65' Twin Reverb motorboating

[SMSgtCC]

Need help with 65’ Twin Reverb:

Two problems: Can’t increase bias without it motor-boating, and volume only comes up when I raise treble, mid and bass pots – both channels.

Problems noted when I received amp:

Bias was set low.

Wire between pins 1 on V 7 and 8 was shorting to chassis just underneath the fuse holder.

Volume pots were so scratchy it wouldn’t be played.

Other than new three prong plug and both mids pots being replaced, amp was all original.

Work I’ve done:

Unsoldered and checked the value of each blue molded cap and all were spot on as far as tolerance.

Replaced all electrolytics.

Replaced resistors connected to filter caps.

Checked resistance of all resistors on board and replaced those that were out of tolerance.

Replaced screen and grid resistors on tube sockets.

Cleaned volume pots on both channels. No loner scratchy – but could they be playing a role in this problem?

Discovered both mids pots were replaced at some point with 250K – schematic calls for 10K.

Replaced bias cap (electrolytic) and 470 resistor.

Tried new bias pot – made no difference.

Traced wires according to schematic, inspected each solder connection with magnifying glass.

I feel like I’m missing something obvious. Maybe the diode on the bias circuit? Maybe the pots are causing issues?

*NOTE: Although the amp is currently set at about 9 watts dissipation, it sounds amazing. Clean, quiet and tremolo/vibrato and reverb both work fabulously. BUT – raise the bias and it motorboats.

*See video:

i=M6T4Q_Y3mnoeO8pI

Thanks in advance for any help!

[sluckey]

volume only comes up when I raise treble, mid and bass pots – both channels.

That's not an issue. That's how that tone stack is supposed to operate. If you look at the schematic, you should see that the top of the volume pot will be grounded (killing all sound) when all tone pots are set to minimum.

[SMSgtCC]

Thank you - that makes sense. I'm just not familiar with this amp and I just assumed it was similar to what I'm familiar with. Awesome! Thanks!

[passaloutre]

Have you tried swapping the OT leads? Or just disconnect the NFB

[SMSgtCC]

I haven't tried that yet, but I will check it out. I did swap out the feedback resistor as it had drifted out of tolerance.

[Esquirefreak]

50v cap is not enough for the bias supply.

If pin 1 was shorting to chassis, then that pair of tubes would have run very hot. Bias voltage is tied to pin 5 via the grid stopper on pin 1. Possibly the output tubes are worn and maybe even the OT was damaged?

/Max

[AlNewman]

If you have 250k pots at the mids, you are getting a lot more signal through with the mids up, which could cause motorboating.  If they're wired as variable resistors, you should be able to parallel the 2 outside legs to reduce value without buying new pots.  27k in parallel will get you back to 25kish.

As the amp's been modded, you could also double check the values of the NFB loop to make sure they haven't been altered.

[SMSgtCC]

It could be the OT. I've tried two different sets of brand new, apex matched tubes - same results.

NFB circuit looks untouched, but I did replace the 820 ohm resistor as it had drifted.

[SMSgtCC]

What is the recommended method to test the OT?

Perhaps the current increase when bringing the BIAS up is exposing a weakness in the OT. How can I rule that out? I've measured resistance from each leg to center tap for bias calculations. Both are very close, about 38 ohms each side. Thanks.

[mresistor]

Have you replaced the PI ?

[stratomaster]

If you remove the tubes and measure the voltage at the wiper as you rotate the pot, what do you see?

How about the 220k bias leak resistors?

With the tubes removed and the wiper of the pot disconnected what voltages do you get on each side of the 0.1uF caps at the outlet of the PI?

[Dave]

Hmm... Maybe my experience is different than everybody else's, but I've never run across a bad OT where the amp still sounded "awesome".
I have run across bad OT's where there was no sound at all, where there was a tiny thin anemic sound, and where there was a buzzing mechanical vibration sound emanating from the OT itself (not the speaker).
But never one where I would describe the sound as "awesome".


Those mid controls shouldn't be a problem IMHO with 250k pots. It will just allow for a LOT more midrange and if set to 10 would probably make the other two controls have little to no effect. But, as someone else said a paralleled 27k resistor would get you back to normal without replacing the pots.


I would check the functionality of the existing power tubes and then I would have a look at lead dress to start with.


