Gretsch Model 6165 "Variety Plus" Weird Hum

[Perkinsman]

I can't find a schematic for this amp, the one I did find on EL34 World site had a 5U4 rectifier but this one is solid state. It's a very cool 2x12 tube amp from the 60's but it has a very loud hum that subsided after warming up but then had "waves" of hissing. First, I cleaned all the controls and sockets and tightened the socket teeth since they were a little loose but to no avail. I thought for sure the problem was going to be the caps so I replaced the cap can on the power supply with 3 new caps, then restarted. It seemed to help a little but not completely. What was odd was that the hum could now be controlled by the bass & treble controls. Since there was still a little hum, I replaced the 2nd cap can mounted on the preamp with another 3 caps, the 4th tab was a ground...the hum that had subsided only got louder. I believe I made the connections correctly, I tied each + of the new cap to the wires that had been attached to each of the tabs & then soldered each - of the caps to other connections that went to chassis ground. Any advice here would be appreciated.

[vintagekiki]

Happy New Year


http://www.prowessamplifiers.com/schematics/gretsch/Gretsch_6165_Premplifier.html

http://www.prowessamplifiers.com/schematics/gretsch/Gretsch_6165_Power_Amplifier.html

1)
https://reverb.com/ca/item/462684-gretsch-6165-variety-plus-1964-guitar-amplifier

https://www.worthpoint.com/worthopedia/1960s-gretsch-model-6165-variety-plus-30465501

2)
https://reverb.com/item/1542576-gretsch-6165-variety-plus-2x12-35-watt-6l6-gc-vintage-tube-amp-tube-driven-vibrato-reverb-no-mods

[Perkinsman]

Happy New Year!

If you open that schematic, you’ll notice that it’s exactly like the ones I mentioned that I’ve found that include a 5U4 rectifier. This amp does not have that. I suppose the rest of it is correct but I was initially hoping to find one with a solid state rectifier.

[vintagekiki]

It is to be assumed that the difference between the both models is in the "Power Amplifier" module. If it is not a big effort for you, you can compare the existing schematic with your amp, and make any changes.
Pre amp has vibrato in the true sense of the word, unlike a large number amps who have tremolo.
Vibrato is made as a three-stage¹⁾ phaser with tubes V2¹⁾, V3 (12AX7) with optocouplers VR1¹⁾, VR2, VR3, and oscillator V8 (6DR7) with lamp 1819 which drives optocouplers VR2, VR3.

[drewl]

I re-furbed a couple similar amps the past few weeks.

Can you isolate the hum by removing the preamp tubes one at a time.

Does it have a grounded power cord.

The preamp wiring is a mess on these, make sure the AC going to the power switch is routed away from any signal wires.

[Perkinsman]

You guys understand these so well, it just makes me realize how much I have to learn. It still has the 2 prong cord, and I'd like to replace that first. Is it just a matter of soldering the new cord's ground wire to the chassis?

Tomorrow I'll pull each tube and see what happens. If the sound goes away, does that mean it would be on that tube's circuit? The tubes have all been tested good.

Before going much further, I'd also like some help determining whether I installed the caps correctly. I first installed the 3 new caps under the power supply chassis, which reduced the hum a little so I also rebuilt the one on the preamp chassis. The preamp is the same exact can so just a duplication of the work. Every positive (+) was connected correctly, however I'm not absolutely sure each negative (-) was. Is it correct to have connected to chassis ground?

[vintagekiki]

Set all volume control to min. Rev F /S off. Do you have a hum?
Open the volume control one by one to determine which channel is causing the hum.
If the hum exists only on channel 2, this is normal because the input jack is open (no jumpers).
Does the hum change depending on the position of the on/off/on AC main switch¹⁾?

1)
Reversing line switch

[Perkinsman]

Today I installed a 3 prong plug & still had hum. I starting pulling one tube at a time & the hum stopped when I pulled V5!! I put a new tube in & it quieted but I still heard a faint hum & after a few minutes, it now made a sizzling sound. I removed the new tube, tightened each tooth in V5 & deox’d & the hum came back again!!!

Volume control does not control the hum, however when I reverse the On switch, it does gets louder.

edit: after poking around I found this loose wire & im not sure where it went. The other end is attached to one of the terminals of a socket the speakers are connected connected to.

[vintagekiki]

1. The V5 (12AX7) is a phase inverter. Short circuit V5-pin2 with gnd ("lower" resistor 100k). Is there still hum.
2. Remove V4 (12AX7) is a mixer for both cannel. Has the hum disappeared.

Question. Wire marked with ? not soldered. Where does its other end go?

[Perkinsman]

1. The V5 (12AX7) is a phase inverter. Short circuit V5-pin2 with gnd ("lower" resistor 100k). Is there still hum.
2. Remove V4 (12AX7) is a mixer for both cannel. Has the hum disappeared.

Question. Wire marked with ? not soldered. Where does its other end go?

I ran a wire from V5 pin 2 to ground and no sound, no hum. I then ungrounded the wire, pulled V4, no sound, no hum. I then ran a 1/2W 100k resistor between V5pin2 and ground and got sound back, with reduced hum. I then switched the 3 way ON switch and had even less hum, but there is still hum. I'm not sure why considering there shouldn't be a difference due to having installed a 3 prong plug yesterday. Is this just something I need to live with or are we getting closer to a solution?

The other end of the "free hanging" green-white striped wired is attached to the 1/4" jack ground side. I'm not sure where it was attached since poking around made it come loose. Can I attach it to any ground on the chassis?

[Perkinsman]

Bump

[g1]

It's already going to chassis via the jack, isn't it? So the disconnected end was connecting some other point to chassis. You would have to look for something that the schematic shows should go to ground, but is not getting it's connection. It's pretty much impossible for us to guess what.

You said poking around made it come loose. So it would not be possible it is the source of the hum, or are you sure it was connected before?

[vintagekiki]

I ran a wire from V5 pin 2 to ground and no sound, no hum. I then ungrounded the wire, pulled V4, no sound, no hum.

This indicates that humming does not occur in the power amp and mixer (V4)

The 3 way ON switch has a 2 ON position. Is there a hum in both ON positions the same or is there a difference?

I didn't understand best what happens when you set all volume control to min, Rev F / S off, all boost sw off. Is there still a hum, or is it smaller?

Testing ch. 2
Remove tubes V2, V3, V6 and V7. Is there a hum and does it change by changing the volume control ch 2?

Question
Comparing Figures 1 and 2, it can be seen that the Jack J1 (fig 1) was subsequently installed. What is its purpose, what connects to it?
In Figure 2, instead of Jack J1, there is an earth point, to which a wire is connected.
It is very likely that it is the same wire from Figure 1. Do not remove the wire, just insulate it.

[vintagekiki]

Comparing Figures 1 and 2, it can be seen that the Jack J1 (fig 1) was subsequently installed. What is its purpose, what connects to it?

If subsequently installed Jack J1 (fig 1) is used as ext. spkr. jack, mount it to the chassis over an insulated washer, or replace metal jack with a plastic jack.
It is very likely that there is a gnd loop and that is why the humming occurs.

[Perkinsman]

Each speaker has a length of lamp cord from the negative (-) speaker wire to the ground of the non stock jack. I believe I found where the floating wire attached to. After installing the 100k resistor from V5 pin2 to ground, the hum was almost entirely eliminated.