67 Pro Reverb - Ch 2 noise

[fdesalvo]

Hi, all.

Recently recapped/retubed a nice 67 Pro Reverb and am working on a few issues in channel 2. Channel 1 has no issues.

1. There's a strange rustling noise that is intermittent, but I can provoke it by chopsticking the .1uf (brown blob) and .047uf tone caps. If I graze it with or poke it with the stick, the noise is very intense. The other caps in the preamp are not exhibiting this noise.
2. There is almost 385vdc on V2's plates, instead of just on the rail, which is sitting at around 410vdc. The dropping resistors on the filter pack board are all within 5% or less variance and there's 460vdc coming off the JJ GZ34.

Fun fact, I recently found out this is Jimmy Vivino's old amp.

Cheers, all. Hope y'all are safe and healthy!

[Justin Thomas]

What are the cathodes of V2 measuring? I'm guessing somewhere around 0? How are the 100K plate resistors?

As far as the cap goes, replace it & see what happens.

Justin

[fdesalvo]

Thanks for the reply.

I’ve lost my mind. The prob has to be v2b’s plate resistor. V2a’s plate is at 280dvc. V2b’s plate is at 382. Strange they both measure the same resistance. Cathode was around 4v.

[Helmholtz]

Schematic?

[fdesalvo]

Just a plain ol’ AA165

http://www.prowessamplifiers.com/sch...Schematic.html

[TomCarlos]

Schematic Posted...

[Leo_Gnardo]

Thanks for the reply.

I’ve lost my mind. The prob has to be v2b’s plate resistor. V2a’s plate is at 280dvc. V2b’s plate is at 382. Strange they both measure the same resistance. Cathode was around 4v.

That is bizarre. With the cathode voltage riding high, one would think either V2b or V1b is conducting too much. BUT V2b's plate voltage is way higher than normal, indicating it's not conducting enough. Hmmm.... perhaps it's not V2 but V1 that's the culprit - try swapping in a fresh tube & let's see what happens. With shared cathode resistor, if V1a is overconducting - very leaky - it's pushing the cathode voltage high, which is upsetting the bias in V2b, forcing that triode into cutoff, so we see the plate voltage way high.

Another method would be to provide V1b and V2b each with separate cathode resistors, then the duff tube will be obvious.

FWIW I worked on another one of Jimmy Vivino's ex-amps about 3 years ago, a really nice Vibrolux Reverb blackface. 1964 IIRC. Jimmy sold it to Connor Kennedy who had been selected to play in Donald Fagen's touring band, and he wanted it to be "bulletproof" - reduce any chances of breakdown as low as possible. One does not want to have his amp go kablooey on tour, it's a hassle we can do without. Things must have gone well, as Connor was mentioned earlier this year as having been brought on board as a member of the Steely Dan tour band. Good for him! Except . . . nobody's doing much touring while this covid business is going on. Let's hope that settles down so he and all the other musos in the world can get back to working pay gigs.

[The Dude]

...... Cathode was around 4v.

Which cathode is 4V and what's the other cathode voltage?

[fdesalvo]

Pin: V1 | V2

1: 290vdc | 370vdc

3: 2vdc | 4.34vdc

6: 300vdc | 279vdc

8: 2.1vdc | 2vdc

I need to have a look at V1/2’s shared cathode resistor, as well. It’s rumbling at the chopstick. All the plate loads on v1/2 are.

[fdesalvo]

Found the culprit. V2a’s cathode resistor was severed at one leg. This resistor was nearly beneath the cap I suspected as noisy. Whenever I touched that cap, the broken resistor was being moved.

Updated numbers - the noise is gone. Have a few other minor things to look at.

Pin: V1/V2

1: 286vdc/251vdc
3: 2vdc/2vdc
6: 287vdc/267vdc
8: 1.6vdc/1.9vdc

I now have to focus on the channel 2 tonestack caps I believe. Once the volume is moved from 8 to 9, the slight background hum disappears and a very low frequency hum appears. I'll never play at that volume, but I need to see what's going on. Channel 1 has no issue.

[Leo_Gnardo]

Found the culprit. V2a’s cathode resistor was severed at one leg. This resistor was nearly beneath the cap I suspected as noisy.

Excellent news!

I now have to focus on the channel 2 tonestack caps I believe. Once the volume is moved from 8 to 9, the slight background hum disappears and a very low frequency hum appears. I'll never play at that volume, but I need to see what's going on. Channel 1 has no issue.

If you know you're never going to play with the ch 2 volume spun up to 9, then why worry about a little hum there? You can chase that rabbit round and round, but I'd opt for ignoring it.

In a similar observation, often I find an amp - Fenders very much included - where there's a hum/buzz with the volume control dialed all the way down, but the hum/buzz vanishes when the volume is set to about 3 to 4, typical for many Fenders. Then I have to ask the musical question "who is gonna use the amp with the volume dialed down to nothing?" then the answer comes clear: don't worry about it, leave 'er be.

If you do decide to chase that hum, here's something I've found in most of the general run of Fenders, 1960 thru early 80's, ones with norm and vib channels. There's a bundle of high voltage wires that sprout up from the doghouse, thru the chassis, and they run behind the vib channel tone controls. Some reduction of hum can be obtained by placing a grounded copper shield between the hi voltage wire bundle and the back of the treble & volume controls. I glue a piece of copper foil, say 1 x 2 inches onto a similar size piece of fish paper, then solder a short length of solid wire from the copper to the brass ground strip below the pots. Then a dab of RTV to hold the copper foil / fish paper "flag" in place and Bob's yer uncle.

[fdesalvo]

Thank you for those observations. Cheers!

[fdesalvo]

OK, this amp is finished. It's dead quiet in it's cab and away from the equipment on my bench's circuit. I'm completely floored by the responsiveness and tone of this amp, even at quiet volumes. I'm just completely beside myself! My Strat sounds so magical!!! AGHHHHHHH!!!!! These Utah's are AMAZING.

[Leo_Gnardo]

OK, this amp is finished. It's dead quiet in it's cab and away from the equipment on my bench's circuit. I'm completely floored by the responsiveness and tone of this amp, even at quiet volumes. I'm just completely beside myself! My Strat sounds so magical!!! AGHHHHHHH!!!!! These Utah's are AMAZING.

Ain't it wonderful, how a properly working Fender amp can sound !

I should have mentioned, with the chassis out of the box, there's no shielding over the open side of the chassis. I keep a slice of sheet brass handy, to place over the top of the chassis and see whether hum goes away when it's shielded. Once you put the chassis in the box, the screen stapled to the "ceiling" of the box usually drops the hum/buzz level quite a bit.

Glad to hear it's working well.

[fdesalvo]

Thanks, Leo!

I found about 6 cracked CC’s from the reverb recovery back to the inputs. I replaced every CC that went to a plate or was tied to ground and she’s just somethig to behold. Such a rich, 3D, and tactile thing. Haven’t even put in the Mullard 12at7s yet.