped.

Yeah the inside of that amp is a cute little rainbow of capacitors, my OCD calls for a full recap, so I'll follow your advice and indulge!   

I changed the roach for one I ordered on Tubetown, thinking I could take a shortcut into fixing this amp, nope the original roach was working just fine!

Thanks a lot for chipping in, really helpful stuff, I appreciate

[Willabe]

Cathode-to-ground or cathode-to-grid though?

K to ground.

If the tube is working, it pulls it's current up through the K R, so there has to be a dcv across the K R. No dcv there, then the tubes not drawing any current and can not put out any ac signal voltage.

 

[Sansteeth]

Okay, I get it!
With no signal in, I'm getting between 1.62VDC and 1.66VDC on all gain stages.
However, fed with a 2Vac sinewave at 1kHz, the plate on V2.2 (the second gain stage of the tremolo circuit) gives me 111VAC but the plate on V4.1 (the third gain stage) give me a 45.5VAC.
Something is happening between those two stages, something's stealing all my volts!!! 

[PRR]

> 2Vac sinewave at 1kHz, the plate on V2.2 (the second gain stage of the tremolo circuit) gives me 111VAC but the plate on V4.1 (the third gain stage) give me a 45.5VAC.

2 Volts audio at the Instrument input? No guitar makes that much signal normally. Likewise 111 VAC is not a normal internal level, nor is 45V except maybe at the POWER stage of a HUGE amplifier.

1/10th of those levels may be normal. It only takes a couple volts at the driver grid.

[Sansteeth]

Ooops, thanks for your reply, makes me realize I should stick to normal usage conditions when testing.
Sooo, with a .021VAC sinewave @ 1kHz, knobs on 10 on both channels and bright switches off, that's what I'm getting:
NORMAL channel:                 TREMOLO channel:
Vg1.1: .023VAC                    Vg2.1: .023VAC
Vp1.1: .257VAC                    Vp2.1: .230VAC (probed after the HP filter)

Vg1.2: .257VAC                    Vg2.2: .230VAC               
Vp1.2: 16.27VAC                  Vp2.2: 14.4VAC (probed after coupling cap)
                                           
                                           Vg4.1: .37VAC
                                           Vp4.1: 12.55VAC (probed after coupling cap)
All DC voltages on the plates of the gain stages of both channels look right (between 230 and 250VDC)
Still, something is happening between the second and third gain stage of the TREMOLO channel, where the reverb and tremolo are inserted so that's the wild West for me over there.
Will start with a a much needed cap job and I guess I'll figure it out from there!

[PRR]

> that's what I'm getting:

We expect these stages to give gain about 50. What we got?

Vg1.1: .023VAC   
Vp1.1: .257VAC   
Gv = 11
--{ Tone Stack??}
Vg1.2: .257VAC   
Vp1.2: 16.27VAC   
Gv = 63
   
Vg2.1: .023VAC     
Vp2.1: .230VAC (probed after the HP filter)   
Gv = 10
--{ Tone Stack??}   
Vg2.2: .230VAC
Vp2.2: 14.4VAC (probed after coupling cap) 
Gv = 63   

Vg4.1: .37VAC 
Vp4.1: 12.55VAC (probed after coupling cap)
Gv = 34

Yes the first stage gain seems to be only 10.... but your notes show the *same* voltage at first plate as at second grid. But there is a tone-stack between! Should be large loss. With knobs dimed, maybe 5:1?

That aside: i don't know how your tubes are numbered, but I don't see data for Guya's V3, a major difference between straight and tremolo paths.

[Sansteeth]

Thanks for your reply,

I double-checked, I am getting the same voltages on the plate of the stage before the tone stack and the grid after the tone stack, didn't think about it then but you're right, should be super lossy, right?

Here's what I gathered:
V1: Two first and only gain stages on the normal side, tone stack in between the two triodes, signal goes straight to the PI after that
V2: the two first gain stages of the tremolo circuit, tone stack in between the two triodes
V3: reverb driver
V4.2: reverb recovery stage
V4.1: tremolo channel third gain stage
V5.1: oscillator tube for the tremolo circuit
V5.2: optocoupler driver
V6: phase inverter

On the reverb driver, I'm getting 6.91VAC on grid and 7.74VAC on plate (after the reverb transformer)
At the Reverb recovery stage I get .01VAC on the grid, .006VAC on the plate if Reverb knob is at 0 and .34VAC if reverb knob is at 10.
In both case the reverb tank is NOT connected, my understanding was that it was going to put the reverb (and possible problems with it) out of the equation to figure out difference in gain between the two channel, or is my reasoning wrong?
Also, realized my DMM cannot reading AC voltages superimposed on DC (like on plates) so I always probed after a cap or a transformer, am I doing it right?

[Sansteeth]

Hey everyone,
I finally got the parts for a recap job, I messed up my order so I still need to change the .047uF caps in the tonestack and the CC between the Normal channel last gain stage and the PI.
I also decided to modify it to conform to the AA165 version of the Fender Pro Reverb (only as far as caps and resistors are concerned... mine will keep its solid-state rectification) which included replacing some of the 1.5k and 2.2k cathode resistors on gain stage 2 of both Normal and Tremolo channel for 820ohms. Bumping the grid leak resistors of the LTP from 470k to 1M, according to Aiken (https://www.aikenamps.com/index.php/the-long-tail-pair) the value is not critical as long as it's between 100K and 1Meg. And a bunch of other things here and there.

The results I'm getting is this:
- Tremolo: seems to be working as it should now, I do get a slight ticking, regardless of what channel I'm plugged to but hey, at this point it's already an improvement and there is plenty of threads about ticking tremolos.
- Tremolo channel is also now louder/gainier than the Normal channel like it's supposed to. The CC between stage 2 and 3 of the tremolo channel was measuring .03 instead of .02, that is a big difference but I don't know if that's the culprit for the lower gain I was getting on the tremolo channel compared to the normal channel.
Considering my voltages measurement of earlier, there WAS something happening precisely between gain stage 2 and 3 of that channel, but that's where the reverb is too so my mind was on that reverb.

I have worked on several Guyatone amps in the past. In fact, I'm just working on a GA-1050 now.
The first thing to do is changing ALL of the grey Atlas Oil caps. They are leaky and lossy like crazy - you don't see that on your standard measurement device. Use Orange Drops or Mallory 150 or whatever you have sitting around. The electrolytic capacitors should go as well....but that's pretty normal for a > 40-year-old device.

But I'm pretty sure, if you change the capacitors, the tremolo will come back again and the original potentiometer should do the job.

You called it! 
Thanks a lot for the responses!

All the best,
Sansteeth