Dave

[SMSgtCC]

Tried two new sets of quad, apex matched tubes. Lead and dress is all original - unmolested. I have traced wires and compared to schematic and checked solder connections with magnifying glass. But - I still could've missed something. But, yes, at reduced bias that it's at the amp is clean, articulate and reverb and tremolo function nicely. When I increase bias pot, it motor boats. What's the possibility that one ore more diodes in rectifier circuit start passing AC under higher current? What's a normal amount of AC to pass thru the diodes? I'm measruing about 5 volts AC where B+ leaves the diodes. Thanks.

[SMSgtCC]

PI - Phase Inverter: The tube that's in it is brand new. Tried another brand new one, same results.

Rotating bias pot: -36.0 to 59.3

Either side of caps, tubes removed, wiper disconnected: .006 on both ends that are closest to pots. 480 and 479 on side closest to PI.

[SMSgtCC]

220K's were replaced - resistance verified before installing.

[SMSgtCC]

-36.0 to -59.3 (forgot the negative sign on the 59.3 in previous reply).

[AlNewman]

It's fairly easy to check the results of replacing the 25k pot at the mids with a 250k pot. 
Go to the tone stack calculator and see what happens...
There's only 1 path to ground in a Fender tone stack.

http://www.guitarscience.net/tsc/fender.htm#RIN=38k&R1=100k&RT=250k&RB=250k&RM=25k&RL=1M&C1=250p&C2=100n&C3=47n&RB_pot=LogA&RM_pot=Linear&RT_pot=LogA

[sluckey]

Discovered both mids pots were replaced at some point with 250K – schematic calls for 10K.

It's a popular mod to replace the mid pot with a 250K-A pot. If using a 10% taper, the pot behaves just like the original 25K mid pot for the first half of rotation. The second half of rotation begins to 'lift' the tone stack, making it less lossy, and increasing gain for a raw sound. I like it. This is a favorite AB763 mod for me, second only to replacing the 100K-L reverb pot with a 100K-A pot for better reverb control.

     https://sluckeyamps.com/misc/AB_Lite_II.pdf

[SMSgtCC]

Great info. on the pots. The amp only had two changes since it left the factory before it came to me: a grounded power cord and the two pots replaced. I knew there had to be a reason for those pots - makes sense. Thanks!

But - as for the motorboating, the bias was set low when the amp came to me. As I mentioned, with the bias low it produces good tone (9 W disapation), with what I perceive to be slightly less volume. But, try to get the bias up where it should be, and the motorboating kicks in.

To me the motor boating is like the 60 cycle AC somehow reaching the speaker. As the current flow from the wall increases when I adjust the bias pot, I'm wondering what component would react to block the AC until a point where the current increases where it can't block it any longer, such as perhaps the diodes in the rectifier or caps?

[sluckey]

This is a long shot... Temporarily solder a 10K resistor across the mid pot. Any better?

[pdf64]

To me the motor boating is like the 60 cycle AC somehow reaching the speaker.

It may be beneficial to provide a video or sound clip demonstrating this. As motorboating typically a much lower frequency than mains hum.

Have tried tried the suggestions in reply 4 yet?

[SMSgtCC]

By chance did you see the video of in my initial post? I demonstrate the motor boating in that video. I'll have to read back thru all the responses to figure out which one is response 4 and see what it said.

[HotBluePlates]

... raise the bias and it motorboats. ...

Motorboating is usually "power supply oscillation."

When the filter caps are new (and not 60 years old), the problem is often that the "0Ω ground connection" is not zero ohms.  Amp current-demand then creates a voltage drop across the non-zero-ohm ground connection, which leads to the motor boating sound.

Check your wiring/soldering for those filter caps.  Especially check the ground wire running from the doghouse board to the chassis connection.

[SMSgtCC]

"Motorboating is usually "power supply oscillation."

When the filter caps are new (and not 60 years old), the problem is often that the "0Ω ground connection" is not zero ohms.  Amp current-demand then creates a voltage drop across the non-zero-ohm ground connection, which leads to the motor boating sound.

Check your wiring/soldering for those filter caps.  Especially check the ground wire running from the doghouse board to the chassis connection."

I verified wiring and soldering for the filter caps last night - looked good. But, I will verify the ground from dog house to board and I will double check everything there again. Thank you!

