Post Phase Inverter Master Volume Mod.....74 twin reverb has small intermittent problem

[fatz treeboy]

good morning, i am a first time thread poster. . I just got back my 74 SF Quad Reverb ( cut down to a head) from Guitronics in san francisco 2 months ago. we did a Post Phase Inverter Master Volume Mod.( PPIMV) thats a mouth full.

i got a small problem also with this amp..i could use some advice please. After a month of happyness the vibrato channel goes silent after 60 seconds of play.... Then if i wait 10 seconds and re plug it in and it works. i was hoping to fix this at home. Does that intermittent play sound like a V2 tube? a pre-amp tube ? it has to be..... for the first 60 seconds the amp is all there roaring great.... then clicks off for a bit before roaring again when u unplug n replug the guitar.....there is nothing wrong with the sound or the volume.... today i was gonna remove v1 and put it in v2. and just leave v1 out. Thank You

i have owned many SF twins and like everyone else i tried fuzz pedals, boss distortion pedals, boosts, all kinda stuff to get my twin to sound more growly rock n roll. I now found my holy grail of twin reverb tone. i googled this mod and it seems like their are several versions? PPIMV mod is in a class by itself. you will be shocked how bad ass your twin reverb will sound

 

[1stpitch]

Man, that's hard to read.

 

[Lynxtrap]

Yes, try to swap the tubes between V1 and V2. If the problem follows the tube, you know it's the tube.

Seems a bit weird that you can "reboot" the tube by unplugging the guitar. It could be the input jack. Have you tried plugging into both jacks on the normal channel?

 

[fatz treeboy]

Man, that's hard to read.

oops..... i never said i went to school . i'm gonna rewrite it... stay tuned

 

[fatz treeboy]

Yes, try to swap the tubes between V1 and V2. If the problem follows the tube, you know it's the tube.

Seems a bit weird that you can "reboot" the tube by unplugging the guitar. It could be the input jack. Have you tried plugging into both jacks on the normal channel?

i did use deoxit on my cable and worked it bk and forth, in and out. if the tube swap does not fix my problem i will go to the shop . i am just blown away how much this mod changed my amp. In a good way......

 

[Peegoo]

oops..... i never said i went to school . i'm gonna rewrite it... stay tuned

That is a very useful mod.

If the tube swap does not change things, try this: place a fan behind the amp, blowing air into it. Switch the amp on and replicate the problem. If the fan makes the problem goes away or it delays the onset of the issue, it's a thermally-induced fault. In many cases it's a poor solder joint, and in some cases it's a component issue. The only practical way to chase it down to the specific connection or component is to do a signal trace through the amp. Your tech--if he's the best in SF as you claim, should fix this for you because it wasn't doing it before he took a soldering stick to it.

If it's not a heat issue, it may be parasitic oscillation, which can cause an amp to go quiet. This can damage the output transformer or the speaker. The weird thing about this is the amp doesn't actually go quiet; it "runs away," producing a high-frequency tone that feeds back in a loop. This tone overpowers all other frequencies and is higher than the range of human hearing. In extreme cases, this can cook a speaker because the cone does not move to pump air over the voice coil and it overheats, melting the glue that holds it together. It can also overheat the output transformer. The usual cause of parasitic oscillation is a poor ground or improper lead dress. Your tech should know about this too.

 

[fatz treeboy]

That is a very useful mod.

If the tube swap does not change things, try this: place a fan behind the amp, blowing air into it. Switch the amp on and replicate the problem. If the fan makes the problem goes away or it delays the onset of the issue, it's a thermally-induced fault. In many cases it's a poor solder joint, and in some cases it's a component issue. The only practical way to chase it down to the specific connection or component is to do a signal trace through the amp. Your tech--if he's the best in SF as you claim, should fix this for you because it wasn't doing it before he took a soldering stick to it.

If it's not a heat issue, it may be parasitic oscillation, which can cause an amp to go quiet. This can damage the output transformer or the speaker. The weird thing about this is the amp doesn't actually go quiet; it "runs away," producing a high-frequency tone that feeds back in a loop. This tone overpowers all other frequencies and is higher than the range of human hearing. In extreme cases, this can cook a speaker because the cone does not move to pump air over the voice coil and it overheats, melting the glue that holds it together. It can also overheat the output transformer. The usual cause of parasitic oscillation is a poor ground or improper lead dress. Your tech should know about this too.

thanks....... i did switch 1 and 2 and that solved it . it has been runnin fine now ........ as to the PPMVM, for a 1970's sf twin, is the holy grail of what to do with all the under loved twins out there........ if u are gonna play ur twin, may as well mod it to be a monster !!!!