Power supply Dropping resisters question, Silverface Fender Bandmaster

Ossyrocks

I’ve looked up my PT transformer code now, and it’s a Super Reverb tranny. Even though this amp started it’s life as a Bandmaster Reverb, I think they just pulled a Super chassis and stuck it in a head box at the factory. The only thing they changed was the OT, which is correct for a Banmaster/Vibrolux Reverb/Pro Reverb. My amp even has a Super Reverb back panel saying it requires a 2 ohm load, which it clearly does not!

Keefy

Gut shots for us perverts please!

Ossyrocks

Keefy said:

Gut shots for us perverts please!

Ha ha ,ok.

RickLucas

Ossyrocks said:

RickLucas said:

However, times have changed and I’ve got a bit more sensible

How are you doing @RickLucas ? One day, when this is all over, you should have a run up to the coast for a brew and a catch up.

Rob

Very well thank you, Rob. They come, they go, they come back, it seems. I'll certainly take you up on that brew. Keep one hand in your pocket while you're poking around in there, mate. I understand entirely the desire to go down rabbit holes (I'm onto my second lockdown Lucaster Tele build), but surely your Deluxe Reverb will provide what you're looking for here?  

Ossyrocks

RickLucas said:

Ossyrocks said:

RickLucas said:

However, times have changed and I’ve got a bit more sensible

How are you doing @RickLucas ? One day, when this is all over, you should have a run up to the coast for a brew and a catch up.

Rob

Very well thank you, Rob. They come, they go, they come back, it seems. I'll certainly take you up on that brew. Keep one hand in your pocket while you're poking around in there, mate. I understand entirely the desire to go down rabbit holes (I'm onto my second lockdown Lucaster Tele build), but surely your Deluxe Reverb will provide what you're looking for here?  

My Deluxe needs some tlc too, it’s next on the bench, it’s humming badly despite me handing it over to a friend and part time amp tech last year to sort it out. He said he did fix it, and I then let him keep it for six months to use to record an album and do some gigs but on firing it up after I got it back, it was just the same. 

This amp is just for fun, to try some mods etc, and learn more about amps in the process.
Your Tele builds are very entertaining, keep it up.
Roll on the time when we can get together again, perhaps your good lady would come up too and we can have lunch or something.

RickLucas

For sure. P90s and cherry red on this one.

Ossyrocks

Quick update on this. 

I finally finished working on this amp and I’m quite pleased with it.

First, the dropping resistors. I fitted 10k and 10k in place of the 4.7k and 1k which were stock, this dropped my voltages as follows.

B+1 is now 447
B+2 is now 445
B+3 is now 355
B+4 is now 302

This is much more in line with where I wanted to go. It’s taken some of the headroom out and made it a bit warmer, softer and little more harmonic.....I think.

In doing all this work I was chasing noise all over the place and even after changing out all the anode load resistors I was still getting hiss and pops and crackles. In the end, I resorted to the chopstick as I was sure I must have a bad connection somewhere, or a bad ground. This was where I discovered that the entire board was noisy! Tapping the fibreboard anywhere induced lots of banging and crackling and I was on the point of considering replacing the whole eyelet board. Instead I decided to try to dry it out to see if that would work. I removed the three screws holding down the eyelet board and pulled it up away from the board underneath and held it there with three strat knobs underneath. My intention was to get some heat under it for a while to see if it would improve.

Almost as an afterthought, I fired it up in this state, with the eyelet board held up on the strat knobs. All the noise had vanished! Basically the top board and components were interacting with the bottom board somehow. 

So next I cut a sheet of plastic the same size as the board and carefully inserted it between the two. Then screwed it back down after punching three holes in the plastic for the screws to get down through the chassis. It’s quiet as a mouse now! No undue noises and it sounds great. It’s a lesson learned.

Cheers,
Rob

ICBM

Ossyrocks said:

Almost as an afterthought, I fired it up in this state, with the eyelet board held up on the strat knobs. All the noise had vanished! Basically the top board and components were interacting with the bottom board somehow.

That's very common and can be extremely baffling if you've not come across it before. Basically the waxed fibreboard has become slightly conductive - and for some reason it much more commonly affects the lower board. Sometimes the result is odd crackling, sometimes instability giving odd tone or even feedback, depending on where the leakage is.

Ossyrocks said:

So next I cut a sheet of plastic the same size as the board and carefully inserted it between the two. Then screwed it back down after punching three holes in the plastic for the screws to get down through the chassis. It’s quiet as a mouse now! No undue noises and it sounds great. It’s a lesson learned.

I use vintage* cornflakes boxes .




(*A week old is vintage enough .)

Ossyrocks

@ICBM I will know next time! It does make me think I have seen this before but never realised. I probably sold the amp out of disappointment. 

Ossyrocks

Ok, as promised here are some pics for @Keefy ;

The cab is by Gartone, the chassis is a 71 Bandmaster.





Chassis



Rob Robinette Lead Mod + Reverb & Trem on Normal Channel



Negative Feedback mod using the ground switch. 820 ohm (stock) + None + 1.5k (Lighter)



The Lead mod is actually really good. I think it's based on the Marshall 1987 and it certainly sounds like it is. It's much tighter, which will really suit humbuckers and drive pedals.

I also fitted a defeat switch on the tremolo circuit to add more gain. This is a replacement pot for the Intensity control which is switchable when you turn it all the way down and then off.

With Tremelo defeated, NFB defeated, and into the Lead Channel it gets some really nice breakup. Plus with both channels now in phase, I can footswitch between the two channels.

Thanks for looking,
Rob

Ossyrocks

I forgot to say, those speakers are coming out soon. They are very efficient at 100db, and whilst they sound immense, they are making the amp too loud for me and also way too heavy. I have a pair of vintage CTS Alnicos now awaiting recone, so I'm going to fit those when they come back, and that will be the amp finished apart from pre-amp valve selection.

Keefy

Ossyrocks said:

Ok, as promised here are some pics for @Keefy ;

The cab is by Gartone, the chassis is a 71 Bandmaster.





Chassis



Rob Robinette Lead Mod + Reverb & Trem on Normal Channel



Negative Feedback mod using the ground switch. 820 ohm (stock) + None + 1.5k (Lighter)



The Lead mod is actually really good. I think it's based on the Marshall 1987 and it certainly sounds like it is. It's much tighter, which will really suit humbuckers and drive pedals.

I also fitted a defeat switch on the tremolo circuit to add more gain. This is a replacement pot for the Intensity control which is switchable when you turn it all the way down and then off.

With Tremelo defeated, NFB defeated, and into the Lead Channel it gets some really nice breakup. Plus with both channels now in phase, I can footswitch between the two channels.

Thanks for looking,
Rob

Thanks Rob, you've made an old man very happy!