[pdf64]

Sorry, I'm impatient and scrolled through the video, missing the motorboating demo :)

... I will verify the ground from dog house to board ...

There's usually 2 ground wires from the HT cap board back to the chassis innerds.
One for the reservoir cap (series pair) that goes to a lug near the mains transformer, another for the rest, usually wired to the brass plate.
Obviously all the HT caps need a path back to ground. There's link wires between eyelets under the board, it may be that one of those became dislodged when the caps were replaced.

[SMSgtCC]

Makes sense! I will double and triple check all that sometime today. Thank you!

[SMSgtCC]

Everything was properly connected at the dog house.

For the two grounds, I'm reading 0.2 ohms.

[stratomaster]

Do voltages that are stable at cold bias begin fluctuating when the motor boating start?  Check DC at bias pot wiper while turning with tubes installed.  See if the voltage pulsates with the sound.  Repeat at plate and screen.

If the screen voltage pulsates I'd be tempted to replace the choke with a resistor temporarily to rule out an internal short at higher current.

[Dave]

My eyes are not great, so I'm saying this with a margin for error...
It looks to me like your bias filter cap is in backwards.
The Positive end is the one that should be grounded.


Dave


Edit... Nevermind. After I figured out how to get full screen, it looks correct. It does look a little small though. What are the values?

[stratomaster]

Orientation correct, voltage rating too low.  I think this was addressed earlier--although I'm at a loss for why the cap would only act up at warmer bias.  It should be always exceeding the voltage rating regardless of pot setting. 

[SMSgtCC]

Yes - I've got a new cap on order to make it 100v, but for now the 50v will have to suffice and yes, it's oriented correctly.

I'll check those voltages and I like the idea of ruling out the choke - that was actually suggested to me last night in a phone call.

Thanks everyone!

[stratomaster]

Do voltages that are stable at cold bias begin fluctuating when the motor boating start?  Check DC at bias pot wiper while turning with tubes installed.  See if the voltage pulsates with the sound.  Repeat at plate and screen.

If the screen voltage pulsates I'd be tempted to replace the choke with a resistor temporarily to rule out an internal short at higher current.

Now that I think about it: Get the bias supply right.  Use a higher voltage cap and test the diode.  It's easy enough to just swap it for a 1N4003-4007 for troubleshooting purposes.

If you set your meter to AC, how much ripple are you measuring at the inlet to the bias pot?

[SMSgtCC]

Will do - thanks!

[sluckey]

Pull the PI tube (V6 12AT7). Can you set the bias now?

[SMSgtCC]

With PI pulled, motorboat still there - just not as loud. I will try some of the other suggestions now that the dog house is all put back together.

[SMSgtCC]

New diode - no change. AC voltage goes from .18 (no motorboating) to .30 - .38 when motorboating. Bounces between those two numbers.

100V cap won't be here until Friday. I'll look at the choke next.

[SMSgtCC]

For the choke test, what size and wattage resistor would you suggest I try?

There are two black wires coming from choke and terminate on board at B+1 and B+2 - confirm, is this corect?

Thanks!

[BrianS]

Are you doing all these tests with the amp plugged into a leak light?  What wattage bulb are you using?

[SMSgtCC]

Yes - 300 watt

[sluckey]

100V cap won't be here until Friday.

Do you have any of the doghouse caps laying around? If so, tack solder one across the bias diode for a test.

[sluckey]

Yes - 300 watt

Remove it and plug straight into the wall.

[SMSgtCC]

It was the dim bulb tester!!!! Thank you!!!!!!!!!!!!!!!!!!!

[BrianS]

https://www.amazon.com/P3-P4400-Electricity-Usage-Monitor/dp/B00009MDBU?th=1
If you want to monitor current draw of the amp, maybe a Kill-o-watt device would be better than keeping it plugged into the leak light...assuming that you don't have a Variac with Ammeter built in.
The leak light should really only be used on the initial power up of an amp to check for shorts, and maybe when you are doing the initial testing of an amp that you've just replaced parts in...to be sure you haven't shorted something accidentally.  I don't think it's good practice to set bias or measure voltages throughout a working amp while it's plugged into the light, as it's clearly dropping some voltage.

[SMSgtCC]

Thank you Brian - I learned allot with this. Thank you!

[mresistor]

well I'll be Opie