Fender Tech question

Jon Light · Post by **Jon Light** » 1 Mar 2009 1:04 pm

I'm replacing original electrolytic caps on a BF Showman. I'm also checking resistor values and replacing ones that have drifted---some severe (!) bad readings. Unfortunately I don't have a replacement for this 820 ohm resistor in the photo. It reads around 1K (but it is grey/red/brown). I had to desolder one leg to get a proper reading--it was showing 440 ohms which was messing with my head until I thought to lift one leg.

So the question: should I be concerned about a 1K resistor in this spot that calls for 820 ohms or is this close enough for Leo (and me)?

btw--here's some of the old caps---

Donny Hinson · Post by **Donny Hinson** » 1 Mar 2009 1:21 pm

I wouldn't think it would cause a problem as long as it's within 10%-20% of the required value. Remember to save all the old parts (some people like to have them). Also, 820 ohms is a standard value, so you can easily get one of it bothers you.

Jon Light · Post by **Jon Light** » 1 Mar 2009 1:31 pm

I've got a respectable stock of components but for some reason I missed this value. The only reason I was concerned was that several years ago a valuable advisor mentioned some areas where I should really try to nail the values (if I find drifting) but since I have little or no actual theoretical knowledge, I don't remember and don't know know if this is one of those areas. Anyway, unless informed otherwise, I'll leave it be and move on.

Bert ten Hove · Post by **Bert ten Hove** » 1 Mar 2009 1:57 pm

The 820 resistor is the negative feedback resistor.
Changing the value will effect the tone (as a presence control).

Jim Sliff · Post by **Jim Sliff** » 1 Mar 2009 10:07 pm

What Bert said. 1k is probably fine, but you might pay around with anything from 420-1500 ohms and see how you like the sound. It won't hurt a thing. This is one of the ways of "voicing" a tube amp.

Jon Light · Post by **Jon Light** » 2 Mar 2009 3:17 am

ok---good. I gather that this is where some people will put a pot for voicing purposes.

Jon Light · Post by **Jon Light** » 28 Mar 2009 9:24 am

I've had so many interruptions that I'm only just getting back inside this amp. Just changed the power cord to 3-way.

question: if I were to change the slope resistor in one channel from 100K to 56K, which channel would make the most sense--Ch 1 which only has treble & bass controls or Ch 2 which also has the mid pot?

I figure that it makes no sense to have two identical channels so I'm looking to change flavors (subtly).

I welcome any other suggestions for minor mods. This is a BF Showman---non reverb.

Jon Light · Post by **Jon Light** » 28 Mar 2009 11:36 am

Oh well----moot point. Fired up the amp for the first time in years after a full recapping and replacing a bunch of resistors and it sounds like total crap and won't bias right. In other words someone like me is totally over his head. I can follow instructions on a cap job, I can replace components,no prob, I can read a diagram and do the 3-wire cord job but if anything is off I don't have a prayer of diagnosing it.

So anyway, although I'd be interested in the answer to today's question re: which channel to mess with the slope resistor, it doesn't matter a bit until I get a tech to work on this. Huge disappointment. Rats. Giant rats.

Jon Light · Post by **Jon Light** » 29 Mar 2009 8:59 am

If I'm going to talk to myself at least I've got good news for me. I found the huge but simple problem with the amp and it works like it should--loud and clean! Some idiot forgot to reattach a lead to the inverter when he was replacing a resistor and the amp sounded like a gravel pit.
Oh man, this is sweet.
Hope I am as thrilled to hear about this as I am.
Now to figure what to do about the dumbass tech. Probably go stuff a burrito and beer in his face. Mmmmm.

David Doggett · Post by **David Doggett** » 29 Mar 2009 2:46 pm

Glad you and yourself got this worked out,Jon. If you guys would work a little closer together it would probably minimize these types of problems.

Jon Light · Post by **Jon Light** » 29 Mar 2009 3:29 pm

Don't get me started. You wouldn't believe what I've got to put up with.

Did another good little tweak. The tremolo was too fast even at zero. Changed two .01 caps to .02 and that did the job. May get a new roach to see if I can get a little more swamp throb going. But actually I'm spoiled by outboard trems (I have Pod XT and VoodooLab trem) and their versatility so onboard trem is no big deal.

Jerry Erickson · Post by **Jerry Erickson** » 30 Mar 2009 5:11 am

Glad to hear that you guys got your amp going,Jon. You should get the BYOC trem kit. It's an easy, fun build and sounds great too.

Jon Light · Post by **Jon Light** » 30 Mar 2009 11:08 am

Yeah, Jerry. But it would have to get in line behind the BYOC delay I bought 3 years ago but still haven't taken out of the envelope. I hate when I get ambitious but...the other me keeps saying 'manana'.
Anyway, both the Pod and the Voodoo Lab sound real good. Plus you can shape the wave on both. If the amp had a killer bias trem maybe I'd be more interested in fine tuning it but I honestly don't think I can get this Showman tremolo to be as versatile as the outboards.

Michael Maddex · Post by **Michael Maddex** » 30 Mar 2009 4:52 pm

Ditto on glad to hear you and you found the problem. I was going to ask, "You did double check your work, didn't you?" but I figured you must have, and might have been insulted. Anyway,

Jon Light wrote:... question: if I were to change the slope resistor in one channel from 100K to 56K, which channel would make the most sense--Ch 1 which only has treble & bass controls or Ch 2 which also has the mid pot? ...

I would probably pick Ch 1 because if I'm going to hardwire tone mods, it might as well be the channel with the fewer controls. In other words, get the tone built-in in Ch 1 and leave Ch 2 with more knobs and switches as the flexible one. Just MHO. YMMV.

Whip Lashaway · Post by **Whip Lashaway** » 30 Mar 2009 4:53 pm

Hey Jon, For future reference on the resistors...If there is a gold band it is 5%, silver is 10% and no band is 20%. That's following the value bands. So your 820 ohm resistor with no tolerance band could be anywhere from 656 to 984. So technically it's out of tolerance but I agree that I'd leave it alone unless it was giving you problems. Good luck.

Jon Light · Post by **Jon Light** » 31 Mar 2009 10:35 am

Thanks for sharing thoughts.
Here's where I and I is at now----this amp sounds so good. The normal channel is so right that I'm just not going to touch it. Ch 2----the mid control teases me with some sounds that I really like to be able to dial in. Rather than mess with the slope resistor I'm thinking of changing to a 25K pot there---see if I can get a little more range out of the freq curve currently at work. I'd consider messing with some .22 caps on that channel for a different flavor except that A) it sounds good as is and B) it has those original blue molded caps. I'd probably only pull those if I were not happy with the amp and needed to try something. But considering how people are always oohing and aahing about those old caps I can't see touching them.
This amp does not need tweaking. I'm just thinking of doing stuff because I can.
And I'm just thinking of doing nothing to it.

Brint Hannay · Post by **Brint Hannay** » 31 Mar 2009 11:30 am

FWIW, I think the 25K mid pot is a good idea.

Now, where the heck did I put those parts for the Pro Reverb, how many years ago was it?