help with the bias caps on my pro reverb-pics inside

[sidehatch]

I've been working on my pro reverb for a while. I paid a local tech $180 to recapp it and know he did the electrolytics but I just realized he didn't do these:

I was examining my super and noticed it has just one blue xicon cap instead of these two white mallory caps.

I wonder why he didnt do them and if someone could guide me on what do do.

Thanks

[sidehatch]

Any thoughts on this?

I guess I could just make it like my super reverb but its laid out a little different.

[Firestorm]

What is the voltage rating and uF value of the blue xicon cap? Did the tech replace both Mallory caps with a single Xicon? Was he trying to blackface the circuit?

[billyz]

Those White Mallories seldom fail. They are sealed and may be a different composition. I don't change them unless they are bad. They will probably outlast you.

[ChrisM]

Those White Mallories seldom fail. They are sealed and may be a different composition. I don't change them unless they are bad. They will probably outlast you.

+1

Those screen resistors look like they would fail much sooner than the caps. Maybe it's just me but they look a bit on the small power size.

[sidehatch]

Cool! Thats good to know. I'll check the power rating on the screen resistors.

the tech replaced the super's cap with 1 xicon cap but I'd have to take it apart to see what value.

So I was just wondering why he didnt do it on the pro.

He's kinda a hard guy to just pick up the phone and talk to. He does good work but doesnt like to be bothered. He doesnt usually tell you what he did to your amp too That's why I'm here trying to learn how to work on them myself.

Thanks for the info guys!

Those White Mallories seldom fail. They are sealed and may be a different composition. I don't change them unless they are bad. They will probably outlast you.

+1

Those screen resistors look like they would fail much sooner than the caps. Maybe it's just me but they look a bit on the small power size.

[dano-rator]

He's kinda a hard guy to just pick up the phone and talk to. He does good work but doesnt like to be bothered. He doesnt usually tell you what he did to your amp too

His name isn't Howard Alexander "something or other" is it?

[Structo]

Also what is strange that he replaced the screen resistors but didn't replace the grid resistors.
Those look a bit toasty to me.

Yes, I would have used 5 watt screen resistors or at least 3 watt.
Those may be 3 watt.

It's hard to figure out why some techs do what they do.
Some are very vintage oriented so they only replace what absolutely needs replacing while other less gifted techs will yank all the blued molded caps out and put orange drops in.

The guys that know vintage Fender amps inside and out are the ones to go to as they are familiar with what fails in what amps and know what to replace the components with, spec wise.

Personally, I would have went through it and measured each carbon comp resistor and replaced any that had drifted by more than 20%.

I would also probably use metal oxide resistors for the high voltage areas such as the plate load resistors.
Some may gasp at that but I don't necessarily feel hiss and noise is part of the m0jo.

How about some more gut shots so we can further critique his job?

[briane]

Those white caps would not be considered part of an 'electrolitic replacement'.

Typically the electrolytic repair just covers the main filter caps, plus one expects the tech to look things over for burn marks, overheated components, etc...But not replace signal components that look good, or even look midway (but still work) unless explicitly directed to do so.

I suspect those white caps are cathode caps, but cant tell for sure. No reason to replace them IMHO.

Who knows, maybe those white caps have the mojo

[Structo]

Which circuit is your Pro Reverb?

All the ones I am looking at show a single 50uf 70v bias cap or a 25uf 50v.

[Phil_S]

The pair of white caps is the bias supply circuit. Look...the + side is grounded, and there is a white wire that goes to a pot that is not on the regular control panel. Those caps are probably fine, as is.

As far as a tech who doesn't want to discuss what he's going to do or what he's done...that's just wrong. He's fixing your property. You are entitled to know. I might understand that he doesn't want to engage in a debate with someone who knows less than he does, or just get into a long unproductive exchange, but no conversation at all gives me a bad feeling. A good business person deals with his customers in a forthright manner. Maybe you don't have another choice, but I'd look for someone else who will give you 5 minutes discussion. It isn't much to ask. If he doesn't like to talk, then a written report would be OK.

I realize it isn't exactly the same, but when I get my car serviced, I receive an itemized statement of the work that was performed. Same idea applies here.

[solderstain]

Which circuit is your Pro Reverb?

All the ones I am looking at show a single 50uf 70v bias cap or a 25uf 50v.

With as little as what's showing, I'd guess it's an AA1069 circuit. That's what mine was (a '74) before I converted it into a Bassman. Those bias caps and the location of their mounting in the chassis look just like what mine had before I converted it to a more 'conventional' Fender bias board location.

Again, just a guess, going by my own.

Oh, and I agree with billyz - those things last forever.

[Firestorm]

It looks like the early-mid '70s CBS circuit. You'll find it in master-volume models and ultralinear models as well as the more conventional models made at the same time. The white Mallorys aren't electrolytic (probably tantalum), so they don't automatically need replacement in a recap since they don't wear out and seldom fail. I'd guess that the Super had the conventional single electrolytic in the bias supply so it needed replacement whereas these didn't. This kind of bias supply is similar to what was used in the tweed models. It's interesting that Fender circled back to it so many years later.

[billyz]

Well, since everyone else is chiming in, I will too. ha ha.

Those white caps are your Bias supply capacitors. Normally they would be changed but those type don't fail often like regular electrolytics. Also, the values are pretty robust for that design. Your Screen grid resistors look to be 1 or 2 watt metal oxide, which are good. Being a vintage fender I would have used Carbon comp 470 ohm 1 watt screen grid resistors. the grid stop resistors should be 1.5k CC 1/2watt . There is no extra noise and no sonic reason to go to Metal, or film whatever resistors. There maybe some beneficial mojo tone character to the original type resistors. But, definitely historical accuracy will help the amp maintain its value as well as the much desired vintage tone.

Your tech may have blackfaced the PI . I see some Carbon Film resistors there.

He should be more informative though. I find that the guys who are secretive often have insecurities about their abilities.