Help With: Bassman "Thud" With Standby Switch

[Hazy]

I also posted this at PP...

It seems like my '66 Bassman head just started slamming the power tubes when I switch from Standby to On, to where there a semi-loud speaker "thud" as the amp switches to on.

Also, it seems that the background noise at idle is louder than before. Here's what I've done:

1. 1 month ago I replaced the PI and Normal channel signal caps with NOS CDE caps (all .1uf @ 600v). All seemed normal before and for a while after this.

2. Replaced and re-baised the tubes. (yesterday)

3. Re-tensioned the power tube sockets. (yesterday)

4. Did "tap, tap" around the circuit for loose connections, especially at the tube sockets - nothing. (yesterday)

Any ideas on the "thud". My other amps don't make that sound coming off of standby.

Thanks,
Hazy

[Flames1950]

It's not that uncommon for Fenders to "thud" coming off standby, I'm sure my '66 Bassman and '67 Bandmaster both do it.
Look at a schematic or layout for a 5F6-A Bassman or JTM45, they use a .05uF cap off the standby to eliminate the pop.
Background noise problems in Fenders of the blackface era is usually the stock carbon comp plate resistors (the 100K's.) They make lots of crackles and sizzling noises as they get old. Are yours still stock or had you changed them out?

[Hazy]

Flames,

Thanks for chimin' in. You know, maybe the thuds been about like that the whole time...it just seems louder now.

Here's the board...not original...at all



I replaced/installed most of the current resistors and almost everything on the board's been changed. The inside fire/corrosion happened before my time.

Determined to fix something, I did "tap tap" again and found some loose solders at V3. Re-soldered, re-tensioned and cleaned V3. The thud is still there but I just recovered the sound.

Thanks

[VelvetGeorge]

Please, replace those 470 1 watts on the tube sockets with at least 2 watts!

George

[Hazy]

Please, replace those 470 1 watts on the tube sockets with at least 2 watts!

George

Thanks, George. I'll do it pronto pronto. On the schematic, it has 1W.

[Billy Batz]

Please, replace those 470 1 watts on the tube sockets with at least 2 watts!

George

Thanks, George. I'll do it pronto pronto. On the schematic, it has 1W.

Yeah and under normal circumstances 1W is fine but theres no room to breath. Those things burn up very often in fenders.

[Hazy]

I've got another problem. When I crank it up, I get a crackle mixed in with the OD...it's not pleasant.

Something's not right. It seems as though it was working fine before the CDEs.

Where would I start to check for my infamous "crackle"? Also, it sounds as though it has less power than my '67 Bassman. And, you don't want to be in the same room with that one once it gets much past 3 on the V...that's with a single 12".

Can you give me some spots to start? Would a DMM with capacitance help? I'm always looking for a new tool.

Thanks

[Hazy]

I've got another problem. When I crank it up, I get a crackle mixed in with the OD...it's not pleasant.

Something's not right. It seems as though it was working fine before the CDEs.

I must have fried one of the two 100K resistors (across from the bias splitter resistors) when I installed the CDE caps. I checked last night and finally found one of the two open, replaced it and re-biased.

Back to sparkly/grindy Bassman stuff.

Man, glad that little search is over.

[gnugear]

That's good news. I want to pick up a bassman now to add to my Fender collection.

[Hazy]

That's good news. I want to pick up a bassman now to add to my Fender collection.

You'll love 'em. Try to get a 65 - 67 in need a of a bit of TLC and you'll most likely get a better deal. These amps kick ^$%#e and they're easy to work on...I have a 66 and a 67.